Boiling is a vital process in many applications from nuclear power generation to micro-electronic immersion cooling. Boiling is typically associated with high heat transfer rates but this can be further improved through the use of surface modifications such as cavities. In an industrial context, a better understanding of boiling surface enhancements may lead to more efficient designs, e.g. more energy generation from power plants or less energy usage in air-conditioning systems.

Preliminary study showing how vapour and temperature vary over a plate with structured cavities present. The plate itself is coloured according to the temperature with the vertical planes showing the vapour fraction (dark blue) of the bubbles growing within the cavities.

A numerical investigation was undertaken by Mitchell Whiting  as part of his Master’s degree under the supervision of Dr Marilize Everts  and Prof. Panagiotis Theodorakis (IFPAN, Poland) to determine how the shape and size of microcavities impacts the bubble dynamics of single bubble boiling. Both cylindrical and conical cavities were investigated with various combinations of cavity depth and radius. The computational fluid dynamics simulations were conducted using the open-source software OpenFOAM to model the boiling and associated bubble dynamics of the refrigerant R1234yf.

Cavities aid the boiling process by promoting nucleation (the initial formation of a bubble seed) and by retaining vapour after a bubble detaches. Improved nucleation implies more bubbles and therefore more heat transfer and more efficient operation. Vapour retention is important as it allows for a subsequent bubble to grow immediately after one has detached without waiting for nucleation to occur again, reducing the time between bubble departures.

The results showed that cavity depth was not significantly important when it came to the design of cylindrical cavities. However, for conical cavities the cavity angle (which is a function of the cavity depth and cavity radius) proved to be a key parameter in determining the cavity’s ability to retain vapour with wider cavity angles resulting in a weaker ability to retain vapour.

Cavity radius proved to be of importance for both cylindrical and conical cavities. Bubble size was observed to have a piecewise dependence on cavity radius with bubble departure size being constant for cavities thinner than some critical radius and wider cavities resulting in larger bubbles. For wide cavities, the bubble departure size was linearly dependent on cavity radius. The bubble departure size is of interest in the design process as it is an indication of how far apart to space the cavities such that the bubbles do not impede each other’s growth.

This research formed part of a larger numerical pool boiling study in collaboration with Dr Marilize Everts, Prof. Panagiotis Theodorakis, and Dr Johann van den Bergh in project ThermaSMART, a Horizon 2020 European Union-funded project. The research has been published in the following articles:

• Influence of cavity geometry on the bubble dynamics of nucleate pool boiling
• Nucleate Pool Boiling Bubble Dynamics for R32 and R1234yf on Machined Micro-Structured Surfaces

Bubble departure from a conical and cylindrical cavity with the thermal boundary layer shown in the background.