Turbulent flow in a model superheater

Abstract

An experimental study was conducted of the turbulent flow in a model superheater using laser velocimetry. The primary objective of the model study was to understand aerodynamic behavior of gas flow through a full-scale superheater, in order to help explain heat transfer and ash deposition characteristics of the superheater. The one-quarter scale superheater model used air at standard conditions to obtain model tube and duct Reynolds numbers as close as practical to full-scale. Two-component vector velocity data were obtained with a dual Bragg cell laser velocimeter. Measurements of mean velocity, turbulence intensity, and velocity-time data were obtained at points spanning the flow at many stations along the flow path around and between the tubes. The data have significant practical value, to show design and analysis engineers the general characteristics of flow in multitube and tube-bank heat exchangers. The flow into the superheater model was nonuniform, which is typical of full-scale superheater installations. Flow nonuniformity is expected to reduce the efficiency of the superheater but these experiments showed that the superheater flow became relatively uniform downstream of the first tube bank. Wakes occurring behind each row of tubes increased turbulence intensity, which is expected to increase ash deposition in real superheaters. A frequency analysis of the data revealed weakly periodic flow oscillations behind the first row of tubes in the first tube bank but none downstream. Data analysis also showed that there was no large-scale flow unsteadiness or surging. Turbulence intensities were very high throughout the tube banks, on the order of 30–40% of the local maximum mean velocities.