Horizontal Enclosure Research Articles

Limitation of the 2D parallel flow assumption in thermosolutal convection: 2D-3D transition

In this study, we investigated numerically two- and three-dimensional convective heat and mass transfer in a horizontal rectangular enclosure filled with heterogeneous porous media. The main goal is to underline the limitation of the widely used classical parallel flow assumption. The considered configuration is Cartesian. The horizontal and vertical walls are submitted to different mass and heat transfer. The Darcy model and the Boussinesq approximation are considered. The governing parameters which control the problem are the Darcy-Rayleigh number, Rt, the buoyancy ratio, N, the enclosure aspect ratio, A, the local permeability ratio, Kr and the Lewis number (fixed to ten in the present study). The obtained results with two-dimensional (2D) and three-dimensional (3D) approaches are compared to underline similarities and differences. We demonstrate the limit validity of 2D solution and the transition to 3D solutions when the convective forces or the domain permeable heterogeneity increases. The flow intensity, heat and mass transfer increases with the domain permeable heterogeneity.

Experimental investigation of the effect of inclination angle on convection-driven melting of phase change material in a rectangular enclosure

This paper investigates the dynamic thermal behavior of phase change material (PCM) melting in a rectangular enclosure at various inclination angles. Lauric acid as a PCM with high Prandtl number (Pr≈100) is used. The enclosure is heated isothermally from one side while the other walls are thermally insulated. Experiments were performed with hot wall temperatures of 55, 60 and 70°C (3.6×108⩽Ra⩽8.3×108) for different inclination angles of 0°, 45° and 90°. Image processing of melt photographs along with recorded temperatures were used to calculate the melt fractions, Nusselt numbers and the local interfacial heat transfer rates at the solid–liquid interface. Qualitative time-dependent natural convection flow structures were deduced indirectly from the instantaneous shape of the solid–liquid interface which were confirmed by quantitative data from temperature measurements. The results reveal that the enclosure inclination has a significant effect on the formation of natural convection currents and consequently on the heat transfer rate and melting time of the PCM. As the inclination angle is decreased from 90° to 0°, the convection currents in the enclosure increases and chaotic flow structures appear. When melting commences in the horizontally inclined enclosure, the solid–liquid interface line becomes wavy which implies the formation of Benard convection cells in the liquid PCM. For the same hot wall temperatures, a decrease in inclination angle leads to a considerable enhancement in energy transport from the hot wall of the enclosure to the PCM. It is found that the heat transfer enhancement ratio for the horizontal enclosure is more than two times higher than that of the vertical enclosure.

Numerical modelling the unsteady process of closed rectangular area radiant heating in conjugate formulation with accounting energy distribution along horizontal and vertical enclosure structures

Mathematical modelling of unsteady convective-conductive heat exchange in premises, heated by infrared radiant heater is passed. Heat flux density from infrared radiant heater was calculated accounting energy distribution along horizontal and vertical building envelope. Comparison between zonal method and Lambert's law radiant energy distribution was done.

An experimental investigation of fluid flow in horizontal enclosure with baffle plates on two opposite walls

An experimental investigation of fluid flow in horizontal enclosure with baffle plates on two opposite walls

NUMERICAL ANALYSIS FOR PRANDTL NUMBER DEPENDENCY ON NATURAL CONVECTION IN AN ENCLOSURE HAVING A VERTICAL THERMAL GRADIENT WITH A SQUARE INSULATOR INSIDE

The natural convection in a horizontal enclosure heated from the bottom wall, cooled at the top wall, and having a square adiabatic body in the center is studied. Three different Prandtl numbers (0.01, 0.7 and 7) are considered for the investigation of the effect of the Prandtl number on natural convection. Adiabatic boundary conditions are employed for the side walls. A twodimensional solution for unsteady natural convection is obtained, using an accurate and efficient Chebyshev spectral methodology for different Rayleigh numbers varying over the range of 10 3 to 10 6 . It had been experimentally reported that the heat transfer mode becomes oscillatory when Pr is out of a specific Pr band beyond the critical Ra. In this study, we reproduced this phenomenon numerically. It was found that when Ra=10 6 , only the case for intermediate Pr (=0.7) reached a non-changing steady state and the low and high Pr number cases (Pr=0.01 and 7) showed a periodically oscillatory fashion hydrodynamically and thermally. The variation of time- and surface-averaged Nusselt numbers on the hot and cold walls for different Rayleigh numbers and Prandtl numbers are presented to show the overall heat transfer characteristics in the system. Further, the isotherms and streamline distributions are presented in detail to compare the physics related to their thermal behavior.

