Thermal Enhancement Of Phase Change Material And Its Application In Thermal Management Of High-temperature Logging Tools

In the fields of aerospace,defense,and military,oil exploration,etc.,the electronic devices are often faced with high-temperature environments.Different from the common thermal management system,the heat dissipation becomes more difficult in such severe environments,which requires more advanced thermal management technologies.However,the research of thermal management in the high-temperature environment is lacking.In this study,we developed a passive thermal management system including vacuum insulation and phase-change thermal storage to solve the thermal problem of the logging tools in 200°C downhole environments.The key factors which affect the thermal insulation and thermal storage performance were investigated experimentally and numerically.Some novelty composite materials were developed to improve the thermal insulation and thermal storage performance and then were applied to the thermal management of logging tools.The main research contents and innovations are as follows:1.The heat transfer processes of vacuum thermal insulation and phase change thermal storage were analyzed numerically.The vacuum multi-layer heat transfer model and unsteady phase transition heat transfer model were established to realize optimal performance.The main conclusions are as follows:?1?The residual gas heat conduction is negligible when the vacuum degree lower than 10^-3 Pa,and the effective thermal conductivity of the vacuum layer keeps constant as the reflective screens exceed 30 layers.?2?The deviations of the unsteady phase change heat transfer model are within 11%,which is helpful for guiding the optimization design of the phase change heat storage module.2.The novel composite phase change materials were proposed to improve the thermal conductivity and avoid the long-time re-solidification process of paraffin.The main research contents and conclusions are as follows:?1?The paraffin was adopted to exfoliate expanded graphite?EG?into nanosheets instead of those harmful solvents to prepare the graphite nanosheets?GNs?/paraffin composite phase change materials?PCM?by ultrasonic exfoliation method.The non-monotonous variation between the thermal conductivity and the ultrasonic exfoliation time was discovered for the first time and the underlying mechanism was investigated.The thermal conductivity of paraffin increases from 0.3 W/m·K to 3 W/m·K with 4 wt.%GNs.?2?A thermally enhanced modularized thermal storage unit?MTSU?was proposed to realize the online charging and offline discharging working characteristics.The proposed MTSU avoids the slow re-solidification process and exhibits potential for continuous thermal storage over long periods of time.3.A thermally-enhanced nanoencapsulated PCM was prepared by modifying the shell with SiO₂/BN through the miniemulsion polymerization method to improve the low thermal conductivity and solve the leakage problem.In addition,the nanocapsules were applied in latent functionally thermal fluid?LFTF?to enhance its thermal storage capacity.The main conclusions are as follows:?1?The thermal conductivity of the nanocapsules is0.912 W/m·K,with a thermal conductivity enhancement of 527%compared with pristine PCM,which is the highest on record among all the nanoencapsulated PCM with encapsulation ratio above 50%.The melting enthalpy and encapsulation ratio of the nanocapsules were 136.8 J/g and 60.4%,respectively.?2?The nanocapsules possess excellent thermal stability without obvious leakage during the phase change process.?3?Compared with traditional single-phase thermal fluid,LFTF with 20 wt.%nanoencapsulated PCM reduced the heat source temperature by 12.1°C at the heating power of 80 W,and the convective heat transfer coefficient was increased by 11.5%.4.A passive thermal management system including vacuum insulation and phase change thermal storage was proposed to solve the thermal problems of logging tools in the high-temperature environment.The main conclusions are as follows:?1?The temperature of the electronic component was maintained below 125°C for a six-hour operating period.?2?The heat transfer model of the logging tool system was developed to predict the chip temperature,and the deviations are within 15%.