Experimental Investigation on Heat Transfer Characteristics of Smooth Water Wall Tube of an Ultra-Supercritical CFB Boiler

Abstract

In this study, an experimental investigation was carried out to explore the heat transfer characteristics of the smooth water wall tube of an ultra-supercritical circulating fluidized bed (CFB) boiler. The ranges of the test pressure, mass flux, and heat flux were 23–32 MPa, 550–1200 kg·m−2·s−1, and 200–560 kW·m−2, respectively. The material of the tube used in the test was 12 Gr1MoVg. The diameter and wall thickness were 30 and 5.5 mm, respectively. The length of the test section was 2 m. The effects of the pressure, mass flux, heat flux, buoyancy, and flow acceleration on the heat transfer characteristics were analyzed. The formulas of the heat transfer coefficient were fitted, and the existing classical formula was used to evaluate the experimental data. The mechanism of heat transfer enhancement and deterioration of the tube were also investigated. Results showed that at the area of supercritical pressure, the wall temperature gradually increased with the increase of enthalpy in the pseudo-enthalpy region and sharply increased with the increase of enthalpy in the low-enthalpy region (enthalpy < 1200 kJ kg−1) and high-enthalpy region (enthalpy > 2400 kJ kg−1). This phenomenon indicated that heat transfer enhancement occurs near a pseudo-critical point. The increase of heat flux resulted in rapid heat transfer deterioration. Thereafter, the wall temperature rose immediately. The deterioration was delayed with the increase of mass flux and pressure. The effect of buoyancy and flow acceleration on the heat transfer concentrated on the pseudo-critical temperature of the fluid. Among the five selected heat transfer correlations, the Jackson and Bishop correlations agreed well with the experimental data.