caloric regenerator; heat transfer and pressure drop; heat exchanger design; optimization; hydrodynamic and thermal developing region
Many researchers have used very small hydraulic diameters in regenerators of solid-state caloric cooling cycles, because smaller diameters can generate higher cooling capacity and system COP. However, using very small diameters hardly represent the real performance of the caloric cooling cycle, because they cannot be manufactured for a real system. Therefore, this paper has used more realistic hydraulic diameter which is used in commercial heat exchangers. To get the more accurate heat transfer coefficient and friction factor, the model used in this paper incorporates hydrodynamic and thermal developing regions which are usually neglected in other papers. This paper shows that COP is a function of heat transfer coefficient, pressure drop and displacement ratio. Higher heat transfer coefficient, lower pressure drop and the optimal displacement ratio can generate higher COP in magnetocaloric refrigeration cycle. From simple analytical comparison, it is expected that plate type regenerators can generate the highest cooling capacity and COP. The regenerator with smaller hydraulic diameter has higher performance, but 0.3mm hydraulic diameter has been chosen in this paper due to manufacturing limitation. In addition, this paper has investigated the effect of the length of the regenerator, the cycle frequency and the regenerator’s porosity on heat transfer phenomena in the regenerator. The cycle frequency has a large effect on cooling capacity and system efficiency while the effects of length and porosity of the regenerator are marginal. For the plate-type regenerator with 0.3mm hydraulic diameter, the system with 0.2m length of the regenerator, 0.45Hz cycle frequency and 0.5 porosity has the highest COP of 2.3 to generate 100W kg-1 cooling capacity for a temperature lift of 20°C. This paper provides detailed information of heat transfer phenomena in the solid-state cooling cycles, which need to be understood thoroughly in order to efficiently utilize caloric effects.