Convective Flow Field Research Articles

Experimental investigation on the chaotic phenomena in the wake of a natural thermal convection flow

Chaotic phenomena in the wake of thermal convection flow fields above a heating flat plate were investigated experimentally. A newly developed electron beam fluorescence technique (EBF) was used to simultaneously measure density fluctuation at 7 points in a cross section above the plate. Correlation dimensions, intermittence coefficients, Fourier spectrum have been obtained for different Grashof numbers. Spatial distribution of correlation dimensions are presented. The experimental result shows that there is a certain relationship between the density fluctuation and the Gr number. And time-spacial characteristic of chaos evolution is also given.

907 人口竜巻流による円柱面の伝熱特性(GS-6 熱工学)

Tornado is occurred by combination of swirling flow on the bottom surface and upflow at the center of swirling flow. We reproduced artificially tornado-like flow by a small propeller fan and four blowing tubes with slit. When the column was installed in the center of tornado, we observed heat transfer characteristic of the heated columnar surface with uniform heat flux and flow field characteristic around the column by smoke wire method and LDV. We measured distributions of local heat transfer coefficient of the column and velocity in the tornado chamber. Result of forced convection heat transfer and flow field by tornado-like flow were discussed.

Experimental Investigation to Study Convective Mixing, Spatial Uniformity, and Cycle-to-Cycle Variation During the Intake Stroke in an IC Engine

The work described in this paper focuses on experiments to quantify the initial fuel mixing and gross fuel distribution in the cylinder during the intake stroke and its relationship to the large-scale convective flow field. The experiments were carried out in a water analog engine simulation rig, and, hence, limited to the intake stroke. The same engine head configuration was used for the three-dimensional PTV flow field and the PLIF fuel concentration measurements. High-speed CCD cameras were used to record the time evolution of the dye convection and mixing with a 1/4 deg of crank angle resolution (and were also used for the three-dimensional PTV measurements). The captured sequences of images were digitally processed to correct for background light non-uniformity and other spurious effects. The results are finely resolved evolution of the dye concentration maps in the center tumble plane. The three-dimensional PTV measurements show that the flow is characterized by a strong tumble, as well as pairs of cross-tumble, counter-rotating eddies. The results clearly show the advection of a fuel-rich zone along the wall opposite to the intake valves and later along the piston crown. It also shows that strong out-of-plane motions further contribute to the cross-stream mixing to result in a relatively uniform concentration at BDC, albeit slightly stratified by the lean fluid entering the cylinder later in the intake stroke. In addition to obtaining phase-averaged concentration maps at various crank angles throughout the intake stroke, the same data set is processed for a large number of cycle to extract spatial statistics of the cycle-to-cycle variability and spatial non-uniformity of the concentration maps. The combination of the three-dimensional PTV and PLIF measurements provides a very detailed understanding of the advective mixing properties of the intake-generated flow field. [S0742-4795(00)00103-4]

The flotation rates of fine spherical particles under Brownian and convective motion

The flotation of spherical colloidal particles by small spherical bubbles is considered. The model accounts for the effects of buoyancy motion, Brownian motion, van der Waals attractive forces, and hydrodynamic interactions. Conditions are such that the fluid has negligible inertia. The suspension is sufficiently dilute that the analysis is restricted to pairwise bubble–particle interactions. The quasi-steady formulation of the Fokker–Planck equation for the pair-distribution function is simplified for negligible transversal diffusion and solved numerically. Allowance is made for bubbles with freely mobile or totally immobile interfaces. For size ratios of the captured particle to capturing bubble of 0.1 and higher, and for bubble Péclet numbers greater than approximately 10 5, convective capture dominates. For these conditions, the collision efficiencies calculated through the more complete Fokker–Planck formulation are in good agreement with those predicted by a particle trajectory analysis, both for free and rigid interfaces. For more extreme size ratios of 0.01 and lower, and bubble Péclet numbers less than approximately 10 5, capture is dominated by diffusion of the small particles within the convective flow field created by the rising bubble; however, it is found that the classical mass-transport formula is not entirely accurate, due to the effects of finite particle size and hydrodynamic interactions when the particles are large enough for boundary-layer mass transfer with high Péclet number to be dominant. A minimum flotation efficiency is observed for a given collecting bubble size, while, for a fixed suspended particle diameter, it is always more effective to utilize smaller bubbles. Bubbles with a rigid interface exhibit lower collection efficiencies than those with mobile interfaces, especially in the regime of convective capture. In all instances, the simple additivity approximation for diffusive and convective capture is shown to overpredict the collision efficiencies, in some cases by up to two-fold.

