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Entropy Scaling of Viscosity IV─Application to 124 Industrially Important Fluids
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Thermophysical and Thermochemical Properties

Entropy Scaling of Viscosity IV─Application to 124 Industrially Important Fluids
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Journal of Chemical & Engineering Data

Cite this: J. Chem. Eng. Data 2025, 70, 2, 727–742
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https://doi.org/10.1021/acs.jced.4c00451
Published January 10, 2025

Copyright © 2025 The Authors. Published by American Chemical Society. This publication is licensed under

CC-BY 4.0 .

Abstract

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In our previous work [Yang, X. J. Chem. Eng. Data 2021, 66, 1385–1398], a residual entropy scaling (RES) approach was developed to link viscosity to residual entropy using a 4-term power function for 39 refrigerants. In further research [Yang, X. Int. J. Thermophys. 2022, 43, 183], this RES approach was extended to 124 pure fluids containing fluids from light gases (hydrogen and helium) to dense fluids (e.g., heavy hydrocarbons) and fluids with strong association force (e.g., water). In these previous research studies, the model was developed by manual optimization of the power function. The average absolute relative deviation (AARD) of experimental data from the RES model is approximately 3.36%, which is higher than the 2.74% obtained with the various models in REFPROP 10.0. In the present work, the power function was optimized by iteratively fitting the global (fluid-independent power terms) and local parameters (fluid-specific and group-specific parameters) and screening the experimental data. The resulting equation has only three terms instead of four. Most notably, the AARD of the new RES model is reduced down to 2.76%; this is very close to the various multiparameter models in REFPROP 10.0, while the average relative deviation (ARD) amounts to 0.03%, which is smaller than REFPROP 10.0’s 0.7%. A Python package is provided for the use of the developped model.

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1. Introduction

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With a residual entropy scaling (RES) approach, the residual part of the transport properties (mainly viscosity, thermal conductivity, and self-diffusion) of pure fluids can be expressed using a single variable: residual entropy. The residual entropy is a thermodynamic property and can be easily calculated with many well-developed and accurate equations of state (EOS). With this feature, the RES approach has been attracting attention in recent years and has been developed in many ways and used successfully by different authors. For example for viscosity, RES approaches have been applied to the Lennard-Jones (L-J) fluid and other spherically symmetric simple potentials, (1,2) as well as hundreds of real fluids including hydrocarbons, (3−5) refrigerants, (6−9) and other fluids of industrial interest. (10−13) Specifically, Lötgering-Lin et al. (11) and Dehlouz et al. (14) developed a generalized RES approach for viscosity of more than 100 pure fluids. For the thermal conductivity, RES approaches are also successful for many real fluids. (9,15−19)
In our previous work, (6) an RES approach was developed to link residual viscosity to residual entropy using a 4-term power function for 39 refrigerants. Later, this approach was extended to 124 pure fluids (12,13) containing a variety of fluids from light gases (hydrogen and helium) to dense fluids (e.g., heavy hydrocarbons) and fluids with strong association (e.g., water). This simple 4-term power function consists of different types of parameters: global parameters (i.e., the structure and power terms) for all fluids, group-specific parameters for a group of similar fluids, and fluid-specific parameters for each fluid. These parameters are explained in detail in Section 2.2. In the above-mentioned papers, (6,12,13) the global parameters (structure and power terms) were determined manually by choosing the ones that best fit the approximately 50,000 well-evaluated experimental data points. With such a trial-and-error method, the average absolute relative deviation (AARD) (corresponding to scattering) of these experimental data from the RES model is approximately 3.36%, which is higher than the 2.74% obtained with the various state-of-the-art models in REFPROP 10.0. (20)
The primary aim of this work is to optimize these global parameters of the power function by means of a self-developed constrained numerical optimization routine. We aim at delivering an RES approach that has the same level of accuracy as the state-of-the-art models implemented in REFPROP 10.0, while using fewer parameters and an approach that is reproducible.
The RES approach calculates only the residual part of the viscosity; the remaining dilute gas viscosity ηρ→0(T) still needs to be determined by another method. If the highest accuracy is the aim, ηρ→0(T) should be calculated with the pure-fluid correlations implemented in REFPROP 10.0. These fluid-specific correlations are available when low-density viscosity data are available in the literature. REFPROP 10.0 cannot calculate ηρ→0(T) for some mixtures, especially those involving alcohols. Therefore, in the present work, other methods are also adopted for pure-fluid ηρ→0(T) calculations: (1) ηρ→0(T) is modeled as a polynomial of temperature with parameters fitted against REFPROP 10.0 results, (2) calculation with known L-J parameters as in our previous work, (6) and (3) calculation with L-J parameters which are estimated according to critical point information. The first method aims at achieving high accuracy, while the latter two aim at using fewer parameters for the viscosity calculation, allowing for a broader predictive application.
This work is structured as follows. In Section 2.1, we provide a brief overview of the RES approach and methods to calculate ηρ→0(T). Thereafter, in Section 2.2, we discuss the design of the power function, the different types of parameters, how and when they are used, and how they were determined. In Section 3, the results are presented and compared to previous work and to REFPROP version 10.0. Finally, conclusions are drawn in Section 4. Part of this endeavor was supported by subproject 3 of the KETEC (Research Platform Refrigeration and Energy Technology) project. (21) One important goal within KETEC subproject 3 is to model thermophysical properties of lubricant + refrigerant mixtures, and this work focuses on the viscosity of more general types of fluids.

