US3526268A - Corona discharge heat transfer - Google Patents

Description

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Sept. 1970 M. ROBINSON 3,526,268

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INTERNAL 8 FORCED CONVECTION AT 500 O FT/SEC 4OFT/S T 80 T C 92 FT/SEC IO I 0.2 0.4 0.6 0.8 L0 I2 L4 L5 (CORONA CURRENTYIZ i ATTORNEYS MA" INVENTO United States Patent 01 fice 3,526,268 Patented Sept. 1, 1970 3 526,268 CORONA DISCHARGE HEAT TRANSFER Myron Robinson, Highland Park, N.J., assignor to Research-Cottrell, Inc., a corporation of New Jersey Filed July 18, 1968, Ser. No. 745,906 Int. Cl. F281? 13/16; Hb 7/16 US. Cl. 165-1 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method and apparatus for increasing heat exchange, particularly the exchange of heat between a fluid and a solid. In general, the invention relates to establishing a corona discharge from a first heated conductor in the presence of an adjacent body. A gas flows between them and is at a temperature below that of the heated conductor. The heated conductor may be a Wire and the corona is termed a forward corona. The second body may be a cylinder of relatively large radius surrounding the wire. The cylinder may have a coating of porous and high resistivity material in which case a corona from the second body which arises is termed a back corona. In this case, the second body is at a higher temperature than the gas.

The invention exhibits utility in a variety of applica tions. An electric oven or heater whose elements are corona-activated can deliver increased amounts of heat while operating relatively cold, even without glow. Heating element life is accordingly prolonged. The invention may also be utilized as a heat modulator, effectively varying the heat transfer coefficient to obtain a constant heat output from a variable input. Utility is further found in the application of the invention to heat exchangers.

In the drawings: 1

FIG. 1 is a partially schematic view of an apparatus illustrating the principles of the present invention.

FIG. 2 is a curve showing the increase, at various wire temperatures, of heat transfer between the wire of FIG. 1 and a gas surrounding it, as a function at corona current.

FIG. 3 is a curve showing the increase, for various temperatures of the wire, in heat dissipation with increase in corona power.

FIG. 4 is a view similar to FIG. 3, illustrating the decrease in wire temperature, for various heating power inputs, with increase in corona power for the apparatus of FIG. 1.

FIGS. 5a and 5b are similar to FIG. 2, but showing the case of forced convection at various velocities at the indicated temperatures.

FIG. 6 is a partially schematic view of an embodiment employing back corona.

The luminous sheath enveloping a corona-discharge electrode in a gas is a zone of intense electrical and mechanical activity. Ions and neutral molecules carried a ng by the electric wind are continuously swept in and out of the region of glow adjacent to the electrode surface. If the corona electrode is heated to a temperature above that of the gas in which it is immersed, more eflicient heat transfer to the gas in consequence of the disruption of the,

boundary film by the discharge in realized.

This mechanism of heat-transfer enhancement utilizing a corona discharge is to be distinguished from the electro strictive method of Senftleben and Braun, described in Der Einfluss elektrischer Felder auf den Warmestrom in Gasen Z. Physik 102, 480-506 (1936), and from their followers such as R. Kronig and N. Schwarz, On the; Theory of Heat Transfer From a Wire in an Electric Field, Appl. Sci. Res. Al, 35-46 (1947), and P. H. G. Allen, Electric Stress and Heat Transfer, Brit. J. Appl. Phys. 10, 347-351 (1959), and P. S. Lykoadis and C. P. Yu, The Influence of Electrostrictive Forces in Natural Thermal Conduction, Int. 1. Heat Mass Transfer, 6, 853- 862 (1963). It is also distinguished from the electric-wind technique of Burke described in US. Pat. 1,835,557. Simliarly, distinction is made from the electric-wind techniques of Velkoff, Electrofiuidmechanics: Investigation of the Effects of Electrostatic Fields on Heat Transfer and Boundary Layers, Technical Documentary Report ASD- TD R -62-650, Wright-Patterson Air Force Base, Ohio (196 2) and An Exploratory Investigation of the Effects of Ionization on Flow and Heat Transfer With a Dense Gas. Technical Documentary Report ASD-TDR-63- 842, Wright-Patterson Air Force Base, Ohio (1963). Also the electric-wind technique described by Marco and Velkoif, Effect of Electrostatic Fields on Free-Convention Heat Transfer From Flat Plates, Am. Soc. Mech. Engrs., Paper No. 63-HT-9 (1963); and from the ionic-current process of Mixon, Chem. Eng. Progr. 55, Bhand et al. Effect of Ionic Currents on Heat Transfer, Indian J. Phys. 37, -190 (1963).

