Numerical And Optimization Study On The Flow And Heat Transfer Characteristics Of Compact Heat Exchangers

Compact heat exchangers are advantageous due to its high heat transfer area in volume unit which reduces the space,weight and ultimately the required cost and they are efficient than ordinary heat exchangers.A compact heat exchanger can be either plate-fin type or tube-fin type.Due to low heat transfer coefficient on the gas side flow,the thermal performance is often limited and it is vital to find novel ways to improve its performance.For this purpose,secondary surfaces are connected to the primary surface in the form of fins or vortex generators(VGs)which increases the heat transfer area and disturbs the flow field to increase the heat transfer between the core and near wall fluids.The design of compact heat exchanger considering all parameters is a complex task and its optimization usually involves heuristic based computation methods.In the first part of this thesis,a multi-objective optimization method is introduced which combines Differential evolution(DE),Genetic algorithm(GA)and Adaptive Simulated Annealing(ASA)algorithms.The combined DE-GA-ASA method is intended to generalize and improve the robustness of the three population-based algorithms.In the proposed method,the three algorithms are linked in the variation stage to enrich the searching behavior and enhance the diversity of the population.The performance of this algorithm is tested against the benchmark problems and then successfully implemented for design optimization of offset strip plate-fin compact heat exchanger.Results show that combined DE-GA-ASA method can be used effectively for optimal design of the plate-fin heat exchanger.Moreover,the effect of fin and heat exchanger parameters on the optimal design is also investigated.The input variables of plate-fin heat exchanger considered are fin pitch,fin height,fin offset length,hot stream flow length,cold stream flow length and no-flow length to obtain maximum heat transfer rate and minimum total annual cost.The investment cost and operating costs are independently optimized and a detailed study is carried out on the effect of variation of fin and heat exchanger geometry parameters.For the selection of final optimal solution,a multi-criteria decision-making method,Technique for Order of Preference by Similarity to Ideal Solution(TOPSIS)is implemented.Designing a compact heat exchanger and employing various techniques to enhance its overall performance has been widely investigated and still an active research field,however,few researches deal with thermal optimization.The use of VGs are widespread in compact heat exchangers because they can produce higher heat transfer with relatively less pressure drop.Here,numerical and optimization study on the plate-fin and tube fin heat exchangers with VGs is carried out.For the optimal design configuration,the computational fluid dynamics(CFD)data,artificial neural network(ANN)and a multi-objective optimization algorithm(MOA)are then combined to maximize/minimize the objective functions.By combining numerical simulation and optimization very useful and valuable results can be obtained.For the plate-fin heat exchanger,the effect of angle of attack,attach angle(angle between VG and wall)and shape of VG on the fluid flow and heat transfer characteristics are studied.Due to low Reynolds number(360-1140)the flow is assumed as laminar.Results are presented in the form Nusselt number and friction factor and various flow structures and temperature contours are examined.The importance of attach angle for various shapes of vortex generator is highlighted and results show that attach angle of 90° might not necessary for maximum heat transfer.An optimal design of VG for maximum Nusselt number and minimum friction factor is then proposed in the form of Pareto-front.The numerical and optimization study of corrugation height and angle of attack of delta type VG is then carried out in a wavy fin-and-tube heat exchanger.The SST-K? is used as the turbulence model due to its robustness in adverse pressure gradient.The numerical model is validated by comparing with the experimental results.The effect of delta winglets with varying corrugation height of wavy fin in three rows of tubes with staggered arrangements is investigated.CFD results show that employing delta winglets has augmented the thermal performance for all corrugation heights and superior effect is observed at a higher corrugation.Optimal wavy fin-and-tube heat exchangers with delta winglets are then designed by applying the same Pareto optimal strategy where CFD results,ANN and MOA are combined.Moreover,experimental study on the flat tube compact heat exchangers with and without louvered fins are also performed which are typically used in automotive air conditioning system.For three samples of heat exchangers,air side heat transfer and pressure drop performance are focused under wet surface condition.The three samples consisted of a finless heat exchanger(HX1),a flat type louver fin with 5 mm fin length(HX2)and a corrugated louver fin(HX3)with 6.1 mm fin length.The experimental results indicated that HX2 provides 15.3%higher heat transfer rate and 11%increased pressure drop compared to HX3.Numerical simulations are then performed for the tested heat exchangers considering dry surface condition.Comparing with the experimental results indicated that the sensible heat transfer rate per unit volume of wet surface is increased by 14%and 15%for HX2 and HX3,respectively.This is attributed to generation of streamwise vorticity by the condensate which has enhanced the flow mixing.Moreover,pressure drop for the wet surface condition is also increased by 18%and 21%,respectively,for HX2 and HX3.