The confined TECFLAM swirling natural gas burner

Application Challenge 2-06 © copyright ERCOFTAC 2004

Introduction

This document contains the specification of the application challenge proposed by Partner 17, MTU Aero Engines. The AC is defined by the standard burner for confined natural gas flames that was developed within the German TECFLAM co-operation. The aim of the research program was the establishment of an extensive experimental database from selective flames and the validation and improvement of mathematical combustion models.

Swirling flows are widely applied in technical combustion systems in order to achieve a fast mixing of fuel and air and to stabilise the flame by recirculation of hot combustion products. The correct mathematical simulation of these flames is, however, difficult and belongs to the important and challenging tasks in modern computational fluid dynamics.

In order to improve the experimental and theoretical knowledge of turbulent swirling flames the TECFLAM co-operation between several German universities and the DLR developed a burner for natural gas / air flames with a thermal load of typically 150 kW. The flames are confined by a water-cooled combustion chamber with good optical access for various measuring techniques. A number of well-defined "standard flames" have been investigated in identical copies of the burner at five different institutes with the goal to get an extensive and reliable quantitative characterisation of the flames. Great care was taken to ensure identical operating conditions of the flames and each research group contributed its specialised knowledge to achieve a high level of accuracy and confidence in the measurements. The results comprising velocity, temperature, mixture fraction, species concentrations and radiation provide a detailed insight into complex physical and chemical processes occurring in the combustion chamber and form a comprehensive basis for the validation of CFD codes.

Relevance to Industrial Sector

For gas turbine combustors flame stabilisation usually is realised by swirl induced recirculation. In particular for aero engine application current combustors all use swirl stabilised diffusion flames in order to guarantee an appropriate operating range and ignition performance. Swirl stabilised flames provide a high degree of stability at short flame lengths. An intensive turbulent mixing process which is originated by a curved free shear layer with large strain rates can be observed.

For the combustor designer the resulting flow field in particular the size and location of the recirculation zones are of great interest. However with current CFD tools the prediction of swirling flows is not always satisfactory. But not only the flow field and heat release prediction is of relevance for the design process, also the prediction of minor species like CO is of great importance in terms of overall emission predictions. As the formation of NOx-emissions is strongly dependent on the local temperature the global temperature field in the combustion chamber is another key issue for this type of swirling flow. This TECFLAM test case is of relevance for the validation of models predicting the interaction between turbulence and chemistry and consequently is of high interest for the gas turbine sector.

Design or Assessment Parameters

DOAP for this type of reacting flow are species and temperature profiles at given downstream locations. Beside these values of mean concentrations the distributions (i.e. temperature and species concentrations as a function of the mixture fraction) are also key features to asses the quality of a calculation. As usually the global emissions (NOx, CO, UHC, Soot) are key criteria for the final combustor layout emission concentrations in the exhaust could also be regarded as DOAP. On the other hand it is to mention that a reduction to that degree could be to strong as specific features of the flow field which i.e. has one ore more swirl induced recirculation zone due to vortex breakdown should at least also be captured.

Flow Domain Geometry

The swirl burner, shown in figure 1, consists of a central bluff body (diameter 20 mm), surrounded by one annulus of 3 mm width for the fuel (natural gas) and a second annulus of 15 mm width for the combustion air.

Figure 1: The TECFLAM natural gas swirl burner

The air flow is swirled by a movable block with a variable intensity between S = 0 and S = 2 in terms of a theoretical swirl number. The arrangement of this movable block swirler is shown in figure 2.

Figure 2: Movable block swirler configuration

The thermal load of the combustor amounts 150 kW. The combustion chamber walls are water cooled in the course of which approximately 50 % of the thermal power are transferred. The combustion chamber has a diameter of Dc=500 mm and a height of Hc=1200 mm. An annular slit for the exhaust gas is placed at the top. The burner can be moved vertically within the chamber like a piston by 500 mm in order to change the location of the relative measurement plane. It was experimentally checked that the height of the combustion chamber is of no significance importance for the flow field. The axial symmetry of the flame was also checked experimentally and confirmed. A sketch of the combustion chamber is displayed in figure 3.

Figure 3: Sketch of the combustor

Flow Physics and Fluid Dynamics Data

The characteristics of the overall flow field can be divided into three regimes: (1) the mixing zone between the fuel and the air streams where combustion takes place predominantly, (2) the inner recirculation zone around the flame axis, and (3) the outer recirculation zone.

The exit bulk velocities of the air and the fuel are 23 m/s and 21 m/s respectively. The corresponding Reynolds numbers are 42900 at the air flow inlet and 7900 at the (non swirled) fuel flow inlet.

Contributors: Stefan Hohmann - MTU Aero Engines

Site Design and Implementation: Atkins and UniS