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A capillary flow valve for use in a two phase heat transfer system such as a loop heat pipe, including an inlet port for receiving working fluid in a vapor-phase, an outlet port for outputting working fluid in a vapor-phase, and a porous wick material extending across the interior of the valve. Heat

A capillary flow valve for use in a two phase heat transfer system such as a loop heat pipe, including an inlet port for receiving working fluid in a vapor-phase, an outlet port for outputting working fluid in a vapor-phase, and a porous wick material extending across the interior of the valve. Heating the wick evaporates liquid-phase working fluid from the wick and allows the vapor-phase working fluid to pass through the wick to the outlet port. Removing the heat allows liquid to condense in the wick, preventing flow of the vapor-phase working fluid through the wick to the outlet port.

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1. A temperature-actuated capillary flow valve for use in a two phase heat transfer system, the valve comprising: inlet port for receiving working fluid in a vapor-phase;an outlet port; anda housing extending between the inlet port and the outlet port, the housing defining a flow passage,the inlet p

1. A temperature-actuated capillary flow valve for use in a two phase heat transfer system, the valve comprising: inlet port for receiving working fluid in a vapor-phase;an outlet port; anda housing extending between the inlet port and the outlet port, the housing defining a flow passage,the inlet port and the outlet ports being aligned along a central longitudinal axis of the housing; anda porous wick having a cylindrical shape with a first, open, end facing the outlet port and a second , closed , end formed of the porous wick material extending across the flow passage and facing the inlet port,an inner surface of the cylindrical shape of the porous wick material and an inner surface of the second, closed, end of the porous wick material defining a cylindrical volume open to the outlet port and separated from the inlet port by the closed end,the housing configured to be heated by a heat source to evaporate liquid-phase working fluid from the wick and allow the vapor-phase working fluid to pass through the porous wick material to the outlet port, wherein removal of the heat source allows liquid to condense in the porous wick material, thereby preventing flow of the vapor-phase working fluid through the porous wick material to the outlet port. 2. The flow valve of claim 1, wherein said flow valve is cooled by a thermal strap attached to the housing configured to transfer heat from the valve housing to a heat sink. 3. The flow valve according to claim 2, wherein the heat source is an electrical resistance heating element adhered to the valve housing at a location along the housing between the outlet port and the thermal strap, and the thermal strap is located between the inlet port and the heating element. 4. The flow valve of claim 1, wherein the porous wick material is a sintered porous metal. 5. The flow valve of claim 2, wherein the heat source is an electrical resistance heating element adhered to the valve housing. 6. The valve of claim 1, wherein the exterior surface of the wick is smooth, with a close fit to the interior surface of the housing. 7. The valve of claim 1, wherein the exterior surface of the wick has at least one longitudinal groove extending the length of the wick. 8. A method for controlling the flow of vapor-phase working fluid through a temperature-actuated capillary flow valve into a condenser in a two-phase heat transfer system having a working fluid with a liquid-phase and a vapor-phase, comprising: providing the temperature-actuated capillary flow valve having an inlet port for receiving working fluid in a vapor-phase, an outlet port, a housing extending between the inlet port and the outlet port, the housing defining a flow passage, the inlet port and the outlet ports being aligned along a central longitudinal axis of the housing, and a porous wick material within the housing and having a cylindrical shape with a first, open end facing the outlet port and a second, closed end formed of the porous wick material extending across the flow passage and facing the inlet port, wherein an inner surface of the cylindrical shape of the porous wick material and an inner surface of the second, closed, end of the porous wick material define a cylindrical volume open to the outlet port and separated from the inlet port by the closed end;introducing the vapor-phase working fluid into the inlet port andcontrolling flow of the vapor-phase working fluid through the capillary flow valve by heating the housing to evaporate liquid-phase working fluid from the wick and allow the vapor-phase working fluid to pass from the inlet port through the porous wick material to the outlet port, or by removing a heat source to allow the vapor-phase working fluid in the porous wick material to condense and prevent flow of the vapor-phase working fluid from the inlet port through the porous wick material to the outlet port. 9. The method according to claim 8, wherein the capillary valve is positioned with the output port at a fluid entrance to an evaporator in the two-phase heat transfer system. 10. The method according to claim 8, wherein the capillary flow valve is cold-biased by a thermal strap to a heat sink. 11. The method according to claim 8, wherein said heating the housing comprises activating an electrical resistance heating element adhered to the housing. 12. The method according to claim 11, wherein the capillary flow valve is cold-biased by a thermal strap, and the electrical resistance heating element is positioned closer to the inlet port than the thermal strap. 13. The method of claim 11, wherein the capillary flow valve is cold-biased by a thermal strap, the heating element is located between the outlet port and the thermal strap, and the thermal strap is located between the inlet port and the heating element. 14. The method according to claim 8, wherein the working fluid comprises ammonia. 15. The method of claim 8, wherein the exterior surface of the porous wick material is smooth, with a close fit to the interior surface of the housing. 16. The method of claim 8, wherein the exterior surface of the porous wick material has at least one longitudinal groove extending the length of the wick.