Jun Yao

Direct numerical simulation of jets in cross-flow

Direct numerical simulation (DNS) of jets in cross-flow (JICF) has been carried out in this study, aiming for the investigation of vortex structure formation and evolution process associated with JICF. A recently developed DNS code is used, which solves three-dimensional (3D) compressible unsteady Navier–Stokes (NS) equations using high-order finite differences and multi-block structure grid treatment for complex geometry. Jet flow from a square duct, perpendicular to the mainstream flow, is introduced and the flow Reynolds number is 100 based on the jet duct diameter (D) and free-stream quantities. Two-dimensional (2D) calculations using various jet to free-stream velocity ratio (R = V jet/V free) reveals different vortex patterns and a further 3D study continues focusing on a velocity ratio of R = 2, for which complex vortex structure is produced. It is found from the 3D simulation that a counter-rotating vortex pair (CRVP) forms immediately after the jet exits, as observed from the experimental test and reproduced by other numerical simulations. The CRVP originates from the near wall viscous layer and its core position moves away from the wall as it evolves downstream. For the condition simulated, the CRVP is finally weakened (due to viscous diffusion) at about 1.6D downstream from the centre of the jet exit. No asymmetric CRVP has been observed, which was reported by other researchers for high-Reynolds number simulations.

Unsteady RANS calculation of flow over Ahmed car model

Three-dimensional separated flow around Ahmed car model has been studied by numerical simulation to solve the unsteady Reynolds-averaged Navier-Stokes (URANS) equations. The simulation considers experimental model with two slant angles of 25° and 35°. After some priori tests, baseline meshes have been identified with 1.5–1.9 × 106 elements for each case. After the precursor steady flow calculation, unsteady simulation continued for further two shedding cycles. Both mean and instantaneous quantities of the flow are accumulated and compared with available test data at representative measurement planes/locations. It was found that for two slant angles considered the time-averaged mean streamwise velocity of URANS predictions are compared fairly well with the experimental data with correct profile and same magnitude of peak. It was also observed that the slant angle has considerable influences on the downstream flow, particularly the flow recirculation, turbulence kinetic energy (TKE) distributions. The separated shear layers from the slant edges are merging together to form large size trailing vortex. While the flow structures agree qualitatively well with the measurement, the TKE has been under-estimated in wake region. This is mainly due to the limitation of two-equation turbulence model for massive separated flow with strong vortex shedding. Further advancement to large eddy simulation will provide a solution for this kind of flow.

Simulation of gas water two-phase flow in diesel turbocharger

Numerical simulation of a gas water two-phase flow in Diesel Turbocharger has been carried out using computational fluid dynamics solution of the Eulerian Reynolds-averaged Navier-Stokes equations for the continuum gas phase and the Lagrangian particle tracking method for the discrete water droplets. A generic diesel turbocharger configuration was chosen, which has an upstream duct inlet and a downstream nblade ring outlet. Three identical water injectors were evenly distributed in the circumferential direction and located upstream of the blades. Simulation considered water injection at an angle of 30o from the centerline with two water pump pressures of 4 bars and 8 bars. The process of liquid droplet break-up has been modeled using the Blob model for primary break-up and the cascade atomization breakup (CAB) and the nReitz and Diwakar breakup (RDB) models for secondary break-up. The results show that the predicted water droplet coverage and the blade temperature drop were in good agreement with experimental measurements. nSimulations also showed that for the two water pump pressures considered, the water droplet coverage and ndistribution patterns on the blade ring change little, indicating the need to increase the number of injectors for better water washing performance.

Numerical study of hole shape effect on blade cooling effectiveness

Numerical study of hole shape effect on blade adiabatic cooling effectiveness has been carried out on four geometry models comprising a standard cylindrical hole, a cylindrical hole with an upstream ramp, a shaped diffuser, and a double console slot. In all the cases, the hole centerline has an inclination angle of 35 degree against the mainstream gas flow. Results of the cylindrical hole model are in good agreement with available experimental and other numerical data. For the other three hole geometry variants considered, it was found that the cooling effectiveness has been considerably enhanced by a max factor of 2, compared to that from the base model. The physical mechanism for this is mainly due to the weakening of coolant flow penetration in the vicinity of the hole exit, thus reducing the level of mixing and the entrainment with the surrounding hot gas flow.

Effect of hole shape on blade cooling effectiveness

The high temperature conditions under which turbine blades operate pose a constraint on their service lifetime. One industrial solution is to apply the film cooling and its effectiveness is generally determined by flow condi tions, cooling hole geometry shapes and its orientations. In this study, a total of four co oling hole geometries are considered as a cylindrical tube, a cylindrical tube with an upstre am ramp, a shaped diffuser, and a double console slot. In all cases, the blowing ratio is se t to be unit and the hole centerline has an inclination angle of 35 degrees against the mainstr eam airflow. Simulation starts with a base model of cylindrical hole and results have shown go od agreement with available experimental data and numerical results. Using this configuration as a baseline, studies continued with three remaining geometries. It was f ound that for shaped diffuser and double console models, the cooling effectiveness has consi derable increase, in comparison to that from the base model. The physical mechanism is primarily due to the weakening of the vortex structures in the vicinity of the hole exit, thus reducing the penetration depth of the coolant jet flow and the entrainment of surrounding hot fluids.

CFD simulation of gas–water two-phase flow in turbocharger

A turbocharger is widely used by industry as an efficient thermal performance enhancement device, and its efficiency is often dependent on the conditions and properties of the working fluid. One industry problem is the use of low-grade diesel, that produces various combustion products, and thus causes blade throat blockage, blade corrosion, damage, etc. At present, one solution is to attach an ad hoc online water washing system that operates daily to remove in part any accumulated solid deposits. While the method works well, an in-depth understanding of the washing mechanism is still quite limited. Complementary to essential in-house rig testing, it is now feasible to carry out numerical simulation of flow thus to provide further understanding. A combined experimental and numerical study of gas-water two-phase flow in turbocharger has therefore been proposed with some results presented here.

CFD MODELLING OF WATER INJECTION FOR TURBINE BLADE CLEANING

CFD modeling of water injection for turbine blade cleaning has been carried out to predict the water coverage on a stationary blade row, which will enable a better understanding on the interactions between hot-air flow and cold-water droplets. A generic configuration was used in a priori in-house experiment, which provides test data for CFD validation. The two-phase flow CFD model adopts the Eulerian-Lagrangian approach, in which the air-flow was treated as the continuous phase and water droplets as the dispersed phase. CFD predictions are found in fairly good agreement with test results, particularly the water coverage on the downstream blade row. Moreover, CFD modeling provides further details, including the trajectory of water droplets, which are difficult to be obtained by experiments, and yet extremely useful for understanding the flow physics.

Computational Prediction of Local Distorted Flow in Turbocharger

This paper presents numerical study by performing three-dimensional Navier-Stokes solution for mixture gas flow in a typical industrial turbocharger configuration. The primary focuses are the flow distortions and behaviours in the proximity of the nozzle vanes. Numerical predictions reveal local flow distortions, shown by a considerable total pressure drop of about 7.5%. The possible reason for this is probably due to the influence of the upstream guide vane wake flow. At both design and off-design conditions considered in this study, the flow near the nozzle vanes has noticeable inhomogeneous in the circumferential direction. However, both local flow distortions and inhomogeneous in annulus are gradually reduced and the flow recovers to near uniform at the nozzle exit plane. Thus the predicted flow distortions have negligible effects on downstream turbine blades.