WO1999063293A1

WO1999063293A1 - Temperature control system with electrohydrodynamic heat transfer

Info

Publication number: WO1999063293A1
Application number: PCT/US1999/012270
Authority: WO
Inventors: Chun Ho Lam
Original assignee: Alliedsignal Inc.
Priority date: 1998-06-02
Filing date: 1999-06-02
Publication date: 1999-12-09

Abstract

A temperature control system uses electrohydrodynamic heat transfer enhancement effects. By applying an electric field across a heat transfer fluid, the heat transfer coefficient can be controlled as a function of the applied voltage. This effect can be used to control the amount of heat energy transfer from one fluid to another fluid. By controlling the heat energy transfer, the discharging temperature of the fluid can be controlled. A heat exchange device (11) may include two chambers (13, 15) each having a fluid inlet (17, 19) and a fluid outlet (21, 23). A liquid isolating, heat conducting barrier and common electrode (25) separates the chambers. Fluid is conveyed to the chambers in generally opposed directions to cause the fluids to flow in opposite directions near the surface of the barrier (25). The temperature of one fluid is sensed by a temperature sensor (27) after the one fluid passes from one chamber outlet (21) and the electric field is applied to at least one of the fluids to enhance the rate of heat transfer between the two fluids and achieve the desired temperature of the one fluid.

Description

TEMPERATURE CONTROL SYSTEM WITH ELECTROHYDRODYNAMIC

HEAT TRANSFER

The present invention relates generally to heat exchange methods and apparatus and more especially to such methods and apparatus for controlling the rate of heat exchange between two media in a heat exchange device. The temperature control system and techniques have aerospace, industrial, and commercial applications.

Conventional temperature control systems use a control valve to bypass a fluid around or to limit the flow through the heat exchanger to vary the heat transfer and achieve the temperature control objective, and usually have the following problems. The system dynamics vary significantly. Maintaining control stability usually results in a less responsive system. The valve is a nonlinear control device which adds to the control problems. Among the temperature control system components, the valve, being a mechanical device, is the most unreliable element. The valve increases the total system weight and volume. It also increases the component installation and system development cost of the system. Since no heat transfer enhancement is applied, the heat exchanger is usually sized to cope with the worst operating conditions, resulting in added weight and volume penalties.

Figures 1 A and 1 B illustrate the prior approaches for controlling the fluid temperature, τ is a time constant, a measure of the system response speed in terms of the elapsed time between the initiation of change to 63% of the final value of the change caused by a step change in the excitation to system. NTU is the number of heat transfer units defined as the ratio of

(UA/W) where UA is the total transfer rate, and W is the smaller thermal capacity value of the two heat exchanging fluids. ω is the fluid thermal capacity ratio of the fluids defined as (W1/W2) where W2 is the larger thermal capacity value of the two heat exchanging fluids. _n refers to the hot fluid, _c refers to the cold fluid, ₀ refers to the heat exchanger outlet, and ι refers to the heat exchanger inlet. In both configurations, T_h0 indicates the controlled fluid temperature. The desirable temperature is achieved by varying the flow W_c, at temperature T_ci, using the control valve S1. The response speeds of valve S1 and the heat exchanger dominate the achievable dynamic response of T_h0- Since the response speed is a function of the time constant τ of the component, a longer time constant implies that the response speed of the component is slower.

The heat exchanger time constant is a function of the heat transfer rates of both fluids and its thermal mass such that the time constant increases as the heat transfer rates decrease. As a result, the heat exchanger response time at low flow conditions can be 10 times or longer than those at the high flow conditions. The other effects are the changes in T o induced by changes in W_c. The ratio of change in T_h0 and change in W_c is known as the gain effect of the heat exchanger, and varies widely as a function of W_c. For fluid type applications, the smaller W_c or Wh will also result in an appreciable increase in transit time ( time delay effects) of the fluid passing through the heat exchanger. All these effects have the tendency to degrade the resulting T_h0 control system stability performance. Since valve S1 is a mechanical device, its performance will usually degrade as it ages, resulting in either valve failures or degradation in its hysteresis and backlash characteristics. These hysteresis and backlash effects will significantly degrade the control system stability. To anticipate the above performance degrading, the initial control system design is usually conservative to improve its robustness. This usually will result in a slow responding T_h0 control system, which is undesirable for many temperature control applications.

