Heat transfer enhancement in a turbulent natural convection boundary layer along a vertical flat plate

An experimental study on heat transfer enhancement for a turbulent natural convection boundary layer in air along a vertical flat plate has been performed by inserting a long flat plate in the spanwise direction (simple heat transfer promoter) and short flat plates aligned in the spanwise direction (split heat transfer promoter) with clearances into the near-wall region of the boundary layer. For a simple heat transfer promoter, the heat transfer coefficients increase by a peak value of approximately 37% in the downstream region of the promoter compared with those in the usual turbulent natural convection boundary layer. It is found from flow visualization and simultaneous measurements of the flow and thermal fields with hot- and cold-wires that such increase of heat transfer coefficients is mainly caused by the deflection of flows toward the outer region of the boundary layer and the invasion of low-temperature fluids from the outer region to the near-wall region with large-scale vortex motions riding out the promoter. However, heat transfer coefficients for a split heat transfer promoter exhibit an increase in peak value of approximately 60% in the downstream region of the promoter. Flow visualization and PIV measurements show that such remarkable heat transfer enhancement is attributed to longitudinal vortices generated by flows passing through the clearances of the promoter in addition to large-scale vortex motions riding out the promoter. Consequently, it is concluded that heat transfer enhancement of the turbulent natural convection boundary layer can be substantially achieved in a wide area of the turbulent natural convection boundary layer by employing multiple column split heat transfer promoters. It may be expected that the heat transfer enhancement in excess of approximately 40% can be accomplished by inserting such promoters.     

Application of the laser beam deflection to the study of temperature fields in Rayleigh–Benard convection

An optical integral and unobtrusive method is developed to determine the temperature gradient field by measurement of laser beam deflection induced by optical index changes. The technique is inexpensive and easy to perform. It allows the study of the temperature gradient at a large number of points simultaneously (over 400/cm²). This technique is particularly adapted to two-dimensional convection flow in both unsteady and steady conditions. For illustration purposes, such a technique is applied to the roll pattern of Rayleigh-Bénard convection. The agreement between theory and experiments is fair.     

Linear stability of solutal convection in solidifying mushy layers: permeable mush–melt interface

The linear stability theory is used to investigate analytically the effect of a permeable mush–melt boundary condition on the stability of solutal convection in a mushy layer of homogenous permeability at the near eutectic (solid) limit. The results clearly show that, in contrast to the impermeable mush–melt interface boundary condition, the application of the permeable mush–melt interface boundary condition destabilizes the convection in a mushy layer.     

A threshold for onset of natural convection in binary metallic alloys

Experiments were performed with a new radioscopic flow visualization technique on natural convection during melting of a binary metallic Ga-In alloy. This technique provides visualization of the density fields within opaque low Prandtl number fluids and their solids. Upon applying a horizontal temperature gradient to a gallium melt alloyed with 5 weight percent indium, the binary melt developed a vertical concentrational stratification and heat transfer was by conduction only. Convective flow developed at a higher temperature difference, which may be termed “critical”. After reducing the temperature difference the thermosolutal convection breaks down and a conductive state reappears at ΔT0 K. This threshold for onset of natural convection in binary Ga-In melts is in need of a theoretical explanation.     

Effect of inclined gradients on thermohaline convection in porous medium

Onset of convection in a thermohaline fluid staturating a porous medium subjected to inclined temperature as well as salinity gradients of finite magnitude is analysed using Galerkin technique. Due to unequal horizontal gradients of heat and salt the basic state fluid density varies horizontally also. It is found that (i) when the horizontal temperature gradient is greater than salinity gradient the system becomes unstable and stationary convection is possible in the stable quadrant, (ii) when the horizontal salinity gradient is greater than temperature gradient the stable region extends to the basically unstable quadrant, (iii) in the case of compensating horizontal gradients also the region of stationary convection gets extended to the stable quadrant when horizontal gradient increases, (iv) if only one of the horizontal gradients (heat or salt) is present the point where stationary convection changes over to oscillatory pattern is shifted to the region conductive to stationary convection; the effect of salinity gradient being more than that of temperature gradient.Mit Hilfe der Galerkin-Methode wird das Einsetzen der Konvektion in einem, ein poröses Medium tränkenden Thermohaline-Fluid unter dem gleichzeitigen Einfluß von veränderlichen Temperatur- und Salzkonzentrationsgradienten endlicher Höhe untersucht. Wegen der ungleichen Horizontalgradienten von Temperatur und Konzentration ändert sich auch die Gesamtfluiddichte in Horizontalrichtung. Es zeigt sich: (1) Ist der horizontale Temperaturgradient größer als der Konzentrationsgradient, so wird das System instabil und im stabilen Quadranten ist stationäre Konvektion möglich. (2) Im umgekehrten Fall erstreckt sich das stabile Gebiet bis zum grundsätzlich instabilen Quadranten. (3) Kompensieren sich die Horizontalgradienten, so erstreckt sich auch das Gebiet stationärer Konvektion bis zum stabilen Quadranten, falls der Horizontalgradient zunimmt. (4) Ist nur einer der beiden Horizontalgradienten vorhanden, so verschiebt sich der Punkt, wo stationäre Konvektion in die oszillatorische Mode übergeht, vom Gebiet reiner Wärmeleitung in das stationärer Konvektion. Der Einfluß des Konzentrationsgradienten überwiegt dabei den des Temperaturgradienten.C. Parthiban thanks University Grants Commission for awarding a fellowship.     

