Increased cooling power with nucleate boiling flow in automotive engine applications

摘要: In modern automotive design, the increasing output of power delivered by ever more compact, ie, less space consuming, engines requires a highly efficient liquid cooling system to keep the wall temperatures of the individual components on acceptable levels. A maximum possible cooling power is particularly needed on surfaces exposed to high thermal loads typically located in the periphery of the exhaust gas outlets in the cylinder head. Searching for a most intense transfer of heat across very confined surface areas, the modern layout of liquid cooling systems must make ever more use of the potential of the nucleate boiling regime to provide significantly enhanced heat transfer rates at acceptable wall superheats. It has been long known in the community of engine developers that nucleate boiling locally occurs in cooling jackets near the exhaust valves, especially when the engine is run under thermally critical operating conditions. These are typically reached when the vehicle is brought to a halt after operation at maximum engine load, so that a considerable amount of heat has to be removed with the help of the fan cooling on the air side only. Nonetheless, the traditional design concepts are essentially based on purely convective single-phase cooling, although under real maximum load conditions the peak temperatures would markedly exceed the allowed limits, if boiling did not occur. In view of this fact it appears very reasonable to make a more deliberate use of the boiling phenomenon for further optimization of the cooling system. The prominent goal of this novel concept is to provide a controlled transition from single-phase convection to …