Adiabatic Side Walls Research Articles

On the challenges for reliable measurements of convection in large aspect ratio Rayleigh-Bénard cells in air and sulfur-hexafluoride

Heat transfer through natural turbulent thermal convection is an important mechanism in geophysics and many engineering problems. A canonical well-known experiment is the Rayleigh-Bénard (RB) cell, which consists of an enclosure with adiabatic side walls uniformly heated from below and cooled from above. The direct correlation between the local momentum and heat transfer is currently under investigation, especially for large aspect ratios. In this situation the dominant large-scale circulation roll, as typically found for low aspect ratios, is substituted by a whole pattern of long-living rolls. Nowadays, commonly advanced optical methods are applied to investigate velocity fields, which makes a transparent heating or cooling plate necessary. For the current study glass plates were coated with a thin metal oxide layer enabling heating via the Joule effect. However, since the temperature homogeneity is crucial for the setup, two heating plates had to be combined to enable a homogeneous temperature distribution at the bottom of the convection cell. Additional difficulties arise from the large aspect ratio and thus the large observation angle. This may cause optical aberrations and systematic errors due to the perspective bias. Furthermore, the observation duration has to be in the order of minutes to hours to resolve the dynamics of the slowly evolving large-scale flow. This results in additional challenges for the seeding generation. To prove the reliability of the experimental approach, a convection cell with an aspect ratio Γ=l/h=10 was placed in the SCALEX (Scaled Convective Airflow Laboratory Experiment) facility. This facility enables experiments with sulfur-hexafluoride (SF6) and air under pressures of up to 10 bar to achieve very high Rayleigh numbers. Here, results of measurements for a Rayleigh number of 5×105 in SF6 at ambient pressure and in air at a pressure of 5.7 bar will be discussed and show the applicability of this approach. Stereoscopic particle image velocimetry (PIV) in horizontal planes covering the whole cross section of the cell (300×300mm2) was used for the estimation of all three components of the velocity vector. The optical access for the laser light sheet was provided by transparent sidewalls. The aim of the current study was to prove the possibility of reliable PIV measurements with reproducible homogenous temperature boundary conditions in the SCALEX facility. Issues of the temperature distribution at the heating plate, tracer particles, illumination and data evaluation will therefore be addressed in greater detail.

Effects of radiation on turbulent natural convection in channel flows

A computational study has been carried out to analyse complex interaction of radiation with turbulent natural convective flow of dry and humid air in open-ended channels. Transient flow simulations are undertaken in the channel with one uniformly heated wall and adiabatic side walls for different values of emissivity of active walls with and without participating medium. To adequately present turbulence and radiation, a computational model included large eddy simulations for the turbulent flow coupled with discrete ordinates method for radiation transfer. Spectral line-based weighted-sum-of-grey-gases for the absorption properties of water vapour has been adopted. Complex three-dimensional vortical structures are identified which directly affect the temperature distribution on the heated wall. Including wall to wall radiation resulted in significant changes in the heat transfer, reaching 14 °C temperature drop at the hot wall with wall emissivity of 0.9. Mixing and cooling rates in this case were increased by up to 25%. Including gas radiation for the humid air with the water vapour molar fraction of 0.02 corresponding to saturated conditions at inlet temperature of 25 °C did not have a significant effect on the mean flow and temperature values comparing with wall to wall radiation. However, turbulent statistics have changed significantly resulting in a delayed transition to turbulence near the active wall of the channel and increased turbulent activity near the cold wall. The model developed in the present study is also applicable in fire management, where the aim is to reduce the damage that occurs when a PV module is exposed to high temperatures.

