A SEMINAR REPORT ON DESIGN CONSIDERATION OF VORTEX TUBE

GUIDED BY PREPARED BY Prof .B .K. Bokankar Mr. Nilesh Gohel Lecturer (1rd Semester, Heat & Power) (Mechanical Department)

INTRODUCTION

The vortex tube (also called the Ranque–Hilsch vortex tube) is a mechanical device operating as a refrigerating machine without any moving parts, by separating a compressed gas stream into a low total temperature region and a high one. Such a separation of the flow into regions of low and high total temperature is referred to as the temperature (or energy) separation effect. The vortex tube was first discovered by Ranque, a metallurgist and physicist who was granted a French patent for the device in 1932, and a United States patent in 1934.

Classifications of the vortex tube

Generally, the vortex tube can be classified into two types. One is the counter-flow type (often referred to as the standard type) and the other the parallel or uni-flow type, as shown in Figs. 1a and b, respectively.

Counter-flow vortex tube The counter-flow vortex tube, as shown in Fig. 1a, consists of an entrance

block of nozzle connections with a central orifice, a vortex tube (or hot tube) and a cone-shaped valve. A source of compressed gas (e.g. air) at high pressure enters the vortex tube tangentially through one or more inlet nozzles at a high velocity. The expanding air inside the tube then creates a rapidly spinning vortex. The air flows through the tube rather than pass through the central orifice located next to the nozzles because the orifice is of much smaller diameter than the tube. The length of the tube is typically between 30 and 50 tube diameters, and no optimum value has been determined between these limits. As the air expands down the tube, the pressure drops sharply to a value slightly above atmospheric pressure, and the air velocity can approach the speed of sound. Centrifugal action will keep this constrained vortex close to the inner surface of the tube.

Fig1a. The Ranque–Hilsch standard vortex tube or counter-flow vortex tubeThe air that escapes at the other end of the tube can be varied by a flow-control valve, usually shaped as a cone. The amount of air released is between 30% and 70% of the total airflow in the tube. The remainder of the air is returned through the centre of the tube, along its axis as a counter-flowing stream. Once a vortex is set up in the tube, the air near the axis cools down while the air at periphery heats up in comparison with the inlet temperature. This phenomenon is known as temperature separation effect (also called theRanque–Hilsch effect). As a result, the gas escaping through the orifice is cold and the hot gas flows out in the other direction. A remarkable feature of this device is the absence of moving parts and simplicity of operation.

Uni-flow vortex tube The uni-flow vortex tube (Fig. 1b) comprises an entrance block of inlet nozzles, a vortex tube

and a cone-shaped valve with a central orifice. Unlike the more popular counter-flow version, the cold air exit is located concentrically with the annular exit for the hot air. The operation of the uni-flow vortex tube is similar to the operation of the counter-flow one. The temperatures of the air leaving the hot and cold ends can differ by as much as 140–160 C, but extremes of up to 230 C have been measured by Comassar [2]. In general, the practical low-temperature limit for the cold air stream is -40 C, although temperatures as low as -50 C have been obtained with research equipment. The practical limit for the high temperature is 190 C, but temperatures in excess of 225 C have been observed by Bruno [3,1]. The main applications of the vortex tube are in those areas where compactness, reliability, and low equipment costs are the major factors and the operating efficiency is of no consequence. Some typical applications are cooling devices for airplanes, space suits and mines; instrument cooling; and industrial process coolers.

The uni-flow or parallel flow vortex tube

CONSTRUCTION OF VORTEX TUBE

The main part of a basic vortex tube is a hollow cylinder, in which compressed air or other fluids are injected tangentially.

The exits set at one or both ends of the tube allow hot and cold air to exhaust. The hot nozzle is located at the periphery, and cold nozzle is placed at the centreline of the tube.

WORKING OF VORTEX TUBE

Working of Vortex Tube

• The vortex tube, as shown in Fig consists of an entrance block of nozzle

connections with a central orifice, a vortex tube (or hot tube) and a cone-shaped valve.

• A source of compressed gas (e.g. air) at high pressure enters the vortex tube tangentially through one or more inlet nozzles at a high velocity.

• The expanding air inside the tube then creates a rapidly spinning vortex.• The air flows through the tube rather than pass through the central orifice

located next to the nozzles because the orifice is of much smaller diameter than the tube.

• As the air expands down the tube, the pressure drops sharply to a value slightly above atmospheric pressure, and the air velocity can approach the speed of sound.

• The air that escapes at the other end of the tube can be varied by a flow-control valve, usually shaped as a cone. The remainder of the air is returned through the centre of the tube, along its axis as a counter-flowing stream.

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• As a result, the gas escaping through the orifice is cold and the hot gas flows out in the other direction.

