Cavity Aspect Ratio Research Articles

Numerical Prediction of Nanofluid Flow in Channel with Heated Cavity

The heat transfer characteristics and performance of alumina-water nanofluid flow in a channel with heated cavity are studied numerically. The double distribution function thermal lattice Boltzmann model is applied due to its simplicity and highly stable numerical scheme. The effects of Grashof number variation in different aspect ratio of cavity and volume fraction of nano-particles on the heat transfer characteristics and performance is considered. It was found from the obtained results that the average Nusselt number increased with increase in Grashof number and volume fraction of Al2O3 nanoparticles regardless aspect ratio of the cavity. Moreover the obtained results showed that the rate of heat transfer increased with increase in nanoparticles volume fraction.

Melting process in a rectangular thermal storage cavity heated from vertical walls

Thermal energy storage is critical for reducing the mismatch between energy supply and demand as well as plays an important role in energy conservation. The melting process in a rectangular thermal storage cavity is investigated numerically. The numerical model is developed and verified by the experimental results. The cavity aspect ratios are varied from 0.1 to 10. The vertical walls of the cavity are heated uniformly, while the horizontal walls are considered insulated. The computational results show how the transient phase-change process depends on the aspect ratio. It is found that the aspect ratios affect dramatically not only the time of full thermal energy storage but also convection currents inside the cavity. The optimized thermal storage performance is obtained for the aspect ratios ≥1.

Estimation of transient heat transfer and fluid flow for alloy solidification in a rectangular cavity with an isothermal sidewall

Transient scaling and integral analyses were performed to predict trends in alloy solidification in a rectangular cavity cooled by an isothermal sidewall. The natural convection fluid flow was approximated by a scaling analysis for a laminar boundary layer at the solidification front, and was coupled to scaling and integral analyses of the energy equation to predict the solidification behaviour of the system. These analyses predicted several relevant aspects of the solidification process, including the time required to extinguish the initial superheat and the maximum local solidification time as a function of the system parameters and material properties. These results were verified by comparison to numerical simulations for an Al–4.5 wt% Cu alloy for various initial and boundary conditions and cavity aspect ratios. The analysis was compared to previous attempts to analyse similar fluid flow and solidification processes, and the limitations of the assumptions used for this analysis were discussed.

Natural convection in thermal plumes emerging from a single heat source

We report numerical simulations of confined natural convection from a single heat source, leading to the evolution of thermal plumes in two and three dimensions. Thermal plumes are driven through a single heat source mounted flush at the bottom of a slender cavity where vertical and top walls are isothermal heat sinks. Velocity and temperature fields were obtained for two Prandtl numbers, P=0.025,0.71 at three different values of the Rayleigh number, R=104,5×104,105 and for different box aspect ratios. Two kind of flow solutions were found: (i) Steady states corresponding to stable thermal plumes characterized by a well defined flow circulation inside the cavity and (ii) periodic states where both the flow and thermal fields oscillate in time. Unsteadiness of fluid and thermal flows is favored by choosing low Prandtl number fluids, working at high Rayleigh numbers inside high aspect ratio cavities. Instabilities are characterized by a periodic and propagative motion of the thermal plume in both transverse and vertical direction. It can be attributed to destabilizing shear stresses between ascending and descending fluid layers.

Stratification enhancement for an integrated collector storage solar water heater (ICSSWH)

ICSSWH are advantageous solar thermal systems for residential buildings. Their overall efficiency can be improved by increasing stratification in the storage tank. In this paper the charge of a high aspect ratio cavity representing the storage of an ICSSWH was studied experimentally. A satisfying stratification was obtained by heating the storage tank in the middle and inserting a stratification plate. The study examined various angles of system inclination (30, 45 and 60° relative to the horizontal), heat flux imposition (1800, 2700 and 3600Wm−2) and length of the stratification plate. The experiments lasted for three hours, temperatures were measured using thermocouples and velocities were measured via particle image velocimetry. We showed that stratification was higher with a shorter stratification plate, and that it increased with greater heat flux and lower angles. Flow observations on the presented configuration showed that reverse flow was responsible for this thermal behavior. This phenomenon, however, increases with time and could result in creation a dead volume under the heated area.

An elastocapillary model of wood-fibre collapse.

