A first-order model of electrically driven flows is described and applied to flows driven by Joule heating of a dielectric fluid. An exact solution for the reduced, first-order equations of electrohydrodynamics is developed for the case of a steady, fully developed flow between vertical, parallel, planar electrodes. The solution is shown to depend on four dimensionless groups: the Grashof and Reynolds numbers and two new quantities that represent the magnitude of charge-induced electric field perturbations over the magnitude of the field at an electrode surface and the ratio of the Joule heating rate to the thermal conduction rate, respectively. It is demonstrated that for small field perturbations the solution yields a parabolic velocity profile. The thermal and velocity profiles are only slightly asymmetrical for cases in which the electric field is strongly nonuniform due to space-charge perturbation of the field.<>

This work presents and discusses the results of a series of experiments investigating effects from corona discharge in air on the heat transfer rate and on the pressure drop in tube flows. Two electrode geometries were studied: a single wire electrode, concentric with the grounded tube wall and dual equipotential wire electrodes which were offset 0.4 cm from center in the horizontal plane. Both positive and negative discharge were examined for the single-wire geometry, at Reynolds numbers in the range 1,000 less than or equal to Re-D less than or equal to 20, 000. The dual wire geometry was studied using positive polarity discharge only, over the range Re-D = 1,000 to Re-D = 10, 000. Heat transfer rates were determined at electrode potentials from 6.00 kV (DC) to 7.75 kV (DC), depending on polarity and electrode configuration. Baseline data were also obtained with the electrode(s) at ground potential.Results demonstrate increases in the Nusselt number of more than two hundred per cent over the values obtained in the absence of discharge. Relative increases in the friction coefficients were generally comparable to the corresponding Nusselt number enhancement. The extent of the increase in either quantity was highly dependent on discharge current and on the Reynolds number. The relative enhancements of both Nusselt number and friction loss coefficient were generally reduced at higher Reynolds numbers (Re-D greater than or equal to 5000). However, the fall-off of enhancement with Reynolds number was less pronounced in the offset, dual-electrode geometry.Results suggest the enhancement mechanism may significantly depend on the electrode geometry, independent of the geometry effects on discharge current. The observed trends are discussed in the context of current theory.

The effect of corona discharge on forced-convection heat transfer in a tube is studied experimentally. Results are reported for parametric values of the Reynolds number (1000-15 000), electric field potential (0 kV to near spark-over potential), and number of electrodes (single or double electrode configurations). The working fluid in all experiments is air. It is found that heat transfer enhancements are significant only in the laminar and transitional flow regimes when using a single electrode. On the other hand, with a two-electrode configuration, enhancements extend to the turbulent flow regime as well. At constant pumping power higher enhancements are generally associated with the double electrode configuration. The only exception to this is for the fully laminar Reynolds numbers and at the highest field potentials, where the single electrode results exceed those of the double electrode.

Enhancement of heat transfer in a heat exchanger via a DC corona discharge was studied experimentally using a single-tube shell-and-tube heat exchanger. Air was the working fluid in both the tube and shell sides. Excitation of the tube side was via a single wire electrode, while that of the shell side was via four rod electrodes oriented symmetrically at 90 degrees intervals. Three series of experiments were performed: (1) excitation of the tube side only, (2) excitation of the shell side only, and (3) simultaneous excitation of the tube and shell sides. Both heat transfer and pressure drop measurements were performed, with Reynolds number and electric field potential as parametric quantities in the tube and shell sides. It was found that highest enhancements take place when the tube and shell sides are excited simultaneously, yielding a 322% increase in the overall heat transfer coefficient. Study of the heat transfer enhancements per unit pumping power indicates that for the range of parameters studied. the technique is most efficient at moderate Reynolds numbers and at electrode potentials in the midrange between threshold and sparkover limits.

Electrohydrodynamic effects on forced convection in tubes may have significant implications for enhancement of heat exchanger performance in heat pumps and other devices. Of particular concern in such applications is the possibility of increased pressure drop associated with electrostatic discharge. Large frictional losses could substantially increase the required pumping power, offsetting performance gains associated with improved heat transfer rates.This article describes a series of experiments designed to determine the effects of corona discharge on pressure fields for air flow in cylindrical tubes. Experiments were performed with a single concentric electrode in the tube and with two nonconcentric electrodes. Measurements were performed at potentials from the onset of measurable current to near the spark-over point and at Reynolds numbers from 10(3) to 2 x 10(4). Friction factors were seen to increase as much as 250 percent over the values obtained in the absence of an applied electric field. Results suggest that the electrostatic effect on pressure drop is very sensitive to current density, Reynolds number, and electrode configuration.

Electrohydrodynamic effects on forced convection in tubes can have significant implications for enhancement of heat exchanger performance in heat pumps and other devices. Of particular concern in such applications is the possibility of increased pressure drop associated with electrostatic discharge, which might substantially increase the pumping power required. The authors describe a series of experiments designed to determine the effects of corona discharge on pressure fields for air flow in cylindrical tubes. Experiments were performed with a single concentric electrode in the tube and with two nonconcentric electrodes. Measurements were performed at potentials from the onset of measurable current to near the sparkover point and at Reynolds numbers from 10/sup 3/ to 2*10/sup 4/. Friction factors were seen to increase as much as 250% over the values obtained in the absence of an applied electric field. The results presented suggest that the electrostatic effect on pressure drop is very sensitive to current density, Reynolds number, and electrode configuration.< >

The enhancement of convective heat transfer by an electric field is but one aspect of the complex thermoelectric phenomena which arise from the interaction of fluid dynamic and electric fields. Our current knowledge of this area is limited to a very few experimental studies. There has been no formal analysis of the basic coupling modes of the Navier–Stokes and Maxwell equations which are developed in the absence of any appreciable magnetic fields. Convective flows in enclosures are particularly sensitive because the limited fluid volumes, recirculation, and generally low velocities allow the relatively weak electric body force to exert a significant influence. In this work, the modes by which the Navier–Stokes equations are coupled to Maxwell’s equations of electrodynamics are reviewed. The conditions governing the most significant coupling modes (Coulombic forces, Joule heating, permittivity gradients) are then derived within the context of a first-order theory of electrohydrodynamics. Situations in which these couplings may have a profound effect on the convective heat transfer rate are postulated. The result is an organized framework for controlling the heat transfer rate in enclosures.