Characteristics Of Thermal Boundary Layers Research Articles

Thermal boundary layer of laminar flow of dilute polymer solution

The thermal boundary layer flow is a canonical flow with characteristics that are present in most natural and industrial convection flows. An approximate self-similar solution is proposed for the first time for the thermal boundary layer of steady laminar flow of viscoelastic fluids, described by the finitely extensible nonlinear elastic constitutive equation with Peterlin's closure (FENE-P model). This semi-analytical thermal solution is obtained by performing an order of magnitude analysis and ensuing simplifications of the governing equations by assuming that the fluid properties are independent of temperature therefore decoupling the flow governing equations from the energy equation. The effects of viscoelasticity quantified with the Weissenberg number based on the streamwise coordinate (x) (Wix) up to Wix=1 and viscous dissipation (results are presented for Brinkman numbers between -40 and +40) on thermal boundary layer characteristics are investigated comprehensively for both constant wall temperature and constant wall heat flux. At low elasticity levels (Wix<0.01) the solution exhibits a global self-similar behavior in which flow and thermal quantities collapse on the corresponding Newtonian curves, and the polymer characteristics show a unique behavior if adequately normalized. However, by increasing flow elasticity the unique self-similar behavior of the approximate solution is lost, with the elasticity dependent results exhibiting local variations. In addition, the effects of elasticity are intensified by viscous dissipation. For the present study cases, it is observed that elasticity may change Nusselt numbers by more than 8%, and the thermal boundary layer thickens by up to 10%.

Influence of geometrical parameters on convective flows in non-uniformly heated cylindrical fluid layers

Convection in a fluid layer with a free surface in a cylindrical container non-homogeneously heated from below was studied experimentally. The heater has a circular form and was placed in the center of the vessel. Such a system in the presents of rotation is very promising for studying the nature of tropical cyclones. In current paper we considered the influence of different geometric parameters on the dynamics of convection flows. For it two experimental setups of different sizes were used. Measurements were done for fluids with different values of Prandtl number and heating powers. It was shown that the structure and intensity of mean flow producing by a horizontal temperature gradient are defined by Rayleigh number Ra. The basic flow leads to unstable temperature stratification over the heating area and the formation of a system of secondary flows. The formation of secondary flows depends on characteristics of thermal boundary layer and is described by local Rayleigh number Raδ.

Statistical analysis of turbulent thermal free convection over a small heat source in a large enclosed cavity

To investigate natural convection in an enclosed cavity, a rectangular experimental cavity is established. Transient temperature along the central line (l1) and the vertical axis of the longer side (l2) of the heat source is measured by using a T type thermal couple. Thermal boundary layers are presented. The center and border of the hot plate show different characteristics of thermal boundary layers and fluctuation features. The efficiency of energy transformation from large scales to smaller ones enhances with increased height and reaches an acme at the height of 5–8 mm, the juncture of thermal boundary layers. Flow field could be divided into three regions according to the analysis of power spectrum distribution (PSD). Numerical results show that the unsteady characteristics are extremely distinctive where the two thermal plumes initially form (Z/H = 0.2) and converge (Z/H = 0.55) because of edge effect. Thus, two peaks exist in the skewness and kurtosis of temperature.

Heat transfer in the boundary layer with asymptotic favorable pressure gradient

Numerical modeling of Prandtl equations was undertaken in order to investigate into the effect of mainstream acceleration on characteristics of dynamic and thermal boundary layers. Unlike in dynamic boundary layer, nondimensional characteristics of thermal boundary layer proved to be rather conservative to the action of the favorable pressure gradient. It was found that, unlike the skin-friction coefficient and the momentum-thickness Reynolds number, the Stanton number and the energy-thickness Reynolds number both exhibit no saturation with increasing the Kays acceleration parameter. Also, an analysis of the violation of Reynolds analogy by the stream acceleration is given.

A parametrized three-dimensional model for MEMS thermal shear-stress sensors

This paper presents an accurate and efficient model of MEMS thermal shear-stress sensors featuring a thin-film hotwire on a vacuum-isolated dielectric diaphragm. We consider three-dimensional (3-D) heat transfer in sensors operating in constant-temperature mode, and describe sensor response with a functional relationship between dimensionless forms of hotwire power and shear stress. This relationship is parametrized by the diaphragm aspect ratio and two additional dimensionless parameters that represent heat conduction in the hotwire and diaphragm. Closed-form correlations are obtained to represent this relationship, yielding a MEMS sensor model that is highly efficient while retaining the accuracy of three-dimensional heat transfer analysis. The model is compared with experimental data, and the agreement in the total and net hotwire power, the latter being a small second-order quantity induced by the applied shear stress, is respectively within 0.5% and 11% when uncertainties in sensor geometry and material properties are taken into account. The model is then used to elucidate thermal boundary layer characteristics for MEMS sensors, and in particular, quantitatively show that the relatively thick thermal boundary layer renders classical shear-stress sensor theory invalid for MEMS sensors operating in air. The model is also used to systematically study the effects of geometry and material properties on MEMS sensor behavior, yielding insights useful as practical design guidelines.