Numerical Simulation for Heat Storage Study of the Microencapsulated Phase Change Material Slurry in a Horizontal Rectangular Enclosure

Two-dimensional numerical simulation of heat storage characteristics in a horizontal rectangular enclosure heated from below by natural convection has been investigated with the microencapsulated phase change material (PCM) slurry. Effects of PCM on heat storage and heat transfer are discussed, in which PCM slurry exhibits the pseudoplastic non-Newtonian fluid behavior and a peak value in the specific heat capacity with latent heat in phase temperature range. And their characteristics between the slurries with and without phase change of PCM are compared in the phase change temperature range. And vertical temperature distributions at the middle position are given. This paper shows streamlines and isotherms in the enclosure for phase change process.

사각 단열체가 존재하는 밀폐계 내부에서 Pr수 변화에 따른 자연대류 현상에 대한 수치적 연구

The natural convection in a horizontal enclosure heated from the bottom wall, cooled at the top wall, and having a square adiabatic body at its centered area was studied. Three different Prandtl numbers (0.01, 0.7 and 7) were considered for an effect of the Prandtl number on natural convection. A two-dimensional solution for unsteady natural convection was obtained, using Chebyshev spectral methodology for different Rayleigh numbers varying over the range of <TEX>$10^4$</TEX> to <TEX>$10^6$</TEX>. It had been experimentally and numerically reported [1,2] that the heat transfer mode becomes oscillatory when Pr is out of a specific Pr band beyond the critical Ra. In this study, we reproduced this phenomenon numerically. The variation of time- and surface-averaged Nusselt numbers on the hot and cold walls for different Rayleigh numbers and Prandtl numbers was presented to show the overall heat transfer characteristics in the system. And also, the isotherms and streamline distributions were presented in detail to compare the physics related to their thermal behavior.

Experiment on heat storage characteristic of microencapsulated phase change material slurry

Heat storage experiment by natural convection in rectangular enclosures heated from bottom has been conducted with fluid slurry composed of microencapsulated phase change material (PCM). The microencapsulated PCM is prepared by in-situ polymerization method, where the core materials are composed of several kinds of n-paraffin waxes (mainly nonadecane) and the membrane is a type of melamine resin. Its slurry mixed with water is used in this study, and shows a peak value in the specific heat capacity with latent heat at the temperature of about T=31 °C. The influences of the phase change material on heat storage and the heat transfer process, as well as effects of PCM mass concentration C m on the microcapsule slurry, temperature of heat storage T H and a horizontal enclosure height H are also investigated. Transient heat transfer coefficient α, heat storage capacity Q and completion time of heat storage t c are discussed.

Natural Convection Heat Transfer of the PCM Microcapsule Slurry for a Solid Phase State in a Horizontal Rectangular Enclosure

This experiment on natural convection heat transfer with the PCM microcapsule slurry for a solid phase state of the PCM were performed at a horizontal rectangular enclosure heating from below and cooling from top. In the present study, important parameters affecting the natural convection heat transfer of the slurry with the PCM were taken into account such as the mass concentration of the PCM microencapsulated into the slurry, the temperature difference between heating plate and cooling plate, the Rayleigh number Ra and the aspect ratio (width/height) of the horizontal rectangular enclosure. The heat transfer characteristics of the PCM microcapsule slurry on natural convection were concluded from predicted conditions simplified by classifying the temperature ranges into three sub-regions, in which the PCM in the slurry was in only solid phase state, phase change state or solid-liquid coexistence state and only liquid phase state of the PCM in the microcapsule, respectively. The upper cooling plate was fixed at a constant temperature in each region, and the lower heating plate temperature was varied by carefully adjusting the electric power input of an electric plate heater. And experiment was done under the thermal steady condition in the PCM microcapsule slurry. Emphasis was given on phase changing temperature range, and experiments had been performed for four kinds of enclosures with various heights.

Heat Transfer Characteristics of the Phase Change Material Microcapsule Slurry in Solid Phase State

This experiment is performed to investigate heat transfer characteristics with the PCM microcapsule slurry in a solid phase state at a horizontal rectangular enclosure heating from below and cooling from top. Some important parameters are taken into account such as the mass concentration of the PCM, the temperature difference between heating plate and cooling plate, Nusselt number Nu, Rayleigh number Ra and the aspect ratio (width/height) of the horizontal rectangular enclosure. Experiment is done under the thermal steady condition in the PCM microcapsule slurry. Heat transfer coefficient is measured under various temperature differences in PCM mass concentrations of 10% and 20%. And relationship with Nusselt number Nu and Rayleigh number Ra is summarized to various heights H or the aspect ratio (width/height) Ar of enclosure.

