High Momentum Flux Ratio Research Articles

Overall and Adiabatic Effectiveness Values on a Scaled Up, Simulated Gas Turbine Vane

Recent advances in computational power have made conjugate heat transfer simulations of fully conducting, film cooled turbine components feasible. However, experimental data available with which to validate conjugate heat transfer simulations is limited. This paper presents experimental measurements of external surface temperature on the suction side of a scaled up, fully conducting, cooled gas turbine vane. The experimental model utilizes the matched Bi method, which produces nondimensional surface temperature measurements that are representative of engine conditions. Adiabatic effectiveness values were measured on an identical near adiabatic vane with an identical geometry and cooling configuration. In addition to providing a valuable data set for computational code validation, the data shows the effect of film cooling on the surface temperature of a film cooled part. As expected, in nearly all instances, the addition of film cooling was seen to decrease the overall surface temperature. However, due to the effect of film injection causing early boundary layer transition, film cooling at a high momentum flux ratio was shown to actually increase surface temperature over 0.35 < s/C < 0.45.

Numerical study of wave disturbance in liquid cooling film

Transient numerical simulations are carried out to investigate the liquid-gas interface characteristics associated with liquid film cooling flows. A two-dimensional axisymmetric multi-phase numerical model using finite volume formulation is developed. The model has been validated against available experimental data for liquid-film cooling flows inside tubes. The model has been used to predict the interface characteristics for a variety of imposed parameters and momentum flux ratios under cold flow conditions wherein both the coolant and mainstream are maintained at the same temperature. Disturbance waves are observed at the liquid-gas interface for coolant flows above a critical value and after a finite distance from the inlet. The distance toward the wave inception point increased with the increase of momentum flux ratio. However, at higher momentum flux ratios, the properties of the disturbance waves did not vary significantly. The parameters related to the liquid-gas interface waves, namely, wave velocity, frequency, amplitude and wave length have been analyzed in detail. Analysis indicates that the liquid entrainment is due to the shearing of the disturbance wave crest.

Heat Transfer and Friction Augmentation in High Aspect Ratio, Ribbed Channels With Dissimilar Inlet Conditions

This work is an investigation of the heat transfer and pressure-loss characteristics in a rectangular channel with ribs oriented perpendicular to the flow. The novelty of this study lies in the immoderate parameters of the channel geometry and transport enhancing features. Specifically, the aspect ratio (AR) of the rectangular channel is considerably high, varying from 15 to 30 for the cases reported. Also varied is the rib-pitch to rib-height (p/e), studied at two values, 18.8 and 37.3. Rib-pitch to rib-width (p/w) is held to a value of two for all configurations. Channel Reynolds number is varied between approximately 3000 and 27,000 for four different tests of each channel configuration. Each channel configuration is studied with two different inlet conditions. The baseline condition consists of a long entrance section leading to the entrance of the channel to provide a hydrodynamically developed flow at the inlet. The second inlet condition studied consists of a cross-flow supply in a direction perpendicular to the channel axis, oriented in the direction of the channel width (the longer channel dimension). In the second case, the flow rate of the cross-flow supply is varied to understand the effects of a varying momentum flux ratio on the heat transfer and pressure-loss characteristics of the channel. Numerical simulations revealed a strong dependence of the local flow physics on the momentum flux ratio. The turning effect of the flow entering the channel from the cross-flow channel is strongly affected by the pressure gradient across the channel. Strong pressure fields have the ability to propagate farther into the cross-flow channel to “pull” the flow, partially redirecting it before entering the channel and reducing the impingement effect of the flow on the back wall of the channel. Experimental results show a maximum value of Nusselt number augmentation to be found in the 30:1 AR channel with the aggressive augmenter (p/e = 37.3) and a high momentum flux ratio: Nu/Nuo = 3.15. This design also yielded the friction with f/f0 = 2.6.

Comparative Study of Spray Characteristics of Gas-Centered and Liquid-Centered Swirl Coaxial Injectors

This study investigated the spray and Sauter mean diameter characteristics of gas-liquid swirl coaxial injectors by measuring and analyzing spray angles and mean drop sizes. Two different types of gas-liquid swirl coaxial injectors were designed and tested for this experiment: 1) a gas-centered swirl coaxial injector and 2) a liquid-centered swirl coaxial injector. The spray patterns were obtained for both types of injectors. The spray angles of these liquidcentered swirl coaxial injectors showed a decreasing trend as a function of the momentum flux ratio over the conditions studied. Good agreement was generally achieved between the predicted and measured spray angles. On the other hand, with increasing momentum flux ratio, the spray angle of the gas-centered swirl coaxial injector initially decreased when the momentum flux ratios were relatively low, but the trend reversed over the high momentum flux ratio range studied. The Sauter mean diameter data of these two types of injectors were also obtained using an image-processing method. Results showed that the Sauter mean diameter distribution of the liquid-centered swirl coaxial injector had a solid cone shape, whereas the gas-centered swirl coaxial injector exhibited two distinct Sauter mean diameter distribution regimes. The gas-centered swirl coaxial injector had a solid cone shape along the centerline, and it had a hollow cone shape at the periphery.

