Heat Transfer Kinetics Research Articles

Heat and Mass Transfer Kinetics in Arc Welding Process

The paper presents the investigation results of the heat and mass transfer kinetics during shielded metal arc welding (SMAW). It is shown that instability of the electrode melting and metal transfer to the molten pool is mostly caused by a system of forces acting on the metal droplet at the continuous electrode tip. Experiments show that the process instability and spatter of the electrode metal depend on the energy parameters of the SMAW mode. At the same time, the SMAW technology can be significantly improved through the implementation of the energy parameter control by the adaptive pulsed-arc welding method.

Irreversibility of Heat-Transfer Processes in Double-Flow Cells of Varying Flow Hydrodynamics

Consideration has been given to the influence of the hydrodynamics and phase state of the flows in a double-flow heat-exchange cell on the entropy production in it. For the case of the Newtonian kinetics of heat transfer in such a cell, the authors have obtained formulas for calculating the entropy production in it as a function of the heattransfer coefficient, the heat load, the temperatures, and the water equivalents of the flows in the cell.

Modeling batch melting: Roles of heat transfer and reaction kinetics

AbstractDevelopment of mathematical models of heat and mass transfer in glass‐melting furnaces began in the 1970s and progressed rapidly with advances in sophisticated experimental/numerical techniques and increasing computational power. Today, practically all newly built or rebuilt furnaces are optimized with these models to meet stringent quality requirements, reduce the unit costs of manufacturing, or control emissions. One remaining hurdle is to model the batch‐to‐glass conversion accurately enough to reliably assess the glass production rate. This article summarizes two key aspects of the batch‐conversion modeling—the heat transfer and the kinetics of conversion—and reviews the current state‐of‐the‐art approaches to simulating them. We critically examine the advantages of the commonly used heat transfer approach, but also explain that its predictive capabilities are significantly restricted by the dependence of batch thermal properties on the time‐temperature history. We argue that kinetic approaches to the batch‐conversion modeling would offer a significant improvement when coupled with the heat transfer approach. Finally, we summarize key areas requiring further research on the way toward a realistic model of the batch blanket.

Heat Transfer Characteristics and Kinetics of Camellia oleifera Seeds During Hot-Air Drying

Abstract The heat transfer characteristics and kinetics of Camellia oleifera seeds under hot-air drying were investigated at different temperatures (40, 60, and 80 °C) and loading densities (0.92, 1.22, and 1.52 g/cm2) with a constant air velocity of 1 m/s. Twelve common drying kinetic models were selected to fit the experimental data. The most suitable model was chosen to describe the hot-air drying process of C. oleifera seeds and help in its optimization. The results showed that the drying temperature has a significant influence on the hot-air drying characteristics of C. oleifera seeds. As the drying air temperature increases, the drying time decreases. The effect of the loading density on the drying characteristics of C. oleifera seeds is much smaller than that of temperature. With the increase in the loading density, the drying time slightly increases. The hot-air drying curve of C. oleifera seeds consists of a very short acceleration rate period at the beginning and a long falling rate period, indicating that the drying of C. oleifera seeds is mainly controlled by the diffusion of moisture inside the material. An effective moisture diffusion coefficient of C. oleifera seeds was estimated to range from 0.81256 × 10−9 to 3.28496 × 10−9 m2/s within the temperature range studied. The average activation energy was 28.27979 kJ/mol. The logarithmic model was found to be the best model to describe the kinetics of hot-air drying of C. oleifera seeds.

Numerical study on a new design of solid-fuel ramjet combustor with swirl flow

A new design of solid-fuel ramjet is proposed and examined numerically in this paper. Multi-physics coupling code is developed using FORTRAN and parallel computing to solve the problems of multi-physics coupling of fluid mechanics, solid pyrolysis, heat transfer, thermodynamics, and chemical kinetics. Simulations are carried out for the proposed design then the results are compared with the classic design of the solid-fuel ramjet. It is found that the proposed design has improved the regression rate significantly; besides, the amount of released solid fuel is increased for the same size. A new flame has been observed inside the combustion chamber of the proposed design then the two flamed were emerged in the afterburning chamber.

