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In spray simulations, uncertainties in the collision incidence predictions always exist for droplet collision models under the discrete droplet model framework. In this study, Beer's law, which is used for electromagnetic radiation and light absorption, was improved to evaluate the accuracy of droplet collision models by eliminating the shadow effect among the light-absorbing (stationary) parcels. In addition, the range of parcel numbers in the computational domain was carefully calibrated to ensure the appropriate implementation of Beer's law. Based on the improved Beer's law, a theoretical evaluation method of the computational efficiency of droplet collision models was proposed to conduct a qualitative analysis. Using the above methods, a complete approach for evaluating the accuracy and efficiency of droplet collision models was established. Three representative collision models, including the O'Rourke model, the Nordin model, and the no-time-counter (NTC) model, as well as a new hybrid stochastic/trajectory (HST) collision model, were evaluated comprehensively. The convergence and dependence of the predicted collision incidence on various factors, including parcel radius, grid size, reference frame velocity, and droplet number represented by one parcel, were investigated. It was found that the HST model shows relatively better performance on the collision incidence compared with the other collision models in most cases, especially when one parcel represents multiple droplets. The NTC model achieves the highest computational efficiency, and the computational cost of the construction of the adaptive collision cells in the NTC model and the HST model constitutes a major proportion of the total time consumption. [ABSTRACT FROM AUTHOR]

The reversible elementary reaction mechanism of six components and seven steps of H2/O2 are applied by using a CFD-DSMC coupling iteration method to study the impact of boundary on flow, heat transfer and chemical reaction in a microtube. The microtube consists of a converging section and a straight section, which represents the gap on the contact surface of the pellets in porous media. It shows that after coupling, with the designed conditions in this paper, the influence of wall temperature is more obvious than that of wall slip velocity on the coupling results from the analysis of chemical reaction, yet the velocity field in the boundary layer is more affected by the wall slip velocity. In addition, the velocity in the central region of the flow decreases while the concentration of reaction products increases after coupling, due to the increasing of the velocity in the boundary layer and the influence of wall temperature, respectively. By the coupling of CFD-DSMC methods, more details and influence of the boundary can be considered, and the computational efficiency is higher than that of the single microscopic method. [ABSTRACT FROM AUTHOR]

The random structure model of porous media is constructed by the gravity packed method, which can give a random packed structure that is similar to the actual packed bed structure by simulating the free fall, collision and stacking process of pellets via the OpenFOAM. Different from the existing ordered structure model of porous media, the structure model adopted in this paper has strong randomness since it simulates the actual generation process of the porous media material. Meanwhile, the following batch modeling instructions are completed with the MATLAB, which can ensure that the random structure model of porous media has the advantages of high similarity with the actual porous structure and rapid modeling. [ABSTRACT FROM AUTHOR]

[Display omitted] • A porous burner formed by SiC foam ceramic and alumina pellets is designed. • The packed bed surface flame characteristics are captured by the high-speed camera. • The wake flow fields under different combustion are analyzed by the PIV system. • Combustion zone map as a reference for the industrial application is obtained. A porous media burner composed of silicon carbide foam ceramic material and alumina pellets was developed to delve into the surface combustion. The surface flame characteristics under lean-burn conditions were captured with the high-speed camera. With the K-type thermocouples and the particle image velocimetry (PIV) method, the temperature distribution and the wake flow field in different combustion cases were analyzed. Moreover, by accurately regulating the inlet flow and the premixed equivalence ratio, the wide variety of flame forms are classified (e.g., the centered blue flame, the tiled blue flame, the tapered dark red flame and the tapered bright red flame). Lastly, the combustion zone map was generated to classify the mentioned variety of flame forms, which could be referenced to assess thermal characteristics for surface combustion. [ABSTRACT FROM AUTHOR]

