A mathematical model for drum drying of black liquor slurry using superheated steam impinging jets

A mathematical model is developed and tested for drum drying of an aqueous slurry subjected to impinging jets of superheated steam. The heat and mass transfer within the slurry film was modeled as a one dimensional, pseudo unsteady diffusion problem while the external convective heat transfer rate was obtained by solving the steady full conservation equations of mass, momentum and energy numerically in two and three dimensions. A modified low Reynolds number version of the k-? turbulence model was selected after a careful evaluation of the predictive performance of various k-? models for impingement flow and heat transfer. The computed heat transfer one-dimensional diffusion model for the slurry film. Agreement between experimental data obtained using a black liquor slurry and results of the model was found to be satisfactory.     

Numerical Study on the Effect of Cross-Flow on Turbulent Flow and Heat Transfer Characteristics Under Normal and Oblique Semi-confined Impinging Slot Jets

Impinging jets are commonly used in industrial dryers and electronics chip cooling. Since in industrial practice it is necessary to use multiple jets, the interaction between jets can have important effect on their heat transfer performance. Hence, the study of cross-flow caused by the spent flow of upstream jets is obviously significant. In this study, a computational fluid dynamics simulation was carried out of the flow and heat transfer characteristics for a single semi-confined turbulent slot jet of air impinging normally or obliquely into an imposed air cross-flow of the same or different temperature. The standard k-ε and the Reynolds stress models were used. Effects of the various flow parameter (e.g., jet-to-cross-flow mass ratio) and geometric parameters (e.g., nozzle-to-target spacing and jet angle) were evaluated at a fixed Reynolds number (11,000 and 12,000) for equal and unequal temperatures of the jet and cross-flow. Results indicate the significant degradation of the impingement heat transfer rates due to cross-flow and a relatively minor influence of the temperature difference between the jet and cross-flow over the ranges of parameters studied. Both the turbulence models produced comparable Nusselt number distributions along the impingement surface.     

Numerical Study on the Effect of Cross-Flow on Turbulent Flow and Heat Transfer Characteristics Under Normal and Oblique Semi-confined Impinging Slot Jets

Abstract

Impinging jets are commonly used in industrial dryers and electronics chip cooling. Since in industrial practice it is necessary to use multiple jets, the interaction between jets can have important effect on their heat transfer performance. Hence, the study of cross-flow caused by the spent flow of upstream jets is obviously significant. In this study, a computational fluid dynamics simulation was carried out of the flow and heat transfer characteristics for a single semi-confined turbulent slot jet of air impinging normally or obliquely into an imposed air cross-flow of the same or different temperature. The standard k?ε and the Reynolds stress models were used. Effects of the various flow parameter (e.g., jet-to-cross-flow mass ratio) and geometric parameters (e.g., nozzle-to-target spacing and jet angle) were evaluated at a fixed Reynolds number (11,000 and 12,000) for equal and unequal temperatures of the jet and cross-flow. Results indicate the significant degradation of the impingement heat transfer rates due to cross-flow and a relatively minor influence of the temperature difference between the jet and cross-flow over the ranges of parameters studied. Both the turbulence models produced comparable Nusselt number distributions along the impingement surface.     

Drum Drying of Black Liquor Using Superheated Steam Impinging Jets

ABSTRACT

A novel drum dryer for black liquor utilizing multiple impinging jets of superheated steam was designed and built to evaluate the performance characteristics and effects of various operating parameters thereon. Appropriate ranges of parameters such as steam jet temperature and velocity were examined experimentally lo quantify the optimal operating conditions for the formation of black liquor film on the drum surface as well as the drying kinetics.     

Drum Drying of Black Liquor Using Superheated Steam Impinging Jets

A novel drum dryer for black liquor utilizing multiple impinging jets of superheated steam was designed and built to evaluate the performance characteristics and effects of various operating parameters thereon. Appropriate ranges of parameters such as steam jet temperature and velocity were examined experimentally lo quantify the optimal operating conditions for the formation of black liquor film on the drum surface as well as the drying kinetics.     