A study of fire propagation in coal seam with numerical simulation of heat transfer and chemical reaction rate in mining field

Spontaneous combustion of coal is a problem that affects the mining operation and generates environmental, economic, social and geotechnical impacts. This phenomenon has been divided into two processes: ignition and propagation. Fire propagation develops in coal seams because of a set of factors such as direction and wind speed, fracturing and temperature. In this work, heat transfer and chemical kinetics are studied from conservation equations of energy and species, respectively, using the software COMSOL Multiphysics to simulate the propagation of fires in coal seams. Two possible scenarios were analyzed that usually occur in the walls of the coal seams, such as fire focus and fire complete screens. It was found that the propagation kinetics of the fire changes depending on the temperature, the fracturing of rock mass and the area of fire influence. For temperature values lower than 300 °C, there is consumption around 250 cm3/h, values around 700 °C, the consumption is 1500 cm3/h, and for fires of 1200 °C have values of 3000 cm3/h. Depending on the speed of propagation can vary from 4 to 17 cm/day, considering on the level and fracturing of the final wall of the open pit.

Numerical Study of Mass Transport and Electrochemical Kinetics inside Porous Structure Layers of PEMFC Using Direct Simulation Approach

The enhancement of the mass transports in porous layers of a polymer electrolyte fuel cells (PEFCs) is essential for developing material and design system to improve the PEFC performance and durability. The oxygen transport resistance is a large fraction of the mass-transport overpotential at the cathode. Condensed water in the flow field and porous layers reduces oxygen transport to the oxygen reduction reaction (ORR) region. Experimental investigation of locally saturated porous media in the operating cell is difficult. The Computational Fluid Dynamics (CFD) is mathematical modeling tool, which can be considered the incorporation of theory and experimentation in the field of kinetics, heat, and mass transport of the PEFCs. The purpose of this work is to use direct modeling-based, Lattice Boltzmann Method (LBM) within the detailed structure of porous layers to understand the kinetics and multi-scalar/multi-physics transports in a polymer electrolyte fuel cells (PEFCs). The model geometries consist of gas diffusion layer (GDL), micro porous layer (MPL), and Catalyst layer (CL), as shown in Fig. 1. The studies include the understanding of water evolution, water saturation, breakthrough pressure, heat transfer, species transport, and electrochemical kinetics inside porous layers under different conditions and situations that could occur in fuel cells. This work will show the enhancement of direct CFD simulation with LBM (CFD-LBM) approach [1-4], which incorporates with detailed structure of porous and catalyst layers from both micro- and nano- X-ray CT. This approach will consist of the kinetic model in the catalyst layer, which will involve coupling electrochemical kinetic to investigate the electrical potentials, electrical current, electron transfer, and exchange current in the catalyst layers, as shown in Fig. 2. The output of this work will be used for the optimization of catalyst layer thickness, with durability and water management improvement, for novel porous materials, particularly in the catalyst layer.

Parametric Study of Microstructural Properties at Laser Cladding

Self-consistent numerical model of laser cladding with coaxial powder injection has been developed. The model takes into account the heat transfer, hydrodynamics, solute transport, and phase transition kinetics. The additive process is numerically analyzed using the open-source computational fluid dynamics (CFD) package. The S-CLSVOF program is adapted for modeling heat and mass transfer and free boundary evolution of the melt pool. The phase transition is modeled using the Johnson–Mehl–Avrami–Kolmogorov equation with allowance for nonuniform temperature distribution on the melt pool surface. The clad-layer microstructure is investigated as a function of the process parameters. High scan rate is shown to result in a finer grain size of clad layer structure. The data obtained can be used for construct a cladding process map.

On the Possibility of Improving the Thermodynamic Characteristics of Multiflow Heat Transfer Systems

The influence of the change in the phase state of part of flows on optimal organization and minimum possible entropy production in a multiflow heat-exchanging system is considered. For the Newtonian kinetics of heat transfer, computational formulas for optimal distribution of the total surface between heat exchangers, the algorithm of selection of contacting flows, and the ratio of absolute temperatures of heated and heating flows have been obtained.

