Book Description

Airflow through single- and multi-swirler arrangements for two swirler cup designs with radial-radial counter rotating vanes is computationally investigated using realizable k-e turbulence model on a grid ranging from 4 million (single swirler) to 36 million points (multi-swirler arrangements). Effect of swirler offset (distance between base wall of confinement and swirler exit plane) for a high swirl number (SN) swirler cup design arranged linearly with 0D, 0.02D and 0.04D offset, where D is swirler exit diameter is analyzed. Flow attaches to the base wall for lower offset conditions. Increasing offset leads to formation of distinct central toroidal recirculation zone (CTRZ) surrounded by jets with presence of recirculation between the adjacent jets and corner recirculation zone (CRZ) at walls. At constant offset, while the mass flow rate through each swirler is essentially the same, the flow field downstream of individual swirlers is quite different in a multi-swirler arrangement. At 0.02D offset a single swirler arrangement has a compact CTRZ with presence of a strong jet and CRZs whereas for the three swirler arrangement fluid entrainment into the central swirler jet leads to flow attachment to the base wall. For a five swirler arrangement the central swirler CTRZ length reduces. A 3x3 arrangement with all swirlers either arranged in a co-swirling or in a co/counter swirling pattern shows presence of a strong jet at each swirl cup along with formation of CTRZs for both arrangements. For co-swirling arrangement the CTRZs are longer in the axial direction with jet extending into the flow with much stronger velocity whereas alternate co/counter arrangement produces more swirler-to-swirler interactions. Changing offset to 0.31D, leads to formation of longer but narrower CTRZ with higher velocity gradient which can lead to better combustion performance. Deflection of the near wall swirler CTRZ is observed for alternate co/counter arrangement due to opposing flow from centermost swirler whereas these CTRZs are more or less symmetric in co swirling arrangement. Effect of flare geometry is studied for low SN swirl cup. Maximum positive axial velocity is comparatively higher while tangential velocity is substantially higher in presence of a flare very close to the swirler exit. For no flare case the tangential velocity variation is mostly due to the outer secondary flow as inner primary flow is rather weak resulting in overall weaker swirl that can adversely affect flame anchoring and combustion performance. Placement of a dummy nozzle at three different placement locations is analyzed. Without a nozzle a strong CTRZ is formed that extends inside the swirler. Placing the dummy nozzle in the region where the primary and secondary path flow merges is found to be the most advantageous as it leads to a strong and compact CTRZ that does not extend inside the swirler geometry. Results presented here show that small changes in a geometric feature of a multi-swirler array can lead to major differences in the resulting flow field. These factors should be carefully considered in design and testing of gas turbine combustors.