[en] Highlights: • Heat transfer of spiral water wall tube in an ultra-supercritical boiler is investigated for fast response-characteristic. • The safe range of mass fluxes and heat fluxes is investigated. The tube metal temperature is safe from 30% to 90% BMCR. • The Fr number can give a convincing prediction on the effect of flow stratification in the tested pressure range 12–18 MPa. • Four selected heat transfer correlations are assessed against our experiment data obtained under designed operating loads. • Natural convection influences heat transfer in the low and pseudo-critical enthalpy region significantly. - Abstract: Nowadays, heavy peak-shaving task requires ultra-supercritical boilers having fast and flexible response-characteristic. Wide operating loads and quick load-change mode ask for better flow and heat transfer characteristics of the water wall. Under the above backgrounds, heat transfer to water flowing upward in an inclined smooth bare tube was experimentally investigated under the operating conditions of an ultra-supercritical boiler. The experimental results show that the metal temperature of the tested water wall tube is safe from 30% to 90% BMCR. The heat transfer coefficient of single-phase water at bottom side is 1–2 times bigger than that at the top. The Fr number put forward by Kefer et al. can give a convincing prediction on the effect of flow stratification in the tested pressure range 12–18 MPa. The calculated steam quality difference with the Kefer et al. correlation is an average-value prediction on axial length of the transitional region. At supercritical pressures, four widely used heat transfer correlations are evaluated with the experimental data. The comparison results demonstrate that Swenson et al. correlation gives the best prediction on heat transfer. The effect of flow stratification is evaluated by the modified correlation of Petukhov and Popov, which demonstrates that the effect of flow stratification can be neglected in the high enthalpy region.

[en] Highlights: • The flow instability of ultra-supercritical water in parallel channels was investigated. • Three regions of flow instabilities were found. • The effects of parameters on flow instabilities were analyzed. • The flow stability map of flow instabilities was derived. -- 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.