A Three-Dimensional Numerical Study of the Gas/Particle Interactions in an Industrial-Scale Spray Dryer for Milk Powder Production

Gas/particle interaction plays an important role in modern spray dryers and may have influences on wall deposition, agglomeration, powder degradation, etc. In the present study, the three-dimensional (3-D) transient multiphase flow in an industrial-scale spray dryer has been investigated using the CFD package FLUENT. The Eulerian–Lagrangian approach and two-way coupling method were used in the simulations. The reaction engineering approach (REA model) for milk particles has been implemented. Some new characteristics of the gas flow pattern and the particle behavior (e.g., temperature–time profiles) were identified from the numerical results; for example, the milk particles flow in such a way that makes the central jet oscillation more nonlinear. The discrete phase enhances the turbulence near the air/droplet inlet but damps it downstream. The transient turbulent flow causes significant uncertainties in the particle tracking, which presented some challenges in simulations. The study has highlighted the importance in performing 3-D transient simulations in order to understand the industrial-scale dryers.     

Prospects for the Modelling and Design of Spray Dryers in the 21st Century

It is becoming increasingly clear that the flow patterns in spray dryers are inherently transient and three-dimensional and that these flow patterns are closely linked to the key problem of wall deposition of particles in this equipment. We are also developing the confidence that current Computational Fluid Dynamics simulations, with standard approaches to modelling turbulence, can predict the frequencies of the transient flows and the three-dimensional nature of the flow patterns with adequate accuracy for both laboratory and industrial conditions. This situation demonstrates the potential for incorporating other important physical processes, such as those involved in wall deposition, into such models, leading to simulations that predict all the key physical processes quantitatively and accurately. Further research into the adhesion and cohesion of particles due to stickiness is important for addressing the challenges of predicting wall adhesion and agglomeration, as is the need to measure thermal degradation under conditions that reflect those inside the dryers. It will then be possible to design new dryer systems from first principles for specific product engineering outcomes, rather than relying on trial and error as in the past.     

Simulation of a Spray Dryer Fitted with a Rotary Disk Atomizer Using a Three-Dimensional Computional Fluid Dynamic Model

Spray dryers fitted with a rotary disk atomizer are widely used in many industries requiring high throughputs to produce powders from liquid streams. The interaction between droplets or particles and the drying medium within the drying chamber is still not well understood and hence difficult to model reliably. In this article CFD results are presented to describe the behavior of the performance of a spray dryer fitted with a rotary disk atomizer in a cylinder-on-cone chamber geometry. Four different turbulence models, i.e., standard k - ε, RNG k - ε, Realizable k - ε, and Reynolds stress models were tested and compared to simulate the swirling two-phase flow with heat and mass transfer in the chamber. The results of this investigation can provide further insight into turbulent swirling flow modeling. The predicted results, such as, air flow patterns, air velocity and temperature, distributions, particle/droplet trajectories, drying performance etc., are obtained using the CFD code FLUENT6.1. Comparison with available limited experimental data shows that CFD results display reasonable agreement. Predicted results also show that the RNG k - ε model performs better in this specific case.     

Heat and Mass Flow Control in an Interconnected Multiphase CFD Model for Chemical Looping Combustion

Chemical looping combustion is a feasible option for carbon capture from fossil fuels. Within the process, the oxygen necessary for combustion is provided by a solid carrier material which alternately undergoes oxidation and reduction reactions. Features of the process are that the oxidation reaction of the particulate carrier in the air reactor is strongly exothermic and that the conversion of both oxidation and reduction reactions has to be in balance for stable operation. Simulations of the transient behavior of chemical looping combustion systems are possible through multiphase CFD. To allow for the modeling of chemical looping at steady state, cooling of the reactors and mass flow between fuel and air reactor must be adequately adjusted. Therefore, an interconnected multiphase CFD model was extended by an adjustment control. In this extended modeling framework variations of the operational load, control set points and carrier materials were performed. These simulations allow detailed insight into the dynamic behavior of chemical looping systems.     

