Effect of Variable Properties on the Flow Past a Needle Moving in a Casson Fluid

Effects of variable air properties on transient natural convection for large temperature differences

The effects of the air variable properties (density, viscosity and thermal conductivity) on the buoyancy-driven flows established in open square cavities are investigated, as well as the influence of the stated boundary conditions at open edges and the employed differencing scheme. Two-dimensional, laminar, transitional and turbulent simulations are obtained, considering both uniform wall temperature and uniform heat flux heating conditions. In transitional and turbulent cases, the low-Reynolds k − ω turbulence model is employed. The average Nusselt number and the dimensionless mass-flow rate have been obtained for a wide and not yet covered range of the Rayleigh number varying from 103 to 1016. The results obtained taking into account variable properties effects are compared with those calculated assuming constant properties and the Boussinesq approximation. For uniform heat flux heating, a correlation for the critical heating parameter above which the burnout phenomenon can be obtained is presented, not reported in previous works. The effects of variable properties on the flow patterns are analyzed.

In this paper, the natural convection heat transfer of Al2O3-EG-water nanofluid in a rectangular cavity which is heated from the bottom and is cooled from the top has been investigated numerically. The governing equations for a Newtonian fluid have been solved numerically with a finite volume approach using the SIMPLER algorithm. The main focus of the current study is on the effects of variable thermophysical properties of nanofluid on the heat transfer in natural convection. The influence of pertinent parameters such as Rayleigh number (Ra=105-107) and volume fractions of nanoparticles (0≤Φ≤0.05) on the heat transfer characteristics of natural convection have been studied. The results have shown that the average Nusselt number is reduced by increasing the volume fraction of nanoparticles. To study the significance of temperature effect on thermophysical properties of nanofluid, the results obtained by using variable properties of nanofluid are compared with those of constant properties.

Experiments measuring film cooling performance are often performed near room temperature over small ranges of driving temperature. For such experiments, fluid properties are nearly constant within the boundary layer and radiative heat transfer is negligible. Consequently, the heat flux to the wall is a linear function of driving temperature. Therefore, the convective heat transfer coefficient and adiabatic wall temperature can be extracted from heat flux measurements at two or more driving temperatures. For large driving temperatures, like those seen in gas turbine engines, significant property variations exist within the boundary layer. In addition, radiative heat transfer becomes sufficiently large such that it can no longer be neglected. As a result, heat flux becomes a non-linear function of driving temperature. Thus, for these high temperature cases, ambient temperature methods utilizing a linear heat flux assumption cannot be employed to characterize the convective heat transfer. The present study experimentally examines the non-linearity of heat flux for large driving temperatures flowing over a flat plate. The results are first used to validate the temperature ratio method presented in a previous study to account for variable properties within a boundary layer. This validation highlighted the need to account for the radiative component of the overall heat transfer. A method is subsequently proposed to account for the effects of both variable properties and radiation simultaneously. Finally, the method is validated with the experimental data. While this methodology was developed in a flat plate rig, it is applicable to any relevant configuration in a hot environment. The method is general and independent of the overall radiative component magnitude and direction. Overall, the technique provides a means of quantifying the impact of both variable properties and the radiative flux on the conductive heat transfer to or from a surface in a single experiment.

PurposeThe purpose of this paper is to evaluate differences between the results of constant property and variable property approaches in solving the problem of Al2O3-water nanofluid heat transfer in an annular microchannel. Also, the effect of nanoparticle diameter on flow and heat transfer characteristics is investigated.Design/methodology/approachThermo-physical properties of the nanofluid including density, specific heat, viscosity and thermal conductivity are assumed to be temperature dependent. Governing equations are descritized using the finite volume method and solved by SIMPLE algorithm.FindingsThe results reveal that the constant property assumption is unable to predict the correct trend of variations along the microchannel for some of the characteristics, especially when the range of temperature change near the wall is considerable. In the fully developed region, constant property solution overestimates the values of shear stress near the walls of the microchannel. In addition, the values of Nusselt numbers are different for the two solutions. Furthermore, a decrease in wall’s shear stress has been observed as a result of increasing nanoparticle size.Originality/valueThis paper reflects that how the friction factor and heat transfer vary along the microchannel in temperature dependent modeling, which is not reflected in the results of constant property approach. To the best of the authors’ knowledge, there is no similar investigation of the effect of nanofluid variable properties with Pr=5 or in annular geometry.

Convective heat transfer from a fluid to a surface is an approximately linear function of driving temperature if the properties within the boundary layer are approximately constant. However, in environments with large driving temperatures like those seen in the hot sections of gas turbine engines, significant property variations exist within the boundary layer. In addition, radiative heat transfer can be a significant contributor to the total heat transfer in a high-temperature environment such that it can not be neglected. As a result, heat transfer to the surface becomes a nonlinear function of driving temperature and the conventional linear heat flux assumption cannot be employed to characterize the convective heat transfer. The present study experimentally examines the nonlinearity of convective heat flux on a zero-pressure-gradient flat plate with large freestream to wall-temperature differences. In addition, the need to account for the radiative component of the overall heat transfer is highlighted. Finally, a method to account for the effects of both variable properties and radiation simultaneously is proposed and demonstrated. Overall, the proposed technique provides the means to quantify the independent contributions of radiative and variable property convective heat transfer to the total conductive heat transfer to or from a surface in a single experiment.

