| Tubular heat exchangers are widely used,and it has been a hot research topic for heat transfer enhancement technology being applied to tubular heat exchangers at home and abroad.In this paper,heat transfer enhancement research on improved tubes arrangement and pipes filled with gradient porous materials has been carried out after investigating passive heat transfer enhancement technology of tubular heat exchangers.(1)The Regular Pentagon Tubes Arrangement(RPTA)was presented,and the experimental platform of heat exchangers was designed and built.Then experimental research was adopted to compare heat transfer and flow resistance performances of RPTA with that of the traditional Positive Triangle Tubes Arrangement(PTTA).The experimental results showed that the Nu number of RPTA was increased by an average of 9.4% and the Eu number was increased by an average of 18.5% compared with that of PTTA in the range of 5 000~22 000 of Re number.The comprehensive performance evaluation factor ? was less than 1 and the comprehensive heat transfer performance of RPTA was not as good as that of PTTA in the range of 5 000~9 400 of Re number.The ? was in 1~1.08 and the comprehensive heat transfer performance of RPTA was better than that of PTTA in the range of 9 400~22 000 of Re number.It seemed that the higher Re number was,the better comprehensive heat transfer performance of RPTA became.(2)The numerical model of flow and heat transfer was built and the heat transfer and flow resistance performances of RPTA and PTTA were researched by numerical simulation.The results showed that the inlet air had sufficient heat exchange with tubes' wall by flowing through fewer tube rows in the RPTA.Circuitous flow of fluid existed in the leeward side of a single tube in the RPTA,which promoted sufficient heat exchange between air and the leeward sides of tubes' wall,as well as increased the flow resistance loss of air flowing through the tube bundle.The average field synergy angle ? in the RPTA was 73.9 degrees,which was far less than 77.8 degrees in the PTTA.The field synergy degree of velocity field and heat flow field of air flowing through the RPTA was better than that of PTTA.(3)Pipes filled with non-linear pore-size(dp)gradient porous materials in radial direction was put forward.The numerical simulation model of porous materials of flow and heat transfer was established,and heat transfer and flow resistance performances were studied and compared with pipes filled with linear pore-size gradient porous materials in radial direction,homogeneous porous materials and pipes filled with no porous materials by numerical simulation.The results showed that when pipes completely filled with porous materials,the filling method of dp=1.5r~2+0.001 m had the best heat transfer ability and the highest field synergy degree,and the comprehensive performance was slightly better than that of dp=0.15r+0.001 m.When the porous materials filling method of dp=-1.49947r~2+0.299r+0.001 m was adopted,the least flow resistance was obtained,and the comprehensive performance was better than other filling methods.It played a good effect in improving the heat transfer performance or reducing the flow resistance in pipes when pipes completely filled with parabolic curve pore-size increased gradient porous materials in radial direction.(4)When pipes partially filled with porous materials(the ratio of porous materials was 0.6),the filling method of dp=35(r-0.06)2/18+0.001 m had higher field synergy degree but worse comprehensive heat transfer performance than the filling method of dp=-7r/60+0.008 m.The filling method of dp=-1.74044r3-1.78742r~2-0.0316 r +0.008 m had better comprehensive heat transfer performance but lower field synergy degree than that of dp=-7r/60+0.008 m.The research results in this paper are to provide some reference for the research of tube arrangement and pipes filled with porous material in tubular heat exchangers. |
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