Herman D. Haustein Mem. ASME Institute of Heat and Mass Transfer, Faculty of Mechanical Engineering, RWTH Aachen University, Aachen, NRW, 52056, Germany e-mail: haustein@wsa.rwth-aachen.de Alon Gany Professor Mem. ASME Fine Rocket Propulsion Lab, Faculty of Aerospace Engineering, Technion – Israel Institute of Technology, Haifa, 32000, Israel e-mail: gany@technion.ac.il Georg F. Dietze Mech. Eng. Faculty, Inst. of Heat & Mass Transfer, RWTH Aachen University, Aachen, NRW, 52056, Germany e-mail: dietze@wsa.rwth-aachen.de Ezra Elias Professor Mech. Eng. Faculty, Technion – Israel Institute of Technology, Haifa, 32000, Israel e-mail: merezra@technion.ac.il Reinhold Kneer Professor Institute Head Mech. Eng. Faculty Inst. of Heat and Mass Transfer, RWTH Aachen University, Aachen, NRW, 52056, Germany e-mail: kneer@wsa.rwth-aachen.de The Dynamics of Bubble Growth at Medium-High Superheat: Boiling in an Infinite Medium and on a Wall At high superheat, bubble growth is rapid and the heat transfer is dominated by radial convection. This has been found, in the case of a droplet boiling within another liquid and in the case of a bubble growing on a heated wall, leading to similar bubble growth curves. Based on an experimental parametric study for the droplet-boiling case, an empirical model was developed for the prediction of bubble growth, within the radial convection dominated regime (the RCD model) occurring only at high superheat. This model suggests a dependence of Rt 1/3 —equivalent to a Nusselt number decreasing over time (Nut 1/3 ), as opposed to Rt 1/2 —equivalent to a highly-unlikely constant Nusselt number, in most other models. The new model provides accurate prediction for both the droplet boiling and nucleate pool boiling cases, in the medium-high superheat range (0.26<Ste <0.41, 0.19<Ste<0.30, accordingly). By comparison, the new RCD model shows a more consistent prediction, than previous empirical models. However, in the nu- cleate boiling case, the RCD model requires the foreknowledge of the departure diame- ter, for which a reliable model still is lacking. [DOI: 10.1115/1.4023746] Keywords: droplet boiling, nucleate boiling, bubble growth, RCD model 1 Introduction This study explores the dynamics of bubble growth at high superheats in two cases of direct-contact rapid-boiling. The first case examined is that of a droplet boiling in an immiscible host liquid (three-fluid system). The second case, which has been well studied, is that of single-bubble nucleate boiling on a heated wall at saturation conditions (Figs. 1(a) and 1(b), accordingly). Both of forms of bubble growth are utilized in various applications, such as: heat exchangers, boilers, energy recovery systems, phase sepa- rators, flow agitators, and so forth. Both cases are examined under high heat-flux conditions, where the liquid is brought into a metastable, superheated state. When a sufficiently large nucleus is generated, the liquid has excess energy that fuels the initial stages of bubble growth that is later followed by additional, external heat transfer. This leads to rapid bubble growth, with a characteristic boiling time of the order of 1–100 ms. A good way for evaluating the superheat level is through the Stefan number (or normalized Jakob number), repre- senting the ratio of excess energy to latent energy required for boiling: Ste ¼ c p DT=L ¼ Ja q v =q l ð Þ. The Stefan number can obtain values between 0 and 1, and is used here as a scale for com- parison of different works. For the case of a droplet boiling within an infinite liquid me- dium at medium-high superheat, there are no known previous studies. At the upper limit of this range, numerous researchers have studied the subject of explosive boiling of a droplet, occur- ring near the temperature of homogeneous nucleation—Ste  0.5–0.8 [see Shepherd and Sturtevant [1] or Frost [2]]. While at the lower limit, the problem of quasi-steady boiling of a droplet, at little to no superheat (Ste  0–0.1), has also been comprehen- sively examined (see review in Ref. [3]). Early works on low superheat boiling, at atmospheric conditions, include those of Sideman and Taitel [4], later followed by studies at elevated pres- sures (see, e.g., Shimizu and Mori [5]). The picture that emerges from an extensive review of available literature, is that no experimental work has been conducted on bubble-growth in the range of superheat considered here (0.2<Ste<0.41). However, some theoretical/numerical work exists for the case of a two-fluid system: the growth of a vapor bubble within an infinite liquid medium at medium superheat (see, e.g., Mikic et al. [6], Theofanous and Patel [7], Prosperetti and Plesset [8], or more recently Lee and Merte [9]). The ability of this theory to predict in the three-fluid case (droplet boiling in an immiscible liquid) at high superheat is examined here. For the second case considered here, the growth of a single bub- ble on a wall in the nucleate boiling regime, the literature is Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received February 28, 2012; final manuscript received February 16, 2013; published online June 6, 2013. Assoc. Editor: Bruce L. Drolen. 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