| Heat exchange tubes are the core components of heat exchange equipment.After a long period of operation,there will be different degrees of fouling in the tubes,which will lead to a decrease in heat transfer efficiency,affect the safe operation of the equipment,and increase equipment cleaning and maintenance costs.Tube inserts not only enhance heat transfer but also act as an anti-fouling agent.The work of this dissertation is funded by the National Natural Science Foundation of China(No.21576245).The heat transfer,fouling and topological optimization characteristics of heat transfer tubes were experimentally and numerically studied.The main research results are as follows:(1)An NT-type twisted tape was proposed,and its heat transfer performance was studied by numerical simulation.The results showed that the PEC of NT-type twisted tape in smooth and inner rib tube can reach up to 1.46 and 1.44;the PEC of ST-type twisted tape in smooth and inner rib tube can reach up to 1.42 and 1.43;the PEC of the inner rib tube without twisted tape can reach up to 1.36.According to the eoqrerimennal results,the Nu and f correlatitns are fitted:The smooth tube with NT twisted tape:Nu = 0.3984Re0.5385 Pr1/3 Y-0.08598 f = 1.9393Re-0.2789Y0.0349 The smooth tube with smooth twisted tape:Nu = 0.382 1Re0.5412 Pr1/3 Y-0.0759 f = 1.831Re--0.2868Y-0.0614(2)The fouling resistance of the smooth tube is about 1.6 times the internally enhanced tube in the long-term fouling characteristics experiment of cooling tower system.The shape and composition of fouling were analyzed by SEM and XRD method.The results showed that the fouling composition in this experiment was mainly a mixture of calcium carbonate and magnesium carbonate rather than a single kind of fouling.In the long-period experiment,particle fouling and crystal fouling are mixed and grow for a long time,all kinds of crystal fouling staggered growth,the distribution of fouling in the tube inner sufface is disorder and irregular.The correlations of thermal resistance and the test time are fitted:RE/Rs=-1.503 ?1.74t0.2794LSI-0.8416(3)An experimental study was conducted on the enhancement of heat transfer and fouling characteristics of NT twisted tape and EHT heat exchange tubes.The results showed that the number of Nu and f increased gradually with the increase of Re number and the decrease of twist rate Y.The integrated performance of the NT twisted tape + EHT-2 and EHT-1 heat exchange tubes can reach as high as 1.16 and 1.35 respectively.The experimental results of the fouling characteristics show that the fouling resistance of the smooth pipe is about 1.5 times that of the EHT-1 heat exchange pipe and 1.1 times that of the EHT-2 heat exchange ppe.By co mparing the thermal resistance,the thermal resistance of the fouling gradually decreases with the decrease of the twisted ratio,and the formula for the change of the thermal resistance of the fouling overtime is obtained:Smooth tube R=-8.644e-11t3 +2.268e-8t2-6.758e-7t + 7.518e-6EHT-1 tubeR =-6.526e-10t3 +1.632e-8t2-2.5e-7t + 1.964e-5EHT-2 tube R=-5.036e-11t3+1.612e-8t2-8.239e-7t?1.811e-6(4)Based on the experimental results of enhanced heat transfer and fouling characteristics with NT twisted tape,an SSR element was proposed,and the numerical simulation and experimental study of the rotation,heat transfer and fouling characteristics of the inserted SSR heat exchange tube were carried out.Compared with the smooth tube,the f number of smooth tube inserted with SSR increases by 1.9-2.8 times,the Nu number of smooth tube inserted with SSR increases by 1.05-1.11 times.Based on the experimental results,the speed correlation of SSR,Nu and f correlations are fitted:Nr= 0.0035Re1.2129(?/?)0.9997Nu = 1.4276Re0.2409 Pr1/3(?/?)0.06506f= 0.2666Re-0.08821(?/?)0.01586Accelerated fouling experiment is conducted to study the fouling characteristics of smooth tube inserted with SSR using CaCO3 fouling cooling water.The results show that the fouling resistance of the smooth tube is significantly higher than that of smooth tube inserted with SSR.The fouling resistance of the smooth tube is about 1.8 times that of smooth tube inserted with SSR.The XRD and SEM analyzes show that the main component of the fouling is calcite,and the surface morphology of the fouling mainly consists of hexahedral calcite.The correlations of thermal resistance and the test time are fitted:Smooth tube with SSR R ?5.252e-11t3-0.722e-8t2?1.896e-6t-2.548e-5(5)Based on Porous Medium Assumptions and Variable Material Density Method,the topology optimization of flow field in heat exchanger tube is studied.Based on the topology optimization theory,the topological optimization mathematical model of the heat exchanger tube under constant temperature boundary conditions is established.The topological optimization method is visualized by COMSOLsoftware,and the clear flow path structure is obtained by MMA optimization algorithm.Density filtering and projection processing are used to correct the topological optimization area in the tube.The results of optimization under different projection processing parameters are similar,and there are gray-level units and edge roughness.Therefore,proper density filtering and projection processing parameters can be used effectively reduce the appearance of gray and checkerboard phenomenon.The number of Re is the main reason for the generation and change of flow channel in the topological optimization area of the tube.Calculation and analysis of flow path structure,velocity distribution and temperature distribution at different Re numbers.It is found that the higher the Re number,the greater the pressure difference between the inlet and outlet of the optimized area,and the smaller the area of the fluid in the optimized area,the more the solid area increases.From the results of the topology optimization,it is found that the optimized solidus in the heat transfer tube is mostly acicular rib.As the Re number increases,the acicular rib becomes thicker and solid appears at the center of the outlet region.Finally,the numerical simulation method was used to compare and analyze the heat transfer and flow performance of the flow structure after topology optimization,the PEC can reach up to 3.2. |
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