Bubble characteristics in subcooled flow boiling of seawater

Abstract

Bubble characteristics are important to the two-phase flow dynamics and two-phase flow simulation. Different from the known slug flow in subcooled flow boiling of de-ionized water, fined bubbly flow and foam flow are found to be dominant in that of artificial seawater at high temperatures. Through image analysis, the present study conducts bubble characteristics measurements of such bubbly/foam flow of subcooled flow boiling in artificial seawater, including bubble Sauter mean diameters, bubble equivalent diameters and distributions of bubble sizes, bubble shapes and bubble orientations. The data set is compared with that in de-ionized water. The images for analysis are acquired from the high-speed camera with 4000 frame/s. The results indicate the Sauter mean diameters of de-ionized water could be larger than that of artificial seawater by 16–30%. Good agreements with two existing correlations prove the accuracy of current measurements and provide a way to predict the bubble characteristic in such fined bubble systems. Moreover, the results reveal the two-phase foam flow structure in artificial seawater is comparatively rigid and inflexible, compared to that in the de-ionized water. Increasing liquid temperature to 85℃ may keep the Sauter mean diameter of bubbles in artificial seawater at a stable value. The only peak of the unimodal bubble size distributions of artificial seawater is at 0.15–0.36 mm, while the possible other peak of bimodal bubble size distributions of de-ionized water is at 0.78–0.99 mm or 0.57–0.78 mm. Bubbles are mostly either nearly/perfectly parallel to the heated surface or perpendicular to the heated surface. Spherical shape for bubbles smaller than 0.2–0.25 mm, rigid ellipsoid shape for intermediate bubbles with diameter greater than 0.40 mm and less than 1.0 mm −2.7 mm, and distorted large bubbles, depending strongly on the liquid temperature, are observed. The data of bubble aspect ratio appear to be quite scattering due to the complicated nature of subcooled flow boiling. However, its general variation trend with Eötvös number or the product of Eötvös number and vapor Reynolds number agrees fairly well with some well-known correlations in the literature. Moreover, the bubble shape can also be predicted well with an extrapolated boundary of the Grace diagram on the plane of gas Reynolds number versus Eötvös number.