2: Thermodynamics Review

Ratio of Specific Heats

, so that

 

Define

Thus,

 

Note: At low to moderate temperatures, (200 to 700K), the ratio of specific heats of diatomic gases such as nitrogen and oxygen is equal to 1.4. In the high temperature regions of engines, this value changes because the specific heats change and the gas composition is different from that of cold air. For most calculations of flows in jet engines, it is usual to assume gamma = 1.4 in the cooler regions, and gamma = 1.33 in the hot gases in the turbine and hot nozzle.


 

Variation of specific heats over a large range of temperatures

Where large changes in temperature occur,such as those encountered across a shock wave in hypersonic flow, or through the combustion chamber and turbine of a jet engine, cp, cv and g cannot be assumed to be equal to their low-temperature values.

 

 

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