W. Pakdee - Academia.edu (original) (raw)

Papers by W. Pakdee

Journal of Applied Fluid Mechanics, 2017

In the present study, the unsteady magnetrohydrodynamic (MHD) flow of compressible fluid with var... more In the present study, the unsteady magnetrohydrodynamic (MHD) flow of compressible fluid with variable thermal properties has been numerically investigated. The electrically conducting fluid flows through a porous media channel. The uniform magnetic field is applied perpendicular to the direction of the flow. The wall is assumed to be non-conducting and maintained at two different temperatures. The thermal conductivity and viscosity of the fluid change with temperature. Sixth-Order Accurate Compact Finite Difference scheme together with the Third-order Runge-Kutta method is used to solve a set of non-linear equations. The results of the calculation are expressed in the form of the velocity and temperature at different values of the magnetic field and porosity. The proposed mathematical model and numerical methods have been validated by comparing with the results of previously published studies that the compared results reveal the same trends. The difference is due to the compressibility and property variation effects. The results showed that the magnetic field and variable properties considerably influences the flows that is compressible thereby affecting the heat transfer as well as the wall shear stress.

Food and Bioprocess Technology, 2009

The heating process of water and oil using microwave oven with rectangular wave guide is investig... more The heating process of water and oil using microwave oven with rectangular wave guide is investigated numerically and experimentally. The numerical model is validated with an experimental study. The transient Maxwell's equations are solved by using the finite difference time domain method to describe the electromagnetic field in the wave guide and sample. The temperature profiles and velocity field within sample are determined by the solution of the momentum, energy, and Maxwell's equations. In this study, the effects of physical parameters, e.g., microwave power, the position of sample in wave guide, size, and thickness of sample, are studied. The results of distribution of electric field, temperature profiles, and velocity field are presented in details. The results show that the mathematical models are in agreement with the experimental data. Conclusively, the mathematical model presented in this study correctly explains the phenomena of microwave heating within the liquid layer. Keywords Microwave heating. Rectangular wave guide. Mode TE 10. Maxwell's equation. FDTD method Subscripts 1 ambient condition a air j layer number in input w water Food Bioprocess Technol

40th AIAA Aerospace Sciences Meeting & Exhibit, 2002

ABSTRACT

tsme.org

This paper proposes a mathematical model used to numerically simulate behaviours of the twodimens... more This paper proposes a mathematical model used to numerically simulate behaviours of the twodimensional laminar premixed combustion in porous media. The governing equations proposed include vorticity-stream function equations. This set of the governing equations is non-dimensionalized and numerically solved based on finite difference algorithm. The rectangular computational domain filled with saturated porous media is subjected to premixed reactant mixture coming into the domain from the lateral wall. The top and bottom walls are insulated. The computation is conducted for half of the domain based on a symmetrical boundary with appropriate flow and thermal conditions. The proposed mathematical model is successfully validated against the published work. The computed results agree reasonably well with the previous work. The model is able to correctly describe physical behaviors of a premixed combustion in which preheat, reaction and post combustion zone are included.

Renewable Energy, 2012

Palm olein and palm stearin are co-products of palm oil refining processes having different melti... more Palm olein and palm stearin are co-products of palm oil refining processes having different melting point ranges. This study compares the storage degradation characteristics of biodiesels derived from these two palm products, which are palm olein and palm stearin, in terms of chemical properties, engine performance and exhaust emission. The degradation study was carried out by keeping biodiesels in dark closed-lid containers at room temperature for up to 6 months. It was found that the oxygen present in the container led to slow degradation of biodiesels through oxidative reaction with the double bonds in biodiesel. Within 6 months, the majority of oxidative products were composed of shorter hydroperoxide compounds and other short secondary products. These changes resulted in lower heating value and higher density of biodiesels, which in turn caused reductions in fuel combustion efficiency and fuel economy. In terms of emission, the degraded biodiesel produced more complete combustion as indicated by lower emissions of black smoke and carbon monoxide but with higher emission of NOx. In terms of palm oil type, even though palm olein biodiesel possessed higher degree of unsaturation and produced higher peroxide value and acid values from the degradation, its combustion efficiency and fuel economy were still superior to the biodiesel produced from palm stearin possibly due to its higher chain lengths.