The investigation of an unstable convective flow using optical methods

The convective flow field in a vessel is investigated by laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV, namely Particle Tracking Velocimetry—PTV). The vessel is heated from below along a linear element at a temperature higher than that of the fluid. Hot fluid raises up and generates two counterrotating vortices. For a given aspect ratio, the two vortices become unstable and start to oscillate on a vertical plane (orthogonal to the heating element). This regime is investigated for increasing Rayleigh numbers to analyze the transition from regular to irregular conditions. The main transition mechanism is observed to be mostly connected to type II intermittency, a mechanism not frequently observed in experiments. However, at some Rayleigh numbers the present data does not definitely rule out type III intermittency. The phenomenon is analyzed by looking at the main frequencies in the spectrum of the horizontal velocity component and their changes with the Rayleigh number at a point above the heating element. Modifications in the local energy spectrum are analyzed by using the Wavelet Transform (WT) tool. Data obtained by PTV measurements make it possible to point out the spatial configuration of the flow and to determine the two velocity components on the measurement plane. These data are used to clarify the fundamental mechanisms of the transition. Instabilities are observed as sudden changes between two regimes of oscillations of the two counterrotating vortices: the first is characterized by oscillations centered on the vertical axis and the second by nonsymmetrical oscillations.

NUMERICAL ANALYSIS OF A BURNING DROPLET WITH INTERNAL CIRCULATION IN A GRAVITATIONAL ENVIRONMENT

A theoretical analysis of steady burning droplets with internal circulation in the presence of normal atmosphere and gravity is presented. The normalized system governing the gas phase consists of the complete set of conservation equations in r-z coordinates and includes finite-rate global kinetics. For the liquid phase, it includes continuity and momentum conservation equations. Thermodynamic equilibrium is assumed along the gas-liquid interface, which is not stationary. A body-fitted grid generation technique is used to handle irregular boundaries. The parametric study is performed by changing droplet diameter. The induced convection flow field is presented to demonstrate its interaction with the flame and the liquid flow motion. The predicted results show that there exists one, two, or three cells within the burning droplet, depending on its diameter. The occurrence of an extra cell is due to reverse flow in the gas phase generated by interaction of the induced flow and blowing velocity. Comparison with the case where internal flow is not considered, for a small droplet the effect of internal circulation on mass evaporation rate can be neglected. On the other hand, the internal flow will increase the mass evaporation rate.

Ignition Behaviors of Fuel Droplets in a Hot Convective Flowfield

Ignition Behaviors of Fuel Droplets in a Hot Convective Flowfield

Oscillatory natural convection of water near the density extremum at high Rayleigh numbers

An experimental investigation has been conducted with regard to the natural convection of water near the density extremum in rectangular enclosures with the vertical walls held at different temperatures. The water in the enclosure is initially set at a uniform temperature and then the temperatures of the cold and hot walls are suddenly changed to 0 and 8°C, respectively. The flow patterns of the steady state are presented for the initial temperature range of 2–6°C at high Rayleigh numbers, not considered previously. At an initial temperature of 4°C, the convective flow field consists of a symmetric double-cell circulation forming a downward flow, called a sinking jet in the interior of the enclosure. For Ra > 9 × 10 6, the interior sinking jet shows an oscillatory behavior due to free-shear instability, which enhances the heat transfer rate between vertical walls. However, at an initial temperature other than 4°C, an asymmetric double-cell circulation is revealed. Oscillatory flow is suppressed near the vertical walls. These phenomena are satisfactorily explained by a numerical analysis.

