Dominant Convection Research Articles

Convective Transport through Porous Layers

Convective motions are a multi-physics phenomenon, in which flow and transport processes interact in a two-way coupling. The density of the fluid depends on the value of transport variable and this back-coupling leads to non-linear behaviour. For the classical constellation of a closed fluid container heated from below convective motions appear, when a critical threshold for the Rayleigh number is exceeded. The heat transfer due to convection is much higher than in the case of pure conduction. Here systems of three layers are examined in detail. Using numerical CFD modelling it is shown that in layered systems different convective flow patterns appear than in the single layer case. The number and constellation of convection cells characterize steady flow patterns. Using a parametric sweep over the relevant parameter range of layer Rayleigh numbers and layer thicknesses we determine diagrams that show the excess heat or mass transfer of the dominant convection patterns, measured by the Nusselt- or Sherwood numbers.

Finite element simulations for natural convective flow of nanofluid in a rectangular cavity having corrugated heated rods

In this study, numerical simulations of natural convection in a partially heated rectangular cavity containing water-based copper oxide nanofluid (CuO–water) have been carried out. The flow field and heat transfer inside the cavity are influenced by two corrugated heated rods. The governing partial differential equations are transformed to dimensionless coupled nonlinear partial differential equations using some suitable variables. For the thermophysical properties of nanofluid, Koo and Kleinstreuer–Li model is implemented in the governing equation. Numerical solutions of the resulting system of equations are obtained utilizing finite element method. The simulations for flow field and thermal distribution are portrayed in terms of line graphs, streamlines and isotherms. The results are executed for various Rayleigh numbers $$(10^4\le {\mathrm{Ra}} \le 10^6)$$ , nanoparticle volume fractions $$(0.0\le \phi \le 0.2)$$ , amplitudes of the corrugated heated rods $$(0.05\le A_{\mathrm{m}} \le 0.2)$$ and wavelength numbers $$(0 \le n \le 20)$$ . Results depict that the thermal distribution and flow field are getting stronger because of increasing Ra and n. The impact of nanoparticle volume fraction is found to be useful in intensifying the heat transfer rate because of dominant convection. It is worth mentioning that with an increase in $$A_{\mathrm{m}}$$ the thermal distribution in the entire cavity is control by convection.

Melting modes in laser powder bed fusion

Depending on processing conditions, laser powder bed fusion (LPBF) is known to have two operational regimes – conduction mode and keyhole mode. Heat conduction is the dominant heat transfer mechanism for conduction mode melting, whereas heat convection is the dominant heat transfer mechanism for keyhole mode melting. In addition, there exists a transition mode, which lies between the conduction and keyhole mode, wherein the dominance of conduction or convection depends upon the processing conditions. In this work, normalized processing diagrams are obtained to visualize the three melting modes - conduction mode, transition mode, and keyhole mode. The normalized processing diagrams obtained from this work are shown to be independent of material for specific classes of materials, of LPBF system, of laser modulation, and of powder layer thickness. Additionally, an analytical model is proposed to robustly predict the threshold between the three melting modes for two different classes of materials, (i) materials with low reflectivity and low thermal conductivity such as titanium, ferrous, and nickel alloys, and (ii) materials with high reflectivity and high thermal conductivity such as aluminium alloys. The normalized processing diagrams, alongside the identified melting mode thresholds, can provide a useful tool in diagnosing the origins of porous defects and enable accelerated process optimization efforts towards tailoring material properties in LPBF.

