Reynolds Averaged Navier-Stokes Techniques Research Articles

Influence of Flow Rate Distribution on Combustion Instability of Hypergolic Propellant

Combustion instability is the biggest threat to the reliability of liquid rocket engines, whose prediction and suppression are of great significance for engineering applications. To predict the stability of a combustion chamber with a hypergolic propellant, this work used the method of decoupling unsteady combustion and acoustic system. The turbulence is described by the Reynolds-averaged Navier–Stokes technique, and the interaction of turbulence and chemistry interaction is described by the eddy-dissipation model. By extracting the flame transfer function of the combustion field, the eigenvalues of each acoustic mode were obtained by solving the Helmholtz equation, thereby predicting the combustion stability for the combustion chamber. By predictions of the combustion chamber instability with different flow rate distributions, it was found that the increasing of inlet flow rate amplitude will improve the stability or instability of combustion. The combustion stability of the chamber was optimized when the flow rate distribution for the oxidant was set more uniform in the radial direction. The heterogeneity of the flow rate distribution in the circumferential direction is not recommended, considering that a homogeneous flow rate distribution in the circumferential direction is beneficial to the combustion stability of the chamber.

Numerical Study of Stratified Flames Using Reynolds Averaged Navier Stokes Modeling.

Reynolds averaged Navier Stokes technique was used to develop a validated numerical model for stratified flames. The validation was carried out with the experimental data of the non-swirl flames of the Cambridge dual annulus swirl burner. The RNG k–ε turbulence model along with the SG-35 skeletal chemical mechanism was found to give a good prediction of scalar and vector quantities while resulting in the reduction of computational time by 99.75% in comparison with that required for large eddy simulation techniques used in the literature. The effect of stratification at a constant input power, global equivalence ratio, and Reynolds number was examined. At stratification ratios (SRs = ϕin/ϕout) 1 and 2, intense burning, marked by the higher OH concentration, was observed close to the bluff body. Beyond SR = 2, the region of intense burning shifts downstream away from the bluff body. This is a result of the high equivalence ratio in the inner annulus, which is beyond the rich flammability limit of methane–air flames, and as a result, the primary flame region is shifted downstream after the mixtures from inner and outer annulus have mixed properly to produce a mixture with the equivalence ratio in the flammability limit. The maximum temperature was found to increase by 24.1% when the SR is increased from 1 to 2 and the combustion efficiency was found to significantly improve by 267%. The highest maximum temperature of 2249 K is observed for the mildly stratified flame at SR = 2. Beyond SR = 2, the maximum temperature decreases, while the combustion efficiency increases slightly.

Large yacht resistance reduction by hydrodynamic multi-objective shape optimization

ABSTRACT This paper presents the results of the hydrodynamic optimization of a large yacht concept design that is meant to be used as a reference for future buildings. The optimization framework is used to improve the calm water resistance performance of a generic baseline design while satisfying a set of geometrical constraints. The shape of the hull is modified according to a geometrical interpolation approach where two shapes are combined to obtain a third design. The search towards the optimum is driven by a global convergence, multi-objective, genetic algorithm where the resistance of each candidate is evaluated by a Boundary Element Method. The final optimal design is then verified by means of a high-fidelity Reynolds Averaged Navier–Stokes technique, preliminary validated against available experimental data. The obtained results show the strong influence of the stern shape of the hull on the final resistance performance, highlighting the effects of the pressure recovery due to a so-called S-shape bottom. The bow shape is modified consistently to fulfil the geometric constraints and to further improve the resistance performance by inducing a positive interference effect on the generated wave pattern.

Optimization of Air Cooling System Using Adjoint Solver Technique

Air cooling systems are currently the most popular and least expensive solutions to maintain a safe temperature in electronic devices. Heat sinks have been widely used in this area, allowing for an increase in the effective heat transfer surface area. The main objective of this study was to optimise the shape of the heat sink geometric model using the Adjoint Solver technique. The optimised shape in the context of minimal temperature value behind the heat sink is proposed. The effect of radiation and trapezoidal fin shape on the maximum temperature in the cooling system is also investigated. Simulation studies were performed in Ansys Fluent software using the Reynolds—averaged Navier–Stokes technique. As a result of the simulation, it turned out that not taking into account the radiation leads to an overestimation of temperatures in the system—even by 14 ∘C. It was found that as the angle and height of the fins increases, the temperature value behind the heat sink decreases and the heat source temperature increases. The best design in the context of minimal temperature value behind the heat sink from all analysed cases is obtained for heat sink with deformed fins according to iteration 14. The temperature reduction behind the heat sink by as much as 25 ∘C, with minor changes in heat source temperature, has been achieved.

