Abstract
A simplified physical model of a single bubble growth during nucleate pool boiling was developed. The model was able to correlate the experimentally observed data of the bubble’s growth time and its radius evolution with the use of the appropriate input parameters. The calculated values of separated heat fluxes from the heater wall, thermal boundary layer, and to the bulk liquid gave us a new insight into the complex mechanisms of the nucleate pool boiling process. The thermal boundary layer was found to supply the majority of the heat to the growing bubble. The heat flux from the thermal boundary layer to the bubble was found to be close to the Zuber’s critical heat flux limit (890 kW/m2). This heat flux was substantially larger than the input heater wall heat flux of 50 kW/m2. The thermal boundary layer acts as a reservoir of energy to be released to the growing bubble, which is filled during the waiting time of the bubble growth cycle. Therefore, the thickness of the thermal boundary layer was found to have a major effect on the bubble’s growth time.
Highlights
Nucleate boiling is one of the most efficient heat transfer mechanisms
The thermal boundary layer acts as the main accumulator of energy from the heater surface before this energy is transferred to the bubbles and, to a certain extent, to the bulk liquid during nucleate pool boiling
The thermal boundary layer of superheated liquid was identified as the main contributor of the heat supplied to the bubble
Summary
Nucleate boiling is one of the most efficient heat transfer mechanisms. The reason for that is the phase change—evaporation of the working fluid, which causes the bulk liquid to maintain its temperature at or below the saturation temperature corresponding to the system pressure. We are well aware that these quantities change with the changing of the input heat flux Both Kim [11] and Liao et al [12] stated that a substantial contribution of the heat gained by a growing vapor bubble during nucleate boiling is through the dome of the bubble from the superheated liquid layer known as the thermal boundary layer. On the other hand, uses the bubble’s growth characteristics (growth rate and growth time) from experimental database in order to separate/estimate the contributions of the different regions (e.g., thermal boundary layer) of the working fluid to the total heat transferred to/from the bubble. The model involves multiple assumptions and simplifications, e.g., neglecting of the microlayer below the growing bubble, in order to keep the calculations relatively simple with low computational cost and to enable us to draw conclusions, which extend our fundamental knowledge of the nucleate pool boiling process. Fluids 2022, 7, 90 draw conclusions, which extend our fundamental knowledge of the nucleate pool boiling process
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