Chapter 2 Literature Review
2.2 Flame measurement techniques
Today, several techniques are available for accurately measuring the laminar burning velocity of gaseous mixtures at a wide range of initial pressure, temperature and composition conditions. Some popular and widely explored techniques are Flame tube, Soap bubble, Bunsen burner, Slot burner, Flat flame, Heat flux, expanding spherical flame, Stagnation flame, Counter-flow flame, Diverging channel method, etc. In the following sections, certain contemporary flame measurement methods are briefly discussed.
2.2.1 Burner method
This method falls in the category of stationary flames. Bunsen burner, nozzle/orifice burner, and slot burner are popular variants that anchor a conical flame to measure LBV.
This technique has been widely adopted for quite a long time owing to its simplicity. The main challenge is to anchor the flame for: (a) mixtures having high burning velocity at
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laminar flow conditions, (b) ultra-lean/rich mixtures may result in buoyancy affected flames or blow-off, and (c) mixtures at elevated pressures result in very short flames at high flow rates. Other common challenges are heat loss from the flame to the burner rim, flame interaction with ambient air, flame stretch effects leading to tip opening, etc.
A burner, flow regulator, and gas cylinders constitute the main essential elements of a generic burner setup, as presented in Figure 2-1. Gas is supplied to the burner with the help of a booster pump from the storage cylinder. A digital flow regulator is placed between the gas reservoir and burner to achieve desired flow rate to maintain the mixture composition. An exclusive mixer is used for attaining multicomponent mixtures depending on the requirement. The measured values of the flow rate and the surface area of the flame front are used to calculate the laminar burning velocity. It is very helpful in obtaining first-hand information on the LBV of any combustible mixture.
Figure 2-1 Schematic diagram of Bunsen burner setup and burning velocity definition based on the half cone angle
Advantages:
• The test rig is relatively inexpensive due to its simple configuration and experimental procedure.
• Less post-processing time.
• Effective real-time control over the flame.
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Challenges:
• Flame anchoring.
• Accurate determination of flame structure, cone angle, and surface area.
• Heat loss to surroundings and burner rim are significant at the bottom of the flame.
• Tip of the flame prone to stretch and thermo-diffusional effect.
• To achieve initial elevated pressure and temperature conditions.
2.2.2 Planar flame method
Figure 2-2 Schematic diagram of Flat flame burner setup
One dimensional planar flame is achieved using a flat flame burner, as shown in Figure 2-2. It is also referred to as the heat flux method. The burner plate design plays a crucial role in anchoring the flat flame over the burner plate. Uniform flow of premixed combustible mixture at the upstream of burner plate is obtained by allowing the gases to pass through flow straighteners such as honeycomb structure. In this method, a special heating provision in the plenum chamber preheats the unburned gases before reaching the burner plate. The key feature is to supply thermal energy by preheating the unburned gases to compensate for the heat loss from the flame to the burner. Later, using an interpolation technique, the LBV of an adiabatic flame is measured in this method. The post-processing is simple compared to other techniques.
Advantages:
• One dimensional flame anchored normal to the unburned gas mixture.
• Significant reduction in heat loss from the flame to burner plate.
• Free from the flame stretch.
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• Provision for probe intrusion for collecting chemical samples under special conditions.
Challenges:
• Fabrication of burner plate is challenging.
• Flame anchoring.
• To achieve laminar flow conditions at high pressures.
• Testing of liquid fuel mixtures.
2.2.3 Stagnation flame method
In this method, a premixed air-fuel mixture is passed through a nozzle at velocities higher than the expected laminar burning velocity for that particular mixture. The flow is directed towards a stagnation plate where the mixture flow decelerates to a stagnation regime upon reaching the plate surface. The counter-flow flame method is a special case of the stagnation flame method. A stagnation plate is replaced with a jet of similar flow characteristics and mixture composition. It impinges on the already established jet, and as a result, two planar flames are formed, as shown in Figure 2-3. This will create an adiabatic flame condition omitting the necessity of a hot plate and the associated wall reactions. At a specific height from the nozzle, the flame front gets stabilized. Here, the particle image velocimetry technique is popularly used to measure the unburned gas velocity fields, and from that, the strain rate and LBV are estimated.
Figure 2-3 Schematic diagram of Stagnation flame burner setup
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Advantages:
• Nearly one-dimensional planar flame is anchored.
• High flame speed mixtures can be tested.
• Measurements at elevated pressures and temperatures are possible.
Challenges:
• Fuel-air mixture in large volume is necessary for an experimental run.
• Flame experiences a positive stretch.
• Achieving two similar jets with identical flow properties.
• Complicated measurement techniques such as particle image velocimetry or Laser Doppler velocimetry are needed to measure the unburned gas velocity.
• Uncertainties associated with post-processing methods.
2.2.4 Spherically expanding flame method
In this method, air and fuel components are filled into a chamber sequentially and left ideal for achieving quiescent conditions. Two high voltage electrodes with a minimum gap are typically used to generate a spark to establish a flame kernel that propagates outwardly, consuming the fresh reactants. It extinguishes once it reaches the inner surface of the chamber. The pressure development inside the chamber and radial propagation of flame are captured. High-speed imaging techniques such as shadowgraph, etc., capture the flame front propagation. Pressure-time and radius-time data are processed to obtain laminar burning velocity using mathematical models developed based on underlying physics. Depending on the facility, the vessel used in this method can be operated in constant-volume or constant-pressure mode. Constant-pressure mode facilitates combustion at high initial pressure conditions. Typical test facility connections and propagating flame images can be seen in Figure 2-4 and Figure 2-5.
Advantages:
• A small quantity of fuel is sufficient for a single experimental run, given that the chamber size is relatively small. Liquid and gaseous fuels can be tested.
• Measurements at higher initial temperatures and pressures can be performed.
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Challenges:
• A sudden spike in chamber pressure during combustion in constant-volume mode.
• Prone to multiple flame instabilities, including buoyancy, thermo-diffusive, hydrodynamic, acoustic, etc.
Figure 2-4 Schematic diagram of Spherically expanding flame setup [32]
Figure 2-5 Shadowgraph images of expanding flame from a spark generated flame kernel
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2.2.5 Summary of Flame measurement techniques
The most common experimental techniques used for flame propagation measurements are the conical flame burner, stagnation/counter-flow flame, heat flux burner, and spherical flame methods. Each method comes with a set of merits and challenges. The steady flow systems (stagnation, heat flux, Bunsen) can achieve higher initial temperatures, as these systems can easily control residence time and temperature.
However, these devices are typically limited to pressures below 5, 10, and 15 atmospheres for stagnation flames [33], heat flux burners [34], and Bunsen burners [35]. In contrast, the spherical flame method is most suited for higher pressure measurements up to 71 atm [36]. The other systems require a steady flow with a relatively large volume of gas at high pressures. Burning rates typically increase with pressure, leading to high Reynolds number and flow instabilities in these systems and difficulty in cooling the burner and associated equipment. For these reasons, the outwardly propagating spherical flame is the best suitable method for laminar burning velocity studies at high pressure [33].