An improved dynamical approximation to Boussinesq equation using Karhunen-Loève basis

Karhunen-Loève (K-L) basis is used to provide an efficient relatively low dimensional dynamical approximation to Boussinesq equation governing thermal convection phenomena. K-L basis is empirically generated from an ensemble of numerically obtained realizations of turbulent thermal convection flow field. Fourier collocation spectral method is used to numerically integrate Boussinesq equation with stress-free boundary conditions at Pr = 0.72 and Ra ≈ 10000. An algorithm, which incorporates the lost dissipative effects of the truncation into the dynamical approximation without increasing the number of degrees of freedom involved, is proposed and numerically tested.

Convection in ice-covered lakes: effects on algal suspension

Convection occurs in ice-covered lakes if solar radiation warms near-surface water from the freezing point towards the temperature of maximal density. One effect of convective mixing may be to suspend non-motile phytoplankton in the upper water column, providing cells with enough light for growth during ice-covered periods. Observations of the diatom Aulacoseira baicalensis under the ice cover of Lake Baikal, Siberia, support the hypothesis that convective mixing causes net suspen- sion of cells. This paper presents a theoretical examination of the conditions under which convective flow fields can suspend algae in the photic zone of the upper water column. It is shown that the efficiency of algal suspension depends on the ratio of the still-water algal sinking rate, Wp, to con- vective updraft speed, Wu. The suspension efficiency is also shown to be affected by asymmetries in the flow field and night-time cessation of convection, but only if Wp and Wu are comparable in value. It is concluded that convection in Lake Baikal should be vigorous enough to increase the mixed-layer residence time of A.baicalensis from a few days to over a month, at least during years with thin snow

On the Dynamics of Small-Scale Solar Magnetic Elements

We report on the dynamics of the small-scale solar magnetic field, based on analysis of very high resolution images of the solar photosphere obtained at the Swedish Vacuum Solar Telescope. The data sets are movies from 1 to 4 hr in length, taken in several wavelength bands with a typical time between frames of 20 s. The primary method of tracking small-scale magnetic elements is with very high contrast images of photospheric bright points, taken through a 12 A bandpass filter centered at 4305 A in the Fraunhofer 'G band.' Previous studies have established that such bright points are unambiguously associated with sites of small-scale magnetic flux in the photosphere, although the details of the mechanism responsible for the brightening of the flux elements remain uncertain. The G band bright points move in the intergranular lanes at speeds from 0.5 to 5 km/s. The motions appear to be constrained to the intergranular lanes and are primarily driven by the evolution of the local granular convection flow field. Continual fragmentation and merging of flux is the fundamental evolutionary mode of small-scale magnetic structures in the solar photosphere. Rotation and folding of chains or groups of bright points are also observed. The timescale for magnetic flux evolution in active region plage is on the order of the correlation time of granulation (typically 6-8 minutes), but significant morphological changes can occur on timescales as short as 100 S. Smaller fragments are occasionally seen to fade beyond observable contrast. The concept of a stable, isolated subarcsecond magnetic 'flux tube' in the solar photosphere is inconsistent with the observations presented here.

Distribution of particles suspended in convective flow in differentially heated cavity

Our aim is to explore, both experimentally and theoretically, the cumulative effects of small particle–liquid density difference, where the particles are used as tracers in recirculating flow. As an example we take a flow field generated in a differentially heated cavity. The main flow structure in such a cavity consists in one or two spiraling motions. Long-term observations of such structures with the help of tracers (small particles) indicated that accumulation of the particles may set in at some flow regions. For theoretical insight into the phenomenon, a simple analytical model of recirculating (rotating) flow was studied. It was assumed that particles are spherical and rigid, and their presence does not affect the flow field. The particle Reynolds number is negligibly small, hence only the effects of particle–liquid density difference are of importance. Besides buoyancy, the effects of Saffman’s force and the inertial forces are also taken into account when calculating particle trajectories. Both cases were analyzed, particles with density slightly higher and lower than the fluid. It was found that in our case the inertial forces are egligible. In the numerical experiment trajectories of particles were investigated. The particles were allocated at random in the flow field obtained by numerical solution of the natural convection in the differentially heated cavity. In the experimental part, behavior of a dilute particle suspension in the convective cell was explored. In the model-analytical study of a simple spiraling motion, it was found that due to the interaction of the recirculating convective flow field and the gravity-buoyancy force, the particles may be trapped in some flow regions, whereas the rest of the flow field becomes particle-free. This prediction agrees fairly well with the numerical and experimental findings.

