Effects Of Cavity Aspect Ratio Research Articles

Effect of cavity aspect ratio on mixed convective heat transfer phenomenon inside a lid-driven cavity due to elastic turbulence

This study performs extensive numerical simulations to investigate how the aspect ratio (AR) of a lid-driven cavity influences the onset of elastic instability and elastic turbulence and the subsequent mixed convective heat transfer rate inside it. To this end, we utilize the finite volume method based open source code OpenFOAM along with Rheotool to solve the mass, momentum, energy, and viscoelastic constitutive equations. We find that the dependency of the cavity AR on the heat transfer rate is highly complicated depending upon the values of the Richardson (Ri) and Prandtl numbers (Pr). At low values of Ri, the heat transfer rate continuously decreases with AR irrespective of the value of the Prandtl number and the fluid type, i.e., Newtonian or viscoelastic. The same trend is also observed at high values of Ri and low values of Pr. At these combinations of Ri and Pr, the heat transfer rate is always higher in viscoelastic fluids than in Newtonian fluids due to the presence of elastic turbulence in the former fluids. However, a different trend is observed at high values of both Ri and Pr. At this combination of Ri and Pr, the heat transfer rate increases with AR in Newtonian fluids, whereas it decreases in viscoelastic fluids. Therefore, at high values of AR, Ri, and Pr, the heat transfer rate is higher in Newtonian fluids than that in viscoelastic fluids despite the presence of elastic turbulence in the latter fluids. This is in contrast to the assumption that the elastic turbulence phenomenon always increases the rate of transport processes. A possible explanation for this behavior is provided in this study. Along with the heat transfer aspects, we also provide a detailed discussion on how the cavity aspect ratio influences the corresponding flow dynamics inside the cavity. In particular, we find that the onset of the elastic instability (and the subsequent elastic turbulence) phenomenon is delayed to higher values of the Weissenberg number as the cavity aspect ratio increases. This is in line with prior experimental studies reported in the literature.

Numerical Investigation of Flow and Heat Transfer over a Shallow Cavity: Effect of Cavity Height Ratio

Flow over shallow cavities is used to model the flow field and heat transfer in a solar collector and a variety of engineering applications. Many studies have been conducted to demonstrate the effect of cavity aspect ratio (AR), but very few studies have been carried out to investigate the effect of cavity height ratio (HR) on shallow cavity flow behavior. In this paper, flow field structure and heat transfer within the 3-D shallow cavity are obtained numerically for two height ratio categories: HR = 0.0, 0.25, 0.5, 0.75, and 1.0 and HR = 1.25, 1.5, 1.75, 2.0, 2.25, and 2.5. The governing equations, continuity, momentum, and energy are solved numerically and using the standard (K-ε) turbulence model. ANSYS FLUENT 14 CFD code is used to perform the numerical simulation based on the finite volume method. In this study, the cavity aspect ratio, AR = 5.0, and Reynolds number, Re = 3 × 105, parameters are fixed. The cavity’s bottom wall is heated with a constant and uniform heat flux (q = 740 W/m2), while the other walls are assumed to be adiabatic. For the current Reynolds number and cavity geometry, a single vortex structure (recirculation region) is formed and occupies most of the cavity volume. The shape and location of the vortex differ according to the height ratio. A reverse velocity profile across the recirculation region near the cavity’s bottom wall is shown at all cavity height ratios. Streamlines and temperature contours on the plane of symmetry and cavity bottom wall are displayed. Local static pressure coefficient and Nusselt number profiles are obtained along the cavity’s bottom wall, and the average Nusselt number for various height ratios is established. The cavity height ratio (HR) is an important geometry parameter in shallow cavities, and it plays a significant role in the cavity flow behavior and heat transfer characteristics. The results indicate interesting flow dynamics based on height ratio (HR), which includes a minimal value in average Nusselt number for HR ≈ 1.75 and spatial transitions in local Nusselt number distribution along the bottom wall for different HRs.

Mathematical modeling of macrosegregation during solidification of binary alloy by Control Volume Finite Element Method

• A mathematical model for solidification with CVFEM is implemented in a computer code. • Coupled heat and mass transfer in solidification of lead-tin alloy is described. • Thermo-solutal convection and freckle formation in phase-change are reported. • Effects of cavity aspect ratio and cooling direction in solidification are studied. • Macrosegregation maps for Pb-48%Sn alloy in rectangular cavities is investigated. Unsteady coupled natural convective heat and mass transfer with liquid to solid phase change is described to analyze the macrosegregation of a Pb–Sn alloy in a rectangular two-dimensional cavity. The effect of cooling direction is investigated, by comparing the numerical simulation of solidification in rectangular molds with one vertical side cooled and with bottom wall cooling, both with the cold wall temperature changing with time. Fluid mechanics, heat and mass transfer coupled equations with unsteady, convective, diffusion and sources terms are solved with the Control Volume Finite Element Method (CVFEM) in cavities with aspect ratios A = H / B = 1 / 3 , 0.6 and 3. Results for the evolution of the distribution of streamlines, solidification front, isotherms and iso-concentrations are obtained with a code developed to characterize coupled fluid, heat and mass transport phenomena. Complex physical processes such as thermo-solutal convection during solidification, freckle formation and the final pattern of macrosegregation are described in terms of the cavity aspect ratio by the mathematical model. A good description of freckle formation and its relation to direction of cooling and aspect ratio has been obtained by the transient-convective–diffusive mathematical model solved by CVFEM.

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.

