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Compact heat exchangers are employed in many different applications because of their high surface area density. Plate-fin heat exchangers in particular are well suited for gas-to-gas and air-to-air recuperators and heat recovery units, among many other applications. In this thesis, constant property, fully or periodically developed laminar flows of air (Pr = 0.72) inside a variety of different inter-fin channels of plate-fin heat exchangers are studied computationally, with the goal of achieving better understanding of plate-fin heat exchangers and providing new designs with superior performance to the existing ones. Majority of plate-fin channels have rectangular, trapezoidal or triangular cross-sectional shapes. Their convective behavior for air flows is investigated and solutions and polynomial equations to predict the Nusselt number are provided. Besides the limiting cases of a perfectly conducting and insulated fin, the actual conduction in the fin is also considered by applying a conjugate conduction-convection boundary condition at the fin surface between partition plates. For the latter, new sets of solutions and charts to determine the heat transfer coefficient based on the fin materials, channel aspect ratio, and fin density are presented.Furthermore, while large fin density increases the heat transfer surface area, the convection coefficient can be increased by geometrical modification of the fins. To this end, two different novel plate-fin configurations are proposed and their convective behavior investigated in this thesis. These include (1) slotted plate-fins with trapezoidal converging-diverging corrugations, and (2) offset-strip fins with in-phase sinusoidal corrugations. The enhanced heat transfer performance of the plate-fin compact core with perforated fin-walls of symmetric, trapezoidally profiled, converging-diverging corrugations is modeled computationally. Air flow rates in the range 10=Re=1000 are considered in a two dimensional duct geometry described by the trapezoid inclination angle, the convergent-divergent amplitude ratio, the dimensionless corrugation pitch, and a surface porosity ß of 10%. The fin-wall flow transpiration is seen to promote enhanced heat transfer by inducing cross stream mixing, and periodic disruption and restarting of boundary layers. With uniform heat flux [H1] at the fin walls, an unusual performance is obtained where higher Nusselt number is accompanied with reduction in the corresponding friction factor, relative to a non-slotted geometry of the same dimensions. In the case of sinusoidal wavy offset-strip channels, the performance enhancement is evaluated for air flows in the range of 10=Re=1000, with fins at constant wall temperature [T], the effect of the wavy-fin amplitude, inter-fin spacing, and fin offset position on the thermal-hydraulic performance is reported. It is generally seen that S-shaped offset channels perform better than C-shaped ones. An average of 400% reduction in volume of a plate-fin heat exchanger can be achieved with S-shaped offset fins when compared to that with plain parallel fins. |