Effect of side wall conditions and aspect ratio on convective pattern formation in Rayleigh-Benard convection

Effect of side wall boundary conditions and aspect ratio in very small boxes on patterns formed during Raleigh-Benard (RB) convection is studied using an indigenously developed flow solver based on the finite difference method. While keeping the bottom (hot) and top (cold) walls isothermal, the side wall thermal conditions are kept either constant heat flux or isothermal type. For constant heat flux side wall condition, the considered value of heat flux lies between 0 and maximum corresponding to conductive heat flux between top and bottom walls. For isothermal side wall condition, the chosen value of temperature is between the top and bottom wall temperatures. The computational domain is a cube or a square prism with aspect ratio in longitudinal direction (ARx) varied between 1.2 and 2 in steps of 0.2. For fixed value of ARx = 1.6, aspect ratio in transverse direction (ARz) has been varied for adiabatic side wall conditions. Effect of side wall condition on orientation of roll axis and the number of rolls has been investigated with observations well supported by stream traces and temperature contours. Moreover, for selected parameters, based on nature of pattern formation for steady flow, onset of oscillatory convection is explained.

Natural convection in a square enclosure with a circular cylinder with adiabatic side walls according to bottom wall temperature variation

Two-dimensional numerical simulations were used to analyze the temperature-driven natural convection phenomena in a square enclosure with an inner cylinder in the Rayleigh number range of Ra =105 ~ 106. The immersed boundary method based on the finite volume method (FVM) was applied to generate and treat inner cylinders, which remained at constant high temperature with no-slip boundary condition. The cylinder remained at a constant high temperature with no-slip boundary conditions. The detailed phenomena of natural convection were investigated according to the bottom wall temperature with a cold and isothermal top walls and adiabatic side walls. The numerical solutions at a Rayleigh number of 105 reached steady state once the solutions fully converged. When the Rayleigh number increased to 106, the solution showed unsteady characteristic depending on the bottom wall temperature. In addition, the distribution of the isotherms in the lower part of the enclosure depended on the temperature variation of the bottom wall and the Rayleigh number. These effects had a significant influence on the convection structure in the lower part of the enclosure and on the corresponding heat transfer on the walls. The convection structures in the enclosure changed significantly in several cases at 106 with a high bottom wall temperature.

Experimental and numerical study on the onset of natural convection in a cavity open at the top

The onset of natural convection in a 2D air filled cavity open at the top with adiabatic side walls is studied. The numerical model shows the existence of weak convective flow near the top corner of a cavity due to the thermal gradient between the walls and the atmosphere even at low Rayleigh numbers, as also confirmed by the interferometry-based experimental data. Additionally, a thermally stratified layer is formed on the lower side of the cavity. The onset of convection is seen to be dependent on the interaction of these two features in the cavity. Results of the study show that in low aspect ratio cavities, the thermally stratified layers are clearly formed and are not significantly disturbed by the flow at the corners. The onset of convection takes place in these earlier thermally stratified layers beyond a certain Rayleigh number. This convective movement is characterized by a sudden jump in the heat transfer coefficient at a critical Rayleigh number. However, for high aspect ratio cavities, the flow at the corners has significant influence on the stratified layers and results in a decrease in the value of critical Rayleigh number. Beyond a certain aspect ratio, these layers cannot be formed and hence there is no onset of convection. Simulations as well as the interferometric measurements show an inherent symmetry in the corner flows, which was seen to breakdown due to the flow-induced instabilities in the thermally stratified layers for Rayleigh numbers greater than the critical value.

Adiabatic partition effect on natural convection heat transfer inside a square cavity: experimental and numerical studies

A steady state and two-dimensional laminar free convection heat transfer in a partitioned cavity with horizontal adiabatic and isothermal side walls is investigated using both experimental and numerical approaches. The experiments and numerical simulations are carried out using a Mach-Zehnder interferometer and a finite volume code, respectively. A horizontal and adiabatic partition, with angle of θ is adjusted such that it separates the cavity into two identical parts. Effects of this angel as well as Rayleigh number on the heat transfer from the side-heated walls are investigated in this study. The results are performed for the various Rayleigh numbers over the cavity side length, and partition angles ranging from 1.5 × 105 to 4.5 × 105, and 0° to 90°, respectively. The experimental verification of natural convective flow physics has been done by using FLUENT software. For a given adiabatic partition angle, the results show that the average Nusselt number and consequently the heat transfer enhance as the Rayleigh number increases. However, for a given Rayleigh number the maximum and the minimum heat transfer occurs at θ = 45°and θ = 90°, respectively. Two responsible mechanisms for this behavior, namely blockage ratio and partition orientation, are identified. These effects are explained by numerical velocity vectors and experimental temperatures contours. Based on the experimental data, a new correlation that fairly represents the average Nusselt number of the heated walls as functions of Rayleigh number and the angel of θ for the aforementioned ranges of data is proposed.

