In a choked nozzle, velocity increases, and which of the following also changes according to the data?

• A. Density decreases
• B. Pressure decreases
• C. Pressure increases
• D. Temperature increases

Answer: (C)

In a choked nozzle the flow reaches Mach 1 at the throat, and as the gas continues to accelerate, most of the energy goes into increasing velocity rather than raising pressure. For an ideal gas in isentropic flow, the stagnation temperature remains constant, so the static temperature falls as velocity rises (through the relation V^2/2 = cp(T0 − T)). Since the static pressure is tied to temperature through the isentropic relation p/p0 = (T/T0)^(gamma/(gamma−1)), a drop in static temperature causes the static pressure to drop as well. In other words, as velocity increases in a choked nozzle, the static pressure decreases. The issue described by the data would show a decrease in pressure accompanying the rise in velocity, not an increase. The temperature would also decrease, not increase, under these ideal choked-flow conditions.