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Poster Paper Proc. of Int. Colloquiums on Computer Electronics Electrical Mechanical and Civil 2011

Study of Active Heat Transfer Enhancement Technique that Utilizes Electrical Field R.Srinivasa Sugash SSN College Of Engineering/Department Of Mechanical Engineering, Chennai, India rssugash1006@gmail.com Abstract—— The greatest challenge faced by the current scholars and researchers of thermal engineering field is to improve the performance of the heat exchanger at the same heat transfer area and also decrease the size of the heat exchanger without compromising on the performance. The heat transfer duty of heat exchangers can be improved by heat transfer enhancement Techniques which can be divided into two groups in general. The active techniques require external forces, e.g. electric field, acoustic or surface vibration etc. The passive techniques require fluid additives or special surface geometries. An Electrohydrodynamic (EHD) technique is one of the types of active heat Transfer enhancement techniques. This paper is based on design and study of EHD enhancement Model which consist of three parts; they are vaporization Section, adiabatic section and condensation section. The Insulating liquid was selected as working fluid and the Copper wire whose diameter is 1mm was selected as the High voltage electrode. The temperature Difference in the inlet and outlet of the coolant in the Vaporization section and the condensation section, and the saturation vapor pressure in the model were Measured under different dc high voltage and different tilt angle. The experiment results indicate that the EHD enhancement heat transfer technology can be employed pragmatically in the heat exchangers used in refrigeration and has special reference value for the investigation of antigravity heat pipes.

engineers concern this heat transfer enhancement technique and conceive that EHD effect can be used to improve the traditional property of heat pipe [3]. Base on these researches, the enhancing effects of the electrostatic force and the polarization force upon heat transfer of the liquid working fluid have been researched under high voltage and using insulated liquid R11 as working fluid. And the experimental result obtained is valuable to the research and application of the refrigerating and heat transfer engineering.

Index Terms— Temperature difference driving force, Electric Field, Electrohydrodynamic Enhancement, vaporization section, adiabatic section, Condensation section, and Antigravity heat pipes.

The above equation can be written in a more detailed form for non-polar fluids as follows:

II. GENERAL PRINCIPLE The EHD enhancement of heat transfers refers to the coupling of an electric field with the fluid field in a dielectric fluid medium. In this technique, either a DC or an AC high-voltage low-current electric field is applied in the dielectric field medium flowing between a charged and a receiving (grounded) electrode [2]. The physical basis of the electrically enhanced condensation and boiling are due to the EHD force, (fe), generated by an electric field and is given by below equation (1):

I. INTRODUCTION The first term of the equation f 1, is known as the electrophoretic force, is the Coulomb force acting on the free charges in a fluid. An electrophoretic force exists once a net charge is created in the fluid and it becomes dominant in applications. The electric field induces a fluid motion called ‘‘corona wind’’. The second term f2 is a consequence of inhomogeneity or spatial change in the permittivity of the dielectric fluid due to non-uniform electric fields, temperature gradients, and phase differences. The third term f3 comprises the dielectophoretic and electrostrictive forces, within the fluid. Dielectophoretic force presents the non-uniformity of the electric field (Figure 1). Condensate is pushed by this force into higher electric field strength. The fourth term f4 is the electrostriction force. This force depends on non-uniformity of electric permittivity. A similar principle applies when a charged needle electrode is brought

Besides the improvement of the performance of the heat exchanger at the same heat transfer area, heat transfer enhancement enables a considerable decrease in the size of the heat exchanger at the same performance. In general, enhancement techniques can be divided into two groups: namely active and passive techniques. The active techniques require external forces, e.g. electric field, acoustic or surface vibration. The passive techniques require special surface geometries (such as rough surface, extended surface for liquids etc.) or fluid additives. Both techniques have been used by researchers for 140 years for increasing heat transfer rate in heat exchangers. Over the past 70 years, the heat transfer enhancement by using a strong electric field has been continuously studied. According to the references reported, the applied electrostatic field can make the heat flux density increase 1.6-6 times than no field [1], and the condensation heat transfer coefficient is 1.5-2 times larger than no field [2]. So at home and abroad many scholars and © 2011 AMAE DOI: 02.CEMC.2011.01. 517

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Poster Paper Proc. of Int. Colloquiums on Computer Electronics Electrical Mechanical and Civil 2011 quantity of flow can be controlled by

close to a liquid surface. The liquid surface extends into the gas toward the electrode. This phenomenon is called the liquid-extraction phenomena. This occurs again due to the EHD-induced surface instability on the gas-liquid interface.