Thermo-gravitational convection in a continuous optical discharge

The relative importance of such processes as the free convective motion of the gas, the absorption of the laser radiation and radiative heat transfer is discussed. The burning of a continuous optical discharge under experimental conditions [6] is theoretically investigated. The two-dimensional problem of the convective motion of the gas in an optical discharge burning in a vertical CO₂ laser beam inside a cylindrical chamber is solved. The principal characteristics of thermogravitational convection of the radiating air under conditions of local thermodynamic equilibrium at atmospheric pressure are studied on the temperature interval from 300 to 20 000°K.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 124–129, July–August, 1989.     

Unsteady flow structure and vorticity convection over the airfoil oscillating at high reduced frequency

Unsteady vortex structures and vorticity convection over the airfoil (NACA 0012), oscillating in the uniform inflow, are studied by flow visualization and velocity measurements. The airfoil, pivoting at one-third of the chord, oscillates periodically near the static stalling angle of attack (AOA) at high reduced-frequency. The phase-triggering and modified phase-averaged techniques are employed to reconstruct the pseudo instantaneous velocity field over the airfoil. During the down stroke cycle, the leading-edge separation vortex is growing and the vortex near the trailing edge begins to shed into the wake. During the upstroke cycle, the leading-edge separation vortex is matured and moves downstream, and the counter clockwise vortex is forming near the trailing edge. Convection speeds and wavelength of the unsteady vortex structure over the airfoil equal to that of the counter clockwise vortex shed into the wake. This kind of vortex structure is termed as “synchronized shedding” type. The wavelength of unsteady vortex structure over the airfoil is significantly different from that at low reduced-frequency. Consistent convection speeds of the leading-edge separation vortex are acquired from the spatial-temporal variations of local circulation and local surface vorticity generation, and equals that predicted from flow visualization. Spatial-temporal variations of the local surface vorticity generation clearly reveal the formation and passage of the leading-edge separation vortex only in the region where the flow does not separate completely from the surface. Significant amounts of the surface vorticity are generated within the leading-edge region of the airfoil during the upstroke cycle. Only negligible amount of surface vorticity is produced within the region of complete flow separation. During the down stroke cycle, the surface vorticity generation is mild along the airfoil surface, except the leading-edge region where a small scale leading-edge separation vortex is forming and growing.     

Enhanced heat transfer performances of molten salt mixed convection in a vertical annular duct

The mixed convection heat transfer of upward molten salt flow in a vertical annular duct is experimentally and numerically studied. The heat transfer performances of mixed convection are measured under Reynolds number 2,500–12,000 and inlet temperature 300–400 °C, and Nusselt number of molten salt flow with cooled inner wall monotonically increases with buoyancy number. The mixed convection is further simulated by low-Reynolds number k-ε model and variable properties, and the heat transfer tendency from numerical results agrees with that from experiments. At low Reynolds number, the natural convection plays more important role in the mixed convection. As the buoyancy number rises, the thickness of flow boundary layer near the inner wall increases, while the effective thermal conductivity remarkably rises, so the enhanced heat transfer of mixed convection is mainly affected by the effective thermal conductivity due to turbulent diffusion.     

