Bubble characteristics during flow boiling at artificial and natural cavities
File(s)
Author(s)
Yang, Dapeng
Type
Thesis
Abstract
Flow boiling is an efficient heat transfer mechanism widely used in many applications. In the current work, a novel parametric experiment has been performed to investigate how the nucleation site geometry of artificial cavities, such as cavity depth, diameter and, for the case of two cavities tested, spacing, affect the ebullition dynamics during flow boiling. Understanding the effects of these parameters can help the design of improved flow boiling heat transfer surfaces.
The bubble dynamics at artificial cavities are compared to those at natural cavities to understand whether the underlying physical mechanisms for both sites remain the same. It was discovered that the bubble dynamics at artificial cavities develop differently to those at natural sites, an original finding. A different bubble growth and condensation mechanism is found at artificial cavities compared to natural cavities which might explain the observed disparities. The widely reported energy balance bubble growth mechanism at natural sites fails to predict the behaviour of bubbles at artificial sites. The current work found and further developed suitable models to predict the bubble dynamics at artificial cavities, something which has not been attempted before.
It was possible to fit the bubble growth profile at most single artificial cavities to a dimensionless power equation which uses the departure diameter and growth period. Two newly developed correlations provided the bubble ebullition frequency and its departure diameter. The departure diameter was accurately calculated from a novel force balance model originally suggested by Du, Zhao & Bo (2018) and further developed in the current work. As such, the bubble growth profile, departure diameter and frequency at an artificial cavity in flow boiling can be predicted based on the experimental boundary conditions, which is a new finding.
The application of the developed models to bubbles growing at a pair of cavities was also investigated. This parametric study showed that an optimum spacing and cavity depth exists which could be explored to achieve improved critical heat flux values or greater heat transfer at lower heat fluxes, which can assist the development of augmented flow boiling heat transfer surfaces.
The bubble dynamics at artificial cavities are compared to those at natural cavities to understand whether the underlying physical mechanisms for both sites remain the same. It was discovered that the bubble dynamics at artificial cavities develop differently to those at natural sites, an original finding. A different bubble growth and condensation mechanism is found at artificial cavities compared to natural cavities which might explain the observed disparities. The widely reported energy balance bubble growth mechanism at natural sites fails to predict the behaviour of bubbles at artificial sites. The current work found and further developed suitable models to predict the bubble dynamics at artificial cavities, something which has not been attempted before.
It was possible to fit the bubble growth profile at most single artificial cavities to a dimensionless power equation which uses the departure diameter and growth period. Two newly developed correlations provided the bubble ebullition frequency and its departure diameter. The departure diameter was accurately calculated from a novel force balance model originally suggested by Du, Zhao & Bo (2018) and further developed in the current work. As such, the bubble growth profile, departure diameter and frequency at an artificial cavity in flow boiling can be predicted based on the experimental boundary conditions, which is a new finding.
The application of the developed models to bubbles growing at a pair of cavities was also investigated. This parametric study showed that an optimum spacing and cavity depth exists which could be explored to achieve improved critical heat flux values or greater heat transfer at lower heat fluxes, which can assist the development of augmented flow boiling heat transfer surfaces.
Version
Open Access
Date Issued
2023-11-03
Date Awarded
01/12/2023
License URL
Advisor
Hardalupas, Yannis
Sergis, Antonis
Sponsor
China Scholarship Council
Imperial College London
Publisher Department
Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)