Role of thermal and flow characteristics on entropy generation during natural convection in porous enclosures with curved walls subjected to Rayleigh-Bénard heating

The comprehensive analysis of natural convection within various enclosures with the concave and convex horizontal walls involving the classical Rayleigh-Bénard heating (hot bottom wall, cold top wall and adiabatic side walls) is carried out via the entropy generation approach. Galerkin finite element method is employed to obtain the solutions in terms of the isotherms (θ), streamlines (ψ), entropy generation (Sθ,Sψ and Stotal) and average heat transfer rates (Nut‾) for a selected range of Darcy numbers (10-5⩽Dam⩽10-2), Prandtl numbers (Prm=0.015 and 7.2) at Rayleigh number, Ram=106 with various test cases with different wall curvatures (concave and convex). The magnitudes and spatial locations of local entropy generation due to heat transfer and fluid friction are studied for various thermal and geometrical parameters. The active zones with larger Sθ are found along near core region and middle portions of the curved walls with less and moderate concavities whereas, those are found only near the middle portions of the curved walls with the high concavity at the low Dam involving all Prm. In the convex domain, the active zones with larger Sθ are seen near the corner regions for the low Dam at all Prm. The zones prone to the entropy generation with the high Sθ are seen along the mid-horizontal axis of the cavity involving all the concave and convex domains at the high Dam and all Prm. Also, the left and right portions of the curved walls act as the active zones with the high Sθ for the concave domains at the high Dam and all Prm. The larger values of Sψ are seen near all the solid walls (at all Dam) and at the interior zone (at the high Dam) for all Prm involving all concave and convex cases. Based on the comparison of all the concave and convex cases, the concave case with the high wall concavity may be preferred over the specific concave cases (with the less and moderate wall concavities) and all the convex cases due to the larger Nut‾ and less Stotal values for the entire range of Dam and Prm.

Investigations on the effect of non-uniform temperature on fluid flow and heat transfer in a trapezoidal cavity filled with porous media

Simulations were carried out for natural convection in a trapezoidal cavity filled with porous media to investigate the effect of uniformly and non-uniformly heated bottom wall using finite element computational procedure. The enclosure used for flow and heat transfer analysis has hot bottom wall, constant temperature cold top wall and adiabatic side walls. The bottom wall is subjected to uniform/linear/sinusoidally varying temperatures. Results are presented in the form of stream lines, isotherms, heatline plots, local and average Nusselt numbers. Heatline visualization technique is a useful method that gives information on heat transport from the heated to cold region inside the porous trapezoidal enclosure with varying temperature boundary conditions. Nusselt numbers are computed for Darcy-modified Rayleigh numbers or Rayleigh number (Ra) ranging from 100 to 2000 for an aspect ratio (H/L) of 0.5. It is observed from this study that the uniform temperature at the bottom wall of the enclosure gives higher Nusselt number as compared to the linear and sinusoidally varying temperature cases. The average Nusselt numbers increases monotonically with Rayleigh number for bottom wall and top walls. The power law correlations between average Nusselt number and Rayleigh numbers are presented for convection dominated regions.

Real-Size Experiments on Reverse Natural Air Convection Between Inclined Parallel Plates for New Insulation Methods in Solar Flat-Plate Collectors