Modelling and dynamic mode analysis of compressor impeller spike-type stall with global stability approach

Flow instability associated with spike-stall restricts the development of advanced aero-engines with a high-speed centrifugal compressor. However, given the irregular structure of centrifugal compressor, few approaches can be applied to stability analysis. Considering the typical features of spike-stall, an implement of stability analysis method on the flow field of impeller inlet was proposed innovatively. Thus, in-depth comprehension of stall perturbation structure and accurate prediction of stall perturbation propagation can be obtained. In this work, an unsteady full annular simulation of a centrifugal compressor with a volute coupled with experimental validations was carried out by using the Reynolds averaged Navier–Stokes technique. Then, an advanced dynamic mode decomposition method named compressed DMD was employed to extract and analyze the dominant stall perturbation structure. Finally, considering the effects of eddy viscosity and molecular viscosity, a two-dimensional global stability analysis method based on the θ-z cylindrical surface at the impeller inlet is established to reveal the propagation behaviors of stall perturbation. The results indicate that the compressed DMD approach can capture low-frequency perturbation caused by stall accurately but not dependent on samplings. The flow structure near stall can be decomposed into the influenced mean flow, flow perturbation associated with rotor frequency and blade passing frequency with the frequencies of 1333 Hz and 9333 Hz, respectively, and stall perturbation with the frequency of 510 Hz. In the view of perturbation propagation features, it is mainly two propagations along the θ-z cylindrical surface. One is the reverse propagation to the upstream axially; the other is the secondary propagation to the blade leading edge with the effect of the main flow during the first propagation process. Both propagation paths deflect to the rotation direction, and the deflection degree of the second propagation paths is larger than that of the first one, thus, a triangular instability region is formed. The researches not only provide a new thought for the application of stability analysis in the centrifugal compressor but also clarify the propagation characteristics of stall disturbance, which lays a foundation for accurate flow control.

Errors Characterization of a RANS Simulation

Abstract This research uses data from direct numerical simulation (DNS) to characterize the different errors associated with a Reynolds-averaged Navier–Stokes (RANS) simulation. The statistics from DNS (Reynolds stresses, kinetic energy of turbulence, κ, and dissipation of turbulence, ε), are fed into a RANS simulation with the same Reynolds number, geometry, and numerical code used for DNS. Three integral metrics error based on the mean velocity, the moduli of the mean rate-of-strain tensor, and the wall shear stress are used to characterize the errors associated with the RANS technique, with the RANS model, and with the linear eddy viscosity model (LEVM). For developed and perturbed flow, it is found that the mean velocity of the RANS simulations with the DNS statistics is almost the same as the mean velocity from DNS data. This procedure enables the study of the relative importance of the different Reynolds stresses in a particular flow. It is shown that for the bounded perturbed turbulent flows studied here, almost all the necessary effects of turbulence are contained in the Reynolds shear stress.

Evaluation of Aerodynamic Characteristics of Mega-Yacht Superstructures by CFD Simulations

Air wake distribution around the superstructure of a mega-yacht is a key concern for the designer because of various reasons such as comfort expectations in recreational deck areas, self-noise generation, air pollution and temperature gradients due to exhaust interactions, and safety of helicopter operations such as landing/take off and hovering. The Reynolds-averaged Navier-Stokes (RANS) technique in computational fluid dynamics (CFD) is frequently used in studies on mega-yacht hydrodynamics and aerodynamics with satisfactory results. In this article, a case study is presented for the utilization of CFD in a mega-yacht's superstructure design. The flow field in recreational open areas has been analyzed for the increase in velocity due to the existence of the superstructure. A reduction in self-noise of the mast structure has been aimed by reducing flow separation and vorticity. Time-dependent velocity data obtained with scale-resolving simulations are presented for the evaluation of helicopter landings. The capabilities and limitations of the RANS technique are discussed along with recent developments in modeling approaches.

