| As a more efficient heat exchanger,serrated spiral finned tube bundle is widely used in large heat exchange equipment such as waste heat boiler and boiler economizer.In this paper,the bent serrated spiral fins were obtained by twisting and bending the traditional serrated spiral fins.The heat transfer and resistance characteristics of serrated spiral fin tube bundles with different working fluids were studied by means of experiment and simulation.In this paper,the heat transfer and resistance characteristics of bent finned and traditional finned tube bundles with water(Re=4000～8000)inside tube were experimentally studied,and the effect of fin pitch on their performance was studied.The experimental results showed that the heat transfer and resistance of bent finned tube bundle are higher than those of traditional finned tube.The heat transfer rate of bent finned tube with 4.23mm fin pitch was 2.23-3.15%higher than that of traditional finned tube.The heat transfer rate of bent finned tube with6.35mm fin pitch was 0.93-5.64%higher than that of traditional finned tube.This was because the bent fins enhanced the disturbance of the fluid around the fins and increased the contact opportunity between the flue gas and the fins,so as to improve the heat transfer performance and resistance.When the fin shape was the same,the heat transfer and resistance characteristics of finned tubes with smaller fin pitch were higher than those with larger fin pitch.Then,the performance of spiral finned tubes with different fin pitches(3.91～7.26 mm)of bent fin and traditional fin was studied by numerical simulation,and the principle of heat transfer enhancement was analyzed by temperature and velocity field.The simulation results showed that the heat transfer and resistance of the bent fin were stronger than those of the traditional fin.The Nu of the bent finned tube was 13.6–15.9%higher than that of the traditional finned tube,and the Eu of the bent finned tube was 5.4–24.6%higher than that of the traditional finned tube.The heat transfer of both the bent fin and the traditional fin was concentrated in the windward section.Compared with the traditional fin,the heat transfer of the bent fin was more uniform,and the temperature of the fin tail was higher,so the heat transfer was better.This was because the bent fin increase the disturbance of flue gas,making the flue gas flow direction more diffuse,and this phenomenon was more obvious in the front tube bundle.The heat transfer and resistance of finned tube with small fin pitch had been improved.Nu of bent finned tube with 3.91 mm fin pitch was 4.8%～5.1%higher than that of traditional finned tube with 7.26mm fin pitch and Eu was increased by about 34.5%.In this paper,the heat transfer and resistance characteristics of bent and traditional spiral finned tube bundles with superheated steam(Re=5000～10600)inside the tube were studied experimentally for the first time,and the effects of different steam flow rates on their performance were studied.The experimental results showed that the bent spiral fin improved the turbulence degree of the flue gas,thus improving the heat transfer performance and increasing the flue gas side resistance.The heat transfer rate and pressure drop of the heat exchanger with smaller fin pitch were greater than that of the heat exchanger with larger fin pitch.The steam velocity had an important influence on the heat transfer characteristics of finned tubes.The heat transfer coefficient on the fin side increased with the increase of steam flow,but the steam velocity had little effect on the resistance characteristics.The comprehensive performance evaluation showed that the j/f of bent finned tube with 4.23 mm fin pitch was 15.18-17.05%higher than that of traditional finned tube.The j/f of bent finned tube with 6.35 mm fin pitch was7.43-15.23%higher than that of traditional finned tube.The comprehensive performance improvement of bent finned tube with 4.23 mm fin pitch was more significant than that of finned tube with 6.35mm fin pitch.Therefore,the economic benefit of applying bent fin with4.23mm fin pitch was higher.Comparing the heat exchange conditions of the steam side and the water side,it could be found that due to the small specific heat capacity of steam,the heat exchange of all the four types of fins was much smaller than that when the working fluid is water.When the working fluid in the tube was steam,the heat transfer rate of the bent finned tube was slightly higher than that of the traditional finned tube,and the heat transfer coefficient of the former was greatly improved compared with that of the latter.The pressure drop of flue gas on the water side and steam side has little difference.The working fluid in the tube affected the flow of the flue gas side by changing the temperature,density and other physical properties of the flue gas,and this change had little effect on the flow of the flue gas outside the tube.Subsequently,this paper studied the effect of tube bundle arrangement on the performance of bent spiral fin tube bundles for the first time through simulation.The simulation adopted bent finned tube with 4.23mm fin pitch in a staggered arrangement with better overall performance in the experiment.The simulation results showed that within the Reynolds number range of 5000to 10600,the relative error between simulation and experiment was within 16%.Combined with waterside simulation,it could be considered that the simulation method in this paper had a certain degree of accuracy and universality.With the decrease of the transverse tube pitch of the tube bundles,the Nu number decreased and the Eu number increased.The overall performance of the heat exchanger decreased with the increase of the transverse tube pitch.The Nu number first increased and then decreased with increasing the longitudinal tube pitch,but the overall change was not obvious.Compared with the influence induced by the transverse tube pitch,the longitudinal tube pitch had little influence on Eu,and with the increase of Re,the influence of the longitudinal pitch became smaller.As the longitudinal tube pitch increased,the overall performance of the heat exchanger first improved and then decreased.When S_T=83 mm and S_L=85 mm,the overall performance of the bent fin tube was the best.In practical applications,the compactness and volume of the fin tube bundle should also be considered,and fin tube bundles of different sizes should be selected according to different applications. |