Morphology of films and nanostructures grown on trenched substrates by Monte Carlo simulations

We performed numerical simulations of two-dimensional models of film deposition on a substrate with a square trench. The particle flux mixes convection and diffusion in a broad range of the Peclet number (Pe). Two models, formerly termed SARM (Standard Adherence Rule Model) and PARM (Potential Adherence Rule Model), are considered. SARM considers aggregation of the incident particle at the first contact with the deposit, while PARM considers aggregation if a potential barrier is overcome during the particle motion. Both models represent a broad range of growth conditions in which effects of adsorbate diffusion can be neglected. The surface roughness converges to values similar to those for growth on flat substrates when the deposit is up to 5 times thicker than the trench depth. In some growth conditions, we observe the formation of funnel-like structures on the trench bottom, with dimension similar to those of the trench. This can be anticipated by an initial large aggregation rate at the upper corners of the trench. In SARM, the funnels are observed with Pe~1 (balanced diffusion and convection) and Pe> > 1 (dominant convection). In PARM, it occurs only with Pe~1, and the substrate coverage is larger in comparison with SARM deposits. Thus, a larger adsorption barrier may improve the adherence of the funnels to the substrate, with the cost of a more careful control of the properties of the incoming flux.

A Unified Eddy-Diffusivity/Mass-Flux Approach for Modeling Atmospheric Convection

Abstract A fully unified parameterization of boundary layer and moist convection (shallow and deep) is presented. The new parameterization is based on the stochastic multiplume eddy-diffusivity/mass-flux (EDMF) approach, which distinguishes between convective plumes and nonconvective mixing. The convective plumes represent both surface-forced updrafts and evaporatively driven downdrafts. The type of convection (i.e., dry, shallow, or deep) represented by the updrafts is not defined a priori, but rather depends on the near-surface updraft properties and the stochastic interactions between the plumes and the environment through lateral entrainment. Consequently, some updrafts may contribute only to the nonlocal transport within the subcloud layer, while others may condense and form shallow or even deep convection. Such a formulation is void of trigger functions and additional closures typical of modular parameterizations. The updrafts are coupled to relatively simple warm-, mixed-, and ice-phase microphysics. Each precipitating updraft forms a downdraft driven by the evaporation of detrained precipitation. The downdrafts control the development of cold pools near the surface that can invigorate convection. The new parameterization is tested in a single-column model against large-eddy simulations (LESs) for cases representing weakly precipitating marine convection and the diurnal cycle of continental deep convection. The results of these EDMF experiments compare well with the LES reference simulations. In particular, the transitions between the different dominant convection regimes are realistically simulated.

Effect of Uniform Magnetic Field on Melting at Various Rayleigh Numbers

Melting phenomena occurs in various industrial applications, such as metal castings of turbine blades, environmental engineering, PCM-based thermal storage devices, etc. During the design of these devices, they are designed for efficient heat transfer rate. To improve the heat transfer rate, understanding of the important flow processes during the melting (and solidification) is necessary. An objective of the present work is to study the effect of natural convection and magnetic field on interface morphology and thereby on melting rate. In this work, therefore, an effect of uniform transverse magnetic field on the melting inside a cavity, filled initially with solid gallium, at various Rayleigh numbers (Ra=3×105, 6×105, and 9×105) is presented. A 2D unsteady numerical simulation, with the enthalpy-porosity formulation, is performed using ANSYS-Fluent. The magnetic field is characterized by the Hartmann number (Ha) and the results are shown for the Ha = 0, 30 and 50. The horizontal walls of the cavity are considered insulated and vertical walls are respectively considered hot and cold. It is observed that the role of natural convection during the melting is significant on the temperature distribution and solid-liquid interface. The increased magnetic field (Ha = 30 and 50) found to have a suppressing effect on the dominance of natural convection at all Rayleigh numbers (Ra=3×105, 6×105, and 9×105).