Flow and Heat Transfer Characteristics of Confined Noncircular Turbulent Impinging Jets

A three dimensional computational fluid dynamic investigation is carried out to predict the turbulent flow and surface heat transfer under an impinging air jet issuing normally from a single noncircular orifice in a plate held parallel to the target surface. Static pressure distributions, velocity fields and local as well as average Nusselt number on the impinged surface are presented for square, elliptic, and rectangular orifices and compared with those for a circular orifice. Effects of jet Reynolds number as well as spacing between the nozzle plate and the impinged surface are examined using a two-layer κ-η turbulence model. Results show flow structure similarities between the characteristics of rectangular and elliptic jets of equal aspect ratio. Further, it is observed that noncircular impinging jets can provide higher average heat transfer rates than corresponding circular jets for certain geometric parameters viz. nozzle-to-plate spacing and the size of the averaging area used to compute the average Nusselt number.     

Flow and Heat Transfer Characteristics of Confined Noncircular Turbulent Impinging Jets

Abstract

A three dimensional computational fluid dynamic investigation is carried out to predict the turbulent flow and surface heat transfer under an impinging air jet issuing normally from a single noncircular orifice in a plate held parallel to the target surface. Static pressure distributions, velocity fields and local as well as average Nusselt number on the impinged surface are presented for square, elliptic, and rectangular orifices and compared with those for a circular orifice. Effects of jet Reynolds number as well as spacing between the nozzle plate and the impinged surface are examined using a two-layer κ–η turbulence model. Results show flow structure similarities between the characteristics of rectangular and elliptic jets of equal aspect ratio. Further, it is observed that noncircular impinging jets can provide higher average heat transfer rates than corresponding circular jets for certain geometric parameters viz. nozzle-to-plate spacing and the size of the averaging area used to compute the average Nusselt number.     

Superheated Steam Drying of a Single Particle in an Impinging Stream Dryer

ABSTRACT

Impinging stream contactors provide a novel configuration for drying and/or chemical reactions of particulates, pastes or suspensions which can be dispersed in a flowing gas stream. Essentially they consist of one or more highly turbulent “impingement” zones formed by collision of two opposing jets (OJ) in a confied channel or duct. The objective of this paper is to present computational fluid dynamic predictions for two-dimensional turbulent opposing jets over a range of nozzle-to-nozzle separations and jet Reynolds numbers for the simulation of single particle drying in these systems using superheated steam. A number of different turbulence models were tested ( e.g. high Reynolds, Lam-Bremhorst, Launder and Sharma models etc.). Predictions are performed in two distinct parts. In the first part a power law, finite volume method based on the “SIMPLEC” algorithm is used to solve the momentum and energy conservation equations for air in OJ systems in order to gain insight into their     

EFFECT OF NEAR-WALL MODELLING ON PREDICTION OF IMPINGEMENT HEAT TRANSFER

Due to enhanced transport characteristics, impinging jets are widely used in industry to dry large surface area products such as paper and textiles. The present numerical study concerns the modelling of convective heat transfer for impingement drying. Flow and heat transfer under a confined two dimensional turbulent air jet impinging on a flat surface were modelled by solution of two-dimensional Navier-Stokes and energy equations. The turbulence model used was the high-Re number version of the well known two-equation (κ-?) model and numerical solution was by the upwind finite difference scheme. The specific objective was to evaluate the accuracy of schemes for modelling the near-wall turbulent flow.

The mean flow properties such as centerline velocity decay and the pressure distribution at the impingement surface show no dependence on the near-wall model used. Heat transfer predictions were found to be quite sensitive to the choice of near-wall model. Best agreement between predictions and experiments was obtained for a Chieng-Launder type model with a new modification, use of k_p instead of k_v in the calculation of τ_w.     

Experimental and Numerical Simulation Study of Heat Transfer Due to Confined Impinging Circular Jet

An experimental and numerical simulation study of heat transfer due to a confined impinging circular jet is presented. In this research, a stainless steel foil heated disk was used as the heat transfer surface of a simulated chip, and the thermocouples were mounted symmetrically along the diameter of the foil to measure the temperature distribution on the surface. Driven by a small pump, a circular air jet (1.5 mm and 1 mm in diameter) impinged on the heat‐transfer surface with middle and low Reynolds numbers. The parameters, such as Reynolds number and ratio of height‐to‐diameter, were changed to investigate the radial distribution of the Nusselt number and the characteristics of heat transfer in the stagnation region. Numerical computations were performed by using several different turbulence models. In wall bounded turbulent flows, near‐wall modeling is crucial. Therefore, the turbulence models enhanced wall treatment, such as the RNG κ‐? model, may be superior for modeling impingement flows. The numerical results showed reasonable agreement with the experimental data for local heat transfer coefficient distributions. The impinging jet may be an effective method to solve the cooling problem of high power density electronic packaging.     

