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.