Delft-Jet-in-Hot-Coflow (DJHC) burner
Application Challenge AC2-11 © copyright ERCOFTAC 2021
The DJHC is a laboratory scale burner designed to mimic conditions found in relatively high-temperature combustion with low oxygen concentration oxidizers. A sketch of the geometry of the burner is given in Figure 1. In essence it consists of a central fuel pipe surrounded by an annular flow of vitiated oxidizer (coflow). The coflow is generated by a secondary burner. To prevent heating of the fuel by the hot coflow, in between the wall of the fuel pipe and the wall of the coflow channel a double annular channel with cooling air is present, air flowing upwards via outer annulus and downward via the inner annulus.
|Figure 1: Geometry of the DJHC burner (Oldenhof, 2012).|
The secondary burner consists of a ring of rich partially premixed flames forming. A small part of the air (24 nl/min) is fed to the ring burner flame, the rest of the air is injected via the air inlets at the bottom and bypasses the ring-burner (at the inner side and at the outer side). When unconfined, the flame created by the secondary burner has a length of about 0.4 m. But in the burner the flame is confined by the outer wall and by a redistribution grid placed horizontally in the coflow annulus. This grid is located 0.11 m upstream of the burner exit. It keeps the fuel pipe centered and plays an essential role in reducing the temperature of the coflow via convective and radiative heat loss. Figure 2 shows photographs of the injection holes of the ring burner (left), the flame created by the ring burner in absence of the outer wall (middle) and the view from above to the redistribution grid.
The configuration of the DJHC has similarities with the Adelaide burner (Dally et al., 2002) and the Cabra-Dibble burner (Cabra et al., 2005), two examples of burners with a central fuel jet surrounded by oxidiser coflows. The DJHC differs from the Adelaide jet-in-hot-coflow burner in the way the secondary burner of the DJHC is constructed so as to allow seeding particles to be added to the flow for velocity measurements using LDA or PIV. On the other hand, the Adelaide burner can operate at lower oxygen levels in the coflow (with a mass fraction as low as 3%) by the use of nitrogen addition. This addition of N2 is also used to control the temperature of the coflow.
|Figure 2: View of interior of the burner. Left: ring burner surrounding cooling pipe and outer ring of air injection holes. Middle: Flame from co-flow burner in unconfined conditions. Right: top view of burner showing the redistribution grid (De et al, 2011).|
Photos of the jet flames created by the combustion of the fuel jet with its surroundings for three different jet Reynolds numbers are shown in Figure 3. The flames have low luminosity compared to a standard lifted flame in cold air. More detailed specification of the cases is given in the section describing the test data. The photos show the gradual start of combustion in the mean but the real dynamics of the ignition process is not seen. The high-speed camera observations and the laser diagnostic measurements have revealed an ignition process centred around the presence of ignition kernels. (See Figure 4.)
|Figure 3: Visual appearance of the DJHC flames for three different jet Reynolds numbers. Red glowing interior wall of the coflow annulus and the protruding fuel nozzle are visible. Coordinate system with axial coordinate z and radial coordinate r is shown. (Oldenhof et al., 2010)|
|Figure 4: Snapshots of ignition kernels from high-speed camera observations (Oldenhof et al., 2010). The scale gives the position relative to the centre of the tip of the fuel nozzle. Difference between the subsequent pictures is 1 ms.|
Relevance to Industrial Sector
MILD (or Flameless) Combustion has been an important technology for industrial furnaces and is promising for other applications, such as gas turbines. The fundamental understanding of this combustion regime is not yet consolidated and experimental studies are key to enhance understanding and validate computational modelling.
The configuration usually called as Jet-in-Hot-Coflow offers several advantages and unique features for the study of Flameless Combustion. The use of vitiated gases in the coflow eliminates the need for aerodynamic recirculation of combustion products in a furnace volume, and it provides a good control of the local composition. By applying low O2 concentrations in the coflow, it became clear that this type of experiment could be suitable to reach the Flameless Combustion regime. The open configuration as opposed to a flame enclosed by walls is directly accessible for making detailed laser diagnostic measurements.
Jet-in-Hot-Coflow setups have many degrees of freedom as there might be several variations: fuel and coflow temperatures, ratio between fuel and coflow velocities, ratio between jet and coflow widths, fuel types, coflow composition, etc. For this reason, the overlap between different experimental setups reported in the literature is uncommon.
Differently from previous experiments of the same type, the DJHC allowed for velocity measurements and was the first to employ CARS (Coherent anti-Stokes Raman Spectroscopy) temperature measurements. Therefore, the experimental data provides a unique set of variables for modellers.
In summary the JHC burners are a tool to study the flame structure of industrial burners using various forms of flue gas recirculation into the reaction zone but they are not meant for to be used directly in industrial practice. It can be expected that conclusions based on the study of JHC burners have to be consolidated by follow-up studies on actual industrial burners. But for those less experimental data will be available.