It has been demonstrated that the fluid heat transfer rates can be enhanced by applying an electric field to the fluid. The degree of enhancement is a function of the field strength, which is proportional to the voltage applied to the fluid. These techniques are commonly known as the electrohydrodynamic (EHD) heat transfer enhancement. Since the control voltage can be controlled with high accuracy and repeatability, the heat transfer rates can also be controlled with high accuracy and repeatability, eliminating the undesirable hysteresis and backlash effects commonly found in control valves. Because the maximum time constant of the heat exchanger occurs when there is no heat transfer enhancement, it will always decrease as the heat transfer rate increases due to the EHD effects. Since the fluid flow rates can remain constant for EHD type control operation, the constant time delay effects upon the control system dynamics can be more easily compensated for as compared to the valve type control systems. These improvements indicate that EHD type control system performance will be superior to the valve type control systems.

In U.S. Patent 4,651 ,806 there is disclosed a heat exchanger having a casing through which pass a plurality of spaced-apart tubes. Heat exchange takes place through the tube walls between a first fluid within the tubes and a second fluid outside them but within the casing. The rate of heat exchange is enhanced by electrohydrodynamic effect by means of an electrode comprising a sheet-form first part which encompasses the tubes, and connected second parts which run lengthwise through the spaces between the tubes. The first part may be mesh-like and the second parts may be mesh-like and/or rod-like. The electrode is excited to high voltage and the casing and tubes are grounded. The effect of the second part is to make the electric field around the individual tubes more uniform than would be the case if the electrode consisted of the first part alone. It is desirable to utilize electrohydrodynamic heat transfer enhancement to provide temperature control in a compact, economical heat exchanger and to provide such control in a way to avoid the above noted problems. The present invention provides solutions to the above by providing an improved method of controlling the time rate of heat transfer between two fluid media and includes sensing the temperature of one of the media and comparing that sensed temperature to a preferred temperature to determine the difference between the sensed and preferred temperatures. An electric field is then selectively applied, as a function of the sensed temperature error, to at least one and sometimes both of the liquid media to increase the time rate of heat transfer between the media whenever the comparison indicates differences between the sensed and preferred temperatures. The electric field may be of a varying direct current and applied continuously, as a function of the sensed temperature error, or may be a variable pulse duration according to the difference between the sensed and preferred temperatures. For temperature control systems using heat exchangers, the maximum outlet temperature achievable is the inlet temperature Tm, and the minimum can approach T_Cj. The maximum temperature control range is therefore the difference between Tw and T_ci. For discussion purposes, the temperature control range achievable for a given temperature control system is defined as the percentage of the maximum temperature control range. In heat transfer literature, the performance of a heat exchanger is expressed by the parameter ε, known as the heat exchanger effectiveness, and is defined as: ε = (Th, - T_h0 ) / ( T_hl - To, ) Figure 2 shows typical trends of ε as functions of the cold and hot fluid flow thermal capacities. The value of ε for a given set of operating condition can be determined using the effectiveness curves of a particular heat exchanger, and Tho is given by:

This equation indicates that ε is never greater than 1 , since the lowest possible temperature attainable is T_cι. For approaches shown in Figures 1 A and 1 B, T_h0 can vary from T_hl to values approaching the sink temperature T_cι. This implies that ε varies from 0 to a value approaching 1 . The lowest ε limit is achieved by using the valve S1 to cut off the flow to the heat exchanger. The ε high limit occurs when maximum W_c is allowed to flow through the heat exchanger and the value is a function of the heat exchanger design.