• The flow inside the vortex tube can be described as rotating air which moves as a spring shaped vortex track. The peripheral flow moves towards the hot and the axial flow moves in the opposite direction towards the cold end. The angle between the circular helix and vertical axis is an important parameter since it decides the length of the vortex track and peripheral velocity of the flow which influence the efficiency of the vortex tube.

• In the middle section of the tube is the inlet for the compressed air. Note that the inlet is much closer to the cold outlet then the outlet. There is very important aspect of the tube related to this feature. From the above figure . one can see the middle part which says spiral chamber in this part. The spiral chamber is the essential component of the tube because it is the source of hot and cold separation of the gas. How it works based primary on the Physics of rotational motion and on mazwell’s law states the basis of kinetic theory of gases which in turn helps explain the fundamental properties of gases such as diffusion and pressure.

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• Distribution profile is much different because of the shape of the spiral tube itself. As compressed air enters into spiral chamber, it immediately desires to diffuse based on this diffusion function.

• The velocity of all molecules will have both a z component and a (thita) component, where z is the direction along the tube axis and (thita) is the angular direction above the axis itself. The majority of molecules will diffuse in such a way to create a gas vortex within the tube which expands as the molecules diffuse down the tube. We know that molecules spiralling down the tube far from the tubes Z- axis will have a higher velocity. In other words. The molecules rotating about the tube axis are moving faster as they travel down the tube.

The first law of thermodynamics state that the increase of internal energy of a thermodynamicSystem is equal to the amount of heat energy added to the system minus the work done on the system by its surrounding. MathematicallyOr written in differential form,U=Q-W ...................................Eq 1Or written in differential form,dU=dQ-dW...........................Eq 2in the equations ,U refers to the internal energy of the system, Q refersto the heat of the system and W refers to the workdone on the system by it surrounding. Applying Eq 1 to the vortex tube , the thermodynamics law reduced to Eq 3 below-dU=dQ........................................Eq 3since no work is done on the system. This means that any change in internal energy related to the change in heat of the system.

• One can now understand why the cold outlet valve is much closer to the inlet than the hot outlet valve. As the molecules travel down the tube, they begin slowing down in the both Z and thita direction. This slowing is the reduction in the axial convection of the vortex as it moves down the tube. In other worlds the molecules diffusion rate slows down as the gas fills more space and relieves pressure causing a slower propagation of the molecules down the line. As this happens, less kinetic energy is used and converted back to internal enegy which in turn increase the temperature of the gas, Since the original radial pressure gradient caused a flow of kinetic energy towards the outer molecules, they have more kinetic energy to convert back to internal energy and hence have higher temperature .This is known as viscous dissipation of the kinetic energy.

• The reason the cold outlet is closer to the inlet is due to the desire to reduce the effect of viscous dissipation of kinetic energy. The outlet music must be close to the inlet because the further away it gets, the more viscous dissipation will occur and hence higher the gas temperature will be. The Reason the cold outlet is not directly next to the inlet is to give the system pace to utilize the radial pressure gradient and to transfer kinetic energy to the molecules at the tube wall. The outlet then is logically poisoned to remove gas from the center of the tube

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• The hot outlet is positioned further from the inlet than the cold outlet is for similar reasons. Instead of getting the air out quickly to keep it cool, the scientist lets it travel farther down the tube warming up by prolonged viscous dissipation of kinetic energy. As the gas travels down the tube, axial convection also decreases, reducing the gradient of temperature of pressure down the tube, to make a more unified stream. By the time the stream reaches the outlet, nearly all of it has the same temperature and has a higher temperature than the inlet due to the increased kinetic energy it had initially. The outlet is drawn from the tube walls to obtain the molecules with maximum high temperature.

• "Viscous dissipation" usually refers to the work done by the velocity against the viscous stresses - ie an irreversible process where mechanical energy (useful) is turned into thermal engery (not so useful).

• define viscous dissipation as the transformation of kinetic energy to internal energy (heating up the fluid) due to viscosity. This includes both turbulent kinetic energy and mean flow kinetic energy.

IMPORTANT DEFINITIONS

5.1.1 Cold mass fractionThe cold mass fraction is the most important parameter indicating the vortex tube performance and the temperature/energy separation inside the vortex tube. Cold mass fraction is defined as the ratio of cold air mass flow rate to inlet air mass flow rate. The cold mass fraction can be controlled by the cone valve, which is placed at the hot tube end. This can be expressed as follows

where Mc is the mass flow rate of cold air and Mi is the mass flow rate of the entry air.

5.1.2. Cold air temperature dropCold air temperature drop or temperature reduction is defined as the difference in temperature between entry air temperature and cold air temperature

Ti is the entry air temperature and Tc is the cold air temperature.

