Flow pattern evolution in natural convection cooling from an array of discrete heat sources in a rectangular cavity at various orientations

An experimental study on convection heat transfer from an array of discrete heat sources

Conjugate free convection from an array of discrete heat sources with water and nano-encapsulated phase change particle in a cold enclosure

Enhancement of natural convection heat transfer from an array of discrete heat sources

Formation mechanism and evolution of flow patterns for thermal convection in an inner cylinder-heated annular pool

Buoyancy-driven convection in a differentially heated rectangular cavity containing layers of air—water, air—silicone oil, and silicone oil—water has been experimentally studied using laser-interferometry. The cavity has dimensions 32 × 32 mm2 in the vertical plane and length 447 mm. The layer thicknesses were taken to be equal. Experiments are conducted with three cavity temperature differences of 10, 15, and 18 K. The flow field is mapped in the direction parallel to the cavity length using a Mach-Zehnder interferometer. The flow regimes (such as steady two-dimensional, time-dependent, etc.) established in the individual fluid layers were compared with the published regime diagrams of single fluid layer at different Rayleigh numbers. Quantities such as the interface temperature, average Nusselt number, and the temperature profiles were determined from the interferograms. The interface shapes are recorded in the form of shadowgraphs. These results were also looked upon in terms of the coupling mechanism established at the interface. The major conclusions arrived in this study are as follows. In the experiments involving air, the layers were found to be thermally coupled. The unsteadiness in water could however be transmitted to air in the mechanical coupling mode. The presence of silicone oil over water led to mechanical coupling in the sense that the convective field in water was visibly retarded. The interface temperature determined from the experiments matched those from correlations for a single fluid whenever the coupling was thermal in origin. The differences were higher during mechanical coupling. The interface deformation correlated well with the roll movement visible in the fringes.

The onset and nonlinear regimes of the Soret-induced convection in a mixture of dodecane, isobutylbenzene, and tetralin, taken in equal mass fractions and placed in a rectangular porous cavity with rigid impermeable boundaries elongated in the horizontal direction and heated from below, have been numerically investigated. The vertical boundaries of the cavity are thermally insulated. The components of the mixture under consideration are representatives of the main groups of chemical compounds that are oil components. The values of the porosity and permeability of the medium were chosen to be close to the values of real media, such as sand, sandstone, or limestone. The area of such a configuration simulates a hydrocarbon field. Due to the Soret effect (thermal diffusion), the light components of the mixture with positive separation ratios (dodecane and isobutylbenzene) are accumulated in the hot region, whereas the heavy component (tetralin) is accumulated in the cold region. In the case of heating from below (provided by the presence of geothermal gradient under natural conditions), this process may induce convection. It was found that a steady flow occurs in the cavity at a certain value of Rayleigh number; this flow becomes oscillatory with an increase in the Rayleigh number. With a further increase in the Rayleigh number, irregular oscillations arise. Several monotonic and oscillatory regimes, characterized by different spatial scales (from one to ten vortices), were found in the range of Rayleigh numbers under consideration. The oscillations of instantaneous flow characteristics in oscillatory regimes have a complex shape. A steady-state flow regime with an asymmetric structure was found. The ranges of Rayleigh numbers in which the found regimes may exist and the time dependences of the integral characteristics during the formation of steady flows were determined. The effect of supercriticality on the flow structure and concentration distributions of mixture components were analyzed.

Convective behaviour of a uniformly Joule-heated liquid pool in a rectangular cavity

A comprehensive numerical analysis has been conducted to investigate unsteady natural convection (UNC), bifurcation behavior, and heat transfer (HT) in a rectangular enclosure containing thermally stratified air. The enclosure comprises a uniformly heated bottom wall, thermally stratified vertical sidewalls, and a cooled top wall. To assess thermal performance, square and rectangular cavities with identical boundary conditions and working fluid are considered. The finite volume method (FVM) is used to solve the governing equations over a wide range of Rayleigh numbers (Ra = 101 to 109) for air with a Prandtl number (Pr) of 0.71. Flow dynamics and thermal performance are analyzed using temperature time series (TTS), limit point–limit cycle behavior, average Nusselt number (Nuavg), average entropy generation (Savg), average Bejan number (Beavg), and the ecological coefficient of performance (ECOP). In the rectangular cavity, the transition from steady to chaotic flow exhibits three bifurcations: a pitchfork bifurcation at Ra = 3 × 104–4 × 104, a Hopf bifurcation at Ra = 3 × 106–4 × 106, and the onset of chaotic flow at Ra = 9 × 107–2 × 108. The comparative analysis indicates that Nuavg remains nearly identical for both cavities within Ra = 105 to 107. However, at Ra = 108, the HT rate in the rectangular cavity is 29.84% higher than that of the square cavity, while Savg and Beavg differ by 39.32% and 37.50%, respectively. Despite higher HT and Savg in the rectangular enclosure, the square cavity demonstrates superior overall thermal performance by 13.52% at Ra = 108. These results offer significant insights for optimizing cavity geometries in thermal system design based on energy efficiency and entropy considerations.