2. Computational Methods

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2.1. Fundamentals

In this section, we provide an overview of the RES approach and various methods to calculate the dilute gas viscosity ηρ→0(T). The fluid viscosity η at temperature T can be expressed as the sum of the dilute gas part ηρ→0(T) and the residual part ηres(sr)
η=ηρ0(T)+ηres(sr,T,ρN)
(1)
Here sr denotes the molar residual entropy.
Table 1. Brief Descriptions for Each of the Fluid Groups
Gr.description
1light gaseous fluids with quantum effects in the low temperature range, mainly hydrogen, its spin isomers, and helium
2gaseous fluids, e.g., the noble gases
3a majority of light hydrocarbons and halogenated hydrocarbons (most of refrigerants belong to this group)
4fluids with benzene rings and similar fluids
5medium hydrocarbons and similar fluids
6heavy hydrocarbons and dense fluids
7fluids with light intermolecular association among molecules like methanol
8fluids with strong intermolecular association among molecules like water
Table 2. Group-Specific Parameters ng,1, ng,2, and ng,3 for the Eight Groups of Fluids
groupng,1ng,2ng,3
11.297005–3.1042172.257168
20.363576–0.0749380.005159
30.392983–0.1675280.037984
40.305935–0.1283910.028658
50.286312–0.1110900.022642
60.220249–0.0702320.011963
70.305932–0.1717620.050058
80.297539–0.2099490.068151
Table 3. Parameters for All Pure Fluids Considered in This Worka
REFPROP fluid namebest availablegroupξzan1n2n3
13BUTADIENEREFPROP 10.021.284710.258856–0.0788860.017208
1BUTENEboth50.835610.430921–0.2031910.047206
1BUTYNEpredictive40.972300.305935–0.1283910.028658
1PENTENEpredictive40.993800.305935–0.1283910.028658
22DIMETHYLBUTANEproposed model30.967410.644029–0.3885940.104904
23DIMETHYLBUTANEproposed model31.036910.328817–0.1181970.023611
3METHYLPENTANEREFPROP 10.050.868210.516076–0.2829090.0711
ACETONEREFPROP 10.040.935810.442736–0.2244320.054166
ACETYLENEproposed model30.997700.392983–0.1675280.037984
AMMONIAREFPROP 10.070.910210.238911–0.077760.017714
ARGONREFPROP 10.020.95710.441631–0.1983070.064599
BENZENEREFPROP 10.040.837710.252163–0.0431190.000423
BUTANEREFPROP 10.031.026310.369686–0.1486510.032041
C11proposed model51.059710.298732–0.1196670.024296
C12REFPROP 10.051.102110.26995–0.102850.020241
C16REFPROP 10.051.238510.236788–0.0853330.015984
C1CC6REFPROP 10.030.978310.418776–0.1883870.044392
C22proposed model61.15910.191955–0.0626550.01086
C2BUTENEproposed model50.819510.512632–0.2722620.067693
C3CC6predictive31.115400.392983–0.1675280.037984
C4F10predictive31.062800.392983–0.1675280.037984
C5F12few data50.85210.409441–0.1856610.04174
C6F14both50.890210.443823–0.2118720.048157
CF3Ifew data30.935810.697509–0.4808120.145142
CHLORINEREFPROP 10.030.886600.392983–0.1675280.037984
CHLOROBENZENEproposed model40.924910.323888–0.1301180.028328
COproposed model2100.363576–0.0749380.005159
CO2REFPROP 10.031.017410.23350.007837–0.023471
COSpredictive30.932500.392983–0.1675280.037984
CYCLOBUTENEpredictive30.978900.392983–0.1675280.037984
CYCLOHEXproposed model30.897310.741397–0.4723540.132227
CYCLOPENREFPROP 10.030.949510.41325–0.1757640.040135
CYCLOPROfew data30.951700.392983–0.1675280.037984
D2proposed model11.351810.396609–0.5105620.40281
D2OREFPROP 10.081.044510.280658–0.1821780.056524
D4both60.842610.3353–0.1318420.026054
D5both60.850110.33123–0.1247030.023571
D6predictive51.095900.286312–0.111090.022642
DEAproposed model71.30110.365242–0.1964520.04811
DECANEREFPROP 10.051.02610.306021–0.1251380.02603
DEEREFPROP 10.031.14810.319833–0.1246790.025516
DMCREFPROP 10.031.186510.322188–0.1397030.031902
DMEREFPROP 10.031.077210.357695–0.151620.03393
EBENZENEREFPROP 10.040.967810.371309–0.1689430.038494
EGLYCOLboth71.217610.422328–0.228430.056447
ETHANEREFPROP 10.030.945610.404667–0.1685290.0384
ETHANOLproposed model71.136210.292038–0.1601780.043583
ETHYLENEboth30.934810.391366–0.1595540.036436
ETHYLENEOXIDEproposed model31.04610.599879–0.3673310.099849
FLUORINEboth21.063210.453053–0.2066860.053423
H2Sproposed model70.758510.225851–0.00749–0.007898
HCLproposed model70.727610.0285740.241739–0.097719
HELIUMboth11.266101.297005–3.1042172.257168
HEPTANEREFPROP 10.050.928310.321695–0.1309730.027822
HEXANEproposed model50.891310.355662–0.1511150.032927
HYDROGENREFPROP 10.011.013612.272439–6.2777654.310689
IBUTENEboth31.014610.406355–0.1884490.045383
IHEXANEproposed model50.8710.368669–0.1581430.034655
IOCTANEproposed model50.832310.416715–0.1912160.043679
IPENTANEproposed model31.034410.391104–0.1689770.038118
ISOBUTANproposed model30.950410.443856–0.2039790.048521
KRYPTONREFPROP 10.020.949710.574447–0.4328660.18256
MD2Mpredictive61.001300.220249–0.0702320.011963
MD3Mpredictive61.094700.220249–0.0702320.011963
MD4Mpredictive61.173300.220249–0.0702320.011963
MDMproposed model60.904310.272782–0.0942780.01691
MEAproposed model71.17910.394192–0.2006010.048884
METHANEREFPROP 10.021.030110.436026–0.2084910.062506
METHANOLproposed model71.073610.3169–0.1942290.056838
MILPRF23699predictive61.348200.220249–0.0702320.011963
MLINOLEAREFPROP 10.061.255810.151958–0.0413490.006241
MLINOLENREFPROP 10.061.272810.153776–0.0440090.006933
MMfew data60.824810.575147–0.2991090.069042
MOLEATEREFPROP 10.061.193810.183461–0.0551580.008903
MPALMITAboth61.070410.254341–0.0901170.016221
MSTEARATfew data61.1110.078226–0.007601–0.000082
MXYLENEREFPROP 10.040.99610.317989–0.1355730.030302
N2Oproposed model21.200910.1025610.326024–0.210582
NEONREFPROP 10.020.856110.375722–0.010707–0.018149
NEOPENTNboth30.892210.415494–0.1534630.030807
NF3proposed model30.825300.392983–0.1675280.037984
NITROGENREFPROP 10.021.009210.340291–0.0910780.021042
NONANEproposed model50.993710.35713–0.1591160.034753
NOVEC649predictive41.04800.305935–0.1283910.028658
OCTANEREFPROP 10.050.959210.327911–0.1381810.029726
ORTHOHYDpredictive1101.297005–3.1042172.257168
OXYGENboth20.587900.363576–0.0749380.005159
OXYLENEREFPROP 10.040.947510.381394–0.1786260.041899
PARAHYDpredictive11.028401.297005–3.1042172.257168
PENTANEproposed model31.080610.404638–0.1848150.042214
POE5predictive61.267500.220249–0.0702320.011963
POE7predictive61.449200.220249–0.0702320.011963
POE9predictive61.70600.220249–0.0702320.011963
PROPADIENEpredictive30.972400.392983–0.1675280.037984
PROPANEREFPROP 10.030.974410.406055–0.1783570.041571
PROPYLENproposed model40.822810.511568–0.2699740.067762
PROPYLENEOXIDEproposed model31.07410.490791–0.2658740.067684
PROPYNEfew data31.006900.392983–0.1675280.037984
PXYLENEREFPROP 10.040.981410.35906–0.166160.038731
R11proposed model30.967410.424119–0.1889060.04421
R1123predictive31.118800.392983–0.1675280.037984
R113proposed model30.952810.470361–0.2254420.054716
R114both30.973310.615057–0.3786720.10544
R115proposed model30.949710.416234–0.1852890.044498
R116both31.198100.392983–0.1675280.037984
R12proposed model30.975210.336945–0.0941210.011411
R1216predictive31.101700.392983–0.1675280.037984
R1224YDZREFPROP 10.031.126410.1467730.050263–0.031134
R123proposed model31.043410.34979–0.1354780.028526
R1233ZDEREFPROP 10.031.122710.3948–0.1840150.042551
R1234YFREFPROP 10.031.067610.267769–0.0553410.001703
R1234ZEEproposed model31.088710.307227–0.1076870.020815
R1234ZEZpredictive31.105100.392983–0.1675280.037984
R124proposed model31.028810.41212–0.189450.044478
R1243ZFpredictive31.040600.392983–0.1675280.037984
R125REFPROP 10.031.029810.326005–0.1091540.019807
R13REFPROP 10.030.986610.385255–0.1584150.035273
R1336MZZZREFPROP 10.031.151310.0953820.091688–0.043208
R134AREFPROP 10.031.058810.310059–0.1061550.020416
R14both31.019410.450154–0.307210.112828
R141Bproposed model30.998110.368577–0.1467870.032034
R142BREFPROP 10.031.005410.1986860.025088–0.024214
R143AREFPROP 10.031.030610.1514590.08917–0.050429
R150proposed model31.026810.535701–0.3052610.080679
R152AREFPROP 10.031.058710.265764–0.0666090.008754
R161proposed model31.052700.392983–0.1675280.037984
R21both31.029410.335415–0.1127030.019639
R218REFPROP 10.030.996510.361546–0.1427150.031279
R22both31.034310.392154–0.1742820.040375
R227EAproposed model31.057110.333557–0.1230020.02453
R23REFPROP 10.031.085110.23995–0.0533480.006059
R236EAproposed model31.046710.336455–0.1268990.026339
R236FAREFPROP 10.031.089710.323674–0.1190770.023505
R245CAproposed model31.044210.3904–0.1685510.037777
R245FAREFPROP 10.031.080410.354663–0.1531910.035329
R32REFPROP 10.031.110210.0152840.220607–0.095156
R365MFCpredictive31.075500.392983–0.1675280.037984
R40REFPROP 10.030.981110.1423190.110023–0.057116
R41few data30.987100.392983–0.1675280.037984
RC318both30.979210.490322–0.2477410.061422
RE143Apredictive31.014100.392983–0.1675280.037984
RE245CB2predictive31.0900.392983–0.1675280.037984
RE245FA2proposed model31.124910.386539–0.1763110.040336
RE347MCCboth31.11910.314588–0.1114380.020902
SF6REFPROP 10.021.096910.1822820.122179–0.070003
SO2proposed model21.335510.0003760.167659–0.062185
T2BUTENEpredictive31.006200.392983–0.1675280.037984
TOLUENEREFPROP 10.040.919110.377943–0.1749810.041297
VINYLCHLORIDEpredictive31.042900.392983–0.1675280.037984
WATERREFPROP 10.081.003810.294605–0.211610.069198
XENONproposed model20.951410.421622–0.1019080.008931
a