Experimental analysis on free convection effect using two different thermal performance enhancers in absorber tube of a forced circulation flat plate solar water heater

The objective of this experimental work is to compare the convection effect of the flat plate solar water heater (FPSWH), both with and without thermal performance enhancer, and to find the optimum mode of convection in the FPSWH. Two different types of thermal performance enhancers have been used, viz., rod and tube, and they were frictionally contacted with a solar absorber tube. The water flow rate in the FPSWH was varied from 0.008 kg/s to 0.025 kg/s. The Grashof number, Rayleigh number and Richardson number, which are based on solar irradiance, have been considered in this study for evaluating the effects of different modes of convection in a FPSWH. Based on the analysis, which a mixed convection heat transfer phenomena occurred in the FPSWH with the free convection domination. The plain copper tube FPSWH has significantly higher Grashof number, Rayleigh number and Richardson number in flows were compared to rod and tube thermal performance enhancer fitted collector.

Melting Modes in Laser Powder Bed Fusion

Depending on processing conditions, laser powder bed fusion (LPBF) is known to have two operational regimes - conduction mode and keyhole mode. Heat conduction is the dominant heat transfer mechanism for conduction mode, whereas heat convection is the dominant heat transfer mechanism for keyhole mode. In addition, there exists a transition mode, which lies between the conduction and keyhole mode, wherein the dominance of conduction or convection depends upon the processing conditions. In this work, normalized processing diagrams are obtained to visualize the three melting modes - conduction mode, transition mode, and keyhole mode. The normalized processing diagrams obtained from this work are shown to be independent of material for specific classes of materials, of LPBF system, laser modulation, and of powder layer thickness. Additionally, an analytical model is proposed to robustly predict the threshold between the three melting modes for two different classes of materials, (i) materials with low reflectivity and low thermal conductivity such as titanium, ferrous, and nickel alloys, and (ii) materials with high reflectivity and high thermal conductivity such as aluminium alloys. The normalized processing diagrams, along side the identified melting mode thresholds, provide a useful tool in diagnosing the origins of porous defects and enable accelerated process optimization efforts towards tailoring material properties in LPBF.

Influence of mixed convection in atmospheric pressure chemical vapor deposition of graphene growth

Chemical vapor deposition (CVD) is a well-known process as the most favourable for high-quality large area graphene production. However, it is a complex method where each parameter will contribute to significant effect on the deposition process. One of them, fluid dynamics, was reported to be one of the major governing factors for better deposition. Heat transfer process at high temperature will influence gas phase dynamics during the deposition process. In this study, the influence of mixed convection in atmospheric pressure CVD (APCVD) on the graphene quality has been systematically investigated. Reynolds number was calculated to identify the gas flow regime as well as to see its effects on the graphene quality. Then, the type of dominant convection inside the reactor was determined using Richardson number (Grashof number/Reynolds number squared). Changes in graphene quality were then observed by Raman spectra at various values of Reynolds and Richardson numbers. Additionally, computational fluid dynamics (CFD) calculations were also performed to observe the temperature and velocity contour during deposition. We then discuss the vital thermal-fluid dynamic characteristics required for an optimal graphene growth. We highlighted that a force convection in APCVD was vital for better quality of graphene.

Water functionalized CuO nanoparticles filled in a partially heated trapezoidal cavity with inner heated obstacle: FEM approach

This frame work is established to investigate the thermal management of free convection enclosed in trapezoidal cavity filled with the water based copper oxide (CuO) nanofluid. As nanoparticles volume fraction play a significant role to handle the thermal conductivity of any working fluid, so we have addressed the complex nature real world model that widely used at the industrial level and many other mechanisms. An identical trapezoidal shape cavity is placed inside the big trapezoidal cavity that have three various constraints at the surface (cold, insulated and heated). Since bottom wall of the outer cavity is partially heated so various heated portion tests are applied to analyze the influence of heat transfer within the entire cavity. Aspect ratio that depends upon the size of the inner cavity is also determine. Complete and compatible mathematical model is constructed in the form of nonlinear coupled partial differential equation. These set of equations are characterized under the law of conservation of mass, momentum and energy equation along with the restricted domain of the cavity. Koo and Kleinstreuer-Li (KKL) model is used for effective thermal conductivity and viscosity of the nanofluid. A Galerkin based Finite Element method (FEM) is implemented to attain the suitable results in term of stream function and isotherms within the restricted domain of the cavity. Results are also obtained for velocity and temperature of the nanofluid at vertically mean position of the cavity. The simulations are performed for nanoparticles volume fraction 0⩽ϕ⩽0.2 heated portion length 0⩽LT⩽1 aspect ratio 0.5⩽AR⩽3.0, Rayleigh number 104⩽Ra⩽105.7, and three heated conditions (cold, adiabatic and hot) for inner trapezium. It is found that flow and thermal field are getting stronger due to increase in Rayleigh number. However, fluid velocity is decreasing with increasing nanoparticles volume fraction ϕ as the fluid is getting dens. Heat transfer rate is decreasing with the increase in ϕ and LT due to dominant convection.

