Kathy Simmons

Computational fluid dynamic modeling of gas flow characteristics in a high-velocity oxy-fuel thermal spray system

A computational fluid dynamics (CFD) model is developed to predict gas dynamic behavior in a high-velocity oxy-fuel (HVOF) thermal spray gun in which premixed oxygen and propylene are burnt in a 12 mm combustion chamber linked to a parallel-sided nozzle. The CFD analysis is applied to investigate axisymmetric, steady-state, turbulent, compressible, and chemically combusting flow both within the gun and in a free jet region between the gun and the substrate to be coated. The combustion of oxygen and propylene is modeled using a single-step, finite-rate chemistry model that also allows for dissociation of the reaction products. Results are presented to show the effect of (1) fuel-to-oxygen gas ratio and (2) total gas flow rate on the gas dynamic behavior. Along the centerline, the maximum temperature reached is insensitive to the gas ratio but depends on the total flow. However, the value attained (∼2500 K) is significantly lower than the maximum temperature (∼3200 K) of the annular flame in the combustion chamber. By contrast, the centerline gas velocity depends on both total flow and gas ratio, the highest axial gas velocity being attained with the higher flow and most fuel-rich mixture. The gas Mach number increases through the gun and reaches a maximum value of approximately 1.6 around 5 mm downstream from the nozzle exit. The numerical calculations also show that the residual oxygen level is principally dependent on the fuel-to-oxygen ratio and decreases by approximately fivefold as the ratio is varied from 90 to 69% of the stoichiometric requirement. The CFD model is also used to investigate the effect of changes in combustion chamber size and geometry on gas dynamics, and the results are compared with the nominal 12 mm chamber baseline calculations.

Improved Cooling in the End Region of a Strip-Wound Totally Enclosed Fan-Cooled Induction Electric Machine

Computational fluid-dynamics modeling has been used to investigate the cooling of the end region of a two-pole strip-wound totally enclosed fan-cooled induction motor. The modeling is validated by experimental measurements. The changes in airflow and heat transfer that each configuration gives are discussed, and recommendations are made of features that can be used to achieve lower convective thermal resistance in the end region.

Modelling windage power loss from an enclosed spur gear

Abstract Within a gearbox the majority of transmission losses can be attributed to bearing losses, meshing losses, or losses due to windage/churning. In this paper the commercial computational fluid dynamics (CFD) code Fluent 6.2.16 is applied in a two-dimensional study of windage power loss (WPL) from a single spur gear rotating in air. By comparing CFD data to published experimental data appropriate grid density and modelling parameters are identified. The model is used to investigate how peripheral shrouding affects WPL and whether WPL can be reduced through minor modifications to tooth tip geometry. Non-dimensional shroud spacings (ratio of gap to gear PCD) of between 0.005 and 0.05 were investigated at shaft speeds between 5000 and 20 000 r/min. Although CFD data compared reasonably well to experimental data, trends were not reproduced and an optimum shroud could not be identified. A full three-dimensional study is recommended. Modifying the tooth tip by adding a small chamfer on the leading edge reduced WPL by ∼6 per cent. A small fillet increased total WPL by a similar amount suggesting that WPL may increase as a gear wears. This preliminary study suggests further work in this area would be beneficial.

Numerical modeling of in-flight characteristics of inconel 625 particles during high-velocity oxy-fuel thermal spraying

A computational fluid dynamics (CFD) model is developed to predict particle dynamic behavior in a high-velocity oxyfuel (HVOF) thermal spray gun in which premixed oxygen and propylene are burnt in a combustion chamber linked to a long, parallel-sided nozzle. The particle transport equations are solved in a Lagrangian manner and coupled with the two-dimensional, axisymmetric, steady state, chemically reacting, turbulent gas flow. Within the particle transport model, the total flow of the particle phase is modeled by tracking a small number of particles through the continuum gas flow, and each of these individual particles is tracked independently through the continuous phase. Three different combustion chamber designs were modeled, and the in-flight particle characteristics of Inconel were 625 studied. Results are presented to show the effect of process parameters, such as particle injection speed and location, total gas flow rate, fuel-to-oxygen gas ratio, and particle size on the particle dynamic behavior for a parallel-sided, 12 mm long combustion chamber. The results indicate that the momentum and heat transfer to particles are primarily influenced by total gas flow. The 12 mm long chamber can achieve an optimum performance for Inconel 625 powder particles ranging in diameter from 20 to 40 µm. At a particular spraying distance, an optimal size of particles is observed with respect to particle temperature. The effect of different combustion chamber dimensions on particle dynamics was also investigated. The results obtained for both a 22 mm long chamber and also one with a conical, converging design are compared with the baseline data for the 12 mm chamber.

Prediction of air/oil exit flows in a commercial aero-engine bearing chamber

Abstract An understanding and optimization of three-dimensional air/oil flows in aero-engine transmission systems forms an integral part of future designs. This especially applies to bearing chambers, which contain a complex two-phase flow formed by the interaction of sealing airflows and lubrication oil. A critical design quantity is the composition of the liquid (oil) and gas (air) phases in the exit flows. Using a previously validated numerical model, the air/oil flow in a commercial bearing chamber is computed with particular focus on the flow exiting the chamber. The division of oil exiting the chamber through the vent and scavenge ports is determined for three shaft speeds and two configurations of the vent port. Comparison with available experimental data shows that consistent trends are predicted, but further model development is necessary in the vicinity of the scavenge port.