Design or Assessment Parameters
The first design parameters that can be assessed in JHC burners is presence or absence of flameless combustion regime as function of fuel composition, oxygen concentration and temperature of recirculated flue gases and Reynolds number. Next comes the type of ignition (auto-ignition or flame propagation) and its consequences for flame stabilisation. And finally the absence of high temperature zones because of their disadvantageous effect on thermal NOx formation. To assess all that the CFD models should be capable to predict the flow patterns and the flame structure. In RANS and LES modelling this concerns the statistical properties of velocity and temperature (mean values, second moments, and also the probability of high temperature fluctuations). In validation studies the model performance should be investigated for different conditions, e.g. fuel-jet Reynolds numbers, as the models should be capable of capturing different operating conditions.
The application challenge here lies mainly in the study of the flow and flame properties downstream of the burner. The experimental setup comprised systems to measure velocity fields (Laser Doppler Anemometry and Particle Image Velocimetry), temperatures (Coherent Anti-Stokes Raman), OH* signal, and composition. The properties of the flow leaving the burner are directly influenced by the design of the burner and this is explained first. The burner was set in the vertical position to avoid bending of the flame away from the direction of the central axis due to buoyancy effects. The secondary burner, responsible for the vitiated gases of the coflow, created flames with a length of about 0.4 m. A grid was located 0.11 m upstream of the burner exit plane to keep the fuel pipe centred and to enhance the heat loss of the flow to the surroundings. During operation, the outer burner tube radiated most strongly at the height of the distribution grid, indicating that this cooling mechanism was effective. The central fuel pipe (with internal diameter 4.5 mm, as shown in Figure 1.) was cooled by constantly flushing air through the concentric cooling air ducts, thus preventing excessive heating of the main fuel jet. The main flow of air passed through the air inlets at the bottom and it is this flow that may carry the seeding particles for velocity measurements. In relation to the DJHC, the Cabra-Dibble burner has a larger diameter (0.21 m), and the flames generating the coflow are different in number and in type, namely 2200 pre-mixed jet flames. With that design, a very homogeneous temperature field is achieved. It is operated at higher oxygen levels than the DJHC burner, namely with an oxygen mole fraction of around 12% to 15% (oxygen mass fractions from 15.6% to 18.5%) and at high jet Reynolds numbers, 25,000 or more. The Adelaide JHC burner is similar in Reynolds number to the DJHC and generates oxygen levels between 9% and 3%. For a more systematic comparison of the different JHC burners in the literature we refer to Perpignan et al (2018).
A sketch of the geometry of the DJHC burner is given in Figure 1. Key dimensions are:
- Outer tube internal diameter: 82.8 mm
- Outer tube outer diameter: 89.2 mm
- Central fuel pipe internal diameter: 4.5 mm
- Central fuel pipe outer diameter: 5.0 mm
The fuel pipe extends 15.0 mm in axial direction in relation to the end of the outer tube. The fuel cooling tube, in which cooling air flows, envelops the fuel pipe up to an axial distance of 60 mm upstream of the end of the coflow tube. Towards its end the 22 mm diameter cooling tube is reduced in a conical shape until it merges with the central fuel jet outer diameter in 45 mm, as shown in Figure 1.
The DJHC is relatively small (coflow diameter 0.09 m, typical power consumption of around 20 kW). The central fuel jet (inner diameter 4.5 mm) enters an oxidant stream with high temperatures (with a maximum temperature between 1390 K and 1540 K) and a low oxygen mass fraction (typically between 7.6% and 10.9%). Fuel jet Reynolds numbers were about 3000, 4500, or 8500, while coflow Reynolds numbers are estimated to be between 1650 and 1820.
The important flow characteristics of the DJHC are i) jet spreading, ii) mixing between fuel and coflow, and iii) entrainment of ambient air into the coflow. Additionally, the accurate prediction of ignition delay is key for the overall behaviour.
- Cabra R, Chen JY, Dibble RW, Karpetis AN, Barlow RS. Lifted methane-air jet flames in a vitiated coflow. Combustion and Flame 2005;143:491-506.
- Dally BB, Karpetis AN, Barlow RS. Structure of turbulent non-premixed jet flames in a diluted hot coflow. Proceedings of the Combustion Institute 2002;29:1147-1154.
- Oldenhof E, Tummers MJ, van Veen EH, Roekaerts DJEM. Ignition kernel formation and lift-off behaviour of jet-in-hot-coflow flames. Combustion and Flame 2010;157:1167-1178.
- Oldenhof E, Tummers MJ, van Veen EH, Roekaerts DJEM. Role of entrainment in the stabilisation of jet-in-hot-coflow flames. Combustion and Flame 2011;158:1553-1563.
- Perpignan A.A.V., Gangoli Rao A., Roekaerts, D.J.E.M., Flameless Combustion and its Potential Towards Gas Turbines, Progress in Energy and Combustion Science 2018; 69: 28-62
Contributed by: André Perpignan, Dirk Roekaerts, E. Oldenhof, E.H. van Veen, M.J. Tummers, Hesheng Bao, Xu Huang — TU Delft
Contributed by: Jeffrey W. Labahn — Stanford University
© copyright ERCOFTAC 2018