Study On Key Problems Of Fluid Flow And Heat Transfer Of Large-Scale Longitudinal Flow Shell-and-Tube Heat Exchanger

Generally, the heat transfer tube length and shell diameter of the heat exchanger were determined by the technique conditions and capacities of production respectively. However, with the enlargement of equipments for industrial production, the diameter of shell-side is ever-increasing and the length-diameter ratio L/D would decrease correspondingly, which will lead to such key technical problems as mal-distribution of fluid flow, pressure drop increasing sharply, the overall performance deterioration of heat transfer and fluid flow of shell-side and so on. In order to solve the problems, two ideas as a novel structure of multi-parallel-channel inlet and outlet (in short: MPC) in shell-side and installing fluid flow distribution baffles (in short: FDB) in the region of inlet and outlet in shell-side, were proposed.In first, it presented numerical emulation of fluid flow and heat transfer in shell-side of large-scale longitudinal flow shell-and-tube heat exchanger by porous-medium and distributed resistance model. Under the condition of constant length and constant outer temperature of heat transfer tube, fluid flow distribution, pressure drop of shell-side, heat transfer performance, and overall performance factor were studied with the change of L/D and Re. The result showed that when L/D from 6.0 decreased to 1.5 with the same Re, shell-side fluid flow mal-distribution became seriously, pressure drop increased sharply, overall performance factorηdecreased sharply, and Nu of shell-side increased slightly. Especially, under the condition of L/D≤2.0, the problems became more and more seriously.It also presented numerical simulation of fluid flow and heat transfer in shell-side of a new type heat exchanger with MPC by same method above. With the number of MPC (M) change, such characteristics of MPC heat exchanger as shell-side fluid flow distribution, pressure drop and heat transfer performance, and overall performance factorηwere studied. Study result showed that under condition of the same Re and L/D, according to increasing of M, shell-side fluid flow mal-distribution was improved, shell-side pressure drop decreasing sharply and the largest decline for more than 50%, mean surface Nu has a certain increase but very limited, and overall performance factor increased. All characteristics of heat exchanger with the changes of both L/D and Re were studied, too.According to energy and material balance principle, A structural optimal mathematical model of FDBs was advanced by resistance analysis method. Based the theoretical model, this paper presented the investigation of FDBs by both numerical and experimental methods. Both fluid flow distribution and pressure drop were studied with different N and FDBs respectively. Study result showed that shell-side fluid flow mal-distribution be improved greatly by FDBs, especially under the condition of N=11, and FDB of No.1 has better effects of fluid redistribution compare with the FDB of No.2 and No.3. It also showed that compare with the condition of no FDB, FDB of No.1 has the largest pressure drop and an increase of 10~15%, but FDB of No.2 and FDB of No.3 have the same pressure drop and an increase of 6~8%, than the condition of no FDB. With the same Re and FDBs, pressure drop would increase rapidly according with increase of N, the correlation of pressure drop and N is: p∝N2.1~2.2.A new method for measuring local heat transfer coefficient of shell-side of shell-and-tube heat exchanger with gas flow in tube bundle is developed creatively. By error analysis and result comparison, it concluded that this method is simply, convenience, reliable. Shell-side local heat transfer performances were investigated by above method. The real shell-side local surface Nu distribution pattern was revealed, and reasonable interruption be provided. Combination with experimental and numerical investigation, it showed that installing FDB not only improved the uniform of shell-side surface Nu distribution but also enhanced the globe heat transfer performances, the effects of N=11 is the most prominent. Comparing with different FDBs, FDB of No.1 is the most effective to enhance globe heat transfer Nu and about increase 8~12% than the condition of no FDB, but FDB of No.2 and FDB of No.3 have the same globe heat transfer surface Nu enhanced and an increase of 6~8%.Comprehensive studies showed that MPC and FDBs were very effectively not only in optimizing the fluid flow distribution of shell-side and enhancing globe heat transfer performance of heat exchanger, but also in decreasing shell-side pressure drop. Comparing with FDB of No.2 and No.3, FDB of No.1 has a more superior performance of both fluid flow distribution and heat transfer enhancement...