scholarly journals Progress in understanding the four dominant intra-particle phenomena of lignocellulose pyrolysis: chemical reactions, heat transfer, mass transfer, and phase change

2019 ◽  
Vol 21 (11) ◽  
pp. 2868-2898 ◽  
Author(s):  
M. Brennan Pecha ◽  
Jorge Ivan Montoya Arbelaez ◽  
Manuel Garcia-Perez ◽  
Farid Chejne ◽  
Peter N. Ciesielski
Keyword(s):  
Heat Transfer ◽  
Mass Transfer ◽  
Phase Change ◽  
Chemical Reactions ◽  
Phase Changes ◽  
Lignocellulosic Materials

Four principal intra-particle phenomena occur in a highly concerted manner during the pyrolysis of lignocellulosic materials: heat transfer, mass transfer, chemical reactions, and phase changes.

Heat and Mass Transfer With Phase Change and Chemical Reactions in Microscale

2010 ◽  
Author(s):  
Vladimir V. Kuznetsov
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Mass Transfer ◽  
Phase Change ◽  
Heat And Mass Transfer ◽  
Chemical Reactions ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Microscale Flow ◽  
Explosive Evaporation

In recent years considerable attention has been paid to the study of microscale flow and heat transfer with phase change and chemical reactions. This article reviews the patterns of the microscale two-phase gas-liquid flow, the statistical parameters of slug flow and capillary phenomena in annular flow for a rectangular microchannel. The evaporative and condensing heat transfer model for the curved liquid microfilm in microchannel and near contact line is developed and discussed. The influence of forced convection, nucleate boiling and thin film evaporation on microscale flow boiling heat transfer is reviewed and analyzed. The model of forced boiling heat transfer in microchannel is developed and compared with the existing experimental data. The mechanism and patterns of microscale explosive evaporation in the MEMS system is determined at high external heat flux density and the acousto-thermal model of the explosive evaporation is considered. The results of calculations are compared with the experimental data. The peculiarities of heat and mass transfer in a micro channel with surface catalytic reactions producing the hydrogen are presented. The kinetics of sequence of chemical reactions at nanoscale catalyst under conditions of significant nonuniformity of temperature and species concentration fields is considered.

scholarly journals Numerical Study of a Cooling System Using Phase Change of a Refrigerant in a Thermosyphon

Energies ◽  
2021 ◽  
Vol 14 (12) ◽  
pp. 3634
Author(s):  
Grzegorz Czerwiński ◽  
Jerzy Wołoszyn
Keyword(s):  
Mass Transfer ◽  
Phase Change ◽  
Heat Dissipation ◽  
Cooling System ◽  
Numerical Study ◽  
Relaxation Parameter ◽  
Phase Changes ◽  
Transfer Time ◽  
Two Phase ◽  
Time Relaxation

With the increasing trend toward the miniaturization of electronic devices, the issue of heat dissipation becomes essential. The use of phase changes in a two-phase closed thermosyphon (TPCT) enables a significant reduction in the heat generated even at high temperatures. In this paper, we propose a modification of the evaporation–condensation model implemented in ANSYS Fluent. The modification was to manipulate the value of the mass transfer time relaxation parameter for evaporation and condensation. The developed model in the form of a UDF script allowed the introduction of additional source equations, and the obtained solution is compared with the results available in the literature. The variable value of the mass transfer time relaxation parameter during condensation rc depending on the density of the liquid and vapour phase was taken into account in the calculations. However, compared to previous numerical studies, more accurate modelling of the phase change phenomenon of the medium in the thermosyphon was possible by adopting a mass transfer time relaxation parameter during evaporation re = 1. The assumption of ten-fold higher values resulted in overestimated temperature values in all sections of the thermosyphon. Hence, the coefficient re should be selected individually depending on the case under study. A too large value may cause difficulties in obtaining the convergence of solutions, which, in the case of numerical grids with many elements (especially three-dimensional), significantly increases the computation time.

