In an adiabatic pipe, a gas flows through a pipe and expands. This expansion leads to:

• A. An increase in velocity of the gas
• B. A decrease in velocity of the gas
• C. An increase in pressure
• D. A decrease in temperature

Answer: (A)

In adiabatic flow, the total (stagnation) enthalpy remains constant. As the gas expands, its static enthalpy h decreases because some of the internal energy is converted into kinetic energy. This relationship is captured by h0 = h + V^2/2 for steady, adiabatic flow. When h drops due to expansion, the kinetic energy term V^2/2 must rise to keep h0 constant, so the velocity increases.

At the same time, the expansion causes the gas to cool and its pressure to fall, since lower temperature and pressure accompany expansion in an adiabatic process. The key takeaway is that, without heat transfer, expanding gas can convert internal energy into motion, accelerating the flow.