Nuclear Engineering and Design 237 (2007) 1016–1024 Simultaneous measurement of void fraction and fundamental bubble parameters in subcooled flow boiling Tomio Okawa ∗ , Hayato Kubota, Tatsuhiro Ishida Department of Mechanical Engineering, Osaka University, 2-1, Yamadaoka, Suita-shi, Osaka 565-0871, Japan Received 4 July 2006; received in revised form 14 December 2006; accepted 15 December 2006 Abstract Visualization was performed for the vapor bubbles in water subcooled flow boiling in a vertical heated tube to measure simultaneously the void fraction and the four fundamental bubble parameters: nucleation site density, bubble release frequency, bubble lifetime and bubble size. Using the mass flowrate and liquid subcooling as the experimental parameters, the changes of void fraction and bubble parameters with the wall heat flux were measured. The results of image analysis showed that the vapor void fraction could be approximated by the function of nucleation site density and bubble lift-off diameter; the bubble lift-off diameter was more influential than the nucleation site density. It was hence concluded that the bubble lift-off diameter could be regarded as the key parameter to determine the vapor void fraction under the present experimental conditions. The strong relation of bubble lift-off diameter to superheated liquid layer thickness was indicated for the future model development studies of bubble lift-off diameter. © 2007 Elsevier B.V. All rights reserved. 1. Introduction The vapor void fraction in water subcooled flow boiling is of significant importance in predicting the inception of two-phase instability and the onset of critical heat flux condition in boiling and pressurized water reactors (Saha and Zuber, 1974; Weisman and Pei, 1983). Furthermore, Bosma et al. (2004) and Wu et al. (2004) indicated that subcooled nucleate boiling occurring in core region of pressurized water reactors causes the accumu- lation of boron compounds on fuel surfaces that leads to an unexpected deviation in axial power distribution or axial offset anomaly (AOA). Primarily due to the considerable interest to the nuclear reac- tor technology, many models were developed to predict the void fraction during the subcooled flow boiling. Levy (1967) and Saha and Zuber (1974) calculated the axial void fraction distri- butions assuming the simple relation of true local vapor quality to the thermal equilibrium qualities at the point of interest and the point of net vapor generation (NVG); Levy (1967) considered the bubble detachment from heated surface as the criterion of NVG whilst Saha and Zuber (1974) interpreted that sufficient decrease ∗ Corresponding author. Tel.: +81 6 6879 7257; fax: +81 6 6879 7257. E-mail address: t-okawa@mech.eng.osaka-u.ac.jp (T. Okawa). of local subcooling initiates NVG at low mass flowrates. Lahey (1978) used the model by Saha and Zuber (1974) to deter- mine the point of NVG but calculated the axial development of true vapor quality using the empirical correlations for the rates of vapor formation and condensation. These models provided good predictions in many conditions of practical interest, though Kroeger and Zuber (1968), Lin-wen and Chin (1995) and Chang and Chapman (1996) reported severe limitations in some ther- mal hydraulic conditions. Multidimensional calculations were also carried out to predict the void fraction in subcooled flow boiling (Kurul and Podowski, 1990; Lai and Farouk, 1993; Tu and Yeoh, 2002; Kljenak and Mavko, 2006); these models are generally more mechanistic comparing with the conventional one-dimensional ones and consequently require detailed exper- imental information to evaluate the fundamental parameters characterizing the vapor bubble behavior such as nucleation site density, bubble release frequency and bubble size. Many experimental results were reported separately in literature for nucleation site density (Treshchev, 1969; Del Valle and Kenning, 1985; Zeng and Klausner, 1993; Basu et al., 2002), bubble release frequency (Von Ceumern-Lindenstjerna, 1977) and bub- ble size (Gunther, 1951; Zeitoun and Shoukri, 1996; Thorncroft et al., 1998; Prodanovic et al., 2002; Okawa et al., 2005a). These experimental data are useful in developing the mechanistic or phenomenological models but thorough understanding has not 0029-5493/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2006.12.010