Critical Heat Flux in Non-uniformly Heated Tube under Low-Pressure and Low-Mass-Flux Condition

Abstract

In an actual boiling channel, e.g. boililer water-tubes, the circumferential heat flux is not uniform. Thus, critical heat flux (CHF) of non-uniformly heated tube becomes an important design factor of conventional boilers, especially for the compact water-tube boiler with tube-nested combustor. The small compact boiler is operated at low-pressure and low-mass flux condition compared with the large scale boiler, thus the redistribution of liquid film will strongly affect on the characteristics of CHF. In this investigation, non-uniformly heat flux distribution along the circumferential direction was generated by using the Joule heating of SUS304 tubes which had the wall thickness distribution. The heated length of test-section was 900mm, inner diameter was 20mm and outer diameter was 24mm. The center of the inner tube surface was shifted by ε=0, 0.5, 1.0, 1.5mm from the center of the outer tube surface. The heat flux ratio between maximum and minimum heat fluxes of these tubes correspond to 1.0, 1.7, 3.0 and 7.0, respectively. Experimental conditions were as follows ; system pressures were 0.3 and 0.4 MPa, mass flux range were 10-100kg/m2s, inlet temperatures were 30 and 80 deg. C. The experimental results showed an increase in the critical heat flux by the existence of the redistribution of the flow. These characteristics have been well explained by using the similar concept of Butterworth's spreading model.