The critical mass flux for positive flow characteristic of heated tubes in the vertical water wall

Abstract

Positive flow characteristic is favorable in design of vertical water wall of large-scale once-through boilers, since it is beneficial for reducing potential temperature differences and thermal imbalance distribution across the water wall. The identification of the critical mass flux (G0) at which the flow characteristic transits from positive to negative is important for design of supercritical boilers with vertical water wall. In this paper, theoretical analysis was performed to investigate G0 under both supercritical and subcritical pressures. A theoretical solution of G0 was derived through thermal-hydraulic models. The impacts of operation parameters and tube structure parameters on G0 were quantified through numerical analysis. The results show that fluid outlet temperature, fluid pressure, tube inner diameter and tube friction factor have significant impacts on G0. G0 decreases as the increase of the fluid outlet temperature and the decrease of the fluid pressure. Under the condition of identical fluid inlet and outlet temperatures, operating pressure and tube length, G0 is proportional to D/λ. With the proposed model, G0 of a 600 MWe supercritical CFB boiler under different loads were calculated and validated.