The column “best available” indicates whether the proposed model is the best available based solely on our data set. The entries are as follows: If ARD and AARD are both lower compared to using REFPROP 10.0’s default model, “proposed model” is assigned. If the AARD of the proposed model is lower, while the ARD is higher, “both” is indicated. For fluids, where REFPROP 10.0’s default models perform better on both indicators, “REFPROP 10.0” is written. In cases, where our data set contains less than 20 data points, “few data” is specified. “predictive” is used in cases, where the coefficients are determined without any data. The column za indicates whether fluid-specific parameters are available (za = 1) or not (za = 0). If fluid-specific parameters are available, then ξ = 1 is used in the calculation instead of the shown value for ξ. The equation with appropriate global exponents is .

Figure 1

Figure 1. AARD (upper plot) and ARD (lower plot) of the filtered (or analyzable) experimental data from the model predictions. REF. models refers to the default viscosity models in REFPROP 10.0. For the remaining models, the labels comprise two parts connected by a “+” symbol. The former part denotes the method for dilute gas viscosity ηρ→0(T) calculation (see Section 2.1): oLJ uses L-J parameters, oREF uses REFPROP 10.0, oP3 uses a third-order polynomial, oP4 uses a fourth-order polynomial, and oCP uses the critical point information. The latter part refers to the residual viscosity calculation method: rP4 is a 4-term power function used in the previous work (6) and rP3 is the 3-term power function developed in the present work.

Figure 2

Figure 2. AARD of the REFPROP 10.0 models and the oREF + rP3 model predictions to the filtered experimental data for each pure fluid. The fluids are further classified according to the model types used by REFPROP 10.0: (a) reference correlations, (32−67) (b) ECS model with fitted parameters or friction theory models, (47,58,61) and (c) ECS model without fitted parameters and other models. (68−70) Fluid names in REFPROP 10.0 are adopted (see the Appendix for their respective IUPAC chemical names and CAS registry numbers). The entries are sorted primarily by whether the proposed RES model has a lower AARD than that of REFPROP 10.0 and secondarily according to increasing AARD of the proposed model. The green background indicates that the proposed model is better.

Figure 3

Figure 3. ARD of the REFPROP 10.0 models and the oREF + rP3 model predictions to the filtered experimental data for each pure fluid. The fluids are further classified according to the model types used by REFPROP 10.0: (a) reference correlations, (32−67) (b) ECS model with fitted parameters or friction theory models, (47,58,61) and (c) ECS model without fitted parameters and other models. (68−70) Fluid names in REFPROP 10.0 are adopted (see the Appendix for their respective IUPAC chemical names and CAS registry numbers). The entries are sorted primarily by whether the proposed RES model has a lower absolute ARD than that of REFPROP 10.0 and secondarily according to increasing ARD of the proposed model. The green background indicates that the proposed model is better.

Figure 4

Figure 4. Plus-scaled dimensionless residual viscosity ηres+ as a function of s+/ξ for each group of pure fluids, where s+ is the plus-scaled residual entropy and ξ is the fluid-specific scaling factor. The group-specific parameters ni,g are used to calculate the curves. All groups are shown at the bottom. Each group is also illustrated and annotated stacked by a power of 20 at the top to highlight the qualitative differences.

Figure 5

Figure 5. Ratio ξ/scrit+ of all considered pure fluids, where ξ is the fluid-specific scaling factor and scrit+ is the plus-scaled dimensionless residual entropy at the critical point, which is obtained from REFPROP 10.0. The group numbers are annotated in the top right of each box. At ξ/scrit+=0.7, a vertical dashed dotted line is drawn. Values for group 1 exceed the plot limits, i.e., PARAHYD: 1.67, ORTHOHYD: 1.61, HYDROGEN: 1.64, HELIUM: 4.09, D2:1.86.

Figure 6

Figure 6. ARD and AARD of the experimental data of 351 binary mixtures from predictions of models. “All selected data”: for calculations with the RES model, similar filters as used in pure fluids were applied for mixture data. “Further filtered data”: one more filter is applied to the “All selected data” so that calculations are also available with REFPROP 10.0 models. For combinations with no available data, 0.0 is given.

