Direct numerical simulation of mass transfer in bidisperse arrays of spheres

In this study, an efficient ghost cell‐based immersed boundary method is used to perform direct numerical simulations of mass‐transfer processes in bidisperse arrays. Stationary spherical particles, with a size ratio of 1.5, are homogeneously distributed in a periodic domain in the spanwise directions. Simulations are performed over a range of solids volume fractions, volume fraction ratios of small‐to‐large particles, and Reynolds numbers. Through our studies, we find that large particles have a negative influence on the overall mass‐transfer performance; however, the performance of individual particle species is independent on the relative volume fraction ratios. We propose two correlations: (a) a refitted Gunn correlation for a better description of the interfacial transfer performance based on individual particle species; (b) a fractional calculation for a simple estimation of the overall performance in bidisperse systems using characteristics of individual particle species. We also investigate how well the overall mass‐transfer coefficient can be predicted by defining an appropriate equivalent diameter

Direct numerical simulation of fluid flow and dependently coupled heat and mass transfer in fluid-particle systems

\u3cp\u3eIn this paper, an efficient ghost-cell based immersed boundary method (IBM) is used to perform direct numerical simulation (DNS) of reactive fluid-particle systems. With an exothermic first order reaction proceeding at the exterior particle surface, the solid temperature rises and consequently increases the reaction rate via an Arrhenius temperature dependence. In other words, the heat and mass transport is dependently coupled through the particle thermal energy equation and the Arrhenius equation, and they offer dynamic boundary conditions for the fluid phase thermal energy equation and species equation respectively. The fluid-solid coupling is enforced at the exact position of the particle surface by implicit incorporation of the boundary conditions into the discretized momentum, species and thermal energy conservation equations of the fluid phase. Different fluid-particle systems are studied with increasing complexity: a single sphere, three spheres and a dense array consisting of hundreds of randomly generated particles. In these systems the mutual impacts between heat and mass transport processes are investigated.\u3c/p\u3

Computing interface curvature from volume fractions: a hybrid approach

The Volume of Fluid method is extensively used for the multiphase flows simulations in which the interface between two fluids is represented by a discrete and abruptly-varying volume fractions field. The Heaviside nature of the volume fractions field presents an immense challenge for the accurate computation of the interface curvature and induces the spurious velocities in the flows with surface-tension effects. A 3D hybrid approach is presented combining the Convolution and Generalized Height Function method for the curvature computation. The volumetric surface tension forces are computed using the balanced-force continuum surface force model. It provides a high degree of robustness at lower grid resolutions with first-order convergence and high accuracy at higher grid resolutions with second-order convergence. The present method is validated for several test cases including a stationary droplet, an oscillating droplet and the buoyant rise of gas bubbles over a wide range of Eötvös (Eo) and Morton (Mo) numbers. Our computational results show an excellent agreement with analytical/experimental results with desired convergence behavior

Performance study of heat and mass transfer in an adsorption process by numerical simulation

In this work, a detailed three dimensional model is employed for the quantitative description of flow and coupled heat and mass transfer in a gas channel coated with porous adsorbent layer. The flow field of the gas stream is obtained by solving the Navier-Stokes equation. The highly coupled mass and heat transfer in both the gas channel and the adsorbent layer are locally described, and the accompanying adsorption/desorption dynamics in the adsorbent layer are modelled as well. A parametric study has been carried out, in which the influences of thermal conductivity of the adsorbent layer, specific heat, porosity, tortuosity, layer thickness and geometrical shape on the performance of moisture adsorption processes are investigated in an exhaustive way. The heat and mass transfer mechanisms in the investigated cases are thoroughly analysed taking advantage of the rigorous 3D model, which deepens our understanding on the intricately coupled transport processes. The results reported in this work are useful for rational design and optimization of adsorption processes in adsorbent coated gas channels

Direct numerical simulation of non-isothermal flow through dense bidisperse random arrays of spheres

Extensive direct numerical simulations were performed to obtain the heat transfer coefficients (HTC) of bidisperse random arrays of spheres. We have calculated the HTC for a range of compositions and solids volume fractions for mixtures of spheres with a size ratio of 1:2. The Reynolds numbers are in the range of 30–100. It was found that the correlation of the monodisperse HTC can estimate the average HTC of bidisperse systems well if the Reynolds and Nusselt numbers are based on the Sauter mean diameter. We report the difference between the HTC for each particle type and the average HTC of the bed in the bidisperse system as function of solids volume fraction, Sauter mean diameter of the mixture, Reynolds number and investigate the heterogeneity of the individual particle HTCs

Effect of flow and fluid properties on the mobility of multiphase flows through porous media

