Bubble nucleation site density, generation frequency and departure diameter in flow boiling of HFE-7100

Abstract

Bubble nucleation and dynamics can play a significant role in the nucleate boiling mechanism during flow boiling. Understanding the behaviour of nucleating bubbles at different operating conditions can help identify the control parameters that should be included in proposed heat transfer models and correlations. This paper presents an experimental work on measurements of active nucleation site density, bubble generation frequency and departure diameter during flow boiling of refrigerant HFE-7100 in a microgap heat exchanger. The microgap heat exchanger had a heated flat surface of 20 mm width, 25 mm length and an adiabatic transparent cover located 1 mm above the heated surface. This allowed direct flow visualisation using a high-speed, high-resolution camera of a relatively large observation area. The effect of heat flux, mass flux and system pressure on the active nucleation site density and bubble dynamics (frequency and departure diameter) was examined. All experiments were carried out at inlet sub-cooling of 5 K, inlet pressure of 1 and 2 bar, mass flux of 100−200 kg/m² s and wall heat flux up to 84 kW/m². The experimental results were then compared with existing models and correlations predicting nucleation site density, bubble generation frequency and departure diameter with limited success. The dominant parameters were also identified, and new correlations were proposed based on the experimental results. The results of the current work can help develop accurate prediction heat transfer models and encourage and enable researchers working in numerical modelling to consider nucleation from multiple sites, rather than simulating one single nucleation site.