EFFECT OF NEAR-WALL MODELLING ON PREDICTION OF IMPINGEMENT HEAT TRANSFER

Due to enhanced transport characteristics, impinging jets are widely used in industry to dry large surface area products such as paper and textiles. The present numerical study concerns the modelling of convective heat transfer for impingement drying. Flow and heat transfer under a confined two dimensional turbulent air jet impinging on a flat surface were modelled by solution of two-dimensional Navier-Stokes and energy equations. The turbulence model used was the high-Re number version of the well known two-equation (κ-ε) model and numerical solution was by the upwind finite difference scheme. The specific objective was to evaluate the accuracy of schemes for modelling the near-wall turbulent flow.

The mean flow properties such as centerline velocity decay and the pressure distribution at the impingement surface show no dependence on the near-wall model used. Heat transfer predictions were found to be quite sensitive to the choice of near-wall model. Best agreement between predictions and experiments was obtained for a Chieng-Launder type model with a new modification, use of k_p instead of k_v in the calculation of τ_w.     

HEAT TRANSFER IN A CONDUCTIVE-HEATING AGITATED DRYER

Abstract

The heat transfers between the heating plane and the granular materials in both the “Stationary heating-plane type” and the “Moving heating-plane type” of the conductive-heating agitated dryers were discussed mainly from the view point of the scale-up of the dryer. Under the condition of complete mixing in the bed of bulk materials, the heat transfer models proposed for both the two types of dryers can predict the heat transfer coefficients in any sizes of the dryers. However, the complete mixing is not usually accomplished in the large scale of dryer. Hence, an “Incomplete mixing model” was proposed to estimate the effect of the incompleteness of mixing on the heat transfer coefficient. In this model, the incompleteness of mixing can be apparently taken into consideration only by increasing the contact time.     

Numerical analysis of penetration lengths in submerged supercritical water jets

“Hydrothermal spallation drilling” is a possible alternative drilling technology that uses the properties of certain rock types to disintegrate into small fragments when heated up rapidly by a hot impinging fluid jet. Hot supercritical water jets are favored to provide the required heat for thermal rock fragmentation. However, the indispensable presence of a dense water-based drilling fluid during operation can cause considerable heat losses in the supercritical water jet before impingement on the rock surface. To predict these heat losses from the hot jet to the cold aqueous environment, a numerical model based on the commercial CFD tool ANSYS FLUENT^® was established. Penetration lengths of the supercritical jet plume at near-critical pressures were determined numerically and validated with experimental values for a wide range of conditions. Experiments and simulations showed an acceptable agreement and the experimental trends were satisfactorily predicted by the model.     

Correlation of the Area-Mass Transfer Coefficient Inside the Drum of a Clothes Dryer

Abstract

The mass transfer process inside the drum of a clothes dryer affects its performance. The area-mass transfer coefficient was defined and correlated to the weight of the clothes, drum speed, the Reynolds number, Schmidt number, and Gukhman number using the experimental results of 32 runs on a Frigidaire clothes dryer. The coefficient of correlation was 0.994. A separate correlation for the Sherwood number was evaluated for a single piece of cloth stretched perpendicular to the flow in a pipe. This second correlation was used to compare the mass transfer inside the actual dryer with that of the “idealized” mass transfer process for a single piece of cloth. The results indicated that the average mass transfer efficiency for clothes in the dryer was only 26.4% of the ideal process. Means to enhance the area-mass transfer coefficient and the reasons for the poor performance of an actual dryer are discussed.     

