Variable Thermal Conductivity Research Articles

Effects of variable thermal conductivity and curvature parameter on the peristalsis of hybrid nanofluid through a curved channel with curvature dependent channel walls

This study focused on the peristaltic motion of hybrid nano-liquid via a curved geometry with curvature-dependent channel walls. The hybrid nanofluid is composed of alumina (Al2O3) nanoparticles mixed with multi-wall carbon nanotubes (MWCNTs) and suspended in thermal oil. This combination is based on an experimental examination conducted by Asadi et al. [58]. Keeping in view the industrial applications of such flows, the hybrid nanofluid thermal conductivity is considered to vary with temperature. Furthermore, heat sink/source is also taken into account in the energy equation. Lubrication approximation is utilized to simplify the flow problem. The resulting model is treated using a numerical scheme NDSolve based on the shooting method (“a built-in command in Mathematica”). The primary focus remains on analyzing the impacts of the curvature parameter, flow rate, and variable thermal conductivity parameter. Outcomes indicate that there is a critical “flow rate” value for such a type of channel. The thermal transfer rate increases with an improvement in the values of the curvature parameter. Furthermore, graphical results reveal that hybrid nanofluid’s temperature decreases for higher values of flow rate, while it rises with heat generation/absorption parameter. It is observed that temperature of the hybrid nanofluid increases as the values of curvature parameter improve. A higher flow rate reduces the rate of heat transfer. It is also noted that velocity profile remains constant at a flow rate value of 7.40. Moreover, the variable thermal conductivity has a remarkable impact on temperature profile.

Variable Thermal Conductivity Metamaterials Applied to Passive Thermal Control of Satellites

Abstract Active materials like the proposed variable thermal conductivity metamaterial enable new thermal designs and low-cost, low-power, passive thermal control. Thermal control of satellites conventionally requires active thermal control systems that are expensive, large, inefficient, energy-intensive, and unavailable for CubeSats. The high-temperature operation case is the thermal system’s primary design consideration for CubeSats. The thermal path is designed to reject as much heat as possible to ensure the system does not overheat. In other cases, such as during a power anomaly, the oversized thermal path results in rapid cooling, culminating in mission failure due to thermal limits on the electronics or batteries. Improving the thermal control of CubeSats can enable new thermally challenging missions, increase satellite longevity, and increase mission success rate by controlling the dynamic thermal environment. The materials available for thermal management are inherently limited, but new engineered materials provide unique opportunities to change how satellites adapt to thermal loads. This paper investigates using an adaptive metamaterial designed to passively change its thermal conductivity as a function of temperature to meet the needs of the satellite. The thermal performance of a CubeSat is evaluated with a variable thermal conductivity metamaterial located in the critical thermal path from the satellite to the radiator. The system’s performance using two metamaterial configurations is compared to a baseline copper thermal path. Multiple satellite thermal operation cases are investigated to determine the operation ranges, and the metamaterial’s performance in various conditions is quantified.

Magneto-Thermo Heat Transfer of a Chemically Reactive and Viscous Dissipative Casson Nanofluid Thin Film Over an Unsteady Stretching Surface with Variable Thermal Conductivity

This paper addressed unsteady magnetohydrodynamic flow and heat transfer of an incompressible Casson nanofluid thin film past a stretching sheet by considering the features of thermal radiation, chemical reaction, and viscous dissipation. The problem is modeled mathematically, and the governing basic equations are brought into nonlinear ordinary differential equations by utilizing appropriate similarity transformations. Then the transformed equations are then solved numerically by using the bvp4c solver. The influences of pertinent physical variables are performed on velocity, temperature gradient, and nanoparticle concentration gradient profiles. It is seen that the profile of the nanoparticle concentration gradient enhances by increasing the values of the Schmidt number, whereas the opposite trends are observed by increasing the values of the thermophoresis parameter. It is also analyzed that by increasing the values of the thermophoresis parameter, there is an increase in the profiles of the temperature and concentration distributions. The computed results are obtained by giving main consideration to the convergence process and comparing them with the results existing in the literature.

