3D RANS Research Articles

Performance enhancement of a small-scale organic Rankine cycle radial-inflow turbine through multi-objective optimization algorithm

An effective methodology that encompasses a mean-line design, three-dimensional CFD analysis and optimization and ORC system modelling of the small-scale ORC radial-inflow turbine is presented. Three-dimensional CFD analysis and a multi-objective optimization algorithm were achieved using ANSYS®17 CFX and Design Exploration based on 3D RANS with a k-omega SST turbulence model. The 3D optimization technique combines a design of the experiment, a response surface method and multi-objective method. The optimization of the blade geometry was performed using 20 design points for both nozzle and rotor blades, based on the B-splines’ technique to represent the blade angles and thickness distribution. The number of blades and rotor tip clearance were included as design parameters. The isentropic efficiency and power output were introduced as an optimization objective with two organic working fluids, namely isopentane and R245fa. The results of the optimized geometry with R245fa showed that the turbine's and cycle's thermal efficiencies were higher by 13.95% and 17.38% respectively, compared with a base-line design with a maximum power output of 5.415 kW. Such methodology is proved to be effective as it allows the enhancing of the turbine's and the ORC's system performance throughout to find the optimum blade shape of the turbine stage.

Plume Interaction and Base Flow Analysis of a Twin Engine Flight Vehicle

3D RANS simulations are performed to study the multi jet interactions of a twin engine gimbal configuration of an aerospace vehicle at different time instants. Simulations captured all the essential features of the flow field and interaction between the neighboring jets did not occur because of low altitudes and moderate under-expansion of the jets considered in the simulations. For higher gimbal angle, two jets were the closest but still did not interact. Detail exploration of the downstream flow field revealed that the distinct features of the jets are retained at the farthest downstream locations; although the pressure field reached the uniformity. Average base pressure ratios for the three different time instances are 0.91, 0.547 and 0.522 and maximum base temperature is of the order 800 K.

A Low-Order Model for Predicting Turbocharger Turbine Unsteady Performance

In this paper, a low-order model for predicting performance of radial turbocharger turbines is presented. The model combines an unsteady quasi-three-dimensional (Q3D) computational fluid dynamics (CFD) method with multiple one-dimensional (1D) meanline impeller solvers. The new model preserves the critical volute geometry features, which is crucial for the accurate prediction of the wave dynamics and retains effects of the rotor inlet circumferential nonuniformity. It also still maintains the desirable properties of being easy to set-up and fast to run. The model has been validated against a experimentally validated full 3D unsteady Reynolds-averaged Navier–Stokes (URANS) solver. The loss model in the meanline model is calibrated by the full 3D RANS solver under the steady flow states. The unsteady turbine performance under different inlet pulsating flow conditions predicted by the model was compared with the results of the full 3D URANS solver. Good agreement between the two was obtained with a speed-up ratio of about 4 orders of magnitude (∼104) for the low-order model. The low-order model was then used to investigate the effect of different pulse wave amplitudes and frequencies on the turbine cycle averaged performance. For the cases tested, it was found that compared with quasi-steady performance, the unsteady effect of the pulsating flow has a relatively small impact on the cycle-averaged turbine power output and the cycle-averaged mass flow capacity, while it has a large influence on the cycle-averaged ideal power output and cycle-averaged efficiency. This is related to the wave dynamics inside the volute, and the detailed mechanisms responsible are discussed in this paper.

Multi-Regime Shape Optimization of Fan Vanes for Energy Conversion Efficiency using CFD, 3D Optical Scanning and Parameterization

An enhanced reverse engineering procedure was developed for roof fan re-design. An original numerical workflow for robust shape optimization based on maximum energy conversion efficiency was developed. It operates using a sample of multiple operating regimes coupled with CFD simulations. The initial shape solution was originally obtained in point cloud form by optical 3D scanning and subsequent B-spline based parameterization of shape. The CFD simulation of the scanned shape using 3D RANS based software was shown to agree very well with the measured features, experimentally obtained in our lab with the actual initial-shape fan. By manipulating the control points of parametric curves, the developed evolutionary optimization workflow was subsequently able to create shape-optimized vanes. This original procedure was applied to cases of constant-thickness and profiled single curvature vanes, both for single-regime and robust multi-point operating conditions. The corresponding increase in efficiency gained by our computational procedure was correlated with respective velocity and pressure distributions and suppression of flow separation. The novel numerical procedure developed here therefore provides a numerical framework for generic object geometry to re-shape itself autonomously. The change in shape ensures maximum energy conversion efficiency for a given composition of operating regimes. The gain in efficiency with optimized vane shapes proves to be significant in the wide range of flow rates around the best efficiency point.