Figure 3: Sketch of experimental system

Fig. 1 Uniformity of the electric field distribution.

overflow valve and throttle valve, the highest flow rate can reach to 1.5x105 m3/s. The high voltage power supply is a power frequency transformer, and bridge type rectification was adopted, DCV can be obtained after filtering and the voltage amplitude is adjustable continuously. The water temperature at the inlets and outlets of the condensation and vaporization sections is detected by the automatic temperature measurement system. The vacuity in the model is measured by the vacuum manometer, which is set at the end of the condensation section.

II. EXPERIMENTAL MODEL AND SYSTEM A. Experimental model The electric field enhancement heat transfer model of tube fluid is shown in Figure 2. The total length of the copper tube is 325mm, and is divided into three parts: the vaporization section (a), the adiabatic section (b) and the condensation section (c).

III. PROCEDURE OF THE EXPERIMENTAL The following three experiments were conducted when the heating temperature of the water bath is 66OC and the quantity of flow is 2.6X10-6 m3/ s. a. Applied positive DCV to the side of high voltage of the power supply gradually, then measured the temperature differences between the inlets and the outlets of the condensation and vaporization section respectively at different tilt angles. b. Changed the tilt angle of the model under different voltages, that is raised the vaporization section and made the vaporization section higher than the condensation section, then measured the temperature differences between inlets and outlets of condensation and vaporization section respectively. c. Changed the tilt angle under different voltages, and then measured the gaseous working fluid vapor pressure of the condensation section.

Figure 2: Schematic diagram of model pipe

The length of the vaporization, condensation and adiabatic section is 150mm, 150mm and 25mm respectively, and the whole outer surface was wrapped with adiabatic asbestos layer which thickness is 8mm. The bare copper wire electrode which diameter is 1mm was placed in the model tube and paralleled with the tube, and the distance between the wire and the bottom of the inner wall is 3mm. The vacuity in the tube is 5x104 mmHg, and the working fluid is R11 which boiling point is 24oC at normal pressure.

IV. EXPERIMENTAL RESULTS The following experimental results can be obtained using the above experimental model and method: a. The relation between the temperature difference T and the applied voltage U at different tilt angles is shown in Figure 5. It can be seen that T increased with the increase of the applied voltage U and decreased with the augment of the model tilt angle.

B. Experimental system As shown in Figure 3, the heat resource of the vaporization section is a super water bath thermostat which type is DL501, and its highest heating temperature is 95oC .The condensation section was cooled down by tap water, and the velocity and ÂŠ 2011 AMAE DOI: 02.CEMC.2011.01.517

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Poster Paper Proc. of Int. Colloquiums on Computer Electronics Electrical Mechanical and Civil 2011 b.

The relation between the temperature difference T and the tilt angle at different voltages is shown in Figure 6. It can be seen that T decreased with the augment of the model tilt angle and increased with the increase of the applied voltage U. The relation between the saturated vapor pressure in condensation section and the tilt angle is shown in Figure 7. It can be seen that the saturated vapor pressure decreased with the increases of the tilt angle and the applied voltage respectively. This is accordant with the changement of the temperature in the condensation and vaporization section.

V. DISCUSSION OF EXPERIMENTAL RESULTS It can be seen for the experimental results that the electrostatic field can enhance the boiling and condensation heat transfer of the insulating fluid in the model, and this enhancement effect becomes more obvious with the increase of the applied voltage. This can be explained with electrohydrodynamic (EHD) theory

Figure 5: The dependence of the temperature

Figure 6: The dependence of vapor pressure

A. Effect of EHD enhancement condensation heat transfer Generally, the EHD force exerted on the non-uniform fluid or the interface between two phase fluids can be written as:

Figure 4: The dependence of the temperature difference

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Poster Paper Proc. of Int. Colloquiums on Computer Electronics Electrical Mechanical and Civil 2011 This formula shows the solution for the dielectric electrophoretic force exerted on spherical Micro particles (the bubbles in boiling fluid) embedded in different dielectrics. In the process of boiling heat transfer, the spheres are bubbles, at this point, .Then Fd is in the direction of the decrease of electric field, that is, the bubbles move in the direction of the decrease of electric field. So the vapor bubbles are squeezed on the heat transfer surface, the area of the film that the bubbles surface touched the heat transfer surface is enlarged, and then boiling heat transfer is enhanced.

