11. What is a vortex tube?

            A vortex tube is a gas thermal separation device. For example, compressed air at room temperature tangentially enters the closed end of the tube and it rotates and advances along the tube in a helical (coil spring-like) path and exits the open end. The exit air at the periphery can be many tens of degrees hotter than the entering air and the exiting air at the center can be many tens of degrees cooler than the entering air. It is believed that the centrifugal force on the individual particles provides the mechanism for separating the particles by their temperature.

           The steady state analysis of gas in a drum rotating at constant speed gives the result that the thermal velocity varies linearly with the radius. Measurements of the temperature as a function of the radial distance from the vortex longitudinal axis indicate that the temperature approximately varies as the square of the radius which means that the thermal velocity varies approximately linearly with the radius. Thus, measurements are in fair agreement with the theoretical prediction that the thermal downstream velocity varies linearly with the radius.