With the growing demand for clean energy, there has been a surge of interest in direct steam/vapour generation for solar power applications. These systems utilise solar energy to boil liquid directly, which can then be used for power generation or heating. However, a significant challenge in these systems is the non-uniform heat flux on the tubes through which the flow boiling occurs. This non-uniform heating poses challenges for direct steam/vapour generation, as the influence of non-uniform heating on flow boiling in tubes is currently not well understood.

An example of a parabolic trough solar concentrating plant, that can generate steam directly within the concentrator tubes ( Solarlite CSP Technology GmbH, SL TSE1 Print, CC BY 3.0)

In response to this need, research was conducted at the University of Pretoria by Andrew Martin for his Master’s thesis, under the supervision of Prof. Jaco Dirker. The study focused on how naturally occurring variations in the heat flux in solar applications affect the efficiency of flow boiling heat transfer. By exploring the intricacies of multiphase flow and heat transfer, this research aimed to understand, characterise, and pave the way for more effective utilisation of direct steam/vapour generation in solar power applications.

Images of vapour-liquid flow structures (flow patterns) observed in the horizontal tube during the experiments.

Flow boiling experiments were conducted with various non-uniform heating loads applied around the circumference of a horizontal tube. These experiments were designed to closely replicate the conditions found in solar power applications that employ refrigerants.

The experimental results revealed that a non-uniform heat flux significantly influenced both the local and average heat transfer in flow boiling. Under certain conditions, applying the heat flux non-uniformly enhanced thermal efficiency, while in other cases, it proved detrimental. The study also identified a link between the flow pattern and the non-uniform heat transfer characteristics of flow boiling.

For more information, the corresponding journal paper can be accessed here: https://doi.org/10.1016/j.applthermaleng.2023.122071