An elastocapillary model for drying-induced collapse is proposed. We consider a circular elastic membrane with a hole at the centre that is deformed by the capillary pressure of simply and doubly connected menisci. The membrane overlays a cylindrical cavity with rigid walls, trapping a prescribed volume of water. This geometry may be suitable for studying structural failures and stiction in micro-electromechanical systems during wet etching, where capillary surfaces experience catastrophic transitions. The dry state is determined using the dihedral-angle and volume-turning-point stability criteria. Open and collapsed conformations are predicted from the scaled hole radius, cavity aspect ratio, meniscus contact angle with the membrane and cavity walls, and an elastocapillary number measuring the membrane stretching rigidity relative to the water surface tension. For a given scaled hole radius and cavity aspect ratio, there is a critical elastocapillary number above which the system does not collapse upon drying. The critical elastocapillary number is weakly influenced by the contact angle over a wide range of the scaled hole radius, thus indicating a limitation of surface hydrophobization for controlling the dry-state conformation. The model is applied to the drying of wood fibres above the fibre saturation point, determining the conditions leading to collapse.

Solidification of Cu-Water nanofluid in a trapezoidal cavity: A CFD study

A numerical study has been carried out to investigate the solidification of a binary mixture of water and Cu nanoparticles inside a horizontal trapezoidal cavity of different aspect ratio under specific given boundary conditions for temperature and concentration gradients. The vertical side walls of the cavity are insulated while the top wall temperature is kept lower than that of the bottom wall. The effect of parameters such as the ratio of the cavity length to height (aspect ratio), the cold wall temperature (-5 to -30 °C) and the initial temperature of the nanofluid (0 °C to16 °C) on solidification time is investigated. The moving solid-liquid interface is obtained using Enthalpy-porosity technique in the model. We found that the solidification time decreases with the increase of the aspect ratio (i.e., a longer trapezoidal cavity) and the decrease of cold wall temperature. Aspect ratio is found to give a prominent effect. However, the initial temperature of fluid does not affect the solidification time much.

Numerical study of magneto-hydrodynamic (MHD) mixed convection flow in a lid-driven triangular cavity

In the present paper, the effect of magneto-hydrodynamic (MHD) on mixed convection flow within a lid-driven triangular cavity has been numerically investigated. The bottom wall of the cavity is considered as heated. Besides, the left and the inclined wall of the triangular cavity are assumed to be cool and adiabatic. The cooled wall of the cavity is moving up in the vertical direction. The developed mathematical model is governed by the coupled equations of continuity, momentum and energy to determine the fluid flow and heat transfer characteristics in the cavity as a function of Rayleigh number, Hartmann number and the cavity aspect ratio. The present numerical procedure adopted in this investigation yields consistent performance over a wide range of parameters Rayleigh number Ra (103-104), Prandtl number Pr (0.7 - 3) and Hartmann number Ha (5 - 50). The numerical results are presented in terms of stream functions, temperature profile and Nussult numbers. It is found that the streamlines, isotherms, average Nusselt number, average fluid bulk temperature and dimensionless temperature in the cavity strongly depend on the Rayleigh number, Hartmann number and Prandtl number.

Laminar-Turbulence Transition over the Rotor Disk in an Enclosed Rotor-Stator Cavity

Large eddy simulation (LES) of transitional and turbulent flow are performed for a range of rotational Reynolds numbers, 4 × 104 ≤ Reω = b2ω/ν ≤ 4 × 105, in both an enclosed rotor-stator annular and an enclosed rotor-stator cylindrical cavities of aspect ratio G = (b − a)/h = 5, where a and b are, respectively, the inner and outer radii of the rotor disk and h is the distance between the rotor and stator disks. Although it is well-known that the Cross-Flow instability plays an important role in the transition process to turbulence in the rotor boundary-layer, the spiral patterns related to the Streamline-Curvature (S-C) instability became dominant in this study. In addition, the structures due to secondary instability appear in the outward side of the primary spiral patterns. These structures obtained from LES are in agreement with experimental visualizations performed in this study. The results suggest that S-C instability is excited and becomes dominant in the rotor side since the rotor boundary-layer is perturbed by disturbances emanating from the stator side. Subsequently, secondary instabilities result from the interaction of the dominant S-C instability with the main flow. These instabilities seem to be the probable source of laminar-turbulence transition in the rotor boundary-layer for the present case.

