Chidambaram Narayanan

Mechanisms of particle deposition in a fully developed turbulent open channel flow

Particle dispersion and deposition in the region near the wall of a turbulent open channel is studied using direct numerical simulation of the flow, combined with Lagrangian particle tracking under conditions of one-way coupling. Particles with response times of 5 and 15, normalized using the wall friction velocity and the fluid kinematic viscosity, are considered. The simulations were performed until the particle phase reached a statistically stationary state before calculating relevant statistics. For both response times, particles are seen to accumulate strongly very close to the wall in the form of streamwise oriented streaks. Deposited particles were divided into two distinct populations; those with large wall-normal deposition velocities and small near-wall residence times referred to as the free-flight population, and particles depositing with negligible wall-normal velocities and large near-wall residence times (more than 1000 wall time units), referred to as the diffusional deposition population....

Two-Phase Convective Heat Transfer in Miniature Pipes Under Normal and Microgravity Conditions

Detailed numerical simulations have been performed to study the effect of flow orientation with respect to gravity on two-phase flow heat transfer (without phase change) in small diameter pipes. The Nusselt number distribution shows that the bubbly, slug, and slug-train regimes transport as much as three to four times more heat from the tube wall to the bulk flow than pure water flow. The flow blockage effect of the inclusions results in a circulating liquid flow superimposed on the mean flow. For upflow, the breakup into bubbles/slugs occurs earlier and at a higher frequency. The average Nusselt numbers are not significantly affected by the flow orientation with respect to gravity. A mechanistic heat transfer model based on frequency and length scale of inclusions is also presented.

Linear stability analysis of particle-laden mixing layers using Lagrangian particle tracking

Abstract This paper presents salient features of the temporal stability of particle-laden mixing layers under uniform particle loadings. Results show the strong stabilizing influence of particles with Stokes numbers of the order of unity. The particles also significantly change the most unstable wavenumber which dominates the initial evolution of the mixing layer. Results from the linear stability analysis were used to evaluate the Eulerian–Lagrangian (particle tracking) methodology and excellent agreement was observed for the instability growth rates. Two methods of implementing the fluid–particle coupling, both based on projecting the coupling force onto the fluid nodes, were compared and found to give almost identical results; indicating little practical difference between the two in the present case. Analyzing the accuracy requirements for the velocity interpolation found second-order interpolation to be inadequate. Converged results were obtained with fourth- and sixth-order accurate interpolation schemes.

Temporal instabilities of a mixing layer with uniform and nonuniform particle loadings

Temporal stability analysis of particle-laden mixing layers with uniform and nonuniform particle loadings is presented. New analytical results have been derived in the limit of small and large Stokes numbers using small-parameter expansions. A dichotomy in the behavior of the fluid–particle system, based on whether the particle diameter or density is increased to achieve large Stokes numbers, is clarified. Good agreement between the limiting analytical expressions and numerical results is obtained. Additional unstable modes are observed for nonuniform particle loadings for large Stokes numbers and high mass loadings conditions. The effect of the steepness of the nonuniformity is presented for the first time. The primary effect of nonuniformity, is an increase in the range of unstable wave numbers for small to intermediate Stokes numbers, and a change in the nature of the dominant Kelvin–Helmholtz instability from stationary to dispersive. The value of the most unstable wave number is shown to remain unaff...

Numerical analysis of the continuum formulation for the initial evolution of mixing layers with particles

Abstract Numerical analysis of the standard continuum description of a dilute dispersed phase as applied to a laminar, particle-laden, mixing layer during its initial evolution has been performed. The flow has been previously analyzed under the framework of linear stability analysis where both the continuous and the dispersed phases are considered as continua. Earlier studies had neglected the closure terms resulting from the averaging of the nonlinear transport term involved in the derivation of the dispersed-phase momentum equations. In this work, Lagrangian particle tracking was coupled to an incompressible Navier–Stokes solver to directly estimate the closure terms (referred to as the averaging-stress terms) and compare them to the other terms balancing the dispersed-phase continuum equations. Calculations were performed for particle Stokes numbers of 1, 10, and 50, and for a mass loading of one. Dispersed-phase flow quantities such as the number density and velocity were determined by averaging the data in the spanwise direction. A parametric study of the influence of the number of particles, for Stokes number of one, showed that an improved approximation to a continuum can be obtained by increasing the number of particles. Examining the momentum balance in detail revealed that the main contributors were the time-derivative, convective, and the interfacial force terms. The averaging stress was at least two orders of magnitude smaller for all the Stokes numbers studied. However, the averaging stress, though negligible in magnitude, showed a deterministic variation in the center of the mixing layer. The results lend support to the currently used continuum equations for analyzing the stability of laminar, particle-laden mixing layers, and possibly other free-shear flows such as jet and wake flows.

