Experimental And Numerical Study On Heat Transfer Enhancement Enforced By Inner Axial Straight Micro Fin In Circular Tube

Petrochemical industry is one of the most energy consumption part in China,and thereinto,heat exchangers are the main energy consuming equipment.In the petrochemical industry,most of the cooling medium is poor thermal performance,and the ribized tube bundle air cooler widely used in this industry has a large fin effect coefficient among many heat exchangers.When the cooling medium is single phase,the heat transfer performance of the air cooler is limited by the heat transfer coefficient of the inner tube surface,which makes the total heat transfer coefficient of the air cooler much lower than that of the air side surface,hence,it is necessary to improve the total heat transfer coefficient of the air cooler by strengthening the convection heat transfer process of the inner tube.Machining the axial straight micro fin on the inner wall is adopted to reduce the rib coefficient under the condition that the heat transfer coefficient of the inner surface is reduced in order to improve the overall heat transfer coefficient of air cooler.The thesis uses a combination of experimental research and numerical analysis to prove that the flow and heat transfer characteristics of micro-straight fins in the tube determine the conditions that can achieve enhanced heat transfer.45#transformer oil was used as the cooled medium in the experimental research,6staggered alignment finless tube rows assembled with micro straight fin on the inner tubes?outer diameter:8 mm,bottom wall thickness:0.5 mm,fin height:0.25 mm,number of fins:62?are the experimental specimens.Based on the Zhukauskas correlation,the air-side heat transfer coefficient of staggered arrangement finless tube bundle can be obtained.Then,the convective heat transfer coefficient of the inner surface of the tube can be calculated.The experiment found that the 8×0.5×0.25/62 compared with smooth tube bundle,Nusselt number Nu_oilil of the inside of the micro straight fin tube increased significantly with a maximum increase of 18%in turbulent state and a flow resistance of the inner tube is also slightly increased.In turbulent flow state,the heat transfer of the inner tube can be strengthened effectively;In the laminar flow state,the resistance coefficient f increased greatly,and the maximum increase rate is 19%.Business-oriented software ANSYS Fluent was utilized in the numerical simulation to analysis the thermal and resistance characteristics of axial inner micro fin models with different fin heights,different fin numbers,and different fin shapes.This paper is based on the heat exchange area in the axial inner micro fin tube,and the thermal enhancement effect is evaluated with h_iF_i/h_isF_is>1.The study shows:?1?The fluid of the inner tube is in the laminar flow when the Reynolds number Re is low,which leads to the result of h_iF_i/h_isF_is<1,and the overall heat transfer coefficient K of the air cooler does not increase;?2?The fluid in the tube is in a turbulent state,h_i F_i/h_isF_is increases with the augment of Reynolds number Re,the growth rate slows down after the Reynolds number attaining a certain range.?3?h_i F_i/h_isF_iss increases with the increasing of the fin height and reaches the maximum 1.68at the fin height e=0.3 mm;h_i F_i/h_isF_is growth rate slows down significantly as the fin height increases.?4?With the increasing the fin number,h_iF_i/h_isF_iss increases.Compared with the increase of the fin number,changing the fin height has little effect on the growth rate of h_i F_i/h_isF_is.The maximum h_iF_i/h_isF_is is 1.45 at fin number N_f=120.And the resistance coefficient of micro-finned tubes with different fin numbers does not increase significantly compared to smooth tubes.?5?Compared with three kinds of fin shape-sine function,quadratic function and arc tangent function,the area increase coefficient of quadratic function fin is the largest,and h_iF_i/h_isF_is of sine function and arc tangent function fin is less different;h_iF_i/h_isF_is of quadratic function fin shape is the highest among the three shapes at high Reynolds number,and the resistance coefficient is also the largest.?6?The relationship between the circumferential local Nusselt number and the radian?of the micro-fin,the thermal performance is the worst at the depression,while it is the best at the protruding.The circumferential local Nusselt number of the quadratic function micro-fin is better than that of the other two.