Aircraft Propulsion, 2nd Edition

CHAPTER 2Compressible Flow with Friction and Heat: A Review

2.1 Introduction

Schlieren visualization of waves around an X-15 model in a supersonic wind tunnel. Source: Courtesy of NASA

The study of propulsion is intimately linked to the understanding of internal fluid mechanics. This means that duct flows are of prime interest to propulsion. The physical phenomena in a real duct include the following effects:

• Friction
• Heat transfer through the walls
• Chemical reaction within the duct
• Area variation of the duct
• Compressibility effects, e.g. appearance of shocks

The laws of thermodynamics govern the relationship between the state variables of the gas, namely density ρ, pressure p, absolute temperature T, entropy s, internal energy e, and derived properties such as enthalpy h and specific heats at constant pressure and volume, c_p and c_v, respectively. In addition to the laws of thermodynamics, the fluid flow problems need to obey other conservation principles that were introduced in Newtonian mechanics. These are conservation of mass and momentum as described in classical mechanics. Since the study of gas turbine engines and propulsion in undergraduate curricula in mechanical and aerospace engineering follows the introductory courses in thermo-fluid dynamics, we shall review only the principles that have a direct impact on our study of jet engines.

The purpose of this chapter ...