The Senftleben-Braun effect obtains by virtue of the fact that the dielectric constant of the convecting fluid is a function of its mass density, i.e., the phenomenon is electrostrictive rather than simple coulombic, and there is no corona. The Burke-Velkoif-Marco method, on the other hand, does employ corona, but in this instance the corona wind is directed from its electrode of origin against the heat-transfer surface. In the subject invention it is an essential feature that the corona discharge be established at the heat transfer surface itself, the object being the breakup of the boundary film by fluid activity within the region of corona glow. The investigations of Mixon, Bhand et al. deal with the influence of electrolytic bubble evolution in the cooling of a heated electrode.

Apparatus demonstrating the invention is shown schematically in FIG. 1. A platinum wire 10 of 20 mils diameter and 12 inches long is mounted coaxially in a metal tube 12 of the same length and 2 inch inside diameter. Each end of the tube is joined to a Plexiglas insulator 14, the upper insulator supportin-g a T-shaped member from which the wire hangs, and the lower insulator carries a member which centers the wire, yet permits it to be pulled taut in case of thermal expansion. The upper and lower vertical wire supports 16 are of iron Ma inch in diameter and in the general shape of a dowell. The diameter of the wire is smaller, insuring that the corona discharge is confined to the wire alone, over the entire range of corona voltages employed. Despite the need to minimize the electrical resistance of connecting leads in the low-voltage heater circuit external to the platinum wire, iron supports 16 rather than copper are employed in order to reduce heat loss by conduction from the ends of the wire.

Heating power is supplied to the wire by a 6-volt storage battery and regulated by a potentiometer as shown. Instrumentation is provided as illustrated for measuring heater current through the platinum wire and the voltage across it. The temperatu e of the wire 10 is determined at any heating-power input by relating the wire resistance (as found from the voltage/current ratio) to the temperature, by means of a previously obtained calibration curve.

Platinum is preferred for the heated wire because that metal shows no temperature-resistance hysteresis over repeated cycling, has a much greater electrical resistivity than the copper used elsewhere in the circuit, and possesses a satisfactorily high temperature coefficient of resistivity. In addition, the emissivity of platinum wire is low, thereby minimizing radiative heat losses. D-C heating power is employed to prevent the inductive reactance of the iron supports 16, present with A.C., from masking the small changes in platinum-wire temperature which are measured. An orifice 18 is positioned between the legs of a manometer to determine gas flow parameters.

The resistance of the connecting leads, supports and junctions of the heating circuit is less than one percent of that of the platinum wire, Wire-temperature measurements are almost always reproducible to within 120 F. over the range of 75 F. to 1500 F.

The platinum wire is electrically grounded and the tube 12 raised to a high potential. A discharge of positive polarity (i.e. tube 12 at negative potential) is selected for two reasons: 1) the Crookes dark space adjacent to the electrode surface in the negative corona conceivably corresponds to a zone of relative aerodynamic quiescence, and (2) the more evenly distributed positive corona posisibly yields a more uniform temperature distribution over the surface of the discharge electrode. A 6 megohm current-limiting resistor between the high-voltage power supply and the corona wire protects the wire from breakage in the event of sparkover. High voltage is read on an electrostatic voltmeter and the voltage is monitored by an oscilloscope to insure D-C of negligible ripple.

A As-inch diameter ceramic rod 20, mounted at right angles to the wire at midlength, functions as a vibration damper and suppresses in the wire any oscillations which tend to occur for certain combinations of wire tension and electrostatic forces.

The term forced convection refers to that component of convection arising from a pressure differential externally applied to the wire-tube system. Internal convection on the other hand, designates convection due to forces originating within the wire-tube system, namely, natural convection and convection produced by the electric wind.

The apparatus is placed in room air (75 The ductwork shown in FIG. 1 may be used to demonstrate cases of forced convection; on internal convection that portion of the apparatus above the upper insulator 14 is removed.

FIG. 2 illustrates for the case of internal convection (zero forced draft) the enhancement of heat dissipation by the practice of the invention. The term P represents heating power dissipated with corona and P represents heating power dissipated without corona, for the wire of FIG. 1. The corona is represented by the parameter square root of corona current, the latter derived from the indicated corona milliameter of FIG. 1. While complete theoretical explanation for the three distinct regions of the family of curves is not as yet possible, the fact that the ratios are greater than unity demonstrates even at the higher temperatures the remarkable utility and reality of the invention.

Reference to FIG. 3 shows that at constant temperatures and low corona-power levels small increments in corona power yield sizeable increases in thermal power transferred. At the lower end of the bottom curve', for example, at 500 F., /2 watt of corona power raises the thermal output from about 8 to 20 watts, an effective amplification ratio of 24.

