Abstract
Industrial system that comprises flow of suspended particles in fluid generally requires an understanding of the multiscale physics occurring from micro/nanoscale to mesoscale and eventually to the macroscale phenomena. Because of the inherent complexities that are prevalent in such flow, investigations are certainly at the crossroads of intense research and development in the environment of significant advancements in experimentation as well as in computing power and performance. Much concerted development is nonetheless still needed to gain a better understanding of the complicated physics through the advancement of experimental techniques and computational methodologies and models and to specifically meet the increasing demand of improving efficiency of industrial multiphase flow system.
This special issue consists of nine papers. The advances of experimental and modelling investigation of multiphase flow system are detailed in the following.
The paper, “Heat transfer coefficient during Evaporation of R-1234yf, R-134a, and R-22 in Horizontal Circular Small Tubes,” by K. I. Choi et al. provided an experimental study where convective boiling heat transfer coefficients were obtained. Effect of heat flux, inner tube diameters, and saturation temperatures was investigated. Heat transfer coefficients were found to increase with increasing heat flux due to the presence of nucleate boiling augmenting the heat transfer process. Heat transfer coefficients were also found to increase with increasing saturation temperature and decreasing inner tube diameter. Correlations obtained in this study can be applied with sufficiently good accuracy to improve the compact heat exchanger design.
The paper, “Flow characterisation of dense-phase pneumatic conveying system of pulverized coal through electrostatic sensor arrays,” by F. Fu et al. focused on experiment investigation being performed on dense-phase pneumatic conveying powder utilising electrostatic sensor array (ESA) and electrical capacitance tomography (ECT). Distribution images over the cross section of the pipeline were obtained for different superficial gas velocities. Measurements obtained through the experiment demonstrated that characteristic of particles motion and its change with increasing superficial gas velocity were different in the dense and dilute phase region of the pipe.
The paper, “Particle dispersion behaviors of dense gas-particle flows in bubble fluidized bed,” by S. Lv et al. depicted the application of Euler-Euler two-fluid model, an improved momentum transfer empirical coefficient and wavelet analysis method as well as large eddy simulation (LES) for complex dense gas-particle flow system. The study revealed that particle collision frequencies at bubble vibrant movement regions along axial direction were found to be much higher than those of radial direction and attenuated along height increase. Also, representing bubble movement, low frequency components of pressure signal in the centre were stronger than wall regions.
The paper, “Solid suspension by an upflow mixture of fluid and larger particles,” by R. Di Felice and M. Rotondi provided an experimental investigation to shed light on the fluid dynamic interactions between the solid and the fluid phases in binary solid mixture suspensions, with the smaller particles fluidized by a mixture of larger neutrally buoyant particles and fluid. The experimental information reported can be used to determine a proper relationship for the drag force in binary-solid suspensions and relationship of relevant importance when computational fluid dynamics (CFD) simulations can be performed.
The paper, “Modeling and experimental investigation of pressure field in the grinding zone with nanoparticle jet of MQL,” by C. H. Li et al. focused on solid nanoparticles being added in minimum quantity lubrication (MQL) to make nanofluids and injected in the grinding zone in the form of jet flow. A mathematical model of two-phase flow pressure field of grinding zone with nanoparticle jet flow of MQL was established. The speed of grinding wheel, the gap between work piece and grinding wheel, jet flow velocity, and spraying angles of nozzles on the pressure field of grinding zone were explored. Experimental results were found to be generally consistent with theoretical simulations.
The paper, “Experimental characterisation and modelling of homogeneous solid suspension in an industrial stirred tank,” by S. Calvo et al. depicted a numerical study based on the development of a CFD model to describe the particle distribution of aluminium salts in an industrial scale tank. This model, validated against experimental data, was further applied to formulate scale-up and scale-down correlations to predict the minimum impeller speed needed to reach homogeneous solid distribution of aluminium salts. Results obtained from different scales allowed correlating values of aluminium salts with the volumetric power consumption of the multiphase flow system.
The paper, “Large-eddy simulation of particle-laden turbulent flows over a backward-facing step considering two-phase two-way coupling,” by W. Bing et al. provided a fundamental numerical study of two-phase turbulent particle-laden flows over a backward-facing step. Particle dispersions in large-scale eddy coherent structures and statistical mean and fluctuating velocities were presented to illustrate particle modulations to turbulence. Influences of particle size and material density on changes in turbulence were analysed.
The paper, “A robust asymptotically based modeling approach for two-phase flows,” by M. M. Awad and Y. S. Muzychka focused on a theoretical study based upon an asymptotic modelling method. A robust compact model was developed by taking into account the important frictional interactions that occur at the interface between liquid and gas as the liquid and gas phases have been assumed to flow dependently of each other. The only unknown parameter in the asymptotic modelling method in the two-phase flow is the fitting parameter which corresponds to the minimum root mean square (RMS) error for any data set.
The paper, “Bounds of two-phase frictional pressure gradient and void fraction in circular pipes,” by M. M. Awad and Y. S. Muzychka depicted a theoretical study based on simple expressions to determine the bounds of two-phase frictional pressure gradient and void fraction in circular pipes. Such an approach was found to be useful for practical design and analysis of system performance. Also, the knowledge of the upper and lower bounds of two-phase frictional pressure gradient and void fraction can assist in the development of new experiments since it can provide a reasonable envelope for the data to fall within.