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experimental investigation on energy separation by vortex tubes

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					Experimental Investigation on Energy Separation by Vortex Tubes
Ting-Quan MA, Qing-Guo ZHAO, Jian YU, Fang YE, and Chong-Fang MA
College of Environmental and Energy Engineering, Beijing Polytechnic University, Beijing 100022,P. R. China

Abstract: The performances of energy separation have been experimentally studied in vortex tubes with compressed air as the working medium. This paper presents experimental results of the energy separation in the vortex tube under different operating conditions. It is experimentally evidenced that the inlet pressure greatly influences the separation performance while the effect of the inlet temperature can be negligible. The most important point revealed in this paper is that an optimum energy separation can be achieved by varying the fraction of the flow rate of the cold stream to the total flow rate at entrance. Key-words: Vortex Tube, Vortex Flow, Energy Separation

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Introduction

Early in the nineteenth century, the great physicist James Clerk Maxwell imagined that someday we might be able to get hot and cold air with the same device with the help of a "friendly little demon" who would sort out and separate the hot and cold molecules of air (Cockreill, 1995). Later his dream had come true. The “friendly little demon” is called vortex tube. In 1928, a French physics student George Ranque occasionally found the phenomenon of energy separation in the vortex tube during his experiment with a vortex-type pump developed by himself. He noticed that the warm air would be drawn from one end, and the cold air from the other. Later it was discovered that the mechanism is closely related to the swirling flow of the air within the tube. Air molecules in the swirl near the wall of the tube tend to have higher velocity compared to those in the central region of the tube. After energy separation in the vortex tube, the inlet air stream was separated into two air streams: hot air stream and cold air stream, the hot air stream left the tube from one end and the cold air stream left from another end. The outlets where the hot and cold air streams leaving the tube are called the hot and cold end respectively. In 1945, Rudolph Hilsch published his systemic experimental results on the thermal performances of vortex tubes with different geometrical parameters and under different inlet pressures. Since then, the vortex tube has been a subject of much interest. In the following years, many experimental studies have been carried out in which attempts were concentrated on explaining the mechanism of energy separation in the vortex tube. Takahama’s (1965, 1981) study resulted in several formulas for determining the performance and efficiency of vortex tubes under a variety of operating conditions, which induced the optimum ratios of vortex tube dimensions corresponding to the highest efficiency of temperature separation. Kurosaka (1982) first studied the effect of acoustic streaming on the energy separation in the Ranque-Hilsch tube. Gutsol (1997) performed experiments with a vortex tube similar to those used by Ranque and Hilsch to determine the possible causes of energy separation in the vortex tube. K. Stephan and his coworkers (1983) measured the temperature profiles at different positions along a vortex tube axis, leading them to the conclusion that the length of the vortex tube would have an important influence on the mechanism of the energy separation. In recent years it has been accepted that the vortex tube is a simple, reliable, low cost and effective solution to a wide variety of industrial spot cooling problems, but the actual mechanism of the energy separation in vortex tubes has not been completely understood. The objective of this study is to provide some new insights into the performance of vortex tubes under different operating conditions.

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1 Pressurizer

2 Bypass valve 3 Thermocouple 4 Pressure sensor 5 Regulating valve Figure 1 Schematic diagram of the experimental set-up

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Experiment

The schematic diagram of the experimental set-up is shown in Figure 1. The compressed air from the compressor passes through the pressurizer (1) and enters the cooler in which the temperature of the air can be regulated. After the mixing chamber, it is led tangentially into the vortex tube. The air is expanded in the vortex tube and separated into hot air stream and cold air stream. The cold stream in the central region flows out of the tube through the central orifice nearer to the inlet nozzle, while the hot stream in the outer annulus leaves the tube through another outlet farther from the inlet. The flow rate of the inlet air is regulated with the pressurizer (1) and bypass valve (2) and that of the hot stream can be adjusted by changing the axial location of the control valve. The volumetric flow rates of the inlet and cold streams are both measured by flow meters. The temperature of the inlet and outlet flows are measured with three thermocouples (3) and the pressure of the inlet and outlet flows are measured with three pressure sensors (4). In order to obtain the reliable average temperature of the inlet or outlet air, a mixing chamber has been set just before each of the temperature measuring section. In the present study, the vortex tube is made of stainless steel with inner diameter of 10mm. The tangential inlet nozzle has a diameter of 3mm. The whole length of the tube is 137mm. The outlet diameters of the cold and hot ends are both 8mm. The flow rate is in the range of 2~12.8m3/h at 18°C under atmospheric pressure. The inlet pressure is adjustable from 2.5 bar to 7.0 bar and inlet temperature is from 12℃ to 36℃.

