Numerical Investigation of Turbulence Models for a Superlaminar Journal Bearing

With rotating machineries working at high speeds, oil flow in bearings becomes superlaminar. Under superlaminar conditions, flow exhibits between laminar and fully developed turbulence. In this study, superlaminar oil flow in an oil-lubricated tilting-pad journal bearing is analyzed through computational fluid dynamics (CFD). A three-dimensional bearing model is established. CFD results from the laminar model and 14 turbulence models are compared with experimental findings. The laminar simulation results of pad-side pressure are inconsistent with the experimental data. Thus, the turbulence effects on superlaminar flow should be considered. The simulated temperature and pressure distributions from the classical fully developed turbulence models cannot correctly fit the experimental data. As such, turbulence models should be corrected for superlaminar flow. However, several corrections, such as transition correction, are unsuitable. Among all the flow models, the SST model with low-Re correction exhibits the best pressure distribution and turbulence viscosity ratio. Velocity profile analysis confirms that a buffer layer plays an important role in the superlaminar boundary layer. Classical fully developed turbulence models cannot accurately predict the buffer layer, but this problem can be resolved by initiating an appropriate low-Re correction. Therefore, the SST model with low-Re correction yields suitable results for superlaminar flows in bearings

Investigations of film cooling and its nonuniform distribution for the conjugate heat transfer passage with a compound inclined angle jet

Film cooling effectiveness and its distribution characteristics on the gas turbine blade surface were investigated numerically. Seven rectangular ribs were located on the internal wall, and a film cooling hole had a compound inclined angle. The case with a compound angle of 45 degrees was designed to improve the lateral film cooling effectiveness on the downstream surface. The secondary flow jet from the internal channel was affected by the flow velocity ratios, and an investigation of the external film cooling was considered for various values of thermal conductivity and heat conduction. Two different plate materials were used to analyze the effect of plate thermal conductivity. The results showed that the compound angle of 45 degrees causes an asymmetric distribution of the film cooling effectiveness. However, the bending effect can be of advantage to increase the film cooling effectiveness in the horizontal direction, so a better distribution of the lateral film cooling effectiveness can be achieved. The area-average cooling effectiveness is improved by an increase of the flow rate at the internal passage inlet. A new definition of D-n is introduced to evaluate the nonuniformity of the distribution of the local film cooling effectiveness quantitatively and conveniently. The downstream cooling effectiveness distribution becomes more uniform using a compound angle of 45 degrees. For the high thermal conductivity case, higher blade-wall heat flux is transferred by heat conduction, which drops the area-average wall temperature along the secondary flow direction

Conjugated heat transfer analysis of a film cooling passage with different rib configurations

Purpose - The purpose of this paper is to analyze the effect of thermal conductivity on gas turbine blades, and to investigate the contribution of different rib configurations to the heat flux and the film cooling effectiveness. Design/methodology/approach - The Renormalization Group (RNG) model with enhanced wall treatment was used for the turbulence modeling, and the SIMPLE algorithm was used to handle the pressure-velocity coupling. Findings - A flame-shape distribution on the internal wall provides high heat flux compared to a hawk-shape distribution; the film cooling effectiveness on the external wall is enhanced for the lateral film cooling effectiveness by heat conduction and film cooling (convection); by comparing the square-rib and pin-rib configurations, the circular-rib configuration offers a higher film cooling effectiveness on the Aluminum wall. Research limitations/implications - In the present research, the combination of internal cooling and external cooling is used to predict cooling effectiveness on film-cooled flat plate; two kinds of different plate materials are used to obtain the influence of the thermal conductivity. The successful computational method should give guidelines for potential CFD users in engineering sciences. Practical implications - The results of the paper are of engineering interest where film cooling and ribbed surfaces are applied. The successful computational method will also serve as guidelines for potential users of CFD in design as well as research and development work. Originality/value - In the present research, the combination of internal cooling and external cooling is used to predict cooling effectiveness on film-cooled flat plate; two kinds of different plate materials are used to obtain the influence of the thermal conductivity

Three-dimensional optimal design of a cooled turbine considering the coolant-requirement change

Cooling technology is widely applied in modern turbines to protect the turbine blades, and extracting high-pressure cooling air from a compressor exerts a remarkable influence on the gas-turbine performance. However, the three-dimensional optimal design of a turbine in modern industrial practice is usually carried out by pursuing high component efficiency without considering possible changes in coolant requirement; hence, it may not exactly lead to improvement in the gas-turbine cycle efficiency. In this study, the turbine stator was twisted and leaned to achieve higher comprehensive efficiency, which is the cycle-based efficiency definition for a cooled turbine that considers both turbine aerodynamic performance and coolant requirement. First, the influence of twist and compound lean on turbine aerodynamic performance, considering stator-hub leakage, was investigated. Then, a method to predict the coolant requirement for turbines with different stator designs was applied, to evaluate coolant-requirement change at the design condition. The optimized turbines were finally compared to demonstrate the necessity of considering the coolant-requirement change in the optimal design. This indicated that proper twisting to open the throat area in the stator hub and compound lean to the pressure surface side could help improve the cooled-turbine comprehensive efficiency

Unsteadiness of Tip Leakage Flow in the Detached-Eddy Simulation on a Transonic Rotor with Vortex Breakdown Phenomenon

Tip leakage vortex (TLV) in a transonic compressor rotor was investigated numerically using detached-eddy simulation (DES) method at different working conditions. Strong unsteadiness was found at the tip region, causing a considerable fluctuation in total pressure distribution and flow angle distribution above 80% span. The unsteadiness at near choke point and peak efficiency point is not obvious. DES method can resolve more detailed flow patterns than RANS (Reynolds-averaged Navier–Stokes) results, and detailed structures of the tip leakage flow were captured. A spiral-type breakdown structure of the TLV was successfully observed at the near stall point when the TLV passed through the bow shock. The breakdown of TLV contributed to the unsteadiness and the blockage effect at the tip region

Numerical Investigation and Optimization of Variable Guide Vanes Adjustment in a Transonic Compressor

In the present work, numerical simulation and optimization was carried out to analyze the mechanism of the variable guide vanes (VGVs) of a transonic compressor. A seven-stage transonic compressor including three-stage VGVs was studied. The VGVs were adjusted individually and jointly under different IGV opening degrees. Changes in performance and shock wave were analyzed, and the coupling effect of the VGV joint adjustment was summarized. Aiming at the maximum efficiency, the joint turning angles were optimized. A novel phenomenon was found wherein the VGV adjustment can affect not only its own performance and that of adjacent downstream blades, but also that of upstream blades. Incidence and performance of upstream blades are improved, but those of the VGV and its adjacent downstream blades are deteriorated. VGV adjustment weakens the shock wave and shock-induced boundary layer separation. The optimal solution for VGV joint adjustment is the combination of the optimal solutions for single VGV adjustments. The joint adjustment optimization improves the efficiency by 0.2–1.93% under different IGV opening degrees