The aim of this study is to provide a better insight into the heat transfer mechanisms involved in single bubble growth in forced convection. In a set-up with vertical upflow of demineralized water under saturation conditions special bubble generators (BGs) were embedded at various positions in the plane wall. Power to a BG, local mean wall temperature and high-speed camera recordings from two viewing angles were measured synchronously. An accurate contour analysis is applied to reconstruct the instantaneous three-dimensional bubble volume. Interface topology changes of a vapour bubble growing at a plane wall have been found to be dictated by the rapid growth and by fluctuations in pressure, velocity and temperature in the approaching fluid flow. The camera images have shown a clear dry spot under the bubbles on the heater surface. A micro-layer under the bubble is experimentally shown to exist when the bubble pins to the wall surface and is therefore dependent on roughness and homogeneity of the wall. The ratio of heat extracted from the wall to the total heat required for evaporation was found to be around 30 % at most and to be independent of the bulk liquid flow rate and heat provided by the wall. When the bulk liquid is locally superheated this ratio was found to decrease to 20 %. Heat transfer to the bubble is also initially controlled by diffusion and is unaffected by the convection of the bulk liquid.

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a b s t r a c tAn experimental setup was designed to perform nucleate boiling experiments in upward saturated flow conditions, in order to investigate the influence of vertically aligned vapor bubble nucleation sites on one another. Experiments were performed by activating two bubble generators, of which the inter-site distance can be varied by steps of 2 mm. Depending on mass flow rate, flow direction and heat fluxes to both bubble generators, nucleation sites have been shown to interact at any nucleation site distance. The results have shown two major trends. The first trend is caused by additional convection (not by bubbles) from the upstream bubble generator, BG2, to the downstream bubble generator, BG1, increasing its nucleation frequency and bubble detachment diameter. The influence of additional convection on bubble frequency and diameter diminishes with increasing inter-site distance, S, and initial bubble nucleation frequency at BG1. The influence of added convective heat is enhanced by increasing the liquid bulk flow rate. The second trend is seen when BG2 initiates its own bubble nucleation. Vapor bubbles that nucleate at BG2 and pass by BG1 in close proximity have an inhibitive effect on bubble nucleation at BG1. The effect of this inhibitive trend diminishes with increasing nucleation frequency at BG1. Because of the significance of the effect of hydrodynamic interaction on the number of active nucleation sites and bubble size at detachment, mechanistic modeling of nucleate flow boiling is expected to benefit from the quantifications presented in this study.

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Abstract. The main topic of this paper is the influence of vertically aligned nucleation sites on each other in upward flow boiling. A setup was constructed to facilitate vertical up-flow of deminiralized water under saturation conditions. The main test section is a glass channel with a set of vertically aligned bubble generators. Each bubble generator is operated independently, where power and wall temperature are registered and the vapour bubbles are visualized by a high-speed camera. During the experiments, the downstream bubble generator (BG1) power is kept constant, while the power fed to the upstream bubble generator (BG2) is incrementally increased. Two main trends have been identified. The first trend is dominated by added convection from one site to the other. Both bubble frequency and detachment diameter on BG1 increase with increased power fed to upstream BG2. This effect decreases with increasing inter-site distance and becomes more significant with increasing liquid flow rate. When vapor bubbles start nucleating from BG2, these vapor bubbles inhibit bubble nucleation BG1 and can even lead to deactivation of this nucleation site. This second trend is only weakly dependent on inter-site distance, since the inhibition originates from bubbles flowing past BG1 in close proximity.