Reynolds-averaged Navier-Stokes Methodology Research Articles

Application of Boundary Layer Combustion in High-Mach-Number Scramjets

Boundary layer combustion is an approach to the reduction of skin friction in hypersonic propulsion systems based on reducing the Reynolds shear stress in the turbulent boundary layer by combustion heat release. In this study, the Reynolds-averaged Navier–Stokes methodology is adopted to study the drag reduction characteristics of boundary layer combustion in high enthalpy flow. Two novel explanations of the local increase in skin friction caused by ignition delay are given in terms of flow structure and the Kármán momentum integral equation, respectively. In addition, it is found that the inhibitory effect of combustion heat release on Reynolds shear stress is gradually weakened with increasing total enthalpy, which is detrimental to drag reduction. Finally, a combination of radical farming and boundary layer combustion is proposed to improve the drag reduction effect. This approach is verified by using a two-dimensional simplified M12-02 scramjet model, with the results indicating that radical farming can considerably suppress ignition delay and thereby further benefit drag reduction and combustion.

Flame Surface Density based mean reaction rate closure for Reynolds averaged Navier Stokes methodology in turbulent premixed Bunsen flames with non-unity Lewis number

The Flame Surface Density (FSD) based mean reaction rate closure proposed by Prof. K.N.C. Bray and co-workers in the context of Reynolds Averaged Navier-Stokes simulations has been a-priori analysed using a Direct Numerical Simulation (DNS) database of turbulent premixed Bunsen flames with different characteristic Lewis numbers representing the strict flamelet regime (i.e. high Damköhler number and low Karlovitz number combustion). The statistical behaviours of stretch factor, orientation factor and wrinkling length scale have been assessed for non-unity Lewis number conditions to identify their Lewis number dependencies. The assumption of presumed bimodal distribution has been found to be rendered invalid close to the nozzle exit where the unresolved wrinkling remains relatively small even when the flow parameters at the nozzle exit represent high Damköhler number and low Karlovitz number conditions. Although the PDF of reaction progress variable shows some resemblance to a bimodal distribution away from the nozzle exit, the Bray-Moss-Libby expressions which can be derived for infinitely large values of Damköhler number have been found to show considerable deviations from the Reynolds averaged reaction progress variable and reaction progress variable variance extracted from DNS data even though these cases represent the wrinkled/corrugated flamelets regime combustion based on nozzle exit conditions. This suggests that it might be necessary to solve a modelled scalar variance transport equation even in the strict flamelet regime instead of using the algebraic relation for the scalar variance. It has been found that the characteristic Lewis number has a major influence on the orientation factor, wrinkling length scale, and the stretch factor. Furthermore, these parameters are found to be (sometimes strong) functions of the axial distance from the nozzle exit. Known parameterisations for the wrinkling length scale and the stretch factor have been shown to be unable to capture the correct variation across the flame brush based on a-priori analysis of DNS data. The variability of the orientation factor and the inadequacy of existing relations to approximate other quantities such as the stretch factor, wrinkling factor and the Reynolds averaged reaction progress variable have the potential to severely limit the performance of algebraic FSD based mean reaction rate closures in turbulent premixed flames within the flamelet regime.

Modeling of In-Line Tube Banks Inside Advanced Gas-Cooled Reactor Boilers

The objective of this research is the prediction of forced convection across confined in-line tube banks found in Advanced Gas-Cooled Reactor (AGR) boiler systems. The focus of this work is periodic flow about a 2 × 2 array. A number of Reynolds-Averaged Navier–Stokes (RANS) turbulence modeling closures have been applied to the challenging case of 1.6 pitch-to-diameter spacing at which flow undergoes transition from straight to diagonal. In contrast to Large Eddy Simulation (LES) predictions, those of RANS models fail to predict transition to diagonal flow. As LES computations are not anticipated to become routine enough for the purpose of flow diagnostics in industrial contexts for many years, further refinement to the RANS methodology in the form of an analytical wall function is one of the main objectives. Its eventual inclusion is not only expected to improve the representation of turbulent boundary layer behavior, but also has scope to account for augmented surface roughness effects experienced by AGR technologies nearing their end-of-life. This work is ultimately in aid of EDF Energy’s policy of AGR lifetime extensions, to allow the UK Government sufficient time to formulate a credible Nuclear New Build scheme to secure our energy requirements in the coming decades. It was found that all but dynamic LES adequately reproduces flow behavior across all criteria considered, and that standard wall functions left much to be desired in the case of flow and thermal predictions, while introducing the analytical wall function generates improvements for flow and heat transfer predictions that are not universally shared between high-Re models utilized.

Effect of adverse pressure gradient on supersonic compressible boundary layer combustion

Influences of adverse pressure gradient (APG) on supersonic turbulent boundary layers are numerically studied using Reynolds-averaged Navier–Stokes (RANS) equations. The RANS methodology is validated by comparing the numerical results with the existing experimental data. Although the flame is restricted in the boundary layer, the heat flux is reduced rather than increased for the suppressed turbulence momentum transportation ability. Moreover, APG contributes more to the heat flux than combustion heat release does. Compared with no-injection case, a large skin friction reduction can be obtained by boundary layer combustion, and further reduction can be achieved in APG state. Additionally, the effects of combustion in APG on the velocity laws of the wall show that White's velocity law is close to the numerical results in the outer region of boundary layer, and Nichols's velocity law is appropriate in the whole boundary layer unless the APG is too strong. Turbulence intensity influences are analyzed in the end. Results show that the additional reduction of skin friction due to induced combustion cannot offset the skin friction increase caused by high turbulence intensity.