Vortex trajectory prediction and mode analysis of compressor stall with strong non-uniformity

Given the heightened requirements of understanding the flow structure under critical condition and improving flow control technique for the turbocharger compressor, further insight into the unsteady behaviors of spike-stall inception associated with tip leakage flow was vital. In this work, an unsteady full annular simulation of a centrifugal compressor coupled with experimental validations was carried out by using the Reynolds averaged Navier–Stokes technique. A spike-type stall in the impeller was determined with multiple criterion principles. Then, a dynamic mode decomposition procedure was applied to the numerical data to capture and visualize the dominant flow structure in both absolute and relative coordinate. The temporal and spatial development of coherent perturbations was reconstructed. Finally, based on the results of mode decomposition, a modified prediction model of vortex trajectory concerning the asymmetrical flow, which can determine the circumferential location of spike-stall inception, was put forward. The results indicate that the low-frequency perturbation associated with stall inception can be captured by the DMD approach under both absolute and relative frames with the perturbation frequencies of 0.44 RF and 0.5 RF, respectively. Under absolute coordinate, the stall inception can be decomposed into three patterns: stationary stall cell in the mean flow, propagated stall cell in a certain region associated with BPF and oscillated stall cell in the origin place with the stall frequency of 0.44 RF. Considering the flow reconstruction, the flow instability in the impeller is relatively stationary stall inception without a significant phase shift and the stall perturbation occurs at 120∘ circumferential location. The specific location of stall inception is influenced by the blade leading edge, which can be predicted by the prediction model of the vortex trajectory accurately.

Numerical study of the effect of jet velocity on methane-oxygen confined inverse diffusion flame in a 4 Lug-Bolt array

Abstract The effect of jet injection velocities in a methane-oxygen confined inverse diffusion flame is numerically analyzed. The case study considers a 4 Lug-Bolt array burner where oxygen is injected by one central nozzle and where methane is injected by four peripheral nozzles. The simulations are conducted with the Reynolds-averaged Navier-Stokes technique, and the turbulence effect is modeled with the realizable k-e model. In addition, the eddy dissipation model is implemented to calculate the effect of the turbulent chemical reaction rate. Results show that the essential mixture mechanisms are the recirculation zone and the central injection velocity, having a relevant participation on perturbations called instabilities. However, the perturbations could interfere with the mixing process on the braid zone through the drag-impulse effect (ReO2 less than 2500). The instabilities could also affect the flame front far away from the reaction zone at injection velocities of ReO2 between 2500 and 5000. If ReO2 is greater than 5000, the reaction layer may induce intense perturbations that affect the combustion ignition zone, and ReO2 being greater than 10000 might lead to the local quench-flickering discontinuity effect, ripping apart the reaction layer and the complete flame blowout (ReO2 greater than 15000).

A RANS simulation toward the effect of turbulence and cavitation on spray propagation and combustion characteristics

A multidimensional computational fluid dynamic code was developed and integrated with probability density function combustion model to give the detailed account of multiphase fluid flow. The vapor phase within injector domain is treated with Reynolds-averaged Navier–Stokes technique. A new parameter is proposed which is an index of plane-cut spray propagation and takes into account two parameters of spray penetration length and cone angle at the same time. It was found that spray propagation factor (SPI) tends to increase at lower r/d ratios, although the spray penetration tends to decrease. The results of SPI obtained by empirical correlation of Hay and Jones were compared with the simulation computation as a function of respective r/d ratio. Based on the results of this study, the spray distribution on plane area has proportional correlation with heat release amount, NO x emission mass fraction, and soot concentration reduction. Higher cavitation is attributed to the sharp edge of nozzle entrance, yielding better liquid jet disintegration and smaller spray droplet that reduces soot mass fraction of late combustion process. In order to have better insight of cavitation phenomenon, turbulence magnitude in nozzle and combustion chamber was acquired and depicted along with spray velocity.

Numerical simulation of turbulent flame–wall quenching using a coherent flame model

