Numerical Simulation And Structural Optimization Of Heat Transfer Characteristics Of Air-sand Heat Exchanger

There are a variety of solutions to meet the huge demand for centralized solar thermal power station heat storage.The sand with low cost and excellent performance stands out from many heat storage media.The air-sand heat exchanger matched with the sand heat storage system of tower solar thermal power generation is a new type of heat exchanger with low cost and high efficiency.The study of the flow and heat transfer characteristics of the air-sand heat exchanger has important theoretical reference and practical value for its application and further design in the sand heat storage system of tower solar thermal power generation.Therefore,numerical simulation and experimental research were used to study the flow and heat transfer characteristics of air-sand heat exchanger under different operating and structural parameters.First,experimental study on heat transfer characteristics of air-sand heat exchanger,and analyses the influence of air flow and sand flow on the average outlet temperature of air and sand,effectiveness,exergy changes and exergy efficiency.In the experiment,the air flow is 0.0082kg/s～0.0130kg/s,and the sand flow is 0.0110kg/s～0.0188 kg/s.The results show that the ratio of sand flow to air flow has a great influence on the average air,sand temperature at the exit,which can be choosed according to the requirements of industry in practical application.Exergy efficiency is positively correlated with the air flow and negatively correlated with the sand flow.When the air flow is 0.0130kg/s and the sand flow is 0.0110kg/s,the exergy efficiency reaches the maximum value(0.7449).Secondly,based on the local non-thermal equilibrium model of porous media,the flow and heat transfer characteristics in the air-sand heat exchanger were studied by numerical simulation method,and the effects of sand-air flow ratio,Reynolds number,the width of the sand bed and the length of the sand bed on the flow and heat transfer were analyzed.In the simulation,the Reynolds number is 0.73～5.83,the sand-air flow ratio is 0.98～3.27,the width of sand bed is 0.05m～0.1m,and the length of sand bed is0.5m～0.95 m.The results show that the internal temperature field is approximately diagonal isotherm and the air pressure of the sand bed decreases along the air flow direction when the air and sand cross flow directly contact to heat transfer in the airsand heat exchanger.The value of sand-air flow ratio determines whether the sand can meet the heat storage requirements and the safe operation requirements of the fan.When the sand-air flow ratio is 1.31,the Reynolds number,the width of the sand bed and the length of the sand bed all play a positive role in promoting the heat transfer capacity of the air-sand heat exchanger.Finally,combining with the orthogonal experimental analysis,the four influencing factors of the sand-air flow ratio,Reynolds number,the length of the sand bed and the width of the sand bed were optimized by numerical simulation method,and the proportion of each influencing factor in the improvement degree of heat transfer capacity of heat exchanger was obtained.The interaction between the four influencing factors was calculated and analyzed,and the calculated results showed that the sand-air flow ratio had obvious interaction with the Reynolds number and the width of the sand bed.Therefore,in order to simplify the calculation,the sand-air flow ratio was set as 1.31,and a 5-level orthogonal analysis was performed on the remaining three factors that did not have interaction.The results show that the Reynolds number has the largest influence on the experimental results among the three factors,followed by the width and length of the sand bed.When the Reynolds number is 5.46,the length of sand bed is 0.96 m,and the width of sand bed is 0.084 m,it is the optimal experimental combination.