Simulation of a Spray Dryer Fitted with a Rotary Disk Atomizer Using a Three-Dimensional Computional Fluid Dynamic Model

Abstract

Spray dryers fitted with a rotary disk atomizer are widely used in many industries requiring high throughputs to produce powders from liquid streams. The interaction between droplets or particles and the drying medium within the drying chamber is still not well understood and hence difficult to model reliably. In this article CFD results are presented to describe the behavior of the performance of a spray dryer fitted with a rotary disk atomizer in a cylinder-on-cone chamber geometry. Four different turbulence models, i.e., standard k ? ε, RNG k ? ε, Realizable k ? ε, and Reynolds stress models were tested and compared to simulate the swirling two-phase flow with heat and mass transfer in the chamber. The results of this investigation can provide further insight into turbulent swirling flow modeling. The predicted results, such as, air flow patterns, air velocity and temperature, distributions, particle/droplet trajectories, drying performance etc., are obtained using the CFD code FLUENT6.1. Comparison with available limited experimental data shows that CFD results display reasonable agreement. Predicted results also show that the RNG k ? ε model performs better in this specific case.     

Experimental and computational fluid dynamic study of reacting gas jet in liquid: Flow pattern and heat transfer

The flow behavior of a jet reactor (consisting of a gaseous jet submerged in a molten-metal bath) is very complex. These are operated at high temperatures (1500–3000 K) and need to be contained within a heavy metal enclosure. The design of such reactors requires a prior knowledge of the jet dimensions, flow pattern and heat transfer characteristics. However, the fuel opaqueness and the high temperature of the jet create difficulties in observing the reaction mass visually and therefore the literature contains a very brief account of the experimental measurements of the flow pattern. Hence, a systematic study has been undertaken with a reaction pair (HCl gas jet submerged in aqueous NH₃), which has the potential for simulating the real systems. The present work is concerned with the CFD simulations by employing k–ε turbulence model and large eddy simulations (LES). The measurements and simulations have been carried out over a wide range of gas velocities (53–323 m/s) and these have been compared with the CFD simulations. A comprehensive comparison has also been made between the k–ε and the LES for the mean flow, temperature and the turbulent kinetic energy. An attempt has been made to understand the relative performance of these models. Further, complete energy balance has been established between the energy supply rate through the jet and the energy dissipation rate within the reactor. The plume characteristics obtained from CFD simulations have been compared qualitatively with the photographic images.     

CFD simulation of flow pattern and plume dimensions in submerged condensation and reactive gas jets into a liquid bath

Submerged gas jets into a liquid bath are widely used in metal processing and thermal processes. These systems are classified as (a) condensation jet and (b) reaction jet systems. This paper presents the CFD simulation of both the types of jets. The CFD model considers phase change, gas-liquid and gas-gas reactions and the accompanied rates of mass transfer. Mass transfer coefficient was estimated using small eddy model where the value of mass transfer coefficient is calculated based on the local values of turbulent kinetic energy (k) and the dissipation rate (ε). A good agreement with the available experimental data of plume length validates the CFD model. The CFD simulations have also been compared with the available experimental data on velocity and temperature profiles which shows excellent agreement. A comparison between the condensation and the reaction jets has been presented in terms of plume dimensions, flow and temperature patterns. The relative predictions of the present model and the rational correlations have been presented for the estimation of plume length for both the types of jet systems.     

