Action of electrohydrodynamic flow on heat transfer at boiling

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Abstract

The influence of electric field intensity and interelectrode spacing on the main parameters of boiling process under the conditions of electrohydrodynamic flow has been studied. It has been established that the effect of field falls with the increase in the heat flow density. The optimal interelectrode spacing when the highest heat transfer can be achieved is revealed. The hydrodynamic picture of the two-phase flow in relation to the heat transfer rate is analyzed on the basis of visual observations and high-speed filming of the process. The calculation relationships that satisfactorily agree with the experimental results are obtained.

Introduction

Investigations and efforts to find technical applications of electrohydrodynamic phenomena that appear in boiling dielectric liquids have been attempted for a long time. It is worth noting, the first patent where electric fields were used with the aim to intensify convective heat exchange was issued in Great Britain more than a hundred years ago [1]. However, this effective, easily controllable method with a simple technical embodiment still did not find a proper practical application, despite interesting results that deserve attention and valuable technical developments were obtained.

The interest to the technical application of electrohydrodynamic (EHD) flows in boiling liquids has increased since it is possible to develop technical facilities that can deliver liquid dielectrical heat-transfer agents to heat generating surfaces with the aim to maintain the specified thermal regimes [[2], [3], [4]]. Investigations related with this problem were accomplished, in particular, in monotube evaporators [1,[5], [6], [7], [8], [9]] and tubular evaporators used in heat pipes [10,11] and refrigerating installations [12,13]. The main attention was focused on the investigation of the regularities of boiling of dielectric liquids in an electric field and derivation of calculation formulas.

The peculiar features of heat transfer at boiling in an electric field are usually associated with electroconvective perturbations induced by electric forces [[2], [3], [4], [5], [6], [7], [8], [9],[14], [15], [16], [17]]. The efficiency of the field action at the specified conditions on the heating surfaces is determined by the heat load, thermo- and electrophysical properties of the boiling medium in the saturation line, the shape and dimensions of the electrodes, strength and degree of inhomogeneity of the field [[14], [15], [16], [17]]. However, the electroconvective method of cooling does not always ensure the specified temperature regimes. This problem is particularly critical for high thermal loads on the heating surface. Therefore, taking into account that these surfaces do not function sufficiently stable the development of new type of evaporative cooling systems – closed electrohydrodynamic systems with active regulation – remains an urgent issue. In connection with this, investigations of the regularities of boiling in conditions induced by electroconvective fluxes are very interesting for the engineering applications of the results.

Section snippets

Formulation of the problem

The investigations on the hydrodynamics and heat transfer at phase transformations in dielectric liquids accomplished in the Institute of Applied Physics of the Academy of Sciences of Moldova [[18], [19], [20]] have confirmed the physical background of the intensification of heat transfer by an electroconvective flux. The electroconvective method facilitates the conditions of circulation and boiling of the dielectric liquid on the heat transfer surface. At the same time, the increasing of the

Experimental installation and method of measurements

The experimental installation for investigation of the influence of EHD flux on the heat exchange at boiling in a large volume consists of a closed hermetic chamber filled by with a dielectric liquid at atmospheric pressure, working zone, and measuring devices (Fig. 1). The working chamber is made as a metal cylindric case 1 with its inner diameter of 300 mm, height 200 mm, and wall thickness of 12 mm. The liquid in the working chamber is heated to the saturation temperature by an auxiliary

Results and discussion

Fig. 3 presents the experimental data for the dependence of the specific thermal load value on the temperature drop at different field intensities E. The more is ΔT=Tw−Ts, the more intense is the formation of vapor bubbles and boiling of the liquid, and the higher are the values of the heat transfer coefficients. The growth in the heat transfer rate under the action of an electric field occurs in the case when the turbulent mixing caused by electroconvection becomes more intensive than

Generalization of the experimental data

Rate of pool boiling heat transfer. The analysis of the conditions of pool boiling heat transfer using the ∏−dimensional method [26] allowed the deriving of the calculation relationship for the heat transfer coefficient in the following form:α=C0c′p3(ρ′−ρ″)r3/2⋅(Tw−Ts)2.

With the account for the Newton's law of convective heat transfer q=α(Tw−Ts) we have for the heat flow densityq=C0c′p3(ρ′−ρ″)r3/2⋅(Tw−Ts)3.Here C0 is the coefficient received via the statistical processing of the experimental

Conclusion

The directed motion of the electrodynamic liquid flow to the heat exchange surface is an effective method of enhancement of heat transfer at boiling. For its optimization more clear concepts are necessary on the mechanism of the liquid delivery to the heating surface. Therefore, the main attention should be paid to the investigation of hydrodynamics, to the influence of the geometry and arrangement of the electrodes. The results can be used to design closed electrohydrodynamic systems of active

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

Authors acknowledge the financial support by the Moldavian National Agency for Research and Development, grant No. 20.80009.5007.06.

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