Characterization of Low Frequency Combustion Dynamics of Hot Blast Stoves by Means of a Flame Transfer Function Based on CFD Forced Response Simulations: Comparison of Different Hot Blast Stove Designs

Abstract

The combustion dynamics of thermo-acoustic systems like gas turbine combustors at elevated pressure and atmospheric industrial furnaces can be studied using a forced response approach. In this approach, the flame is excited by external perturbation of the upstream fuel or air mass flow. The flame transfer function can then be determined, which describes the response of the heat release rate in the combustor or furnace to the upstream velocity fluctuations. Subsequently, the flame transfer function can be used as an input for acoustic network models to further analyze the stability behavior of a given combustion system. Most of the applications of the flame transfer function analysis are for natural gas fired systems with dimensions such, that most of the relevant combustion dynamics is in the frequency range 100–500 Hz. The situation is different for hot blast stoves as used in the iron making process. Here the fuel is low calorific coal gas and the dimensions of the stove are huge, with heights of 30 m at a diameter of 5 m. This leads to a relevant frequency range for the combustion dynamics in interaction with acoustics of about 3–80 Hz. In order to cope with this combination of a large computational domain and extreme low frequent combustion dynamics in the response simulation, special attention was devoted to computational efficiency. In order to allow for a sufficient mesh resolution to capture the combustion characteristics while keeping the computational demands in a feasible range, the computational domain is to be drastically reduced by the use of symmetry assumptions. In a first step, the mesh dependency is studied and different combustion models are analyzed for a reference geometry on the basis of steady states results. The burning velocity model with adapted laminar flame speed description is subsequently chosen for the transient simulations. Transient numerical simulations are performed using a URANS turbulence model. The combustor is excited by a multi-harmonic perturbation of the fuel mass flow, to further reduce computational time. The flame transfer function is determined and compared for two different burner designs. The results show significant impact of combustor design on the acoustic behavior and combustion time scales. While the reference design acts like a low pass filter with a cut-off frequency of about 6 Hz, the modified design shows band-pass filter characteristics with a lower and higher cut-off frequency of 30 and 60 Hz, respectively.