Study On Natural Convection Heat Transfer Performance And Structure Optimum Of Longitudinal Finned Tube In Vertical

As an element of heat transfer enhancement,finned tube is already widely used in Petroleum,Chemical industry,Energy,Metallurgy and Nuclear power and so on. Because of the various structure shapes and changefully application fields,little heat transfer report on longitudinal outer finned tube in free(natural)convection is found presently.Furthermore,contribution laws of structure parameters of longitudinal outer finned tube upon heat transfer performance and heat transfer equation of longitudinal outer finned tube aren't found in literatures.Based on present situations,profound study on longitudinal outer finned tube in vertical is carried.Numerical model of longitudinal finned tube in vertical is established.On the condition of analyzing transition of fluid flow in vertical duct from laminar to turbulent, flow state of vertical longitudinal finned tube in natural convection is determined to laminar flow.For computational temperature difference of the study object is lower than 40℃in general,the air physical parameters are set as Boussinesq hypothesis.Since Discrete Ordinate Radiation Model is a fit model for local heat source existent, symmetric boundary and well optical thickness compatibility,Discrete Ordinate Radiation Model is selected based on comparison with others.For coupling heat transfer of liquid and solid,shadow face is used to ensure heat boundary continuity on coupling interface.Based on large parameters gradient in calculation domain,hybrid grids division method and grid self adapting are both adopted to improve accuracy with less computational cost,then independent solution for grid is got.The other convection questions can benefit from this solving method.The multi-combination of end face shape,parent tube height,fin length,fin included angle and temperature difference between parent tube and ambient of longitudinal finned tube are all studied with numerical simulation.The research on end face indicates;heat dissipating capacity per unit mass,heat dissipating capacity per unit area,average heat transfer coefficient and surface heat transfer coefficient of opening finned tube is the biggest besides 3 type finned tubes,yet heat dissipating capacity of opening finned tube is the smallest; Heat dissipating capacity of arc-faced closed finned tube is the biggest,heat dissipating capacity per unit mass,heat dissipating capacity per unit area,average heat transfer coefficient and surface heat transfer coefficient of curved-faced closed finned tube is larger than that of flat-faced finned tube; For flat-faced closed finned tube,beside heat dissipating capacity is higher than opening finned tube,the other parameters is the lowest besides 3 type finned tubes.The velocity fields,temperature fields,average velocity in duct and average temperature in duct of 3 kinds of finned tubes with different end face are compared, velocity of opening finned tube is increased uniformly,outlet velocity is the highest; velocity increase of closed finned tube focus on duct entrance,variety of velocity is little,outlet velocity have little drop; The difference of temperature difference of 3 end face shape finned tubes between inlet and outlet of duct is great,the temperature difference of plat-faced closed finned tube and arc-faced closed finned tube are higher 98.4% and 79.6% than that of opening finned tube.The research on fin length and fin included angle show;(1)As fin included angle is changed,the maximum value of heat dissipating capacity per unit mass,average heat transfer coefficient,surface heat transfer coefficient and metal heat intensity of finned tubes with constant fin length mostly appear when fin included angle is 60°.(2)Heat dissipating capacity per unit mass,average heat transfer coefficient,surface heat transfer coefficient and metal heat intensity of finned tubes are increased with fin length increase.(3)The proportion of convection is improved with fin length increase.When fin length changed,the position which proportion of convection is maximum is changed with fin included angle.The research on parent tube height indicates;(1)Heat dissipating capacity per unit mass,heat dissipating capacity per unit area, average heat transfer coefficient,surface heat transfer coefficient,metal heat intensity and convection proportion of finned tubes are decreased when parent tube height is increased. (2)Except heat dissipating capacity per unit area of finned tube with 100mm parent tube height is decreased with increase of fin length,the maximum heat dissipating capacity per unit area all appear when fin length is 30mm.When temperature difference between parent tube and ambient is augmented,heat dissipating capacity per unit mass,heat dissipating capacity per unit area,average heat transfer coefficient,surface heat transfer coefficient,metal heat intensity are always increased.Yet convection proportion have maximum with temperature difference increase.Based on numerical simulation results of 65 different structure flat-faced closed finned tubes,non-dimensionless factor d_D/H which is modified is proposed,fitting criterion equation is;Nu=0.302(Gr Pr)^0.312(d_D/H)^0.160 Where,2.29×10⁶≤Gr Pr≤1.38×10⁹,3.8mm≤d_D≤49.2mm; 100mm≤H≤800mm. The average deviation between fitting criterion equation and computational results is 3.53%.Compared with the non-modified fitting criterion equation,the accuracy of new fitting criterion equation has improvement either in maximum deviation or average deviation.Based on built finned tube heat transfer table,experimental study on plat-faced closed finned tubes with constant parent tube temperature is proceeded by orthogonal experiments.The following interaction factors are considered;parent tube height,fin length & fin included angle.Numerical simulation on experimental finned tubes show that,numerical simulation results and experimental results agree well,simulation results is lower 3.3% than experimental results in average,maximum deviation is -15.9%.The methods adopted in numerical simulation are correct; the accuracy of simulation is high.Range analysis and variance analysis when heat dissipating capacity and heat dissipating capacity per unit mass with tube average temperature 60℃are selected as experiment index are carried.The analysis show,the influence order upon heat dissipating capacity is parent tube height＞fin length＞fin included angle.Finned tube when parent tube height is 800mm,fin length and fin included angle is 50mm and 90°respectively have maximum heat dissipating capacity.The influence order upon heat dissipating capacity per unit mass is fin included angle＞parent tube height＞fin length. Finned tube when parent tube height is 400mm,fin length and fin included angle is 50mm and 60°respectively have maximum heat dissipating capacity per unit mass.Numerical simulation and experiment on finned tube bundle and single column finned tube are done.The results show,numerical results are lower 6.4% than experimental results,numerical and experimental results are in good agreement; total heat dissipating capacity of tube bundle is increased with fin pitch increase,yet growing velocity is decreased; the variable tendency of side single column fumed tube heat dissipating capacity is uniform with that of tube bundle basically; but variable tendency of middle single column finned tube have difference with two sides column finned tubes, when fin pitch is larger than 20mm,augment in heat dissipating capacity of tube bundle mostly arose from increase in heat dissipating capacity middle single column finned tube.When plate space changing,simulation analysis on flow and heat transfer in vertical duct with constant wall temperature is carried.Based on definition of critical heat saturation and heat saturation coefficient determination,universal fitting equation of heat saturation space and vertical plate height is given;S_sb=2.714H_sb^0.250×((T_w-T_∞)/T_∞)^-0.250 where,200mm≤H_sb≤1000mm,60≤T_w≤90,℃.Elaborate research on structural parameters of finned tube and fin pitch of tube bundle is been done,which have significant meaning for design and manufacture of finned tube heat exchanger.