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Journal of Heat Transfer, Vol. 125, No. 2, pp. 349–355, April 2003 ©2003 ASME. All rights reserved.

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TheEffects of Air Infiltration on a

Large Flat Heat Pipeat Horizontal

and Vertical Orientations

M. Cerza

B. Boughey

US Naval Academy, Mechanical Engineering Department, Annapolis, MD 21402

Received: July 12, 2001; revised: May 13, 2002

In the satellite or energyconversion industries flat heat pipes may be utilized to transfer heat to the thermal sink. In this investigation, a large flat heat pipe, 1.22 m×0.305 m×0.0127 m, fabricated from 50 mil Monel 400 metal sheets and Monel 400 screens was videographed at

horizontal and vertical orientations with an infrared video camera. The heat pipe evaporator section consisted of a 0.305 m×0.305 m area (one heated side only) while the side opposite the heated section was insulated. The remaining area of the heat pipe served as the condenser. In the horizontal orientation the heated section was on the bottom. In the vertical orientation the evaporator was aligned below the condenser. The sequence of photographs depicts heat inputs ranging from 200 W to 800 W, and the effect of air infiltration on heat pipe operation for both

orientations. For the horizontal orientation, the air is seen to recede towards the small fill pipe as the heat input is increased. For the vertical orientation, the air and water vapor exhibit a buoyant

interaction with the result that the air presence inhibits heat transfer by rendering sections of the condenser surface ineffective. The effects depicted in this paper set the stage for future analytical and

experimental work in flat heat pipe operation for both normal and variable

conductance modes.

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Contributedby the Heat Transfer Division for publication in the JOURNALOF HEAT TRANSFER. Manuscript received by the Heat Transfer DivisionJuly 12, 2001; revision received May 13, 2002. Associate Editor:G. P. Peterson.

Contents

Infrared (IR) Videographic Results o A. Horizontal Orientation o Nomenclature

Introduction

Figure 1 depicts a conceptual thermophotovoltaic (TPV) energy conversion system utilizing flat heat pipes. Combustion gases from a heat source such

as a gasturbine combustor flow through channels on which heat pipes are

mounted. These hot side heat pipes serve as emitter surfaces. Across from the hot side heat pipes, TPV cells can be mounted to cold side heat pipes which are heat pipes in contact with the thermal sink. An isothermal emitting surface is needed in TPV energy conversion systems because the voltage outputs of the TPV cells are very sensitive to the wavelength bandwidth of the emitting surface. The emitter's wavelength bandwidth is a function of temperature. On the cold side, the TPV cells could utilize a flat heat pipe, but this is less critical. Flat heat pipes are not new

to the industry, several companies have designed them for spaceor

computer applications [1][2][3][4]

. Figure 1.

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Flat heat pipes are similar to cylindrical heat pipes. The only real difference between the two is geometrical. While this may seem a minor difference, it presents many challenges from an engineering standpoint. Typically, heat pipes are used to transfer quantities of heat across a distance with only a slight temperature loss from end to end. The cylindrical design works well to serve this purpose. However, when designing an emitter for a TPV energy conversion system, it is

advantageous to have a large surface area to volume ratio in order to maximize the power density of the system. A flat heat pipe was conceived for this purpose. Flat heat pipes also have different internal flow and structural design considerations than those of cylindrical heat pipes. Flowproperties in cylinders are different from those in rectangular geometries such as flat plates and/or boxes. The flow of a thin film through a flat wick (such as in the liquid return path of a flat heat pipe) is not the same as the flow of a cylindrical circumferential film. Also, vapor flow through a cylindrical space differs from vapor flow through a rectangular cross section. The flow geometrical differences can alter the

steady-state limitations in flat heat pipe design.

The limit most affected in the design of a moderate temperature (100°C) flat heat pipe utilizing water as the working fluid is the capillary limit.

The capillary limit involves theability of the wick to develop the

necessary pumping head to overcome the vapor and liquid pressure losses as the working fluid circulates through the heat pipe.

In this investigation, it was desired to qualitatively examine the effects of what would happen if air infiltrated a hermetically sealed flat heat pipe containing only water. In order for a flat heat pipe to withstand pressure differences across its flat surfaces, the flat surface structure

needs to be supported. Monel pins wereused as support structures in this

flat heat pipe design. These pins were welded to the sheet metal surfaces, and the welds, should they crack, would be a source for air infiltration for a heat pipe containing water as the working fluid and operating below 100°C in an atmospheric environment.

It should be pointed out that this flat heat pipe was not designed as a variable conductance or gas loaded heat pipe. There was no noncondensable gas reservoir at the condenser end, however, there was a short 5 cm in length, 18 mm in diameter fill pipe attached to the condenser end. In a gas loaded variable conductance heat pipe (VCHP), Fig. 2, a reservoir which contains a am …… 此处隐藏：33120字，全部文档内容请下载后查看。喜欢就下载吧 ……