CFD study of turbulent jet impingement on curved surface

The heat transfer and flow characteristics of air jet impingement on a curved surface are investigated with computational fluid dynamics(CFD)approach.The first applied model is a one-equation SGS model for large eddy simulation(LES)and the second one is the SST-SAS hybrid RANS-LES.These models are utilized to study the flow physics in impinging process on a curved surface for different jet-to-surface(h/B)distances at two Reynolds numbers namely,2960 and 4740 based on the jet exit velocity(U_e)and the hydraulic diameter(2B).The predictions are compared with the experimental data in the literature and also the results from RANS k-εmodel.Comparisons show that both models can produce relatively good results.However,one-equation model(OEM)produced more accurate results especially at impingement region at lower jet-to-surface distances.In terms of heat transfer,the OEM also predicted better at different jet-to-surface spacings.It is also observed that both models show similar performance at higher h/B ratios.     

Flow characteristics of a laminar swirling impinging jet: A numerical study

The primitive Navier-Stokes equations were solved to predict the flow field induced by a partially confined swirling laminar jet impinging normally on a flat surface. A study is made of the influence of the jet Reynolds number, nozzle-to-plate spacing, axial and swirl velocity profiles at nozzle exit plane, size of the confining and impingement plate and uniform suction applied at the plate. An interesting feature of the flow configuration is the predicted development of a recirculation bubble in the stagnation region which influences the heat and mass transfer characteristics. Axial and swirl velocity profiles at entry have dominant influence on the development of the flow field.     

Selected Aspects of Developing a Simulation Model of a Spouted Bed Grain Dryer Based on the Eulerian Multiphase Model

This article discusses the key aspects of developing a simulation model of a spouted bed grain dryer for an analysis of fluid dynamics. The following aspects have been analyzed: selection of computational space, type of geometry and computational grid, configuration of the mathematical model (based on the Eulerian multiphase model) through the selection of “closures,” turbulence modeling, the simulation model's sensitivity to changes in phase and flow parameters, as well as the configuration of numerical parameters. The presented study makes reference to an earlier experiment carried out by the author, and it sums up the author's previous work in the modeling of a spouted bed grain dryer. This article presents a synthetic overview of the problems that are frequently encountered in the process of developing simulation models of a spouted bed grain dryer and proposes several solutions.     

圆形自由水射流冲击换热及喷嘴布置

引言 水射流冲击冷却由于具有较高的换热能力,广泛应用于机械和化工行业,以实现工件的快速冷却和控制工件的温度变化.在大型轴类工件(工件直径D=1000～3000 mm)喷水冷却装置中,多喷嘴圆孔自由水射流以特定阵列布置冲击至工件表面,相对短暂的沸腾换热结束后,阵列自由水射流即以强制对流方式实现工件冷却.因此,自由水射流冲击换热特性及多喷嘴布置形式对喷水冷却装置的结构设计至关重要.     

Performance of stress-transport models in the prediction of particle-to-fluid heat transfer in packed beds

Computational fluid dynamics (CFD) as a simulation tool allows obtaining a more complete view of the fluid flow and heat transfer mechanisms in packed bed reactors, through the resolution of 3D Reynolds averaged transport equations, together with a turbulence model when needed. This tool allows obtaining mean velocity and temperature values as well as their fluctuations at any point of the bed. An important problem when a CFD modeling is performed for turbulent flow in a packed bed reactor is to decide which turbulence model is the most accurate for this situation. Turbulence models based on the assumption of a scalar eddy viscosity for computing the turbulence stresses, so-called eddy viscosity models (EVM), seem insufficient in this case due to the big flow complexity. The use of models based on transport equations for the turbulence stresses, so-called second order closure modeling or Reynolds stress modeling (RSM), could be a better option in this case, because these models capture more of the involved physics in this kind of flow.To gain insight into this subject, a comparison between the performance in flow and heat transfer estimation of RSM and EVM turbulence models was conducted in a packed bed by solving the 3D Reynolds averaged momentum and energy equations. Several setups were defined and then computed. Thus, the numerical pressure drop, velocity, and thermal fields within the bed were obtained. In order to judge the capabilities of these turbulence models, the Nusselt number (Nu) was computed from numerical data as well as the pressure drop. Then, they were compared with commonly used correlations for parameter estimations in packed bed reactors. The numerical results obtained show that RSM give similar results as EVM for the cases checked, but with a considerably larger computational effort. This fact suggests that for this application, even though the RSM goes further into the flow physics, this does not lead to a relevant improvement in parameter estimation when compared to the performance of EVM models used.