Natural Convection in a Series of Thermally Interacting Cavities with Wall Mounted Heaters

Laminar natural convection in a series of thermally interacting cavities is numerically studied. Each cavity consists of a conducting bottom wall with a surface mounted heater. The side walls of the cavities are isothermally cooled. Each cavity thermally interacts with its adjacent cavities through the conducting walls. Flow and heat transfer characteristics are studied in detail for various Rayleigh numbers. The convection characteristics in multiple cavities are compared with those in single independent cavity. The thermal interaction between the cavities results in lower temperatures compared with those in independent cavities. While heat is rejected into the adjacent upper cavity through some portion of the conducting wall, heat is received from the adjacent cavity through the remaining portion of the wall. The influence of substrate conductivity on heat exchange between adjacent cavities are examined. Substrate conductivity shows strong effect on temperature distribution. When cooling at both vertical sides is changed to one side cooling, the heat transfer characteristics are changed drastically and many interesting flow features are observed. Effects of cavity aspect ratio is studied and higher heat transfer rates are observed at higher aspect ratios. Correlations for dimensionless temperature maximum and average Nusselt number are presented in terms of Rayleigh number.

Conjugate turbulent natural convection in the roof enclosure of a heavy construction building during winter

This paper presents the results of a study of conjugate turbulent natural convection inside a building attic in the shape of a rectangular enclosure bounded by realistic walls made from composite construction materials under winter day boundary conditions. The effects of cavity aspect ratio, Rayleigh number ( Ra), depth of the external concrete beam, and external wall construction materials on the flow and heat transfer characteristics were the main focus of the investigation. The Shear stress transport k– ω turbulence model is implemented to calculate air-flow velocities and temperatures in a steady, turbulent, two-dimensional conjugate natural convection heat transfer inside an attic. The governing equations were solved by employing the line-by-line tri-diagonal matrix algorithm (TDMA) control volume method. For Ra ranging from 10 7 to 10 10, steady-state results of the streamline and temperature contours in addition to local and mean Nusselt numbers at all surfaces of the cavity were obtained. The results show that the values of Ra, attic aspect ratio and the composite wall materials have significant effect on the temperature and stream function contours within the enclosure, and the heat flux out of the room through the enclosure.

Conjugate natural convection in the roof cavity of heavy construction building during summer

This paper presents the results of a study of conjugate natural convection inside a building attic in the shape of a rectangular enclosure bounded by realistic walls made from composite construction materials under summer day boundary conditions. The effects of cavity aspect ratio, Rayleigh number ( Ra), and orientation of the external surfaces on the flow and heat transfer characteristics were the main focus of the investigation. The problem was formulated in terms of the vorticity-stream function procedure, and the governing equations for steady, laminar, two-dimensional conjugate natural convection heat transfer were solved by employing the Alternating Direction Implicit (ADI) control volume method along with under-relaxation factors for temperature, vorticity, and stream functions. For Ra ranging from 10 3 to 10 10, steady state results of the streamline and temperature contours in addition to local and mean Nusselt number at all surfaces of the cavity were obtained. The results show that the values of Ra and the aspect ratio have significant effect on the temperature and stream function contours within the enclosure. Another important finding of the study is that heat flux into the room increases with the increase of both the aspect ratio and Rayleigh number.

Effect of cavity aspect ratio on flow and heat transfer characteristics in pipes: a numerical study

Using the standard k–e turbulence model, a two-dimensional turbulent pipe flow was simulated with and without square cavities. Effect of cavity aspect ratio on flow and heat transfer characteristics was investigated. Uncertainty was approximated through experimental validation and grid independence. The simulation revealed circulation inside the cavities. Cavity boundaries were shown to contribute significantly toward turbulence production. Cavity presence was shown to enhance overall heat transfer through the wall, while increasing pressure drop significantly across the pipe. It was predicted that cavities with higher aspect ratio enhance heat transfer more while increasing pressure drop.

Laser-induced breakdown spectroscopy: flat surface vs. cavity structures

Cavity formation is a common phenomenon involved in solid-state analysis when repetitive laser pulses are applied to induce breakdown spectra. While previous LIBS investigations have been mostly focused on laser ablation on flat surfaces, the development of a laser-induced plasma inside cavity structures is of both fundamental and practical significance for quantitative chemical analysis using LIBS. In this paper, we attempt to answer the question, to what extent cavity formation would influence the temperature and electron density of laser-induced plasma. We found a significant effect of cavity aspect ratio on plasma characteristics, in particular, the measured high temperature and electron density of laser-induced plasma inside cavity structures.

Laser-induced plasmas in micromachined fused silica cavities

Cavity formation is a frequent result in many laser ablation applications. Although most theoretical investigations have been devoted to laser ablation on a flat surface, the development of a laser plasma inside a cavity is of both fundamental as well as practical significance. In this study, the temperature and electron number density of laser-induced plasmas in fused silica cavities were determined using spectroscopic methods. The effects of cavity aspect ratio on plasma properties were investigated. The temperature and electron number density of laser-induced plasma were measured to be much higher and to decrease faster for a plasma inside a cavity than on the flat surface. Cavity wall influences on the plasma expansion are discussed.

Cavity flows of elastic liquids: Purely elastic instabilities

Experimental observations of a purely elastic flow instability occurring in the lid-driven cavity flow of two semi-dilute polymer solutions are reported and the effect of cavity aspect ratio on the dynamical structure of the unstable flow is quantitatively investigated. The spatial and temporal characteristics of the secondary flow are measured using flow visualization, laser Doppler velocimetry, and digital particle image velocimetry. At the onset conditions the disturbances appear in the form of spatially periodic flow cells which propagate along the neutral direction of the cavity. The secondary flow structure is analogous to the Taylor–Görtler vortices observed in inertially driven hydrodynamic instabilities. The critical onset conditions for two elastic test fluids and five different aspect ratios correlate with a recently proposed dimensionless stability criterion which incorporates measures of the local streamline radius of curvature and the non-Newtonian normal stresses in the flow domain.