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Positioning of jet along the axis of a swirl jet is one of very commonly used application in the gas turbine combustors. Such combination leads to very different flame shapes depending upon the interaction between two concentric jets. Gas turbine combustion engineers thrive to obtain very high flame stability at low power conditions (idle); whereas low emissions are desired at high power conditions (Take off or Cruise). High flame stability can be obtained if the fuel coming out axial jet has least interaction with concentric jet flow. Such kind of flame are known as Jet type flames and denoted as Type 1 flame. On the other hand if fuel-air mixture coming out the central jet has good mixing with main flow, then recirculating type of flames are obtained. Such flames are denoted as Type 2 flame and have characteristics of low emissions due to very good mixing. In this work the occurrences of these very two different flame types (Type 1 and Type 2) has been demonstrated using both computational analysis and as well experimental techniques. Using a commercial solver Fluent, isothermal turbulent flow simulations are performed to understand the mixing process. The measurement techniques Laser Doppler Anemometry and OH* Chemiluminescence have been used to measure flow field and characterize the flame, respectively. Measurements performed at industry relevant conditions show that by having an equivalence ratio and inner axial jet momentum combination, desired combustion characteristics in the combustor can be obtained.
Burner used in this work is depicted in Figure 1. Multi-swirled nozzle has two coannular jets, a pilot jet in the centre and a main swirler placed coaxial to the central jet. Gaseous fuel is allowed to mix with pilot air flow as shown.
By changing the mass flow through the central jet and by keeping main flow constant, the flow field in the combustor can be obtained as shown in Figure 2. Top half of the figure shows streamlines resulting from Type 1 flame flow field, where postive axial velocities exist along the axis, which means axial jet is able to penetrate the recirculation zone. Bottom half streamlines show the flow field resulting from non-penetration of axial jet into the recirculation bubble, and recirculating flow exits along the axis (Type 2 flow field).
Figure 3 shows the iso-surfaces of zero axial velocities ( i.e. surface of recirculation zone) coloured with fuel distribution. It is clearly noticeable that fuel distributes evenly in Type 2, overall lower values of mole fraction is observed at recirculation bubble surface as compared to the case in Type 1 flow field.
As seen above with change in recirculation characteristics, the fuel distribution also gets affected which further leads to differences in flame shape and its positioning in the combustor as shown in Figure 4 and Figure 5. The flame shape changes from a jet flow to a recirculating jet flow in the combustor by changing the momentum of pilot jet. At large values momentum of pilot jet, Type 1 flame is observed due to penetration of jet into the recirculation zone.
It is demonstrated in this work that in order to fully characterize occurance of such type of flames, one has to take both air flow rate (momentum of pilot jet) as well as the stiochiometric ratio of the pilot mixture into account. As very high fuel rich mixture along the axis can lead to fuel-air distribution beyond flammability limits, flame stabilizes itself at higher radial locations as shown in Figure 6.
Effect of introducing swirl to the pilot flow on the flow field is also studied in this work. With some modifications the burner has been successfully tested with liquid fuel as well. Details regarding flames of Type 1 with liquid fuel can be found in Literature.