Experimental and numerical study into the effects of passive flow disturbance on conversion and C7+ yield of the packed bed Fischer Tropsch reaction

This experimental and numerical study investigates the effect of flow field alteration via bluff body flow disturbers in the packed bed Fischer-Tropsch (FT) reaction, which is known to be limited by heat and mass transfer and chemical kinetics as prescribed by the reactor geometry, flow and temperature fields, and catalyst properties. Experiments were carried out using a 188 cm3 reactor with 30.8 aspect (length-to-diameter) ratios loaded with 20% cobalt/Al2O3 catalyst and controlled at 200 °C at the exterior wall. Numerical simulation was carried out in the commercial ANSYS Fluent package. Experimental results show that the bluff bodies increased the CO (carbon monoxide) conversion and C7+ yields (hydrocarbons with 7 carbon chains or longer) across the factorial design space by as much as 1.8× and 3.3× of the baseline values, respectively. Numerical simulations suggest that these increases can be attributed to higher thermal conductivities and local velocities for greater heat rejection and lower external mass transfer resistance, respectively. Results suggest that the use of internal flow disturbers in the packed bed had significant influence in lowering the heat and mass transfer resistances of the FT process, and can improve the cost-competitiveness of FT liquids production in the small scale.

A spatially resolved physical model of an ion transport membrane reactor for system development

The ion transport membrane (ITM) reactor has been developed as a potential novel syngas production technology that includes both air separation and fuel conversion. To successfully commercialize the ITM reactor, it is necessary to optimize its operation.In this study, a spatially resolved physical model of the ITM reactor for syngas production has been developed to investigate the ITM reactor characteristics using Aspen Plus®, which will be extended to the system level simulation. The approach captures spatial variations in the crucial physics of heat transfer, oxygen permeation and reaction kinetics in a manner that is simple enough to make the model amenable to ITM reactor system simulations development. To simulate the partial oxidation of methane (POM), methane oxidation, steam reforming of methane, and dry reforming of methane have been considered. By varying the carbon space velocity, CH4 conversion, CO selectivity, H2–CO ratio, and species molar flow rates have been calculated. Moreover, oxygen partial pressure and temperature distribution on the feed and sweep sides are presented in the paper. This study provides the basic insight to establish the optimal system designs and operating schemes of the ITM reactor by analyzing the reaction kinetics distribution of the POM and oxygen permeation rates.

Analytical Solution of the Time-Dependent Microfluidic Poiseuille Flow in Rectangular Channel Cross-Sections and its Numerical Implementation in Microsoft Excel.

We recently demonstrated that the Navier–Stokes equation for pressure-driven laminar (Poiseuille) flow can be solved in any channel cross-section using a finite difference scheme implemented in a spreadsheet analysis tool such as Microsoft Excel. We also showed that implementing different boundary conditions (slip, no-slip) is straight-forward. The results obtained in such calculations only deviated by a few percent from the (exact) analytical solution. In this paper we demonstrate that these approaches extend to cases where time-dependency is of importance, e.g., during initiation or after removal of the driving pressure. As will be shown, the developed spread-sheet can be used conveniently for almost any cross-section for which analytical solutions are close-to-impossible to obtain. We believe that providing researchers with convenient tools to derive solutions to complex flow problems in a fast and intuitive way will significantly enhance the understanding of the flow conditions as well as mass and heat transfer kinetics in microfluidic systems.

Mathematical modeling and experimental assessment of the cast-tape drying

Cast-tape drying (CTD) process was evaluated by an experimental approach, obtaining drying curves, convective coefficients of heat (hh) and mass (hm) transfer, time-temperature, and pulp shrinkage kinetics for tomato pulp at 2, 3, and 4 mm thick. A heat and mass transfer model with moving border described CTD. Pulp thickness was reduced by 90%, under hh and hm of 55 W m−2 K−1 and 0.05 m s−1, respectively. Evaporative capacity ranged between 8 and 15 kg m−2 h−1, and drying time between 8 and 24 min for 2 and 4 mm thick, respectively. Coupled experimental and theoretical analysis of CTD clarified the relative importance of different phenomena controlling the process.

