Effect of Swirling Secondary Flow on the Under-expanded Non-circular Supersonic Jets

Effects of swirling secondary flow on the decay characteristics of non-circular under-expanded supersonic primary jets were experimentally studied. Primary jet is issuing from a convergent-divergent nozzle with different exit geometry such as circle, cruciform, triangle and rectangle. In this study, two different methods were used to impart swirl on the secondary flow as follows, (i) introduction of- angular vanes inside the secondary flow duct and (ii) using tangential inlets at the secondary flow duct. This swirling secondary flow influences the boundary condition of the primary jet and results in reduction of jet core length and shock train length, it also enhances the jet decay rate. Maximum shock train length reduction of 70 % is achieved when swirling secondary flow (tangential injection method) is introduced over the circular primary jet.

Gas Turbine Combustor Flow Structure Control Through Modification of the Chamber Geometry

As combustors are put in service, problems such as erosion, hot spots, and liner oxidation occur, and a solution based on lessons learned is essential to avoid similar problems in future combustor generations. In the present paper, a combustor flow structure control via combustor geometry alteration is investigated using laser Doppler velocimetry. Mainly, three configurations are studied. The first configuration is that of a swirl cup feeding a dump (rectangular cross section) combustor. The rectangular chamber is configured with a width to breadth (w/b) ratio of 85%. The second configuration is similar to the first one, but a combustion dome is installed. The dome is configured with a 9 deg difference in the expansion angle on both sides (asymmetric dome). The third configuration is that of a swirl cup and a combustion dome installed in a prototype combustor (single annular combustor (SAC) sector), with both primary and secondary dilution jets blocked. The SAC is configured with a cross sectional area that decreases toward the exit through the tilting of the inner combustor liner. The results show that the combustion dome eliminates the corner recirculation zone and the low velocity region close to the combustor walls. The combustion dome asymmetry results in a significant asymmetry in the velocity magnitude, as well as the turbulence activities and the tilting of the central recirculation zone (CRZ) toward the surface with the higher expansion angle. The liner tilting results in a 40% reduction in the length of the CRZ. However, once the primary jets are open, they define the termination point of the CRZ. The chamber w/b ratio results in a CRZ with the same diameter ratio (85%) in all configurations. Interestingly, the maximum reverse flow velocity is roughly constant in all measurement plans and configurations up to a downstream distance of 1R (R is the flare radius). However, with open primary jets, the CRZ strength increases appreciably. It appears that the confinement dictates both the flow field outside the CRZ and the size of the CRZ, while the swirl cup configuration mainly influences the strength of the CRZ. Regarding turbulence activities, the presence of the dome damps the fluctuations in the expanding swirling jet region. On the other hand, the primary jets increase the turbulence activities appreciably in the jet impingement region, as well as the upper portion of the CRZ (60% increase).

Influence of the Primary Jets and Fuel Injection on the Aerodynamics of a Prototype Annular Gas Turbine Combustor Sector

Transverse dilution jets are widely used in combustion systems. The current research provides a detailed study of the primary jets of a realistic annular combustion chamber sector. The combustor sector comprises an aerodynamic diffuser, inlet cowl, combustion dome, primary dilution jets, secondary dilution jets and cooling strips to provide convective cooling to the liner. The chamber contracts toward the end to fit the turbine nozzle ring. 2D PIV is employed at an atmospheric pressure drop of 4% (isothermal) to delineate the flow field characteristics. The laser is introduced to the sector through the exit flange. The interaction between the primary jets and the swirling flow as well as the sensitivity of the primary jets to perturbations is discussed. The perturbation study includes: effect of partially blocking the jets, one at a time, the effect of blocking the convective cooling holes, placed underneath the primary jets and shooting perpendicular to it. In addition, the effect of reducing the size of the primary jets as well as off-centering the primary jets is explained. Moreover, PIV is employed to study the flow field with and without fuel injection at four different fuel flow rates. The results show that the flow field is very sensitive to perturbations. The cooling air interacts with the primary jet and influences the flow field although the momentum ratio has a 100:1 order of magnitude. The results also show that the big primary jets dictate the flow field in the primary zone as well as the secondary zone. However, relatively smaller jets mainly influence the primary combustion zone because most of the jet is recirculated back to the CRZ. Also, the jet penetration is reduced with 25% and 11.5% corresponding to a 77% and 62% reduction of the jet’s area respectively. The study indicates the presence of a critical jet diameter beyond which the dilution jets have minimum impact on the secondary region. The jet off-centering shows significant effect on the flow field though it is in the order of 0.4 mm. The fuel injection is also shown to influence the flow field as well as the primary jets angle. High fuel flow rate is shown to have very strong impact on the flow field and thus results in a strong distortion of both the primary and secondary zones. The results provide useful methods to be used in the flow field structure control. Most of the effects shown are attributed to the difference in jet opposition. Hence, the results are applicable to reacting flow.

