Optimization And Prediction Of Coal Fire Gravity Heat Pipe Extracting Heat Performance

Coal spontaneous combustion(CSC)is called as a global disaster,which seriously threatens the natural,human healthy and economic growth.In order to realize the "green"control of CSC,there are lots of researchers have been applied Gravity Heat Pipe(GHP)on extracting the heat energy generated by CSC.In this study,9 GHPs have been designed based on working fluid(Al2O3-H₂O nanofluids,CuO-H₂O nanofluids,and TiO2-H₂O nanofluids),working fluid filling rates(15%,25%,35%),and length-diameter ratio(16,11,8.5)helped with orthogonal experiment.9 GHPs had been applied on coal pile with different temperature to explore the influence of GHP on the temperature and temperature distribution of condensation section.Four index parameters(cooling range,cooling rate,effective radius,and extracting heat performance),which characteristic the heat transfer performance of GHP had been calculated.The weight of each index parameter is determined by the combination of analytic hierarchy process(AHP)and entropy weight method.And the method of Technique for Order Preference by Similarity to Ideal Solution(TOPSIS)is used to evaluate the comprehensive extract heating performance of the 9 GHP under different temperature.And then,the range analysis was used to analyses the results of orthogonal experiment.Moreover,based on the VOF model,the multiphase flow process inside the GHP is established,and the total thermal resistance and equivalent convective heat transfer coefficient of 9 GHPs are calculated when the working temperature is 350?.The main research contents and results are as follows:GHP can effectively reduce the temperature of coal pile.The cooling process of measuring points can be divided into two stages:large decline stage and gentle decline stage,and the high temperature coal pile have longer large decline stage.With the increase of working time,the temperature of condensation section increases sharply at first,then decreases slowly and tends to decrease.Besides,the closer the measuring point of condensation section is to evaporation section,the higher and more stable the temperature is.The cooling range and cooling rate of coal pile measuring points are positive correlation with the distance between measuring points and GHP.Furthermore,the extracting heat performance of GHP is positive with working temperature.However,the effective radius of GHP does not changed regularly with the temperature increased.According to range analysis,when the temperature is 70?,the best extracting heat performance of GHP by experiment and comprehensive optimization is 1#.When the temperature rises to 140?,the best GHP by experiment and comprehensive optimization is 5#.When the temperature is 210?,the GHP obtained by experiment is 3#,and the optimal parameter combination obtained by comprehensive optimization:working fluid is Al2O3-H₂O nanofluids,working fluid filling rates is 15%,and length-diameter ratio is 8.5.The VOF model is established to simulate the optimal parameters combination of the GHP under 210?.The results show that the effective convective heat transfer coefficient is 12.94%higher than the experimental optimum.Moreover,combined with numerical simulation and orthogonal experiment,the equivalent convective heat transfer coefficient of 9 GHPs was been calculated.And the influence degree of each factor and best parameter combination was obtained:aspect ratio>working medium type>liquid filling rate,working fluid is Al2O3-H₂O nanofluids,working fluid filling rates is 25%,and length-diameter ratio is 8.5.