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Visualization study on premature CHF during two phase flow instability in narrow, vertical, rectangular channel   HE Haisha, CAO Xiaxin1, HU Jian, LI Na, and YANG Peixun   1. College of Nuclear Science and Technology, Harbin Engineering University, Harbin, 150000, China (caoxiaxin@hrbeu.edu.cn)   Abstract:As one of the three major criteria for reactor design, the prediction of critical heat flux (CHF) is of great concern. Traditionally, CHF can be roughly divided into departure from nucleate boiling (DNB) and dryout according to exit equilibrium quality (xe). DNB mostly happens when xe<0, while dryout occurs when xe>0. Recently, however, another type of CHF, which is called premature CHF (PM-CHF), is observed during two-phase flow instability through high-speed photography. Although premature CHF is not as high as stable CHF, it can cause an abrupt rise of heating wall temperature, which may do much damage to heater. Plate type fuel assemmbly is widely used in research reactors and integration reactors because of its large cooling area and compactness. But few study focused on premature CHF in narrow rectangular channel. Therefore, visualization study on premature CHF during two phase flow instability for upward vertical flow in narrow rectangular channel heated from one side was experimentally performed. The flow channel is 700 mm long, 70 mm wide and 2.7 mm in gap. The coolant is deionized water. The premature CHF was achieved by rising heating power step by step in low inlet subcooling(15~34 K), low mass flux (117~337 kg/(m2•s)) and high heat flux (~177 kW/m2) under atmospheric pressure. Results showed that in the process of gradually increasing the heating power, small bubbles, bubble growth, bubble coalescence and intermittent flow can be successively observed at the channel outlet. During the intermittent flow, the fluid and vapor alternately passed through the channel. With the increase of heating power, the intermittent oscillation period got longer and longer and the amplitude of heating plate temperature got larger and larger. Eventually, the system got to the state of premature CHF. Besides, before the occurrence of premature CHF there existed the fluctuation of pressure drop and flow rate. Therefore, it is reasonable to monitor the fluctuation of pressure drop and flow rate to prevent premature CHF. A boundary map to prevent premature CHF is depicted in terms of dimensionless inlet subcooling number and dimensionless phase change number based on the present experimental configuration. Keyword:premature CHF; visualization; narrow; rectangular channel
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