Comparison of the combustion dynamics of two hot blast stove designs using the flame transfer function

The forced response of two different hot blast stove burner designs is determined. This is done by forcing the flow by perturbations in fuel mass flow rate and monitoring the response in terms of the heat release rate. The resulting flame transfer function describes this response in terms of the gain and phase. A great deal of literature on the flame transfer function is available for gas turbine systems, which exhibit combustion dynamics in the 100-500 Hz range. Due to the large size of hot blast stoves, the coupling between combustion and acoustics in these systems normally occurs below 100 Hz. The hot blast stoves are characterised by lengths of 30 m and diameters of 5 m and are fired by the low-calorific exhaust gas of the blast furnace. The fluid domain of the stoves is reduced by means of symmetry in periodicity, in order to keep the computational cost within a feasible range. The reacting flow is simulated using the SAS-SST URANS turbulence model and a non-adiabatic, diffusion flamelet-based FGM model for combustion. To reduce the computational cost further, the flow is forced using a superposition of four sine waves at distinct frequencies. Results show two very different flame shapes, due to the difference in burner dimensions. The larger C1 produces a much longer flame than C2 with a more distributed heat release rate. The flame transfer functions similarly show differences: Burner case C1 is more responsive in terms of the gain over a larger frequency range than C2. The combustion time delay is also longer for C1 than C2: 50 ms to 30 ms, respectively.