NUMERICAL INVESTIGATION OF FLOW STRUCTURE AND MIXED CONVECTION HEAT TRANSFER OF IMPINGING RADIAL AND AXIAL JETS

Abstract Mixed convection flow fields and heat transfer of partially enclosed axial and radial laminar jets impinging on a heated flat plate have been investigated from the numerical solution of incompressible unsteady Navier-Stokes and energy equations with a Boussinesq approximation. For mixed convection flow at Re = 200, steady flow has not been observed for either the radial or the axial jet. For the smallest Crashof number (Gr = 10,000), periodic solutions have been obtained. With Gr = 40,000, nonsteady nonperiodic (chaotic) flow appears. Free convection may increase the heat transfer by more than 200 %.

A Study of Natural Convection in a Rotating Enclosure

The local and mean natural convection heat transfer characteristics and flow fields were studied experimentally and numerically in an air-filled, differentially heated enclosure with a cross-sectional aspect ratio of one. The enclosure is rotated above its longitudinal horizontal axis. A Mach-Zehnder interferometer was employed to reveal the entire temperature field, which enable the measurement of the local and mean Nusselt numbers at the hot and cold surfaces. Laser sheet flow visualization was employed to observe the flow field. The result showed that the Coriolis and centrifugal buoyancy forces arising from rotation have a remarkable influence on the local heat transfer when compared with the nonrotating results. Local heat fluxes were obtained as a function of Taylor (Ta≤4×105) and Rayleigh numbers (104<Ra≤3×105), at different angular positions of the enclosure. In addition, a series of interferograms, stream function and isotherm plots demonstrated the strong effect of rotation on the flow field and heat transfer. A correlation of Nusselt number as a function of Taylor and Rayleigh numbers is presented.

Burning of a spherical fuel droplet in a uniform flowfield with exact property variation

An analytical/numerical model is developed for single droplet evaporation and burning in a convective flowfield. The model is based on the boundary-layer approach, and chemical reaction kinetics are represented by a one-step, finite-rate reaction mechanism, while variation of gas properties with temperature and gas composition is based on the kinetic theory of gases. Four droplet models differing in the degree of complexity concerning property variation and chemistry are compared. Comparisons are also provided with existing empirical correlations for convective droplet evaporation and burning.

Large-eddy simulations of compressible convection on massively parallel computers

We report preliminary implementation of the large-eddy simulation (LES) technique in 2D simulations of compressible convection carried out on the CM-2 massively parallel computer. The convective flow fields in our simulations possess structures similar to those found in a number of direct simulations, with roll-like flows coherent across the entire depth of the layer that spans several density scale heights. Our detailed assessment of the effects of various subgrid scale (SGS) terms reveals that they may affect the gross character of convection. Yet, somewhat surprisingly, we find that our LES solutions, and another in which the SGS terms are turned off, only show modest differences. The resulting 2D flows realized here are rather laminar in character, and achieving substantial turbulence may require stronger forcing and less dissipation.

Transient Natural Convection From a Leadless Chip Carrier in a Liquid Filled Enclosure: A Numerical Study

The passive cooling, including the transient start-up process, of a leadless chip carrier package mounted on a vertical substrate in a liquid filled cubic enclosure is investigated numerically. A relatively detailed thermal model of the electronic package is included. The governing three-dimensional unsteady equations for natural convection within the fluid and for the coupled heat conduction in the package are solved numerically using a finite difference method. The transient heat up of the system can be characterized by four stages: 1) initial heat up of the chip with some conduction through the package and the surrounding fluid; 2) increased conduction characterized by symmetric thermal spreading through the substrate, accompanied by the development of the natural convection flow field; 3) increased convection effects, as the buoyant driving force reaches its peak level, leading to asymmetric spreading through the substrate and a strong plume above the package; 4) gradual approach to steady state as the fluid in the enclosure is slowly heated, with reduced buoyant driving force and hence decreasing velocities. Steady-state conditions are characterized by thin boundary layers along the hot and cold surfaces, well stratified temperature in the core, a strong plume above the package, and oval shaped isotherms along the substrate surface. With FC-75 as the coolant, a large fraction of the heat generated in the chip is conducted to the substrate and then transferred to the fluid, due to the high thermal conductivity of the substrate relative to that of the fluid. Even when the upper lid of the package is removed, exposing the chip directly to the fluid, the chip temperature drops only by 5 percent, indicating the importance of substrate conduction. Use of a chip that has twice the cross-sectional area as the base case results in a 25 percent drop in chip temperature, with little effect on the remainder of the flow. When water is used as the coolant instead of FC-75, the nondimensional temperature of the chip increases, due to the lower Rayleigh number and lower ratio of substrate-to-fluid thermal conductivity. In dimensional units, however, the actual chip temperature is 25°C cooler.