Particle-image-velocimetry applied to thermocapillary convection

Thermocapillary convection is studied experimentally using particle-image-velocimetry for flow visualization and analysis. This method offers the advantage of measuring the entire flow field (velocity field, streamlines etc.) in a selected plane within the fluid at a given instant of time in contrast to point by point methods like laser-Doppler-velocimetry (LDV). The paper describes the method and presents quantitative results for different Marangoni numbers.Presented in part at the VIIth European Symposium on Materials and Fluid Sciences in Microgravity, Oxford University, UK, September 10–15, 1989     

Numerical investigation of a spatially developing turbulent natural convection boundary layer along a vertical heated plate

Large-eddy simulation (LES) on a spatially developing natural convection boundary layer along a vertical heated plate was conducted. The heat transfer rate, friction velocity, mean velocity and temperature, and second-order turbulent properties both in the wall-normal and the stream-wise direction showed reasonable agreement with the findings of past experiments. The spectrum of velocity and temperature fluctuation showed a -2/3-power decay slope and -2-power decay slope respectively. Quadrant analysis revealed the inclination on Q1 and Q3 in the Reynolds stress and turbulent heat flux, changing their contribution along the distance from the plate surface. Following the convention, we defined the threshold region where the stream-wise mean velocity takes local maximum, the inner layer which is closer to the plate than the threshold region, the outer layer which is farther to the plate than the threshold region. The space correlation of stream-wise velocity tilted the head toward the wall in the propagating direction in the outer layer; on the other hand, the correlated motion had little inclination in the threshold region. The time history of the second invariant of gradient tensor Q revealed that the vortex strength oscillates both in the inner and the outer layers in between the laminar and the transition region. In the turbulent region, the vortex was often dominant in the outer layer. Instantaneous three-dimensional visualization of Q revealed the existence of high-speed fluid parcels associated with arch-shape vortices. These results were considered as an intrinsic structure in the outer layer, which is symmetrical to the structure of canonical smooth/rough wall bounded layer flow in forced convection.     

Experimental techniques in natural convection

Experimental techniques in natural convection heat transfer employed in the author's laboratory are introduced. The techniques are mostly related to visualization of flow, temperature field, and heat flux distribution in fluids. Three topics are presented, the first being natural convection in a horizontal rectangular liquid layer driven by surface tension and buoyancy. The patterns of flow were visualized by suspending fine aluminum flakes in the liquid. At the same time, the distribution of the temperature gradient in the liquid was visualized by an optical method making use of the refraction of light. The second topic is the onset of oscillatory convection in the Czochralski growth melt. In this case a forced flow due to rotation of the crystal and the vessel is superimposed on the buoyancy convection, resulting in an oscillatory flow under certain circumstances. The flow pattern and the temperature distribution in the liquid were visualized simultaneously by suspending in the liquid a microencapsulated temperature-sensitive liquid crystal. Periodical oscillation of the flow and the temperature was clearly recognized. The third topic is the rollover of double liquid layers that were stratified stably due to a density difference. A small-scale experiment was carried out to clarify the basic mechanism of rollover. The tracer method was used to visualize boundary layer flow along the vertical side wall and the shadowgraph technique to visualize the density distribution in the liquid layers. The article emphasizes the importance of visual observation in the investigation of natural convection phenomena.     

Yttria-stabilized-zirconia hollow spheres prepared by atmospheric plasma spray

Yttria-stabilized-zirconia (YSZ) hollow spheres are widely utilized for their novel physical and chemical properties. However, developing a simple and low-cost method for preparing such hollow spheres still remains a great challenge. In this paper, an atmospheric plasma spray (APS) method is introduced, and the formation mechanism of hollow 7YSZ (ZrO₂-7wt%Y₂O₃) spheres is presented. The hollow sphere morphology was observed by scanning electron microscopy (SEM) when agglomerated and sintered 7YSZ powders were used. Additionally, additive composition changes, phase transformations, and the thermal behavior of 7YSZ powders were analyzed by energy dispersive spectroscopy (EDS), X-ray diffractometry (XRD), thermogravimetric analysis (TG) and differential scanning calorimeter analysis (DSC). Furthermore, the phase transformations of agglomerated and sintered 7YSZ powders, 7YSZ hollow spheres that annealed at various temperatures for different times are analyzed.     