Flow visualisation experiments for turbine film cooling

AbstractFlow visualisation experiments related to turbine film cooling have been conducted. These investigated the fluid mechanics of coolant ejection using a large-scale, flat-plate model at engine-representative Reynolds numbers in a low-speed tunnel with ambient-temperature mainstream flow. The coolant trajectories were captured using a fine nylon mesh covered with thermochromic liquid crystals, allowing measurement of gas temperature contours in planes perpendicular to the flow. Three injection geometries were assessed: cylindrical holes with stream-wise injection, cylindrical holes with cross-stream injection, and fan-shaped holes. The data demonstrated that the cylindrical holes produced discrete jets, which lifted off the surface at high coolant-to-mainstream momentum flux ratios; these jets were characterised by the kidney-shaped stream-tubes expected for injection into cross-flow. The jets injected with cross-stream momentum exhibited a more obvious kidney-shaped cross-section, which rotated with distance downstream of injection. The jets from the fan-shaped holes were attached to the surface even at high momentum flux ratios, were more diffuse, and exhibited two cores of high temperature. The trajectory visualisation data were used to interpret the adiabatic cooling effectiveness measured at the surface.

Mixing Zone Optimization of a Rich-Burn/Quick-Mix/Lean-Burn Combustor

The mixing process of hot primary zone gases with secondary air must be rapid and intense for an advanced low NOX gas turbine based on a rich-burn/quick-mix/lean-burn (RQL) combustor. The injection of multiple jets normal into a confined crossflow is a key technology for this combustion concept. Therefore, an experimental investigation of a nonreacting mixing process of jets in a crossflow was conducted. The jets were injected through one stage of opposed rows of circular orifices into a slightly heated crossflow within a rectangular duct with no annular bypass. All geometries were tested with in-line and staggered arrangements of the centerlines of the opposed jets. Using the analogy of heat and mass transfer the temperature distribution was measured, and from that the mixing rate was determined for parametric variation of flow and geometric conditions. In accordance with the application to RQL combustion, emphasis was put on high momentum-flux ratios with high mass flow addition. The mixing process was found to be minimally affected by mainstream Reynolds number and mainstream turbulence, but significantly influenced by the addition of swirl to the mainstream. Correlations based on the experimental data were developed describing best mixing depending as a function of geometric conditions (duct height to hole diameter ratio, relative spacing of adjacent jets) and jet to crossflow momentum-flux ratio.

Adiabatic Effectiveness, Thermal Fields, and Velocity Fields for Film Cooling With Large Angle Injection

The film cooling performance and velocity field were investigated for discrete round holes inclined at an injection angle of 55 deg. Results are compared to typical round film cooling holes, with an injection angle of 35 deg. All experiments in this study were performed at a density ratio of DR = 1.6, using cryogenic cooling of the injected air. Centerline and lateral distributions of effectiveness were obtained for a range of momentum flux ratios. Thermal field and two component mean velocity and turbulence intensity measurements were made at a momentum flux ratio that was within the range of maximum spatially averaged effectiveness. Compared to round holes with 35 deg injection angle, the 55 deg holes showed only a slight degradation in centerline effectiveness for low momentum flux ratios, while a significant reduction in effectiveness was seen at high momentum flux ratios. The thermal field for the 55 deg round holes indicated a faster decay of cooling capacity for the 55 deg round holes. The high turbulence levels for the 55 deg round hole coincided with the sharp velocity gradients between the jet and free stream, and the decay of turbulence levels with downstream distance was found to be similar to those for a 35 deg hole.

