An Euler-Lagrange model is presented that describes the dynamics of liquid droplets emerging from a high-pressure spray nozzle in a relatively large volume (of the order of almost a cubic meter). In the model, the gas phase is treated as continuum, solved on an Eulerian grid, and the liquid phase is treated as a dispersed phase, solved in a Lagrangian fashion, with interphase coupling through state-of-the-art drag relations obtained from direct numerical simulations. The droplets are introduced into the system at high velocities, leading to a turbulent self-induced gas flow which is solved using large eddy simulation. Despite the relatively low liquid volume fraction in the spray, the number density of droplets at the nozzle is still more than 10¹⁰m^-3, which is why we employ a highly efficient stochastic Direct Simulation Monte Carlo approach to track collisions between droplets. The droplet collision frequency is calculated on the basis of local droplet number density, droplet size and relative velocities of neighbouring droplets within a dynamically adapting searching scope, as described in Pawar et al. (2014. Chem. Eng. Sci. 105, 132-142). We use known correlations from literature to determine the outcome of a binary droplet collision, which depending on characteristic dimensionless numbers can be coalescence, bouncing or, for high velocity impacts, stretching or reflexive separation leading to formation of satellite droplets. Our simulation model is compared with droplet velocities and size distributions obtained from phase Doppler interferometry experiments on an industrial scale hollow-cone pressure swirl nozzle spray. We find semi-quantitative agreement for spray characteristics such as the axial and radial spray velocity, spray jet width, and the dependence of the droplet size distribution on position within the spray. The simulation model enables us to study the relative importance of different droplet collision events occurring in the spray volume.

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In this paper, a modified Direct Simulation Monte Carlo (DSMC) algorithm is introduced which is tailored towards achieving quantitative agreement with deterministic Discrete Particle Model (DPM) simulations for the collision frequency between particles in dilute granular flows. To avoid lattice artifacts, we use a spherical searching scope in which a particle searches for a collision partner during each particle time step. The particle collision frequency is calculated on the basis of particle concentration, particle sizes and relative velocities of neighbouring particles within the searching scope, similar to existing DSMC methods found in the literature. However, when the particle time step is limited by an external time step, such as the time step for the solver of the gas equations, without additional measures, the resulting searching scope often contains a single or even no neighbours, with detrimental effects on estimates of the average collision frequency. We modified the method to automatically and self-consistently increase the searching scope until it contains a minimum number of neighbouring particles to ensure that a statistically accurate and unbiased estimate of the average collision frequency is made. The developed stochastic-DSMC model is verified qualitatively and quantitatively with DPM simulations of two colliding streams of elastic as well as inelastic monodisperse spheres. The major advantage of the stochastic-DSMC model is its capability to handle many millions of particles for simulation in a reasonable computation time. This number increases even more when each simulated particle represents a large group of real particles, called a parcel. We investigate how far the parcel size can be increased before the DSMC approach breaks down.@en

Research on the dynamic flow behaviour in spray dryers has a long history. Interest in describing these flows originates from problems like roof and wall fouling. The aim of the present study is to experimentally investigate the dynamic jet behaviour and turbulent flow in a scaled-down cold flow model of a spray dryer in order to better understand and optimize spray drying units. Dynamic jet behaviour and turbulent flow features (i.e., RMS velocities) were studied by particle image velocimetry (PIV) using water as the continuous phase. To obtain more insight in the jet dynamics, we analyzed the turning point, the width and shape, and the velocity profiles of the turbulent jet at different heights and the turbulence characteristics. We found that at higher Reynolds numbers, the jet penetrates further along the downward direction with a time-averaged profile which is symmetric at the centre. In addition, we investigated the effect of the expansion ratio via proper orthogonal decomposition (POD). Outcomes of different characteristics of the dynamic jet, like steady, transient, regular, and complex precession, can be collapsed by proper scaling. These results can be used for validation of computational fluid dynamics simulations and facilitate the design (identification of jet operation boundaries) of new spray dryer configurations.

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