Effect of geometric modification on flow behaviour and performance of reverse flow combustor

Numerical investigation had been performed on the reverse flow combustor of a mini gas turbine engine so as to investigate the performance characteristics of the combustor by means of geometry modifications. In order to enhance the thrust performance of the reverse flow combustor, the baseline combustor (Model A) was previously modified by increasing its chamber volume by 15%, the fuel-air ratio (FAR) by 40% and by raising the injection point density to two (Model B). However, the thrust optimization of the baseline combustor resulted in high combustor exit temperature that could potentially damage the combustor liners. To rectify the adversity of high exit temperature, the combustor cooling effects were achieved by subsequently adding additional passage holes at the dilution zone of the Model B combustor so as to direct the incoming cold flow from the compressor exit towards the outgoing hot flow in the reverse flow combustor (Model C). The commercial software ANSYS Fluent 17.0 was adopted in this study and to solve the turbulence model, Reynolds Averaged Navier–Stokes methodology was adopted by employing standard k-ɛ turbulence model with standard wall function. A probability density function model was generated to introduce the combustion species and a discrete phase Model was employed to specify the kerosene fuel-based injector properties. The numerical results of Model A combustor were validated against the previous experimental results using grid convergence test. The numerical results were observed to be in good agreement with the experimental results. However, ineffective mixing was found to be a setback for the baseline combustor (Model A) indicating the need for combustor performance improvement. The comparative results of the revised combustor model (Model C) showed that better cooling effects at the combustor exit could be achieved by adding supplementary passage holes at the downstream of the combustor outer liner, respectively. The addition of dilution holes also resolved the issue of high pressure loss that was observed in Model A combustor with no significant change in the specific fuel consumption. The present paper confirms that the performance of the reverse flow combustor model could be affected by slight geometric modification. The performance characteristics of the combustor models are presented in terms of thrust, thrust-to-weight ratio, specific fuel consumption, pressure loss and pattern factor.

A cavitation aggressiveness index within the Reynolds averaged Navier Stokes methodology for cavitating flows

The paper proposes a methodology within the Reynolds averaged Navier Stokes (RANS) solvers for cavitating flows capable of predicting the flow regions of bubble collapse and the potential aggressiveness to material damage. An aggressiveness index is introduced, called cavitation aggressiveness index (CAI) based on the total derivative of pressure which identifies surface areas exposed to bubble collapses, the index is tested in two known cases documented in the open literature and seems to identify regions of potential cavitation damage.

Large Eddy Simulation of Diesel Engine In-cylinder Flow

The numerical simulation of the in-cylinder flow of a realistic Diesel engine is presented. Over the past three decades most of the CFD research for internal combustion engines (ICE) has been carried out using the Reynolds Averaged Navier Stokes (RANS) approach. Despite the achievements obtained in engine design, numerical investigations with RANS models can only give insight into the mean behaviour of the in-cylinder flow. By contrast, Large–Eddy Simulation (LES) allows a better description of the in-cylinder flow motion and gives access to the cycle-to-cycle variations. Multi-cycle simulations of a motored case using LES and a hybrid LES/RANS approach are compared against experimental data and results obtained with a RANS methodology. The cycle-to-cycle variations in global in-cylinder characteristics such as pressure, swirl number and tumble number are reported and an analysis of the in-cylinder flow for each stroke of the engine cycle is presented. The interaction between spray and turbulence is also analysed with closed-chamber simulations. The effects of the hybrid RANS/LES approach on mixing of the vapour phase of the fuel is described and compared to the results obtained with a standard RANS approach. This work gives an insight into the numerical simulation of Diesel engine and illustrates some of the advantages of the LES methodology over RANS. The trade-off between computational cost and details in the flow description using a hybrid RANS/LES approach is discussed.

LES Predictions of Noise Emissions From a Low-Bypass Ratio Military Gas Turbine Engine

A practical framework for predicting jet structure and noise from military aircraft is described, which is developmental and has been examined for some fundamental jet flow problems. The framework currently utilizes Reynolds-averaged Navier Stokes (RANS) methodology for geometrically complex internal propulsive flowpaths and large eddy simulation (LES) methodology for the jet structure downstream of the nozzle exit. Temporal data from the LES solution is stored on a flared-cylindrical surface surrounding the jet, to be used for noise propagation to the farfield. Earlier applications of RANS methodology combined with the use of analogy-based jet noise codes proved inadequate due to the inability of the noise codes to treat complex 3D flows, such as those associated with multiple nozzles and/or with varied jet noise reduction concepts. Restricting the use of LES (or RANS/LES), methodology to free shear flows remedies the severe grid resolution issues that would be encountered with utilization of LES for modeling internal propulsive flows. The issue of “adequately” initiating the LES solution from a RANS solution profile just downstream of the nozzle exit has been the focus of our exploratory studies and is clearly more complex than standard procedures, such as recycling and rescaling techniques used for simple wall bounded flows. Approaches examined are discussed and unified RANS/LES solutions for several flows are described. The application of this framework to more complex flows requires no fundamental modifications as will also be discussed.