EXPERIMENTAL STUDIES OF CAPABILITIES FOR ENHANCEMENT OF VAPORIZA-TION HEAT TRANSFER AT THE MICROCHANNEL SURFACES

The use of grooves, as the structure of flat plate heat pipe is widespread and is a popular and effective technical solution in the practical implementation of various design systems for thermal control of electronic equipment. Advanced thermal control system must meet two criteria: 1) have a high heat transfer coefficient; 2) remove heat of high density. However, the last criterion leads to the fact that the high density heat flux may lead to a boiling limit, which leads to a drastic decrease in heat transfer coefficient, and conducts to a rapid increase of surface temperature. The paper presents an experimental investigation of the heat transfer characteristics of the microgrooved structures at vaporization of water and ethanol. The structures are represented in the form of axial grooves with a square section dimensions of 0.3, 0.5 and 0.7 mm, and a rectangular cross section with the same width and pitch parameters of the channel but with double depth of channels. Studies showed that in the grooved structures three modes of vaporization occur as follows: evaporation, evaporation-pulsation and boiling. The experiment revealed the area having constancy of the heat transfer coefficient at varying specific heat flux that is typical for evaporation. At certain heat flux value the nucleate boiling in the grooves is initiated, and heat transfer coefficient is growing at increasing heat flux. Boiling in the channels of micro-grooved surface is accompanied by a slug boiling regime, and formation of a thin liquid film on the surface of the channel. It is essentially increasing the heat transfer characteristics of the micro-grooved structures. Double of the channel depth leads to increase in heat transfer coefficient up to 30 - 40% for both water and ethanol. Increase of the heat transfer coefficient could be explained by the enlargement of the heat transfer surface of the channels. Increasing of the effective pore radius by the increasing of the channels depth leads to reduction in the hydraulic resistance of the channels that accompanies increase of the critical heat flux up to 50 - 60%.