搅拌槽内黏性流体流动的DPIV测量与CFD模拟

搅拌槽是化学工业及其相关工业广泛应用的设备之一,由于其内部流动的复杂性,搅拌混合操作目前尚未形成完善的理论体系．对搅拌槽的设计和放大,主要是依赖半经验的方法,对其内部流场有必要进行更深入的研究．目前对不同黏性流体的流动测量及计算流体力学模拟工作见诸报道较少,而     

USE OF COMPUTATIONAL FLUID DYNAMICS TECHNIQUES TO ASSESS DESIGN ALTERNATIVES FOR THE PLENUM CHAMBER OF A SMALL SPRAY DRYER

The inlet region of a pilot-scale, co-current spray dryer was simulated using the proprietary Computational Fluid Dynamics (CFD) codes, CFX4 and CFX5. Several design alternatives were considered for correcting uneven inlet air distribution, which is known to influence spray dryer performance and airflow patterns. The simulations were used to assess each alternative prior to construction, assuming isothermal and incompressible flow conditions. Experimental measurements were compared with the simulation results for the original and one modified design.

Drying air is supplied to this dryer via an overhead pipe feeding an annular plenum chamber, of diameter 400 mm, surrounding the atomiser. A distributor plate with two concentric rings of 50 holes, each of 5 mm diameter, forms the base of the plenum chamber. A three-dimensional grid was required to model each of the 100 holes separately and to consider the asymmetric flow behaviour. The resulting grid consisted of about 532,000 cells. The CFD simulations proved useful in predicting the trends in flow distributions in each of the designs.     

USE OF COMPUTATIONAL FLUID DYNAMICS TECHNIQUES TO ASSESS DESIGN ALTERNATIVES FOR THE PLENUM CHAMBER OF A SMALL SPRAY DRYER

The inlet region of a pilot-scale, co-current spray dryer was simulated using the proprietary Computational Fluid Dynamics (CFD) codes, CFX4 and CFX5. Several design alternatives were considered for correcting uneven inlet air distribution, which is known to influence spray dryer performance and airflow patterns. The simulations were used to assess each alternative prior to construction, assuming isothermal and incompressible flow conditions. Experimental measurements were compared with the simulation results for the original and one modified design.

Drying air is supplied to this dryer via an overhead pipe feeding an annular plenum chamber, of diameter 400 mm, surrounding the atomiser. A distributor plate with two concentric rings of 50 holes, each of 5 mm diameter, forms the base of the plenum chamber. A three-dimensional grid was required to model each of the 100 holes separately and to consider the asymmetric flow behaviour. The resulting grid consisted of about 532,000 cells. The CFD simulations proved useful in predicting the trends in flow distributions in each of the designs.     

Incorporating detailed reaction mechanisms into simulations of coal-nitrogen conversion in p.f. flames

The simulation strategy described in this paper provides an alternative to conventional CFD post-processing to estimate exhaust NO_X emissions. The method first analyzes a conventional CFD simulation to specify temperature histories and mixing rates. The bulk flow patterns are then represented with an equivalent network of idealized reactor elements. Detailed reaction mechanisms are then applied over the reactor network. The analysis was able to depict all the important tendencies among the major intermediates and products from a selection of coals that spanned almost the entire rank spectrum, under reaction conditions that spanned the domain of stoichiometric ratio in p.f. flames. The main practical benefit of the mechanistic complexity is that simulations based on detailed mechanisms require far fewer parameter adjustments than conventional CFD simulations whenever different fuels are considered.     

CFD Modeling of Air Flow on Wall Deposition in Different Spray Dryer Geometries

Wall deposition is one of the most conventional problems in the spray drying process. The operation of a spray dryer is affected by the wall deposition fluxes inside the equipment. In this study, computational fluid dynamic (CFD) simulation was used to investigate the effect of spray dryer geometry on wall deposition. A CFD model was developed for different geometries of spray dryer with a conical (case A) or a parabolic (cases B and C) bottom. The results implied that the parabolic geometry resulted in a lower deposition rate on the spray dryer walls. A comparison of results using the P-values (F-test) of the air velocity, in the conical and parabolic geometries, showed that there was a significant difference in air stability between them. The flow field in conical geometry case A was significantly more unstable, and parabolic geometry case C produced the most uniform airflow patterns. Moreover, the higher wall shear stress in case C, with lower values of the vorticity, would result in less wall deposition.     

