Vortex tube

The vortex tube, also known as the Ranque-Hilsch vortex tube, is a mechanical device that separates a compressed gas into hot and cold streams. The air emerging from the "hot" end can reach temperatures of 200 °C (392 °F), and the air emerging from the "cold end" can reach −50 °C (−58 °F). It has no moving parts.

Pressurised gas is injected tangentially into a swirl chamber and accelerated to a high rate of rotation. Due to the conical nozzle at the end of the tube, only the outer shell of the compressed gas is allowed to escape at that end. The remainder of the gas is forced to return in an inner vortex of reduced diameter within the outer vortex.

To explain the temperature separation in a vortex tube, there are two main approaches:

This approach is based on first-principles physics alone and is not limited to vortex tubes only, but applies to moving gas in general. It shows that temperature separation in a moving gas is due only to enthalpy conservation in a moving frame of reference.

The thermal process in the vortex tube can be estimated in the following way: 1) The adiabatic expansion of the incoming gas, which cools the gas and turns its heat energy into the kinetic energy of rotation. The total enthalpy (which is the sum of the enthalpy and the kinetic energy), however, is being conserved. 2) The peripheric rotating gas flow moves towards the hot outlet. Here the heat recuperation effect takes place between the quickly rotating peripheric flow and the opposite slowly rotating axial flow. Here the heat transfers from axial flow to the peripheric one. 3) The kinetic energy of rotation turns into the heat energy by the means of the viscous dissipation. The temperature of the gas rises. As the total enthalpy has been increased during the heat recuperation process, this temperature is higher than the incoming gas. 4) Some of the hot gas leaves the hot outlet, carrying away the excess heat. 5) The rest of the gas turns towards the cold outlet. As it passes its way to the cold outlet, its heat energy is transferred to the peripheric flow. Although the temperature at the axis and at the periphery is about the same everywhere, the rotation is slower at the axis, so the total enthalpy is lower as well. 6) The low total enthalpy cooled gas from the axial flow leaves the cold outlet.

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