SACOMAR Technologies for Safe and Controlled Martian Entry€¦ · SACOMAR D 6.3 Report and library...

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SACOMAR Technologies for Safe and Controlled Martian Entry SPA.2010.3.2-04 EU-Russia Cooperation for Strengthening Space Foundations (SICA) Re-entry Technologies and Tools Theme 9 - Space Activity 9.3 - Cross-Cutting-Activities Area 9.3.2 - International Cooperation Deliverable Reference Number: D 6.3 Deliverable Title: Report and library on gas phase chemistry Due date of deliverable: 1 st December 2011 Actual submission date: 10 th February 2012 Start date of project: 20 th January 2011 Duration: 18 months Organisation name of lead contractor for this deliverable: DLR Revision #: 0 Project co-funded by the European Commission within the 7 th Framework Programme Dissemination Level PU Public X PP Restricted to other programme participants (including the Commission Services) RE Restricted to a group specified by the consortium (including the Commission Services) CO Confidential, only for members of the consortium (including the Commission Services)

Transcript of SACOMAR Technologies for Safe and Controlled Martian Entry€¦ · SACOMAR D 6.3 Report and library...

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SACOMAR

Technologies for Safe and Controlled Martian Entry

SPA.2010.3.2-04

EU-Russia Cooperation for Strengthening Space Foundations (SICA)

Re-entry Technologies and Tools

Theme 9 - Space Activity 9.3 - Cross-Cutting-Activities Area 9.3.2 - International Cooperation

Deliverable Reference Number: D 6.3 Deliverable Title: Report and library on gas phase chemistry

Due date of deliverable: 1st December 2011

Actual submission date: 10th February 2012

Start date of project: 20th January 2011

Duration: 18 months

Organisation name of lead contractor for this deliverable: DLR

Revision #: 0

Project co-funded by the European Commission within the 7th Framework Programme

Dissemination Level

PU Public X

PP Restricted to other programme participants (including the Commission Services)

RE Restricted to a group specified by the consortium (including the Commission Services)

CO Confidential, only for members of the consortium (including the Commission Services)

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Table of Contents

APPROVAL 2

Table of Contents i

List of Figures ii

Nomenclature iii

Abstract 1

1 Introduction 1

2 Modelling of Chemical Kinetics 2

3 Transport Properties 3

4 Verification and Validation 4

4.1 Equilibrium Composition 4

4.2 Transport Properties 4

4.3 HEG Nozzle Flow 5

4.4 Flow Around Wind Tunnel Probe 6

5 Summary and Conclusions 9

References 9

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List of Figures

Figure 1 Equilibrium composition of pure CO2 (left) and of a mixture of 97% CO2 and 3% N2 (right) at 105 Pa 4

Figure 2 Viscosity (left) and thermal conductivity (right) of pure CO2 at 105 Pa versus temperature. Black, blue and red lines indicate results for Schmidt numbers Sc = 0.5, Sc = 0.7 and the Chapman-Cowling mixture rule, respectively. 5

Figure 3 Viscosity (left) and thermal conductivity (right) of a 97% CO2, 3% N2 mixture at 105 Pa versus temperature. Black, blue and red lines indicate results for Schmidt numbers Sc = 0.5, Sc = 0.7 and the Chapman-Cowling mixture rule, respectively. 5

Figure 4 Mach number contours along the HEG nozzle. Based on experience, transition from laminar to turbulent was set at x = 0.758m. 6

Figure 5 Species mass fractions along the centreline of the HEG nozzle (left) as well as pressure and velocity at the nozzle exit (right). 6

Figure 6 Axisymmetric grid used for the simulation of the flow around a stagnation probe after six adaptation Steps. 7

Figure 7 Pressure (left) and temperature (right) contours for the flow around the stagnation probe 8

Figure 8 Mass fractions along the stagnation line (left) as well as heat flux and pressure versus radial direction (right) of the stagnation probe 8

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Nomenclature

A/ (K) Arrhenius activation temperature

cp (J/kg/K) Specific heat capacity at constant pressure

H (J/kg) Enthalpy

Ma (-) Mach number

p (Pa) Pressure

r (-) Arrhenius reaction rate exponent

Re (-) Reynolds number

s (m³/mole/s) Arrhenius reaction rate coefficient

Sc (-) Schmidt number

T (K) Temperature

v (m/s) Velocity

x, y, z (m) Coordinate directions

(kg/m³) Density

(-) Mass fraction

(-) Mole fraction

Subscripts

0 Stagnation condition

Free stream

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Acronyms

CFD Computational Fluid Dynamics

DLR German Aerospace Centre

HEG High Enthalpy Facility, Göttingen

WP Work Package

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Abstract

This document describes the extension of the TAU CFD modelling with respect to CO2 flows performed within WP 6.3 of the SACOMAR study.

