Studies on rotating heat pipe and its application to cooling of electrical machines

The coaxial rotating heat pipe, which is an efficient heat transfer device with capability to handle large heat fluxes with small temperature difference, is studied from the point of view of its application to cooling rotors of electrical machines.; The numerical work concentrates on developing a thermal model of the heat pipe cooled rotor based on the steady state heat conduction in the rotor coupled with energy balance equations for different heat transfer modes occurring within the heat pipe.; A three dimensional finite element model of the heat pipe rotor is built using a standard computer program capable of automatic mesh generation and facility to incorporate the volumetric heat generation in the rotor windings with imposed convective boundary conditions on the heat pipe. Thermal conditions of the rotor of a typical 11 kW motor are simulated and numerical solutions are obtained for the temperature distribution in the rotor and the saturated vapour temperature inside the heat pipe. The rotating heat pipe is found to bring about a marked change and redistribution in the rotor temperature and the thermal gradients within the rotor are substantially lower compared to conventional cooling.; Experiments on a carefully instrumented laboratory-scale rotating heat pipe, 35 mm O.D., 25.4 mm I.D., 850 mm long, have yielded measurements which show how the temperature rise of the heat pipe critically depends on the rotational speed. For a specific heat input and fill ratio of 0.3, the temperature difference between evaporator and condenser has steeply increased beyond 1600 rpm. During decreasing speeds, the heat pipe temperature remained at an elevated level, up to about 800 rpm at which the temperature abruptly decreased exhibiting a kind of 'hysteresis phenomena'. This is attributed to deteriorating condenser performance at higher speeds due to an increased internal thermal resistance at the condenser end while the evaporator heat transfer rates remained fairly uniform, independent of rotational speed.; Tests conducted when the rotating heat pipe is slightly inclined to the horizontal with condenser above evaporator, have shown that smaller inclinations of about 2{dollar}spcirc{dollar} have beneficial effects on its performance while higher inclinations caused serious instabilities indicated by rapidly fluctuating temperature recordings.; To assess the effectiveness of rotating heat pipe in the thermal control of electrical machines, two 11 kW, 1440 rpm induction motors in two different frame sizes are developed using heat pipe shafts. The condenser in each case is built from thin circular aluminium fins and is press-bonded to the bimetallic heat pipe shaft made of hollow carbon steel shaft and copper-water heat pipe. Provision is made for external forced cooling of the condenser. Temperature rise tests at different loads showed that the machines operated at considerably reduced temperatures. For the frame sizes tested, about 25% more output is obtained without exceeding permissible temperature limits. The stator and rotor temperatures are more uniform and the thermal response time has decreased to almost one-third of the time required to reach steady state operation in conventional machines. (Abstract shortened by UMI.)...