Where the first term is the Coulomb force exerted on the free charges in unit volume dielectric; the second term, called the dielectric electrophoretic force, is due to the force exerted on a non-homogeneous dielectric liquid by an electric field; the third term, called the electrostrictive force. Its effect on incompressible fluid can be neglected. At condensation section, when electric field is weak, EHD force drags the condensation film on the tube wall and makes it become bead or quasi bead, then the heat resistance decreases and condensation heat transfer is enhanced. B. Refrigerating effect of corona discharge When the electric field is increased further, the corona discharge of the high voltage wire electrode makes the vapor molecules around the wire ionize, then the wire electrode will emit a beam of ion stream and the corona wind will be created, it will enhance the disturb of the condensing surface, so it will enhance the condensation heat transfer too.

VI. CONCLUSION From the above experimental investigation of electrostatic field enhancing heat transfer of dielectric fluid, the following conclusions can be obtained: a) The applied electrostatic field enhances heat transfer capacity of the dielectric fluid in the model; b) When the location of the heat source is higher than cold source, EHD force accelerates the fluid flow back to the vaporization section; it makes the effect of electrostatic field enhancement heat transfer more obvious relatively; c) A kind of efficient EHD heat pipe will be applied in heat transfer and refrigerating engineering practically.

C. Flow of dielectric fluid between condensation section and vaporization section As shown in Figure 5, when the model is inclined without external electric field, i.e. vaporization section is lifted, the quantity of liquid working fluid in vaporization section will decrease, it will result in the decrease of evaporation capacity, and then the external temperature difference will be diminished. When vaporization section is lifted at a critical height, working fluid in condensation section will be unable to flow back, that will make vaporization section dry up, and then the model will lose heat transfer capability. After apply an electric field, in condensation section, because of the electric drag effect, the distance between the liquid level and the electrode becomes closer, and the electric field strength between them increases, so the electric field force increases too. And because the temperature is low, the polarization force is relatively large. So the EHD force in condensation section is larger than vaporization section, and the liquid level is higher than vaporization section. Then the height difference on the applied dc voltage on the gradient of the model pipe of the liquid level will make the working fluid flow from condensation section to vaporization section, so the increase of the circumfluence force makes the heat transfer capacity of the model increase.

REFERENCES [1]Zongbiao Lin, Heat transfer enhancement and its engineering application, China mechanical industry press, 1987. [2] Choi H. Y., “Electrohydrodynamic Conden- sation Heat Transfer,” Transactions of the ASME, pp. 98-102, Feb., 1968. [3] T. B. Jones, “Electrohydrodynamic heat pipes, Int. J. Heat Mass Transfer,” vol. 16, pp. 1045-1048, 1973. [4] V. D. Shkilyev, M. K. Bologa, “Heat transfer characteristics and constructive peculiarities of heat pipes utilizing the effect of electric fields,” Proc.4th international Heat conference. 7-10 London, UK, pp. 603-672, Sept., 1981. [5] Ohadi M. M., et al. “Heat Transfer Enhance- ment of Laminar and Turbulent Pipe Flow via Corona Discharge,” Int. J. Heat Mass Transfer, Vol. 34, pp. 1175-1187, 1991. [6] Atten P., et al. “Electroconvection and its Effect on Heat Transfer,” IEEE Trans. on Electrical Insulation, Vol. 23, No. 4, pp. 659-667, 1988. [7] Pohl H. A., “DielectrophoresisÀÛüÜThe behavior of neutral non-uniform electric fields [M],” Cambridge: Cambridge University Press, 1978. [8]Chang, H.C., Yeo, L. (2009).” Electrokinetically Driven Microfluidics and Nanofluidics”. Cambridge University Press. [9]Patterson, Michael; Kesner, Raymond (1981). Electrical Stimulation Research Techniques. Academic Press.. [10]Kirby, B.J. (2010).” Micro- and Nanoscale Fluid Mechanics: Transport in Microfluidic Devices”.. Cambridge University Press. [11] Kasayapanand N, Kiatsiriroat T. EHD enhanced heat transfer in wavy channel. vol. 32 2005 p. 809–21. [12] Seyed-Yagoobi J. Electrohydrodynamic pumping of dielectric liquids. J Electrostat 2005;63:861–9. [13] Al-Ahmadi A, Al-Dadah RK. A new set correlation for EHD enhanced condensation heat transfer of tubular systems. Appl Therm Eng 2002;22:1981–2001.

D. Explanation about The effect of EHD enhancement boiling heat transfer The effect of EHD enhancement boiling heat transfer can be explained from the following two aspects: the effect of the dielectric electrophoretic force on bubbles and the dragging effect of the EHD force on the charges on the liquid vapor interface and the ones in condensation film. The effect of electric field on bubbles is mainly dielectric electrophoretic force. Suppose that there is a sphere which radius is R and permittivity is in the working fluid which permittivity is then the dielectric electrophoretic force d F exerted on the sphere can be denoted as :

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