Numerical analysis of the heat transfer of radiant tubes and the slab heating characteristics in an industrial heat treatment furnace with pulse combustion

For a heat treatment furnace, the precise study of the flow characteristics, combustion and heat transfer in the furnace is essential to know the temperature distribution in a slab during the heating process. In this context several different three-dimensional CFD (Computational Fluid Dynamics) models have been developed to simulate the combustion and heat transfer process in the furnace. These models were developed based on direct fired reheating furnaces with continuous fuel input. In this paper, a novel three-dimensional transient CFD model used to investigate the slab heating characteristics in the heat treatment furnace with pulse combustion was developed. The transient calculation in the furnace was performed based on the steady-state simulation of the furnace temperature, and the movement of slabs on the rollers in the furnace was simulated using dynamic mesh. By comparison, the results from the EDC with two different reaction mechanisms showed good consistency. However, the EDC with the two-step reaction mechanism required one fifth the calculation time of the EDC with the GRI-Mech3.0, and was thus the preferred model. In contrast to direct fired reheating furnaces, the outer radiant tube walls were treated as the coupled boundary conditions in the model to realize indirect heat transfer between the process gas in the furnace and the combustion gas in the radiant tubes. Furthermore, a pulse combustion method was proposed to control the working states of the burners based on UDF (User-Defined Function), in order to realize the pulse combustion process in the heat treatment furnace. By the validation of “Black Box”, the temperature variation of the slab and the gas temperature in the furnace predicted by the model agreed well with the experimental results. The simulation revealed that the temperature distribution of the radiant tubes and furnace gas was uniform and the maximum temperature difference on the outer radiant tube walls was less than 84 K. During the slab transient heating process, the slab temperature distribution became more uniform in the soaking zone and the maximum temperature difference along the thickness of the slab was below 13 K at discharge.