Plasma confinement to enhance the momentum coupling coefficient in ablative laser micro-propulsion: a novel approach

We introduce for the first time the novel idea of manipulating the momentum coupling coefficient using plasma confinement and shock wave reflection from the cavity walls. The plasma was confined using cylindrical geometries of various cavity aspect ratios to manipulate the momentum coupling coefficient (Cm). The Nd: YAG laser (532 nm, 5 ns pulse duration) was focused on the ferrite sample surface to produce plasma in a region surrounded by cylindrical cavity walls. The multiple reflections of the shockwaves from the cavity walls confined the laser-induced plasma to the central region of the cavity that subsequently resulted in a significant enhancement of the momentum coupling coefficient values. The plasma shielding effect has also been observed for particular values of laser fluencies and cavity aspect ratios. Compared with the direct ablation, the confined ablation provides an effective way to obtain high Cm values.

Numerical investigation of collector cooling for a 1 MW ITER gyrotron operated with vertical sweeping

The present gyrotron designs for EC plasma heating in nuclear fusion reactors require the safe exhaust of a power comparable to that injected into the plasma, in order to keep the maximum temperature below the acceptable value of 300°C. In this paper, the commercial computational fluid dynamics (CFD) software STAR-CCM+® is used to analyze the thermal performance of the annular copper collector of a 1MW ITER gyrotron, equipped with a hypervapotron structure made of annular fins with rectangular cavities of aspect ratio (depth/width)=3, cooled by highly subcooled (90–100°C) pressurized water flowing at ∼4m/s. It is assumed that the simple vertical sweeping strategy is used to reduce the very high peak heat flux on the collector (up to 30MW/m2 transient, 5MW/m2 time average), due to the spent electron beam. The 2D steady-state conjugate heat transfer problem is solved assuming azimuthal symmetry and accounting for 2-phase flow. The single-cavity flow and heat transfer problem is considered first, to optimize the mesh and the selection of the turbulence model. For the operating conditions considered in this paper, the full collector (100+ cavities) solution shows that boiling occurs only in a limited number of cavities close to the peaks of the heat flux, with the vapor remaining trapped in the bottom of the cavities, i.e. no full hypervapotron regime should be achieved in these operating conditions. The steady-state analysis allows the numerical evaluation of the heat transfer coefficients between Cu and water; these are then used as input for the simplified, purely thermal (solid only) analysis of the actual transient problem for the full collector. The results of the simplified model, which allows a huge reduction of the computational effort, are successfully benchmarked against those of a comprehensive thermal–hydraulic simulation. The computed peak Cu temperature is below the acceptable limit under the steady-state (time averaged) heat load, but becomes unacceptably high if the actual transient heat load is considered, confirming the need for more effective sweeping strategies and/or optimized collector geometries with improved heat transfer capabilities.

An explicit formula for the coherent SH waves’ attenuation coefficient in random porous materials with low porosities

In this paper, the attenuation coefficient of coherent SH waves in random porous material with uniformly randomly distributed elliptical cavities of different aspect ratios is studied. Based on an analysis of the mechanism for attenuation, a simple macro model for the attenuation coefficient is proposed. The macro model says that the attenuation coefficient can be expressed as a function of the mean scattering cross section and the number density of cavities at low porosities. Then, large-scale numerical simulations using the pre-corrected Fast Fourier Transform (pFFT) algorithm accelerated Boundary Element Method (BEM) are conducted to specify this macro model. Finally, this macro model is compared with four theoretical models derived for composite/porous materials with circular inclusions at the porosity p=3.17% and 5%. Results show this macro model agree well with three of them. Compared to the existing theoretical models, the form of this macro model is simple and has a clear physical meaning. In addition, it is applicable to cases with relatively complex cavities.

Investigation on the relative performance of various low-Reynolds number turbulence models for buoyancy-driven flow in a tall cavity

The present study deals with the numerical investigation of turbulent buoyancy driven flow in a differentially heated rectangular cavity with adiabatic horizontal walls. The aspect ratio of cavity is 5 and the Rayleigh number based on the cavity height is 4.56 × 1010. The computations have been carried out using the finite volume method on a staggered grid and SIMPLEC algorithm for pressure–velocity coupling. The low-Reynolds number \(k-\epsilon\) model proposed by Yang and Shih (YS), low-Reynolds number \(k-\omega\) model proposed by Wilcox, and \(k-\omega\) shear stress transport (SST) model of Menter have been applied for turbulence closure. The performance comparison of different models have been carried out using the experimental, LES and various RANS results available in the literature. The computation of turbulent natural convection flow is numerically challenging due to complex flow involving laminar, transition and turbulent regions, coupling of velocity with the energy equation, and some other problems reported in literature e.g. grid dependency of solution, numerical stability problem, etc. The flux Richardson number is calculated to get an estimate of relative importance of buoyancy and shear force in different regions of flow. The shearing and swirling zones have been identified in the entire flow domain using the \(\lambda _{2}\) criterion. Based on the comparison of mean flow, heat transfer and turbulence characteristics with the available results, it has been found that YS model performs better. The better performance obtained from YS model may be due to peculiarity of model that takes into account the Kolmogorov time scale near the wall and the conventional time scale \((k/\epsilon )\) away from the wall.