For pure fluids, four methods are used for the ηρ→0(T) calculation in the present work. The first method (denoted as oREF) is simply using REFPROP 10.0, which implements a variety of different methods depending on the particular fluid. The second method (denoted as oP3 and oP4) is to use a polynomial as a function of temperature
ηρ0(T)=m4T4+m3T3+m2T2+m1T+m0
(2)
where T is the temperature in K. The parameters m0, m1, m2, m3, m4 are fitted against calculations of REFPROP 10.0 from the triple point temperature to generally 1000 K or the highest calculable temperature and are implemented in the software package oilmixprop 1.0. (22) The parameters are also included in the Supporting Information (SI) of the present work. Note that the fourth-order polynomial for ηρ→0(T) (denoted as oP4) yields better results than the third-order polynomial (denoted as oP3), while a fifth-order polynomial does not achieve notably better results. The third method (denoted as oLJ) is the same as the one used in our previous work (6) and is described below. Assuming the molecular interactions can be approximated by those of L-J particles, ηρ→0(T) can be calculated using the Chapman–Enskog (23) solution of the Boltzmann transport equation
ηρ0(T)=516mkBTπ1σ2Ω(2,2)*
(3)
where m is the molecular mass in units of kg, kB = 1.380649 × 10–23 J/K is the Boltzmann constant, σ is the collision diameter of the L-J particle in unit of m, and Ω(2,2)* is the reduced collision integral obtained by integrating the possible approach trajectories of the particles. An empirical correlation of Ω(2,2)* for the Lennard–Jones fluid as a function of T was used
Ω(2,2)*=1.16145·T*0.14874+0.52487·e0.77320·T*+2.16178·e2.43787·T*
(4)
where T* = kBT/ϵ is the dimensionless temperature, and ϵ/kB is the reduced L-J pair-potential energy. This equation is also used in REFPROP 10.0 (20) and is a simplification of the original one proposed by Neufeld et al. (24) In the third method, σ and ϵ were obtained from REFPROP 10.0 and are listed in the SI of our previous work. (6) The fourth method (denoted as oCP) is almost the same as the third one, except that the L–J parameters σ and ϵ are estimated according to the predictive correlations of Chung et al. (25)
ϵ/KB=Tc/1.2593
(5)
σ3=0.3189/ρc
(6)
where Tc and ρc are critical temperature in units of K and critical density in units of m–3, respectively. These fluid constants were obtained from REFPROP 10.0 and can be found in the SI of our previous work. (13) Please note that in the third and fourth methods of calculating the dilute gas viscosity, the molecular interactions are approximated by those of L-J particles. However, this does not mean that all molecules are assumed to be spherical in this work. It only means that only the dilute gas portion is treated in that manner.
Now, we discuss the residual part of viscosity ηres(sr), which, as introduced by Bell et al., (3,10) can be calculated by
ηres(sr,T,ρN)=ηres+ρN2/3mKBT(s+)2/3
(7)
s+=sr/R
(8)
Here, ρN is the number density in unit of 1/m3, sr is the molar residual entropy in unit of J/(mol K), defined as the difference between the real fluid entropy and the ideal gas entropy at the same temperature and density, i.e., sr(T, ρ) = s(T, ρ) – s0(T, ρ), and R ≈ 8.3145 J/(mol K) is the molar gas constant. (26) The plus-scaled dimensionless residual viscosity ηres+ (see eq 7) is linked to the plus-scaled dimensionless residual entropy s+ (see eq 8) using power functions to be discussed in the following section; see eq 13.
The Python interface of package CoolProp 6.4.1 (27) was used to call REFPROP 10.0 (20) and calculate the number density ρN and molar residual entropy sr for both pure fluids and mixtures in the present work. The reference EOS used for each pure fluid are listed in Table S2 in the SI in our previous work. (13)
For mixtures, a predictive mixing rule was utilized to allow the RES model to predict the viscosity of mixtures without any additional adjustable parameters. The dilute gas viscosity for mixtures ηρ→0,mix is obtained with the approximation of Wilke (28)
ηρ0,mix=f=1Nxf·ηρ0,fj=1Nxj·ϕfj
(9)
with
ϕfj=[1+(ηρ0,f/ηρ0,j)1/2·(mj/mf)1/4]2{8·[1+(mf/mj)]}1/2
(10)
where xf and mf are the mole fraction and the mass of one molecule of component f. In eqs 7 and 8, the effective mass of one particle m is substituted by the mole fraction weighted average mmix
mmix=fxf·mf
(11)
Then, the mole fraction weighted average coefficients ni,mix are utilized to substitute the fluid-specific and group-specific parameters ni, i.e.
ni,mix=fxf·ni,f
(12)
where ni,f (i = 1, 2, 3) are fitted parameters of pure fluid f (see the following section). The utilized mixing rules allow predictions of not only binaries but also multicomponent mixtures.

2.2. Equation Design, Predictions, and Parameter Identification

As mentioned in our previous work, (6,12,13) the plus-scaled dimensionless residual viscosity ηres+ is linked to the plus-scaled dimensionless residual entropy s+ with the following equation
ln(ηres++1)=n1,X(s+/ξf)p1+n2,X(s+/ξf)p2+n3,X(s+/ξf)p3+n4,X(s+/ξf)p4
(13)
In eq 13, pi (i = 1, 2, 3, 4) are global parameters that are the same for all fluids. ni,X (i = 1, 2, 3, 4) are either fluid-specific (X = f) parameters, which are different for each fluid f, or group-specific parameters (X = g), which are different for each group of fluids g. The classification of the studied 124 pure fluids into eight groups is exactly the same as in our previous work. (13) In Table 1, a short description for each group is provided, while the group numbers for each fluid are indicated in Table 3.
For a pure fluid, when there are sufficient good-quality data, fluid-specific parameters ni,f can be obtained; otherwise, group-specific parameters ni,g must be used. ξf is the fluid-specific scaling factor, which is equal to 1 when ni,f is in use, or is a fitted value when ni,g is in use.
In the previous work, pi values (i = 1, 2, 3, 4) were taken equal to 1.0, 1.5, 2.0, 2.5, respectively, which were determined by the trial-and-error method. In the present work, pi values were optimized with a comprehensive optimization procedure as summarized in Algorithms 1 and 2.
The same data filters F1, F2, F3 as in the previous work (13) were adopted. The first filter F1 removes data that exceed the limitation of the used EOS in calculating density and residual entropy. These data amount to approximately 3.5% of all data. The second filter F2 removes data that suggest conflicting phase information. For instance, a high viscosity value suggests the fluid is in the liquid phase while the given temperature and pressure indicates that the fluid is in the gas phase. About 1.3% of the data were screened out with this filter. To handle outliers, the third filter F3 removes data whose relative deviation from the model prediction is larger than a set threshold. We used 30% for the threshold, which is the same as in the previous work. Filter F3 is involved in the iteration of Algorithm 1, and, in the end, only approximately 1.1% of the data are filtered out with this filter. Due to the very few data filtered out, there is limited risk that filter F3 is too selective, making the model appear to have a small deviation while actually steering the model in the wrong direction. For each fluid, the number of all data and of those filtered by each filter are summarized in a table in the SI of the present work.
As shown in Algorithm 1, the third filter F3 is carried out in every optimization loop. In each loop, the method to determine ni,f, ni,g, and ξf is the same as in the previous work, (13) while the method to determine pi is inspired by the literature (29−31) and summarized in Algorithm 2.
In Algorithm 2, for pi optimization, only fluids with fluid-specific parameters ni,f, i.e., with data of sufficiently good quality, are involved. This reduces the complexity of Algorithm 2 while there are still more than 90% of all screened data in use. For each studied fluid in Algorithm 2, an objective function is defined
Minimize
1nk=1n(ηres,k+f(sk+,pf,nf))2+σji=13(pi,fp¯i)2
(14)
w.r.t. pf, nf, where pi,f are the fluid-specific pi parameters for fluid f, i is the mean of pi,f across all fluids, σj is a penalty weighting factor in the iteration step j, and ηres,k+ and sk+ are the k-th data points to be fitted. This objective function couples the individual optimization problems (the first term in the objective function) by applying a penalty (the second term in the objective function) to the parameters intended to be global for each individual problem. The strategy to push fluid-specific pi,f toward their mean i is to start with a very small penalty weighting factor, i.e., σj = 10–5, and then increase it step by step σj = σj · σincrease with an increasing factor σincrease = 1.2.