Particle transport and flow modification in planar temporally evolving laminar mixing layers. I. Particle transport under one-way coupling

Simulations of two-dimensional, particle-laden mixing layers were performed for particles with Stokes numbers of 0.3, 0.6, 1, and 2 under the assumption of one-way coupling using the Eulerian-Lagrangian method; two-way coupling is addressed in Part II. Analysis of interphase momentum transfer was performed in the Eulerian frame of reference by looking at the balance of fluid-phase mean momentum, mean kinetic energy, modal kinetic energy, and particle-phase mean momentum. The differences in the dominant mechanisms of vertical transport of streamwise momentum between the fluid and particle phases is clearly brought out. In the fluid phase, growth of the mixing layer is due to energy transfer from the mean flow to the unstable Kelvin-Helmholtz modes, and transport of mean momentum by these modes. In contrast, in the particle phase, the primary mechanism of vertical transport of streamwise momentum is convection due to the mean vertical velocity induced by the centrifuging of particles by the spanwise Kelvin-...

Computational Modelling Of Gas–liquid Multiphase Flows With DQMOM And The N-phase Algebraic Slip Model

In the present work the Direct Quadrature Method of Moments (DQMOM) has been implemented into the CFD code TransAT. The TransAT code is a finite volume solver, based on structured multiblock grids, with a focus on multiphase flow modelling: including two-phase interface tracking, Lagrangian particle tracking and multiphase mixtures with an algebraic slip model generalized for an arbitrary number of phases (N-phase ASM). The DQMOM technique was combined with the turbulent N-phase ASM model in order to extend its ability to handle dispersed phase populations with each class having its own velocity field. In the scope of this work the DQMOM implementation has been validated by performing 0D, 1D and 2D test cases – from unit tests to very complex problems such as bubble columns. The drag force acting on the bubble population, turbulent dispersion, bubble aggregation, breakage and growth phenomena were considered. The results show that DQMOM is an efficient method for solving complex multiphase problems, and allows more sophisticated modelling than ASM. However attention should be paid to proper implementation into a CFD framework, especially when it comes to mass conservation, realizability of DQMOM abscissas and discretization of stiff particle interaction kernels.

Prediction of Droplet Tear-Off and Meniscus Formation in the Top-Spot Experiment Using TransAT

For the design and development of new microfluidic devices reliable modeling and simulation tools must be made available. Many extensions to conventional computational fluid dynamics are required, especially multiphase fluid dynamics simulation capability. A new dynamic contact angle model is presented here, which does not require the specification of a contact angle or contact-line velocity. The level-set method is used for interface capture. The model is tested for unit problems such as relaxation to equilibrium of a contact line. It is then applied to the problem of fluid filling in a prototypical microdevice to show its utility as a design tool.Copyright

Application of N-phase algebraic slip model and direct quadrature method of moments to the simulation of air-water flow in vertical risers and bubble column reactor

Abstract In the present work the Direct Quadrature Method of Moments (DQMOM) has been implemented into the commercial CFD code TransAT. DQMOM has recently become a very attractive approach for solving population balance equation (PBE) due to its capability of representing the most interesting properties of the population, eg. Sauter mean diameter, void fraction, number of particles. The DQMOM technique was coupled with the turbulent N-phase Algebraic Slip Model (ASM) model in order to extend the model to handle dispersed phase populations such that each class has its own velocity field. The results compared to experimental data show that the developed numerical model accurately predicts void fraction profile in a long riser within a bubbly flow regime. Moreover the model is used for the simulation of bubble column, proving that it accurately predicts the gas hold up and the Sauter mean diameter.

Four-Way Coupling of Dense Particle Beds of Black Powder in Turbulent Pipe Flows

The modeling of particle deposition and transport in pipes is one of the most challenging problems in multiphase flow, because the underlying physics is multi-faceted and complex, including turbulence of the carrier phase, particle-turbulence interaction, particle-wall interactions, particle-particle interactions, two-way and four-way couplings, particle agglomeration, deposition and re-suspension. We will discuss these issues and present new routes for the modeling of particle collision stress. Practical examples like black powder deposition and transport in gas pipelines will be presented and discussed. The model employed is based on dense-particle formulation accounting for particle-turbulence interaction, particle-wall interactions, particle-particle interactions via a collision stress. The model solves the governing equations of the fluid phase using a continuum model and those of the particle phase using a Lagrangian model. Inter-particle interactions for dense particle flows with high volume fractions (from 1% to close packing ∼60%) have been accounted for by mapping particle properties to an Eulerian grid and then mapping back computed stress tensors to particle positions. Turbulence within the continuum gas field was simulated using the V-LES (Very Large-Eddy Simulation) and full LES, which provides sufficient flow unsteadiness needed to disperse the particles and move the deposited bed.Copyright