6.2.2: Inlet

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截屏2022-01-21 下午8.08.32.png
Figure 6.4: Types of inlets.

The free-stream air enters the jet engine at the inlet (also referred to as intake). There exist a variety of shapes and sizes dependent on the speed regime of the aircraft. For subsonic regimes, the inlet design in typically simple and short (e.g., for most commercial and cargo aircraft). The surface front is called the inlet lip, which is typically thick in subsonic aircraft. See Figure 6.4.a.

On the contrary, supersonic aircraft inlets have a relatively sharp lip as illustrated in Figure 6.4.b. This sharpened lip minimizes performance losses from shock waves due to supersonic regimes. In this case, the inlet must slow the flow down to subsonic speeds before the air reaches the compressor.

An inlet must operate efficiently under all flight conditions, either at very low or very high speeds. At low speeds the free-stream air must be pulled into the engine by the compressor. At high speeds, it must allow the aircraft to properly maneuver without disrupting flow to the compressor.

Given that the inlet does no thermodynamic work, the total temperature through the inlet is maintained constant, i.e.:

\[\dfrac{T_{2t}}{T_{1t}} = \dfrac{T_{2t}}{T_0} = 1.\]

The total pressure through the inlet changes due to aerodynamic flow effects. The ratio of change is typically characterized by the inlet pressure recovery (IPR), which measures how much of the free-stream flow conditions are recovered and can be expressed as follows:

\[IPR = \dfrac{p_{2t}}{p_{0t}} = \dfrac{p_{2t}}{p_0}; M < 1.\nonumber\]

As pointed out before, the shape of the inlet, the speed of the aircraft, the airflow characteristics that the engine demands, and aircraft maneuvers are key factors to obtain a high pressure recovery, which is also related to the efficiency of the inlet expressed as:

\[\eta_i = \dfrac{p_{2t}}{p_{1t}} = \dfrac{p_{2t}}{p_0}.\nonumber\]