Flow, Turbulence and Combustion最新文献

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.

A new approach using optimization techniques for controlling water flows is proposed in this work. The investigated problem is related to shallow water flows where a given time-evolution of outflow should be determined in order to control water elevation at some region. Typical applications are problems involving the control of movable barriers or water flowing through floodgates to prevent inundation. Usually, this type of problem is solved using gradient-based control techniques which can provide complex solutions that can be difficult to be implemented in practical situations. Here, the shape of the outflow discharge along time is predefined by a curve parametrization and used as design variable of an optimization problem. The shallow water equations are evaluated using the Finite Element Method (FEM). Numerical applications of water height control are presented and the different shapes of water outflow are investigated and discussed. As a result, the present framework can solve optimal flow control problems where an outflow discharge must satisfy a given type of variation along time.

From the recent DNS database (Yang in Journal of Fluid Mechanics) of channel flows with rough walls in the presence of heat transfer, the impact of the skewness of the roughness elevation map on the velocity and temperature profiles within the roughness sublayer is analysed. The separation zones observed near the wall in the sublayer are shown to play a significant role when the skewness is negative. The (k-omega)-based turbulence model (Chedevergne and Forooghi in Journal of Turbulence 21:463–482, 2020); (Chedevergne in Journal of Turbulence 22:713–734, 2021, Chedevergne in Journal of Turbulence 24: 36–56, 2023), capable of capturing roughness effects and incorporating the Double Averaged Navier–Stokes (DANS) equations and the Discrete Element Method (DEM), is tested against this DNS database, showing some limitations in the description of the roughness sublayers, especially for configurations with negative skewness. To reproduce the observations made in the DNS database, the pressure gradient imposed in the simulated channel using the DANS/DEM model is adjusted based on the distance to a reference wall in the roughness sublayers. Additionally, the increase in turbulent mixing observed in the DNS database for rough configurations with negative skewness is accounted for in the DANS/DEM model by modifying the source terms in the transport equations of the turbulent scalars with respect to the skewness, improving the prediction the roughness effects.

The objective of the present article is to address the influence of turbulence-radiation interactions (TRI) on parameters associated with the evaporation rate of fuel droplets in the spray combustion of a fuel mixture containing ({text{C}}_{10}{text{H}}_{22}) within a model combustor. Variables such as turbulence kinetic energy, TRI factors, and temperature distributions, particularly at the sub-grid scale, are investigated utilizing the large eddy simulation approach. Also, parameters including the pattern factor and (text{NO}) concentration at the combustor outlet are assessed. The Eulerian approach to simulate the gaseous phase and the Lagrangian approach to model the liquid phase are employed. A two-way is used to couple their interactions, excluding the secondary breakup due to the Weber number being less than unity. The wall-adapting local eddy-viscosity model is adopted to simulate the eddy viscosity. The discrete ordinates method with the weighted-sum-of-gray-gases model is applied for thermal radiation calculating absorptivity and emissivity. The probability density function is utilized for modeling combustion. Results indicate that considering TRI facilitates the vaporization of fuel droplets due to accelerating the breakup process of the largest droplets by 3.36%, increasing their volumetric heat capacity by 4.50%, and reducing the penetration length by 10 mm. Furthermore, the maximum (text{NO}) pollutant concentration at the combustor outlet decreases from 11.64 to 9.84 ppm, and PF reduces from 0.034 to 0.011 in the presence of both resolved and sub-grid scale TRI.

Planar velocity measurements using the particle image velocimetry technique have been performed at a repetition rate of 10 kHz in the prechamber of a large bore gas engine mounted on a rapid compression machine (RCM), to visualize the velocity fields in the non-reacting gas flow during a compression stroke. The prechamber investigated in this work is a prototype with modifications made to facilitate optical access, and it is mounted axially on the RCM combustion chamber. The parameters of the compression stroke in the RCM are set to achieve a compression ratio of 10. After removing outlying data based on pressure and piston displacement curves, PIV data from compression strokes were analyzed. The time-resolved velocity fields capture the formation and motion of a tumble vortex in the imaged plane. Mean flow fields obtained by phase averaging across the datasets are presented, showing the development of the flow field in the prechamber throughout the compression stroke. The data obtained will be used to validate CFD simulations.