Inherently Safe Looped Thermosyphon Cooling System for Aircraft Applications using Dielectric Fluid H-Galden

This paper presents experimental results of an inherently safe liquid cooling system. A test rig is operated at Hamburg University of Technology in order to prove the concept and gather first data for electronics cooling in modern civil aircraft. The cooling system uses a dielectric working fluid for the natural circulation in a looped thermosyphon operating in both oneand two-phase mode. First the mass flow of this natural circulation system is investigated comparing the measurement data to calculations. After that the cooling performance of the system is evaluated by taking a closer look at the heat loads and corresponding temperatures. Finally heat transfer coefficients in the cold plate are calculated. The results are discussed with respect to the following parameters, which are varied in the test series conducted: heat load, the heat sink temperature and the system orientation. 1. INHERENTLY SAFE COOLING SYSTEMS The power densities of electronic components are increasing continuously, thus conventional air cooling systems are replaced with liquid cooling to remove the waste heat (Dietl et al., 2008). Liquid cooling systems can transfer much higher waste heat flux densities. State-of-the-art liquid cooling systems use a closed cooling loop basically consisting of a cold plate and a cooler connected with pipes. The liquid is circulated by a pump. In the cold plate the waste heat from the electronic components is transferred to the working fluid. This hot liquid is pumped to the cooler, where the waste heat is discharged in most cases to sink the ambient air acting as the final heat. 1.1 Objective in Aircraft Applications Crucial flight systems (e.g. avionics) need highly reliable cooling systems. In common civil aircraft these systems are air cooled using forced convection. The electronics are air-ventilated by fans. Nevertheless the cooling systems are designed to ensure a minimum cooling performance without fans for some time to allow a safe landing at the nearest airport. With increasing power densities of microprocessors and power electronics the waste heat flux densities increase and air cooling has to be replaced by liquid cooling systems like described above. The reliability is a critical issue for these active liquid cooling systems compared to the conventional air cooling. With a failure in a coolant pump, which can have many reasons, the liquid stops circulating and the electronics get overheated very quickly. Thus, analogically to air cooling, it is the aim of an inherently safe liquid cooling system to ensure a minimum cooling performance without a coolant pump. In this passive configuration the circulation of the working fluid has to be actuated by buoyancy forces, which result from density differences. In one-phase operation this is a critical issue, as the density does not vary much for most liquids. A significant change in density is achieved by evaporating the working fluid. Therefore two-phase operation is highly attractive for inherently safe liquid cooling and will be the main focus of this paper.