Charging Considerations Effects in Ground Testing Loop Heat Pipes

Investigation of the literature shows that many researchers expect the LHP compensation chamber to become full in the constant conductance regime, while others expect it to remain two-phase at all times. The distinction is hitherto unacknowledged in the open literature, yet affects most aspects of LHP performance. The current work compares operation of a single LHP at two levels of charge, one sufficient to fill the compensation chamber with liquid at high heat loads and the other insufficient. A borescope was used to observe liquid levels in the compensation chamber and evaporator core during operation. Each charge level produced reliable constant conductance operation, yet several performance advantages were found for the lesser charge. The review finds that some have employed what we call Full-CC charge, in which the compensation chamber is allowed to become completely liquid filled, others Partial-CC charge in which the loop contains insufficient liquid to fill the compensation chamber under any operational condition. Authors using Partial CC charge have typically used large compensation chambers with sufficient volume that they can not be depleted of liquid when the loop is shut down. The successful use of much more modest compensation chambers under Full-CC charge suggests that this requirement may unnecessary. Our experimental objective is to explore the relative merits of Full-CC and Partial-CC charge via comparative testing in the same loop with one compensation chamber of modest size. Of particular interest are: • the state of the compensation chamber in the constant conductance regime. • performance in the constant conductance regime • hysteresis phenomena associated with the regime transition • the ability of the loop to start at low heat load with the lower liquid inventory at Partial-CC charge