Effects of Rotation at Different Channel Orientations on the Flow Field inside a Trailing Edge Internal Cooling Channel

The flow field inside a cooling channel for the trailing edge of gas turbine blades has been numerically investigated with the aim to highlight the effects of channel rotation and orientation. A commercial 3D RANS solver including a SST turbulence model has been used to compute the isothermal steady air flow inside both static and rotating passages. Simulations were performed at a Reynolds number equal to 20000, a rotation number (Ro) of 0, 0.23, and 0.46, and channel orientations ofγ=0∘, 22.5°, and 45°, extending previous results towards new engine-like working conditions. The numerical results have been carefully validated against experimental data obtained by the same authors for conditionsγ=0∘and Ro = 0, 0.23. Rotation effects are shown to alter significantly the flow field inside both inlet and trailing edge regions. These effects are attenuated by an increase of the channel orientation fromγ=0∘to 45°.

Fan Root Aerodynamics for Large Bypass Gas Turbine Engines: Influence on the Engine Performance and 3D Design

The exit flow field of the fan root of large turbofan engines defines the inlet conditions to the core compressor. This in turn could have significant impact to the performance of the core compressor. This study is aimed to resolve two related issues concerning the impact of the fan root flow on the core compressor performance: to establish the effect of an increased loss at the inlet on the engine specific fuel consumption (SFC) and to assess the effect of the radial distribution of the fan root flow on the engine performance. With understanding of these issues, the geometric parameters and design details which can produce a more uniform core flow at the exit of the fan stage module can be identified. The fan root flow is analyzed with methods of different complexity and fidelity. A simple cycle analysis is used to assess the impact on engine SFC of a stagnation pressure deficit at the fan root; a throughflow code is used for the preliminary study of the curvature effect of the root flow path, and 3D RANS CFD calculations are then used to simulate the flow path from the inlet of the fan to the first stage of the core compressor. The adequacy of the application of the numerical code in this case has been assessed and confirmed by the comparison with the experimental data for two rig configurations. The results of this study show that the flow at the fan hub region is very complex and dominated by 3D effects. The interaction of the secondary flow with real geometries, such as leakage flows, is found to have a strong detrimental effect on the core performance. The curvature of the hub end wall is a key parameter controlling the fan root flow topology; it influences the strength of the secondary flow, the spanwise distribution of the flow, and its sensitivity to leakage flow. With this understanding, it is possible to redesign of the fan hub flow path to reduce the loss generation by a significant amount.

Effects of Terrain Slope on Nacelle Anemometry

A numerical analysis of the effects of sloped terrain on the reading of a nacelle anemometer is investigated. Simulations of the turbulent flow around a 2.5 MW wind turbine in an atmospheric boundary layer are made by resolving 3D RANS equations. In addition to flat terrain, four escarpments (at slopes of 7.5%, 11%, 14%, and 20%) are studied for various inlet velocities in three cases: terrains with no wind turbine, with nonoperating turbines and with operating turbines. The slope of the ground has two major effects on flow: speed-up and an increase in flow inclination. The presence of the nacelle enhances the flow speed-up caused by the escarpment, especially outside the anemometer’s position. However, the horizontal velocity at the location of the anemometer tends to decrease with increasing ground slope. This trend is due in large part to the nacelle wake. This disturbed area is characterized by the presence of separated flow and two opposing vortices which are sensitive to the flow inclination. The evaluated nacelle transfer function is influenced by the terrain slope but this sensitivity is reduced by displacing the position of the anemometer upward the nacelle body.

EXPERIMENTAL AND NUMERICAL COMPARISION OF 3D DAM-BREAK FLOOD WAVE PROPAGATION

Dam-break flood wave is an irregular, time-dependent and rapidly varying free surface open channel flow and therefore, is one of the complex and difficult problems. The research for solving this problem is mainly directed at experimental and numerical studies due to the difficulties in collecting field data. Despite that, the experimental and numerical studies on 3-dimensional dam-break problem are limited in the literature. In this study, the propagation of expanding dam-break flood wave in the downstream direction is investigated using a vertical plate placed in front of a reservoir built in the laboratory. The propagation of the wave front was observed using high speed camera and obtained results were compared with FLOW-3D numerical model which uses 3D RANS equations. The results were in quite good agreement.