Mathematical Model for Heat Transfer Simulation of Rotary Alumina Kiln

2013 ◽  
Vol 423-426 ◽  
pp. 881-884
Author(s):  
Xiao Yan Yang ◽  
You Gang Xiao ◽  
Xian Ming Lei
Keyword(s):  
Heat Transfer ◽  
Mathematical Model ◽  
Temperature Distribution ◽  
Phase Change ◽  
Heat Loss ◽  
Chemical Reactions ◽  
Outer Shell ◽  
Test Results ◽  
Distribution Rule ◽  
Heat Transfer Simulation

According to kiln structure and material movement features, considering convective, radioactivity, conductivity and various phase change and chemical reactions, a series of comprehensive models are built for quantifying the thermal fluxes from the gas to the material bed and the heat loss from outer shell to the atmosphere in the rotary alumina kiln. The results show that the temperatures of outer shell accord with test results; the temperature distribution rule of gas is the same with that of materials, but the gas temperatures are higher; it is feasible to use the model to improve alumina kiln performance.

Advanced Computational Modeling of Steady and Unsteady Cavitating Flows

2008 ◽  
Author(s):  
Huiying Li ◽  
Frank J. Kelecy ◽  
Aleksandra Egelja-Maruszewski ◽  
Sergio A. Vasquez
Keyword(s):  
Heat Transfer ◽  
Mass Transfer ◽  
Phase Change ◽  
Unsteady Flows ◽  
General Purpose ◽  
Phase Volume ◽  
Cavitating Flows ◽  
Mesh Motion ◽  
Vapor Mass ◽  
Liquid Vapor

The numerical simulation of steady and unsteady cavitating flows presents unique challenges in the development of robust numerical methodologies. The major difficulties are associated with the large density variations due to the phase change processes, and the modeling of liquid-vapor mass transfer (along with potentially strong interfacial heat transfer). The multiphase cavitation modeling approach described in this paper has been found to be capable of addressing these issues in an accurate and robust fashion, and is therefore suitable for inclusion in an advanced general purpose CFD solver. In the present approach, cavitation can be modeled within the framework of either the multiphase mixture model or the Eulerian multifluid model. The governing equations are the mixture (mixture model) or phase (Eulerian multifluid model) momentum, energy, turbulence, and phase volume fraction equations. The liquid-vapor mass transfer (evaporation and condensation) is modeled using two baseline cavitation models: Schnerr-Sauer model, and Zwart-Gerber-Belamri model. An advanced numerical scheme has been developed for solving the model equations which can handle large liquid-vapor density ratios, provide for mass transfer source terms in phase volume equations, and address the coupling between the phase change rates and the pressure correction equation. In addition, the cavitation models have been extended to compressible multiphase liquid and vapor flow regimes, and to problems involving convective heat transfer. The numerical algorithm has been implemented in an advanced, general-purpose CFD code, FLUENT, and validations have been carried out for a range of steady-state and unsteady flows, including a 2D axisymmetric orifice, a 3D fuel injector, a radial liquid pump, and a vane pump. The results demonstrate that the cavitation models are able to correctly predict the location and size of vapor bubbles, pressure distributions, and bulk flow parameters. Tests also indicate that the present implementation is both fast and robust, as compared to previous approaches. For unsteady simulations, the method can employ large time steps (limited only by physical or mesh motion considerations), making it efficient for unsteady flows driven by either boundary conditions or mesh motion.