An effective scale-dependent dispersivity deduced from a purely convective flow field

Abstract In the case of straight flow but with hydraulic conductivity varying in a transverse direction, the distribution of hydraulic conductivity has been determined for which the breakthrough curve due to convection only will have the same analytical form as the onedimensional convection/dispersion equation solution at the outlet end of a porous medium. That distribution is found exactly and it is very similar to the lognormal distribution. This result is significant since field evidence indicates that the logarithm of hydraulic conductivity is normally distributed. For the case considered, a simple relation between dispersivity and the coefficient of variation of hydraulic conductivity is found. One can thus determine very simply dispersivity in terms of the parameters of the distribution of hydraulic conductivity. This is particularly useful to estimate dispersivity in various cells of finite difference or finite element models when the distribution of hydraulic conductivity is not stationary, i.e. v...

Flame sheet algorithm for use in numerical modeling of ramjet combustion instability

A new two-dimensional flame sheet algorithm has been developed for use in numerical modeling of the unsteady combustion dynamics found in ramjet combustion instability. The flame representation allows for convoluted flames with a large number of points of abrupt change of orientation and separated flamelets. To demonstrate the effectiveness of the algorithm, it was coupled with a low-Mach-number, constant-density vortex model of the internal combustor flowfleld. The algorithm is robust and cost-effective. Sample numerical results show that the flow in the combustor is dominated by the inlet shear-layer instability, which generates largescale vortical structures. There is a strong relationship between the vorticity field and the flame geometry. It is shown that in the absence of an external forcing of the inlet velocity and without density change across the flame, the flowfleld evolves to a stationary state. Thus, the low-frequency ramjet combustor instability arises out of the interaction between the acoustic and convective flowfields. HE purpose of the work reported in this paper was to develop and demonstrate the effectiveness of a new flame sheet computational model that can be utilized in direct numerical simulations and semianalytic analysis of ramjet combustion instability. We have been studying instabilities in ramjet combustors, with the goal of understanding the fundamental instability mechanisms. Our objective is to develop the capability to predict observed instability frequencies and amplitudes. In previous work,15 it has been shown that the combustion instability is convectively/acoustically coupled. The details of unsteady combustion dynamics are critical in determining limit cycle behavior. The reaction zone is strongly convected by a large-scale vortex-dominated flowfield. Thus, predictive capability hinges on the ability to model adequately the reaction behavior in the unsteady combustor environment. The vortex-dominated flow in the combustor and its effect on reaction cannot be accounted for satisfactorily in closed-form linear stability models4 because they are inherently nonlinear and multidimensional. A stability analysis that includes the flow in the combustor and the combustion chemistry must therefore be at least partially numerically based. Previous works in numerical modeling of the ramjet instability problem include that of Kailasanath et al.6 and Menon and Jou.7 The goal of these workers was to predict experimental observations by numerical simulation where possible. The details of their numerical methods differed considerably; they attempted direct or large eddy simulation of the entire flowfield, including the inlet and exhaust nozzle, and acoustic

Three-Dimensional Temperature Measurement by Laser-Holographic Interferometry. Numerical Simulation of Light Deflection and Its Quantitative Compensation for Measurement Error.

A numerical simulation is performed on the deflection characteristics of the incident measuring light rays passing through the thermal boundary layer developing along an isothermally heated flat plate. The additional fringe order shift and fringe position displacement due to light deflection are calculated, and compensation for the deflection measurement uncertainty is mede by analyzing the fringes of real holographic interferograms. The validity of this compensation is confirmed by experiment on the combined free and forced convection flow field in the thermal entrance region of a horizontal rectangular duct with a constant wall temperature.