Reverse natural air convection (hot plate top) was experimentally investigated between two inclined parallel aluminum plates (1 m × 2 m × 3 mm) with a separation distance of 20 mm to 100 mm. The inclination ϑ to the horizontal was varied from 0 deg to 90 deg. The mean temperatures of the plates have been adjusted to 90 °C and 30 °C resulting in Rayleigh numbers Ra between 2.7 × 104 and 3.3 × 106. The experimental conditions correspond to the back side of an absorber in a typical solar flat-plate collector, where the conventional insulation has been removed. The upper hot plate simulates the absorber and was electrically heated by an area heater, while the temperature distribution over the plate was measured. The lower cold plate was held isothermally by integrated water tubes and a thermostat. The side walls of the rectangular cavity were thermally connected to the colder plate and had a distance of 10 mm to the hot plate, comparable to a typical collector casing. The experimentally obtained results for Nu (Ra,ϑ) were mathematically described and compared to rare reverse convection data of other authors, gained at smaller aspect ratios/flow lengths and for adiabatic side walls: The formula of Elsherbiny approximately (within 10%) describes solar flat-plate collectors between 0 deg and 60 deg inclination, while the relations of Arnold, Ozoe, and Inaba show large errors up to 50%. Additionally, we experimentally showed that pure air gap insulation (30–50 mm) has surprisingly acceptable loss coefficients between 1.3 and 2.5 W/m2K depending on collector slope. It can be used as a cheap insulation method for low temperature collector applications. Additionally, inserting an 25–50 μm thick aluminum film symmetrically between the plates, a new and efficient insulation method for the absorber of a solar flat-plate collector was experimentally investigated: At plate distances of 30–50 mm, temperatures below 100 °C and slopes below 45 deg, this compact and cheap film insulation was proven to be equivalent to dry mineral wool and avoids its disadvantage of worsening insulation properties due to humidity.

Critical role of blockage ratio for flame acceleration in channels with tightly spaced obstacles

A conceptually laminar mechanism of extremely fast flame acceleration in obstructed channels, identified by Bychkov et al. [“Physical mechanism of ultrafast flame acceleration,” Phys. Rev. Lett. 101, 164501 (2008)], is further studied by means of analytical endeavors and computational simulations of compressible hydrodynamic and combustion equations. Specifically, it is shown how the obstacles length, distance between the obstacles, channel width, and thermal boundary conditions at the walls modify flame propagation through a comb-shaped array of parallel thin obstacles. Adiabatic and isothermal (cold and preheated) side walls are considered, obtaining minor difference between these cases, which opposes the unobstructed channel case, where adiabatic and isothermal walls provide qualitatively different regimes of flame propagation. Variations of the obstructed channel width also provide a minor influence on flame propagation, justifying a scale-invariant nature of this acceleration mechanism. In contrast, the spacing between obstacles has a significant role, although it is weaker than that of the blockage ratio (defined as the fraction of the channel blocked by obstacles), which is the key parameter of the problem. Evolution of the burning velocity and the dependence of the flame acceleration rate on the blockage ratio are quantified. The critical blockage ratio, providing the limitations for the acceleration mechanism in channels with comb-shaped obstacles array, is found analytically and numerically, with good agreement between both approaches. Additionally, this comb-shaped obstacles-driven acceleration is compared to finger flame acceleration and to that produced by wall friction.

Radial and Axial Magnetic Fields Effects on Natural Convection in a Nanofluid-filled Vertical Cylinder

This work aims to study numerically the steady natural convection in a vertical cylinder filled with an Al2O3 nanofluid under two different external magnetic fields (Br, Bz) either in the radial or axial directions. The cylinder having an aspect ratio H/R0=1 is bounded by the top and the bottom disks at temperatures Tc and Th, and by an adiabatic side wall. The equations of continuity, Navier-Stokes and energy are non-dimensionalized and then discretized by the finite volume method. A computer program based on the SIMPLER algorithm is developed and compared with the numerical results found in the literature. The effects of nano-size solid volume fraction ranging from 0 to 0.1 and application of the magnetic field in either directions axial and radial for various values of Hartmann numbers on flow and thermal fields, and on local and average Nusselt numbers are presented and discussed for two values of Rayleigh numbers (Ra=10 3 and 10 4) . The behaviors of average Nusselt number, streamlines, temperature contours, and the both components of velocity are illustrated. The results indicate that for small values of the Hartmann number, where the flow remains due to the convection, the average Nusselt number decreases when increasing the solid volume fraction and this decrease is more important if the magnetic field is applied in the axial direction and by increasing the Hartmann numbers. The increasing in the solid volume fraction increases the performance of heat transfer in the nanofluid.

