Critical heat flux in non-uniformly heated tube under low-pressure and low-mass-flux condition

Abstract

In an actual boiling channel, e.g., a boiler water-tube, the circumferential heat flux is not uniform. Thus, the critical heat flux (CHF) of a non-uniformly heated tube becomes an important design factor for conventional boilers, especially for a compact water-tube boiler with a tube-nested combustor. A small compact boiler is operated under low-pressure and low-mass-flux conditions compared with a large-scale boiler, thus the redistribution of liquid film strongly affects the characteristics of CHF. In this investigation, non-uniform heat flux distribution along the circumferential direction was generated by using the Joule heating of SUS304 tubes with the wall thickness distribution. The heated length of test-section was 900 mm with an inner diameter of 20 mm and an outer diameter of 24 mm. The center of the inner tube surface was shifted by e=0, 0.5, 1.0, 1.5 mm from the center of the outer tube surface. The heat flux ratio between maximum and minimum heat flux of these tubes corresponded to 1.0, 1.7, 3.0, and 7.0, respectively. The experimental conditions were as follows: system pressure at 0.3 and 0.4 MPa, mass flux of 10–100kg/(m2s), inlet temperatures at 30° and 80°. The experimental results showed an increase in the critical heat flux substantiated by the existence of the redistribution of the flow. These characteristics are explained by using a concept similar to that of Butterworth's spreading model. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(1): 47–60, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20095