Thermal and Hydraulic Characterization of Arrays of Hook-Shaped Fins and Cavities for Enhancing Convective Heat Transfer

A popular method for augmenting convective heat transfer from surfaces is altering the flow field by adding extended features (fins) and/or cavities (dimples). The size, shape, interfin spacings and configuration of these arrays play a crucial role in their performance. Many studies have investigated ways to leverage novel manufacturing techniques to create different fin shapes to improve thermal-fluidic performance. NUCAP Industries has developed a technology that creates a unique array of hook-shaped raised features (hooks) and cavities (dimples) on metal surfaces (GRIPMetal). However, understanding the associated thermal and hydraulic performances of these newly developed arrays represents a distinct technical gap. Consequently, the primary objective of this work is to develop models and design tools that will allow engineers to design these GRIPMetal arrays with the optimum performance fitted into each application. This will be addressed through three specific objectives with a synergistic combination of experimentations and numerical simulations. First, the enhancement in heat transfer and the associated pressure drop for rectangular channels fitted with typical GRIPMetal designs is accurately quantified through experimentations. Two distinct testing facilities were designed, open-circuit wind tunnel and closed fluid loop, and constructed to carry out these experiments employing air and water as working fluids. The outcome of these experiments demonstrated that the presence of GRIPMetal arrays in rectangular channels promotes the convective heat transfer when compared to flat surfaces, however, with an inevitable increase in the required pumping power. The magnitude of the enhancement depends on the flow rate, tunnel height with respect to the hooks height but not the interfin spacings between hooks and their size. Nevertheless, more comprehensive investigations are required to complete the performance map of these arrays. Second, the overall Nusselt numbers and friction factors were calculated for the arrays, and empirical correlations were developed through nonlinear multivariable regression to act as design tools for fitting these arrays into real-life applications. Finally, a numerical model is developed using a commercially available CFD software package to simulate the flow across these arrays. The model will aid in providing better insight of the fluid flow and heat transfer characteristics associated with these unique arrays. Then consequently, optimizing their performance by exploring a wider range of geometrical configurations. The model was validated against the data obtained from the closed fluid loop. The model underpredicted the heat transfer from the array, while it overpredicted the incurred pressure drop; predictions worsen at higher Reynolds numbers. This is attributed to the incorrect resolving of the boundary layer and the lower estimation of turbulence intensity in the array as well as the distinction between the actual manufactured array geometry and the developed computational model. A spatial variation in the heat transfer coefficient of the array was concluded from the velocity and temperature distributions of the fluid flow.