(Industrial Electrochemistry and Electrochemical Engineering Division H. H. Dow Memorial Student Achievement Award) Transport Study Inside Porous Layers of PEFC Using Direct Numerical Simulation

The purpose of this work is to use direct modeling based Lattice Boltzmann Method (LBM) [1-4] to understand the multi-scalar/multi-physics transports of various porous layers in a polymer electrolyte fuel cells (PEFCs). The studies include the understanding of water evolution, water saturation, breakthrough pressure, heat transfer, species transport, and electrochemical kinetics inside porous layers under different conditions and situations that could occur in fuel cells. Condensed water in the flow field and porous layers reduces oxygen transport to the oxygen reduction reaction region. Current platinum group metal (PGM) catalysts tend to experience transport resistance, which results in kinetic limitations within the hydrogen oxidation reaction (HOR) and the oxidation reduction reaction (ORR). Due to these limitations, the enhancement of mass transport is critical to improving fuel cell performance and durability, and therefore the development of a PGM-free catalyst is vital in understanding multi-scalar transport. This will enrich the development of novel structures that address local oxygen transport resistance in the PGM-free catalyst layer, and will outline the urgent need to further investigate multi-physical phenomena and the electrochemical nature of a fuel cell. The model geometries of porous layers [gas diffusion layer (GDL), micro porous layer (MPL), and catalyst layer (CL)] provided in this study were obtained by a 3D, reconstructed microstructure from both micro and nano X-ray computed tomography (CT) as shown in Figure 1. Current research outcomes reveal that the liquid water saturation profiles inside the GDL, breakthrough pressures, heat, and mass transport are dependent on GDL type, operating conditions, cell assembly specifications, and flow field geometry [3]. This work also shows the enhancement of my approach that include the kinetics model in the catalyst layer, which will involve coupling electrochemical kinetics with LBM computational fluid dynamics (CFD) as shown in Figure 2. This kinetic model simulates the chemical reaction in the cathode side in order to investigate the electrical potentials, electrical current, electron transfer, and exchange current in the catalyst layers. It shows the potential capability of a CFD-based investigation of multi-physical transport to find solutions of design and operational conditions in the PEFC. The output of this work will be used for the optimization of catalyst layer thickness, with durability and water management improvement, for novel porous materials, particularly in the catalyst layer. Moreover, the predictions from CFD simulations can help to optimize the design of porous layer structures and other components, such as flow fields, thereby improving the PEFC’s overall performance, cost of production, and development time.

Improvement of Evaluation of 3-D Characteristics of a Diode-Pumped Alkali Laser

It has been proved that a diode-pumped alkali laser (DPAL) is one of the potential candidates to achieve the output power with kW-order or more. However, the theoretical analyses of a DPAL are still incomplete although some regimes have been developed to evaluate the spatial characteristics of a DPAL. It is necessary to build a mathematical model to analyze the tridimensional features of a DPAL system with the much higher output power. By combining the procedures of heat transfer and laser kinetics, we developed a new algorithm to simultaneously evaluate the three-dimensional (3-D) lasing characteristics and the 3-D thermal features of a cesium-vapor laser end-pumped by a laser diode. The 3-D population density distributions of the quasi-five-level of atomic alkali, temperature distribution, and distributions of the absorbed power and laser photon density inside a vapor cell have been systematically obtained and analyzed. The theoretical results of the output characteristics coincide with the experimental ones. The results are thought to be meaningful for the realization of a high-powered DPAL in the future.

Numerical Analysis of Solid Fuel Regression Rate Distribution Characteristics in Solid-fuel Ramjet with Different Fuel Types

This paper proposes and examines numerically a new design of solid-fuel ramjet motor. Multi-physics coupling code is developed using FORTRAN and parallel computing to solve the problems of multi-physics coupling of fluid mechanics, solid pyrolysis, heat transfer, thermodynamics, and chemical kinetics. Simulations are carried out for the proposed design then the results are compared with the classic design of the solid-fuel ramjet. It is found that the proposed design has improved the regression rate significantly; besides, the amount of released solid fuel is increased for the same size.

