The influence of the length-to-diameter ratio on two-phase flow distribution and stability in microchannels under entrance effects

Entrance effects are a key factor governing the stability and flow maldistribution in parallel microchannels. In this study, a flow model that accounts for the entrance-region friction factor is developed to characterize how developing flow in the entrance section influences the pressure-drop-flow-rate characteristics of an individual channel and the operating point of the parallel system. On this basis, the role of entrance effects in regulating the stability range and flow distribution uniformity of parallel microchannels is systematically investigated. Furthermore, the length-to-diameter ratio is introduced as a critical geometric parameter to examine its influence on system behavior under different length-to-diameter ratio. The results show that decreasing the length-to-diameter ratio significantly increases the fraction of the entrance region relative to the total channel length, thereby strengthening the flow-resistance augmentation induced by entrance effects. Consequently, the instability range of the parallel system is effectively narrowed, inter-channel flow redistribution is suppressed, and the degree of flow maldistribution is reduced.