3D RANS Simulation of Turbulent Flow and Combustion in a 5 MW Reverse-Flow Type Gas Turbine Combustor

This study is concerned with 3D RANS simulation of turbulent flow and combustion in a 5 MW commercial gas turbine combustor. The combustor under consideration is a reverse flow, dry low NOx type, in which methane and air are partially mixed inside swirl vanes. We evaluated different turbulent combustion models to provide insights into mixing, temperature distribution, and emission in the combustor. Validation is performed for the models in STAR-CCM+ against the measurement data for a simple swirl flame (http://public.ca.sandia.gov/TNF/swirlflames.html). The standard k-ε model with enhanced wall treatment is employed to model turbulent swirl flow, whereas eddy break-up (EBU), presumed probability density function laminar flamelet model, and partially premixed coherent flame model (PCFM) are tried for reacting flow in the combustor. Independent simulations are carried out for the main and pilot nozzles to avoid flashback and to provide realistic inflow boundary conditions for the combustor. Geometrical details such as air swirlers, vane passages, and liner holes are all taken into account. Tested combustion models show similar downstream distributions of the mean flow and temperature, while EBU and PCFM show a lifted flame with stronger effects of swirl due to limited increase in axial momentum by expansion.

A Resistance Reduction Study of a Trimaran using Waterline Parabolization

Waterline parabolization or addition of side bulbs about the ship's midbody can significantly reduce the wave-making resistance of a vessel [Calisal, S. M., Goren, O., and Danisman, D. B., 2002 Resistance reduction by increased beam for displacement-type ships, Journal of Ship Research, 46, 3, 208–213]. These side bulbs are designed to create a wave pattern that interacts with the ship wave system of the hull at the desired speed range. This concept was first successfully tested on a coaster tanker and then extended to the UBC series hull, a series typical of Canadian West Coast fishing vessels. Systematic tow tank experiments revealed that while parabolization decreases the total resistance, the form factor suffered an increase. An integral boundary layer solver and a 2D RANS solver both showed that the increase in viscous resistance was mainly caused by an increase in form drag or viscous pressure drag. The parabolization concept was subsequently extended to a high-speed NPL trimaran to determine whether resistance reduction using parabolic side bulbs could be achieved for a very slender multihull vessel. A Rankine source panel method was used to predict the wave-making characteristics of the trimaran, and an integral boundary layer solver and a RANS solver were used to calculate the viscous drag. A parametric study, varying the size and location of bulbs, was first performed on the center hull to design the side bulb. The study was then extended to the trimaran to evaluate the additional wave interactions caused by the outriggers. Experimental model tests validated the numerically predicted wave interactions, as well as the change in viscous drag. Based on the numerical work, a modified NPL trimaran hull form was designed that reduced the total resistance of the vessel by up to 6% in the design speed range and providing critical additional engine or accommodation space.

Further Development of a Dynamic Intermittency Model For Wake-Induced Transition

The transition model studied in this paper uses an equation for free-stream intermittency and one for near-wall intermittency, combined with the SST turbulence model. The model was already assessed in earlier work for wake-induced transition in boundary layers in separated state and in attached state. For separated state transition, the model predicts the start and growth of transition very well, except for the impossibility to describe the breakdown of the roll-up vortices in a 2D RANS simulation. In attached state transition, the model has the tendency to generate a delayed start of transition and a too slow growth rate in the initial phase of transition. We demonstrate that a delayed start of transition is inherent for a RANS simulation. We propose a repair for the too slow growth rate in the initial stage of the transition. Additionally, both for wake-induced transition in separated state and in attached state, the original model has the tendency to predict a state too near to laminar in between wakes at the trailing edge. We demonstrate that this is caused by the too rapid destruction of the near-wall intermittency during relaminarization and in the lack of activation of near-wall intermittency for low turbulence level in the free stream. We propose a repair for both deficiencies. The fourth improvement is that we take into account the influence of the free stream turbulence length scale in the criteria for onset of transition. We demonstrate the high quality of the improved model for wake-induced transition on a steam turbine stator blade for outlet Reynolds number 600,000 and two levels of turbulence in the background flow: 3% and 0.4%.

A Comparison of Approximate Versus Exact Geometrical Representations of Roughness for CFD Calculations of cf and St

