ABOUT DETERMINATION OF BOILING CURVE AND CHF FOR EXTREMELY HIGH HEAT FLUX DENSITY

摘要

Two forms of two-phase heat transfer mechanism may be regarded at subcooled flow under high mass velocities. The first is the process of conventional boiling (CB). It is observed in the part of q_(w) -(DELTA)T plane ((DELTA)T velence T_(W) -T_(S)). The second, which was called "pseudo-boiling" (PB) is observed at extremely high heat flux densities q_(w) (up to 250-280 MW/m~(2)). in small diameter tubes (0.4-2 mm) for high mass velocities and subcoolings. When wall temperature T_(w) achieves values substantially more than saturation temperature T_(s) q_(w) ((DELTA)T) curve for CB is closed to related part (sometimes practically coincide) of conventional pool boiling curve or its smooth expansion. So one may suppose that wall sublayer structure is governed of boiling process and forced convection provides heat removal to bulk flow only. Other situation occurs for PB regimes. Lines q_(w) ((DELTA)T) can't be related with conventional pool boiling curves and flow structure is controlled by vapor generation and condensation. The approach was used that convective heat transfer under turbulent flow conditions may be considered as a result of some cross flow from a bulk flow to wall vicinity and return in a core after its enthalpy increase in the wall layer. It is corresponded to so-called Reynolds flow conception. As a result new expressions for boiling curve and CHF are obtained for PB case.