In the past, the majority of TAU applications focused on the earth atmosphere including air flows in thermal and chemical non-equilibrium. With the increased interest in missions to Mars the modelling of the Martian atmosphere consisting of 97% carbon dioxide and 3% nitrogen became a task. The thermodynamic properties of the carbon related species, the reaction rates within the chemical kinetics as well as the transport properties for the new species had to be added. Whereas most of the improvements towards a state of the art model for the Martian atmosphere could be done by adjusting the parameter files, the extension of TAU concerning CO2 containing flows required the implementation of a new species type with multiple vibrational degrees of freedom.

The reaction scheme for pure CO2 flows in chemical non-equilibrium accounts for 27 independent reactions. In addition, an advanced reaction scheme for CO2, N2 mixtures has been compiled which consists of 103 independent chemical reactions.

The transport properties are described by either simple viscosity based mixture rules or by the advanced Chapman-Enskog modelling employing collision integrals. The collision integrals have been available at DLR from a previous DLR collaboration with VKI under ESA funding. Unfortunately, these collision integrals are not available from literature.

In a final step, results of the enhanced modelling will be demonstrated for equilibrium conditions and selected SACOMAR related flow cases.

1 Introduction

The DLR TAU code is a finite volume method formulated for unstructured meshes consisting of tetrahedra, prisms, hexahedra and pyramids. The time-accurate three-dimensional Navier-Stokes equations are marched for steady or unsteady conditions by an explicit multi-stage Runge-Kutta scheme or an implicit LUSGS scheme. State of the art acceleration techniques such as local time stepping, residual smoothing and multi-grid are implemented. All modules of the code (pre- and post-processing as well as the solver and the grid adaptation) run efficiently in massive parallel applications. Different one- and two-equation turbulence models can be used as well as many different flux discretizations.

The thermodynamic properties of each species can be specified in TAU either by defining the required input data for an internal calculation of the partition functions or by giving tabulated values. Calculating the partition functions for diatomic molecules the internal degrees of freedom can be either assumed as separable (harmonic oscillator, rigid rotator and electronic excitation) or fully coupled (including anharmonic effects). Only a model with separated vibrational, rotational and electronic modes is available for the partition functions of multi-atomic species in TAU.

For neutral species a set of chemical reaction rates (dissociation reactions are counted separately for each collision partner) as parameters for modified Arrhenius fits is made available in TAU format.

In TAU two approaches exist to describe the transport properties for a mixture of perfect gases. The first approach uses viscosity fits for each species and employs a mixture rule in order to determine the viscosity of the gas mixture. The thermal conductivity in this model is calculated per species via a modified Eucken correction and again a mixture rule is applied to achieve the thermal conductivity of the mixture. The diffusion is accounted for by Fick’s Law with a common diffusion coefficient for all species related to the common viscosity via a given Schmidt number.

The second approach starts from fits of the collision integrals for each binary collision and calculates the mixture viscosity and translational part of the thermal conductivity via more accurate mixture rules.

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In order to verify the implementation and the parameter files a number of different test have been conducted starting from the investigation of the equilibrium properties and two different flows conditions related to SACOMAR:

2 Modelling of Chemical Kinetics

The thermodynamic properties of each species can be specified in TAU either by defining the required input data for an internal calculation of the partition functions or by giving tabulated values of the three values for phi, H-H0 and cp over the temperature like presented by Gurvich et al.[1] The thermodynamic properties of the Martian atmosphere species are described according to Capitelli et al.[2]

A reaction scheme for pure CO2 flows consisting of six neutral species (CO2, CO, C2, O2, C, O) in chemical non-equilibrium accounting for 27 independent reactions has been compiled. The reaction set together with the associated Arrhenius parameters is given in Table 1. In the Table “M” stands for an arbitrary collision partner, “mol” for a molecular collision partner and “atom” for an atomic collision partner.

