Determining boiling heat transfer and bubble diameter for water in deep minichannels

•Object of analysis: minichannels 5.5, 6.0 and 10 mm deep and 0.5–1.2 mm wide.•Boiling curves and heat transfer coefficients for distilled water.•Visualization studies: growing bubble diameter changes and bubble departure diameter.•Contact lines and contact surfaces determination for two cases of bubble detachment.•Determining the bubble departure diameter assuming a balance of four or five forces. The use of narrow channels or confined spaces is a relatively simple way for boiling heat transfer enhancement. The deep minichannels and the minifins surrounding them form a high heat capacity array that provides thermal stabilization and regulation to systems or devices which require transferring large heat fluxes. A bubble departure diameter is one of the basic parameters used to determine the heat flux and heat transfer coefficient theoretically. The existing correlations for the prediction of bubble departure diameter were developed for plain smooth surfaces and are not suitable for extended surfaces. Pool boiling heat transfer experiments were conducted for minichannels 5.5–6 and 10 mm deep and 0.5–1.1 mm wide using water at atmospheric pressure. Boiling visualization aimed to study phases of vapor bubble growth in the minichannel space and on minifin tops and measure the bubble departure diameter. The images were recorded using high-resolution photographs and high-speed imaging techniques across the entire surface of the samples. Contact lines and contact surfaces were defined for two cases of bubble detachment: from the space between the minifins bordering the channel and from the top of a single minifin. The bubble diameter at departure was determined analytically, adjusting for the balance of four or five forces acting on the bubble before departure. The computational model provided satisfactory accuracy for isolated bubbles in the 5.5–6 mm deep minichannels and heat fluxes in the range of 40–160 kW/m2.