Nuclear Engineering and Design ELS EVI ER Nuclear Engineering and Design 172 (1997) 137-155 Critical heat flux and flow pattern for water flow in annular geometry Jae-Wook Park, Won-Pil Baek, Soon Heung Chang * Department of Nuclear Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong-dong, Yusong-gu, Taejon 305-701, South Korea Received 23 February 1996; received in revised form 12 September 1996 Abstract An experimental study on critical heat flux (CHF) and two-phase flow visualization has been performed for water flow in internally-heated, vertical, concentric annuli under near atmospheric pressure. Tests have been done under stable forced-circulation, upward and downward flow conditions with three test sections of relatively large gap widths (heated length = 0.6 m, inner diameter = 19 ram, outer diameter = 29, 35 and 51 ram). The outer wall of the test section was made up of the transparent Pyrex tube to allow the observation of flow patterns near the CHF occurrence. The CHF mechanism was changed in the order of flooding, churn-to-annular flow transition and local dryout under a large bubble in churn flow as the flow rate was increased from zero to higher values. Observed parametric trends are consistent with the previous understanding except that the CHF for downward flow is considerably lower than that for the upward flow. In addition to the experiment, selected CHF correlations for annuli are assessed based on 1156 experimental data from various sources. The Doerffer et al. (1994); Barnett (1966); Jannsen and Kervinen (1963); Levitan and Lantsman (1977) correlations show reasonable predictions for wide parameter ranges, among which the Doerffer et al. (1994) correlation shows the widest parameter ranges and a possibility of further improvement. However, there is no correlation predicting the low-pressure, low-flow CHF satisfactorily. © 1997 Elsevier Science S.A. 1. Introduction The research on critical heat flux (CHF) has been extensively carried out during the last few decades for various geometries: round tubes, rod- bundles, annuli, rectangular channels, etc. A sig- nificant amount of work has been done for * Corresponding author. Fax: +82 42 8693810; e-mail: shchang@sorak.kaist.ac.kr annular channels in the nuclear context, because they are, essentially, a 'one-rod bundle' and may therefore be deemed to simulate more closely the rod bundle geometry than do round tubes (Hewitt, 1982). The investigation on the effect of the un- heated outer wall in an internally heated annulus also provides insights into the cold wall effects in rod bundles. Some of the early CHF correlations for rod bundles were developed based on the annulus CHF data (Jannsen and Kervinen, 1963). 0029-5493/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0029-5493(97)00032-0