Experimental study of low pressure pool boiling of water from narrow tunnel surfaces

Abstract

This article aims at studying pool boiling of water at sub-atmospheric pressure which takes place on narrow tunnel structures (NTS). This kind of surface consists of multiple horizontal and vertical channels that can remain open, or be covered with perforated copper foil. The structures were immersed under 5, 28.7, and 86.2 mm of distilled water. The range of saturation pressure range was 0.75–4 kPa (corresponding to a temperature range of 2.8–28.9 °C), and that of imposed heat flux was 0.25–3 W/cm2. The process of bubble creation and detachment, the bubble departure diameters and the frequencies were analyzed for three different NTS structures. Visual observations were complemented by measurements of temperatures and pressures, which were used to determine heat flux, superheat, and heat transfer coefficients.The boiling curves at sub-atmospheric pressure were shifted towards higher superheats in comparison to atmospheric pressure (this in agreement with the literature). NTS surfaces reduced this negative effect. Indeed, the NTS structures enhanced with perforated copper foil substantially improved heat transfer at low pressures, i.e. they increased the heat transfer coefficient and the bubble departure frequency.