5-6. Transient Characteristics of a Gas-loaded Liquid Metal Heat Pipe with a Long Adiabatic Section R. PONNAPPAN, L. I. BOEHMAN & E. T. MAHEFKEY Summary High temperature liquid metal heat pipes (LMHP) have exhibited startup difficulties when attempting to start from a frozen state due to the inherently low nearroom temperature vapor pressures associated with the working fluids. Inert gas loading is a recommended method to help frozen state startup. The applicability of this to special heat pipe designs, particularly those with arterial grooves and long adiabatic lengths, is unknown. In the present work, a composite wick sodium heat pipe of the double walled artery type with a grooved artery channel and a long adiabatic section without screen wick was designed, fabricated and tested. The 2 m long, 2.22 cm diameter experimental heat pipe was made of stainless steel tubes and screen and filled with 93.4 g of sodium. This scaled down design has only one artery groove (in place of 12 possible grooves) capable of transporting 1800 W at 1000 K through the 74.5 cm transport length. The inert gas used was argon and charged to 1.35 Torr at room temperature. A 12.5 cm long reservoir wick in the evaporator met the fluid supply during startup. The experimental heat pipe was tested inside a vacuum chamber for startup transients at several power inputs. The frozen state startup of the gas-loaded heat pipe with a long adiabatic section was successfully demonstrated. The transient hot zone temperature variation and hot front propagation were as predicted. Transient temperature profiles for frozen state startup and liquid state startup were compared. Introduction High temperature liquid metal heat pipes are used in the waste heat rejection systems of space-borne power plants [1, 2]. An area of major concern for these heat pipes is the behavior of these devices under transient load conditions during startup, shutdown and operational variation of the thermal loads. The startup process may not be successful when the working fluid is in the frozen state since there will be no liquid return from the condenser to the evaporator. A sudden removal of the heat input to the evaporator without removing the condensor load may freeze the working fluid at the condenser and create evaporator dryout [3]. The transient operating conditions imposed on the heat pipe may be one or more of the several conditions schematically explained in Fig. 1. These transient operating problems are further complicated by special artery wick R. Ponnappan, Senior Scientist, Scientific Services Division, Universal Energy Systems, Inc., 4401 Dayton- Xenia Road, Dayton, OH 45432, USA. L. I. Boehman, Professor, Mechanical Engineering, University of Dayton, Dayton, OH 45469, USA. E. T. Mahefkey, Deputy Chief, Aerospace Power Division, AFWAL/POO, Wright-Patterson AFB, OH 45433, USA. Paper number IAF-ICOSP89-5-6.
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