Simulation of Polydisperse Gas-Droplet Mixture Flows with Chemical Transformations

В статье представлен обзор результатов теоретических, численных и экспериментальных исследований процессов горения и инициирования детонации в гетерогенных полидисперсных смесях. Обсуждаются проблемы распыления, испарения и горения капель топлива, а также неравновесные эффекты при распылении капель и фазовых переходах. Влияние неоднородности размеров капель и неоднородности пространственного распределения на воспламенение смеси и ускорение пламени было исследовано для сильного и мягкого инициирования детонации: ударной волной и искровым зажиганием с последующим переходом от дефлаграции к детонации (ДДТ). Изучены особенности впрыска и зажигания струи в реакционной камере. The paper presents the results of theoretical, numerical and experimental investigations of combustion and detonation initiation in heterogeneous polydispersed mixtures. The problems of fuel droplets atomization, evaporation and combustion, and the nonequilibrium effects in droplets atomization and phase transitions are discussed. The effects of droplets size nonuniformity and spatial distribution nonuniformity on mixture ignition and flame acceleration were investigated for strong and mild initiation of detonation: by a shock wave and spark ignition followed by deflagration to detonation transition (DDT). The features of jet injection and ignition in a reaction chamber are studied.

Influence of aqueous foam syneresis on the shock wave propagation velocity

Numerical simulation of the spherical shock pulse propagation in aqueous foam with volumetric liquid fraction of 0.0083 has been carried out in accordance with the published experimental data on the explosion of HE in aqueous foam. The assumption is used that the foam structure is destroyed by the shock wave, which leads to the transformation of the foam into a monodisperse gas-droplet mixture. The system of equations for the two-phase gas-droplet model of aqueous foam includes the laws of conservation of mass, momentum, energy for each phase and the equation for the dynamics of the volumetric liquid fraction in a single-pressure, two-velocity, two-temperature approximations in a three-dimensional formulation and takes into account the forces of the Schiller-Naumann interfacial drag, the Ranz-Marshall interphase contact heat exchange and the effect of foam syneresis on the initial distribution of its volumetric liquid fraction. Realistic equations of state in the form of Peng-Robinson and Mie-Gruneisen are used to describe the thermodynamic properties of air and water that make up a gas-droplet mixture. Numerical modeling of the processes under consideration was carried out in the open software of computational fluid dynamics OpenFOAM using the finite volume method based on the iterative two-step PIMPLE algorithm. The analysis of the effect of foam syneresis on the dynamics of shock pulse in aqueous foam is given. It was found that the uneven distribution of the liquid fraction in the foam, caused by its sedimentation under the gravity, leads to the increase in the shock pulse velocity in upper layers of the foam. In comparative analysis of numerical solutions and experimental data at sensor locations, the importance of taking into account syneresis phenomena in modeling the dynamics of shock wave in aqueous foam is shown. The reliability of calculations obtained by the proposed model is confirmed by their agreement with experimental data.

Modeling of grain processing in a pneumomechanical dresser

To improve the quality and effectiveness of pre-sowing treatment of seeds of grain crops, a hypothesis was put forward on the possibility of using the air flow to continuously perform a set of technological operations in the process of dressing. The proposed approach is implemented in a unrivalled pneumomechanical deresser, in the processing chamber of which a multiphase flow of dispersed mixture “air-seeds-droplet” is formed. For the theoretical justification of the main parameters of the processing process, a mathematical model of the movement of seeds and fine droplets of the working fluid was developed and numerically solved taking into account the decrease in their concentration along the length of the working chamber. As a result, it was found that the speeds of seeds and drops of different sizes as they move in the processing chamber naturally increase, and the concentration of drops decreases asymptotically. It was revealed that to ensure high quality processing at the entrance to the processing chamber, the seeds should move in suspension with the speed of the corresponding droplet speed of the working fluid. The minimum path length of the “air-seed-droplet” mixture is substantiated, where the maximum decrease in the concentration of drops occurs due to the adhesion of seeds suspended in the air flow to the surface. The results obtained are important for optimizing the length of the processing chamber, substantiating the modes and parameters of the movement of air, grain and drops, at which full coverage of seeds with a minimum flow rate of the working fluid is achieved. In the long term, the research results will serve as the basis for optimizing the design and technological parameters of pneumomechanical dressers, which ensure high quality and efficiency of pre-sowing treatment of seeds of grain crops.