Experimental Study On Heat Transfer Characteristics Of A New Fluidized Bed Heat Exchanger With Heat Pipe

The essence of the gas-solid fluidized bed heat exchanger is a kind of heat exchanger which combines the particle fluidization technology with the heat collection tube.It is widely used in industry because of its high efficient fluid solid mixing characteristics,remarkable enhancement of heat transfer and anti-scaling property.Heat pipe is one high efficient heat transfer element.Its greatest advantage is to take the heat of great load with small section area,but without the need of external power to transport the heat in a long distance.It is now permeated in many fields,such as petrochemical,metallurgy,space,electronics.But at present,the heat exchanger mostly uses the traditional water supply heat exchange tube,and the strengthening means only change the operating conditions and the geometry parameters of the heat transfer tube.Researches about heat pipe heat transfer are mainly focused on using the heating wires as the heat source,but the multiphase heat source or the gas solid two phase to heat pipe heat transfer regularities are very lack,which greatly restrict the new heat collection innovation and performance optimization.Based on this,this study attempts to combine the heat pipe with the fluidizing heat transfer technology.In the fluidized bed,heat pipe is introduced to replace the water-supply pipe to verify the heat transfer feasibility in the gas-solid two phase.A set of?352 mm×2000 mm thermal model device was designed and built.The working condition gas velocity in the bed and experimental parameters were measured and corrected by using the thermocouple,differential pressure meter,compensation flowmeter and control program.The effects of bed temperature,gas velocity,static bed height,cooling water volume and medium type(FCC catalyst,quartz sand and empty bed)on heat transfer coefficient of dense phase heat pipe and condensing section were investigated.In addition,the characteristics of tube wall temperature distribution,starting temperature and minimum heat transfer quantity were measured,and the influencing factors of heat pipe characteristics in starting and stable heat transfer stage were explored.The results reflect that the heat pipe-fluidized bed coupled heat exchanger has superior heat transfer effect than the water supply tube in the traditional bed.The maximum dense phase heat transfer coefficient can reach 700 W/(m~2·K)under the experimental conditions,and the condensing heat transfer coefficient is about 600~1600W/(m~2·K).The dense phase heat transfer coefficient grows with the increase of gas velocity,which is in accordance with the particle mass updating theory.But with the bed temperature increasing,the heat transfer coefficient increases first and then descends.With FCC catalyst static bed height rising,the heat transfer coefficient grows,but the quartz sand bed is reversed.Among the effect of operation conditions on the dense phase heat transfer coefficient,gas velocity occupies first place,static bed height second and bed temperature last.In the same operating condition,the dense phase heat transfer coefficient of FCC catalyst is higher than quartz sand.Increasing the flow rate of cooling water can improve the heat transfer but has no obvious improvement on dense phase heat transfer coefficient.The heat transfer coefficient of the condensing section and the wall temperature distribution are only dominated by heat transfer,barely affected by operation conditions.Additionally,the heat pipe heat transfer quantity has an obvious linear rule with the wall temperature or bed temperature,which is determined by the thermal resistance of the heat pipe.The slope of the linear relationship is the reciprocal of the heat pipe dominant heat resistance.As for starting stage,the heat pipe wall temperature tends to 35~40 ~oC,and the minimum heat transfer is about 50~100 W.The starting temperature and the minimum heat transfer quantity increase with the raise of gas velocity and static bed height,but the influence of gas velocity is weak.However,the effects of the gas velocity and static bed height on starting time perform opposite trend.