Reaction s (m³/mole/s) r (-) A/ (K)

CO2 + mol À CO + O + mol 6.9 · 1015 -1.5 63275

CO2 + atom À CO + O + atom 1.4 · 1016 -1.5 63275

C2 + M À 2 C + M 1.5 · 1010 0 71600

CO + mol À C + O + mol 2.3 · 1014 -1.0 129000

CO + atom À C + O + atom 3.4 · 1014 -1.0 129000

O2 + mol À 2 O + mol 2.0 · 1015 -1.5 59750

O2 + atom À 2 O + atom 1.0 · 1016 -1.5 59750

CO + O À C + O2 3.9 · 107 -0.18 69200

CO + C À C2 + O 2.0 · 1011 -1.0 58000

CO2 + O À CO + O2 2.1 · 107 0 27800

Table 1: Reaction scheme and Arrhenius parameters for six species CO2 model accounting for CO2, CO, C2, O2, C, O

In addition, an extended reaction scheme for CO2, N2 mixtures has been compiled which consists of 103 independent chemical reactions. The reaction scheme accounts for the eleven neutral species CO2, CO, C2, O2, N2, NO, CN, CNO, C, N and O. The additional reactions as well as the associated reaction parameters are presented in Table 2.

Reaction s (m³/mole/s) r (-) A/ (K)

N2 + mol À 2 N + mol 7.0 · 1015 -1.6 113200

N2 + atom À 2 N + atom 3.0 · 1016 -1.6 113200

CN + M À C + N + M 2.53 · 108 0 71000

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Reaction s (m³/mole/s) r (-) A/ (K)

NO + mol À N + O + mol 5.0 · 109 0 75500

NO + atom À N + O + atom 1.1 · 1011 0 75500

NCO + M À CO + N + M 6.3 · 1010 -0.5 24000

C + N2 À CN + N 5.24 · 107 0 22600

CN + C À C2 + N 5.0 · 107 0 13000

C2 + N2 À 2 CN 1.5 · 107 0 21000

NO + O À N + O2 8.4 · 106 0 19450

N2 + O À NO + N 6.4 · 1011 -1.0 38370

CO + N À CN + O 1.0 · 108 0 38600

CN + O À NO + C 1.6 · 107 0.1 14600

CN + O2 À NCO + O 6.6 · 106 0 -200

CN + CO2 À NCO + CO 4.0 · 108 0 19200

CN + NO À NCO + N 1.0 · 108 0 21200

CO + NO À NCO + O 3.8 · 1011 -0.87 51600

CN + CO À NCO + C 1.5 · 1010 -0.49 65800

Table 2: Additional reactions and Arrhenius parameters for the eleven species CO2, N2 model accounting for CO2, CO, C2, O2, N2, NO, CN, CNO, C, N and O

3 Transport Properties

In TAU two approaches exist to describe the transport properties for a mixture of perfect gases. The older model uses viscosity fits according to Blottner [3] for each species and Wilke’s [4] mixture rule. The thermal conductivity in this model is calculated per species via a modified Eucken [5] correction and again a mixture rule is applied to achieve the thermal conductivity of the mixture. The diffusion is accounted for by Fick’s law with a common diffusion coefficient for all species related to the common viscosity via a given Schmidt number. Following Bottin [6] and Hirschfelder [7], the newer model starts from fits of the collision integrals for each binary collision and calculates the mixture viscosity and translational part of the thermal conductivity via Yos’ [8] mixture rule or the lowest non vanishing approximation of Chapman and Cowling [7, 9]. It must be noted that thermal diffusion is neglected in the modelling of thermal conductivity. The contribution of the internal modes of energy to the thermal conductivity is treated separately like an Eucken correction. The binary diffusion coefficients are expressed by the collision integrals. An effective diffusion coefficient for each species in the mixture is calculated from the binary coefficients and used for a first approximation of the diffusion flux depending on the gradient of the species mole fraction. This approximation is then corrected following Ramshaw [9] to ensure that the sum of all diffusion fluxes is exactly zero.

