06-12-2012, 11:38 AM
Parametric Cycle Analysis of Real Engines
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Introduction
In Chapter 5, we idealized the engine components and assumed that the
working fluid behaved as a perfect gas with constant specific heats. These idealizations
and assumptions permitted the basic parametric analysis of several types
of engine cycles and the analysis of engine performance trends. In Chapter 6, we
looked at the variation of specific heat with temperature and fuel/air ratio and
developed component models and figures of merit. This allows us to use realistic
assumptions as to component losses and to include the variation of specific heats
in engine cycle analysis. In this chapter, we develop the cycle analysis equations
for many engine cycles, analyze their performance, and determine the effects of
real components by comparison with the ideal engines of Chapter 5. We begin
our analysis with the turbojet engine cycle and treat the simpler ramjet engine
cycle as a special case of the turbojet (Trc = 1, Zc = 1, err ---- 1, ~'t = 1).
Turbojet
We will now compute the behavior of the turbojet engine including
component losses, the mass flow rate of fuel through the components, and the
variation of specific heats. Our analysis still assumes one-dimensional flow at
the entrance and exit of each component. The variation of specific heats will
be approximated by assuming a perfect gas with constant specific heat Cpc
upstream of the main burner (combustor) and a perfect gas with different constant
specific heat Cpt downstream of the main burner.
The turbojet engine with station numbering is shown in Fig. 7.1, and the T-s
diagram for this cycle with losses is plotted in Fig. 7.2. Figure 7.2 shows the total
states for all engine stations along with the static states for stations 0 and 9.
Cycle Analysis
In this section we develop a system of equations to analyze the turbojet engine
cycle. The steps of cycle analysis are applied to the turbojet engine and presented
next in the order listed in Section 5.4.
Turbojet with Afterburner
The turbojet engine with afterburner is shown in Fig. 7.8, and the temperatureentropy
plot of this engine with losses is shown in Fig. 7.9. The numbering
system indicated in these figures is the industry standard. 3