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.     

Heat transfer and skin-friction in a turbulent boundary layer under a non-equilibrium longitudinal adverse pressure gradient

The experimental data on the effect of weak and moderate non-equilibrium adverse pressure gradients (APG) on the parameters of dynamic and thermal boundary layers are presented. The Reynolds number based on the momentum thickness at the beginning of the APG region was Re^** = 5500. The APG region was a slot channel with upper wall expansion angles from 0 to 14°. The profiles of the mean and fluctuation velocity components were measured using a single-component hot-wire anemometer. The friction coefficients were determined using two methods, namely, the indirect Clauser method and the direct method of weighting the lower wall region on a single-component strain-gage balance. The heat transfer coefficients were determined by a transient method using an IR camera. It is noticed that in the pressure gradient range realized the universal logarithmic region in the boundary layer profile is conserved. The values of the relative (divided by the parameters in zero gradient flow at the same value of Re^**) friction and heat transfer coefficients, together with the Reynolds analogy factor, are determined as functions of the longitudinal pressure gradient. The values of the relative friction coefficient reduced to c_f/c_f0 = 0.7 and those of the heat transfer to St/St₀ = 0.9. A maximum value of the Reynolds analogy factor (St/St₀)/(c_f/c_f0) = 1.16 was reached for the pressure gradient parameter β = 2.9.     

Film cooling by injection into a turbulent boundary layer

Most papers on film cooling concern injection of a homogeneous gas. Stollery et al. [1] examined the case of tangential injection of gas into a boundary layer, the specific heat_63-01 differing little from that of the main flow,_63-02.Here we examine the effectiveness of film cooling of a thermally isolated planar wall by local supply to a turbulent boundary layer.     

Displacement in the turbulent boundary layer on a permeable plate with supercritical injection

The displacement thickness in a turbulent boundary layer is determined for supercritical injection parameters. Experimental relations between the displacement thickness and the injection parameter are obtained for air, helium, and freon-12 injected into air.     

Heat mass transfer in a turbulent layer above permeable membranes

The temperature and concentration fields in a boundary layer above perforated membranes are presented, and their relationship with the velocity fields given in [1] is established. Measurements of the thermal state of membranes are made with various geometric and thermophysical properties and various coolant drafts. Empirical formulas are also presented for thermal flux and temperature of the permeable walls.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 22–31, July–August, 1973.     

Study of a three-dimensional turbulent boundary layer with allowance for coupled heat transfer

Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 3, pp. 118–124, May–June, 1989.     

Heat transfer experiments in rotating boundary layer flow

The influence of Coriolis force on heat transfer in a rotating transitional boundary layer has been experimentally investigated. The experiments have been conducted for local Görtler numbers up to 150. Heat transfer measurements have been performed for a flat plate with nearly uniform heat flux applied to the surface, where the temperature was measured by the thermochromic liquid crystal method. The results indicate that heat transfer is enhanced when Coriolis force acts towards the wall, i.e., on the pressure surface. The velocity measurements under equivalent conditions show that Coriolis instability induces counter-rotating longitudinal vortices which augment the lateral transport of the fluid on the pressure surface. On the other hand, the heat transfer on the suction surface remains at the same level as compared to the case without system rotation. As a consequence, the heat transfer coefficient on the pressure surface is 1.8 times higher than that measured on the suction surface when averaged over the measured surface.     

Wake layer in a thermal turbulent boundary layer with pressure gradient

The open equations of thermal turbulent boundary layer subjected to pressure gradient have been analysed by method of matched asymptotic expansions at large Reynolds number. The flow is divided into outer wake layer and inner wall layer. The asymptotic expansions are matched by Millikan-Kolmogorov hypothesis. The temperature profile in overlap region yields composite law which reduce to log. law for moderate pressure gradient and inverse half power law for strong adverse pressure gradient. In case of a shallow thermal wake, the matching result of outer wake layer reduces to composite temperature defect law, which is more general than the classical log. law. The comparison of data for thermal boundary layer with strong adverse pressure gradient is also considered. Received on 26 May 1998     

Heat and mass exchange in a turbulent boundary layer on a disintegrating plate surface

Heat- and mass-exchange processes in a turbulent boundary layer on a disintegrating surface during aerodynamic heating are investigated. The flow in the layer is considered frozen, and the gas composition is determined only by the reactions on the disintegration surface. The problem is solved under the usual assumptions for turbulent-boundary-layer theory. Computation of the disintegration of an asbestos-textolite surface is presented as an example.     

