Abstract:

Zeotropic mixed refrigerant is widely used in Joule-Thomson refrigeration cycles, and refrigerator performance is greatly affected by the composition shift of mixed refrigerant. Based on conservation equations,a mathematical model of mixed-refrigerant composition shift in gas-liquid two-phase flow is proposed in this paper. The deduction result shows that different velocity of gas and liquid in the two-phase flow is the necessary condition of local composition shift. Then, the composition shift principle is obtained, and with the composition of circulated refrigerant, local refrigerant composition is calculated and verified by a mixed-refrigerant shift experiment in the evaporator of Joule-Thomson refrigerator. The results show that in gas-liquid two-phase flow, if the jth refrigerant component fraction sharing in gas is greater than that in liquid, the local concentration will be less than the circulated concentration, vice versa. Therefore,the local concentration of low boiling point component is less than circulated concentration in Joule-Thomson refrigeration cycle, while that of high boiling point component is greater than the circulated one. Further analysis shows that circulated concentration of low boiling point component is greater than charged fraction, while that of high boiling point component is less than the charged fraction. Thus,in order to achieve the predetermined mixed-refrigerant composition, more high boiling point component needs to be injected into the system.