This study investigates the effect of variable thermophysical properties on radiative Casson fluid flow around a stretching cylinder. The governing partial differential equations for momentum and energy are changed into ordinary differential equations with suitable similarity transformations. Our mathematical model analyses the impact of variable thermal conductivity, viscosity, and radiation parameters on the fluid flow system. The resulting coupled nonlinear equations are solved numerically using the Runge-Kutta fourth-order method with shooting technique. The effect of key parameters including Casson fluid parameter, thermal radiation parameter, magnetic parameter, Grashof number, Prandtl number, Eckert number, ohmic heating parameter and Biot number on velocity and temperature profiles is examined. Results indicate that increasing the Casson parameter reduces fluid velocity while increasing the temperature distribution. The thermal boundary layer thickness is seriously affected by the radiation parameter and variable thermal conductivity. In addition, the study reveals that heat transfer rates at the surface increase with higher values of the Biot number. These findings provide valuable insights into heat transfer optimization in industrial applications involving non-Newtonian fluids with radiative effects and variable properties.

“Variable property effects” is a theoretical construct since a real fluid always is subject to variable properties when changes in temperature or pressure occur. Their influence compared to a corresponding situation but with artificially constant properties may be small and thus neglected in a first approximation. Those artificial “constant property results” may then be corrected with respect to the initially neglected effects due to the variability of the fluid properties. This concept assumes small variable property effects and therefore is not applicable when the flow itself is basically generated by a variable property (like natural convection, generated by density variations) or strongly affected by it (like strongly compressible flow, determined by density variations). Therefore we define “variable property effects” as those (small) artificial effects that would be present if fluid properties could change from constant to variable. They have to be added to a solution which is gained under the assumption of constant properties in order to account for the fact that real fluids always have properties which are temperature and pressure dependent. The variable property correction of a constant property solution can be accomplished in different ways. Basically there are three methods which are widely used in this context. They are

In this study, the effects of variable fluid properties on heat transfer in MHD Casson fluid melts over a moving surface in a porous medium in the presence of the radiation are examined. The relevant similarity transformations are used to reduce the governing equations into a system of highly nonlinear ordinary differential equations and those are then solved numerically using the Runge–Kutta–Fehlbergmethod. The effects of different controlling parameters, namely, the Casson parameter,melting and radiation parameters, Prandtl number,magnetic field, porosity, viscosity and the thermal conductivity parameters on flow and heat transfer are investigated. The numerical results for the dimensionless velocity and temperature as well as friction factor and reducedNusselt number are presented graphically and discussed. It is found that the rate of heat transfer increases as the Casson parameter increases.

Effects of variable properties and non-uniform heating on natural convection flows in vertical channels

A direct numerical simulation of water at a supercritical pressure is carried out to study the fundamental characteristics of turbulence and heat transfer in the flow between two parallel plates. The top and bottom plates are differentially heated to achieve stable thermal stratification. Both forced and mixed convection are studied to investigate buoyancy and variable property effects in a stably-stratified horizontal flow. The simulation results show that the variable property effect causes turbulence deterioration and flow laminarization near the cold wall while increasing the turbulence stress intensity near the hot wall. The direct buoyancy effect weakens the turbulence development throughout the channel. The indirect buoyancy effect strongly enhances flow laminarization near the cold wall and it increases turbulence near the hot wall. However, the net buoyancy effect near the hot wall is insignificant due to a cancelling of the competing direct and indirect buoyancy effects. The turbulence behaviour near the hot wall is dominated by the variable property effect. The heat flux transferred through the wall under the condition of fixed wall temperature is strongly reduced by buoyancy, while it is less sensitive to property variation.

Purpose The purpose of this study is to consider the dynamics of Casson–Walters-B alongside gyrotactic microorganisms through the investigation of antibacterial and antiviral mechanisms using silver nanoparticles (AgNPs). The Casson fluid and Walters-B flow from the penetrable plate to the boundary layer (BL) in this analysis. The antiviral and antibacterial mechanisms of AgNPs were separately examined in this study. Design/methodology/approach The physical phenomenon of this problem was analyzed with partial differential equations (PDEs). These PDEs were changed into ordinary differential equations (ODEs) to further explain the significance of pertinent control parameters. The set of equations is solved numerically by implementing the spectral relaxation method (SRM). SRM is a numerical technique that uses the basic techniques of Gauss-Seidel. The SRM first decouples and linearizes the coupled nonlinear set of ODEs. Findings In this finding, it is found that the thermal radiation parameter produces higher temperatures within the BL to cause blockage in viral replications. It is found in this study that the magnetic parameter assisted in disinfection by lowering the antiviral and antibacterial mechanisms within the momentum BL. This is evident from the reduction in the velocity and momentum BL as the Casson and Walters-B parameters increase. Originality/value This paper is unique because it examined the antiviral and antibacterial mechanisms by using AgNPs. Prior to the authors’ understanding, no study of this type was conducted in the past. To the best of the authors’ knowledge, no other study in the past has examined the mechanisms of antiviral and antibacterial separately within the BL. Also, the simultaneous flow of Casson (honey) and Walters-B fluids were considered flowing through the vertical porous plate to the BL.

The effects of variable properties and hall current on steady MHD laminar convective fluid flow due to a porous rotating disk

The effect of variable properties on momentum and heat transfer in a tube with constant wall temperature

The effects of variable thermophysical properties on the ignitability of a single-component fuel droplet in a hot oxidizing environment were investigated by a matched asymptotic analysis of large activation energy. It is shown that the effects of variable thermophysical properties on the ignition are partly via the mass evaporation rate and partly through the thermal driving force for droplet evaporation, beta, which directly influences the ignition Damkoehler number. It was found that the ignition Damkoehler number increased both with an increase of beta and with a decrease in the fuel reaction order. 17 refs.