Within an ESA TRP VKI has supported FGE and DLR by providing the MUTATION library as a state of the art model for the transport coefficients of various species. A direct linking of the library - like it is done for the VKI fluid solvers - seemed not to be efficient due to different algorithms for

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calculating the diffusion fluxes. The benefit of the MUTATION library used in TAU is the set of data for the collision integrals used in connection with the collision integral based models described above.

4 Verification and Validation

In order to verify and validate the parameter set and the implementation the models were applied in equilibrium and in non-equilibrium conditions. In Section 4.1 the chemical composition in equilibrium is presented versus temperature. Based on the equilibrium composition the transport properties in equilibrium are shown in Section 4.2. Non-equilbrium results are shown in Sections 4.3 and 4.4.

4.1 Equilibrium Composition

In order to verify and validate the thermodynamic species data, the equilibrium composition of pure CO2 as well as of the typical Martian atmosphere consisting of 97% CO2 and 3% N2 have been computed at 105 Pa and compared to other sources. The computed equilibrium compositions shown in Figure 1 are plotted versus temperature. The symbols filled in red plotted for each thousand K mark results from the CEA2 program of Gordon and McBride [11, 12].

T (K)

mo

le f

ract

ion

(-)

0 2000 4000 6000 8000 1000010-5

10-4

10-3

10-2

10-1

100

ψCO2 ( )ψC2 ( )ψC ( )ψCO ( )ψO2 ( )ψO ( )

T (K)

mo

le f

ract

ion

(-)

0 2000 4000 6000 8000 1000010-5

10-4

10-3

10-2

10-1

100

T (K)

mo

le f

ract

ion

(-)

0 2000 4000 6000 8000 1000010-5

10-4

10-3

10-2

10-1

100

ψCO2 ( )ψC2 ( )ψC ( )ψCO ( )ψO2 ( )ψO ( )ψNCO ( )ψN2 ( )ψN ( )ψNO ( )ψCN ( )

T (K)

mo

le f

ract

ion

(-)

0 2000 4000 6000 8000 1000010-5

10-4

10-3

10-2

10-1

100

Figure 1 Equilibrium composition of pure CO2 (left) and of a mixture of 97% CO2 and 3% N2 (right) at 105 Pa. Symbols filled in red are CEA2 results.[11, 12]

Very good agreement was obtained when comparing our result to the results of the CEA program of Gordon and McBride and with the results of Barbante and Magin [13] up to temperatures of about 8000 K. At temperatures above 8000 K the amount of ionized species, which are not considered in our model, become significant.

4.2 Transport Properties

Based on the equilibrium composition reported above the transport coefficients for the two mixtures have been computed. Viscosity and thermal conductivity of pure CO2 and the Martian atmosphere mixture versus temperature are shown in Figure 2 and Figure 3. The black and red curves are the results of the old species viscosity fit based transport coefficients model employing Schmidt numbers Sc = 0.5 and Sc = 0.7, respectively. The blue curves are the results of the collision integral based Chapman-Cowling approach. While the influence of the small nitrogen fraction in the Martian atmosphere is low, the more sophisticated modelling of the transport coefficients via collision integrals alters the transport coefficients significantly. The viscosity computed according to the Chapman-Cowling mixture rule is about 20% higher at 10000K. Since the Schmidt number is used for the determination of the contribution of mass diffusion, the viscosity is not affected by the different Schmidt numbers.

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T (K)

visc

osi

ty (

kg/m

/s)

0 5000 100000

5E-05

0.0001

0.00015

0.0002

0.00025

0.0003

η (kg/m/s), Sc = 0.5η (kg/m/s), Sc = 0.7η (kg/m/s), CC

T (K)

ther

mal

co

nd

uct

ivit

y (W

/m/K

)

0 2000 4000 6000 8000 100000

1

2

3

4

5

6ktot (W/m/K), Sc = 0.5ktot (W/m/K), Sc = 0.7ktot (W/m/K), CCkfr (W/m/K), Sc = 0.5kfr (W/m/K), Sc = 0.7kfr (W/m/K), CC

Figure 2 Viscosity (left) and thermal conductivity (right) of pure CO2 at 105 Pa versus temperature. Black, blue and red lines indicate results for Schmidt numbers Sc = 0.5, Sc = 0.7 and the Chapman-Cowling mixture rule, respectively.

