Bluff body burner for CH4-HE turbulent combustion

Application Challenge 2-01 © copyright ERCOFTAC 2004

Description

The bluff-body burner, like the piloted burner, provides a flame suitable for the study of turbulence-chemistry interactions. Bluff-body burners also bear a great similarity to practical combustors used in many industrial applications. This geometry is, therefore, a suitable compromise as a model problem because it has some of the complications associated with practical combustors while preserving relatively simple and well-defined boundary conditions. The burner is centered in a co-flowing stream of air and generally consists of a circular bluff-body with an orifice at its center for the main fuel. A complex flow pattern forms downstream of the face of the bluff-body where one and possibly two recirculation zones are formed and these must produce enough hot gases to stabilise the flame to the burner. At sufficiently high fuel jet velocity, the flow penetrates through the recirculation zone and forms a jet-like flame further downstream, which is not unlike the piloted jet flame. The jet flame can be extinguished in a region downstream of the recirculation zone where turbulence is well developed and the finite rate chemistry effects are significant. The flame may also reignite further downstream where turbulent mixing rates are relaxed. It should therefore be noted that both the bluff-body jet flames discussed here consist, generally, of three main zones: stabilisation, extinction and reignition zones.

Introduction

The bluff-body burner here considered has been suggested and investigated experimentally by Masri, 1985. The burner is centred in a coflowing stream of air (perfect gas with k=1.4, R=287 J/kgK) and consists of a circular bluff-body with an orifice at its centre for the main fuel Figure 1. The diameters are 3.6 mm for the fuel nozzle and 50mm for the bluff-body. The main fuel jet composition is 50% CH4 and 50% H2 for a bulk Jet velocity of 118, 178, 214 m/s. The outer flowing air has a 40 m/s velocity and temperature of 300 K. The Reynolds number based on the air bulk velocity and the outer diameter of the bluff-body results in 141800. As described by Masri a complex flow pattern forms downstream of the face of the bluff-body where one and possibly two recirculation zones are formed and these must produce enough hot gases to stabilize the flame to the burner. At sufficiently high fuel jet velocity, the flow penetrates through the recirculation zone and forms a jet-like flame further downstream, which is not unlike the piloted jet flame.

Figure 1: picture of the bluff-body burner

The jet flame can be extinguished in a region downstream the recirculation zone where turbulence is well developed and the finite rate chemistry effects are significant. The flame may also reignite further downstream where turbulent mixing rates are relaxed. It should therefore be noted that, as observed by Masri the bluff-body jet flame discussed here consists, generally, of three main zones: stabilization, extinction and reignition zones.

Relevance to Industrial Sector

The bluff-body burner, provides a flame suitable for the study of turbulence-chemistry interactions. The bluff-body burner also bears a great similarity to practical combustors used in industrial applications such as gas combustors. This model geometry is, therefore, a suitable compromise because although it has some of the complications associated with practical combustors it is still preserving relatively simple and well-defined boundary conditions.

A complex flow pattern forms downstream of the face of the bluff-body where one and possibly two recirculation zones are. At sufficiently high fuel jet velocity, the flow penetrates through the recirculation zone and forms a jet-like flame further downstream. The jet flame can be extinguished in a region downstream of the recirculation zone where turbulence is well developed and the finite rate chemistry effects are significant. The flame may also reignite further downstream where turbulent mixing rates are relaxed. It should therefore be noted that the bluff-body jet flames discussed here consists, generally, of three main zones: stabilization, extinction and reignition zones which represent a relevant challenge for CFD simulations. Conversely from piloted flames, usually predictions for bluff-body flames are not yet satisfactory concerning the agreement with experiments. This is especially the case for the flame region starting about two diameters downstream the bluff-body. The inaccuracies are usually found regardless the numerical method used or the specific modeling constants.

Detailed chemical kinetics are needed to adequately compute the mass fraction of minor species such as OH and NO even in region where local extinction is not dominant such as in the recirculation zone.

Design or Assessment Parameters

The experimental data available were originally designed for combustion flame but are very attractive also for the validation of CFD code capability to capture the aerodynamic flame features and species composition. The parameters relevant in this regard are represented firstly by detailed parameters such as velocity and temperature profiles and mixture fraction in radial direction for different distance z in vertical direction. Morever scalar and kinetic energy profiles can be computed and compared with experimental for different jet Reynolds number i.e. jet bulk velocity. Measurements include species such as CO, CO2, H2, H2O, O2, N2, Hydrocarbon, as well as OH and NO radial distribution for different z level are finally available to test the chemical model and turbulence model. These values are relevant to evaluate the flame structure and the main region of pollutant emission production.

Stability characteristics of these flames are given in terms of the fuel jet velocity, u_j and the coflow velocity, u_e; and have been published elsewhere [1,10].