For temperature control systems using EHD effects, the temperature control range achievable is usually less than those achievable by approaches 1 A and 1 B, because there is no valve to cut off the cold flow W_c to the heat exchanger. As a result, the lowest limit of ε is greater than zero, implying that the maximum temperature attainable is less than the hot fluid inlet temperature Tm. The achievable temperature control ranges also depend on the type of fluids used in the system. The temperature control range will be larger for applications using liquids or refrigerants than the gaseous applications, because the electrohydrodynamic enhancement effects on liquid and refrigerant type fluids are higher than those of gaseous type fluids. Figure 2 shows that the electrohydrodynamic temperature control approach is most effective when the heat exchanger is designed to operate at moderate NTU values and under the conditions that the cold side and hot side fluids have similar thermal capacities. Ohadi, M.M., Nelson, D.A., Zia, S, "Heat transfer enhancement of laminar and trubulent pipe flow via corona discharge". Int. J. Heat Mass transfer, Vol 34, No. 4..5, pp. 1 175-1 187, 1991 , and Ohadi, M.M., "Heat transfer enhancement in heat exchangers", ASHARE Journal, December 1991 , report that for PAO, oil, and refrigerant type fluids, the heat transfer enhancement effects can reach 15 times or higher, while that for air is about 3 times. Based on these enhancement data, the temperature control range for the liquid to liquid type heat exchanger applications is in the order of 60% and that for the air to air applications is in the order of 25%. For the air to liquid type operation, the control range is in the order of 30%.

The present invention comprises in the transfer of heat between a fluid media and a second media, an improved method of controlling the time rate of heat transfer between the media comprising: sensing the temperature of one of the media; comparing the sensed temperature to a preferred temperature to determine the difference between the sensed and preferred temperatures; applying selectively an electric field to the fluid media to control the time rate of heat transfer between the two media as a function of the determined difference.

Figure 1A illustrates a prior temperature control method which utilizes flow limiting; Figure 1 B illustrates a prior temperature control method which utilizes flow bypass;

Figure 2 illustrates typical trends of heat exchanger effectiveness ε;

Figure 3 is a simplified cross-sectional view of a plate-fin type heat exchanger incorporating the invention; Figure 4 is a schematic representation of a temperature controlled heat exchange system according to the invention in one form; and

Figure 5 is a schematic representation of the control function for the heat exchange system of Figure 3.

Corresponding reference numerals indicate corresponding parts throughout views of the drawings.

The present invention applies electric fields across the heat transfer fluids to enhance heat transfer within the heat exchanger. The fluids are dielectric and the heat transfer is a function of the field strength. Consequently, temperature control can be achieved by changing the electric field magnitude, eliminating the control valve. The approach should be most effective for liquid and working type fluids in terms of system size and weight impacts. For gas, the control concept is still applicable, but the weight and size impact will be less since the electrical field enhanced heat transfer effect is not as high as for either the fluid or refrigerant cases.

Briefly, the system includes an electrohydrodynamic heat exchanger 1 1 with a set of electrodes 25, 29 and 25, 31 (Figure 3) for each fluid passage, a temperature sensor 27, and one temperature controller 33 (Figure 4) with two independent controllable high voltage supplies 35, 37 (Figure 5) connected to the respective sets of electrodes.

The controllable heat exchange device has two chambers illustrated in Figure 3, but will typically have several adjacent chambers having opposed directions of fluid flow in each adjacent pair. Chamber 13 has a fluid inlet 17 and a fluid outlet 21 while chamber 15 has inlet 19 and outlet 23 to give this opposing direction of flow effect as illustrated by the arrows within the chambers. A heat conducting separator 25 functions as a common electrode and to separate one chamber from the other. An electrode 29 is disposed in chamber 13 and electrically isolated from the common electrode 25. The voltage V1 from power supply 37 (Figure 5) is applied between electrodes 29 and 25. Similarly, electrode 31 is disposed in chamber 15 and voltage V2 from power supply 35 is applied between it and common electrode 25. Pumps convey their respective fluids to the corresponding inlets so as to cause the fluids to flow near the surfaces of the separator 25. The temperature of the first fluid is sensed by sensor 27 after it passes from the first chamber outlet 21. When the sensed temperature differs from a desired temperature sufficiently, power supplies 35 and 37 apply selectively electric fields to one or both of the fluids to control the rate of heat transfer there between. Preferably, both fluids are liquids and the fluid to which the electric field is applied is a dielectric fluid. The temperature control error signal e_r (Figure 5) will be compensated for control performance and stability by K(s) which provides two output signals to drive the high voltage power supplies 35 and 37.

There are at least two approaches for controlling voltages VI and V2. In one approach, initially, VI will remain below the electrohydrodynamic (EHD) effectiveness threshold while V2 will vary to achieve the desirable discharge temperature Tp as sensed at 27. That effectiveness threshold will vary with materials and dimensions, and may range from a few kilovolts up to more than 10 kilovolts. If Tp fails to reach the desirable set point after V2 reaches its maximum value, then VI will be allowed to vary to control Tp. In the second approach, both VI and V2 will be allowed to rise and fall simultaneously at the same rate as required by K(s).

