Flow And Heat Transfer Analysis Of A Novel Heat Exchanger With Rotated Aligned Tube Bank

Cement industry has the characteristics of high energy consumption and high pollution.It is best for cement industry to make full use of the waste heat during the production process,which can not only meet the power demand,but also reduce the cement cost.It is also a good way to reduce heat emission and to protect the environment,which is a trend of the worldwide cement industries.China is the largest cement producer and consumer in the world.It is imperative to make full use of the waste heat of cement production.In terms of heat recovery system in a cement plant,the heat transfer surface of the waste heat recovery steam generator is a crucial equipment for thermal energy conversion.Hot flue gas flows across tube banks,and water flows through the tubes and turns into steam for generating electricity.There are usually two ways of tube arrangements,namely aligned arrangement and staggered arrangement.The aligned arrangement has less flow resistance due to the smooth flow paths within the tube bank,and is preferable when cleaning and soot blowing are needed due to the ash content in the flue gas;but the heat transfer efficiency is relatively lower.For the staggered arrangement,the fluid flows through bend paths among the tube bank which are alternately contracting and expanding,forming a more turbulent flow than aligned arrangement;as a result,the convective heat transfer is stronger,but the flow resistance is high and the amount of ash deposition is large.In terms of the problems existing in the utilization of waste heat in a cement plant,a novel cross-flow heat exchanger was developed and manufactured,and applied to the industrial waste heat integrated utilization system of a cement plant.A waste heat boiler experimental platform was established,which was dependent on the production of the cement industry while relatively independent to conduct experimental studies.In this thesis,the flow and heat transfer characteristics of the novel heat exchanger were analyzed by the combination of numerical simulation and experimental method,as well as by multi-objective optimization approach.A theoretical model of the heat exchanger was established.The models of flow and heat transfer,entropy generation and economy were formulated,and the SSTκ-ω turbulent model was applied to the numerical calculation of the heat exchanger.By using the commercial software ANSYS Fluent 15.0,numerical simulation was conducted to investigate the flow and heat transfer characteristics of the heat exchanger,as well as the performance of the traditional aligned and staggered heat exchangers.The numerical results showed that the heat transfer performance was significantly better than the other two comparative heat exchangers.Under the same inlet velocity,the convective heat transfer coefficient was 37.5 to 78.8%higher than that of the aligned tube bank,and 32.7 to 66.7%higher than that of the staggered tube bank.However,the pressure drop and flow resistance were much higher than the other two configurations.The effect of tube pitch on the parameters of the heat exchanger and the flow and heat transfer characteristics was also numerically analyzed.When reaching ST/do=2.0 and SL/do=2.5,it had little effect on the flow and heat transfer performance when increasing the tube pitches.According to the simulation results,the correlations of the heat transfer and the friction factor were obtained.By evaluating the comprehensive performance criteria of the heat exchanger,it was concluded that the tube pitch ratio combinations of ST/do=2.0,SL/do=2:0 and ST/do＝2.0,SL/do= 2.5 had the optimum performances.The experimental heat exchanger platform was designed and established.The experimental research on heat transfer and flow resistance performance of the heat exchanger was carried out on the basis of the cement production.The experiment was conducted by using different hot air temperatures,hot air flow rate and water flow rate,and the effects of different working conditions on temperature,pressure and flow rate were analyzed.Under the same mass flow rate,the convective heat transfer coefficient of the heat exchanger was 47.9～65.6%higher than the aligned tube bank,but the pressure drop of each tube was 1.6～2.5 times higher than the aligned one.The studied tube bank had characteristics of high heat transfer coefficient and high pressure drop.Under the same Reynolds number,the Nusselt number was 3.2～9.9%higher,and friction factor was 7.8～36,1%higher,than the aligned tube bank.The PEC value was also higher,showing a better thermo-hydraulic performance.The special structure resulted in very high velocity within the tube bank;it greatly improved the heat transfer performance,but at the expense of high flow resistance.Based on the experimental data,the experimental correlations of the shell side Nusselt number and friction factor were fitted,which was in good agreement with the numerical simulation results,and the reliability and accuracy of the numerical method was verified.Considering the heat exchanger’s evaluation criteria,seven parameters including the outer tube diameter,the number of serpentuator panels,the number of tube passes,tube length,transverse pitch,longitudinal pitch,and the inlet air temperature were involved as the design variables,and a multi-objective optimization was carried out using Matlab genetic algorithm toolbox.For objective functions being heat transfer rate and pressure drop,under the same heat transfer condition,the heat transfer area,total pressure drop and total cost of the heat exchanger were reduced by 25.1%,85.7%and 32.8%,respectively,compared with the original design.It was proposed to use the method of hypothesis ideal point to locate the nearest optimal point on the Pareto front which had the smallest distance to the ideal point,and to compare the optimal point with the maximum value of Q/Np.Compared with the original design under the same heat transfer rate,the pressure drop decreased by 67.9%and 69.7%,respectively,and the total annual cost decreased by 2.4%and 16.3%,respectively,demonstrating a very good optimization effect.For objective functions being modified entropy generation number caused by heat transfer and flow resistance,in addition to the ideal point method,the optimal point with the maximum total entropy generation number could be considered.Design parameters and optimization results of the given optimal points were listed and compared,which enabled designers to flexibly choose the best suited optimization design according to the actual demands.