Bubble growth on a smooth metallic surface at atmospheric and sub-atmospheric pressure

Abstract

Bubble growth rate is one of the most important parameters required for the development of accurate mechanistic nucleate boiling heat transfer models. It is also very important for understanding the hydrodynamic forces and the mechanism of bubble departure. This paper presents an experimental study on bubble growth measurements in saturated pool boiling of deionized water on a plain copper surface at atmospheric and sub-atmospheric pressure. The measurements were conducted using a high-speed, high-resolution camera with a microscopic lens. The mechanisms of bubble growth are discussed, while the microlayer evaporation mechanism has been evaluated and discussed using the measured bubble growth curve. The estimated contribution of microlayer evaporation to a single bubble growth is about 70 %, while the contribution of latent heat transfer (evaporation) to the total heat transfer rate from the surface is about 30 %. The remaining 70 % is due to other mechanisms, i.e. conduction and convection. These values were obtained based on the analysis of the bubble growth curve only and agreed with some researchers who conducted local heat transfer measurements using integrated sensors or infrared thermography. These detailed measurement techniques cannot be used with the thick copper block tested in the current study, which was also tested by many researchers in literature and is representative of industrially used surfaces. It was also found that the bubble departure mechanism at atmospheric pressure is due to a static balance between surface tension and buoyancy forces while at sub-atmospheric pressure, it was between buoyancy and liquid inertia forces. The pressure did not have a significant effect on the characteristics of the dynamic contact angle, which was also measured from the instantaneous images of the bubble. It was concluded also that the force balance required for the accurate prediction of departure diameter should be conducted when the two forces are equal, which occurred at time less than the departure time and dynamic contact angle of about 45. In most bubble departure models, researchers recommended the balance to be conducted at the moment of departure when the bubble forms a neck with contact angle of 900 (underestimation to the surface tension force). The analysis of one of the commonly used homogeneous growth models indicated that for homogeneous bubble growth models to be applicable in nucleate boiling, an allowance must be made for the fact that the degree of superheat varies with time during a bubble growth period.