Air flow patterns in an industrial milk powder spray dryer

The air flow patterns in an industrial milk powder spray dryer have been investigated. Isothermal three-dimensional transient simulations in the absence of atomised liquid droplets have been carried out using the commercial CFD code (CFX10.0) in which the transient Navier–Stokes equations are solved. The shear stress transport (SST) turbulence model was implemented to model the effects of turbulence.     

聚丙烯非织造布熔喷过程的计算流体力学模拟研究进展

聚丙烯非织造布采用熔喷工艺制成,在过滤、阻菌、吸附、防水等方面性能优异,有着非常广阔的市场前景。熔喷工艺中的聚合物拉伸过程复杂且迅速,难以用实验观察。因此,为降低熔喷布生产成本,提高成品质量,计算流体力学（computational fluid dynamics, CFD）方法被广泛应用于该工艺过程分析之中,包括模头中的流道分析、喷射流场分析等。其中,喷射流场分析是主要应用方向,可为喷丝板结构优化、喷射流场优化等问题提供解决方案。本文简要介绍了聚丙烯非织造布熔喷工艺原理及其特点,主要对该领域中应用CFD模拟的相关研究进展进行了综述。文中指出,目前熔喷过程的CFD模拟一般基于喷射流场中的气流,没有考虑黏性聚合物纤维对其影响。然而,黏性聚合物纤维在高速高温气流条件下会发生振动,对射流流场的影响不容忽视。熔喷非织造布的关键问题是减小聚合物纤维的直径,提高熔喷非织造布的质量。因此,研究的重点应逐渐由气流场转变为纤维流场。虽然CFD已被广泛应用于熔喷过程模拟,但熔融状态下聚合物纤维流场模拟研究仍需要在未来的工作中进行展开。     

Numerical probing of a low velocity concurrent pilot scale spray drying tower for mono-disperse particle production – Unusual characteristics and possible improvements

A pilot scale micro-fluidic-spray-dryer (MFJSD-II) utilizing a single-stream atomization to produce mono-disperse particles has been developed at Monash University. A unique feature of this unit is the use of relatively low air velocities in the range of 10⁻¹-10⁻² m s⁻¹, which increases the residence time of the particles during the drying process, in comparison with conventional spray dryers. In this study, Computational Fluid Dynamics (CFD) simulation revealed that the effects of natural convection, caused by heat loss from the wall, were significant in deflecting the flows of air from the central jet-side recirculation pattern, causing them to spread towards the wall. The flow patterns formed a layer of relatively high velocity region adjacent to the wall. The understanding of these flow patterns would be crucial for future designs of low-velocity spray dryers. The spreading of the central jet towards the wall also created recirculation regions in the center of the tower, thus affecting the residence times particularly for smaller particles. More numerical simulations are required to optimize the design of the bottom bustle to be effectively used as a particle-separator.     

Gas management in flow field design using 3D direct methanol fuel cell model under high stoichiometric feed

This study presents a 3D CFD model for modeling gas evolution in anode channels of a DMFC under high stoichiometric feed. The improved two-phase model includes a new submodel for mass source and interphase transfer in anode channels. Case studies of typical flow field designs such as parallel and serpentine flow fields illustrate applications of the CFD model. Simulation results reveal that gas management of typical flow fields is ineffective under certain operating conditions. The CFD-based simulations are used to visualize and to analyze the gas evolution and flow patterns in anode channels. The developed CFD model is useful in flow field design for improving gas management in DMFC.     

A NUMERICAL STUDY OF THE AIRFLOW PATTERNS IN A SAUSAGE DRYER

This paper describes the use of a computational fluid dynamics code (Star-CD)in the food industry. We assessed the capability of CFD to predict airflow patterns in a modern sausage dryer by comparing numerical results with experimental air velocity data. Qualitative and quantitative comparisons showed that simulated values agreed closely with measurements although some discrepancies appeared for specific flow conditions. Using CFD, it was established that these differences couid be attributed to slow time-variations in the blower airflow rate. These comparisons also revealed errors in measurement.