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Experimental Results and Discussions

When compressed air is introduced into the vortex chamber of the tube, the linear momentum of the air is converted to the angular momentum and it flows spirally in the form of forced vortices. The tangential velocity in the tube will increase with radial location and approach its maximum near the tube wall. Because of the high velocity in the outer annulus, it will have high kinetic energy. When this fluid annulus flows out of the annulus outlet at the hot end again in the form of linear movement with a low velocity, its original high angular momentum energy will be converted into thermal energy and thus a high temperature is attained. In comparison, the air in the central region with a low tangential velocity will have a low angular momentum. When it leave the outlet at the cold end with a velocity of the same order as that of the air flowing out of the hot end, it will have a low thermal energy and therefore a low temperature. Though the mechanism of the energy separation has not been well understood, the phenomenon itself has been verified. The possible flow pattern in the vortex tube is shown in Figure 2.

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Figure 3 presents the changes of the temperature of cold and hot streams with inlet pressure where P1 and T1 are the air pressure and temperature at the entrance of the vortex tube, and η is the cold fraction. Let Gc and Gt denote the volumetric flow rate of the cold stream and the total one at the entrance, then the cold fraction η = Gc G t . It can be clearly seen from Figure 3 that the temperature of the hot stream increases with inlet pressure increasing, and that of cold stream decreases with inlet pressure. This indicates that the energy separation will be greatly enhanced by increasing the inlet pressure. The compressed air begins its vortex flow as soon as its introduced into the vortex tube. Because of the centrifugal at T1=26°C and η=44.6% characteristics of the forced vortex flow, the tangential velocity of the air near the tube wall would be larger than that in the central region. This would naturally cause the temperature near the tube wall to be higher than that in the central region. Also, the higher frictional force among fluid particles as well as among the fluid particles and the tube wall near the wall region is responsible for part of this phenomenon. The higher the inlet pressure, the greater the centrifugal force. Then the difference between the tangential velocity in the near-wall region and that in the central region would be larger, and hence the difference between the temperatures of the two regions. The influences of inlet temperature on the outlet temperatures are shown in Figure 4. It is evident that both
Figure 3 The outlet temperature vs. inlet pressure P1

Figure 4

Outlet temperature vs. inlet temperature at P1=0.7bar, η=48%

Figure 5

Outlet temperature difference vs. inlet temperature at P1=0.7bar, η=48%

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Figure 6

Cold stream temperature vs. cold fraction η at T1=19℃

Figure 7

Temperature difference vs. cold fraction η at T1=19℃

the hot stream temperature and cold stream temperature increase slightly with inlet temperature rising in Figure 4. The outlet temperature difference between the hot air stream and the cold air stream is shown in Figure 5. This temperature difference between the hot and the cold air stream decreases slightly with inlet temperature rising. The changes are all negligible from our experiments. It can be concluded from these figures that the inlet temperature is not a strong factor influencing the energy separation just in our experimental range. Figure 6 illustrates changes of the cold stream temperature with η under different inlet pressures. It can be seen from Figure 6 that the cold stream temperature generally increases slightly with η and attains its extreme value in the range of about η = 30 ~ 43% , depending upon the inlet pressure. Clearly, this region should be avoided in practical operations. Figure 7 illustrates changes of the temperature difference between the temperature of the hot stream and that of the cold stream. The most important point has been revealed in Figure 7 is that the temperature difference between the hot stream and the cold one approaches its maximum value in the range of about η = 70 ~ 80% . The maximum temperature difference seems to take place at the lower η value when the inlet pressure becomes higher. As a priori assumption, the maximum would tend to happen at η = 50% if the inlet pressure continued to increase, but this is a subject of further extensive investigation that has exceeded the range of the present study.

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Concluding Remark

It is clear that the pressure of the air at entrance is the necessary driving force for the energy separation. It is clear from our experiments that the higher the inlet pressure, the greater the temperature difference of the two outlet air streams. According to the law of energy conservation, the higher inlet temperature will result in higher temperatures both in the hot stream and in the cold stream. In spite of the fact that the higher inlet temperature will more or less decrease the degree of the energy separation, this effect can be negligible, based on the experimental results. The present study indicates that the cold fraction is an interesting operating parameter influencing the performance of the energy separation of the vortex tube, as there is a point of η , in the range of about 70~80%, in which the optimum temperature separation can be achieved. Though not conclusive, this phenomenon is worthy receiving attention in the future study. Acknowledgement This work was financially supported by the National Basic Research Priorities Programme, the Ministry of Science and Technology, People’s Republic of China. The project number is G2000026304.

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References Cockreill T., Ranque-Hilsch vortex tube, the M.S. thesis, Engineering Department at Cambridge University, 1995. Gutsol A. F., The Ranque effect, Physics-Uspekhi, Vol. 4, pp. 639-658, 1997. Kurosaka M., Acoustic streaming in swirling flow and the Ranque-Hilsch (vortex-tube) effect, Journal of Fluid Mechanics, Vol. 124, pp. 139-172, 1982. Stephan K., Lin S., Durst M., Huang F., and Seher D., An investigation of energy separation in a vortex tube, Journal of Heat and Mass Transfer, Vol. 26, No. 3, pp. 341-348, 1983. Takahama H., Studies on vortex tube, Bulletin of JSME, Vol. 8, No. 31, pp. 433-440, 1965. Takahama H., and Yokosawa H., Energy separation in vortex tubes with a divergent chamber, Transactions of ASME, Vol. 103, pp. 196-203, 1981.

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