This paper reports progress on a project to develop a design tool for large arrays of nucleation sites at specified locations to achieve high rates of cooling by pool boiling. The tool will be based on an improved version of a hybrid simulation, in which the 3-D temperature field in the wall is solved numerically, along with simple sub-models for bubble-driven heat transfer that require experimental calibration. Improvements to the computer code and progress with the experiments are reported briefly. The paper focuses on the development of a sub-model for the lateral coalescence of bubbles, which is shown to cause irregularity in the bubble production by a regular array of nucleation sites