Flame quenching by fine mesh is one of the oldest known methods for mitigating flame propagation. Sir Humphry Davy pioneered the study of flame–wall quenching while developing the mining safety lamp. Installing a quenching mesh around the equipment is an effective preventive technique against hazardous flame propagation. Quenching of flame at the wall is due to the coupled thermo physical process involving heat transfer, flame stretch and preferential diffusion. While laminar flame–wall quenching has been extensively studied both theoretically and numerically, the reported studies of turbulent flame–wall interactions are limited to some Direct Numerical Simulations (DNS), which have subsequently led to improvement of models for predicting flame characteristics in the vicinity of the wall.Large Eddy Simulation (LES) of turbulent flows is considered as a powerful tool to predict the occurrence of instabilities due to heat release, hydrodynamic flow fields and acoustic waves. It provides a better description of turbulent–combustion interaction than the classical Reynolds Averaged Navier Stokes techniques (RANS). In the present study, the single mesh quenching of turbulent flame deflagration is investigated in stoichiometric methane–air mixture, which is equi-diffusive with unit Lewis number. It is well known that flame stretch and preferential diffusion has negligible influence in flame quenching for equi-diffusive flames. Therefore a unity Lewis number flamelet formulation can be used for simulating the turbulent combustion process within the premixed flamelet regime.In the present study, LES predictions are performed using the OpenFOAM CFD toolbox solver. The Coherent Flame Model (CFM) in the LES context as proposed by Richard et al. (2007) is implemented for modeling flame deflagration. During the flame/wall interactions, enthalpy loss through the wall affects the flamelet speed, flamelet annihilation and flame propagation; and the decrease in turbulence scales near the wall affects turbulent diffusion and flame strain. These flame–wall interactions are accounted for through extending the closures proposed by Bruneaux, Poinsot, and Ferziger (1997) for the CFM–RANS model to the LES context. Preliminary testing has demonstrated good potential of the modified CFM to capture the quenching effect of the wall.

Numerical sensitivity analysis of 3- and 2- dimensional rib-roughened channels

Rough surfaces have been used as a tool to enhance heat transfer by increasing the level of turbulence mixing in the flow. In numerically simulating such flows, it is common to simulate a 3D rib-roughened channel with a 2D domain in order to reduce the computational time and power. The main purpose of the present work is to investigate the accuracy of the above approximation. In order to do so, initially a 3D channel is simulated using Reynolds-Averaged Navier–Stokes technique and comparison is made against 2D simulations as well as experimental data. In addition, the effects of rib thermal boundary condition and near-wall treatments are also investigated. All computations are undertaken using the commercial CFD code ‘STAR-CD’. The Reynolds number, based on the channel bulk velocity and hydraulic diameter, is 30,000. Two low-Reynolds-number linear Eddy–Viscosity Models, namely the Lien–Chen–Leschziner k − e model and a variant of Durbin’s v 2 − f formulation are used. In the CFD simulations reported here, the focus is on the experimental data of Rau et al. (ASME J Turbomach 120:368–375, 1998). It was found that the present results for a 3D channel are in relatively good agreement with the data. It was also shown that a 2D channel can be used to represent the flow in the centre-line of a 3D channel with relatively good accuracy.

Large eddy simulation of particle-laden flow in a duct with a 90° bend

Large eddy simulation (LES) of particle-laden turbulent flow is studied for a square duct with a 90° bend and a radius of curvature of 1.5 times the duct width, and for a Reynolds number based on the bulk flow velocity of 100,000. A Lagrangian particle tracking technique is used to study the motion of particles experiencing drag, shear lift, buoyancy and gravitational forces in the flow. LES predictions capture important physical aspects of these flows known to occur in practice, unlike alternative Reynolds-averaged Navier-Stokes (RANS) approaches, such as flow separation in the boundary layers around the bend entrance on the concave wall of the bend, and around the bend exit on the convex wall. The LES predicted flow and particle statistics are generally in good agreement with both experimental data used for validation purposes and RANS solutions, with r.m.s. fluctuating velocity predictions from the LES in particular being superior to values derived using the RANS technique.

Spray vaporization in nonpremixed turbulent combustion modeling: a single droplet model

The injection of liquid fuel is a common procedure in turbulent combustion devices operating in the nonpremixed regime. Various numerical models may be found in the literature to calculate such turbulent flames, using either Reynolds averaged Navier-Stokes techniques (RANS) or large eddy simulation (LES). The typical inputs of nonpremixed turbulent combustion modeling are the mean and the fluctuations of the mixture fraction. In computational fluid dynamics codes, the mean source of mixture fraction may be provided by Euler-Lagrange spray modeling. However, the sources of fluctuations of mixture fraction due to vaporization require more closures. Direct numerical simulation (DNS) provides a way of estimating these sources and, using DNS of droplets evaporating in a turbulent flow, it is described how they play an important role in the time evolution of fuel/air mixing in a dilute spray. The statistical properties of the spray and of the scalar field are analyzed to propose a single droplet model (SDM) to evaluate these sources. SDM calculates mean values of the Eulerian source of fuel conditioned on the mixture fraction.