【正文】 urnal of Heat Transfer, Vol. 125, No. 2, pp. 349–355, April 2003169。2003 ASME. All rights reserved. Up: Issue Table of ContentsGo to: Previous Article | Next ArticleOther formats: HTML (smaller files) | PDF (297 kB) The Effects of Air Infiltration on a Large Flat Heat Pipe at Horizontal and Vertical OrientationsM. CerzaB. BougheyUS Naval Academy, Mechanical Engineering Department, Annapolis, MD 21402Received: July 12, 2001。 revision received May 13, 2002. Associate Editor: G. P. Peterson. IntroductionFigure 1 depicts a conceptual thermophotovoltaic (TPV) energy conversion system utilizing flat heat pipes. Combustion gases from a heat source such as a gas turbine bustor 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 emitter39。C) flat heat pipe utilizing water as the working fluid is the capillary limit. The capillary limit involves the ability of the wick to develop the necessary pumping head to overe 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 were used 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 100176。s operating temperature is expected. Figure 2. Several investigators have examined the effects of noncondensable gas levels on VCHP operation. These have been predominately for cylindrical VCHPs. Kobayashi et al. [7] have conducted an experimental and analytical study to examine the flow field behavior of the vapor/noncondensable gas mixture. They determined that gravity and noncondensable gas level had a strong effect on the location and profile of the gas/vapor interface layer. Peterson and Tien [8] examined the mixed double diffusive convection in gas loaded heat pipes and twophase thermosyphons. They showed that temperature and concentration gradients can redistribute the gas within the condenser. This redistribution, however, did not greatly alter the overall condenser heat transfer. Peterson et al. [9] also showed that double diffusive convection changes the noncondensable gas flow structure as the Rayleigh number is increased. The heat pipe employed in this study was a very large flat heat pipe since in the energy conversion industry large surface areas are required to cool large power producing devices. Initially, a small amount of air was loaded into the heat pipe. An attempt was made to pare the performance for this air loaded heat pipe to one without air, but unfortunately, air was believed to have infiltrated the second case. This investigation also presents the use of infrared videography as a diagnostic measurement tool to record the external surface temperatures of the heat pipe39。C to 130176。C by using thermocouple data on the heat pipe and calibrating the infrared camera with these data. Later, during operational tests, the surface emissivity was measured at at 100176。C per thermocouple channel and the heater input could be recorded +/–5W. A stand was fabricated so that the heat pipe could be operated at various angles of inclination. Experimental InvestigationData was taken for two types of heat pipe orientation and two different noncondensable gas loadings. Two gas loadings were selected in order to discern the effects caused by minor and major air leaks into the heat pipe. The first heat pipe orientation was horizontal. In this orientation the flat side of the heat pipe was parallel to the ground. In addition, the evaporator heater was on the side facing the ground, thus, the evaporator section was heated from below while the top portion of the evaporator section was adiabatic (thermal insulation was wrapped around the entire evaporator section). The second heat pipe orientation was vertical with the evaporator section placed below the condenser section. At 25176。C, or approximately kPa for water. Since this pressure represents a partial vacuum, it was very easy to bleed a little air into the heat pipe for the initial gas loading. For the first case, air was bled in until the pressure gage read 33 kPa. Assuming that the air would initially occupy the entire inside volume ( m3, approximately) at a partial pressure of kPa and temperature of 25176。C (internal pressure conditions equal to atmospheric conditions with water as the working fluid). The emissivity of the infrared camera was then dialed in until the IR camera was depicting a near isothermal condenser region at a temperature of 100176。C of the thermocouple data. With the heat pipe charged with water and air, the apparatus was set in the horizontal orientation and the heat input was set at 200 W. After stabilization of the heat pipe, which took approximately three hours due to the large thermal mass of the heat pipe, temperatures stabilized and IR video data was taken. The heat input was then changed to 400 W and IR data was taken again when the heat pipe temperatures appeared to stabilize. This procedure was followed up to a heat input value of 800 W. The heaters were then shut down and the heat pipe allowed to cool overnight. The next day, it was noticed that the heat pipe temperature was at 24176。C with increasing