The plots on the right in Figure 2 and Figure 3 show the thermal conductivity of the two mixtures in equilibrium versus temperature. The lines with symbols show the frozen thermal conductivity neglecting mass diffusion. As for the viscosity the frozen conductivity computed by the species viscosity based mixture rules is too low by about 20% at 10000 K. However, around T = 3500 K and T = 7300 K the major portion of the total thermal conductivity arises from diffusion due to CO2 and CO dissociation, respectively. As can be seen from the plots, Sc = 0.7 is closer to the Chapman-Cowling result for CO2 dissociation while Sc = 0.5 is closer to the Chapman-Cowling result for CO dissociation.

T (K)

visc

osi

ty (

kg/m

/s)

0 5000 100000

5E-05

0.0001

0.00015

0.0002

0.00025

0.0003

η (kg/m/s), Sc = 0.5η (kg/m/s), Sc = 0.7η (kg/m/s), CC

T (K)

ther

mal

co

nd

uct

ivit

y (W

/m/K

)

0 2000 4000 6000 8000 100000

1

2

3

4

5

6ktot (W/m/K), Sc = 0.5ktot (W/m/K), Sc = 0.7ktot (W/m/K), CCkfr (W/m/K), Sc = 0.5kfr (W/m/K), Sc = 0.7kfr (W/m/K), CC

Figure 3 Viscosity (left) and thermal conductivity (right) of a 97% CO2, 3% N2 mixture at 105 Pa versus temperature. Black, blue and red lines indicate results for Schmidt numbers Sc = 0.5, Sc = 0.7 and the Chapman-Cowling mixture rule, respectively.

The results for CO2 presented in Figure 2 agree very well with the results from Barbante and Magin. [13]

4.3 HEG Nozzle Flow

This section shows the results of an expanding HEG nozzle flow. The total conditions of the inflow boundary were set to p0 = 350 · 105 Pa and T0 = 6000 K according to the expected reservoir conditions for the HEG campaign within SACOMAR. In order to resolve the flow appropriately, an axisymmetric, structured grid with 1 million was used. However, it must be noted that only a pure

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CO2 flow has been computed. Usually, a shock is triggered at the nozzle walls of HEG due to the transition from laminar to turbulent flow. The turbulent properties of the flow have been modelled using the Wilkox k- two equation turbulence model.[14] In the simulation, the location of the transition was fixed at x = 0.758 m based on experience.

Figure 4 shows the Mach number contours in the HEG nozzle. Due to the expansion the flow is accelerated up to about Ma = 7.

Figure 4 Mach number contours along the HEG nozzle. Based on experience, transition from laminar to turbulent was set at x = 0.758m.

The plot on the left of Figure 5 shows the mass fractions along the centreline of the HEG. As can bee seen from the plot, the composition freezes at about x = 0.8 m. At this location, the Mach number is Ma = 4.3, the pressure is p = 39750 Pa and the temperature is T = 2654 K.

x (m)

mas

s fr

acti

on

(-)

1 2 30

0.2

0.4

0.6

0.8

1 ξCO2

ξC2

ξC

ξCO

ξO2

ξO

r (m)

pre

ssu

re

axia

l vel

oci

ty (

m/s

)

rad

ial v

elo

city

(m

/s)

0 0.1 0.2 0.3 0.4500

600

700

800

900

1000

1100

1200

-1000

0

1000

2000

3000

4000

5000

6000

0

100

200

300

400

Figure 5 Species mass fractions along the centreline of the HEG nozzle (left) as well as pressure and velocity at the nozzle exit (right).

The plot on the right of Figure 5 shows the radial variation of static pressure and velocity in the exit section of the HEG nozzle at x = 3.84 m. The core flow has a radius of about 32 cm and is accelerated to about vx = 4.5 km/s. The local maximum of the pressure which can be observed at about 10 cm in radial direction is due to the transition to turbulent mentioned above.