Effect of Convection Heat Transfer on Performance of Waste Heat Thermoelectric Generator

Efficiency of energy conversion processes can be improved if waste heat is converted to electricity. A thermoelectric generator (TEG) can directly convert waste heat to electricity. The TEG typically suffers from low efficiency due to various reasons, such as ohmic heating, surface-to-surrounding convection losses, and unfavorable material properties. In this work, the effect of surface-to-surrounding convection heat transfer losses on the performance of TEG is studied analytically and numerically. A one-dimensional (1-D) analytical model is developed that includes surface convection, conduction, ohmic heating, and Peltier, Seebeck, and Thomson effects with top and bottom surfaces of TEG exposed to convective boundary conditions. Using the analytical solutions, different performance parameters (e.g., heat input, power output, and efficiency) are calculated and expressed graphically as functions of thermal source and sink temperatures and convection heat transfer coefficient. Finally, a two-dimensional (2-D) mathematical model is solved numerically to observe qualitative results of thermal and electric fields inside the TEG. For all calculations, temperature-dependent thermal/electric properties are considered. Increase in thermal source temperature results in an increase in the power output with adiabatic side wall conditions. A change in boundary condition to convection heat transfer from adiabatic boundary has a large impact on thermal efficiency.

Bifurcation analysis of steady natural convection in a tilted cubical cavity with adiabatic sidewalls

AbstractNatural convection in an inclined cubical cavity heated from two opposite walls maintained at different temperatures and with adiabatic sidewalls is investigated numerically. The cavity is inclined by an angle $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}\theta $ around a lower horizontal edge and the isothermal wall set at the higher temperature is the lower wall in the horizontal situation ($\theta = 0^\circ $). A continuation method developed from a three-dimensional spectral finite-element code is applied to determine the bifurcation diagrams for steady flow solutions. The numerical technique is used to study the influence of ${\theta }$ on the stability of the flow for moderate Rayleigh numbers in the range $\mathit{Ra} \leq 150\, 000$, focusing on the Prandtl number $\mathit{Pr} = 5.9$. The results show that the inclination breaks the degeneracy of the stable solutions obtained at the first bifurcation point in the horizontal cubic cavity: (i) the transverse stable rolls, whose rotation vector is in the same direction as the inclination vector ${\boldsymbol{\Theta}}$, start from $\mathit{Ra} \to 0$ forming a leading branch and becoming more predominant with increasing $\theta $; (ii) a disconnected branch consisting of transverse rolls, whose rotation vector is opposite to ${\boldsymbol{\Theta}}$, develops from a saddle-node bifurcation, is stabilized at a pitchfork bifurcation, but globally disappears at a critical inclination angle; (iii) the semi-transverse stable rolls, whose rotation axis is perpendicular to ${\boldsymbol{\Theta}}$ at $\theta \to 0^\circ $, develop from another saddle-node bifurcation, but the branch also disappears at a critical angle. We also found the stability thresholds for the stable diagonal-roll and four-roll solutions, which increase with $\theta $ until they disappear at other critical angles. Finally, the families of stable solutions are presented in the $\mathit{Ra}-\theta $ parameter space by determining the locus of the primary, secondary, saddle-node, and Hopf bifurcation points as a function of $\mathit{Ra}$ and $\theta $.

