Elongated bubbles in microchannels. Part II: Experimental study and modeling of bubble collisions

The collision of elongated bubbles has been studied along adiabatic glass microchannels of 509 and 790 μm internal diameters for refrigerant R-134a. The slug flow regime obtained here comes from the nucleation process inside a micro-evaporator located upstream. Using an optical measurement technique based on two lasers and two photodiodes, it was possible to determine the vapor bubble length distributions at the exit of the micro-evaporator and 70 mm downstream and thus characterize both diabatic and adiabatic bubble collisions. The database includes 412 coupled sets of distributions involving thousands of bubbles. Half of the database has been obtained under diabatic conditions and the second half under adiabatic conditions.A model for predicting the collision of elongated bubbles in microchannels (and their coalescence into longer bubbles) is proposed here and applied to the bubble length distribution at the exit of the micro-evaporator to determine the bubble length distribution along the glass microchannel. Presently, 81% of the entire database are predicted by the model with a tolerance of ±20%±20% on the lengths of vapor bubbles. The test of Kolmogorov–Smirnov is used to compare the experimental and theoretical distributions. Assuming an initial mean bubble length, it was possible to simulate the sharp peak in bubble frequencies observed experimentally, capturing both the location and magnitude of these peaks.