Numerical and Experimental Investigation of Air-Water Two-Phase Flow in a Pipe with Three Cavities of Different Aspect Ratios

The recirculating air vortex inside an orthogonal cavity with direction vertical to the flow is the motivating idea to investigate numerically with Ansys Fluent and with laboratory experiments the possibility to use the street canyon phenomenon as a novel gas-liquid separation mechanism. Three orthogonal Plexiglas cavities are designed and constructed for laboratory experiments in a recirculating air-water mixture laboratory loop. The air separation efficiency of each cavity is monitored in different operating conditions. The Volume of Fluid (VOF) and the Eulerian multiphase models combined with the Realizable k-e and the k-ω SST turbulence models are used for the numerical calculations employed with the commercial code Ansys Fluent 14.5. The comparison between the numerical and the experimental data proves that the simple two dimensional model is capable of approaching the laboratory air separation values especially in high air flow rates. Moreover a flow map is created to observe the flow pattern region and to compare with numerical contours of the air volume fraction. Alterations based on the conclusions are to be considered for further research.

Bifurcations and eddy genesis of Stokes flow within a sectorial cavity PART II: Co-moving lids

This paper is a complementary study to earlier work (Gürcan, 2013), concerning eddy genesis and bifurcations in a sectorial driven cavity for Stokes flow, consisting of a pair of curved stationary side-walls and two straight translating lids moving in the same radial direction with speeds U1 and U2. The flow is governed by two physical control parameters: the cavity aspect ratio A=r2r1 where r1 and r2 are the radii of the inner and outer curved side-walls, respectively and the ratio of the upper and lower lid speeds (S=U1U2). In previous work (Gürcan, 2013), focus was on lids moving in the opposite direction or a single moving lid.By considering different A values for each S, the streamline patterns and their bifurcations are studied; occurrence of stagnation point bifurcations and transformation of the local flow topology are observed at critical values of A and S. The key results are shown in an (S,A) control space diagram which exhibits an intricate structure due to the intersection and confluence of eight bifurcation curves representing flow bifurcations at degenerate critical points. Six different eddy genesis mechanisms are observed for S∈(0,1] and A∈(1.45,20). In each case the flow structures and eddy genesis mechanisms are illustrated in detail with figures and bifurcation diagrams. The study also serves as a benchmark problem for testing new numerical algorithms.

Convection of Cu-water nanofluid in a vented T-shaped cavity in the presence of magnetic field

This paper presents the results of a numerical study on the mixed convection of Cu-water nanofluid in a T-shaped cavity in the presence of a uniform magnetic field. Some sections of the bottom walls of the cavity are heated at a constant temperature and the other walls are thermally insulated. The nanofluid at a relatively low temperature enters from the bottom and exits from the top of the cavity. The governing equations are solved numerically with a finite volume approach using the SIMPLE algorithm. The effects of parameters such as Reynolds number (10 ≤ Re ≤ 400), Richardson number (0.01 ≤ Ri ≤ 10), Hartmann number (0 ≤ Ha ≤ 80), solid volume fraction (0 ≤ φ ≤ 0.06), and cavity aspect ratio (0.1 ≤ AR ≤ 0.4) on the fluid flow and the thermal performance of the cavity are studied. The results indicate that the presence of nanoparticles enhances the heat transfer except at Re = 100 and Ha < 10 as well as Re = 400 and Ha < 60, where pure water has a slightly higher heat transfer rate compared to the nanofluid. The influence of nanofluid on the heat transfer enhancement increases as AR increases. For Ri = 0.01 and 1, the maximum heat transfer rate is obtained at AR = 0.4; however, for Ri = 10, the maximum heat transfer rate occurs at AR = 0.1.

