Research On Structure Optimization Design And Performance Characteristics Of Serrated Spiral Finned Tube Bundles

The serrated spiral finned tube bundles,which have the advantages of more outstanding heat transfer performance and easier to manufacture,are developed from the conventional solid spiral finned tube bundles.To further improve the performance characteristics of the serrated spiral fins,it is proposed to twist and bend the segmented part of the fin.There are few research reports on the serrated fins of this treatment up to now.In this paper,a set of self-designed and built thermal performance test system of the finned tube heat exchanger is used to investigate the heat transfer and flow resistance characteristics of these serrated spiral finned tube bundles.The structural optimization design of the tube bundles is further studied through three-dimensional numerical simulation.To study the performance of different structure serrated fins,the experiments are conducted in a high-temperature wind tunnel to investigating the performance characteristics of serrated spiral fins and twisted serrated spiral fins.The overall thermal-hydraulic performances of the twisted fins are better than the serrated fins in the range of Re=6000-11000,and increasing the water vapor content of the flue gas is beneficial to improve the performance of the heat exchangers,which provides a theoretical basis for selecting the appropriate fin structure and operating conditions for engineering applications.Subsequently,the high-temperature flue gas tunnel was modified to study the heat transfer and resistance performance of the bent serrated finned tube bundles at a high velocity range(v_g=8-16 m/s,v_g is the velocity at the minimum free flow area of bundles).The performance comparison of the bent serrated spiral fin and serrated spiral fin were studied,and the influence of operation conditions(including the flue gas velocity,flue gas temperature,and cooling water flow rate)on the bent serrated spiral fins was discussed.The results show that the total heat transfer rate,flue gas side heat transfer coefficient and pressure drop of bent serrated fins are higher compared with the same structure of serrated fins.The heat transfer factor j and friction factor f of the bent serrated fin increased by 7.0-16.6%and7.9-23.3%compared with the serrated spiral fins.The experiment results also point out that the bent serrated spiral fins with small fin pitch can promote the improvement of the overall thermal-hydraulic performances in the present work.The total heat transfer rate of bent serrated spiral fins improved significantly with the increasing of the test section inlet flue gas temperature,the total heat transfer rate improved about 64%when the flue gas inlet temperature increased from 250℃to 350℃.The flue gas temperature has little influence on the total heat transfer coefficient,Nu,and flow resistance of the bent serrated spiral fins.It is observed that both Nu and flow resistance increased when the flue gas velocity at the minimum free flow area increased from 8 m/s to 16 m/s.The heat transfer performance of the bent serrated spiral fins is better under a high cooling water flow rate,when the cooling water flow rate increases from 0.5m/s to 0.6 m/s,the total heat transfer date,total heat transfer coefficient,and Nu of the bent serrated spiral fins increased by 2.9%、3.7%and 1.7%,respectively.The change of cooling water flow rate has little effect on the resistance of the tube bundles and can be ignored.The effects of the different structures of the segment parts and fin parameters(fin pitch:4.23-8.47 mm,fin thickness:0.8-1.2 mm)on the performance characteristics of the bent serrated spiral fins were investigated by numerical simulation methods.In the present work,9samples were modeled to optimize the design of bent serrated spiral fin.The results pointed out that the heat transfer performance can be strengthened by bend and twist of the segment part of fins,the strengthening effect of twist and bent the segment part at the same time is stronger than bend only.The bent angle and twist have a more significant effect on the heat transfer performance than the bent directions.The reduction of the fin pitch is conducive to strengthening the heat transfer,but the resistance characteristics are worse.The analysis of j/f shows that the overall thermal-hydraulic performance of small fin pitch is better.However,the influence of fin pitch on heat exchanger weight and working environment should be considered when choosing fin pitch.When other structures remain unchanged,increasing the thickness of the fin is beneficial to improve the heat transfer performance,but has the opposite effect on the pressure drop.The thickness of the fin has no obvious influence on the overall thermal-hydraulic performances of the bent serrated spiral fin.Besides,this paper also uses numerical simulation to study the effect of reducing material on the serrated spiral fin tube(that is,cutting off the wake part of the fin to reduce fin)on its performance.The removal of the wake fins has little effect on the total heat transfer rate and the flue gas flow characteristics,and its effect on the overall thermal-hydraulic performances is also very small.Therefore,in the subsequent design of the spiral fin tube bundle,it is possible to consider the appropriate removal of the fin in the wake flow area to reduce the weight of the fin tube bundle and save production costs.