A numerical analysis of the effect of heat recovery burners on the heat transfer and billet heating characteristics in a walking-beam type reheating furnace

The present study presents a numerical simulation of the effects of using self-recuperative burners on the performance of a walking-beam reheating furnace. The study was done using CFD (Computational Fluid Dynamics) simulations where a low computational cost method was implemented to simulate the billet heating as a steady state system. The preheating temperature of the air was defined as a function of the air mass flow and the flue gas temperature in each burner, using a UDF (User-Defined Function). The results of the billet heating profile were validated with experimental measurements in a furnace not utilizing heat recovery and showed good agreement with a maximum deviation of 54 K. Efficiency was found to increase from 32.7% to 48.5% with the use of self-recuperative burners. This result was reflected in a fuel consumption decrease of 31.3%, or an increase in furnace production of 51.3%. The heat transfer in the furnace and the billet heating characteristic were analyzed, which were observed to have changed with the different burners. This effect was lesser when the thermal input was decreased, and greater when the production was increased or when both remain constant. With and without heat recovery, radiation represents about 90% of the total heat flux on the billets. Due to the importance of radiation, solid and gas radiation were also analyzed. An improvement upon a methodology found in the literature was proposed. Using this new methodology it was found that, for the furnace evaluated, around 25% of the radiation absorbed by the billets comes from the flue gases. The effect of defined a constant billet emissivity on the results of the simulation was also discussed.