Heat Transfer and Air Diffusion Phenomena in a Bed of Polymer Powder Using Apparent Heat Capacity Method: Application to the Rotational Molding Process

2007 ◽  
Author(s):  
M. Boutaous ◽  
E. Pe´rot ◽  
A. Maazouz ◽  
P. Bourgin ◽  
P. Chantrenne
Keyword(s):  
Heat Transfer ◽  
Mass Transfer ◽  
Heat Flux ◽  
Phase Change ◽  
Heat And Mass Transfer ◽  
Granular Media ◽  
Transfer Model ◽  
Melting Front ◽  
Polymer Powder ◽  
Air Diffusion

The process of rotational moulding consists in manufacturing plastic parts by heating a polymer powder in a biaxial rotating mould. In order to optimise the production cycle of this process, a complete simulation model has to be used. This model should describe the phenomena of heat and mass transfer in a moving granular media with phase change, coalescence, sintering, air evacuation and crystallization during the cooling stage. This paper focus on the study of heat and mass transfer in a quiescent polymer powder during the heating stage. An experimental device has been built. It consists in an open plane static mold on which an initial thickness, e, of a polymer powder is deposited. This powder is then heated until it melts. An inverse heat conduction method is used to determine the heat flux and temperature at the interface between the mold and the powder. This interfacial heat flux is taken as a boundary condition in a numerical heat transfer model witch takes into account the heat transfer in granular media with phase change, coalescence, sintering, air bubbles evacuation and rheological behaviour of the polymer. For the numerical simulation of the heat transfer, the apparent specific heat method is used. This approach allows to solve the same energy equation for all the material phases, so one do not have to calculate the melting front evolution. This fine modelling, close to the real physical phenomena makes it possible to estimate the temperature profile and the evolution of the polymer powder characteristics (phase change, air diffusion, viscosity, evolution of the thermophysical properties of the equivalent homogeneous medium, thickness reduction, air volume fraction...). Several results are then presented, and the influence of different parameters, like the thermal contact resistance, the process initial conditions and the polymer’s rheological characteristics are studied and commented. Indeed the predictions of the temperature rises in the polymer bed, agree well with the experimental measurements.

Study of Heat and Mass Transfer in MgCl2/NH3 Thermochemical Batteries

2017 ◽  
Vol 139 (3) ◽  
Author(s):  
Seyyed Ali Hedayat Mofidi ◽  
Kent S. Udell
Keyword(s):  
Heat Transfer ◽  
Mass Transfer ◽  
Phase Change ◽  
Heat And Mass Transfer ◽  
Waste Heat ◽  
Experimental Studies ◽  
Reaction Rates ◽  
Solid Matrix ◽  
Dominant Factor ◽  
Gaseous Ammonia

Intermittency of sustainable energy or waste heat availability calls for energy storage systems such as thermal batteries. Thermochemical batteries based on a reversible solid–gas (MgCl2–NH3) reactions and NH3 liquid–gas phase change are of specific interest since the kinetics of absorption are fast and the heat transfer rates for liquid–vapor phase change are high. Thus, a thermochemical battery based on reversible reaction between magnesium chloride and ammonia was studied. Two-dimensional experimental studies were conducted on a reactor in which temperature profiles within the solid matrix and pressure and flow rates of gas were obtained during discharging processes. A numerical model based on heat and mass transfer within the salt and salt–gas reactions was developed to simulate the NH3 absorption processes within the solid matrix, and the results were compared with experimental data to determine dominant heat and mass transfer processes within the salt. It is shown that for high permeability salt beds, the reactor uniformly adsorbs gaseous ammonia until the bed reaches the equilibrium temperature, then adsorbs gas near the cooled boundaries as the reaction front moves inward. In that mode, the heat transfer is the dominant factor in determining reaction rates.

Progress in the Modeling and Simulation of Flow Boiling Heat Transfer of Binary Fluids Using Advanced Multi-Fluid Modeling Techniques

2012 ◽  
Author(s):  
Vedanth Srinivasan ◽  
Rok Kopun
Keyword(s):  
Heat Transfer ◽  
Mass Transfer ◽  
Heat Flux ◽  
Phase Change ◽  
Bubble Dynamics ◽  
Flow Boiling ◽  
Three Dimensional ◽  
Transfer Model ◽  
Transfer Coefficients ◽  
Wide Range