3. Results and Discussion

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3.1. New RES Model

With the algorithms described in the previous section, the final optimized global parameters pi (i = 1, 2, 3) are 1.8, 2.4, and 2.8, respectively. The fluid-specific parameters ni,f are listed in Table 3, the group-specific parameters are listed in Table 2 and their fluid-specific scaling factors are listed in Table 3.
To note, the 4-term power function is reduced to a 3-term one. The 4-term power function is denoted as rP4, while the 3-term one is denoted as rP3. There are two key arguments for this simplification. First, to avoid negative slope of the ln(ηres+ + 1) vs. s+ curve at the condition s+ = 0, the lower bound of n1,X in the lowest power term is zero in the fitting. Although a negative slope is physical, (3) a positive slope helps to avoid overfitting for those fluids without data in the gas phase. As a result, n1,X = 0 holds in many fluids and groups. Second, compared to the 4-term equation, a 3-term one allows more fluids, particularly those without gas phase data, to have fluid-specific parameters. This is because, when there are no data in the gas phase, the 4-term equation overfits the data and delivers unreasonable parameters, while this problem is avoided by using the 3-term one.
To evaluate the new RES model, two statistical values AARD and the average relative deviation (ARD) are defined
AARD=1nk=1n|ηexp,kηcalc,kηexp,k|
(15)
ARD=1nk=1nηexp,kηcalc,kηexp,k
(16)
where ηexp,k is the experimental viscosity of data point k and ηcalc,k is the prediction by a model. The AARD is a measure of the scattering, while the ARD denotes systematic offsets. The AARD (upper plot) and ARD (lower plot) plots of the filtered (or analyzable) experimental data from different models are presented in Figure 1. REFPROP 10.0, as the state-of-the-art software package to accurately calculate thermophysical properties, was adopted as a baseline. For the fluids studied here, the default viscosity models in REFPROP 10.0 are reference correlations (32−67) for 43 pure fluids, extended corresponding states models (58,68−70) for 77 pure fluids, and friction theory models (47,61) for three pure fluids. For fluid NF3, REFPROP 10.0 does not provide a viscosity model. The various RES-based models with different ηρ→0(T) calculation methods (see Section 2.1) and residual viscosity calculation with different power functions (the 4-term one from the previous work and the 3-term one in the present work) are also shown in Figure 1.
As can be seen in Figure 1, REFPROP 10.0 yields the lowest AARD (2.74%), i.e., the lowest scattering of experimental data from model, but with almost the largest ARD (0.7%), i.e., the largest systematic offset from experimental data to the model. The best RES approach is the one with ηρ→0(T) calculated using REFPROP 10.0 (oREF in Figure 1) and with residual viscosity calculated using the 3-term power function (rP3) from the present work. Compared to REFPROP 10.0, it yields slightly higher AARD (only 0.02% higher) but almost zero absolute ARD and with 1% more data analyzable (or 1% fewer data filtered out). Of course, the comparison to REFPROP 10.0 models is not entirely fair. A fair comparison should be that only data used to develop the REFPROP 10.0 models were adopted to assess our models. Such a comparison is however not straightforward to achieve given the documentary standards in the open literature, so it was not attempted. The parameters listed in Tables 2 and 3 are all based on this RES approach (oREF + rP3). A Python package containing the proposed model including the corresponding parameters is provided in the SI.
If ηρ→0(T) is calculated with polynomials (oP3 or oP4), the AARD slightly increases (less than 0.1%). This method is favored as the ηρ→0(T) is a relatively simple correlation with few parameters, and it helps to develop an RES approach more independent of REFPROP 10.0 in the future. If ηρ→0(T) is calculated with reliable L-J parameters (oLJ), the AARD reaches up to 3.4%; the benefits here are that fewer parameters are needed and fluids not implemented in REFPROP 10.0 can also be studied. If ηρ→0(T) is calculated with L-J parameters estimated using critical point information (oCP), the AARD reaches up to 4.5%. In this case, if critical point, mole mass, and acentric factor of a pure or quasi-pure fluid are known, only three additional ni,f (i = 1, 2, 3) parameters are needed to enable reliable viscosity prediction (with an estimated standard uncertainty of 4.5%). Such applications have been seen in our previous work in fluid optimization (71) and oil property modeling. (72)
Compared to the previous work (13) in which a 4-term power function is used for residual viscosity calculation (rP4), the 3-term equation, in the present work, rP3 yields improved performance. The key reasons are (1) the function form with power terms is optimized and (2) overfitting is avoided so that more fluids have individual fluid-specific parameters.

3.2. Details for Pure-Fluid Correlation

The previous section provides the overall statistics of the performance of the best RES approach (oREF + rP3), while this section presents the details of its correlation of each pure fluid.
The AARD and ARD of the filtered experimental data from model predictions for each pure fluid are illustrated in Figures 2 and 3, respectively. Although overall REFPROP 10.0 has a lower AARD than the proposed RES model (see Figure 1), the proposed RES model shows a smaller AARD than REFPROP 10.0 for more than half of the 124 fluids. The proposed approach yields both, a lower AARD and ARD, for 48 of the pure fluids. The new RES model could be considered as the best available one according to the considered data. Direct comparison with the primary data sets selected by the authors of the reference correlation is required to make a more authoritative comparison. Such a comparison is not feasible to do because many of the data sets are not included in the NIST Thermo Data Engine database. This is also indicated in Table 3. The standard uncertainty of the RES model is estimated to be 2.8%, as approximately 68.4% (corresponding to one standard deviation of a normal distribution) of the data agree with the model within this threshold. The ARD of the proposed RES model for most of the pure fluids is significantly lower than that of REFPROP 10.0. One hundred and one fluids exhibit an absolute ARD of less than 1%, while 117 are below 2%. Most fluids with more than 1000 points have an absolute ARD of below 1%, while all are 1.5% or lower. Fluids with an absolute ARD larger than 2% are generally those with data only in the gas phase (e.g., R161), with very few data (e.g., CYCLOPRO, R41), or with less accurate dilute gas viscosity calculations (e.g., KRYPTON). The AARD and ARD of some fluids seem to be zero because either no model is available, e.g., for NF3 in REFPROP 10.0, or only dilute gas viscosity data are available, e.g., for CO.
Figure 4 shows the values of ηres+ + 1 as a function of s+/ξ for each group of pure fluids. As expected and similar to the previous work, (13) each group’s fluids collapse into one curve. The fluid-specific scaling factors ξf of each fluid are listed in Table 3 and the ratio ξ/scrit+ are shown in Figure 5, where scrit+ is the plus-scaled dimensionless residual entropy at the critical point obtained from REFPROP 10.0. The ratios in group 3 are approximately 0.7, and for groups with heavier components, the ratios generally decrease, which is in good agreement to literature. (4,13,73) With the group-specific parameters and the almost-constant ξ/scrit+ ratio in each group, one may obtain rough estimates of viscosity for fluids without data if the EOS yields a reasonable value for the residual entropy at the critical point. This can be done by classifying a fluid into one of the groups and use its constant ξ/scrit+ ratio to estimate its ξf, based on the similarity of molecular structure, or on existing models. For example, the latter method has been applied to some fluids in the present work. These are indicated with “predictive” in the “best available” column in Table 3. For these 27 pure fluids, which are available in REFPROP 10.0 but for which no viscosity were available in the NIST ThermoDataEngine, the group number was determined by the following steps: (1) carrying out calculations using REFPROP 10.0 in both liquid and gas phases and using these data as “experimental data”; (2) fitting ξf using each group’s group-specific parameters with these “experimental data”; (3) assigning the group number of this fluid where its ξf is the most close to 1. These results are listed in Table 3. Finally, we would like to mention, compared to our previous work, (13) group 1 is still a noticeable outlier in terms of fluid-specific scaling factors ξf as shown in Figure 5.
Group 1 comprises fluids with quantum effects at low temperatures, i.e., hydrogen, deuterium, and helium. Not only is the data availability low but also it is crucially low in areas where we expect the RES approach to worsen for those fluids. A simple empirical means of handling at least some of the quantum correction to the RES approach for the cryogens has been proposed, (74) but further study is required. Furthermore, the results for water look better than they actually are, as the available data in the NIST ThermoDataEngine is not comprehensive. More experimental data of water and maybe alcohols need to be collected to further evaluate the RES model for these fluids.

