Test Setup The heat pipe was instrumented with electrical resistance heaters for evaporator heating and chromel-alumel thermocouples for external wall temperature measurement. Multiple layers of polished stainless steel radiation shields insulated the evaporator and adiabatic sections. Rotatable set of vanes (six) mounted over the condensator helped to vary the condenser heat rejection. The test article was mounted concentrically within a 20 cm diameter 230 cm long vacuum chamber. Cooling water was circulated through the copper coil wrapped around the chamber at 2.5 cm pitch. Thermocouples installed on the cooling coil at the inlet, intermediate positions and outlet together with the rotameter type flowmeter in-line provided data for calorimetry. The vacuum in the chamber was maintained at 10-6 Torr or better. About 2.5 cm thick insulation over the chamber minimized the heat exchange between the chamber and ambient. A refrigerated constant temperature bath with centrifugal pump provided the coolant circulation. A schematic diagram of the setup is shown in Fig. 3. The thermocouple numbering and positions are also indicated in this sketch. Test Procedure The heat pipe was tested in the horizontal position with the adiabatic artery at the 3 O'clock orientation. Transient and steady state temperature profiles were obtained in the vacuum mode first. After these tests, the pipe was filled with 1.35 Torr argon at room temperature conditions and tested as before. Frozen state startup tests were usually started in the beginning of the day when the pipe had cooled down to room temperature. In a typical test run, a desired constant input power was applied to the evaporator and the temperatures along the length of the heat pipe were recorded at intervals of two minutes for nearly three hours. Performance tests were done by allowing the transient tests to continue till a steady state was reached. At steady conditions, axial temperatures, power input, cooling water inlet and outlet temperatures and flow rate were recorded. During these transient tests an important precaution
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