Steady state post dryout experiments at low quality and medium pressure

Steady state post dryout heat transfer data have been obtained for vertical flow inside a tube of inside diameter 10 mm and length 920 mm. The data were obtained using the ''hot patch'' technique in which indirectly heated copper blocks brazed onto either end of the test section tube operate at above the critical heat flux. These hot patches stabilise the quench front beyond the tube, which may then be operated in post dryout down to relatively low heat fluxes and steam qualities. The range of data obtained was limited by the ability of the hot patches to prevent the quench front entering the tube. In upflow data were obtained at up to 200 kg/m2s at 20 bar and 1000 kg/m2s at 5 bar. Downflow data were restricted to 200 kg/m2s at 5 bar and 500 kg/m2s at 2 bar. In many runs it was possible to measure vapour superheat at the tube exit. Typical results are presented and their trends discussed. These trends may be explained qualitatively by assuming that in upflow the flow pattern is initially one of inverted annular flow which gradually breaks down into dispersed flow. In upflow, raising the pressure improved heat transfer along the whole length of the test section. Raising the flow rate also improved heat transfer. Heat transfer in downflow was very much worse than in upflow. The data are compared with several models and correlations. (author)