\u3cp\u3eIn this paper we quantify the effect of capillary number (Ca), contact angle (θ) and viscosity ratio (M) on the mobility of multiphase flow through porous media. The focus is mainly on oil-water flows through porous rocks observed during the water flooding process. Simulations are performed using a finite volume method employing a staggered grid formulation. Interactions between fluids and complex solid boundaries are resolved by a direct forcing, implicit and sharp interface immersed boundary method (IBM). The fluid-fluid interface is tracked by a mass conservative sharp interface volume of fluid (VOF) method. IBM and VOF are coupled by imposing the contact angle as a boundary condition at the three phase contact line. Our methodology has been verified/validated for several test cases including multiphase Poiseuille flow in a channel, a viscous finger in a channel and mesh convergence of the contact force. Two types of porous structures are considered: (i) a repeated single pore and (ii) a random multi-pore arrangement. Temporal evolution of phase pressure difference and oil saturation have been studied as viscous fingers penetrate the pores. We observed that the residual oil saturation for different capillary numbers shows exactly the opposite trend for the single and multi pore arrangement. The residual oil saturation for multi-pore shows a well defined linear trend with logCa,θ and logM.\u3c/p\u3

Numerical modelling of flow and coupled mass and heat transfer in an adsorption process

In this paper, a detailed three dimensional mathematical formulation and a simplified one dimensional model for the numerical simulation of the flow and coupled heat and mass transport in a gas channel coated with adsorbing material are presented. In the three dimensional model, the velocity distribution of the gas flow is obtained by solving the momentum equation. The coupled heat and mass transport phenomena are locally described in both the gas channel and the adsorbent layer, and the concomitant adsorbate adsorption and desorption processes are taken into account. In the one dimensional model, the gas flow is assumed as a plug flow, and the heat and mass transfer across the solid-fluid interface is estimated by empirical transfer coefficients. A comparative study between the detailed three dimensional model and a simplified one dimensional model is carried out. Both model predictions are compared with experimental data available from literature. The heat and adsorbate concentration gradients observed in both radial and circumferential directions indicate that the detailed three dimensional model is desired. The time-dependent variations of temperature and heat flux distributions at the interface between the gas channel and the adsorbent layer justify the use of the more detailed three dimensional model. Three dimensional modelling is essential to obtain accurate predictions for cases where the solid side transport resistances are dominating

Direct numerical simulation of reactive fluid-particle systems using an immersed boundary method

\u3cp\u3eIn this paper, direct numerical simulation (DNS) is performed to study coupled heat and mass-transfer problems in fluid-particle systems. On the particles, an exothermic surface reaction takes place. The heat and mass transport is coupled through the particle temperature, which offers a dynamic boundary condition for the thermal energy equation of the fluid phase. Following the case of the unsteady mass and heat diffusion in a large pool of static fluid, we consider a stationary spherical particle under forced convection. In both cases, the particle temperatures obtained from DNS show excellent agreement with established solutions. After that, we investigate the three-bead reactor, and finally a dense particle array composed of hundreds of particles distributed in a random fashion is studied. The concentration and temperature profiles are compared with a one-dimensional heterogeneous reactor model, and the heterogeneity inside the array is discussed.\u3c/p\u3

Computing interface curvature from volume fractions:a machine learning approach

\u3cp\u3eThe volume of fluid (VOF) method is widely used to simulate the flow of immiscible fluids. It uses a discrete and sharp volume fractions field to represent the fluid-fluid interface on a Eulerian grid. The most challenging part of the VOF method is the accurate computation of the local interface curvature which is essential for evaluation of the surface tension force at the interface. In this paper, a machine learning approach is used to develop a model which predicts the local interface curvature from neighbouring volume fractions. A novel data generation methodology is devised which generates well-balanced randomized data sets comprising of spherical interface patches of different configurations/orientations. A two-layer feed-forward neural network with different network parameters is trained on these data sets and the developed models are tested for different shapes i.e. ellipsoid, 3D wave and Gaussian. The best model is selected on the basis of specific criteria and subsequently compared with conventional curvature computation methods (convolution and height function) to check the nature and grid convergence of the model. The model is also coupled with a multiphase flow solver to evaluate its performance using standard test cases: i) stationary bubble, ii) oscillating bubble and iii) rising bubble under gravity. Our results demonstrate that machine learning is a feasible approach for fairly accurate curvature computation. It easily outperforms the convolution method and even matches the accuracy of the height function method for some test cases.\u3c/p\u3

Direct numerical simulation of fluid-particle heat transfer in fixed random arrays of non-spherical particles

Direct numerical simulations are conducted to characterize the fluid-particle heat transfer coefficient in fixed random arrays of non-spherical particles. The objective of this study is to examine the applicability of well-known heat transfer correlations, that are proposed for spherical particles, to systems with nonspherical particles. In this study the spherocylinders are used to pack the beds and the non-isothermal flows are simulated by employing the Immersed Boundary Method (IBM). The simulations are performed for different solids volume fractions and particle sizes and low to moderate Reynolds numbers. Using the detailed heat flow pattern, the average heat transfer coefficient is calculated for the different operating conditions. The numerical results show that the heat-transfer correlation of spherical particles can be applied to all test beds of spherocylinders by choosing a proper effective diameter. Our results reveal that the diameter of a spherocylinder is the proper effective diameter for characterizing particle-fluid heat transfer