Influence of the Primary Jets and Fuel Injection on the Aerodynamics of a Prototype Annular Gas Turbine Combustor Sector

Transverse dilution jets are widely used in combustion systems. The current research provides a detailed study of the primary jets of a realistic annular combustion chamber sector. The combustor sector comprises an aerodynamic diffuser, inlet cowl, combustion dome, primary dilution jets, secondary dilution jets, and cooling strips to provide convective cooling to the liner. The chamber contracts toward the end to fit the turbine nozzle ring. 2D PIV is employed at an atmospheric pressure drop of 4% (isothermal) to delineate the flow field characteristics. The laser is introduced to the sector through the exit flange. The interaction between the primary jets and the swirling flow as well as the sensitivity of the primary jets to perturbations is discussed. The perturbation study includes: effect of partially blocking the jets, one at a time, the effect of blocking the convective cooling holes, placed underneath the primary jets and shooting perpendicular to it. In addition, the effect of reducing the size of the primary jets as well as off-centering the primary jets is explained. Moreover, PIV is employed to study the flow field with and without fuel injection at four different fuel flow rates. The results show that the flow field is very sensitive to perturbations. The cooling air interacts with the primary jet and influences the flow field although the momentum ratio has a 100:1 order of magnitude. The results also show that the big primary jets dictate the flow field in the primary zone as well as the secondary zone. However, relatively smaller jets mainly influence the primary combustion zone because most of the jet is recirculated back to the CRZ. Also, the jet penetration is reduced with 25% and 11.5% corresponding to a 77% and 62% reduction of the jet’s area, respectively. The study indicates the presence of a critical jet diameter beyond which the dilution jets have minimum impact on the secondary region. The jet off-centering shows significant effect on the flow field though it is in the order of 0.4 mm. The fuel injection is also shown to influence the flow field as well as the primary jets angle. High fuel flow rate is shown to have very strong impact on the flow field and thus results in a strong distortion of both the primary and secondary zones. The results provide useful methods to be used in the flow field structure control. Most of the effects shown are attributed to the difference in jet opposition. Hence, the results are applicable to reacting flow.

Experimental Investigation of the Flow Inside a Water Model of a Gas Turbine Combustor: Part 2—Higher Order Moments and Flow Visualization

The measurements of mean and turbulent quantities presented in Part 1 showed a strong influence of the primary jet system and evidence of the existence of bimodal distributions of the azimuthal velocity was also noted. Due to the importance of this phenomenon for combustor operation, a further study was carried out and measurements of higher order moments (skewness and flatness) were taken, followed by spectral analysis and high-speed flow visualization. These showed that, under the present flow conditions, the time behavior of the six radially impinging primary jets is similar to that encountered in single jet instability studies, with a dominant frequency corresponding to a Strouhal number of 0.27, when correlated with the primary jet characteristics. The unsteady nature of the flow around the primary jets and the high turbulence anisotropies observed suggest that accurate calculations of gas turbine combustor flows are likely to be impossible with models based on time-averaged version of the governing equations, even with closure at the second moment level.

