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https://ntrs.nasa.gov/search.jsp?R=19650024642 2018-06-22T11:53:17+00:00Z

. NASA SP-3020

By Leland H. Jorgensen

Ames Research Center Moffett Field, California

Scientific and Technical lnformation Diviszon 1 9 6 5 NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

Washington, D.C.

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For sale by the Clearinghouse for Federal Scientific and Technical Information Springfield, Virginia 22151 - Price $0.50

SUMMARY

For enthalpies to 28,000 Btu/lb and pressures from to l o 2 atmospheres, curves of isentropic exponent as a function of enthalpy and speed are presented for equilibrium air, nitrogen, carbon dioxide, and a composition of 20-percent CO2 and 80-percent NE (by volume). For a pressure of 1 atmosphere, curves are also pre- sented for compositions of 10-percent CO2 - 90-percent N2 and 50-percent C 0 2 - 5 0 -percent NE.

INTRODUCTION

The isentropic exponent o r dimensionless speed of sound parameter, y is present in many gasdynamic equations. In the field of planetary entry aerodynam- ics where real-gas phenomena are important, it is convenient to have plots of 7 as a function of enthalpy for specified pressures. This enables the often desired value of 7 at the stagnation point to be read directly from the plots after the speed is con- verted to total enthalpy.

a2p/p,

Because of interest in flight within the Martian and Venusian atmospheres which are believed to be primarily nitrogen and carbon dioxide, these gases, as well as air, should be considered. It is the purpose of this report to present curves of isentropic exponent versus enthalpy and speed for air, nitrogen, carbon dioxide, and several mixtures of nitrogen and carbon dioxide. The gases are assumed to be in equilibrium.

NOMENCLAT TJRE

isentropic speed of sound

enthalpy (h = 0 for molecular gas at 0' K)

pressure

dynamic pressure, pu2

speed

1

y isentropic exponent

p density

Subscripts

s constant entropy

s t stagnation

co free s t ream

Conversion From Units in Figures 1-6 to "SI Units" (lnternational System of Units, NASA TT F-200)

Physical quantity To convert from Multiply by To obtain

density slugs/ft3 5. 154x10, k g / s

enthalpy Btu/lb 2 . 3 2 4 ~ 1 0 ~ J/kg

speed ft/ sec 0.3048 m/ s

METHODS AND RESULTS

For enthalpies to 28,000 Btu/lb and pressures from to l o 2 atmospheres, values of isentropic exponent were computed fino= the defining relation

Al l calculations were made on an IBM 7094 system, with the thermodynamic proper- t ies having been previously recorded on magnetic tape by Dr. Harry E. Bailey of Ames Research Center. reference 1, Dr . Bailey recently computed the equilibrium thermodynamic properties of various gases and mixtures. H i s results for carbon dioxide are reported in reference 2.

Following the assumptions and approximations made in

In figures 1 through 5, curves of isentropic exponent y versus enthalpy h are presented for a i r , nitrogen, carbon dioxide, and a 20-percent CO, - 80-percent N, composition (by volume). The velocity scale at the top of each figure is applicable only when the values of y , P, and h are taken at the stagnation point. To obtain y at the stagnation point, stagnation-point pressure pst and enthalpy list must be determined. For hypersonic flight,

2

an&

The variation of y with h for low values of h (fig. 5) in air and C02 is useful for interpretation of test results from many wind tunnels. In figure 6, the previous curves for the different gases at p = 1 atm a r e compared with each other and with additional curves for compositions of 10-percent C 0 2 - 90-percent N2 and 50-percent C 0 2 - 50-percent Ne. The effect of gas composition on y (fig. 6) appears to be the greatest at enthalpies below about 10 ,000 Btu/lb.

REFERENCES

1. Marrone, Paul V. : Inviscid, Nonequilibrium Flow Behind Bow and Normal Shock Waves. Part I. General Analysis and Numerical Examples. CAL Rep. QM-1626-A-12 (I), May 1963.

2. Bailey, Harry E. : Equilibrium Thermodynamic Properties of Carbon Dioxide. NASA SP-3014, 1965.

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