Flow, Turbulence and Combustion最新文献

Based on a (synthetic) turbulent signal which obeys a Gaussian probability density function (PDF) together with some form of prescribed two-point statistics (i.e. integral length or time scales or turbulent energy spectrum), a simple algorithm is proposed to transform the original signal, such that it follows a new target PDF. It is shown that for many practical applications the transformation does not change the integral length or time scale more than a few per cent. The algorithm can be combined with any turbulence generator. It has applications for prescribing boundary or initial conditions of non-Gaussian signals in scale resolving simulations of turbulent flows, such as passive scalars like temperature, bounded passive scalars occurring in reactive flows or velocity signals close to walls.

The drag-type Savonius rotor, a type of vertical-axis wind turbine, is designed to capture wind energy and convert it into rotational torque. However, their efficiency is limited, which restricts their commercial viability. This inefficiency is primarily due to the negative torque produced by the returning blades, which results in minimal power output. This study examines the effect of the aspect ratio on a new elliptic-shaped deflector using three-dimensional (3D) computational fluid dynamics (CFD) modeling and an optimization approach. The aim of this novel deflector is to enhance the aerodynamic performance of the Savonius turbine by reducing negative torque during blade sweeping on the return side. Although there is extensive literature on elliptic-shaped bodies, there is a notable lack of research on the interaction between airflow over such a body used as a deflector and the Savonius rotor. This research uses an optimization methodology based on the design of experiments to determine the optimal design. Using the Taguchi method and analysis of variance, the number of blades is identified as the most significant factor, accounting for 77% of the rotor performance near the deflector. At a Tip Speed Ratio (λ) of 0.8, the optimal deflector achieves the highest average power coefficient of 0.34, representing a significant 42% improvement compared to the maximum average power coefficient without a deflector.

Diffusers are devices found in several engineering applications and their performance and design are object of numerous investigations. However, relatively few investigations have been dedicated to diffusers operating at low and moderate Reynolds numbers. In this regime, the flow could be laminar, turbulent or transitional, and the aerodynamic performance of the diffuser becomes highly dependent on the specific value of the Reynolds number and inlet conditions. In particular, the present study focuses on evaluating the role of inlet conditions on the performance and flow behaviour of two-dimensional diffusers on this specific Reynolds number regime ((Re approx 8000)). Furthermore, the diffuser discharges in a stationary chamber and it does not present a tail-pipe configuration, a condition that has not found a clear presence in the existing literature so far. A numerical investigation of two-dimensional plane diffusers was performed at (Re = 8163) for 9 different cases, combined varying the inlet turbulence intensity (0.05, 3, and 10 percent), and the velocity profile, characterised by different blockage factors (0, 0.05 and 0.33). For each case, the divergence angle ranged from 0 to 30 degrees, and several URANS simulations were performed using the (k-omega) Transitional SST model that accounts for the possible transition of the boundary layer. The results show that the design recommendations valid for high Reynolds number diffusers with a thin boundary layer are not always applicable, and extreme caution must be exercised when dealing with operating conditions that do not ensure a sufficiently high turbulence level at the inlet. The divergence angles of the stall regimes are shown, and performance indicators (e.g. pressure-recovery coefficients) are reported. These reveal a strong decrement (up to 60 percent) of the pressure recovery on reducing turbulence intensity from 10 percent to 0.05 percent. The blockage factor of the velocity profile has an important effect on performance as well. In order to simplify the comparison between the different blockage factors, a modified effectiveness was employed to account for the distortion introduced by a non-uniform inlet velocity profile.

Improving mixing between two coaxial swirled jets is a subject of interest for the development of next generations of fuel injectors. This is particularly crucial for hydrogen injectors, where the separate introduction of fuel and oxidizer is preferred to mitigate the risk of flashback. Raman scattering is used to measure the mean compositions and to examine how mixing between fuel and air streams evolves along the axial direction in the near-field of the injector outlet. The parameters kept constant include the swirl level (S_e = 0.67) in the annular channel, the injector dimensions, and the composition of the oxidizer stream, which is air. Experiments are carried out in cold flow conditions for different compositions of the central stream, including hydrogen and methane but also helium and argon. Three dimensionless mixing parameters are identified, the velocity ratio (u_e/u_i) between the external stream and internal stream, the density ratio (rho _e/rho _i) between the two fluids, and the inner swirl level (S_i) in the central channel. Adding swirl to the central jet significantly enhances mixing between the two streams very close to the injector outlet. Mixing also increases with higher velocity ratios (u_e/u_i), independently of the inner swirl. Additionally, higher density ratios (rho _e/rho _i) enhance mixing between the two streams only in the case without swirl conferred to the central flow. A model is proposed for coaxial swirled jets, yielding a dimensionless mixing progress parameter that only depends on the velocity ratio (u_e/u_i) and geometrical features of the swirling flow that can be determined by examining the structure of the velocity field. Comparing the model with experiments, it is shown to perform effectively across the entire range of velocity ratios (0.6 le u_e/u_i le 3.8), density ratios (0.7 le rho _e/rho _i le 14.4), and inner swirl levels (0.0 le S_i le 0.9). This law may be used to facilitate the design of coaxial swirled injectors.