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PARAMETER AFFECTING THE EFFICIENCY OF THE VORTEX TUBE

BASED ON RESEARCH by Zeegers and waele (2009)

Westley ,(1995) experimentally optimized the geometrical parameter of the RHVT system. He found that optimum configuration can be described asrelationship between the inlet area , the tube length , the vortex tube cross sectional area, the cold nd orifice area size , hot end plug dimension and the inlet pressure

Zeegers and waele (2009) measured the flow parameters in the tube. The air velocity inside the tube was measured and the perfomance of the vortex tube with different parameter like cold fractions, pressure of the injected air, and shape of vortex tube.

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BASED ON RESEARCH by Arjomandi and XueEFFECT OF THE HOT END PLUGS ON THE EFFICIENCY

• Results

• The thermal efficiency of the tube has been used for comparing the different hot end plug. to calculate the thermal efficiency eq is represented by Fulton represented by

where η is the vortex tube thermal efficiency, Γ=(γ-1)/γ, γ is the specific heat ratio=1.4, ε is the cold mass fraction and is defined as the cold mass flow rate divided by the input mass flow rate, ∆T is the temperature difference, Tin is the input temperature, pin and pc represent the air pressure at the inlet and at cold air nozzle, respectively.

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Ap/At is the ratio between plug area and tube cross section area. As it is seen the cold mass fraction increases from 0.17 to 1 when Ap/At changes from 0.75 to 1 which is the result of varying the plug diameter from 26mm to 30mm.

Nozzle parameters affecting vortex tube energy separation performance

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OBSERVATION• The increase of the number of inlet nozzles leads to higher temperature separation in the

vortex tube.• Using a small cold orifice (d/D ¼ 0.2, 0.3, and 0.4) yields higher backpressure while a large

cold orifice (d/D ¼ 0.6, 0.7, 0.8, and 0.9) allows high tangential velocities into the cold tube, resulting in lower thermal/energy separation in the tube.

• Optimum values for the cold orifice diameter (d/D), the angle of the control valve (f), the length of the vortex tube (L/D), and the diameter of the inlet nozzle (d/D) are found to be approximately d/DE 0.5, , L/DE 20, and d/DE 0.33, respectively, which are expected to be fruitful for vortex tube designers.

• The inlet gas pressure should be 2 bar (for optimal efficiency) while the higher inlet pressure is due to high temperature separation. Inlet gas with helium gives higher temperature difference than those found from the oxygen, methane, and air

• The inlet pressure is the necessary driving force for the energy separation. Experiments shows that the higher the inlet pressure, the greater the temperature difference of the outlet stream.

• The cold fraction is an important parameter influencing the performance of the energy separation in the vortex tube and there is an optimum value to obtain maximum temperature difference and it is not the same for maximum enrgy separation.

• The effect of number of nozzle is very important. For Constant inlet pressure test. It is clear that increasing number of nozzle increase the temperature difference between inlet and outlet. Also the study indicates that there could be optimum number of jet maximum load that require further investigation.

• The vortex tube length ( vortex stopper location) has direct effect on the performance of the vortex tube. The data shows that as vortex stopper moves far from the vortex generator the energy separation performance increase which is true as long the distanced moved is below 14 times the vortex tube diameter ( which was the investigated range)

• Insulation has minimal effect on vortex tube cooling and heating capabilities.

ADVANTAGES OF VORTEX TUBE

No moving parts to wear out

No electricity or chemicals

Small, lightweight

Low in cost

Maintenance free

Instant cold air

Durable, stainless steel construction

Adjustable temperatures from -45°C to +126°C

APPLICATIONS OF VORTEX TUBECNC Machines

Refrigerators

Cooling of firemen’s suits

Heating processes

Nuclear reactor

Cooling of equipments in laboratories

Temperature control of divers air suppliers

Manned underwater habitants

Hyperbaric chambers

Liquefactions of natural gas

BIBLIOGRAPHY Ranque GJ. “Experiments on expansion in a vortex with simultaneous exhaust of hot air and cold air”. J Phys Radium

(Paris) 1933;4:112–4 S-115, June. Also translated as General Electric Co., Schenectady Works Library 1947; T.F. 3294.

Ranque GJ. “Method and apparatus for obtaining from a fluid under pressure two outputs of fluid at different temperatures”. US patent 1:952,281, 1934.

Hilsch R. “The use of expansion of gases in a centrifugal field as a cooling process”. Rev Sci Instrum 1947;18(2):108–13.

Westley R. “A bibliography and survey of the vortex tube”. College of Aeronautics. Cranfield note, UK, 1954.

Curley W, McGree Jr R. “Bibliography of vortex tubes”. Refrigeration Engineering .

Kalvinskas L. “Vortex Tubes: An extension of Wesley’s bibliography”. Jet Propulsion Laboratory, California Inst of Technology Literature Search, 56, Part 2, 1956.

Dobratz BM. “Vortex tubes: a bibliography”. Lawrence Radiation Laboratory UCRL-7829, 1964.

Nash JM. “The Ranque–Hilsch vortex tube and its application to spacecraft environmental control systems”.

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