Summary

This work presents a theoretical and experimental analisys of a novel compact heat exchanger surface geometry for refrigeration applications. The so-called peripheral finned-tube evaporator is a cross-flow heat exchanger whose air-side is composed by a hexagonal arrangement of open-pore cells formed by radial fins whose bases are attached to the tubes and whose tips are connected to the peripheral fins. Each fin arrangement is made up of six radial fins and six peripheral fins forming a hexagon-like structure. The air-side fin configuration is composed of three levels of fin arrangement, each characterized by the length of radial fin and mounted with a 30o offset from its neighboring level. An experimental apparatus was used to measure the air-side pressure drop and the heat transfer characteristics in 5 evaporators prototypes as a function of the air flow rate and the heat exchanger geometric parameters, such as the radial length of fins, distribution and evaporator lenght. The test facility consists of an open wind tunnel connected to a water loop. A one-dimensional theoretical model based on the theory of porous media has been developed to predict the thermal-hydraulic behavior of the heat exchanger. The model incorporates the actual fin geometry into the calculation of the air-side porosity. The air-side permeability is calculated according to the Kozeny-Carman model with the particle diameter definition due to Whitaker. The correlations due to Whitaker (1972) and Handley and Heggs (1968) for the Nusselt number and due to Ergun (1952) and Montillet et al. (2007) for the friction factor have been implemented in the model. They have predicted the experimental data with an acceptable level of agreement, showing a maximum deviation of 10% for the heat transfer and 20% and 30% for the pressure drop, respectively. The optimum overall dimensions of the peripheral finned-tube evaporator have been determined for the limiting cases of constant wall temperature and constant wall heat flux based on a minimization of the entropy generation (due to fluid friction and heat transfer) on the air-side for a given air heat transfer rate.

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