Photobioreactors are used to increase microalgae productivity. As an improvement of the photobioreactor, the horizontal baffle is used to increase mixing capabilities. The phenomenon that occured using a horizontal baffle was bubble coalescence. A phenomenon of joining some bubble that affects mass transfer capabilities. This study aimed to reveal the effect of the horizontal baffle on the bubble size distribution and bubble behavior but only quantitatively. Bubbles distribution data was taken using a high-speed camera on three different vertical position processed by using ImageJ. Based on the result of the study, bubble coalescence occurred because of velocity difference, and the bubble was trapped, which led the bubble to coalescence. The higher position of the data retrieval shows that the more bubble with larger diameter because of the coalescence that occurs in the H/0.25, H/0.5, and below h/0.75. The bubble size 200-400 μm seems to be constant because of its not carried away by the wake of the larger bubble.
Keywords: Photobioreactor, Horizontal Baffle, Bubble Coalescence, Microalgae