3.3. Prediction for Mixtures

For fluid mixtures, the developed RES approach is a pure prediction method, i.e., no additional adjustable parameter is needed. In total, 33,109 data were collected from 351 binary mixtures as used in our previous work. (13) An overview of statistical measures of the prediction performance of the developed RES approach is shown in Figure 6. Please note: (1) REFPROP 10.0 does not calculate dilute gas viscosity of many mixtures, especially those involving alcohols. Therefore, for the dilute gas viscosity of a mixture, the fourth-order polynomial (eq 2) was adopted for pure component calculation with the mixing rule of the approximation of Wilke (28) as described in Section 2.2. (2) For all the collected mixture data, similar filters as used in pure fluids were applied, and the remaining data were named “all selected data” in Figure 6. REFPROP 10.0 cannot calculate about 20.5% of “all selected data”; therefore, one more filter was applied so that calculations were also available with REFPROP 10.0 models, and the remaining data are named “Further filtered data” in Figure 6. In summary, the proposed model agrees with 68.2% of “all selected data” within 7.8%.
With the RES model, for binary mixtures of the same group, the ARD is generally less than 2.7%. Mixtures within group 5 show particularly low ARD, namely 0.2%. The ARD for mixtures of different groups varies widely, ranging from 0.8% for groups 1 and 2 (with 1982 data points) up to 16.3% for groups 7 and 8 (with 3625 data points). As shown in Figure 6, the AARD of the all selected data has a relatively wide range depending on the group combination. The scatter for different groups are, however, mostly in the same order of magnitude as REFPROP 10.0’s, while some are lower. In total, the proposed model has a lower AARD than REFPROP 10.0 for 185 out of the 351 considered mixtures.
Modeling the viscosity of mixtures has been a challenging issue, especially for those composed of molecules of very different sizes. For instance, the AARD are very high for both the new RES model and the models in REFPROP 10.0 in comparison to experimental data of groups 1 and 8 (light gas and liquid with strong association force) and those involving group 6 (heavy molecules). One of the key reasons is that the viscosity changes thousands of times from a pure component of small molecular size (for example, a refrigerant R32 in group 3) to a pure component in group 6 of large molecular size (for example, dodecane as a lubricant oil in group 6). This, on the one hand, makes accurate measurements very hard and, on the other hand, makes the model generally yield very large deviations (e.g., on the order of 30%) if the given composition has a 1% uncertainty. Modeling mixtures using proper mixing rules is an important and interesting topic of future work.

4. Conclusions

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The RES approach developed in our previous work was successfully improved. The approach demonstrates an accuracy on par with that of the fluids in REFPROP 10.0 not modeled by fluid-specific correlations. The key improvement lies in the optimization of the relations between the residual viscosity and the residual entropy. In the previous work, they are related by a 4-term power function determined manually, while in the present work they are related by a 3-term power function determined with a comprehensive fitting procedure. As a result, the average absolute relative deviation, corresponding to scattering of the experimental data from the model, is reduced down to 2.76%; this is very close to 2.74% obtained with the various multiparameter models in REFPROP 10.0. Meanwhile, the average relative deviation, corresponding to systematic offset of the experimental data to the model, is 0.03%, which is significantly better than REFPROP 10.0’s 0.7%. Based solely on the data available to us, for 48 of the 124 pure fluids, the proposed approach yields both a lower AARD and ARD. For mixture predictions, the RES approach is a pure prediction method, which agree with more than 68.4% of the data within 7.8%. Therefore, we could roughly estimate our RES having a standard uncertainty of 2.8% for pure fluids and 7.8% for mixtures.
The present work also investigates various methods to calculate the dilute gas viscosity. The method using REFPROP 10.0 calculation yields the best agreement with the experimental data. A fourth-order polynomial for dilute gas viscosity was developed in the present work which yields slightly worse results compared to REFPROP 10.0 but has a simple functional form which can be used for all studied fluids. Methods based on L-J parameters either obtained from reliable sources or estimations with critical point information were evaluated, as well. Both yield higher AARD. Nonetheless, with the last method, no additional parameters are needed for dilute gas viscosity calculation, while it yields AARD only twice (4.49% compared to 2.76%) of the best RES method here. To use the model developed in the present work, a Python package is provided in the SI. Fluid-specific parameters are suggested to be used, and only if they are not available should group-specific parameters with the fluid-specific scaling factor be used.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jced.4c00451.

  • main.py (PY): implementation of the core of the RES model; Dilute_gas_viscosity.txt (TXT): parameters for the calculation of the dilute gas viscosity; RES_Parameter.txt (TXT): parameters for the models; Samples_* (TXT): all files beginning with “Samples_” are sample calculations that may be used to verify the model; Data_evaluation_REF_15.txt (TXT): various statistics concerned with the data used for the pure fluid calculations; Data_evaluation_mix.txt (TXT): various statistics concerned with the data used for the mixtures calculations; Table_multi.txt (TXT): information about the experimental data of mixtures; Table_pure_REF_15.txt (TXT): various statistics of the model qualities for the pure fluids; Fluid_Constants.txt (TXT): fluid constants used; pure-fluid data and literature.docx (DOCX): detailed information on pure fluid data and the literature; figure_pure_devs (folder/*PNG): relative deviation for all used data points for the RES model and REFPROP models for all substances; figure_pure_groups (folder/*PNG): ηres+ + 1 as a function of s+/ξ for one group each; mix_dev_exp_res_ecs (folder/*PNG): relative deviations for all mixtures for the RES model and the REFPROP models individually, as well as their data sources; mix_s_eta_all_data (folder/*PNG): ηres+ + 1 as a function of s+/ξ for each mixture individually for all data, as well as the data sources; mix_s_eta_select_data (folder/*PNG): ηres+ + 1 as a function of s+/ξ for each mixtures individually for the filtered data, as well as the data sources; pure_dev_exp_res_ecs (folder/*PNG): relative deviations for all pure fluids for the RES model and the REFPROP models individually, as well as the data sources; pure_s_eta_all_data (folder/*PNG): ηres+ + 1 as a function of s+/ξ for each pure fluid individually for all data, as well as the data sources; pure_s_eta_fitted_data (folder/*PNG): ηres+ + 1 as a function of s+/ξ for each pure fluid individually for the filtered data, as well as the data sources (ZIP)

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Author Information

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  • Corresponding Author
  • Authors
    • Viktor Martinek - Interdisciplinary Center for Scientific Computing, Heidelberg University, 69120 Heidelberg, GermanyOrcidhttps://orcid.org/0000-0001-6215-4783
    • Ian Bell - Applied Chemicals and Materials Division, National Institute of Standards and Technology, Boulder, Colorado 80305, United States
    • Roland Herzog - Interdisciplinary Center for Scientific Computing, Heidelberg University, 69120 Heidelberg, Germany
    • Markus Richter - Faculty of Mechanical Engineering, Applied Thermodynamics, Chemnitz University of Technology, 09107 Chemnitz, GermanyOrcidhttps://orcid.org/0000-0001-8120-5646
  • Author Contributions

    Study conception and design: X.Y.; data collection: X.Y., I.B.; optimization methodology: V.M., R.H., X.Y.; analysis and interpretation of results: V.M., X.Y.; draft manuscript preparation: V.M.; manuscript major revision: I.B., X.Y.; funding acquisition: R.H., M.R. All authors reviewed the results and approved the final version of the manuscript.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)─HE 6077/14-1 and RI 2482/10-1─within the Priority Programme “SPP 2331: Machine Learning in Chemical Engineering”. Partial support was provided by NIST. This work was also supported by the German Federal Ministry of Education and Research on the basis of a decision by the German Bundestag (Funding Code 03SF0623A).

