【正文】 t the volume that would occupy the fittings as well as the heat pipe. Once evacuation was plete, the valve attached to the vacuum pump and the valve attached to the heat pipe were closed and the vacuum pump was shut off. The valve attached to the fluid column was then opened, and the vacuum inside the fittings drew the water in to fully fill the pipe volume between the fittings. The heat pipe valve was then opened slightly to bleed in the necessary charge (as marked on the column). All valves were then closed and the charging apparatus removed. The heat pipe was charged to 125 percent of the porous volume that the screen wick contained. Thirtyeight (38), 20 AWG type K thermocouples were mounted on the heat pipe as shown in Fig. 4. The thermocouples were soldered in place. The entire heat pipe was then painted flat black using Krylon paint. The emissivity of the black surface was measured as at 25176。s condenser region and to infer what was internally happening between the air and water vapor in the condenser end. The primary focus of this paper is on the qualitative effects of air infiltration in a large, flat heat pipe. Flat Heat Pipe FabricationA flat heat pipe, m m m, was fabricated from 50 mil Monel R400 metal sheets and Monel R400 screens, [10][11]. The heat pipe was designed to utilize water as the working fluid in an operational temperature range of 25176。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 amount of a noncondensable gas is added to the heat pipe condenser end. Marcus [5], and Marcus and Fleischman [6] give an excellent review of a simplified VCHP. A primary goal for a VCHP operating with a constant heat sink temperature is to achieve a steady internal operating temperature at varying heat input conditions. This is acplished for increasing evaporator heat input by the working fluid vapor pressing the noncondensable gas towards the reservoir, thus, lengthening the active condenser length. The condenser length that contains the gas essentially prohibits heat rejection from that portion of the heat pipe condenser. With proper design, this increase in heat pipe condenser area with increasing heat input can achieve a nearly isothermal vapor operating condition. Generally, this calls for the gas reservoir volume to be much larger than the condenser volume. In this investigation, the ratio of the fill pipe volume to the condenser volume was . So if air infiltrates the heat pipe, it will not behave as a traditional VCHP, ., for an increasing heat input, a rise in this heat pipe39。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 panies have designed them for space or puter applications. Figure 1. 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. Flow properties 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 steadystate limitations in flat heat pipe design. The limit most affected in the design of a moderate temperature (100176。 revised: May 13, 2002In the satellite or energy conversion industries flat heat pipes may be utilized to transfer heat to the thermal sink. In this investigation, a large flat heat pipe, m m 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 m 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. Contributed by the Heat Transfer Division for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received by the Heat Transfer Division July 12, 2001。Jo