Effects of Temperature-Dependent Conductivity and Magnetic Field on the Radiated Carreau Nanofluid Flow and Entropy Generation

This investigation is related to this study of entropy generation during Carreau nanofluid flow under variable thermal conductivity conditions. The heat and mass transfer phenomena are observed in the presence of thermal radiation and activation energy. The flow is induced by a porous stretching surface. Appropriate variables are used in order to simplify the problem into dimensionless form. The numerical simulations are performed by using the shooting technique. The physical aspects of the problem in view of different flow parameters are reported. It is observed that consideration of variable fluid thermal conductivity enhances heat transfer. An enhancement in heat and mass transfer phenomena is observed with increasing the Weissenberg number. Moreover, entropy generation increases with Weissenberg and Brinkman numbers. Current results present applications in thermal processes, heat exchangers, energy systems, combustion and engine design, chemical processes, refrigeration systems, etc.

High-performance magnetic thermal switch based on MnFe[formula omitted]O[formula omitted]/Ethylene Glycol:Water refrigerant dispersion

The use of magnetic nanoparticles for the remote control of heat transfer in electronic devices can overcome the current limitations of appliance engineering. In this work, we demonstrate that a thermal switch based on a low-cost and stable MnFe2O4/Ethylene Glycol:Water (MFO/EG:W) dispersion can increase the span temperatures as high as 60% in the 0.01–0.60 Hz operating frequency range under the same heat supply for different recipient filling ratios (FR). Under the optimum condition of FR = 80%, the efficiency of our new MFO/EG:W colloidal dispersion is twice the obtained for the commercial Fe3O4/paraffin oil fluid. From numerical calculation, we demonstrate that the improved heat exchange efficiency relates to the three-steps effective thermal conductivity variation during operation, expanding the contact time between the heat and cold sources. Thus, the combination of an EG:W refrigerant solution and superparamagnetic MFO nanoparticles with high saturation magnetization allows their use for heat management control of electronic systems for long operation periods.

Experimental Study of the Dimensional and Hygrothermal Properties of Hemp Concrete under Accelerated Aging

In this article, the functional properties of hemp concrete are studied. Hemp concrete stands to reduce the carbon impact and improve the energy consumption of houses. Hence, numerous properties are measured: mass and dimension (volume) variations are found, as is the variability in hygrothermal properties (density, thermal conductivity, heat capacity, moisture buffer value, and water vapor permeability). This entry proposes three different characterization campaigns. The first is a short introduction to the spatial variability in thermal conductivity; the second is dedicated to the study of univariate variations in the mass, volume, and hygrothermal properties of hemp concrete samples. The last one tackles the aging evolution of the properties characterized during the second campaign, in which the samples follow several aging protocols, including exposure to outdoor conditions, the application of immersion-drying cycles, and the application of freeze–thaw cycles. A set of samples is kept under control conditions to allow for comparison. As the main result, spatial variability was found in the material. This is related to the random manufacturing variability or the spatial position regarding the height of the manufactured element. A high univariate variability is found across hemp concrete samples. Moreover, the storage of samples under stable reference conditions implies very little change in the studied materials’ properties, whereas all accelerated aging protocols implied major changes of properties. In particular, we observed an evolution of the thermal conductivity of the samples kept under control conditions for 4 months, with the thermal conductivity ranging from −2.7% to +6.3% with a mean evolution of +1.22%. We observed an increase in the same property, ranging from +2.7% to +18.3%, with a mean of +9.0% for samples kept for 4 months under natural outdoor conditions, an increase ranging from +7.3% to +23.6% with a mean of +15.2% for samples that had undergone 20 cycles of immersion-drying, and an evolution of this property ranging from −5.6% to +12.3% for samples that had undergone 20 freeze–thaw cycles.