Appendix: Fluid Name Glossary

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In the following, the REFPROP 10.0 names of all considered fluids are listed with their corresponding IUPAC chemical names and their CAS Registry numbers:
13BUTADIENE: buta-1,3-diene; 106-99-0 | 1BUTENE: 1-butene; 106-98-9 | 1BUTYNE: but-1-yne; 107-00-6 | 1PENTENE: pent-1-ene; 109-67-1 | 22DIMETHYLBUTANE: 2,2-dimethylbutane; 75-83-2 | 23DIMETHYLBUTANE: 2,3-dimethylbutane; 79-29-8 | 3METHYLPENTANE: 3-methylpentane; 96-14-0 | ACETONE: propanone; 67-64-1 | ACETYLENE: ethyne; 74-86-2 | AMMONIA: ammonia; 7664-41-7 | ARGON: argon; 7440-37-1 | BENZENE: benzene; 71-43-2 | BUTANE: n-butane; 106-97-8 | C11: undecane; 1120-21-4 | C12: dodecane; 112-40-3 | C16: hexadecane; 544-76-3 | C1CC6: methylcyclohexane; 108-87-2 | C22: docosane; 629-97-0 | C2BUTENE: cis-2-butene; 590-18-1 | C3CC6: n-propylcyclohexane; 1678-92-8 | C4F10: decafluorobutane; 355-25-9 | C5F12: dodecafluoropentane; 678-26-2 | C6F14: tetradecafluorohexane; 355-42-0 | CF3I: trifluoroiodomethane; 2314-97-8 | CHLORINE: chlorine; 7782-50-5 | CHLOROBENZENE: chlorobenzene; 108-90-7 | CO: carbon monoxide; 630-08-0 | CO2: carbon dioxide; 124-38-9 | COS: carbon oxide sulfide; 463-58-1 | CYCLOBUTENE: 1-cyclobutene; 822-35-5 | CYCLOHEX: cyclohexane; 110-82-7 | CYCLOPEN: cyclopentane; 287-92-3 | CYCLOPRO: cyclopropane; 75-19-4 | D2: deuterium; 7782-39-0 | D2O: deuterium oxide; 7789-20-0 | D4: octamethylcyclotetrasiloxane; 556-67-2 | D5: decamethylcyclopentasiloxane; 541-02-6 | D6: dodecamethylcyclohexasiloxane; 540-97-6 | DEA: 2,2′-iminodiethanol; 111-42-2 | DECANE: decane; 124-18-5 | DEE: diethyl ether; 60-29-7 | DMC: dimethyl ester carbonic acid; 616-38-6 | DME: methoxymethane; 115-10-6 | EBENZENE: phenylethane; 100-41-4 | EGLYCOL: 1,2-ethandiol; 107-21-1 | ETHANE: ethane; 74-84-0 | ETHANOL: ethyl alcohol; 64-17-5 | ETHYLENE: ethene; 74-85-1 | ETHYLENEOXIDE: ethylene oxide; 75-21-8 | FLUORINE: fluorine; 7782-41-4 | H2S: hydrogen sulfide; 7783-06-4 | HCL: hydrogen chloride; 7647-01-0 | HELIUM: helium-4; 7440-59-7 | HEPTANE: heptane; 142-82-5 | HEXANE: hexane; 110-54-3 | HYDROGEN: hydrogen (normal); 1333-74-0 | IBUTENE: 2-methyl-1-propene; 115-11-7 | IHEXANE: 2-methylpentane; 107-83-5 | IOCTANE: 2,2,4-trimethylpentane; 540-84-1 | IPENTANE: 2-methylbutane; 78-78-4 | ISOBUTAN: 2-methylpropane; 75-28-5 | KRYPTON: krypton; 7439-90-9 | MD2M: decamethyltetrasiloxane; 141-62-8 | MD3M: dodecamethylpentasiloxane; 141-63-9 | MD4M: tetradecamethylhexasiloxane; 107-52-8 | MDM: octamethyltrisiloxane; 107-51-7 | MEA: ethanolamine; 141-43-5 | METHANE: methane; 74-82-8 | METHANOL: methanol; 67-56-1 | MILPRF23699: mIL-PRF-23699; 1-1-1 | MLINOLEA: methyl (Z,Z)-9,12-octadecadienoate; 112-63-0 | MLINOLEN: methyl (Z,Z,Z)-9,12,15-octadecatrienoate; 301-00-8 | MM: hexamethyldisiloxane; 107-46-0 | MOLEATE: methyl cis-9-octadecenoate; 112-62-9 | MPALMITA: methyl hexadecanoate; 112-39-0 | MSTEARAT: methyl octadecanoate; 112-61-8 | MXYLENE: 1,3-dimethylbenzene; 108-38-3 | N2O: dinitrogen monoxide; 10024-97-2 | NEON: neon; 7440-01-9 | NEOPENTN: 2,2-dimethylpropane; 463-82-1 | NF3: nitrogen trifluoride; 7783-54-2 | NITROGEN: nitrogen; 7727-37-9 | NONANE: nonane; 111-84-2 | NOVEC649:1,1,1,2,2,4,5,5,5-nonafluoro-4-(trifluoromethyl)-3-pentanone; 756-13-8| OCTANE: octane; 111-65-9 | ORTHOHYD: orthohydrogen; 1333-74-0o | OXYGEN: oxygen; 7782-44-7 | OXYLENE: 1,2-dimethylbenzene; 95-47-6 | PARAHYD: parahydrogen; 1333-74-0p | PENTANE: pentane; 109-66-0 | POE5: pentaerythritol tetrapentanoate; 15834-04-5 | POE7: pentaerythritol tetraheptanoate; 25811-35-2 | POE9: pentaerythritol tetranonanoate; 14450-05-6 | PROPADIENE: 1,2-propadiene; 463-49-0 | PROPANE: propane; 74-98-6 | PROPYLEN: propene; 115-07-1 | PROPYLENEOXIDE: 1,2-epoxypropane; 75-56-9 | PROPYNE: propyne; 74-99-7 | PXYLENE: 1,4-dimethylbenzene; 106-42-3 | R11: trichlorofluoromethane; 75-69-4 | R1123: trifluoroethylene; 359-11-5 | R113: 1,1,2-trichloro-1,2,2-trifluoroethane; 76-13-1 | R114: 1,2-dichloro-1,1,2,2-tetrafluoroethane; 76-14-2 | R115: chloropentafluoroethane; 76-15-3 | R116: hexafluoroethane; 76-16-4 | R12: dichlorodifluoromethane; 75-71-8 | R1216: hexafluoropropene; 116-15-4 | R1224YDZ: (Z)-1-chloro-2,3,3,3-tetrafluoropropene; 108 | R123:2,2-dichloro-1,1,1-trifluoroethane; 306-83-2 | R1233ZDE: trans-1-chloro-3,3,3-trifluoro-1-propene; 110 | R1234YF: 2,3,3,3-tetrafluoroprop-1-ene; 754-12-1 | R1234ZEE: trans-1,3,3,3-tetrafluoropropene; 29118-24-9 | R1234ZEZ: cis-1,3,3,3-tetrafluoropropene; 29118-25-0 | R124:1-chloro-1,2,2,2-tetrafluoroethane; 2837-89-0 | R1243ZF: 3,3,3-trifluoropropene; 677-21-4 | R125: pentafluoroethane; 354-33-6 | R13: chlorotrifluoromethane; 75-72-9 | R1336MZZZ: (Z)-1,1,1,4,4,4-hexafluoro-2-butene; 692-49-9 | R134A: 1,1,1,2-tetrafluoroethane; 811-97-2 | R14: tetrafluoromethane; 75-73-0 | R141B: 1,1-dichloro-1-fluoroethane; 1717-00-6 | R142B: 1-chloro-1,1-difluoroethane; 75-68-3 | R143A: 1,1,1-trifluoroethane; 420-46-2 | R150:1,2-dichloroethane; 107-06-2 | R152A: 1,1-difluoroethane; 75-37-6 | R161: fluoroethane; 353-36-6 | R21: dichlorofluoromethane; 75-43-4 | R218: octafluoropropane; 76-19-7 | R22: chlorodifluoromethane; 75-45-6 | R227EA: 1,1,1,2,3,3,3-heptafluoropropane; 431-89-0 | R23: trifluoromethane; 75-46-7 | R236EA: 1,1,1,2,3,3-hexafluoropropane; 431-63-0 | R236FA: 1,1,1,3,3,3-hexafluoropropane; 690-39-1 | R245CA: 1,1,2,2,3-pentafluoropropane; 679-86-7 | R245FA: 1,1,1,3,3-pentafluoropropane; 460-73-1 | R32: difluoromethane; 75-10-5 | R365MFC: 1,1,1,3,3-pentafluorobutane; 406-58-6 | R40: methyl chloride; 74-87-3 | R41: fluoromethane; 593-53-3 | RC318: octafluorocyclobutane; 115-25-3 | RE143A: methyl trifluoromethyl ether; 421-14-7 | RE245CB2: methyl-pentafluoroethyl-ether; 22410-44-2 | RE245FA2:2,2,2-trifluoroethyl-difluoromethyl-ether; 1885-48-9 | RE347MCC: 1,1,1,2,2,3,3-heptafluoro-3-methoxypropane; 375-03-1 | SF6: sulfur hexafluoride; 2551-62-4 | SO2: sulfur dioxide; 7446-09-5 | T2BUTENE: trans-2-butene; 624-64-6 | TOLUENE: methylbenzene; 108-88-3 | VINYLCHLORIDE: chloroethylene; 75-01-4 | WATER: water; 7732-18-5 | XENON: xenon; 7440-63-3.