MHD convection ow in a constricted channel

Abstract We consider the steady, laminar, convection ow in a long channel of 2D rectangular constricted cross-section under the inuence of an applied magnetic field. The Navier-Stokes equations including Lorentz and buoyancy forces are coupled with the temperature equation and are solved by using linear radial basis function (RBF) approximations in terms of the velocity, pressure and the temperature of the fluid. RBFs are used in the approximation of the particular solution which becomes also the approximate solution of the problem. Results are obtained for several values of Grashof number (Gr), Hartmann number (M) and the constriction ratios (CR) to see the effects on the ow and isotherms for fixed values of Reynolds number and Prandtl number. As M increases, the ow is flattened. An increase in Gr increases the magnitude of the ow in the channel. Isolines undergo an inversion at the center of the channel indicating convection dominance due to the strong buoyancy force, but this inversion is retarded with the increase in the strength of the applied magnetic field. When both Hartmann number and constriction ratio are increased, ow is divided into more loops symmetrically with respect to the axes.

Edge Patch-Wise Local Projection Stabilized Nonconforming FEM for the Oseen Problem

Abstract In finite element approximation of the Oseen problem, one needs to handle two major difficulties, namely, the lack of stability due to convection dominance and the incompatibility between the approximating finite element spaces for the velocity and the pressure. These difficulties are addressed in this article by using an edge patch-wise local projection (EPLP) stabilization technique. The article analyses the EPLP stabilized nonconforming finite element methods for the Oseen problem. For approximating the velocity, the lowest-order Crouzeix–Raviart (CR) nonconforming finite element space is considered; whereas for approximating the pressure, two discrete spaces are considered, namely, the piecewise constant polynomial space and the lowest-order CR finite element space. The proposed discrete weak formulation is a combination of the standard Galerkin method, EPLP stabilization and weakly imposed boundary condition by using Nitsche’s technique. The resulting bilinear form satisfies an inf-sup condition with respect to EPLP norm, which leads to the well-posedness of the discrete problem. A priori error analysis assures the optimal order of convergence in both the cases, that is, order one in the case of piecewise constant approximation and 3 2 \frac{3}{2} in the case of CR-finite element approximation for pressure. The numerical experiments illustrate the theoretical findings.

Pressure-robust analysis of divergence-free and conforming FEM for evolutionary incompressible Navier–Stokes flows

AbstractThis article focusses on the analysis of a conforming finite element method for the time-dependent incompressible Navier–Stokes equations. For divergence-free approximations, in a semi-discrete formulation, we prove error estimates for the velocity that hold independently of both pressure and Reynolds number. Here, a key aspect is the use of the discrete Stokes projection for the error splitting. Optionally, edge-stabilisation can be included in the case of dominant convection. Emphasising the importance of conservation properties, the theoretical results are complemented with numerical simulations of vortex dynamics and laminar boundary layer flows.

