True or False: For flowing gases it's generally assumed that the kinetic energy changes are negligible.

True or False: For flowing gases it's generally assumed that the kinetic energy changes are negligible.

• A. True
• B. False
• C. Not sure
• D. Sometimes

Answer: (B)

A flowing gas energy balance must include kinetic energy changes. In steady, adiabatic (no heat transfer) and often inviscid flow, the total enthalpy plus kinetic energy per unit mass, called stagnation enthalpy h0 = h + v^2/2, tends to be conserved along a streamline. This means that when the gas speeds up, some internal energy is converted into kinetic energy, and when it slows down, kinetic energy can be converted back into internal energy (often seen as a temperature change). Because of this exchange between internal energy and kinetic energy, changes in velocity are directly tied to energy changes in the gas. Therefore, assuming kinetic energy changes are negligible is not generally valid. The KE term can be the dominant part of the energy balance in regimes where velocity changes are large—for example, in nozzles, diffusers, or jet propulsion problems—so it must be accounted for. Neglecting it would lead to incorrect predictions of temperature, pressure, and performance. Only in very slow, nearly uniform flows, where velocity changes are truly tiny, might the kinetic energy change be a reasonable approximation, but that’s not the general case.

A flowing gas energy balance must include kinetic energy changes. In steady, adiabatic (no heat transfer) and often inviscid flow, the total enthalpy plus kinetic energy per unit mass, called stagnation enthalpy h0 = h + v^2/2, tends to be conserved along a streamline. This means that when the gas speeds up, some internal energy is converted into kinetic energy, and when it slows down, kinetic energy can be converted back into internal energy (often seen as a temperature change). Because of this exchange between internal energy and kinetic energy, changes in velocity are directly tied to energy changes in the gas.

Therefore, assuming kinetic energy changes are negligible is not generally valid. The KE term can be the dominant part of the energy balance in regimes where velocity changes are large—for example, in nozzles, diffusers, or jet propulsion problems—so it must be accounted for. Neglecting it would lead to incorrect predictions of temperature, pressure, and performance. Only in very slow, nearly uniform flows, where velocity changes are truly tiny, might the kinetic energy change be a reasonable approximation, but that’s not the general case.