A Numerical Model for Oil Film Flow in an Aeroengine Bearing Chamber and Comparison to Experimental Data

The work presented forms part of an ongoing investigation, focusing on modeling the motion of a wall oil film present in a bearing chamber and comparison to existing experimental data. The film is generated through the impingement of oil droplets shed from a roller bearing. Momentum resulting from the impact of oil droplets, interfacial shear from the airflow, and gravity cause the film to migrate around the chamber. Oil and air exit the chamber at scavenge and vent ports. A previously reported numerical approach to the simulation of steady-state two-phase flow in a bearing chamber, which includes in-house submodels for droplet-film interaction and oil film motion, has been extended. This paper includes the addition of boundary conditions for the vent and scavenge together with a comparison to experimental results obtained from ITS, University of Karlsruhe. The solution is found to be sensitive to the choice of boundary conditions applied to the vent and scavenge.

Comparison of Computational Fluid Dynamics and Particle Image Velocimetry Data for the Airflow in an Aeroengine Bearing Chamber

Investigations into the single-phase velocity field of a model aeroengine bearing chamber are presented. Adequately resolving the airflow field is important to subsequent computational modeling of two-phase fluid transport and heat transfer characteristics. A specially designed test rig, representing the features of a Rolls Royce Trent series aeroengine bearing chamber, was constructed. Experimental data for the airflow field was obtained using particle image velocimetry (PIV). The results show a strong influence of shaft rotation and chamber geometry on the flow features within the bearing chamber. A computational fluid dynamics (CFD) simulation was carried out using the commercial CFD code FLUENT 6. Flow features were adequately modeled, showing the features of secondary velocities. Turbulence modeling using the differential Reynolds stress (RSM) model shows good agreement with the experimental data.

The Effect of Initial Injection Conditions on the Oil Droplet Motion in a Simplified Bearing Chamber

Increasing demands on aeroengines to operate at higher shaft speeds and temperatures require an understanding and optimization of the integral systems. The ability to model movement of oil around bearing chambers in the form of droplets and films is a current area of interest. This paper presents a two-phase numerical modeling approach developed for predicting air, oil droplet, and oil film behavior within an aeroengine bearing chamber including the significant droplet/film interactions. In-house code is linked to the commercial CFD package CFX4.3 and a predictive algorithm for determining the outcome of droplet impact with a wall film and the associated transfer of oil mass and momentum is developed. The method is used to simulate the motion of oil droplets shed from a roller bearing in a simplified aeroengine bearing chamber geometry and through a parametric study shows that initial droplet size distribution parameters (mean diameter and spread) have a significant effect on oil deposition location. In contrast, the Sauter mean diameter of droplets within the chamber showed little sensitivity to initial injection parameters. The behavior of oil within a bearing chamber is strongly influenced by the conditions with which it leaves the bearing. There is potential for performance improvement if bearing shed can be controlled or if chamber design can be modified such that oil behavior is insensitive to initial shed conditions.

Computational fluid dynamics applied to a cement precalciner

Abstract This paper describes a study of the use of computational fluid dynamics (CFD) to investigate the performance of a precalciner vessel at a cement works, In this vessel, limestone, held in suspension, is calcined to calcium oxide and the endothermic reaction is supported by the combustion of coal. Results are presented from a CFD model that contains all the essential features of the precalciner as operated when burning coal. The model fully represents the reactions and fluid dynamics of the precalciner. Previously unidentified features are illustrated. Certain key features at points in the precalciner, where some limited measurements can be made, are compared with the parameters indicated by the computational model. The measurements are consistent with the results calculated by the model indicating fair validation. The CFD data show the following 1 The gases undergo distinct recirculation. 2 The coal particles entering at one inlet have significantly different trajectories and temperature histories from those entering at the second diametrically opposed inlet. 3 There is 90 per cent completion of coal combustion at the exit. 4 73 per cent limestone in the raw meal is calcined to calcined to calcium oxide at the exit from the precalciner. 5 The highest reaction rate of the raw meal is closer to one side of the vessel due to interaction with the gas flows. Future work is proposed which, firstly, will provide further validation of the results so far attained by selective measurements on the precalciner and, secondly, will model the combustion and aerodynamic behaviour of waste-derived fuels in the precalciner vessel, commencing with shredded car tyre chips.

The Application of CFD to Model Windage Power Loss From a Spiral Bevel Gear

In some aero-engine applications a spiral bevel gear is mounted in a bearing chamber. The bearings and gears create a highly rotating, turbulent and chaotic flow field within the chamber. In this paper a single-phase computational study of the flow around a spiral bevel gear is presented and the data is compared to available experimental data. Results are presented for clockwise and anticlockwise rotation for both unshrouded and shrouded configurations. Reasonable agreement of the computational data with the experimental data is obtained. Moment coefficients show the same trends in behaviour for clockwise and anticlockwise rotation although numerical agreement is not ideal with a difference of up to 27%, in the unshrouded case. For the shrouded configuration, static pressure profiles along the shroud are compared to experimental data, and these show good agreement although in the worst case there is 23% difference between computational and experimental values. The effect of different computational wall treatments is presented.Copyright