Spray cooling has been proved to be an effective method for treating scald. However, enhancing its cooling effectiveness and improving user’s thermal comfort are the key factors for its practical implementation. In this study, numerical simulation with computational fluid dynamics software and experimental testing, and subjective questionnaire surveys. Factors influencing the heat removal efficiency of spray cooling for scald treatment and the user’s perception under spray cooling conditions were studied. The results showed that spray temperature had a significant impact on cooling efficiency. The distance between the spray and skin, mass flow rate, and spray medium also had noticeable effects. Additionally, the influence of spray cooling on thermal sensation and thermal comfort under different spray temperatures was investigated. By introducing a “temperature correction coefficient”, thermal sensation data closer to scald conditions were obtained. Experimental results demonstrated that compared to splashing, spray cooling exhibited better cooling effectiveness and comfort feelings. Using the Predicted Mean Vote and Thermal Comfortable Vote as indicators and considering both cooling effectiveness and human thermal comfort, the optimal cooling temperature for females was determined to be 13.1 °C and for males 13.5 °C under scald conditions.

In this paper, we report on the efficacy of four different thermal boundary models for Wall-Modelled Large Eddy Simulations (WMLES) of turbulent natural convection. Our test cases consist of Rayleigh-Bénard convection of liquid water at two Rayleigh numbers, (Ra =1.35{times }10^8) and (Ra =10^9), respectively. Two configurations are examined, namely, convection in a box and in a cavity; the latter one involving a free-slip top boundary. For these test cases, the numerical results obtained via WMLES with the thermal boundary models are compared with those of Wall-Resolved Large-Eddy Simulations. According to our comparative studies, a particular version of the so-called Kays & Crawford model provides the most accurate predictions, at least for the test cases considered herein. Additionally, in this paper, we report on WMLES of turbulent convection at a higher Rayleigh number, (Ra =5{times }10^9), with the aforementioned model. For this case, we analyse herein the flow structure and present results for first and second-order statistics of the flow.

Computational singular perturbation (CSP) has been successfully used in the analysis of complex chemically reacting flows by systematically identifying the intrinsic timescales and slow invariant manifolds that capture the essential subprocesses driving the dynamics of the system. In this article, the analytical and computational framework is applied for the first time to analyze the Lagrangian droplets undergoing evaporation and dispersion in the surrounding gases. First, a rigorous mathematical formulation is derived to adapt the CSP tools into the droplet dynamics equations, including the formal definition of the tangential stretching rate (TSR) that represents the explosive/dissipative nature of the system. A steady ammonia and a falling water droplet studies are then conducted to demonstrate the utility of the CSP methodology in identifying various physical mechanisms driving the evolution of the system, such as the distinction of thermal-driven and mass-driven regimes. Various definitions of the importance indices are also examined to provide in-depth analysis of different subprocesses and their interactions in modifying the droplet dynamics.

This study presents an assessment of the Partially Stirred Reactor (PaSR) as a subgrid model for large eddy simulations (LES) of turbulent premixed combustion. The PaSR-LES approach uses a skeletal mechanism for methane/air combustion, and requires the transport of all the species, with a closure for the filtered source terms. The rate of progress for each reaction is given by the mixing and chemical time scales, which are computed from global flame parameters and a turbulent time scale respectively. This model is applied to a swirled combustor exhibiting a V-flame shape attached to the nozzle, subjected to heat loss. LES are carried out for two distinct equivalence ratios at atmospheric pressure. The flow fields and the thermochemical states from PaSR-LES are compared with the experimental data and solutions based on Flamelet Generated Manifolds (FGM). The results show good correlation with the experiments and FGM-LES, though also some sensitivity to the resolution. The approach also reproduces well the effect of heat loss, which is determined by the use of a chemical time scale given by a progress variable. Dedicated analysis of the swirl-stabilized flame on different regions is conducted evaluating the capabilities of the model to reproduce the burning velocity, flame shape and flame structure.

The paper describes the development of a novel transition/turbulence model based on the laminar kinetic energy concept. The model is intended as a base framework for data-driven improvements. Starting from a previously developed framework, mainly aimed at separated-flow transition predictions, suitable terms for model generalization are identified and reformulated for handling different transition modes, namely bypass and separated-flow modes. The ideology for the definition of new terms has its roots in mixing phenomenological and correlation-based arguments, ensuring generality and flexibility and allowing a variety of lines of action for improving model components via machine-learning approaches. The model calibration, carried out with reference to flat plate test cases subjected to different pressure gradients and freestream turbulence levels, is discussed in detail. Although the constructed model is calibrated on a group of classic flat plat cases, the validation campaign, mostly carried out on gas turbine cascades, demonstrates its ability to predict transitional flows with engineering accuracy. Finally, while the model is not specifically developed for natural transition predictions, satisfactory predictions are obtained in scenarios with low freestream turbulence for flat plate and airfoil flows.

The mixing enhancement of a jet and its characteristics are essential for numerous aerospace applications, for example, reducing the infrared radiation of combat aircraft, mitigating noise in passenger aircraft, improving combustion characteristics in conventional jet, ramjet, and scramjet engines, producing vectored thrust for controlling spacecraft, missiles, and satellite. These applications led to studying the compressible jet mixing processes and strategies for controlling them. The mixing process is severely suppressed in high-speed flows (particularly when the jet Mach number is above 0.3) because of the compressibility effects. Jet mixing requires the development of augmentation strategies due to the short flow residence time (about a tenth of a millisecond). This study provides a comprehensive overview of the mixing improvement methods for compressible jets. It begins with an introduction to the compressible flow mixing layer. It examines several methods for enhancing jet mixing, such as active or passive control and unconventional mixing techniques like fluidic oscillators and mixing induced by shock waves. The passive flow control strategies make the flow more unstable and introduce large-scale vortices in the flow direction. The investigators studied the passive jet control configurations based on the above two approaches to increase mixing efficiency while maintaining a tolerable thrust loss and base drag. Active flow control is achieved by inducing instability but are only effective for appropriately selected values of actuating frequency, duty cycle, mass flow ratio, exit diameter of the actuating jet, location of actuators, etc.