Facile aqueous synthesis and thermal insulating properties of low-density glass/TiO2 core/shell composite hollow spheres

Anatase TiO2 shells assembled on hollow glass microspheres(HGM)with tunable morphologies were successfully prepared through a controllable chemical precipitation method with urea as the precipitator. Thus,glass/TiO2 core/shell composite hollow spheres with low particle density(0.40 g/cm3)were fabricated.The phase structures,morphologies,particle sizes,shell thicknesses,and chemical compositions of the composite microspheres were characterized by X-ray diffraction(XRD),scanning electron microscopy (SEM),and energy dispersive X-ray spectroscopy(EDS).The morphology of the TiO2 shell can be tailored by properly monitoring the reaction system component and parameters.The probable growth mechanism and fabrication process of the core/shell products involving the nucleation and oriented growth of TiO2 nanocrystals on hollow glass microspheres was proposed.A low infrared radiation study revealed that the radiation properties of the products are greatly influenced by the unique product shell structures. A thermal conductivity study showed that the TiO2/HGM possess low thermal conductivity that is similar to that of the pristine HGMs.This work provides an additional strategy to prepare low-density thermal insulating particles with tailored morphologies and properties.     

Experimental investigation of evaporation-induced convection in water using laser based measurement techniques

Recent studies have shown that the evaporation of water can induce surface tension gradients along the water surface that ultimately lead to a surface driven flow, known as Marangoni convection. To visualize and characterize the Marangoni convection in water, this study generated evaporation driven convection in pure water with a vacuum pump to control and increase the evaporation rate of water within a rectangular cuvette that was placed within a vacuum chamber, and investigated the velocity and temperature distributions of the generated convection. The investigation was performed as the vacuum chamber pressure ranged from ∼250 Pa to ∼820 Pa. The temperature field obtained from thermocouple measurements and temperature planar laser induced fluorescence (temp-PLIF) measurements indicated that no buoyancy driven motion was generated during the investigation. Velocity vector fields captured with stereo particle image velocimetry (stereo-PIV) demonstrated a convection pattern that was strong and symmetric with the centerline of the cuvette. The strength of the convection was found to be correlated with the mean evaporation rate of water. The estimated Marangoni number exceeded the critical value typically used to characterize the onset of Marangoni convection. The convection had a similar pattern as Marangoni convection observed in volatile liquids evaporated from capillary tubes. In both cases, the convection scaled with the width of the liquid container even though the sizes of the containers differ by an order of magnitude. In addition, the size of the convection in this study was much larger than the Marangoni convection in water that was observed in previous studies.     

Facile aqueous synthesis and thermal insulating properties of low-density glass/TiO2 core/shell composite hollow spheres

Anatase TiO₂ shells assembled on hollow glass microspheres (HGM) with tunable morphologies were successfully prepared through a controllable chemical precipitation method with urea as the precipitator. Thus, glass/TiO₂ core/shell composite hollow spheres with low particle density (0.40 g/cm³) were fabricated. The phase structures, morphologies, particle sizes, shell thicknesses, and chemical compositions of the composite microspheres were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). The morphology of the TiO₂ shell can be tailored by properly monitoring the reaction system component and parameters. The probable growth mechanism and fabrication process of the core/shell products involving the nucleation and oriented growth of TiO₂ nanocrystals on hollow glass microspheres was proposed. A low infrared radiation study revealed that the radiation properties of the products are greatly influenced by the unique product shell structures. A thermal conductivity study showed that the TiO₂/HGM possess low thermal conductivity that is similar to that of the pristine HGMs. This work provides an additional strategy to prepare low-density thermal insulating particles with tailored morphologies and properties.     

A linear stability analysis of thermal convection in spherical shells with variable radial gravity based on the Tau-Chebyshev method

The onset of thermal convection in a non-rotating spherical shell is investigated using linear theory. The Tau-Chebyshev spectral method is used to integrate the linearized equations. We investigate the onset of thermal convection by considering two cases of the radial gravitational field (i) a local acceleration, acting radially inward, that is proportional to the distance from the center r, and (ii) a radial gravitational central force that is proportional to r⁻ⁿ. The former case has been widely analyzed in the literature, because it constitutes a simplified model that is usually used, in astrophysics and geophysics, and is studied here to validate the numerical method. The latter case was analyzed since the case n = 5 has been experimentally realized (by means of the dielectrophoretic effect) under microgravity condition, in the experimental container called GeoFlow, inside the International Space Station. Our study is aimed to clarify the role of (i) a radially inward central force (either proportional to r or to r⁻ⁿ), (ii) a base conductive temperature distribution provided by either a uniform heat source or an imposed temperature difference between outer and inner spheres, and (iii) the aspect ratio η (ratio of the radii of the inner and outer spheres), on the critical Rayleigh number. In all cases the surface of the spheres has been assumed to be rigid. The results obtained with the linear theory based on the Tau-Chebyshev spectral method are compared with those of the integration of the full non-linear equations solved by using the spectral element method. By using the Tau-Chebyshev method, we were able to explore new cases that have not been previously reported in the literature.     

