Numerical Prediction of the Transient Operation of a Gas-Fired Reheating Furnace

Abstract

Abstract The well-known “zone method”, treats thermal radiation in an enclosure in a global manner so that reliable estimates of this mode of heat transfer can be obtained in systems such as gas-fired furnaces. Consequently the paper employs a two-dimensional thermal radiation model in which both the height and length of the furnace was divided into surface and volume zones. An isothermal computational fluid dynamics simulation was used to estimate the relative mass flow rates, and hence enthalpy flows, into or out of each volume zone. This simplified approach was considered to provide a reasonable estimate of the appropriate inter-zone mass flows since the use of small-scale experimental, near ambient temperature models has been shown in the past to give useful data on flow related behaviour of combustion systems. The computing times resulting from the coupling of a multi-zone model with a single isothermal computation of the flows are relatively short so that the transient performance of the reheating furnace can be analysed. The mathematical model was thus used to predict the fuel consumptions and load temperatures in a gas-fired furnace heating steel bars to a nominal discharge temperature of 1250°C over a typical transient operating period of 8 hours including the start-up from cold. The influences of burner geometry as well as the effects of changes to the value of the roof set point temperature which was used to control the thermal input to the burners were studied. The effects of varying the position of the roof temperature control sensor relative to the burners and of changes in the roof construction were also investigated.