Opposed round jets issuing into a small aspect ratio channel cross flow

Round jets (diameter D) discharging into a confined cross flow (dimension 3.16 D × 21.05 D) are investigated experimentally. Two configurations are considered: (1) a single jet (momentum flux ratio, J = 155) and (2) two opposed jets with two different momentum flux ratios ( J = 60, and 155). A two-component laser-Doppler anemometer is used to make a detailed map of the normal stresses and mean velocities in the symmetry plane of the jets. In addition, smoke-wire and laser-sheet visualization are used to study the flow. The rate of bending of the single confined jet is found to be higher than the rate of bending of an unconfined jet with the same momentum flux ratio. In the far field, the jet centerline velocity is observed to decay more slowly than the unconfined jet, indicating poor turbulent diffusion of linear momentum. Annular shear layer vortices are visualized on the upstream edge of the jet in the near field. In the far field, the flow visualization suggests that the jet loses its integrity and fragments into independent regions that are convected by the cross flow. In the opposed jet configuration at the high momentum flux ratio ( J = 155), the jets impinge in the center of the duct, and a pair of vortices is observed upstream of the impingement region. The flow visualization implies that the impingement vortices form quasi periodically and have a finite life span. In the impingement region, the jets are observed to penetrate alternately beyond the symmetry plane of the duct. In the two-jet configuration with J = 60, the jets do not impinge on each other owing to the higher rate of bending. Instead, the flow visualization indicates that the shear layers of the jets penetrate to the central region and periodically pinch off regions of the potential-like cross-flow fluid where they meet. The pinch-off regions of cross-flow fluid are convected by the turbulent flow for large distances, yet remain essentially unmixed.

Film Cooling With Compound Angle Holes: Heat Transfer

Heat transfer coefficients have been measured for film cooling injection from a single row of holes laterally directed with a compound angle of 60 deg. Two hole configurations were tested, round holes and holes with a diffusing expansion at the exit. Streamwise-directed round holes were also tested as a basis for comparison. All the holes were inclined at 35 deg with respect to the surface. The density ratio was 1.0, momentum flux ratios ranged from I = 0.16 to 3.9, and mass flux ratios ranged from M = 0.4 to 2.0. Results are presented in terms of hf/ho, the ratio of film cooling heat transfer coefficient to the heat transfer coefficient for the undisturbed turbulent boundary layer at the same location. Results indicate that for the streamwise directed holes, the heat transfer rates are close to the levels that exist without injection. Similarly, at low momentum flux ratio, holes with a large compound angle had little effect on heat transfer rates. However, at high momentum flux ratios, holes with a large compound angle had significantly increased heat transfer levels. The results were combined with adiabatic effectiveness results to evaluate the overall performance of the three geometries. It is shown that for evaluation of film cooling performance with compound angle injection, especially at high momentum flux ratios, it is critical to know the heat transfer coefficient, as the adiabatic effectiveness alone does not determine the performance. Compound angle injection at high momentum flux ratios gives higher effectiveness values than streamwise-directed holes, but the higher heat transfer levels result in poorer overall performance.

Prediction of opposing turbulent line jets discharged laterally into a confined crossflow

A numerical study is presented for the mixing of opposing heated line jets discharged normally or at an angle into a horizontal cold cross-flow in a rectangular channel. The k- ε turbulence model is adopted and the simulation is performed for the jet-to-cross-flow momentum flux ratio ranging from 0.42 to 5.42 and the incident angle from 60° to 90°. The results show that there is a strong recirculation near the downstream region of the nozzle opening, and the temperature field behaves like a deflected plume. The turbulent kinetic energy is high in the region where the vertical velocity gradient is steep. The vertical temperature profiles can be expressed as the self-similar forms. Correlations for the jet temperature and velocity trajectories, the penetration and circulation depths, the jet half-width, and the reattachment point are derived in terms of the momentum flux ratio, the downstream distance and the incident angle. As compared to the case of a one-side line jet, the opposing jets will hinder the vertical penetration but increase the horizontal velocity when the jets impinge on each other. Better thermal mixing can be achieved at higher momentum flux ratio and incident angle.

Measurement of opposing heated line jets discharged at an angle to a confined crossflow

An experimental study is presented for the discharging of opposing heated line jets at an angle to a confined crossflow. Parametric variations characterizing the mixing processes such as the velocity and temperature trajectories, and the circulation length, are correlated in terms of the momentum flux ratio, the incident angle, and the downstream distance. Measurements show that the turbulence level is high within the region where the mean velocity gradient is steep, and the vertical temperature profiles can be expressed in self-similar forms. Better thermal mixing can be achieved at higher momentum flux ratio and incident angle.

Concentration imaging measurements in turbulent concentric-jet flows

Raman scattering images were used to determine the CH 4 concentration in nonreacting concentric-jet flows. The flow consists of a central CH 4 jet and a concentric air jet separated by a bluff body. Three jet velocity ratios were studied under isothermal conditions. At high momentum-flux ratios, the CH 4 jet penetrates the recirculation zone formed by the airflow behind the bluff body. At low momentum ratios, the jet is stagnated and fuel is dispersed into the recirculation zone