Stability of double-diffusive convection induced by selective absorption of radiation in a fluid layer

Linear and nonlinear stability analyses were performed on a fluid layer with a concentration-based internal heat source. Clear bimodal behaviour in the neutral curve (with stationary and oscillatory modes) is observed in the region of the onset of oscillatory convection, which is a previously unobserved phenomenon in radiation-induced convection. The numerical results for the linear instability analysis suggest a critical value γ _c of γ, a measure for the strength of the internal heat source, for which oscillatory convection is inhibited when γ > γ _c . Linear instability analyses on the effect of varying the ratio of the salt concentrations at the upper and lower boundaries conclude that the ratio has a significant effect on the stability boundary. A nonlinear analysis using an energy approach confirms that the linear theory describes the stability boundary most accurately when γ is such that the linear theory predicts the onset of mostly stationary convection. Nevertheless, the agreement between the linear and nonlinear stability thresholds deteriorates for larger values of the solute Rayleigh number for any value of γ.     

Threshold excitation of photoabsorption convection

The article considers questions of the stability of the equilibrium states of a liquid which absorbs light. Threshold values are found for the intensity of the light in the problem of the stability of the equilibrium of a liquid in a square cavity with three thermally insulated walls. A steady-state integro-interpolation scheme is presented for the numerical calculation of problems of photoabsorption convection. The propagation of light waves in absorbing media is accompanied by the dissipation of radiant energy. In heavy liquids, absorption heating of a substance in the field of a wave may be the reason for the appearance of convection [1–3]. It is important to study the conditions for the appearance and the special characteristics of this type of convection, and its inverse effect on the structure of the light field. The first problem is important when the light beams are regarded only as a source of convection [4], and the second in questions of the directed propagation of light [5] and of self-focusing phenomena [2, 3, 6–10]. For high-energy heat fluxes and a liquid with a strong temperature dependence of its dielectric permeability, the convective self-stress will be very considerable; in this case, both problems are mutually interconnected. The excitation of convection by the absorption of light, without taking account of the inverse effect on the structure of the light beam, was studied numerically in [1, 4]. Equations for photoabsorption convection, taking account of convective self-stress in the Boussinesq approximation and of the geometry of the optics, were formulated in [11]. Several economical finite-difference schemes for solving problems of photoabsorption convection problems in rectangular cavities are discussed in [12]. The present article is devoted to an investigation of the threshold intensities of light for the excitation of photoabsorption convection. The existence of critical intensities of light, above which the mechanically equilibrium states of the liquids absorbing the light become unstable, was demonstrated in [1, 4].Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 128–135, September–October, 1971.The authors thank A. V. Lykov for his continuing interest and aid, and G. I. Petrov and V. I. Polezhaev for their useful evaluation of the work.     

A model for convection in the evolution of under-ice melt ponds

A model for convection in the evolution of under-ice melt ponds is presented. The system exhibits two competing effects namely, a temperature gradient which is destabilising and a salt gradient which is stabilising. Density is assumed to have a dependence quadratic in temperature and linear in concentration. A linear instability analysis and a nonlinear stability analysis are performed. The standard energy method does not yield unconditional stability so a weighted energy analysis is employed to achieve global results. The global stability bound is found to be independent of the salt field and a presentation of the region of possible subcritical instabilities is given. Received May 16, 2002 / Published online September 4, 2002 RID="a" ID="a" e-mail: Magdalen.Carr@durham.ac.uk Communicated by Brian Straughan, Durham     

Yttria-stabilized-zirconia hollow spheres prepared by atmospheric plasma spray

Yttria-stabilized-zirconia （YSZ） hollow spheres are widely utilized for their novel physical and chemical properties. However, developing a simple and low-cost method for preparing such hollow spheres still remains a great challenge. In this paper, an atmospheric plasma spray （APS） method is introduced, and the formation mechanism of hollow 7YSZ （ZrO2-7wt%Y2O3） spheres is presented. The hollow sphere morphology was observed by scanning electron microscopy （SEM） when agglomerated and sintered 7YSZ powders were used. Additionally, additive composition changes, phase transformations, and the thermal behavior of 7YSZ powders were analyzed by energy dispersive spectroscopy （EDS）, X-ray diffractometry （XRD）, thermogravimetric analysis （TG） and differential scanning calorimeter analysis （DSC）. Furthermore, the phase transformations of agglomerated and sintered 7YSZ powders, 7YSZ hollow spheres that annealed at various temperatures for different times are analyzed.