Carol Eastwick

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.

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.

Gear Windage: A Review

The efficiency of power and propulsive systems is increasingly being targeted as a means of reducing environmental impact. This has caused a renewed interest in industry in the losses associated with meshing gears. Gearbox efficiency varies from 98% to 99% for the best designed high power applications, but that can still equate to losses in megawatts. There are different mechanisms for losses that have been identified within gearboxes; these are meshing losses, bearing losses, windage losses, and churning losses. Depending on the application, the relative importance of these mechanisms varies. This paper reviews information on windage power loss. The motivation for this is that for some applications, this power loss can be a significant component, particularly lightly loaded high-speed applications. For instance, within some aeroengines, gears are mounted internally within bearing chambers. The component of windage power loss becomes significant in this case, and the flows associated with windage power loss have a significant impact on the amount of heat transferred to the oil within the chamber, which is a critical design consideration. This paper provides a review of experimental investigations and available models of gear windage power loss for spur, helical, and bevel gears. The aim of the review is to provide a comprehensive compilation of published information on windage power loss to assist gearbox designers in identifying relevant experimental and modeling information. While it is clear from the review of published work that the rotational speed, gear geometrical parameters, degree of confinement, and density of the fluid surrounding the gear are important, the degree of effect and general solutions for reducing power loss are less clear.

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.

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

Mechanical and thermal design of an aeroengine starter/generator

The paper describes the mechanical and thermal design of a high speed, high power density synchronous permanent magnet machine for an aero engine starter generator system with a power rating of 150 kW and maximum speed of 32,000 rpm. The electrical machine is designed to minimise the weight and maximise the efficiency so both mechanical and thermal aspects are considered. The cooling strategy adopts an inner stator sleeve for enhanced cooling of the stationary components whilst minimising the windage loss. Finite Element Analysis (FEA) is used for the static structural analyses of critical components of the machine and the dynamic performance of the rotating shaft. Heat transfer phenomena are also investigated by the means of Computational Fluid Dynamics (CFD) and Lumped Parameter Thermal Network (LPTN) for the optimisation of the proposed cooling arrangement and prediction of the machine temperature distribution.

Study of aero-engine oil-air separators

Abstract For aero-engines, oil-air separation is a key function, and one approach to assessing separator effectiveness is computational fluid dynamics (CFD). The two-phase flow is complex and oil can be present in different forms (for example, droplets, mist, film). However, necessary modelling simplifications may affect solution accuracy and range of validity. This article presents a modelling methodology for oil-air separators; the effect of simplifications is discussed and their relative magnitude assessed. Comparison with available experimental data is presented. It is concluded that although simplification has an impact, the significant features of the oil-air separator are predicted with sufficient accuracy to allow design comparisons. Two separator configurations, one internal to a bearing chamber and one external, are modelled and the data presented. Flow fields are compared and the effectiveness of the separators in removing oil droplets prior to impact on the breather (primary separation) presented. The separation performance of the external design is largely independent of shaft speed, with all droplets >3 μm removed before impact on the breather. The critical droplet diameter of the internal design is larger, varying with breather configuration and shaft speed but the power loss is an order of magnitude lower than for the external design.

Computational Investigation of Torque on Coaxial Rotating Cones

This article looks at a modification of Taylor–Couette flow, presenting a numerical investigation of the flow around a shrouded rotating cone, with and without throughflow, using the commercial computational fluid dynamics code FLUENT 6.2 and FLUENT 6.3 . The effects of varying the cone vertex angle and the gap width on the torque seen by the rotating cone are considered, as well as the effect of a forced throughflow. The performance of various turbulence models are considered, as well as the ability of common wall treatments/functions to capture the near-wall behavior. Close agreement is found between the numerical predictions and previous experimental work, carried out by Yamada and Ito (1979, “Frictional Resistance of Enclosed Rotating Cones With Superposed Throughflow ,” ASME J. Fluids Eng., 101, pp. 259–264; 1975, “On the Frictional Resistance of Enclosed Rotating Cones (1st Report, Frictional Moment and Observation of Flow With a Smooth Surface) ,” Bull. JSME, 18, pp. 1026–1034; 1976, “On the Frictional Resistance of Enclosed Rotating Cones (2nd Report, Effects of Surface Roughness) ,” Bull. JSME, 19, pp. 943–950). Limitations in the models are considered, and comparisons between two-dimensional axisymmetric models and three-dimensional models are made, with the three-dimensional models showing greater accuracy. The work leads to a methodology for modeling similar flow conditions to Taylor–Couette.

Applicability of Mechanical Tests for Biomass Pellet Characterisation for Bioenergy Applications

In this paper, the applicability of mechanical tests for biomass pellet characterisation was investigated. Pellet durability, quasi-static (low strain rate), and dynamic (high strain rate) mechanical tests were applied to mixed wood, eucalyptus, sunflower, miscanthus, and steam exploded and microwaved pellets, and compared to their Hardgrove Grindability Index (HGI), and milling energies for knife and ring-roller mills. The dynamic mechanical response of biomass pellets was obtained using a novel application of the Split Hopkinson pressure bar. Similar mechanical properties were obtained for all pellets, apart from steam-exploded pellets, which were significantly higher. The quasi-static rigidity (Young’s modulus) was highest in the axial orientation and lowest in flexure. The dynamic mechanical strength and rigidity were highest in the diametral orientation. Pellet strength was found to be greater at high strain rates. The diametral Young’s Modulus was virtually identical at low and high strain rates for eucalyptus, mixed wood, sunflower, and microwave pellets, while the axial Young’s Modulus was lower at high strain rates. Correlations were derived between the milling energy in knife and ring roller mills for pellet durability, and quasi-static and dynamic pellet strength. Pellet durability and diametral quasi-static strain was correlated with HGI. In summary, pellet durability and mechanical tests at low and high strain rates can provide an indication of how a pellet will break down in a mill.