Investigation of the Effect of a Nonuniform Electrostatic Field on Heat Transfer and Flow Hydrodynamics
Abstract Generated abstract
This study examines how a nonuniform electrostatic field affects heat transfer and flow hydrodynamics in a vertical tube-in-tube heat exchanger with air flowing through the annular space under laminar conditions. Experiments varied air velocity and electrode voltage while measuring wall and air temperatures, flow rate, and hydraulic resistance during heat transfer from condensing saturated steam. The results indicate that voltages above about 6 kV, where corona discharge was observed, intensified heat transfer, with a reported 3.2-fold increase in the heat-transfer coefficient at 7 kV and 0.73 m/s, while hydraulic resistance increased more modestly. The authors suggest that ionization near the electrode generates an electric wind that promotes additional flow disturbance and thereby enhances heat transfer.
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STUDY OF THE EFFECT OF A NONUNIFORM ELECTROSTATIC FIELD ON HEAT TRANSFER AND FLOW HYDRODYNAMICS
I. P. Chashchin, V. P. Ignatov, V. M. Madzurenko
(Presented by the scientific-methodological seminar of the Department of Processes, Apparatuses, and Cybernetics of Chemical Production)
In recent years a number of works [1, 2] have appeared on the study of the effect of an electric field on the intensification of heat transfer. Although certain positive results have been achieved in this area, they are sometimes contradictory in character and do not make it possible to draw general conclusions.
The aim of the present work was to study the effect of a nonuniform electrostatic field on heat transfer and the hydrodynamics of the flow. The investigations were carried out in an apparatus representing a vertical “tube-in-tube” heat exchanger, into the annular space of which electrodes were introduced.
Heat transfer occurred from saturated water vapor condensing in the inner tube to air flowing in the annular space of the apparatus. The entire length of the heat-exchanger tube was conventionally divided into two sections, in order to reduce the influence of the flow-inlet conditions into the apparatus on heat transfer and flow hydrodynamics \((l/d_{\text{eqv}}>50)\). The temperatures of the wall of the inner tube and of the air flow were measured using chromel-copel thermocouples connected to a PP-63 potentiometer. The accuracy of temperature measurement was \(\pm 0.5^\circ\text{C}\). Measurements of the flow rate of the air entering the heat exchanger were carried out using a throttle flowmeter. The accuracy of air-flow measurement was \(\pm 3\%\). Hydraulic resistances were measured with an inclined draft manometer. The measurement accuracy was \(\pm 0.2\ \text{N}/\text{m}^2\).
Investigations of the effect of a constant nonuniform electrostatic field on heat transfer and hydrodynamics were carried out with forced flow of the air stream in the intertube space of the heat exchanger. The air-flow regime was laminar, \(\mathrm{Re}=(600\div 2000)\). The electric voltage was applied to an electrode 0.565 m long and 0.2 mm in diameter. The experiments differed from one another in the air flow rate and in the magnitude of the voltage applied to the electrode.
The results of the experimental data on heat transfer and hydraulic resistances upon application of an electric field are presented in Fig. 1.
It is evident from the figure that the presence of an electric field promotes intensification of heat transfer. Thus, at a flow velocity \(W=0.73\ \text{m/sec}\), the heat-transfer coefficient, upon application of an electric field of \(7\ \text{kV}\), increases by a factor of 3.2 in comparison with the heat-transfer coefficient without application of an electric field. The effect of the electric field on
...the intensification of heat transfer decreases as the flow velocity increases. Intensification of heat transfer is observed when an electric field with a voltage above 6 kV is applied. The magnitude of the hydraulic resistance in the measured section when the electric field is applied also increases, but to a lesser degree \((E = 7 \text{ kV},\ W = 0.73 \text{ m/s},\ \frac{\Delta p_1}{\Delta p_0} = 1.3)\) than the relative heat-transfer coefficient.
Fig. 1. Effect of the electric field on heat transfer and hydraulic resistance at different flow velocities.
\(\bullet\) — \(W = 0.73\ \text{m/s}\), \(+\) — \(W = 1.105\ \text{m/s}\), \(\blacktriangle\) — \(W = 1.66\ \text{m/s}\), \(\blacktriangledown\) — \(W = 1.97\ \text{m/s}\).
The visual observations carried out showed that when a voltage above 6 kV is applied to the electrode, a corona discharge is observed.
The mechanism by which the electric field affects heat transfer and hydraulic resistance can presumably be explained by the fact that, in the presence of a corona discharge at the electrode, part of the air adjacent to it is ionized. The positive ions, moving toward the heat-emitting surface, collide with neutral gas molecules, creating a pressure gradient that causes an electric wind, which produces additional turbulization of the air flow.
REFERENCES
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V. M. Buznik. Intensification of Heat Transfer in Ship Installations. “Sudostroenie,” Moscow, 1969.
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V. M. Borishanskii. Advances in the Field of Heat Transfer. Moscow, “Mir,” 1970.