Flow And Heat Transfer Performance And Ash Deposition Characteristics Of 3-D Finned Tube Heat Exchanger In The Low Temperature And Ash-laden Flue Gas

Three-dimensional(3-D)finned tube is a new type of enhanced heat exchange element with high efficiency,which has significant application potential for recovering the waste heat of flue gas.However,air-side flow and heat transfer law is incomplete,the distribution characteristics and the influence rules of surface ash deposition are still unclear,and the influence of ash deposition on the flow and heat transfer performance is still in the stage of research gap.These problems hinder the application and popularization of 3-D finned tube heat exchanger for recovering the waste heat of flue gas.In order to make up for the shortcomings of the existing research,the flow and heat transfer performance and fouling characteristics of 3-D finned tube heat exchanger are experimentally investigated.Based on the results in laboratory,an industrial-grade 3-D finned tube heat exchanger with high thermal efficiency and anti-fouling is designed,and the waste heat recovery performance in the medium or low temperature and high particle contained flue gas is evaluated,which can provide the theoretical guidance for the industrial application of 3-D finned tube heat exchanger.The main research contents and conclusions of this thesis are as follows.(1)As for the air-side flow and heat transfer performance of 3-D finned tube heat exchanger,the influence of structure parameters of the heat exchanger on the flow and heat transfer performance is experimentally studied,and the empirical correlations are proposed.The results show that staggered 3-D finned tube heat exchanger has significant heat transfer performance compared with aligned 3-D finned heat exchanger and staggered smooth tube heat exchanger.On the condition of same tube pitch,the heat transfer performance of staggered 3-D finned heat exchanger increases with the increase of fin height,but the flow performance gradually deteriorates.On the condition of same fin height,the heat transfer performance of the heat exchanger reduces with the increase of transverse tube pitch,but the flow resistance performance is improved.Besides,the heat transfer performance and the flow performance gradually deteriorates with the increase of longitudinal tube pitch.However,when the flow and heat transfer performance are considered comprehensively,the comprehensive heat transfer performance of 3-D finned tube heat exchanger can be improved by increasing the transverse tube pitch or decreasing the longitudinal tube pitch and the fin height.Based on the experimental data.Compared with other type of finned tube heat exchangers,the flow and heat transfer performance of 3-D finned tube heat exchanger is more significant,which indicates that 3-D finned tube heat exchanger has significant comprehensive heat transfer performance and significant industrial application potential.(2)As for the distribution characteristics and the influence rules of surface ash deposition,cold tests are performed.The results show that staggered 3-D finned tube bundle has significant anti-fouling characteristics.The ash deposition morphology of staggered 3-D finned tube bundle on windward side only presents one single peak,and most of the fly ash particles concentrated in the leeward side.However,the ash deposition morphology of aligned 3-D finned tube bundle on windward side presents double peaks,and the fly ash particles are mainly concentrated on the windward side.In the process of ash deposition,the ash deposition firstly appears on the leeward side of 3-D fined tube.As time goes on,the ash deposition layer on the leeward side of the tube gradually grows up along the direction of fin height,but the ash deposition layer on the surface of 3-D fins grows gradually along the contour of 3-D fins until the ash deposition completely covers the surface of 3-D fins.However,the change of the ash deposition layer on the windward side of the tube surface and 3-D fin surface is unobvious.When the ash deposition layer grows up to a certain extent,it will fall off and break off under the interaction of gravity and axial velocity.As for the particle size distribution of the ash deposition layer,the larger particles primarily distribute on the surface of the top side and the leeward side,whereas the smaller particles primarily distribute on the fin surface of the leeward side and the windward side.Longer transverse tube pitches or shorter longitudinal tube pitches significantly restrict the ash deposition phenomenon.Fin height first restrains and then promotes the fouling phenomenon as the fin height increases,and the optimal fin height for anti-fouling is 3 mm.(3)As for the flow and heat transfer characteristics of 3-D fined tube heat exchanger under the condition of ash deposition,orthogonal experiments of thermal ash deposition are performed considering the influence of fly ash concentration,flue gas velocity and fin height.The results show that the pressure drop of the 3-D finned tube heat exchanger with fin height of 5 mm increases,but the pressure drop of the 3-D finned tube heat exchanger with fin height of 1 mm and 3 mm decreases when the ash deposition is occurred.The optimized combination scheme obtained by orthogonal analysis is as follows:the velocity of flue gas is 6 m/s,the concentration of fly ash is 4 g/m~3,and the fin height is 5 mm.When compared with other 3-D finned tube heat exchanger or smooth tube heat exchangers,the heat transfer performance of 3-D finned tube heat exchanger under the optimal combination scheme has a small reduction and it maintains at the higher value when ash deposition tends to steady state.(4)Based on the above results,an industrial-grade 3-D finned tube heat exchanger with high thermal efficiency and anti-fouling performance is designed.The waste heat recovery rate of this type of heat exchanger is obtained through experiments under variable working conditions,and the performacne of waste heat recovery is evaluated.Under the condition of constant flue gas flow and cooling airflow rate,the waste heat recovery rate of 3-D finned tube heat exchanger increases from 60.47%to 79.41%with the increase of the temperature of inlet flue gas.Under the condition of constant flue gas flow rate and flue gas temperature,the waste heat recovery rate of 3-D finned tube heat exchanger is further increased from 74.6%to 91.54%with the increase of air flow rate.It can be seen that 3-D finned heat exchanger has significant performance for recovering waste heat of low-temperature flue gas.