Bubble migration in a turbulent boundary layer

The wall void peaking distribution observed in an upward turbulent bubbly boundary layer along a flat plate is generated by bubbles that move towards the plate, come into contact with the wall and then slide along it. This transverse ‘migration’ has been studied using flow visualization, high speed video and particle tracking techniques to measure the trajectories of mono-disperse air bubbles at very low void fractions. Investigations have been performed at four Reynolds numbers in the range 280 < Re_θ < 3000, covering both the laminar and turbulent regimes, with mono-disperse bubbles of mean equivalent diameter between 2 mm and 6 mm. Lagrangian statistics calculated from hundreds of trajectories show that the migration only occurs in the turbulent regime and for bubble diameters below some critical value: 3.5 mm < d_eqcrit < 4 mm. Above this size (We > 3), the interface deformation is such that bubbles do not remain at the wall, even when they are released at the surface. Also, bubble migration is shown to be non-systematic, to have a non-deterministic character in the sense that trajectories differ significantly, to increase with Reynolds number and to take place on a short time scale. A series of experiments with isolated bubbles demonstrates that these results are not influenced by bubble–bubble interactions and confirm that two-way coupling in the flow is limited. Flow visualizations show that the migration originates with the capture of bubbles inside the large turbulent structures of the boundary layer (‘bulges’). The bubbles begin to move towards the wall as they cross these structures, and the point at which they reach the wall is strongly correlated with the position of the deep ‘valleys’ which separate the turbulent ‘bulges’. The analysis of the mean Lagrangian trajectories of migrating bubbles confirms these observations. Firstly, the average time of migration calculated from these trajectories coincides with the mean transit time of the bubbles across the structures. Secondly, once the trajectories have been scaled by this transit time and the boundary layer thickness δ, they all have the same form in the region y/δ < 0.4, independent of the Reynolds number.     

Kinematics of a turbulent boundary layer

In the article an attempt is made, within the framework of the Navier-Stokes equations, to describe the field of the instantaneous velocities of a liquid in the region of a turbulent flow near the wall. It is assumed that the velocities of the liquid are determined by the field of the eddies arising in regions of ejections under the action of pressure pulses in the region near the wall.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 34–40, September–October, 1973.     

Low-Reynolds-number effects in a turbulent boundary layer

Low-Reynolds-number effects in a zero pressure gradient turbulent boundary layer have been investigated using a two-component LDV system. The momentum thickness Reynolds number R is in the range 400 to 1320. The wall shear stress is determined from the mean velocity gradient close to the wall, allowing scaling on wall variables of the inner region of the layer to be examined unambiguously. The results indicate that, for the present R range, this scaling is not appropriate. The effect of R on the Reynolds normal and shear stresses is felt within the sublayer. Outside the buffer layer, the mean velocity is more satisfactorily described by a power-law than by a logarithmic distribution.The support of the Australian Research Council is gratefully acknowledged     

Effect of a pre-attached dynamic section on heat transfer in a turbulent boundary layer,with blowing

This paper gives the results of an investigation of the effect of an initial dynamic section on heat transfer with blowing. The experimental data obtained show that the preattachment of a section can have a considerable effect on the heat transfer. A method is proposed based on the use of the relative laws of heat transfer; in this case, the effect of the initial section is taken into account in terms of the Stanton number at an impermeable surface. The calculation is in satisfactory agreement with the experimental data obtained, as well as with the experimental data of other authors.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 6, pp. 126–131, November–December, 1971.     

Large-eddy simulation of a turbulent boundary layer with blowing

The modifications of a turbulent boundary layer induced by blowing through a porous plate were investigated using large-eddy simulation. The Reynolds number (based on the length of the plate) of the main flow was about 850000. Large-eddy simulations of such a boundary layer needs a turbulent inflow condition. After a review of available turbulent inflow, we describe in details the condition we developed, which consisted of recycling the velocity fluctuations. Then we show the necessity for this inflow to be non-stationary and to be three dimensional with respect to the mass conservation equation. If these properties are not achieved, we found that the velocity fluctuations do not grow as expected along the domain. Finally, the results of simulations of the boundary layer submitted to blowing are compared with experimental measurements. The good agreement obtained validate our turbulent inflow conditions and also the blowing model used. PACS 47.27.Eq, 47.27.Te, 44.20.+b