International Journal of Heat and Mass Transfer, 2011

This paper presents the numerical and experimental analysis of microwave heating in a saturated p... more This paper presents the numerical and experimental analysis of microwave heating in a saturated packed bed by using a rectangular waveguide (TE 10 mode). A complete mathematical model is proposed, which uses comprehensive two-dimensional energy and momentum equations to describe unsteady temperature and flow fields, coupled with a complete solution of the transient Maxwell's equations in time domain. The influences of particle size, sample dimension, placement inside the guide, and frequency on heat transfer and flow patter are studied. The simulation results are in good agreement with experimental data. Flow pattern strongly depends on the bead size and thermal efficiency of the sample changes with sample size and frequency. Furthermore, the middle placed inside the guide aids in uniform heating.

Heat Transfer—Asian Research, 2012

A numerical analysis is carried out to study the performance of natural convection inside a squar... more A numerical analysis is carried out to study the performance of natural convection inside a square open tilted cavity filled with air. An adiabatic circular solid cylinder is placed at the center of the cavity and the sidewall in front of the breathing space is heated by a constant heat flux. The top and bottom walls are kept at the ambient constant temperature. Two‐dimensional forms of Navier–Stokes equations along with the energy equations are solved using the Galerkin finite element method. Results are obtained for a range of Grashof numbers from 103 to 106 at Pr = 0.71 while the tilt angle varies from 0 to 45° and the diameter ratio of the cylinder is considered to be 0.2, 0.3, and 0.4 with constant physical properties. The parametric studies for a wide range of cylinder diameter ratios and cavity tilt angles show significant features of the present problem in terms of stream functions and temperature profiles. The computational results indicate that the heat transfer coefficien...

Experimental Thermal and Fluid Science, 2009

The heating process of dielectric materials by microwave with rectangular wave guide has been inv... more The heating process of dielectric materials by microwave with rectangular wave guide has been investigated experimentally. This experiment is operated in the TE 10-dominated mode at a frequency of 2.45 GHz. It was performed for two dielectric materials, water layer and saturated porous medium. In this work, the microwave powers level, a type of dielectric materials, dimensions and positions of dielectric material on the microwave power absorb and average temperature within dielectric materials were examined in details. The results show that the locations of sample have greater effects than the other parameters. The analyses from this research serve as essential fundamentals to development of mathematic models of heat and mass transfer phenomena.

Drying Technology, 2007

This study develops a mathematical model for coupled heat and mass transfer in an unsaturated por... more This study develops a mathematical model for coupled heat and mass transfer in an unsaturated porous slab exposed to a flowing hot gas. Effects of the initial saturation conditions on associated variables, i.e., total pressure, temperature, moisture content, and multiphase flow, are studied. The Newton-Raphson method based on a finite volume technique is applied. This study emphasizes the influence of initial saturation level and gravitational effect in heat and multiphase flow phenomena associated with this system. Gravity enhances the downward flow of liquid within the porous slab. Pressure buildup occurs near the interface between the wet and the dry zone. However, it appears that the order of magnitude to the total pressure is small. This study explains the fundamental mechanism of multiphase flow that involves heat and mass transfer in a heated unsaturated porous slab.

Drying Technology, 2011

Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf ... more Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, redistribution , reselling , loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

Drying Technology, 2012

The present work numerically investigates the natural convection of fluid in saturated porous med... more The present work numerically investigates the natural convection of fluid in saturated porous media under an electromagnetic field. The porous medium consists of water and spherical solid particles. The Brinkman-Forchheimer extended Darcy model for the momentum equation is employed, and two energy equations are solved with local thermal nonequilibrium (LTNE) models. The study aims at a comprehensive analysis of the influence of material properties, microwave power levels, and operating frequencies on the transport processes in porous media. The results show that all effects have significant consequences on the flow field and heat transfer in porous media. These findings can explain the phenomena taking place inside saturated porous media in electromagnetic wave drying processes using LTNE and local thermal equilibrium (LTE) models.

