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[References: 48 tit.]

It was hypothesized that the intensity of heat transfer (phase transitions in evaporation and condensation zones, heat conduction and convection in vapor channel) in a thermosyphon depends, first of all, on the intensity of heat transfer in coolant layer on the thermosyphon lower cover and on the free surface of this layer. We conducted experiments to determine the thermogravitational convection velocity in the coolant layer. The velocity averaged over a thickness was up to 0.63 mm/s in the range of heat fluxes from 0.18 to 1.3 kW/m2 and thicknesses of the coolant layer from 3.2 to 7.4 mm. We experimentally obtained temperature fields in a vertical thermosyphon in the range of heat fluxes from 0.18 to 2.6 kW/m2 and filling ratios of an evaporation section from 15 to 35%. We developed a mathematical model of heat transfer in the coolant layer on the thermosyphon lower cover based on our experimental studies. Our model differs from previous ones as it accounts for conduction and convection only in the coolant layer on the lower cover and conduction in the evaporation section of the thermosyphon. Calculated temperatures in characteristic points of the coolant layer are in a good agreement with the readings of thermocouples.