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The vortex tube was discovered in 1930 by Georges Ranque, a French physics student, was experimenting with a vortex-type pump he had developed when he noticed warm air exhausting from one end, and cold air from the other. Ranque soon forgot about his pump and started a small firm to exploit the commercial potential for this strange device that produced hot and cold air with no moving parts. However, it soon failed and the vortex tube slipped into obscurity until 1945whenRudolphHilsch, a German physicist, published a widely read scientific paper on the device. Vortec was the first company to develop this phenomenon into practical, effective cooling solutions for industrial applications.
Much earlier, the great nineteenth century physicist, James Clerk Maxwell postulated that since heat involves the movement of molecules, we might someday be able to get hot and cold air from the same device with the help of a "friendly little demon" who would sort out and separate the hot and cold molecules of air.
Thus, the vortex tube has been variously known as the "Ranque Vortex Tube", the "Hilsch Tube", the "Ranque-Hilsch Tube", and "Maxwell’s Demon". By any name, it has in recent years gained acceptance as a simple, reliable and low cost answer to a wide variety of industrial spot cooling problems.
When it was first announced that the device using the compressed air discharged hot and cold stream simultaneously at its two ends, many people thought that this is a case of violation of second law of thermodynamics. Later, Eckert gave a theoretical explanation of its behavior.
Since then, the tube had been the subject of many papers and research projects. But, commercial acceptance has been slow. Today, there are an estimation of more than 300,000 vortex tubes are in use commercially worldwide.
WORKING OF VORTEX TUBE:
A compressed air is passed through the nozzle as shown in figure. Here air expands and acquires high velocity due to particular shape of the nozzle. A vortex flow is created in the chamber and air travels in spiral motion along the periphery of the hot side. Then, the rotating air is forced down the inner walls of the hot tube at speeds reaching 1,000,000 rpm.
The valve restricts this flow. When the pressure of the air near the valve is made more than the outside by partly closing the valve, a reversed axial flow through the core of the hot side starts from high-pressure region. During this process, energy transfer takes place between reversed stream and forward stream and therefore air stream through the core gets cooled below the inlet temperature of the air in the vortex tube while the air stream in forward direction gets heated. The cold stream is escaped through the diaphragm hole into the cold side, while hot stream is passed through the opening of the valve. By controlling the opening of the valve, the quantity of the cold air and its temperature can be varied.
There are several theories, which give the physical explanation of the energy transfer from the colder region to the hotter region. No theory is so perfect, which gives the satisfactory explanation of this whimsical energy transfer.
The explanation given by Van Deemeter is described in shot.
The air enters the main tube through the nozzle and forms a free vortex. Due to the centripetal acceleration, the vortex travels along the periphery of the tube and when it reaches the throttle valve, the rotation almost ceases, so there is a point of atmospheric pressure, a reverse axial flow starts. This flow comes into contact with the free vortex, which is moving with the increasing speed, therefore the axial stream forms a forced vortex.
The energy required maintaining the forced vortex in the reversed axial flow stream is supplied by the force vortex at the periphery. Therefore, there is flow of energy (momentum) from the peripheral layer of air to the reversed axial flow stream at the axis The rotational velocity of the free vortex at the periphery decreases gradually from the plane of the nozzle to the plane of the valve, therefore there is a relative sliding between the two adjacent air-planed, which are moving towards the valve. The result of this is a continuous transfer of energy from the plane of the nozzle to that of the valve. This gives the explanation why the heating of the air takes place as it proceeds towards the valve. The transfer energy from the inner core (from the region of forced vortex) to the periphery (into the region of free vortex) has not been explained satisfactorily.
Theories abound regarding the dynamics of a vortex tube. Here is one widely accepted explanation of the phenomenon:
Compressed air is supplied to the vortex tube and passes through nozzles that are tangent to an internal counter bore. These nozzles set the air in a vortex motion. This spinning stream of air turns 90° and passes down the hot tube in the form of a spinning shell, similar to a tornado. A valve at one end of the tube allows some of the warmed air to escape. What does not escape, heads back down the tube as a second vortex inside the low-pressure area of the larger vortex. This inner vortex loses heat and exhausts thru the other end as cold air.
While one air stream moves up the tube and the other down it, both rotate in the same direction at the same angular velocity. That is, a particle in the inner stream completes one rotation in the same amount of time as a particle in the outer stream. However, because of the principle of conservation of angular momentum, the rotational speed of the smaller vortex might be expected to increase. (The conservation principle is demonstrated by spinning skaters who can slow or speed up their spin by extending or drawing in their arms.) But in the vortex tube, the speed of the inner vortex remains the same. Angular momentum has been lost from the inner vortex. The energy that is lost shows up as heat in the outer vortex. Thus the outer vortex becomes warm, and the inner vortex is cooled.
Prof.Parulekar has proposed the following hypothesis, which is based on experimental observations.
The air enters the tube tangentially and forms a free vortex. The vortex travels along the wall due to centrifugal action. The air almost ceases to rotate in the region of the valve.
The pressure near the valve is more than outside the diaphragm at the other end, a reversal axial flow starts. This reversed flow comes in to contact with the forward moving free vortex along the internal surface of the vortex. The free vortex forces the axial stream to rotate as it rotates at a very high speed. Thus an axial stream forms a forced vortex. The energy required to form the forced vortex of the axial stream is supplied by the outer free vortex. However, the flow of energy is in opposite direction but it is too small compared with the energy transfer from the inner core to the outer periphery. The energy transfer from the inner core to the outer periphery is explained below.
The turbulent mixing in the centrifugal field results in pumping of energy from the low-pressure region at the axis to the high-pressure region at the periphery. The energy is then transferred towards the valve in the form of momentum as already explained above by Van Deemeter. This radial outflow energy due to turbulent mixing is much more than that of the inward flow of energy due to the formation of vortex, there is net transfer of energy radially outward and towards the valve.