Skin friction (cf) and heat transfer (St) predictions were made for a turbulent boundary layer over randomly rough surfaces at Reynolds number of 1×106. The rough surfaces are scaled models of actual gas turbine blade surfaces that have experienced degradation after service. Two different approximations are used to characterize the roughness in the computational model: the discrete element model and full 3D discretization of the surface. The discrete element method considers the total aerodynamic drag on a rough surface to be the sum of shear drag on the flat part of the surface and the form drag on the individual roughness elements. The total heat transfer from a rough surface is the sum of convection on the flat part of the surface and the convection from each of the roughness elements. Correlations are used to model the roughness element drag and heat transfer, thus avoiding the complexity of gridding the irregular rough surface. The discrete element roughness representation was incorporated into a two-dimensional, finite difference boundary layer code with a mixing length turbulence model. The second prediction method employs a viscous adaptive Cartesian grid approach to fully resolve the three-dimensional roughness geometry. This significantly reduces the grid requirement compared to a structured grid. The flow prediction is made using a finite-volume Navier-Stokes solver capable of handling arbitrary grids with the Spalart-Allmaras (S‐A) turbulence model. Comparisons are made to experimentally measured values of cf and St for two unique roughness characterizations. The two methods predict cf to within ±8% and St within ±17%, the RANS code yielding slightly better agreement. In both cases, agreement with the experimental data is less favorable for the surface with larger roughness features. The RANS simulation requires a two to three order of magnitude increase in computational time compared to the DEM method and is not as readily adapted to a wide variety of roughness characterizations. The RANS simulation is capable of analyzing surfaces composed primarily of roughness valleys (rather than peaks), a feature that DEM does not have in its present formulation. Several basic assumptions employed by the discrete element model are evaluated using the 3D RANS flow predictions, namely: establishment of the midheight for application of the smooth wall boundary condition; cD and Nu relations employed for roughness elements; and flow three dimensionality over and around roughness elements.

Modelling turbulent flow past a circular cylinder by RANS, URANS, LES and DES

Incompressible turbulent flow past a circular cylinder is computationally investigated by a cascade of turbulence modelling strategies. Results are compared with experiments, by keeping the focus on the predictability of the drag coefficient. Two-Dimensional Reynolds Averaged Numerical Simulations (2D RANS) and Two-Dimensional Unsteady Reynolds Averaged Numerical Simulations (2D URANS) are performed for a wide range of Reynolds numbers, encompassing the critical regime. It is shown that the drag crisis can qualitatively be predicted by the 2D RANS, provided that the near-wall region is adequately resolved. The 2D RANS underpredicts the experimental drag coefficients for the most Reynolds numbers, seemingly due to the missing contribution of the organised transient motion. The 2D URANS, on the other hand, generally overpredict the drag coefficients, indicating the intimate relationship between the organised transience and three-dimensionality. Computations by three-dimensional transient procedures such as Three-Dimensional Unsteady Reynolds Averaged Simulations (3D URANS), Large Eddy Simulations (LES) and Detached Eddy Simulations (DES) are performed for the Reynolds number of 104. A substantial improvement in the prediction quality is observed. The best agreement with the experiments is obtained by the LES. The DES is also observed to deliver results that are very close to the measurements.

Validation of a 3D RANS model to predict flow and stratification effects related to fish passage at hydropower dams

This paper discusses the validation and application of a three-dimensional RANS model to simulate flow and temperature distributions within the powerhouse units and the forebay of McNary Dam on the Columbia River. The hydrodynamic model is first validated with data collected from an experimental model study of a single turbine intake unit. The capability of the model to predict stratification effects is then investigated by comparing the predicted temperature field with field data collected within the forebay of McNary Dam. It is found that the model is able to accurately predict the salient features of the thermally stratified flow within the forebay for a test case corresponding to strongly stratified conditions without spill and for a case corresponding to moderately stratified conditions with spill. The model is then applied to investigate the effects of introducing a floating curtain wall in the forebay on the gatewell temperatures at the 14 powerhouse units of the dam. It is found that one of the designs appears to alleviate the occurrence of adverse thermal conditions within the gatewells and fish collection channel

Validation of a 3D RANS Solver With a State Equation of Thermally Perfect and Calorically Imperfect Gas on a Multi-Stage Low-Pressure Steam Turbine Flow

A state equation of thermally perfect and calorically imperfect gas is implemented in a 3D RANS solver for turbomachinery flow applications. The specific heats are assumed as linear functions of temperature. The model is validated on a five-stage low-pressure steam turbine. The computational results exhibit the process of expansion in the turbine. The computed and measured distributions of flow parameters in axial gaps downstream of subsequent turbine stages are found to agree reasonably well. It is also shown that the obtained numerical solution gives considerable improvement over the solution based on the thermally and calorically perfect gas model.

Fan Interaction Noise Predictions Using RANS-BEM Coupling

A source-to-far-field computation procedure aiming at predicting the noise generated by rotor-stator fan interactions and inlet noise radiation is presented here. It is applied to the NLR turbofan model, tested in DNW-LLF anechoic chamber under the framework of DUCAT project. The unsteady aerodynamic input for the acoustic computation is obtained using a 3D RANS code, CANARI, developed at ONERA for turbomachinery applications. The CFD solutions are coupled, using a modal expansion approach, to a BEM code, solving the Helmholtz equation in an arbitrary bounded space. A single interacting cut-on mode amplitude is either directly deduced from experiment or provided by post-processing the CFD data. With this approach, the predicted noise radiation can be related to the directivity patterns measured at several axial positions upstream of the turbofan inlet. A fairly good agreement is found using both experimental and CFD input data.