Bubble characteristics in time periodic evaporation flow of R-134a in a narrow annular pipe due to heat flux oscillation

In this work, bubble characteristics of periodic evaporation flow with refrigerant R-134a in a horizontal narrow annular pipe were examined experimentally in details. Attention is focused on the time periodic evaporation flow characteristics affected by the mean levels, amplitudes, and periods of the heat flux oscillation. The photos of the R-134a time periodic evaporating flow taken from the duct side are presented to show the change of the dominant two-phase flow pattern in the duct with the experimental parameters. The results show that at the low vapor quality, the bubbles get smaller with time and become less crowded in the duct in the first half of the cycle in which the R-134a heat flux decreases. The changes of the bubble characteristics with the instantaneous heat flux become more pronounced for an increase in the amplitude of the heat flux oscillation. At the very high mean vapor quality the bubble nucleation can be barely seen in the entire periodic cycle since the liquid film covering the heating surface is very thin. In addition, the duct flow is dominated by the annular two-phase flow at all time.