Improvement of longitudinal fins configuration in latent heat storage systems

Abstract In this study, the consequences of variation of the longitudinal fins angle on the heat transfer improvement during phase change are investigated. Therefore, the melting process of RT 35 as a phase change material is studied for triple-fin and double-fin cases for different angles and the results are compared with of bare tube case. Results indicated that due to the natural convection domination, the upper fin does not leave a great effect on the total melting time. Considering the triple-fin heat exchangers, as the fins angle increases from 60° to 120°, the total melting time reduces. However, when double-fin cases are under consideration, reducing the angle from 150° to 45° results in melting time reduction. More reduction in the angle increases the total melting time. Results also showed that the best cases among triple-fin cases and among the double-fin cases result in 22.5 and 62% reduction in melting time with respect to the simple heat exchanger.

Statistical Characteristics of Convective Clouds over the Western Ghats Derived from Weather Radar Observations

AbstractX‐band radar observations at Mandhardev (18.04°N, 73.85°E) are used to investigate statistics of convective clouds over the Western Ghats during monsoon season (June–September 2014). Convective storms (cells) are identified using an objective‐tracking method to examine their spatiotemporal variability, thus quantifying the time‐continuous aspects of convective cloud population over the region for the first time. An increased frequency of storm location and initiation along the windward mountains compared to coastal and lee side highlights orographic response to southwesterly flow, with superimposed diurnal cycle. An eastward progression of convective activity from upstream the barrier through windward slopes of mountains over to the lee side is observed. Storm area, height, and duration follow lognormal distributions; wherein, small‐sized storms contribute more to total population and unimodal distribution of 35 dBZ top heights (peaking at 5.5 km) depicts the dominance of shallow convection. Storms exhibit a pronounced diurnal cycle with a peak in afternoon hours, while the convective area maximum is delayed by several hours to that of precipitation flux. Cell lifetime and propagation show that cells move with slow speeds and have mean duration of 46 min. They align east‐west nearly parallel to mountain ridges, and their direction of movement is steered mostly by large‐scale winds at lower levels. Based on top heights, convective cells are further classified into cumulus, congestus, and deep clouds. In general, congestus (deep) cells are most abundant in the windward (leeward) side. A lead‐lag relationship between congestus and deep cells indicates midtroposphere moistening by congestus cells prior to deep convection.

Flow reversal of laminar mixed convection for supercritical CO2 flowing vertically upward in the entry region of asymmetrically heated annular channel

Numerical investigation has been performed in order to determine the onset of flow reversal occurrence for buoyancy assisted flow of supercritical CO2 through vertical annular channel of finite length with heated inner wall. The outer wall was considered adiabatic. Only steady state and laminar mixed convection cases were considered in order to study the involved heat transfer phenomena in a fundamental level. Results gave a clear insight of the flow reversal mechanism and highlighted buoyancy as the driving force of the aforementioned phenomenon. Furthermore, according to the results, two different flow reversal maps were constructed with the use of dimensionless parameters. Results clarifies that the dominance of free convection is vital for the onset of the flow reversal. More specifically, free convection must be higher from both conductive and forced convective heat transfer. In addition, mass flux suppresses the mentioned phenomenon, while heat flux aids the presence of flow reversal.

A comprehensive numerical model for double-layered porous air journal bearing at higher bearing numbers

Modeling air/gas lubricated double-layered porous journal bearing requires the solution of compressible Reynolds equation. It is observed that at higher bearing numbers, treatment of Reynolds equation with finite difference method using second-order central difference exhibits instabilities due to convective term dominance. To address such instabilities, Reynolds equation is discretized using finite volume and a third-order interpolation scheme to obtain variable values at grid point centers. Multistage Runge–Kutta method and biconjugate gradient stabilized method are used for solving governing equations at film and porous regions, respectively. Steady state and stability characteristics of finite double-layered porous air journal bearings considering Beavers–Joseph velocity slip at porous-film interface are obtained. Numerically stable results are obtained for bearing numbers up to 150 and feeding parameter values ranging from 0.01 to 10.

