Effects of Real Gases on Calculated Performance

Effects of Real Gases on Calculated Performance

Recall that we have assumed Cp and y to be constants. This concept is
true when the average square of the linear speeds of the molecules accounts
for all of their energy. When molecules consist of two or more atoms,
however, the atoms rotate about each other and vibrate; this energy is
absorbed by the gas. More and more energy is diverted this way as the
temperature is raised; thus, the value of Cp continually increases while ~,
decreases. We note that air consists almost entirely of the diatomic gases
oxygen and nitrogen and that Cp increases and 7 therefore decreases with
increasing air temperatures. After combustion, we also find important
quantities of triatomic elements--carbon dioxide and water vapor. Combustion
thus causes a further increases in Cp and reduction in y. The gas
constant is also lowered. We now need to determine how these changes
affect our thoughts about engine thermodynamic design.
The result of a spot check of the effect of real-gas properties is presented
in Table 1.1. These results are typical. The assumed fixed conditions for the
cycle are noted. The calculated gas properties are: the enthalpy increase for
compression, the energy added by fuel to obtain the stipulated turbine inlet
temperature, the enthalpy converted into mechanical energy during the
expansion process, the energy produced by the engine, and the engine
efficiency. The first column shows the results of assuming Cp to be 0.24 and
to be