A Numerical Study of Heat Transfer Mechanisms in Gas–Solids Flows Through Pipes Using a Coupled CFD and DEM Model

A two dimensional numerical model has been developed to simulate heat transfer in gas–solids flows through pipes, in which the gas phase is modelled as a continuum using the Computational Fluid Dynamics (CFD) approach and the solids phase is modelled by the Discrete Element Method (DEM). This allows interactions between gas, particles, and pipe wall to be accounted at the scale of individual particles and convective and conductive heat transfers to be calculated using local gas and solids parameters. The predicted changes to the flow structures and the various heat transfer mechanisms due to the presence of particles were analyzed and compared with other workers' findings. This study has quantitatively demonstrated the crucial effect of particle transverse motion on heat transfers due firstly to the thermal energy transport by rebounding particles and secondly to the modification of the fluid thermal boundary layer characteristics.

Unsteady free convection flow in the stagnation-point region of a three-dimensional body

The unsteady free convection flow in the stagnation-point region of a heated three-dimensional body placed in an ambient fluid is studied under boundary layer approximations. We have considered the case where there is an initial steady state that is perturbed by a step-change in the wall temperature. The non-linear coupled partial differential equations governing the free convection flow are solved numerically using a finite difference scheme. The presented results show the temporal development of the momentum and thermal boundary layer characteristics.

Passive alteration and control of convective heat transfer utilizing alternate porous cavity-block wafers

Passive heat-transfer augmentation or alteration along an external boundary using alternately emplaced porous cavity-block wafers is analyzed in this work. The Brinkman-Forchheimer-extended Darcy Model, which accounts for the effects of impermeable boundary and inertia, is used to characterize the flow field inside the porous region. The present work constitutes an innovative way of altering and control of the flow and heat-transfer characteristics of an external surface. The formulation of the problem shows that flow and heat-transfer characteristics depend on seven dimensionless parameters, namely, the Reynolds number, Darcy number, the Prandtl numbers, inertial parameter, two pertinent geometric parameters, and the number of porous cavity-block obstacles. Solution of the governing equations is carried out using the steam function-vorticity formulation, and an in-depth discussion of the results for various physical interactions between the recirculating flows inside of the cavity and the external flow is presented. Several interesting phenomena such as the interactions between the blowing and displacement effects from the porous blocks and the vortices penetrating into the porous cavities are presented and discussed, and it is shown that altering some parametric values can have significant effects on the external momentum and thermal boundary-layer characteristics. The present investigation forms a pertinent and basic research investigation for altering the skin friction and heat-transfer characteristics of an external surface.

Lateral heterogeneity in the convecting mantle

Thermally induced lateral heterogeneity in the earth's mantle has been studied in terms of idealized two‐dimensional, constant viscosity, numerical models of mantle convection. Steady model solutions of the temperature and velocity fields were analyzed with respect to both the spatial and spectral signatures of their inherent lateral heterogeneity. By examining the changes in spectral signature produced by varying the Rayleigh number, the degree of internal heating, and the depth in the convection cell, it has been possible to identify those features of the spectra of lateral heterogeneity which are most indicative of these variations. Particular attention has been paid to the spectral characteristics of thermal boundary layers within the convecting layer. The spectral analysis of known temperature fields represents the forward problem corresponding to the inverse problem of inferring the thermal structure of the mantle from the spectral components of its lateral heterogeneity as determined by seismic tomography. The forward problem presented here illustrates a new approach in convection modeling which may ultimately provide important diagnostic criteria for the interpretation of recent tomographic data.

Temperature and Velocity Fields in the Separated and Reattached Flow over Blunt Flat Plates

Temperature, velocity, and turbulence intensity distributions were measured in the separated, reattached, and redeveloped regions of two-dimensional air flow over blunt flat plates of various nose shapes. Especially investigated are the nose shape effects upon the flow and thermal boundary layer characteristics. It is found that they are strongly dependent on the nose shape far downstream from the reattachment point.

Impinging jet flames

An experimental investigation has been made of a flame impinging at an angle of 20° on the hearth of a furnace. Measured velocity and temperature profiles in the flame were found to have the same reduced form as profiles obtained for a plane heated impinging jet, by Mathieu. Direct comparison of results obtained in the furnace with those obtained by Mathieu show that when account is taken of differences in initial and boundary conditions then predictions can be made of fluid dynamic and thermal boundary layer characteristics as well as rates of convective heat transfer in impinging flames. Heat transfer measurements show that total heat transfer can be increased by a factor of three by direct impingement of the flame on the furnace hearth. Convective heat transfer, which is of little significance for flames without impingement, amounts to 70 per cent of the total heat transfer in the region of impingement. A twofold increase in the momentum of the flame resulted in a proportional increase in local heat transfer coefficients. The Reynolds number exponent in the Nusselt-Reynolds number relationship was found to be 0·8.