Optical diagnostics of flow boiling in a vertical miniaturised channel
File(s)
Author(s)
Chen, Zengchao
Type
Thesis
Abstract
Flow boiling of low-boiling-point dielectric fluids in miniaturised channels is one of the most promising solutions for highly-efficient heat dissipation in high-power-density electronic components. Despite recent advances, a fundamental understanding of the hydrodynamic and thermal interactions between the vapour and liquid phases in boiling flows remains lacking, due to the inherent complexity of boiling phenomena. This work presents an experimental study on flow boiling of HFE-7100 in a 5×5 mm vertical square channel using advanced, non-invasive, highly-spatiotemporally-resolved optical diagnostics.
A bespoke experimental apparatus was constructed for flow boiling of HFE-7100 in the miniaturised square channel with one wall heated and all four walls optically accessible. High-speed backlight measurements were performed to visualise various flow regimes and characterise bubble or liquid film dynamics in nucleate bubble, slug and annular flows.
A novel approach, referred to as single-dye multi-spectral planar laser-induced fluorescence, that harnesses the ratio of fluorescence intensities from two spectral bands of a single fluorophore, Nile Red, was developed and applied to obtain first-of-a-kind measurements of the liquid temperature fields in nucleate flow boiling. Bubble-induced thermal mixing was observed, in which bubbles interact with the thermal boundary layer and give rise to hot fluid that extends into the colder bulk flow, visible as thermal bulges, plumes, vortices and waves, contributing to enhanced thermal transport from the heated wall. In turn, thermally-induced bubble oscillation occurs, visible as bubble expansion and shrinkage, due to evaporation and condensation at the bubble bottom and top, respectively.
Through infrared measurements of the wall temperature fields, low-temperature areas were observed in bubble wakes. While the thermal effects of single isolated bubbles are limited, multiple consecutive bubbles exert significant influence on heat transfer from the heated wall. Compared to bubble evaporation, enhanced convection induced by bubble disturbance dominates the heat transfer enhancement mechanism in nucleate flow boiling.
A bespoke experimental apparatus was constructed for flow boiling of HFE-7100 in the miniaturised square channel with one wall heated and all four walls optically accessible. High-speed backlight measurements were performed to visualise various flow regimes and characterise bubble or liquid film dynamics in nucleate bubble, slug and annular flows.
A novel approach, referred to as single-dye multi-spectral planar laser-induced fluorescence, that harnesses the ratio of fluorescence intensities from two spectral bands of a single fluorophore, Nile Red, was developed and applied to obtain first-of-a-kind measurements of the liquid temperature fields in nucleate flow boiling. Bubble-induced thermal mixing was observed, in which bubbles interact with the thermal boundary layer and give rise to hot fluid that extends into the colder bulk flow, visible as thermal bulges, plumes, vortices and waves, contributing to enhanced thermal transport from the heated wall. In turn, thermally-induced bubble oscillation occurs, visible as bubble expansion and shrinkage, due to evaporation and condensation at the bubble bottom and top, respectively.
Through infrared measurements of the wall temperature fields, low-temperature areas were observed in bubble wakes. While the thermal effects of single isolated bubbles are limited, multiple consecutive bubbles exert significant influence on heat transfer from the heated wall. Compared to bubble evaporation, enhanced convection induced by bubble disturbance dominates the heat transfer enhancement mechanism in nucleate flow boiling.
Version
Open Access
Date Issued
2026-02-13
Date Awarded
2026-06-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
Markides, Christos N.
Sponsor
Imperial College London
Engineering and Physical Sciences Research Council
Grant Number
EP/T03338X/1
Publisher Department
Department of Chemical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)