• A new hybrid stochastic/trajectory (HST) droplet collision model was developed. • Collision cell, collision frequency, and collision pair selection were updated. • Extensive validation was performed using measurements of spray-to-spray impingement. • Accuracy of prediction is significantly improved compared to previous models. • Stable performance is achieved by HST model under various calculation parameters. Focusing on the spray simulation in engines using the Lagrangian-Drop Eulerian-Fluid (LDEF) approach, a new hybrid stochastic/trajectory (HST) droplet collision model with the improved collision frequency, the redivided collision cell, and an updated method for selection of the candidate collision pairs was developed in this study. The stochastic method used in the HST model was based on the theoretical framework of the time-counter (TC) direct simulation Monte Carlo (DSMC) method. Besides the stochastic method, the trajectory method was also introduced for the cases with sparse spray conditions in order to improve the reliability of the model predictions. The factor of "every parcel represents a certain number of droplets" was well considered in the calculations of the droplet collision frequency in the present model, which has not been attracted enough attention in previous models. Moreover, an adaptive collision cell technique was used for reducing the dependence of the calculation results on the computational mesh. The formula for selecting the collision pairs was updated in order to accurately reproduce the spatial distribution of the spray droplets after collision. To assess the performance of the droplet collision model, the experimental data with the direct spray-to-spray impingement under wide operating conditions were used for model validations. In addition, the numerical results by the droplet collision models including O'Rourke model, Nordin model, and no-time-counter (NTC) model were presented for comparison. The results indicate that the HST model is capable of satisfactorily reproducing the spray morphology, collision frequency, and Sauter mean diameter (SMD) of the droplets during the spray process. Meanwhile, good independence on the computational grid size, time step, and parcel number can be achieved by the HST model. [ABSTRACT FROM AUTHOR]

In this paper, a three-dimensional (3D) randomly packed bed model is established to research the filtration combustion process. 3D numerical simulation of premixed combustion in the randomly packed bed is carried out, and the temperature distributions and flame propagation characteristics are predicted and discussed. Simulation results show that the change rules of the temperature field and the flame surface in the random model are consistent with the classical theory, and the stochastic packed model is more promising for numerical simulation of filtration combustion in comparison with the traditional regular structures. [ABSTRACT FROM AUTHOR]

In the research of porous media and the interior of microelectronic devices, fluids frequently shuttle among pores and wall cracks, therefore it is of great significance to explore the influence of media wall on fluid flow and heat transfer. In this paper, CFD-DSMC coupled iterative algorithm is first used in the research of the flow and heat transfer of gas near the wall in the porous media, which simplified into the flow and heat transfer in micron-scale tube with smooth and rough wall conditions. It is found that compared with the pure CFD method, the velocity field obtained by the coupled method is quite different in the region close to the wall, and this will affect the flow field in the mainstream region, that is, with the higher roughness of the wall, the velocity in the central region will be smaller. There is no significant difference in the temperature distribution in the microtube before and after coupling, but the temperature in the mainstream decreases with the increase of the roughness of the wall. In addition, the temperature jump was found from some of the coupling results close to the wall. The results will contribute to a deeper understanding of the influence of rough surfaces on gas flow and heat transfer near the wall, and provide guidance for the regulation of flow and heat transfer in porous media. [ABSTRACT FROM AUTHOR]

The low velocity filtration combustion of a lean methane/air mixture in the fixed bed reactors is numerically investigated using the system scale method. The effect of porosity on the flame characteristics, such as temperature profile, flame displacement speed and flame shape is examined in this study. For comparison, porosity is considered in two forms, i.e., volumetric average porosity (VAP) and porosity spatial distribution function (PSDF). The simulation results show that, local distribution of porosity primarily affects the flame shape, while having minimal impact on the peak flame temperature and mean flame displacement velocity. Quantitative analysis indicates that the flame displacement velocity obtained using VAP is closer to the experimental data compared to the PSDF. However, the simulation with PSDF explicitly captures the flame front bending caused by local porosity variation. Velocity and temperature fields suggests that flame front bending predominantly depends on the local flame displacement speed (LFDS), which is the vector sum of local interstitial airflow speed and the laminar flame speed of the combustible gas. The radial difference in LFDS, which is associated with uniformity in velocity and temperature fields, serves as the main driving force for flame front instability. The inclusion of PSDF enables a more accurate prediction of flame front development by capturing the preferential flow near the wall. [ABSTRACT FROM AUTHOR]