4.4 Flow Around Wind Tunnel Probe

As a final test case for the modelling a stagnation flow onto a cylindrical probe was simulated. A typical Mars entry flow condition from HEG with a moderate total enthalpy of about H = 9 MJ/kg was selected for this simulation. At the inflow boundary a temperature of T = 724 K, a density of

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= 8.83 · 10-4 kg/m³ and a velocity of v = 3257 m/s was prescribed. The mole fractions at the inflow boundary are CO2 = 0,4559, CO = 0,3556, O2 = 0,1662 and O = 0,0223 for CO2, CO, O2 and O, respectively. The final axisymmetric grid for the simulation of the flow around a cylindrical probe of 10 cm in diameter is shown in Figure 6. Starting from an initial grid consisting of 121,270 points, six successive adaptation steps were performed resulting in the final grid with 569,518 points.

Figure 6 Axisymmetric grid used for the simulation of the flow around a stagnation probe after six adaptation Steps.

As can be seen from the pressure and temperature contours in Figure 7, the final grid allows for a proper resolution of the shock upstream of the probe. No abnormal runtime behaviour or convergence problems have been encountered during the simulation of the flow.

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Figure 7 Pressure (left) and temperature (right) contours for the flow around the stagnation probe

The plot on the left in Figure 8 shows the species mass fractions along the stagnation line of the probe. Downstream of the shock, the composition of the flow shows a typical relation behaviour. At the surface an fully catalytic boundary condition and a temperature of T = 300 K was applied which results in a significant decrease of the dissociation degree in the boundary layer.

x (m)

mas

s fr

acti

on

(-)

-0.025 -0.02 -0.015 -0.01 -0.005 010-12

10-10

10-8

10-6

10-4

10-2

100

ξCO2

ξC2

ξC

ξCO

ξO2

ξO

z (m)

hea

t fl

ux

(W/m

2 )

pre

ssu

re (

Pa)

0 0.01 0.02 0.03 0.04 0.050

500000

1E+06

1.5E+06

0

2000

4000

6000

8000

10000

Figure 8 Mass fractions along the stagnation line (left) as well as heat flux and pressure versus radial direction (right) of the stagnation probe

The plot on the right of Figure 8 show surface pressure an heat flux versus the radial coordinate. Close to the stagnation point a slight decrease of the heat flux can be observed which indicates a

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problem in the discretization of the axisymmetric flow. However, both the pressure of about p = 9000 Pa as well as the heat flux of about 800 kW/m² are reasonable values.

5 Summary and Conclusions

Within the frame Work package 6.3 the CFD code TAU of DLR has been extended to allow for the simulation of Mars entry flows.

The species properties as well as the transport coefficient modelling have been verified and validated under equilibrium conditions. The more accurate Chapman-Cowling mixture rules for the transport properties result in about 20% difference of the viscosity and the frozen thermal conductivity at T = 10000 K. Significant difference can also be observed when comparing the contribution of diffusion on thermal conductivity. While the collision integral based approach allows for the simulation of arbitrary mixtures a single Schmidt number is not appropriate if multiple different molecules are present in the flow.

An expanding as well as a compressing stagnation point flow have been computed in order to validate the modelling in non-equilibrium flows. Reasonable results were obtained. However, since the flow conditions have been selected before the testing was done, further validation by means of comparison with experiment or with other numerical codes is necessary.

References

1 Gurvich, L., Veyts, I. and Alcock, C. Thermodynamic properties of individual substances, Vol. 1 of Thermodynamic Properties of Individual Substances. Hemisphere Publishing, 1989. ISBN 9780891167617.

2 Capitelli, M., Colonna, G., Giordano, D., Marraffa, L., Casavola, A., Minelli, P., Pagano, D., Pietanza, L. D., Taccogna, F. and Warmbein, B. Tables of Internal Partition Functions and Thermodynamic Properties of High-Temperature Mars-Atmosphere Species from 50K to 50000K. ESA Scientific Technical Review, Vol. 246, Oct. 2005.

3 Blottner, F., Johnson, M. and Ellis, M. Chemically Reacting Viscous Flow Program for Multi-Component Gas Mixtures. Tech. Rep. SC-RR–70-754, Sandia Labs, Albuquerque, New Mexico, USA, Dec. 1971.

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5 Eucken, A. Über das Wärmeleitvermögen, die spezifische Wärme und die innere Reibung der Gase. Physikalische Zeitschrift, Vol. 14, No. 8, 324–332, 1913. (in German)

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