Critical Heat Flux in a Circumferentially Non-Uniformly Heated Tube

Abstract

Critical heat flux is an important design factor of boiling two-phase flow equipments, such as boiler, steam generator, so on. In actual boiling system, the water tubes were suffered from a non-uniform heat flux and a tube inclination. In this series investigation, in order to understand the CHF phenomena in inclined upward flow systematically, CHF experiment was conducted by using tube of 900, 1800 mm in heating length with several tube inclinations and 450 mm in heating length with vertical upward flow. On the basis of evaluation of CHF in vertical upward flow by using the film flow model, the entrainment was drastically increased with a decrease of heating length, because the entrainment due to boiling can not be neglected under high heat flux condition. The objective of this investigation is to systematically understand the influence of the tube inclination on CHF characteristics under such high heat flux condition. In this study, the CHF experiment was conducted by using a circumferentially non-uniform heated tube with the forced convective boiling system. The test section had 20 mm in inner diameter, 24 mm in outer diameter and 450 mm in heating length. In the non-uniform heated tube, the center of inner diameter was shifted by 1.5 mm from the center of outer diameter. The circumferentially non-uniform heat flux distribution was generated by the difference of the circumferential electrical resistance. The test section tube was set at arbitrary inclination angles. The obtained CHF was strongly influenced by the circumferential location of local maximum heat flux point and tube inclination. These CHF characteristics could be categorized by using the CHF ratio against the value of the vertical upward flow and the modified Froude number which took account of the influence of disturbance wave.