Effects Of Variable Fluid Properties Research Articles

The Blasius and Sakiadis flow with variable fluid properties

A theoretical study of the effect of variable fluid properties on the classical Blasius and Sakiadis flow is presented in this paper. The investigation concerns engine oil, water and air taking into account the variation of their physical properties with temperature. The results are obtained with the numerical simulation of the governing equations and cover large temperature differences. Velocity and temperature profiles are presented, as well as values of wall shear stress and wall heat transfer, for a variety of temperatures between the plate and the ambient fluid. It is found that the variation of fluid properties and especially viscosity have a strong influence on the results. The results of oil and water are, in general, similar and are generalized to liquids whereas air results are different and are generalized to gases. Except of the new results found in the present work some inaccurate results existing in the literature have been identified.

The effects of variable fluid properties and thermocapillarity on the flow of a thin film on an unsteady stretching sheet

The effects of variable viscosity, variable thermal conductivity and thermocapillarity on the flow and heat transfer in a laminar liquid film on a horizontal stretching sheet is analyzed. Using a similarity transformation the governing time dependent boundary layer equations for momentum and thermal energy are reduced to a set of coupled ordinary differential equations. The resulting five-parameter problem is solved numerically for some representative value of the parameters. It is shown that the film thickness increases with the increase in viscosity of the fluid. In other words viscosity resists film thinning. Further it is shown that more heat flows out of the liquid through the stretching surface when conductivity increases with temperature than that for the case when conductivity decreases with temperature.

Non-uniform slot injection (suction) into water boundary layer flow past yawed cylinder

The non-uniform slot injection (suction) into water boundary layer flow over a yawed infinite circular cylinder is analyzed including the temperature-dependent viscosity and Prandtl number. The difficulties arising at the starting point of the streamwise coordinate, at the edges of the slot and at the point of separation are overcome by applying the implicit finite difference scheme in combination with the quasilinearization technique. The results indicate that the separation can be delayed by non-uniform slot suction and also by moving the slot downstream but effect of non-uniform slot injection is just the opposite. Further, the effect of variable fluid properties is to move the point of separation downstream but yaw angle has very little affect on the location of the point of separation.

Non-uniform slot injection (suction) into steady laminar water boundary layer flow over a rotating sphere

The influence of non-uniform slot injection (suction) into steady axi-symmetric laminar incompressible boundary layer flows with temperature dependent viscosity and Prandtl number has been examined from the origin of the streamwise coordinate to the exact point of separation. The difficulties in obtaining the non-similar solutions at the origin of the streamwise coordinate, at the edges of the slot and at the point of separation have been overcome by applying an implicit finite difference scheme with the quasilinearization technique and an appropriate selection of finer step size along the streamwise coordinate. The results indicate that the separation can be delayed by non-uniform slot suction and also by moving the slot downstream but the effect of non-uniform slot injection is just the opposite. Further, the effect of variable fluid properties is to move the point of separation downstream but rotation parameter has the reverse effect.

Radial Flow Between Parallel Discs With Downward Center Flow

Radial Flow Between Parallel Discs With Downward Center Flow

Effect of variable fluid properties on impingement heat transfer with submerged circular jets

Effect of variable fluid properties on impingement heat transfer with submerged circular jets

NUMERICAL METHOD FOR LAMINAR CONVECTION IN A CONCENTRIC VERTICAL ANNULAR DUCT WITH VARIABLE PROPERTIES

An implicit finite-difference method is developed to study the effect of variable fluid properties on laminar convection for upflow of air in the entrance region of a heated vertical concentric annulus. Fluid properties are assumed to vary with temperature according to power-law relations. Computations have been made with a radius ratio of 0.2S, and typical results are obtained for pure forced convection with air as the working fluid. The walls of the annulus are subjected to UHF or UWT boundary conditions. The effects of variation of air properties on the local Nusselt number and friction factor are found to be slight at locations close to the channel entrance and far downstream from it, but the effects are shown to be noticeable in locations that are in between. Fluid property variations can cause a reduction of the ratio of the wall and bulk temperatures by about 6%. It is shown that in UWT, the Nusselt numbers for constant and variable properties do not necessarily merge in the entrance region.

Effects of variable fluid properties and boundary conditions on thermal convection

The onset of convection in a fluid layer heated from below is considered. The fluid properties are allowed to vary with temperature according to a linear relation. The fluid layer is bounded by two horizontal rigid plates, and the lower boundary is considered a perfect thermal conductor whereas the upper one is taken with a finite thermal conductivity. With these asymmetric boundary conditions linear variations in density, thermal conductivity and specific heat yield first-order corrections to the critical value of the Rayleigh number whereas linear variations in viscosity and thermal expansion coefficient give rise to second-order corrections to the critical Rayleigh number. The greatest effect is due to thermal conductivity variation. When the boundary conditions are symmetric the corrections are second-order.