Sidewall effects in Rayleigh–Bénard convection

AbstractWe investigate the influence of the temperature boundary conditions at the sidewall on the heat transport in Rayleigh–Bénard (RB) convection using direct numerical simulations. For relatively low Rayleigh numbers $Ra$ the heat transport is higher when the sidewall is isothermal, kept at a temperature $T_c+\Delta /2$ (where $\Delta $ is the temperature difference between the horizontal plates and $T_c$ the temperature of the cold plate), than when the sidewall is adiabatic. The reason is that in the former case part of the heat current avoids the thermal resistance of the fluid layer by escaping through the sidewall that acts as a short-circuit. For higher $Ra$ the bulk becomes more isothermal and this reduces the heat current through the sidewall. Therefore the heat flux in a cell with an isothermal sidewall converges to the value obtained with an adiabatic sidewall for high enough $Ra$ (${\simeq }10^{10}$). However, when the sidewall temperature deviates from $T_c+\Delta /2$ the heat transport at the bottom and top plates is different from the value obtained using an adiabatic sidewall. In this case the difference does not decrease with increasing $Ra$ thus indicating that the ambient temperature of the experimental apparatus can influence the heat transfer. A similar behaviour is observed when only a very small sidewall region close to the horizontal plates is kept isothermal, while the rest of the sidewall is adiabatic. The reason is that in the region closest to the horizontal plates the temperature difference between the fluid and the sidewall is highest. This suggests that one should be careful with the placement of thermal shields outside the fluid sample to minimize spurious heat currents. When the physical sidewall properties (thickness, thermal conductivity and heat capacity) are considered the problem becomes one of conjugate heat transfer and different behaviours are possible depending on the sidewall properties and the temperature boundary condition on the ‘dry’ side. The problem becomes even more complicated when the sidewall is shielded with additional insulation or temperature-controlled surfaces; some particular examples are illustrated and discussed. It has been observed that the sidewall temperature dynamics not only affects the heat transfer but can also trigger a different mean flow state or change the temperature fluctuations in the flow and this could explain some of the observed differences between similar but not fully identical experiments.

Numerical study of flow and heat transfer in differentially heated enclosures

Two-dimensional laminar natural convection is studied numerically for differentially heated air-filled rectangular enclosures with adiabatic side walls and aspect ratios of 1, 2, 4 and 8. The inclination angle of the enclosure was varied from 0? to 180?, and the effect of inclination on flow field and heat transfer was investigated over the range 103 ? Ra ? 106. Correlations of average Nusselt number based on the present results are presented for horizontal and vertical cases. Large discrepancies were found among published results.

Effect of a Side Wall on the Natural Convection Heat Transfer from a Vertical Row of Horizontal Square Rods

Experimental study is carried out to investigate natural convection from a vertical array of three horizontal square rods fixed adjacent to a vertical adiabatic wall and tested under constant heat flux condition. The experimental rig is designed and constructed for this purpose with variable horizontal distance ratio (H/D) in the range (0.125 < H/D < 3.0). The vertical distance between the rods is changed to get ratio (S/D) ranges of (1.5 < S/D < 4). The Rayleigh number tested is in the range of (1.26 × 106 < Ra < 6.6 × 106) with variable heating levels. The finite volume computational method is used to determine the flow and thermal field around the array of the square rods in different experimental configurations. The computed fields confirm the discussion of the experimental behavior of each rod in the array at each test configuration. Results show that the main parameters affecting the heat dissipation rate of each of the rods are position of the rod in the array, relative vertical distance between the rods and horizontal distance to the adiabatic side wall. The chimney effect noticed between the rods and the adiabatic wall plays a big role in heat transfer enhancement at certain (H/D) values. Experiments show that the ability of the rods to dissipate heat was increased at the largest tested vertical distances with the same behavior noticed for the small horizontal distances relative to the adiabatic wall with a tendency of each rod to behave as a free rod as its horizontal distance to the wall is increased. A dimensionless empirical correlation for each rod was found. Another general correlation for the whole array is used to relate the experimental data of each rod according to its position in the array.

Large-eddy simulation of turbulent buoyancy-driven flow in a rectangular cavity

Turbulent buoyancy-driven flow in a rectangular cavity with two differentially heated opposite walls is investigated numerically by means of large-eddy simulation (LES). Different dynamic global-coefficient subgrid-scale models for weakly compressible flows are applied to simulate the natural convective flow. It is shown that transition of the boundary layer is delayed in cases where the model coefficients are fixed or changing dynamically according to the Germano identity. On the contrary, in the ‘global equilibrium’ approach, the result shows an earlier change in flow regime due to lower subgrid-scale viscosity. Further, it is also demonstrated that three-dimensional effects of the natural convective flow may be significant due to the presence of adiabatic side walls.

