Part 2 reports on the analytical study. The free vortex motion of the gas upon entrance to the tube is mathematically superposed to a compressible sink to give a spiral flow in the plane. The characteristic existence of limit circles is corroborated by the experimental flow visualization. The solution in space is obtained by addition of a uniform axial velocity to the spiral flow. When viscosity effects are considered, the free vortex is shown to change into a forced vortex. The latter flow is one of minimum kinetic energy and maximum entropy. Energy considerations enable the determination of an optimum cold air radius to give largest stagnation temperature separation. Significantly, this was the radius that gave best performance in the experimental program.

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