Cooling process of water in a horizontal circular enclosure subjected to non-uniform boundary conditions

Abstract This paper investigates the cooling process of water in a circular enclosure subjected to non-uniform boundary conditions. Half of the enclosure boundary is maintained at a constant temperature, while the other half is well insulated. The enclosure is cooled from its top, bottom, or left side. The problem is solved numerically using the finite element method. The examined Rayleigh numbers are 1 × 10 5 , 1 × 10 6 , and 1 × 10 7 , and the effect of maximum density at 4 °C is considered. The results indicate that the natural convection flow plays a significant role on the cooling process of water. The effects of the cooling side and Rayleigh number on the cooling process are presented. For Ra = 1 × 10 5 , the results indicate that the bottom cooling side has the lowest cooling time, but it has the highest cooling time for Ra = 1 × 10 7 . For Ra = 1 × 10 6 and 1 × 10 7 , the left cooling shows lowest cooling time. For Ra = 1 × 10 5 , the bottom cooling case takes 52.97% less time than the top cooling case to complete the cooling process. The density inversion near 4 °C creates a unique natural convection flow, and high resolution capturing of temperature contours and natural convection flow is presented.

Experimental Study on Natural Convection Heat Transfer From two Parallel Horizontal Cylinders in Horizontal Cylindrical Enclosure

An experimental study on natural convection heat transfer from two parallel horizontal cylinders in horizontal cylindrical enclosure was carried out under condition of constant surfaces temperature for two cylinders and cylindrical enclosure. The study included the effect of Rayleigh number, rotation angle that represent the confined angle between the passing horizontal plane in cylindrical enclosure center and passing line in two cylinders centers, and the spaces between two cylinders on their heat loss ability. 39 An experimental set-up was used for this purpose which consist water container, test section which is formed of plastic cylinder that represent the cylindrical enclosure, and two heating elements which are formed of two copper cylinders with (19 mm) in diameters heated internally by electrical sources that represents transfer and heat loss elements through this set-up. The experiments were done at the range of Rayleigh number between ( ), cylinders rotation angle at ( ), and spacing ratio at ( ). The study showed that the ability of heat loss from two cylinders is a function of Rayleigh number, cylinders rotation angle, and the spaces between them. This ability is increased by increasing of Rayleigh number and it was showed that this ability reaches maximum value at the first cylinder ( ) and minimum value at the second cylinder ( ) at spacing ratio (S/D=3) and rotation angle ( ) for the first and ( ) for the second cylinder respectively. The effective variables on natural convection heat transfer from the above two cylinders are related by two correlating equations, each one explains dimensionless relation of heat transfer from each cylinder that represented by Nusselt number against Rayleigh number, rotation angle, and the spacing ratio between two cylinders.

A Generalized Lattice Boltzmann Method for Three-Dimensional Incompressible Fluid Flow Simulation

In this work, a 19-bit Incompressible Generalized Lattice Boltzmann (IGLB) method has been proposed for threedimensional incompressible fluid flow simulation, for the first time. Equilibrium moments in moment space are derived from an incompressible BGKLB method. The incompressible Navier–Stokes equations can be recovered through the Chapman-Enskog multi-scale expansion without artificial compressible effects. To compare the performance of proposed model, several benchmark problems (such as a cubic lid-driven cavity flow, flow over a backward-facing step, and a double shear flow) are solved and the results are compared with those of both 19-bit Incompressible BGK Lattice Boltzmann (IBGKLB) method and existing CFD simulations. It is shown that the stability and accuracy of the 19-bit IGLB method is better than those of the 19-bit IBGKLB method; in fact with the IGLB model we can increase the Reynolds number by factor of 2.5 and still get stable results. The proposed 3-D IGLB method is successfully expanded and applied to simulation of the 3-D incompressible buoyancy driven flows. The results of the 3-D steady-state natural convection in an air-filled differentially heated cubic cavity obtained by the extended model comply well with the existing data in literature. In addition, natural convection from a discrete heat source which is mounted flush with the bottom wall of a horizontal enclosure is simulated. The obtained results indicate that the proposed method is very convenient for simulation of thermally driven flow problems.

Parallel Flow Convection in a Shallow Horizontal Cavity Filled with Non-Newtonian Power-Law Fluids and Subject to Horizontal and Vertical Uniform Heat Fluxes

Natural convection heat transfer in a shallow horizontal rectangular enclosure is considered. The enclosure contains non-Newtonian fluids and is exposed to cross-gradients of temperatures. The power-law model is used to characterize the non-Newtonian fluid behavior. Analytical solution is obtained in the central region of the cavity in terms of stream function and temperature fields, assuming a parallel flow in the central part of the enclosure. The analytical results are compared against the numerical solution, obtained by solving the full governing equations using a finite difference method. It is demonstrated that multiple steady state convective flows are possible.

