Abstract
The results of researches of condensation processes in the vapour channel similar to the Laval nozzle of short linear heat pipes are presented. Capacitive sensors are additionally installed in cooled top covers of the heat pipes, and electromagnetic pulses were supplied to them from the external generator. At heating the heat pipe evaporator, starting from a certain thermal power threshold value, electromagnetic pulses became modulated. It is related with the formations of the boiling process in the capillary-porous evaporator and large amount of vapour over it. Boiling process results in rapid increase of the pressure under which the average temperature of the evaporator occurs to be less than the boiling temperature of the working fluid under increased pressure. Considering condensation of excess vapour, this leads to repeated initiation and extinction of the boiling process in the evaporator, which reflects in pressure pulsations in the vapour channel. Pressure pulsations cause modulating effect on electromagnetic impulses. Pulsations frequencies are measured as well as their dependence from overheating of the evaporator. Using the capacitive sensors and a special electronic equipment we measured the local thickness of the working fluid at the condensing surface inside the heat pipes. Time-averaged values of the condensate film thickness are measured, depending on the heat load on the capillary-porous evaporator. The measurement error does not exceed 2 × 10–3 mm. It is demonstrated that the condensate film thickness lessens sharply with the increase of the heat load on the evaporator of a Laval-like low-temperature heat pipe, while the heat resistance of the film on the condensing surface reaches 60% of the total heat resistance of heat pipe with the capillary-porous evaporator.
Highlights
Intense advancement and practical use of short low-temperature range heat pipes (HPs) initiate a thorough study of characteristics of the inner flow of the working fluid’s condensate film inside HPs on the condensing surface
Considering condensation of excess vapour, this leads to repeated initiation and extinction of the boiling process in the evaporator, which reflects in pressure pulsations in the vapour channel
It is demonstrated that the condensate film thickness lessens sharply with the increase of the heat load on the evaporator of a Laval-like low-temperature heat pipe, while the heat resistance of the film on the condensing surface reaches 60% of the total heat resistance of heat pipe with the capillary-porous evaporator
Summary
Intense advancement and practical use of short low-temperature range heat pipes (HPs) initiate a thorough study of characteristics of the inner flow of the working fluid’s condensate film inside HPs on the condensing surface. Occurrence of velocity and pressure pulsations in the vapour channel of short HPs leads to the modulation of fluid film thickness on the condensing surface with a pulsation frequency, what allows using the capacitance method to measure the pulsations frequency and average local thickness of the working fluid condensate film. Seryakov 192 condensation processes inside short metal-cased low-temperature HPs, and a measurement of instantaneous values and time-averaged thickness of the working fluid condensate film on the condensing surface, depending on the heat load on the evaporator. The most accurate and reliable results can be obtained when the dielectric permeability of the fluid and vapour (gas) phases differs by five or more time, εl εvp ≥ 5 In such case, even with compact electrodes, the capacitance sensor has sufficient sensitivity of 10 - 30 pF/m (0.01 0.03 pF/mm), which is normally sufficient when using gauges with a measuring sensitivity for electric capacity of 10−4 pF and higher. With a minor difference between the dielectric permeability of the fluid and vapour phases εl εvp ~ 1 , the measuring sensitivity and broad bandness using a compact capacitance sensor are increased due to the frequency growth and application of the differential method, related to the measurement of frequency differences of measuring and reference high-frequency generators and in the measurement control using a PC
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