Heat extraction calculations for deep coaxial borehole heat exchangers: matrix analytical approach

SUMMARY Deep boreholes represent a source of clean energy. Therefore, effective calculations of potential extraction of heat from boreholes for realistic models of the Earth’s crust with variable thermal conductivity and diffusivity are needed. We deal with heat extraction in a quasi-steady state from coaxial boreholes where downward and upward flows of pumped fluid (water) are separated by an inner pipe and connected only at the bottom. We first obtain theoretical estimates of heat extraction for a thermally isolated inner pipe and a model of the ground with constant thermal diffusivity and conductivity. Then, we develop a new analytical matrix method for a general layered ground model that enables us to include depth-dependent ground properties as well as heat exchange between the downward and upward flows of fluid in the borehole. Our straightforward and fast approach is thus suitable for various parametric studies or as a tool for benchmarks of numerical software. A key role in heat extraction from coaxial boreholes is played by the inner-pipe thermal resistance. We apply our method to the parametric study showing the dependence of pumped water temperature and total heat extraction from the borehole on realistic borehole geometries under different amounts of water pumping. The calculations are performed for a 3 km deep borehole as the representative of present deep boreholes used for extraction of geothermal energy and for a 10 km deep borehole. Drilling of such a superdeep borehole has just started in China and our results demonstrate potential limits of geothermal energy extraction from such great depths.

Determination of one unknown coefficient in a two-phase free boundary problem in an angular domain with variable thermal conductivity and specific heat

Two different two-phase free boundary Stefan problems in an angular domain with temperature-dependent thermal coefficients are considered. Analytical similarity solutions are obtained imposing a Dirichlet or Neumann type boundary condition, respectively, by solving functional problems. Moreover, formulas are obtained for the determination of one unknown thermal coefficient in the overspecified problem that consists in adding a Neumann condition to the problem with a Dirichlet one if and only if some restrictions on data are verified.

Influence of variable thermal conductivity and mixed convection on hybrid nanofluid (SWCNT + MWCNT/H2O) flow over an exponentially elongated sheet with slip conditions

Influence of variable thermal conductivity and mixed convection on hybrid nanofluid (SWCNT + MWCNT/H2O) flow over an exponentially elongated sheet with slip conditions

Chemically reactive and thin film flow analysis of cross nano-liquid over a moving surface

Vapour generalized liquids hydrodynamics plays a pivotal role in the absorption columns or design of distillation using structured packing since they indomitable the fluid dynamics restrictions of the columns and control rates of mass balance which is the main purpose of this particular work. Particularly, a first attempt is made regarding this work to explore an investigation for time-dependent thin film flow of Cross nano-liquid over a moving surface. Additionally, some physical aspects during this illustration in terms of heat absorption/generation, thermal radiation, magnetic field, non-constant thermal conductivity, and activation energy with chemical reaction are studied. Some of the suitable similarity variables are introduced to convert the leading problem in the form of ordinary differential equations and then the transformed problem is approximated by one of the built-in collocation methods by using MATLAB. Very interesting results in terms of monotonic reduction in the film thickness for various uplifting values of unsteadiness and magnetic parameters and vice versa for the Wessenberg number are found. Moreover, temperature and concentration are concluded as enhancement functions of variable thermal conductivity and activation energy, respectively. The last comparative work is presented to validate the entire approach and where the method shows an excellent correlation with the previously published results.

Investigation of composed charged particles with suspension of ternary hybrid nanoparticles in 3D-power law model computed by Galerkin algorithm

