The measurement of the condensate film thickness of the working fluid by means of the capacitive sensors in the low-temperature range heat pipes

The description of the automated capacitive meter of local thickness of the condensate film of working fluid in the low-temperature range short heat pipes are presented. The design, calibration results of the open small-sized capacitive sensors and electronics that allows the determination of the local thickness of the film of working fluid on the condensation surface of the within heat pipes designed for cooling the heat-stressed designs of spacecraft are presented. The cooling system of the spacecraft on the basis of heat pipes (HPs) can have complex spatial and different topological configuration. The movement of the working fluid in such systems must be carefully examined in the terrestrial environment and determined boundary and limit settings and ranges of stability of HPs work. For such studies modern measuring equipment, which must be inserted into the outlets of HPs to control the thickness, temperature and later the flow rate of the film coolant, is required. The developed electronic hardware, mounted on small circuit boards directly on the lids of short HPs, consists of a measuring and a reference frequency generators, mixer and low pass filter. Both generators are performed on two identical broadband amplifiers EL4551 (INTERSIL), providing amplification of signals with frequency up to 90 МHz. The scheme of each generator is created on the serial LC resonant circuit, in parallel to the capacitance of which is connect with the measuring and reference capacitive sensors in the measuring and reference HPs. With the outputs of the amplifiers of the measurement and reference signals are fed to the inputs of the balanced type mixer ADE-1, then the low pass filter, oscilloscope and a computer. The average time values of the condensate film thickness depending on the heat load on the capillary-porous evaporator have been measured. The measurement error does not exceed 2·10-3 mm. It is shown that the condensate film thickness decreases sharply with increase of heat load on the evaporator is a short low-temperature range HPs, thermal resistance of film on the condensation surface reaches 60 % of the total thermal resistance of the short HPs with capillary-porous evaporator.