Bubble behavior and departure bubble diameter of bubbles generated over nucleating cavities in flow boiling

Abstract

This paper presents the results of an experimental study of bubbles nucleating over cavities on a localized heater surface under a flow of subcooled water at low bubble frequencies. The experimental apparatus consists of a rectangular flow channel 50 mm wide x 3 mm high with a 10 mm square copper heating surface placed flush in the center of the bottom wall surface. A viewing window located directly above the heater surface provides a clear view of the cavities and the bubbles growing over them. A microscope equipped with a video camera is connected to a digital image grabber board and to a video cassette recorder. The maximum effective magnification attained is approximately 700X which allows a feature dimension recognition of 3 μm. Water at atmospheric pressure is circulated through a constant temperature bath equipped with heating and cooling options. The water temperature is kept between 45 and 80°C. The flow rate through the channel is measured by a rotameter and the flow is controlled with a hand valve. Pressure drop through the channel is measured using an inverted inclined U-tube manometer filled with water. The visual study indicates that, for the flow rates investigated, the departing bubble diameters can be as small as 5 μm over nucleating cavities which are 2-3 μm in diameter. The departure bubble diameters become larger as the flow rate is reduced. Experimental data is presented for departure bubble diameter as a function of flow velocity and cavity diameter. Some interesting features were revealed through the visual observation. The bubbles coming from upstream sites generally do not knock out the bubbles growing at a nucleation site. Rather, the oncoming bubbles follow a streamline around the nucleating bubble if there is enough room to go around. In some cases, bubbles seem to "walk" along the surface in the flow direction. The experiments were conducted at relatively low heater power levels to obtain bubble frequencies less than 500-1000 per second.