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  1. Xiaoxian Yang. Viscosity and Thermal Conductivity Models of 151 Common Fluids Based on Residual Entropy Scaling and Cubic Equations of State. ACS Omega 2025, 10 (6) , 6124-6134. https://doi.org/10.1021/acsomega.4c10815

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  • Abstract

    Figure 1

    Figure 1. AARD (upper plot) and ARD (lower plot) of the filtered (or analyzable) experimental data from the model predictions. REF. models refers to the default viscosity models in REFPROP 10.0. For the remaining models, the labels comprise two parts connected by a “+” symbol. The former part denotes the method for dilute gas viscosity ηρ→0(T) calculation (see Section 2.1): oLJ uses L-J parameters, oREF uses REFPROP 10.0, oP3 uses a third-order polynomial, oP4 uses a fourth-order polynomial, and oCP uses the critical point information. The latter part refers to the residual viscosity calculation method: rP4 is a 4-term power function used in the previous work (6) and rP3 is the 3-term power function developed in the present work.

    Figure 2

    Figure 2. AARD of the REFPROP 10.0 models and the oREF + rP3 model predictions to the filtered experimental data for each pure fluid. The fluids are further classified according to the model types used by REFPROP 10.0: (a) reference correlations, (32−67) (b) ECS model with fitted parameters or friction theory models, (47,58,61) and (c) ECS model without fitted parameters and other models. (68−70) Fluid names in REFPROP 10.0 are adopted (see the Appendix for their respective IUPAC chemical names and CAS registry numbers). The entries are sorted primarily by whether the proposed RES model has a lower AARD than that of REFPROP 10.0 and secondarily according to increasing AARD of the proposed model. The green background indicates that the proposed model is better.

    Figure 3

    Figure 3. ARD of the REFPROP 10.0 models and the oREF + rP3 model predictions to the filtered experimental data for each pure fluid. The fluids are further classified according to the model types used by REFPROP 10.0: (a) reference correlations, (32−67) (b) ECS model with fitted parameters or friction theory models, (47,58,61) and (c) ECS model without fitted parameters and other models. (68−70) Fluid names in REFPROP 10.0 are adopted (see the Appendix for their respective IUPAC chemical names and CAS registry numbers). The entries are sorted primarily by whether the proposed RES model has a lower absolute ARD than that of REFPROP 10.0 and secondarily according to increasing ARD of the proposed model. The green background indicates that the proposed model is better.

    Figure 4

    Figure 4. Plus-scaled dimensionless residual viscosity ηres+ as a function of s+/ξ for each group of pure fluids, where s+ is the plus-scaled residual entropy and ξ is the fluid-specific scaling factor. The group-specific parameters ni,g are used to calculate the curves. All groups are shown at the bottom. Each group is also illustrated and annotated stacked by a power of 20 at the top to highlight the qualitative differences.

    Figure 5

    Figure 5. Ratio ξ/scrit+ of all considered pure fluids, where ξ is the fluid-specific scaling factor and scrit+ is the plus-scaled dimensionless residual entropy at the critical point, which is obtained from REFPROP 10.0. The group numbers are annotated in the top right of each box. At ξ/scrit+=0.7, a vertical dashed dotted line is drawn. Values for group 1 exceed the plot limits, i.e., PARAHYD: 1.67, ORTHOHYD: 1.61, HYDROGEN: 1.64, HELIUM: 4.09, D2:1.86.

    Figure 6

    Figure 6. ARD and AARD of the experimental data of 351 binary mixtures from predictions of models. “All selected data”: for calculations with the RES model, similar filters as used in pure fluids were applied for mixture data. “Further filtered data”: one more filter is applied to the “All selected data” so that calculations are also available with REFPROP 10.0 models. For combinations with no available data, 0.0 is given.

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  • Supporting Information

    Supporting Information


    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jced.4c00451.

    • main.py (PY): implementation of the core of the RES model; Dilute_gas_viscosity.txt (TXT): parameters for the calculation of the dilute gas viscosity; RES_Parameter.txt (TXT): parameters for the models; Samples_* (TXT): all files beginning with “Samples_” are sample calculations that may be used to verify the model; Data_evaluation_REF_15.txt (TXT): various statistics concerned with the data used for the pure fluid calculations; Data_evaluation_mix.txt (TXT): various statistics concerned with the data used for the mixtures calculations; Table_multi.txt (TXT): information about the experimental data of mixtures; Table_pure_REF_15.txt (TXT): various statistics of the model qualities for the pure fluids; Fluid_Constants.txt (TXT): fluid constants used; pure-fluid data and literature.docx (DOCX): detailed information on pure fluid data and the literature; figure_pure_devs (folder/*PNG): relative deviation for all used data points for the RES model and REFPROP models for all substances; figure_pure_groups (folder/*PNG): ηres+ + 1 as a function of s+/ξ for one group each; mix_dev_exp_res_ecs (folder/*PNG): relative deviations for all mixtures for the RES model and the REFPROP models individually, as well as their data sources; mix_s_eta_all_data (folder/*PNG): ηres+ + 1 as a function of s+/ξ for each mixture individually for all data, as well as the data sources; mix_s_eta_select_data (folder/*PNG): ηres+ + 1 as a function of s+/ξ for each mixtures individually for the filtered data, as well as the data sources; pure_dev_exp_res_ecs (folder/*PNG): relative deviations for all pure fluids for the RES model and the REFPROP models individually, as well as the data sources; pure_s_eta_all_data (folder/*PNG): ηres+ + 1 as a function of s+/ξ for each pure fluid individually for all data, as well as the data sources; pure_s_eta_fitted_data (folder/*PNG): ηres+ + 1 as a function of s+/ξ for each pure fluid individually for the filtered data, as well as the data sources (ZIP)


    Terms & Conditions

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