Analysis of heat losses from a trapezoidal cavity used for Linear Fresnel Reflector system

A Computational study to investigate the heat loss due to radiation and steady laminar natural convection flow in a trapezoidal cavity having eight absorber tubes for a Linear Fresnel Reflector (LFR) solar thermal system with uniformly heated tubes and adiabatic top wall and side walls has been performed. The losses due to convection and radiation were considered from the bottom glass cover. The results are validated with experimental data. Radiative component of losses from the cavity was found to be dominant which contributes around 80–90%. Heat loss characteristics have been studied for cavities of different depths. Simulations have been carried out for various values of heat transfer coefficient based on the wind speed below the glass surface. Effect of emissivities of the tubes on the heat loss has also been simulated. Flow pattern and isotherms inside the cavity for various depths have been analyzed. Finally, the correlation between the total average Nusselt number and its influencing parameters has been obtained for the proposed cavity.

Turbulent free convection in a vertical converging channel

This paper presents the results of experimental and numerical investigations of the problem of turbulent natural convection in a converging-plate vertical channel. The channel has two isothermally heated inclined walls and two adiabatic vertical side walls. The parameters involved in this study are the channel geometry represented by the channel width at exit, the inclination of the heated walls and the temperature difference between the heated walls and the ambient. The investigation covered modified Rayleigh numbers up to 108 in the computational study and up to 9.3 × 106 in the experimental work. The experimental measurements focused on the velocity field and were carried out using a PIV system and included measurements of the mean velocity profiles as well as the root-mean-square velocity and shear stress profiles. The experiments were conducted for an inclination angle of 30°, a gap width of 10 mm and two temperature differences (∆T=25.4°C and 49.8°C). The velocity profiles in the lower part of the channel indicated the presence of two distinct layers. The first layer is adjacent to the heated plate and driven by buoyancy forces while the second layer extends from the point of maximum velocity to the channel center plane and driven mainly by shear forces. The velocity profile at the upper portion of the channel has shown the merging of the two boundary layers growing over the two heated walls. The measured values of the Reynolds shear stress and root mean square of the horizontal and vertical velocity fluctuation components have reached their maximum near the wall while having smaller values in the core region. The computational results have shown that the average Nusselt number increases approximately linearly with the increase of the modified Rayleigh number when plotted on log–log scale. The variation of the local Nusselt number indicated infinite values at the channel inlet (leading edge effect) and high values at the channel exit (trailing edge effect). For a fixed value of the top channel opening, the increase of the inclination angle tended to reduce flow velocity at the inlet section while changing the flow structure near the heated plates in such a way to create boundary-layer type flow. The maximum value of the average Nusselt number occurs when θ = 0 and decreases with the increase of the inclination angle. On the other hand, the increase of the channel width at exit for the same inclination angle caused a monotonic increase in the flow velocity at the channel inlet.

사각공동내 자연대류에서 측면 단열벽에 의한 영향

사각공동내 자연대류 열전달 실험에서, 단열벽에 의해 열전달이 저하가 관찰되는 영역을 실험적으로 그리고 수치해석적으로 평가하였다. Gr<SUB>H</SUB> 수 1.53×10?부터 1.01×10¹?까지 변화시키며, 단열벽이 존재할 때와 그렇지 않을 때를 구분하여 열전달률을 측정하였다. FLUENT 실험의 결과를 예측할 수 있는지 확인하고 실험으로 수행하기 어려운 매우 좁은 영역에 대해 FLUENT를 수행하였다. 실험과 FLUENT의 결과를 다른 연구와 비교한 바 일치함을 보였다. 단열벽이 전체 열전달에 미치는 영향은 예상과 같이 단열벽 근처의 매우 좁은 영역에 국한하여 나타남을 확인하였다. 본 연구는 유사성(Analogy) 원리를 이용하여 열전달계를 전기도금계의 물질전달계로 모사하는 방법론을 개발하는 과정에서 실험을 효율화하고자 하는 방안을 강구하기 위하여 추진되었다. 본 연구를 통하여 단열벽간 거리(전극의 폭)를 매우 줄일 수 있는 이론적 근거를 확보하였다.