Passive aircraft cooling systems for variable thermal conditions

• The novel cooling system can handle a variety of thermal conditions as presented by aircraft operations.
• Refrigeration by the fuselage can reduce the air conditioning requirements.
• Cooling system malfunction is not observed when water inside the fuselage condenser is frozen.
• The evaporator temperatures can be as low as 41 °C for 1 kW of input power during flight.
• On ground the air conditioning condenser external area should be optimized with fins.

A novel design for a heat exchanger system [1], for which a patent application has been filed, was experimentally evaluated in the laboratory under variable thermal conditions characteristic of an aircraft in operation. The passive heat exchanger prototype consists of a loop-thermosyphon with two condensers and a common evaporator. Water was applied as the working fluid. Natural convection, forced convection and a combination of forced and natural convection at each condenser were replicated. The fuselage condenser performance was tested at temperatures ranging from −30 to 50 °C with heat transfer coefficients ranging from natural convection to values of around 200 W/(m2 K). Air supply to the air conditioning condenser was kept at 20 °C with coefficients ranging from natural convection to around 50 W/(m2 K). Input power up to 900 W was provided to the evaporator section by an embedded electrical resistance. Cooling system malfunction is not observed when the water within the fuselage heat sink is frozen, for input power below 300 W. Under cruise flight conditions, the cooling system is able to dissipate 900 W maintaining vapor temperatures at around 41 °C. The evaporator temperature increases by approximately 0.014 °C/W when start-up occurs. Refrigeration on the ground requires air duct speeds of around 6.1 m/s to keep the vapor temperatures below 100 °C for an input power of ca. 1 kW. When forced convection acts in both condensers, the heat removal capacity of the fuselage heat sink is dominant (around 90%).