キルティングの伝熱特性の非接触評価

キルティングの伝熱特性の非接触評価

Mechanism of Wax Deposition on Cold Surfaces: Gelation and Deposit Aging

To further develop an understanding of the wax deposition mechanism needed to model its occurrence in subsea oil pipelines, deposition experiments using a pure n-alkane and a binary n-alkane mixture were performed in a cold finger apparatus modified to allow for reliable and accurate visualization of the wax deposit thickness as a function of time. The deposit thickness is found to grow initially but to shrink at long times as the deposit composition enriches in the wax content and the wax concentration in the surrounding oil decreases. This non-monotonic time evolution of the deposit thickness and increase in wax composition are explained and modeled using transient heat and mass transfer kinetics. The ultimate shrinkage of the deposit thickness is traced to depletion of soluble wax in the bulk oil because soluble wax continuously diffuses from the bulk oil into the deposit and precipitates there by fast precipitation kinetics.

Calculation of the kinetics of heat transfer using the experimental data of moisture exchange in the process of convective drying of thin flat materiаls

In the paper, the authors analyzed the solution of the differential equation of non-stationary heat conduction for an unbounded plate during the heat exchange of plate surfaces with the surrounding medium according to Newton’s law at a constant temperature of the medium. To use the results of solving the equations in the drying of thin flat materials, the dependence of the heat transfer coefficients on temperature and moisture content was studied. As a result of studying and analyzing a number of literature sources, the regularities of the change in the heat transfer coefficients during drying are established with high reliability. Studies of drying of thin wet plates of white and red clays with known heat transfer coefficients have shown that for small values of the heat transfer criterion of the Bio and small temperature gradients over the section of a thin material, application of the results of solutions of the heat transfer equations gives completely satisfactory agreement between the calculated and experimental values of the temperatures and the duration of drying. It is established that for small Bio numbers, the main factor is the external heat and mass transfer of the surface of the material with the surrounding medium and the rate of drying depends little on internal mass transfer. It is shown that the use of numerical methods for solving differential equations is possible with varying degrees of approximation only for accurate and reliable dependences of heat and mass transfer coefficients on moisture content and temperature. For a number of materials with known heat transfer coefficients, the use of analytical methods in calculations is of considerable interest and brings the theory closer to the practice of drying.

Methane Hydrate Intrinsic Dissociation Kinetics Measured in a Microfluidic System by Means of in Situ Raman Spectroscopy

The present work investigates the dissociation kinetics of methane (sI) hydrate in a thermoelectrically cooled microfluidic system with in situ Raman spectroscopy. The dissociation profile of methane (sI) hydrate was measured under different Reynolds numbers (0.42–4.16), pressures (60.2 to 80.1 bar), and temperature driving force ([Teq – 0.1 K] to [Teq + 0.3 K]). A theoretical model was derived from first-principles to describe the contributions of heat transfer and intrinsic kinetics on the dissociation rate of methane hydrate. It was observed that the dimensionless ratio of heat transfer to the intrinsic dissociation rate depended on the initial thickness of methane hydrate, temperature driving force, pressure, and the time. Intrinsic kinetics dominated where the initial thickness of methane hydrate was in the range of 10 μm and the temperature driving force was low. Increasing initial thickness of methane hydrate resulted in a switch to a heat-transfer-limited dissociation. Our results support that the “memory effect” previously reported is the result of dissociation limited by intrinsic kinetics. Furthermore, this work introduces a new laboratory method of unusually sensitive microscale control of the dissociation conditions, while generating molecular-level insight. Our findings support that microfluidics with in situ Raman spectroscopy are excellent laboratory tools to understand methane hydrate dissociation with potentially much broader utility to the field. Methane hydrates play an important role in energy production and storage and the atmospheric and oceanic sciences.