There are also two possible approaches to excite the fluids. In one approach, a variable DC voltage is used to excite the fluid. The second approach is to use a pulse width modulation (PWM) scheme. The voltage will be at its maximum value when applied to the fluid. However, the PWM duty cycle duration of the applied voltage will be a function of the control signal from K(s). The PWM frequency should be higher than the control system bandwidth or such that the induced fluid turbulence onset and turn-off can follow the PWM turn-on and turn-off actions. These fluid turbulence on-off effects may further enhance the flow turbulence for additional heat transfer enhancement benefits. This approach could potentially reduce the power supply complexity.

The present invention has the following advantages. The system does not require a control valve normally used for bypassing or restricting the coolant flow to achieve temperature control. With fewer mechanical components, the system reliability will be higher, resulting in lower mechanical component and installation costs. The system can achieve better control stability by eliminating the valve servo hysteresis, backlash, nonlinear valve area, and aging effects. Fluid flow rates through the heat exchanger will remain constant. This reduces the time constant variations and time delay effects of the dynamic system due to the flow bypassing or restricting effects, resulting in a potentially more stable control system. Finally, this approach may reduce the weight and volume for certain types of the heat exchanger.

Claims

WHAT IS CLAIMED IS:

1 . The transfer of heat between a fluid media and a second media includes an improved method of controlling the time rate of heat transfer between the media comprising: sensing the temperature of one of the media; comparing the sensed temperature to a preferred temperature to determine the difference between the sensed and preferred temperatures; applying selectively an electric field to the fluid media to control the time rate of heat transfer between the two media as a function of the determined difference.

2. The improvement of Claim 1 , the improvement further including the step of applying selectively a second electric field to the second media to control the time rate of heat transfer between the two media as a function of the determined difference.

3. The improvement of Claim 1 , wherein the electric field is of a varying magnitude and is applied continuously.

4. The improvement of Claim 1 , wherein the electric field is a pulse width modulated field.

5. The improvement of Claim 4, wherein the width of the applied pulse is determined by the difference between the sensed and preferred temperatures.

6. A controllable heat exchange device (11 ) comprising: at least first (13) and second (15) chambers, each chamber (13, 15) having a fluid inlet (17, 19) and a fluid outlet (21 , 23); a heat conducting separator (25) separating one chamber (13) from the other (15); first means for conveying (P) a first fluid into the first chamber inlet (17), through the first chamber (13) and out the first chamber outlet (21); second means for conveying (P) a second fluid into the second chamber inlet (19), through the second chamber (15) and out the second chamber outlet (23); means for sensing the temperature (27) of the first fluid after the first fluid passes from the first chamber outlet (21 ); and means responsive (33) to the means for sensing (27) and for applying selectively an electric field to at least one of the first and second fluids to enhance the rate of heat transfer between the first and second fluids.

7. The heat exchange device of Claim 6, wherein the fluid to which the electric field is applied is a dielectric fluid.

8. The heat exchange device of Claim 6, wherein the first and second conveying means (P, P) supply fluid in generally opposed directions to cause the fluids to flow in opposite directions near surfaces of the separator (25).

9. The heat exchange device of Claim 6, wherein the means for applying (33) selectively an electric field comprises a high voltage pulse width modulated power supply (35, 37) and includes means responsive to the sensing means (27) for varying the width of the pulse.

10. The heat exchange device of Claim 6, wherein the means for applying (33) selectively an electric field comprises a direct current high voltage generator (35, 37) and includes means responsive to the sensing means (27) for enabling and disabling the generator.

11. The heat exchange device of Claim 6, wherein the means for applying (33) selectively an electric field comprises a two direct current high voltage generators (35, 37), one (37) for applying a direct current electric field to the first fluid and the other (35) for applying a direct current field to the second fluid.

12. The heat exchange device of Claim 6, wherein the means for applying (33) selectively an electric field comprises two high voltage pulse width modulated power supplies (35, 37), one (37) for applying a variable pulse width electric field to the first fluid and the other (35) for applying a variable pulse width electric field to the second fluid.