EFFECTS OF VARIABLE PROPERTIES AND INTERNAL HEAT GENERATION ON NATURAL CONVECTION AT A HEATED VERTICAL PLATE IN AIR

Numerical results are presented for the transient and steady-state velocity field, temperature field, and heat transfer characteristics. These results were obtained by solving the partial differential equations describing the conservation of mass, momentum, and energy by an explicit finite-difference method in time-dependent form. Two cases were studied, namely the variable fluid property (VFP) case and the constant fluid property (CFP) case. Values of the velocity components u, v (absolute) and the temperature 8 in the VFP case are larger away from the plate and smaller near the plate than those in the CFP case. Further, the velocity and temperature fields are more prominent in the presence of heat sources than in their absence. Numerical results indicate that the wall heat transfer coefficient in the VFP case is about 30% higher than in the CFP case. This warrants further analyses of the effect of variable fluid properties on the flow and heat transfer characteristics. Another useful result is that the ...

The effect of variable fluid properties on the free convective flow of air/water confined between two parallel vertical walls

The title problem has been considered for detailed analysis in view of its varied physical applications taking into account the temperature-dependent fluid properties. The governing equations of the problem have been reduced to two coupled nonlinear ordinary differential equations, which have been solved numerically by the Runge-Kutta method as modified by Gill. Several qualitatively interesting behaviours of the flow and heat transfer characteristics have been pointed out after making a comparative study of the VFP-results with the CFP-counterparts and some of them are presented in the Conclusion-section, which results warrant further study of the effect of variable fluid properties on the flow and heat transfer characteristics.

The Effect of Variable Fluid Properties on the Sensitivity of MHD Flowmeters

For ducts operated as MHD flowmeters, it is demonstrated that the sensitivities are substantially affected by variable fluid viscosity and electrical conductivity. The analysis considers a mathematical model based on fully developed laminar MHD flows with constant heat flux boundary conditions. By considering the pertinent equations in dimensionless form and assuming a power law dependency of the fluid properties on temperature, flowmeter sensitivities are calculated as functions of the Hartmann number, duct aspect ratio, and the magnitude of the property variations. It is shown that, even for small property variations, the flowmeter sensitivities deviate significantly from constant property values. For larger variations, they can be as much as 60% below ideality for heated walls and 30% above for cooled walls.

Theoretical results for variable property, laminar boundary layers in water with adverse pressure gradients

Numerical solutions of the two-dimensional laminar boundary-layer equations for flows with heat transfer and adverse pressure gradient are presented for water. Experimental values (in the range 0–100°C) for the viscosity, conductivity, specific heat and density are used. On introducing the Howarth-Dorodnitsyn and Görtier transformations the boundary-layer equations are put in a form suitable for numerical work. The resulting pair of coupled partial differential equations are solved, using the implicit method of Hartree and Womersley, first for the flow past a flat plate with main stream velocity u m = u 0 ( 1— x 8c ) and secondly for the potential flow past a circular cylinder with main stream velocity u m = u 0 sin ( x c ) . Boundary-layer characteristics such as displacement and momentum thicknesses, skin friction and heat-transfer coefficients are evaluated up to the point of separation. The effect of variable fluid properties is to move the position of separation either upstream or downstream of the known incompressible isothermal flow location depending on whether the wall is cooled or heated. For example, with main stream velocity u m = u 0 ( 1 − x 8c ) the numerical evidence obtained indicates that separation is likely to occur at approximately x s c = 0–959 ( Pr m Pr w 1 2 . In the case u m = u 0 sin ( x/ c) the approximate location is given by x sc = π 2 + 0·25 ( Pr m Pr w 1 5 . Here Pr w and Pr m are, respectively, the wall and main stream Prandtl numbers.

Effects of Surface Curvature and Property Variation on Cellular Convection

For a fluid layer heated from below, a calculation is made of the nonlinear effect of surface curvature which turns out to be of the same type as the non-Boussinesq effect. Results are given for the effects of variable fluid properties.