Vibrational convection of heat-generating fluid in a rotating horizontal cylinder. The role of relative cavity length

Thermal convection of fluid with internal heat sources in a rotating horizontal cylinder with isothermal boundary and adiabatic ends is investigated experimentally. Under the action of gravity nonisothermal liquid oscillates in the cavity frame. These tidal oscillations generate the average mass force and, as a result, excite convection. The steady convection developing by this mechanism is called thermal vibrational. Centrifugal force in the considered case plays a stabilizing role. The objects of studying are the excitation thresholds of the averaged convection, heat transfer and the structure of convective flows. The parameters varying in the experiments are heat release rate, relative length of the cylinder and rotation velocity. It is found that the inertial waves which are generated near the ends of the cavity by tidal oscillations of nonisothermal liquid effect the convection. The intensity of flows excited by these waves is relatively low, but significant especially below the threshold of thermal vibrational convection. It is shown that the influence of inertial waves on heat transfer and structure of convective flows strongly depends on the cavity aspect ratio.

Convection-Enhanced Transport into Open Cavities : Effect of Cavity Aspect Ratio.

Recirculating fluid regions occur in the human body both naturally and pathologically. Diffusion is commonly considered the predominant mechanism for mass transport into a recirculating flow region. While this may be true for steady flows, one must also consider the possibility of convective fluid exchange when the outer (free stream) flow is transient. In the case of an open cavity, convective exchange occurs via the formation of lobes at the downstream attachment point of the separating streamline. Previous studies revealed the effect of forcing amplitude and frequency on material transport rates into a square cavity (Horner in J Fluid Mech 452:199-229, 2002). This paper summarizes the effect of cavity aspect ratio on exchange rates. The transport process is characterized using both computational fluid dynamics modeling and dye-advection experiments. Lagrangian analysis of the computed flow field reveals the existence of turnstile lobe transport for this class of flows. Experiments show that material exchange rates do not vary linearly as a function of the cavity aspect ratio (A = W/H). Rather, optima are predicted for A ≈ 2 and A ≈ 2.73, with a minimum occurring at A ≈ 2.5. The minimum occurs at the point where the cavity flow structure bifurcates from a single recirculating flow cell into two corner eddies. These results have significant implications for mass transport environments where the geometry of the flow domain evolves with time, such as coronary stents and growing aneurysms. Indeed, device designers may be able to take advantage of the turnstile-lobe transport mechanism to tailor deposition rates near newly implanted medical devices.

Laminar Wall Jet Flow and Heat Transfer over a Shallow Cavity.

This paper presents the detailed simulation of two-dimensional incompressible laminar wall jet flow over a shallow cavity. The flow characteristics of wall jet with respect to aspect ratio (AR), step length (X u), and Reynolds number (Re) of the shallow cavity are expressed. For higher accuracy, third-order discretization is applied for momentum equation which is solved using QUICK scheme with SIMPLE algorithm for pressure-velocity coupling. Low Reynolds numbers 25, 50, 100, 200, 400, and 600 are assigned for simulation. Results are presented for streamline contour, velocity contour, and vorticity formation at wall and also velocity profiles are reported. The detailed study of vortex formation on shallow cavity region is presented for various AR, X u, and Re conditions which led to key findings as Re increases and vortex formation moves from leading edge to trailing edge of the wall. Distance between vortices increases when the step length (X u) increases. When Re increases, the maximum temperature contour distributions take place in shallow cavity region and highest convection heat transfer is obtained in heated walls. The finite volume code (FLUENT) is used for solving Navier-Stokes equations and GAMBIT for modeling and meshing.

Transient Thermal Analysis of Natural Convection in Porous and Partially Porous Cavities

In this article, the authors numerically investigate dynamic heat transfer related to free convection in porous and partially porous cavities, paying particular attention to the dependence of velocity and temperature fields on Rayleigh (Ra) number, porous layer permeability, and cavity aspect ratio (AR), the ratio between its height and width. Heat and fluid flows inside the cavity are described by using the generalized porous medium model, and the governing equations were numerically solved using the artificial compressibility version of the characteristic-based split scheme. The proposed model was first applied to the simulation of heat and fluid flows in a square porous cavity, and an excellent agreement was found between the results obtained and the data available in the literature. The analysis was then extended, here for the first time, to transient natural convection in partly porous cavities, with AR equal to 4:1 and 8:1 for different Ra and Darcy (Da) numbers. The mean Nusselt (Nu) number was calculated as a function of cavity geometrical characteristics, porous layer permeability, and fluid properties.