Critical heat flux under flow oscillations and natural circulation conditions

Abstract

The critical heat flux (CHF) is a major parameter which determines the cooling performance and therefore the prediction of CHF with accuracy is of importance for the design and safety analysis in boiling systems, such as nuclear reactor, conventional boilers and other various two-phase flow systems. In these boiling systems, it has been observed that premature burnout would occur due to the flow instabilities. Moreover, the CHF correlations based on stable flow conditions(without flow instabilities)would overestimate the flow instability induced CHF. However, the CHF under flow instability conditions is not yet well understood. Therefore, the flow instability induced CHF has been studied to understand the effect of major parameters on the CHF and to predict the CHF under flow oscillation and flow excursion conditions with high accuracy. First, the effects of flow oscillations on CHF are investigated for water flow in vertical round tubes at low-pressure, low-flow(LPLF) conditions. An experimental study has been conducted to investigate the difference in CHF between forced and natural circulations, and between stable and oscillating flow conditions. Tests were performed with three (3) vertical round tube test sections (5.0mm ID × 0.6m long, 6.6mm ID × 0.5 long and 9.8mm ID × 0.6 long) for mass fluxes below 400kg/㎡s under near atmospheric pressure. It is found that flow oscillations reduce the CHF drastically, in particular for natural-circulation conditions. The correction factors for flow oscillation induced CHF are developed at forced and natural circulations separately. The correction factors predict the experimental data at LPLF conditions within 20% error bounds. Based on flow excursion(or Ledinegg instability)criterion and the simplified two-phase homogeneous model a mechanistic CHF model and a CHF formula for water are developed. The relationship between the CHF and the principal parameters such as mass flux, heat of vaporization, heated length-to-diame...