Development of PCM coupled heat pipe for space application
Axially grooved aluminium-ammonia heat pipes are extensively used for the thermal control of satellites. Heat pipes efficiently transfer waste heat from the electronic packages, housed inside the satellite, to thermal radiator, located on the outer surface of the satellite. The use of solid - liquid Phase Change Materials (PCM) to store the waste heat during transient operations are gaining prominence in the recent times. Effective combination of heat storage and heat transfer elements will be very promising for the thermal management of future satellites. The present research on the development of one such efficient combination called ‘PCM coupled heat pipe’ for space application is motivated by following factors: 1. Sparse availability of such units in space application 2. Possible reduction in thermal radiator area, volume and mass 3. Requirement of lesser Heater power for satellite thermal management Though the heat pipes transfer heat very efficiently from the heat source to the heat sink with minimum resistance, it requires large radiator area to emit the heat to outer space. Thermal radiators are designed to radiate certain amount of heat to space. When the heat generation is transient or cyclic in nature, still the same area is required to radiate the heat to space. Also, electrical heater power is required to maintain the temperatures when there is no heat generation. This motivates the development of an efficient PCM coupled heat pipe, where heat is stored in the PCM module during transient / cyclic generation and heat pipe will receive the heat from PCM module gradually and transfers to radiator. This mechanism ensures the continuous supply of smaller quantity of heat to radiator rather than short supply of peak heat. Since the radiator is receiving smaller quantity of continuous heat, area requirement to emit the heat to space is smaller and additional heater power to maintain the temperatures is not called for. This results in the saving of area / volume, mass and electrical power in the satellite Thermal Control System. In the present study, the transient phenomenon of heat pipe operation is investigated. The two significant parameters, viz., heat capacity and equivalent thermal conductance of a heat pipe are obtained simultaneously through analytical estimation and experimental evaluation. Eicosane is chosen as phase change material and a PCM module is designed using Thermal (R-C) network model considering thermal coupling between the heat pipe and the PCM module. Since, PCMs are poor thermal conductors, module top cover plate is designed with square pin fins for enhancing the effective thermal conductivity. Realised PCM module along with Eicosane is subjected to thermal tests to validate the design. The PCM module is thermally coupled with axially grooved aluminium-ammonia heat pipe and the overall thermal performance is experimentally evaluated under vacuum environment. A thermal mathematical model is developed to simulate the test conditions and validate the heat storage capacity of the PCM module. A flight model PCM module is developed and subjected to series of qualification and flight acceptance tests. This module with a volume of approximately 100mm × 100mm × 25mm and mass of 400 g, including the PCM mass of 125 g, is capable to store up to 60 kJ of heat as sensible and latent heat. PCM coupled heat pipe is flown on-board GSAT-29 satellite as flight experiment. Once the satellite operations are stabilised, few milestone experiments are carried out to establish the heat storage capacity. The PCM module coupled with the heat pipe is functioning efficiently under micro gravity environment and storing 60 kJ of heat under transient operations. The peak temperature near the heat source is controlled within 65 °C at a heat load of 130 W for 460 s, as per its intended design. This flight experiment has given enough confidence on the operations of PCM module under micro gravity environment. With the combination of heat pipe to transfer heat to thermal radiator, each PCM module weighing 400 g can reduce the radiator mass by almost 80% in future satellites having transient / cyclic heat generation in the orbit. The major scientific knowledge gained from this research work include a) design and development methodology of a PCM module, b) interfacing the PCM module with a conventional heat pipe for transient heat load applications, c) arriving at an optimum and efficient PCM coupled heat pipe configuration and d) demonstration of the PCM module operation under micro gravity environment though in-orbit Flight experiment. Key Words: Heat pipe, Phase Change Material (PCM), PCM coupled heat pipe, spacecraft thermal control, PCM module, flight experiment, GSAT-29 spacecraft Thesis presents the design, fabrication, qualification and acceptance of PCM coupled heat pipe for space application. It also gives details about flight experiment package flown aboard GSAT-29 spacecraft and thermal performance results of PCM coupled heat pipe under microgravity environment in orbit for one year mission life.