Int. J. Therm. Sci. 41 (2002) 609–618 www.elsevier.com/locate/ijts A photographic study on the near-wall bubble behavior in subcooled flow boiling ✩ Soon Heung Chang a,∗ , In Cheol Bang a , Won-Pil Baek b a Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong ,Yuseong-gu, Daejeon, 305-701, Republic of Korea b Korea Atomic Energy Research Institute, 150, Dukjin-dong, Yuseong-gu, Daejeon, 305-353, Republic of Korea Received 26 October 2001; received in revised form 8 February 2002 Abstract The behavior of near-wall bubbles in subcooled flow boiling has been investigated photographically for water flow in vertical, one-side heated, rectangular channels at mass fluxes of 500, 1500, 2000 kg·m -2 ·s -1 under atmospheric pressure. Primary attention was given to the bubble coalescence phenomenon and the structure of the near-wall bubble layer. The number of near-wall bubbles increased with the increase in the heat flux. At sufficiently high heat fluxes (>60–70% CHF), three characteristic layers were observed in the heated channel: (a) a superheated liquid layer with small bubbles attached on the heated wall, (b) a flowing bubble layer consisting of large coalesced bubbles over the superheated liquid layer, and (c) the liquid core over the flowing bubble layer. In addition, the existence of a liquid sublayer under coalesced bubbles was identified photographically. According to visualization, the CHF mechanism for the present experimental condition could be related to the formation of large vapor clots resulting from coalescences of bubbles and the evaporation of the superheated liquid layer beneath those clots.  2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: CHF; Liquid sublayer; Bubble; Flow structure; Visualization 1. Introduction Forced convective nucleate boiling is very effective in achieving a high heat flux with a small temperature dif- ference between the heated surface and the cooling fluid; however, there is a boundary of this effective heat transfer regime, called the departure from nucleate boiling (DNB). Reliable understanding of this DNB phenomenon is impor- tant for effective and safe operation of nuclear systems and other thermal-hydraulic equipment [1,2]. DNB is a transition of the heat transfer regime from nucleate boiling to film boiling or partial film boiling. This ✩ This article is a follow-up a communication presented by the authors at the ExHFT-5 (5th World Conference on Experimental Heat Transfer, Fluid Mechanics and Thermodynamics), held in Thessaloniki in September 24– 28, 2001. * Correspondence and reprints. E-mail addresses: shchang@mail.kaist.ac.kr (S.H. Chang), wpbaek@kaeri.re.kr (W.-P. Baek). usually involves the transition of the flow regime from bubbly flow to inverted annular flow. Detailed physical mechanisms leading to DNB, however, have not been clearly understood in spite of extensive research, mainly due to the difficulty in observing the near-wall region. Several investigators have tried to get rid of this difficulty by means of various flow visualization tests. Their works have focused mainly on bubble parameters within the bubble boundary layer and bubble behavior related to DNB occurrence. Gunther [3] investigated bubble growth-collapse process and measured maximum bubble size, population and growth rate, etc. It was also reported that when the local vapor film due to bubbles coalescence was formed, the CHF would occur. Jiji and Clark [4] investigated the bubble boundary layer and correlated its thickness. Tong et al. [5] identified the effect of mass flow rate on bubble size and correlated the bubble size for freon-113. In particular, they suggested the existence of a superheated liquid layer on the heated wall. Galloway and Mudawwar [7] suggested that one of the 1290-0729/02/$ – see front matter  2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII:S1290-0729(02)01354-6