Multiple steady flows in confined gaseous double diffusion with discrete thermosolutal sources

A detailed numerical study has been conducted to investigate the steady natural convection in a binary mixture in rectangular enclosures with the simultaneous presence of discrete heat and moisture sources. The Prandtl number and Lewis number used are 0.7 and 0.8, respectively. The heat and mass transfer characteristics of the horizontal enclosure are analyzed using the heat lines and mass lines, and the heat and moisture transfer potentials are also explained by the variations of overall Nusselt and Sherwood numbers. Numerical results are particularly presented to illustrate the effects of the buoyancy ratio, thermal Rayleigh number, and strip pitch on the multiple steady flow patterns and the associated heat and mass transfer, for both destabilizing and stabilizing solutal buoyancy forces. The effects of the buoyancy ratio are found to be rather significant on the flow pattern and heat and mass transfer especially for the destabilizing solutal forces. Additionally, in the regime that both thermal and concentration buoyancy forces are destabilizing, the heat and mass transfer potential can be promoted or inhibited, depending strongly on the thermal Rayleigh number and strip pitch.

Coupled Natural Convection and Radiation in a Horizontal Rectangular Enclosure Discretely Heated from Below

A numerical study is carried out to investigate the interaction between natural convection and thermal radiation in a horizontal enclosure filled with air and heated discretely from below. The results are presented for a cavity having an aspect ratio A r = L′/H′ = 10, while the Rayleigh number and the emissivity of the walls are varied in the ranges 103 ≤ Ra ≤ 106 and 0 ≤ ϵ ≤ 1, respectively. The results of the study, presented in terms of flow and temperature patterns, average convective, radiative and total Nusselt numbers, evaluated on the cold wall, show that the problem has multiple solutions. Each of these solutions is characterized by a specific flow structure, and its appearance and range of existence depend strongly on the parameters Ra and ϵ. The amount of heat evacuated through the cold surface is dependent on the type of solution.

169 円柱細線を伝熱面間に挿入した水平多孔質層内の自然対流(熱工学II)

169 円柱細線を伝熱面間に挿入した水平多孔質層内の自然対流(熱工学II)

Natural Convection in an Enclosure with Distributed Heat Sources

ABSTRACT Presented in this article is a computational analysis of the heat transfer due to an array of distributed heat sources on the bottom wall of a horizontal enclosure. The heat sources are modeled as flush-mounted sources with prescribed heat flux boundary conditions. Optimum heat transfer rates and the onset of thermal instability triggering various regimes are found to be governed by the length and spacing of the sources and the width-to-height aspect ratio of the enclosure. With respect to source spacing, we found that spacing equal to that of the source length provides effective convective heat transfer, and increasing the source spacing further does not result in significant improvements. The transition from a conduction-dominated regime to a convection-dominated regime is found to be characterized by a range of Rayleigh numbers, in contrast to the classical bottom wall heating problem. The range of Rayleigh numbers at which transition takes place decreases as the source length increases. At the transition region for very small source lengths, the Rayleigh-Bénard cell structure grows significantly to form fewer and larger cells, which accounts for higher heat transfer rates compared to configurations with longer heat sources where the cell structure remains the same throughout transition. Following the transition to a convection-dominated regime, bifurcations in the Rayleigh-Bénard cell structures as well as further regime changes are observed, reflecting the instabilities in the physical system.

Numerical simulation of natural convection in a horizontal enclosure with a heat-generating conducting body

The physical model considered here is a horizontal layer of fluid heated below and cold above with heat-generating conducting body placed at the center of the layer. The dimensionless thermal conductivities of body considered in the present study are 0.1, 1 and 50. The dimensionless temperature difference ratios considered are 0.0, 0.25, 2.5 and 25. Two-dimensional solution for unsteady natural convection is obtained using an accurate and efficient Chebyshev spectral methodology for variety of Rayleigh number from 10 3 to 10 6. Multi-domain technique is used to handle square-shaped heat-generating conducting body. The fluid flow, heat transfer and time- and surface-averaged Nusselt number are investigated for various ranges of Rayleigh number, thermal conductivity ratio and dimensionless temperature difference ratio. The results for the case of conducting body with heat generation are also compared to those without heat generation to see the effects of heat generation from the conducting body on the fluid flow and heat transfer in the enclosure.

4616 空隙率の高い水平多孔質体内の自然対流熱伝達(G06-3 伝熱(3),G06 熱工学)

4616 空隙率の高い水平多孔質体内の自然対流熱伝達(G06-3 伝熱(3),G06 熱工学)