Experimental Research On Boiling Heat Transfer In Active Magnetic Regenerator In Magnetic Refrigerator At Room Temperature

Magnetic refrigeration(MR) is an environmentally friendly technology for refrigeration with its high efficient, which is currently being widely concerned. Theoretically operating cycle could be shorten when boiling is introduced to the core component of MR(active magnetic regenerator, AMR), thus leading to a greater cooling capacity. Boiling heat transfer is a high efficient mode. However, mechanism research of magnetic field on boiling and heat transfer and whether it can be used in magnetic refrigeration is limited at present. The innovation points in this paper is to study the influence of magnetic field on flow boiling and to explore the influence factors of fluid boiling under magnetic field, to sum up the influences by magnetic field on water and R134 a.In this paper, experimental platform using magnets from Sichuan University prototype and a visualization experimental section with mass flow meter was established to test water temperature, surface tension and viscosity before and after water go through magnetic field. Water bubble’s period, growth and detachment regular and detachment diameter was observed by camera. The wall temperature of flow boiling of R134 a was tested by thermocouples, wall superheat and the heat transfer rate of each phase was calculated.The results were as follows: under magnetic field, the viscosity of water decreasedabout 3% comparing to no magnetic field condition, and the surface tension decreased about 1% comparing to no mangentic field condition All stages of the bubble cycle had a certain degree of reduction. Magnetic field and super-cooling had greatest influence on detachment stage and growth stage respectively. When it was magnetized, the time when bubble appears was earlier comparing to no magnetic field condition, and the bubble detachment diameter was increased along with a higher detachment frequency. Also, the outlet water temperature is higher than the outlet temperature without magnetic field.For R134 a, a higher wall superheat was needed for boiling occurring and maintaining with magnets. In addition, heat transfer of R134 a was inhibited by magnetic field..Here came the conclusions: 1) the magnetic field enhanced the boiling heat transfer of water by reducing the viscosity of water, as a result, bubble appeared in advance with higher frequency of departure and shorter waiting time. 2) Magnetic field increased the bubble departure diameter by increasing the surface tension of the water and rising the temperature of liquid near heating wall, which was conducive to the initial nucleation of bubbles, leading to more nucleus of boiling in initial stage and larger diameter in departure stage of bubble. The heat transfer rate was enhanced. 3) From the average heat transfer rate it could be seen that flow boiling of R134 a was restrained by magnetic field for boiling occurring and maintaining. The degree of wall superheat with magnetic field was higher than that without magnetic field and heat transfer rate was lower comparing to no magnetic field condition. As the flow rate increased, heat transfer rate of single phase flow increased, while the heat transfer rate decreased as a result of boiling not fully developed in larger flow rate. 4) Though water boiling was enhanced in magnetic field but only negative pressure can realize boiling in room temperature. This is not conducive to system stability. However, the boiling heat transfer rate of R134 a was still far higher than single phase heat transfer rate even it was restrained in magnetic field, so boiling of R134 a can be used in magnetic field.