Experimental investigation on flow instabilities of ultra-supercritical water in parallel channels

Abstract

An experimental study on the flow instabilities of ultra-supercritical water in the water wall tubes of an ultra-supercritical circulating fluidized bed (CFB) boiler was conducted. The material of the tubes is 1Cr18Ni9Ti. The scope of experimental parameters is as follow: pressure from 23 to 30 MPa, mass flow rate from 0.05 to 0.2 kg·s−1; inlet water temperature from 200 to 390 °C, inlet pressure drop coefficient from 0 to 5.5, and heat flux from 0 to 500 kW·m−2. Variations of mass flow rate and pressure in the parallel channels were recorded with the occurrence of flow instability, and oscillation curves of the wall temperature were obtained. The starting points and factors that affect various types of oscillations were analyzed to derive a stability map. Results show that pressure and mass flow rate display a reverse phase pulsation during oscillation. With the increase in heat flux, three regions of oscillations, namely, region1, region2 and region3, appeared. The oscillations in region1 and region2 are observed for the first time under ultra-supercritical pressure. Among them, oscillations in region1 and region2 are system oscillations, and they have long periods and large amplitudes. Oscillation in region3 belongs to density wave oscillation (DWO), which is the intertube oscillation and has a short period and small amplitude. Increases in system pressure, inlet mass flow rate, inlet pressure drop coefficient or decreasing the inlet water temperature are conducive to system stability.