FIG. 4 depicts similar behavior, revealing the influence of corona on wire temperature for the three designated fixed heating power inputs. These curves show that an increase in corona power causes the temperature of the wire to diminish, for a fixed heating power input to the wire.

In the case of forced convection, the corona established at the heated wire still effects increased heat transfer. This is shown at FIGS. 5a and 5b of the drawings, which illustrate the enhancement of heat transfer observed with the apparatus of FIG. 1 and for the two temperatures 500 F. and 1500 F. In these curves, P now represents the heat rate due to all causes including corona and forced draft, and P is the heat transfer rate from all causes except corona. As illustrated, at velocities of forced draft of 10 feet/sec., corona can more than double the heat transfer rate, and at velocities of 92 feet/sea, a 10% increase in heat transfer is still observed.

Referring now to FIG. 6 of the drawings, an embodiment is illustrated which employs the less commonly encountered back-corona discharge. In a heat transfer apparatus such as that shown at FIG. 1, performance is believed to depend upon the presence of a nonuniform electric field of suflicient intensity to establish a corona discharge on the heat transfer surface, this being the platinum wire 10. The required high intensity field can be produced by power sources of reasonable size only for discharge electrodes of relatively small radii of curvature. Accordingly, the illustrated apparatus and method is practical for extracting heat from hot wires. However, difiiculties of a practical nature are encountered in disrupting boundary films on relatively flat surfaces. In such cases, a back corona is utilized. In general, back corona requires a conventional forward corona discharge and a coating of porous high resistivity material on the normally passive electrode. Depending upon the current density and breakdown voltages, resistivities as low as 10 ohm-cm. or less are often suitable, although values even orders of magnitudes greater are commonly required. The coating is preferably one having good thermal conductivity. However the corona-induced turbulance of the fluid in and adjacent the pores will tend to eliminate problems due to poor thermal conductivity.

In FIG. 6, the numeral denotes a copper cylinder to which is cemented on the inside surface thereof a cylinder 62 formed of glass fiber cloth. The numeral 64 denotes a plurality of heating coils adapted to maintain the copper cylinder at a constant temperature. The coils 64 are embedded in the inner portion of insulator cylinder 65! so as to inhibit radially outward flow of heat. The copper cylinder 60 is electrically grounded as indicated at 70. A forward corona discharge wire 72 is mounted at its lower end on electrode holder 74 and at its upper end to electrode 76, the latter leading to a source of high potential.

The cylinder 60 is held at a known elevated temperature and a high voltage is impressed between the wire 72 and the cylinder 60. The appearance of forward corona on the wire 72 is accompanied by back corona at the covering 62 adjacent the copper cylinder 60. As the back corona increases, heat transfer takes place radially inwardly from the copper cylinder 60 and increased heating power from the copper cylinder 60 and increased heating power from the heating coils 64 is accordingly required in order to maintain a constant temperature of the copper. This performance will obtain in the case of both internal and forced convection. Further, it will take place with electrical wave forms of any shape or polarity. Further, the electrode 60 may be cooled and heat flow reversed.

The foregoing examples of apparatus and procedures for the practice of the invention have employed gas. However, the invention also admits of a dielectric liquid as the fluid which passes over the corona discharge surface. Further, the wire-cylinder electrode configuration of FIGS. 1 and 6 represents a wire-cylinder system. Others include wire-wire, point-wire, point-plane, wire-plane and planeplane.

I claim:

1. The method of enhancing heat transfer between the surface of an electrically conductive body and its ambient including the steps of,

(a) covering said surface with a porous, high-resistivity 10 ohm-cm.) material, (b) establishing a forward corona by maintaining a high potential between said surface and a corona discharge ture of said surface is substantially greater than the radius electrode proximate to said surface, of curvature of said forward corona discharge electrode. whereby a back corona is established at said surface. 2. In a device transferring heat from a heat source to a References C e tg g g u f 1 t H du ti 5 UNITED STATES PATENTS m n z ii f g 325 g; i g 6 a y 2,605,377 7/1952 Kaehni et al 165-1 X (b) a porous, high-resistivity material ohm-cm.) gtlelgerwald 1651 over at least a portion of said surface, omgren et a 165 1 X (c) a forward corona discharge electrode proximate to OTHER REFERENCES f 10 Sal Sm 373,051 4/1923 Germany.

((1) means for establishing a high potential between said forward corona discharge electrode and said surface, ROBERT A OLEARY Primary Examiner hrb abak ron tbl'hd t"d alf e y c a es a e a A. W. DAVIS, JR, Assistant Examiner 3. The apparatus of claim 2 wherein said porous mate- U S C1 X R rial is of generally uniform thickness. 4. The apparatus of claim 2 wherein the radius of curval-109; 2193 69, 383, 391, 399'

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