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Proceedings of the 27th National and 5th International ISHMT-ASTFE Heat and Mass Transfer Conference December 14-17, 2023, IIT Patna, Patna-801106, Bihar, India
December, 14-17, 2023, Bihar, India

Effect of Cyclic Exponential Heat Flux-based Power Transients on Bubble Coalescence in Nucleate Pool Boiling

Get access (open in a dialog) DOI: 10.1615/IHMTC-2023.890
pages 547-552


The present study investigates the effects of cyclic exponential heat flux-based power transients on nucleate boiling. For this purpose, the in-house CMFD solver utilises a sharp interface-based level set method on a co-located grid. Excursion time τ is reduced, and the resulting coalescence cycles are observed. No coalescence is observed for a steady heat flux of 10 kW. Coalescence does not occur for transient heat flux cases where excursion time is more than a critical value. When the excursion time is below the critical value, coalescence of vapour bubbles is observed under the effect of exponential power transient, which was not observed under steady heat flux conditions for similar time-averaged heat flux. This indicates that coalescence increases as excursion time is reduced. A unique case of periodic coalescence with identical coalescence cycles is observed when the excursion time is slightly lower than the critical value. Further reduction in excursion time breaks down the periodic nature of coalescence cycles, and the coalescence regime transitions to a chaotic state. As excursion time is reduced, the number of uncoalesced bubbles decreases. Below a particular value of excursion time, uncoalesced bubbles cease to exist and pure coalescence is observed. Vapour columns are observed due to a chaining effect of coalescence events. Hence, four boiling regimes with increasing coalescence are observed due to decreasing excursion time - no coalescence, periodic coalescence, chaotic coalescence & pure coalescence. For the first time in this research field, this work highlights that vertical coalescence patterns in boiling have multiple regimes transitioning from no coalescence to pure coalescence, with some regimes demonstrating periodicity in the coalescence pattern.