Measurement of Heat Flux during Saturated Water Boiling on Surfaces of Different Shapes Using the Gradient Heatmetry Method

The development of technologies in many industries has imposed strict requirements on the control of thermal performance control of power modules. For example, the maximum operating temperature of modern bipolar power transistors of fourth generation exceeds 175°С at a heat flux (HF) above 1 MW/m². Removal of such heat fluxes requires boiling-based cooling systems. The heat release of a power module cannot be controlled without direct measurement of its heat flux. In this work, heterogeneous gradient heat-flux sensors (HGHFS) are employed, which can directly measure local heat fluxes. These sensors are a reliable tool for investigating phase transition processes. Since surface finning considerably increases the heat-transfer surface area, finned models with one, three, and five longitudinal fins are examined. The first critical heat flux during saturated water boiling on a horizontal surface was determined experimentally. The HGHFS signal was compared with a thermocouple signal. It has been established that the onset of a boiling crisis cannot be determined using temperature measurements since the heat flux already exceeds the first critical heat flux by the time the temperature begins to rise. The delay of the thermocouple signal relative to the HGHFS signal is 0.5 s. The local heat flux during boiling on finned surfaces is compared with the heat flux during boiling on a flat surface. Heat-transfer enhancement was obtained for all studied surfaces. The temperature of the simulated power module could be reduced by 11.7–20.5% relative to the temperature of a horizontal plate. With a finning ratio of 7.4, the temperature drop decreased by 20.5%.