Quantitative prediction of overheating risk at the dry-out point for the cooling wall during dynamic processes considering uneven heat flux distribution and flow deviation

Abstract

As renewable energy deployment expands, the demand for thermal power plants to conduct deep peak shaving becomes more urgent. However, it increases risk of overheating tube burst in cooling walls. In view of uneven distribution of heat flux along height direction, a dynamic model of cooling wall is developed. The flow deviation existing within cooling wall augments fluctuation of metal wall temperature, thereby constraining load cycling rate. When flow deviation rates are 3 % and 6 % respectively, maximum power ramp rates are 1.5 % and 1.0 % Pe/min during 30 %-50 % THA loading up processes by adding cooling wall temperature as limiting condition. Under high vapor quality, heat transfer deterioration caused by dry-out is prone to occur. Flow deviation induces the downward displacement of dry-out position, thereby causing augmentation of heat flux at specific location. When flow deviation rate reaches 6 %, dry-out point experiences a descent of 3.0 m and the heat flux undergoes an increment of 479.2 W/m at 30 % THA. Meanwhile, the decrease of mass flow rate in the pipeline leads to a slight reduction in heat transfer coefficient. These two factors cause increase in inner wall temperature. Quantitative relationships between flow deviation and dry-out point height, as well as heat transfer coefficient, are obtained. On this basis, a mathematical equation for predicting metal wall temperature based on flow deviation rate is further constructed. When the vapor quality is 0.95, compared with average flow rate, the increase in wall temperature reaches 37.4 % caused by convection heat transfer, and the wall temperature rises to 397.8 °C.