The Flow Inside a Model Gas Turbine Combustor: Calculations

The present study assesses the ability of the k-ε turbulence model to calculate the flow inside gas turbine combustors. Results of calculations using a cylindrical system of coordinates, hybrid differencing, and a mesh with about 40,000 nodes are compared with velocity measurements of the flow inside a perspex model of can-type gas turbine combustor. The larger discrepancies between measurements and predictions were found in the primary region. The complexity of the flow near the primary jet impingement led to underprediction of the maximum negative axial velocity and turbulence kinetic energy by about 35 and 20 percent, respectively. The calculated results exhibited higher levels of momentum diffusion compared to the experiments and did not show the two contrarotating vortices created between the primary jets; no qualitative agreement with the azimuthal velocity downstream of the primary jets could be achieved. Despite these deficiencies, the model gave acceptable results in other regions of the combustor and correct prediction of the main features of the combustor flow was possible.

The Role of Primary Jets in the Dome Region Aerodynamics of a Model Can Combustor

The role of the primary jets in the aerothermal behavior and overall performance of a gas turbine combustor is explored through an experimental study. The study is performed in a model laboratory combustor that possesses the essential features of practical combustors. The test bed is designed to accommodate optical access for laser diagnostics and overall flow visualization, and is capable of incorporating variable inlet geometries. In the present case, the combustor is operated on JP-4 at atmospheric pressure. A parametric variation in the number of jets per row and axial location of the jet row is performed. The aerodynamic and thermal fields are characterized using laser anemometry and a thermocouple probe, respectively. Species concentrations are acquired via extractive probe sampling. The results demonstrate the importance of primary jet location with respect to the dome swirler. The percent mass recirculated into the dome region, as well as the overall uniformity of mixing and combustion efficiency, are substantially influenced by jet row location. The momentum ratio of the incoming primary jet stream to that of the approaching crossflow of reacting dome gases has a direct impact on the mixing patterns as well. An increase in the number of primary jets leads, in the present case, to more uniform mixing.

The Influence of Air/Fuel Ratio and Swirl Number on the Combustion Characteristics of a Model Combustor

Combustion characteristics of a model can-type combustor are reported for air/fuel ratios encompassing take-off and ground-idle conditions and for two swirlers with gaseous fuel and at atmospheric pressure. Temperatures were obtained with fine-wire thermocouples, and concentrations of UHC, H2, CO, CO2, O2 sampled through a water-cooled probe with a flame ionization detector, a gas chromotograph and infrared and paramagnetic analyzers. The results indicate combustion efficiencies greater than 97%. For the air/fuel ratio corresponding to that of take-off, 45% of the mass flow of the primary jets turned upstream after impingement, combustion occurred in the wakes of the swirler and of the primary jets resulting in a pattern factor of 0.46. At ground-idle condition, 58% of the primary jet flow turned upstream reducing the pattern factor to 0.43. An 18% reduction in the swirl number at the ground-idle condition led to a pattern factor to 0.35 and, for takeoff, to a pattern factor of around 0.37 with combustion occurred mainly in the intermediate zone.

The Role of Primary Jets in the Dome Region Aerodynamics of a Model Can Combustor

The role of the primary jets in the aerothermal behavior and overall performance of a gas turbine combustor is explored through an experimental study. The study is performed in a model laboratory combustor that possesses the essential features of practical combustors. The test bed is designed to accommodate optical access for laser diagnostics and overall flow visualization, and is capable of incorporating variable inlet geometries. In the present case, the combustor is operated on JP-4 at atmospheric pressure. A parametric variation in the number of jets per row and axial location of the jet row is performed. The aerodynamic and thermal fields are characterized using laser anemometry and a thermocouple probe respectively. Species concentrations are acquired via extractive probe sampling. The results demonstrate the importance of primary jet location with respect to the dome swirler. The percent mass recirculated into the dome region, as well as the overall uniformity of mixing and combustion efficiency, are substantially influenced by jet row location. The momentum ratio of the incoming primary jet stream to that of the approaching crossflow of reacting dome gases has a direct impact on the mixing patterns as well. An increase in the number of primary jets leads, in the present case, to more uniform mixing.