Study on solid liquid interface heat transfer of PCM under simultaneous charging and discharging (SCD) in horizontal cylinder annulus

Heat transfer performance during the simultaneous charging and discharging (SCD) operation process for phase change materials (PCM) contained inside the annulus of concentric horizontal cylinder was investigated. In the experimental set-up, the PCM inside the annulus serves as the heat sink along with an externally imposed forced cooling air. The obtained time wise temperature profile was used to determine the effects of different heat fluxes and the imposed forced convection cooling on the melt fraction values and the transition shift time from the observed conduction to natural convection heat transfer patterns. Furthermore, non-dimensional analysis was presented for the heat transfer at the interface to enable generalizing the result. Comparison of the results show that the SCD operation mode establish the condition that enables much PCM phase transition time and thus longer time of large latent heat transfer effect than the Partial and non simultaneous operations. Analysis results show that the variation of the heat flux for the SCD mode did not change the dominance of the natural convection over conduction heat transfers in the PCM. However, it significantly influences the commencement/transition shift time and melting rate while higher heat fluxes yields melt fraction that was 38–63% more for investigated process time. Variation with different cooling air flow rate shows more influences on the melt fraction than on the mode of heat transfer occurring in the PCM during melting. Available non-SCD modes correlation was shown to be insufficient to accurately predict interface heat transfer for the SCD modes.

Local projection stabilized and characteristic decoupled scheme for the fluid–fluid interaction problems

In this article, we propose and analyse a local projection stabilized and characteristic decoupled scheme for the fluid–fluid interaction problems. We use the method of characteristics type to avert the difficulties caused by the nonlinear term, and use the local projection stabilized method to control spurious oscillations in the velocities due to dominant convection, and use a geometric averaging idea to decouple the monolithic problems. The stability analysis is derived and numerical tests are performed to demonstrate the robustness of this new method. © 2016 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 33: 704–723, 2017

Surface tension problems solved with the particle finite element method using large time-steps

In previous works [S. R. Idelsohn, J. Marti, P. Becker, E. Onate, Analysis of multifluid flows with large time steps using the particle finite element method, International Journal for Numerical Methods in Fluids 75 (9) (2014) 621–644. doi:10.1002/fld.3908. URL http://dx.doi.org/10.1002/fld.3908, Juan M. Gimenez and Leo M. Gonzalez, An extended validation of the last generation of particle finite element method for free surface flows, J Comput Phys 284 (0) (2015) 186–205. doi:http://dx.doi.org/10.1016/j.jcp.2014.12.025. URL http://www.sciencedirect.com/science/article/pii/S0021999114008420 ], the authors have presented a highly efficient extension of the Particle Finite Element Method, called PFEM-2, to solve two-phase flows. The methodology which uses X-IVS [S. Idelsohn, N. Nigro, A. Limache, E. Onate, Large time-step explicit integration method for solving problems with dominant convection, Comp Methods in Appl Mech Eng 217–220 (2012) 168–185.] to treat convection terms allowing large time-steps was validated for problems where the gravity forces and/or the inertial forces dominate the flow. Although that is the target range of problems to solve with PFEM-2, most of real problems that fall in these categories also includes other flow regimes in certain regions of the domain. Maybe the most common secondary regime is when the surface tension dominates, as an example when drops or bubbles are released from the main flow, and this feature must be taken into account in any complete numerical strategy. Attending to that, in this work the treatment of the surface tension to PFEM-2 is included. An implicit CSF methodology is employed together with a coupling between the marker function with a Level Set function to obtain a smooth representation of the normal of the interface which allows an accurate curvature calculation. Examples for curvature calculation and isolated bubbles and drops are presented where the accuracy and the computational efficiency are analyzed and contrasted with other numerical methodologies. Finally, a simulation of a jet atomization is analyzed. This case presents the above mentioned features: it is a inertia-dominant flow with a surface tension phenomena on drops and ligaments break up that can not be neglected.