Applied Thermal Engineering, 2006

Numerical investigations of transient natural convection flow through a fluid-saturated porous me... more Numerical investigations of transient natural convection flow through a fluid-saturated porous medium in a rectangular cavity with a convection surface condition were conducted. Physical problem consists of a rectangular cavity filled with porous medium. The cavity is insulated except the top wall that is partially exposed to an outside ambient. The exposed surface allows convective transport through the porous medium, generating a thermal stratification and flow circulations. The formulation of differential equations is non-dimensionalized and then solved numerically under appropriate initial and boundary conditions using the finite difference method. The finite different equation handling the boundary condition of the open top surface is derived. The two-dimensional flow is characterized mainly by two symmetrical vortices driven by the effect of buoyancy. A lateral temperature gradient in the region close to the top wall induces the buoyancy force under an unstable condition. Unsteady effects of associated parameters were examined. It was found that the heat transfer coefficient, Rayleigh number and Darcy number considerably influenced characteristics of flow and heat transfer mechanisms. Furthermore, the flow pattern is found to have a local effect on the heat convection rate.

... Department of Mechanical Engineering, University of California, Riverside, CA 92521 WatitPakd... more ... Department of Mechanical Engineering, University of California, Riverside, CA 92521 WatitPakdee Joint Center for Combustion and Environmental Research Department of Mechanical Engineering University of Colorado at Boulder Boulder, CO 80309-0427 ...

Combustion, Explosion, and Shock Waves, 2007

A fully compressible database of turbulent non-premixed flames of a wood pyrolysis gas is develop... more A fully compressible database of turbulent non-premixed flames of a wood pyrolysis gas is developed by means of direct numerical simulation (DNS). A reduced kinetic mechanism is used to model the combustion of a pyrolysis gas-air mixture. The instantaneous flame surface density evolution equation based on the concept of a displacement speed is examined. The normal component of the displacement speed is nearly constant with respect to curvature, while the curvature-related component tries to restore the flame front to a planar shape. The strain-rate term is mainly a source as the flame is mostly extended. The normal displacement is responsible for both positive and negative contributions to the flame area. The displacement/curvature term is primarily a sink, since it is dominated by its curvature component. Effects of strain and curvature are analyzed by considering their correlations with reaction rates. Reaction rates are enhanced with increased positive strain rates owing to an increase in the flame surface area and to a decrease in curvature. The analyzed results aid in the development of turbulent combustion models. Finally, a new model for a mean variance of the scalar dissipation rate, based on a scale similarity approach, is proposed and examined. A comparison with DNS results shows that the proposed model provides a significant improvement over existing models.

Journal of Computational Applied Mechanics, Feb 12, 2024

This research investigates the numerical analysis of magnetohydrodynamic (MHD) mixed convection f... more This research investigates the numerical analysis of magnetohydrodynamic (MHD) mixed convection flow and heat transfer within a bottom lid-driven cavity filled with water-alumina (Al2O3) nanofluid. The cavity's sidewalls exhibit a wavy profile and are maintained at distinct temperatures. Cavity domain exhibit distinct free and force convections. These wavy walls, characterized by zigzag shapes determined by various wave amplitudes and their ratios (wave form), create a dynamic thermal environment. The top and bottom surfaces remain flat and well-insulated, while forced convection is induced by the drag of the bottom wall from left to right at a constant speed. Additionally, the bottom wall is subjected to a vertical magnetic field. The system of equations is discretized using the finite difference method. The numerical solutions are derived by the Gauss-Seidel iterative method. The study primarily focuses on investigating the effects of key parameters, including the wavy wall geometry, solid volume fraction (0 ≤ φ ≤ 0.0003), Rayleigh number (103≤ Ra ≤105), and Hartmann number (0 ≤ Ha ≤0.6). Numerical solutions are computed across different ranges of these parameters, and the obtained results are successfully validated against previous numerical studies. The findings reveal that higher Hartmann numbers and solid volume fractions lead to lower circulation rates and Nusselt numbers. Convection is markedly enhanced with higher amplitude and its ratios of the wavy sidewalls. The combined two-sinusoidal function with the wave amplitudes of 2.5 and 0.47 of provides the highest mean Nusselt numberof3.204 with the highest dimensionless stream function of 1.638. These results highlight the significant influence of the wave form on both flow and temperature distributions.