In this paper, we discuss the implementation and testing of a novel boiling mass transfer model to simulate the thermal and phase transformation behavior, generated due to boiling of binary mixtures, using the commercial CFD code AVL FIRE® v2011. The phase change model, based on detailed bubble dynamics effects, is solved in conjunction with incompressible phasic momentum, turbulence and energy equations in a segregated fashion, to study the flow boiling process inside a rectangular duct. Full three dimensional validation studies including the effect of flow velocity and exit pressure conditions, acting on a wide range of operating wall (superheat) temperatures, clearly shows the suppression of heat and mass transfer coefficients with enhancement in flow convection. Competing mechanisms such as phase change process and turbulent convection are identified to influence the heat transfer characteristics. In particular, the varying influence of the mass transfer effects on the heat flux characteristics with alteration in wall temperature is well demonstrated. Comparisons of the predicted total heat flux, computed as the sum of the convection and phase change components, indicate a very good agreement with experimental data, wherever available. Description of the flow field inclusive of phasic fraction, temperature and velocity field provides extensive details of the multiphase behavior of the boiling flow. Some preliminary results on the phase change work flow to model heat transfer in cooling jackets, for automotive applications, is also discussed.

scholarly journals Transient Heat and Mass Transfer of Liquid Droplet Ignition at the Spreading over the Heated Substrate

2014 ◽  
Vol 6 ◽  
pp. 269321 ◽  
Author(s):  
Dmitrii O. Glushkov ◽  
Pavel A. Strizhak
Keyword(s):  
Mass Transfer ◽  
High Temperature ◽  
Time Delay ◽  
Heat And Mass Transfer ◽  
Chemical Reactions ◽  
Liquid Droplet ◽  
Phase Changes ◽  
Ignition Process ◽  
Metallic Plate ◽  
Droplet Spreading

The processes of heat and mass transfer accompanied by phase changes and chemical reactions are numerically modeled for the ignition of a liquid droplet formed from a condensed substance hitting the surface of a high-temperature metallic plate (substrate). The time delay of a droplet ignition is determined as well as the influence scope of a substrate, droplet, and oxidizer temperature, together with sizes and speed of droplet spreading on the ignition response. Conditions are revealed when spreading and deformation of a liquid droplet dominate during the ignition process.

Application of Computational Fluid Dynamics to Protective Clothing System Evaluation

2000 ◽  
Author(s):  
Phillip W. Gibson ◽  
Majid Charmchi
Keyword(s):  
Fluid Dynamics ◽  
Mass Transfer ◽  
Computational Fluid Dynamics ◽  
Phase Change ◽  
Heat And Mass Transfer ◽  
Protective Clothing ◽  
Phase Changes ◽  
Nonlinear Material ◽  
Physical Factors ◽  
Hydrophilic Polymer

Abstract Convection, diffusion, and phase change processes influence heat and mass transfer through textile materials used in clothing systems. For example, water in a hygroscopic porous textile may exist in vapor or liquid form in the pore spaces or in bound form when it has been absorbed by the solid phase, which is typically some kind of hydrophilic polymer. Phase changes associated with water include liquid evaporation/condensation in the pore spaces and sorption/desorption from hydrophilic polymer fibers. Certain materials such as encapsulated paraffins may also be added to textiles; these materials are designed to undergo a solid-liquid phase change over temperature ranges near human body temperature, which influences the perceived comfort of clothing. Additional factors such as the swelling of the solid polymer due to water imbibition, and the heat of sorption evolved when the water is absorbed by the polymeric matrix, can all be incorporated into the appropriate conservation and transport equations describing heat and mass transfer through clothing layers. These physical factors, nonlinear material properties, and complex multiphase flows make the task of modeling and predicting levels of protection and comfort of various clothing designs difficult and elusive. Computational fluid dynamics (CFD) has proven to be useful at several levels of material and system modeling to evaluate and design protective clothing systems and material components. This paper summarizes current and past work aimed at utilizing CFD techniques for protective clothing applications.

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