Poiseuille Flow With Variable Fluid Properties

Flow of a fluid through a parallel channel is one of the simplest types of flow. However, the equations of flow and energy are far from simple and can only be solved in a closed form in the simplest cases when nonlinear effects such as the inertia and convective terms can be neglected (i.e., for zero Reynolds number) and when the energy and momentum equations are uncoupled. A numerical iterative method is described in which the coupled momentum and energy equations are solved when the viscosity, thermal conductivity and specific heat are functions of temperature, and the density a function of temperature and pressure; inertia terms are retained in the momentum equation and the convective terms, compression work term and the predominant dissipation term retained in the energy equation. Results are obtained for a variety of boundary temperatures up to about 2400 deg F and the effect of variable fluid properties and various terms in the energy and momentum equation are shown.

Combustion in the turbulent boundary layer on a vaporizing surface

A theoretical treatment of the turbulent boundary layer with heat transfer, mass transfer, and chemical reactions is presented. The analysis differs from earlier ones, primarily in the way the Spalding mass-transfer number B is related to the thermodynamic characteristics of the flow and in the treatment of the heat-transfer “blocking effect” caused by wall mass addition. The dual role of the mass-transfer number as a similarity parameter of the boundarylayer flow, and as a thermochemical parameter characterizing the reactants, is discussed. The Howarth-Dorodnitsyn coordinate transformation accounts for the effect of variable fluid properties, and, in the present treatment, a relatively easy measurement of the boundarylayer thickness serves to define the reference state. The analysis provides good agreement with experimentally observed heat and mass-transfer rates even at relatively large values of B , where earlier analyses using a “thin-film” approximation are not very accurate. Diffusion-limited turbulent boundary-layer combustion calls for certain modifications in the classicaldiffusion-flame model. The analysis shows that the flow field of the turbulent boundary layer may substantially affect the mixture ratio of the flame. Data relevant to flame position and stoichiometry are cited. It is found that the theory and measurements are qualitatively entirely consistent, and the quantitative agreement is remarkably good. For example, both the theory and the data indicate that the oxidizer-fuel ratio of a Plexiglasoxygen flame approaches a value of about 1.4 at high Reynolds Numbers, whereas the stoichiometric ratio is 1.92. More complete experimental information will be required to establish clearly the quantitative limitations of the theory, but the available data support the general conclusion that the combustion mixture ratio depends to a significant degree on the Reynolds Number of the flow.

Variable-property, constant-property, and entrance-region heat transfer results for turbulent flow of water and oil in a circular tube

Measurements have been made of the fully developed and entrance-region heat transfer characteristics of a hydrodynamically-developed flow of oil and water in an electrically-heated tube. The Prandtl number range was 3–75. The effect of variable fluid properties was isolated by varying the local wall-to-bulk temperature difference at fixed values of the local Reynolds and Prandtl numbers. It was found that both the Sieder—Tate and the Colburn corrections overestimated the effect of variable fluid properties. Constant-property heat transfer results were deduced by extrapolating the measured heat transfer coefficients to the condition of zero wall-to-bulk temperature difference. Except at low Reynolds numbers for the oil tests (Pr = 48 and 75), the constant-property results were underestimated by the Dittus-Boelter equation Nu = 0·023 Re 0·8 Pr 0·4. Local heat transfer coefficients were measured in the thermal entrance region. The thermal entrance length, defined in terms of a 5 per cent approach to fully developed conditions, ranged from 1·25 to 3·75 diameters depending upon the Reynolds and Prandtl numbers. Analytical results, computed as an extension of an earlier investigation by S parrow et al. are presented for a wide range of Reynolds and Prandtl numbers.

Investigation of Turbulent Flow and Heat Transfer in Smooth Tubes, Including the Effects of Variable Fluid Properties

Abstract Equations are derived for the prediction of radial velocity distributions for fully developed turbulent flow in smooth tubes both with and without heat transfer. The analysis results in an equation which represents both the conventional buffer layer and the laminar layer. In order to check the analysis and determine values for the constants appearing in the equations, tests were conducted to determine fully developed velocity distributions for air flowing without heat transfer in a smooth tube. The results are correlated by using conventional velocity and distance parameters, and agree closely with those of Nikuradse and other investigators. The analysis is extended to include flow with heat transfer, and equations for this case are obtained for Prandtl number of 1 in which the effects of the variation of fluid properties due to temperature variation across the tube were considered. By use of the equations for velocity and temperature distributions, relations are also obtained among Nusselt number, Reynolds number, and friction factor for a fluid with a Prandtl number of 1 for the case where the variation of fluid properties across the tube is large. The trends predicted analytically were found to be similar to those determined experimentally by measurement of average surface heat-transfer coefficients and friction factors of air flowing in tubes.