Free convection flow and heat transfer in a rectangular cavity with heated triangle and internal heat generation is investigated numerically. A finite element analysis is performed to investigate the effects of uniform heating and is also used for solving the Navier-Stokes and Energy balance equations. The horizontal bottom wall is divided into three equal sections. The middle section of the horizontal bottom wall, bottom side of the triangle and left vertical left wall in the enclosure were kept temperature at Th. The other two parts of the horizontal bottom wall and the other two sides of the triangle were kept thermal insulation while the right vertical walls and the top wall of the cavity were maintained constant temperature Tc with Th>Tc. The physical problems are represented mathematically by different sets of governing equations along with corresponding boundary conditions. The dimensionless Parameters in the equations are performed for Heat generation (λ), Rayleigh number (Ra) and Prandtl number (Pr). The streamlines, isotherms, average Nusselt number, velocity profiles and temperature distribution of the fluid in the enclosure are presented graphically. The numerical results indicate that the Heat Generation and Rayleigh number have strong influence on the streamlines and isotherms. Also the mentioned parameters have significant effect on average Nusselt number at the hot wall and average temperature of the fluid in the enclosure.

In the process of water filling in long-distance water transfer projects, the transient flow of water and air, along with the fluctuation in pressure, indicates the complexity of the hydraulic transition process, particularly when there are curved pressurized pipes, and the movement of air inside the pipes is not easily determined. This study focuses on pipeline systems with significant changes in elevation, such as those crossing rivers or roads. It employs a combined approach of generalized model experiments and numerical simulations to investigate the formation and transformation of flow patterns in the pipeline system during the filling process. It also analyses the fluctuation of pressure on both sides of the curved pipe under different filling modes and reveals the coupling mechanism between the movement of large-scale air bubbles and turbulent vortex structures within the curved pipe. When the water filling velocity at the pipeline inlet is v = 0.8 m s–1, the size of the bubbles formed by the air is larger, the variety of flow patterns is greater, and the evolution of flow patterns is more complex. When impounded water occurs in the river crossing pipeline due to maintenance, the pipeline system forms slug flow at a filling flow velocity of v = 0.8 m s–1. This leads to an abrupt change in the overall pressure of the pipeline system, with the maximum pressure being 1.36 times that of the filling flow velocity of v = 0.4 m s–1. At a filling flow velocity of v = 0.8 m s–1, bends in the pipe readily foster the formation of large-scale bubbles. The interaction between the buoyancy and drag forces acting on these bubbles and the centrifugal forces provided by the bends results in continuous changes in the hydraulic conditions within the pipe. The research findings not only enrich the study of the hydraulic transition process in pipeline systems but also hold significant importance for expediting the construction pace of long-distance water transfer projects.

Three-dimensional natural convection and radiation in a rectangular cavity with one active vertical wall

Three dimensional numerical analysis of natural convection cooling with an array of discrete heaters embedded in nanofluid filled enclosure

Purpose – The purpose of this paper is to study the effect of feeding scheme on melt flow and temperature field during the steady-state of level-pour direct-chill (DC) casting of A390 alloy hollow billet and optimize the design of feeding scheme. Design/methodology/approach – Melt flow and temperature field are investigated by numerical simulation, which is based on a three-dimensional mathematical model and well verified by experiments. Findings – The numerical results reveal that both melt flow and temperature field are obviously affected by the feeding scheme. The homogeneity of melt flow and temperature field in hollow billet with the feeding scheme of modified four inlets are better than the other feeding schemes. Experimental results show that crack can be eliminated by increasing the number of feeding inlets. The primary Si size appears unaffected while the distribution of primary Si particles is highly affected by the change of feeding scheme. Only with the feeding scheme of modified four inlets can fine and uniformly distributed primary Si particles be achieved. Practical implications – The paper includes implications for the design of feeding scheme in level-pour DC casting of hollow billet for practical use. Originality/value – This paper develops different feeding schemes for level-pour DC casting of hollow billet and optimizes the design of feeding scheme.

We optimize and design a new half-ring conductance sensor for measuring two-phase flow in a small diameter pipe. Based on the experimental signals measured from the designed sensor, we using the limited penetrable visibility graph we proposed construct complex networks for different flow patterns. Through analyzing the constructed networks, we find that the joint distribution of the allometric scaling exponent and the average degree of the network allows distinguishing different gas-liquid flow patterns in a small diameter pipe. The curve peak of the degree distribution allows uncovering the detailed features of the flow structure associated with the size of gas bubbles, the average degree of the network can reflect the macroscopic property of the flow behavior, The allometric scaling exponent is very sensitive to the complexity of fluid dynamics and allows characterizing the dynamic behaviors in the evolution of different flow patterns. In this regard, limited penetrable visibility graph analysis of fluid signals can provide a new perspective and a novel tool for uncovering the dynamical mechanisms governing the formation and evolution of different flow patterns.

A numerical study of the effect of fluid-structure interaction on transient natural convection in an air-filled square cavity