Transport of heat visualizes a vital role in many industrial developments. Current study is discussing the role of Joule heating, solar thermal radiation, heat generation/absorption, reactions (homogeneous and heterogeneous) with variable thermal conductivity on partially ionized power law material past over a three-dimensional heated stretched surface. The power law model is assumed to have the thermal characteristics of ethylene glycol material. The phenomenon of momentum and energy balance is derived in Cartesian coordinates and developed PD (partial differential)-equations. Swimming pools, solar collectors, food processing, electronic gadgets, cooling systems, magnetic field measurement, computer chips, thermal enhancement, semiconductor characterization, nuclear fusion research and other physical applications are examples of ongoing research. The principle of boundary layer simplified the governing problem. The complex coupled PD (partial differential)-equations have been converted into ordinary differential equations OD (ordinary differential)-equations by using appropriate similarity transformation. The converted boundary value problem is complex and highly nonlinear which does not have the exact solution. The approximate solution is computed numerically via finite element scheme (FES) which is coded in MAPLE 18.0 symbolic package. The convergence of the scheme is established through grid independent survey and the solution is plotted against numerous involved parameters. Thermal performance produced by Si{O}_{2}-Ti{O}_{2}-{Al}_{2}{O}_{3}/EG is higher thermal performance produced by Si{O}_{2}-Ti{O}_{2}/EG. Ion slip and Hall forces are responsible for generating Joule heating mechanism that is responsible for reduction of velocity curve and generating shear stresses. Hence, tangential stresses are declined against increasing {beta }_{i} and {beta }_{e}.

Experimental research on the thermal conductivity of unsaturated rocks in geothermal engineering

The thermal conductivity of rock is an essential thermophysical parameter in geothermal engineering. Experiments have shown that when rock changes from an unsaturated to a saturated state, thermal conductivity can be increased by 50% at most. In this paper, the variation of thermal conductivity with saturation of 101 samples of six different rock types from Songliao Basin, Gonghe Basin, and Ordos Basin in China was measured in the laboratory. Correlations between saturation, porosity, internal structural characteristics of rocks and thermal conductivity were investigated. The results show that the effect of saturation on the thermal conductivity of rock increases exponentially with porosity. Based on three classical models and the relationship between porosity, saturation, and thermal conductivity, improved models and a fitted model of thermal conductivity are proposed. By comparing the improved models to the fitted model, the applicable scope of each model is obtained. The errors of each model are analyzed in detail. The average MAE value of the improved models is 0.405 and the average RMSE value is 1.207 over the applicable model scopes. The R2 of the fitted model is 0.94. Compared with experimental data from literature, MAE values of the improved models and the fitted model are 0.040 and 0.003 respectively. Unsaturated rock with a single fracture heat transfer model is established by simulation. A thermal-Hydrological coupling model is used to simulate seepage heat transfer through fracture. The effect of variable thermal conductivity simulation is the best with the experimental value, and the MAPE value was 8.3%. This study fills a gap in available models of unsaturated thermal conductivity of rock, and provide theoretical support for the accurate calculation of heat transfer efficiencies in related geothermal engineering applications.

Variable fluid properties and concentration species analysis of a chemically reactive flow of micropolar fluid between two plates in a rotating frame with cross diffusion theory

The importance of this study is that the physical properties no longer to be consider as a constant, therefore dynamics of the mechanism of temperature dependent fluid properties and species concentration variation on the magnetized flow of a micropolar fluid in a rotating frame is studied in this article. The flow is considered between two porous plates with slip boundary conditions on the lower plate. The cross-diffusion theory on the thermal and solutal transportation investigation is presented by the help of non-uniflorm source / sink and nonlinear chemical reaction. The equations of motion and energy are reduced into the set of couple nonlinear ordinary differential equations by the usage of appropriative transformation. These couple nonlinear equations are tackled numerically with the utilization of BVP4C function on MATLAB. The graphical and numerical outcomes are obtained with physical discussion for various parameter. The study concludes that as the material parameter approaches to zero, the flow of micropolar fluids approaches to the Newtonian fluids. It is worth mentioning that the variable viscosity reduces the fluid velocity near the lower plate due to domination of viscous forces, while fluid temperature shows rising trend by the improvement of variable thermal conductivity because of the more heat production. Further, it is noticed that the Soret and Dufour impacts precludes the solutal deposition and coolant the bounding at surface, as a result fluid temperature and concentration enhances.

Entropy generation on MHD pulsatile flow of third grade hybrid nanofluid in a vertical porous channel with nonuniform heat source/sink, variable viscosity, thermal conductivity, and Joule heating: A numerical study

This investigation inspects the entropy generation of magnetohydrodynamic pulsating third grade hybrid nanofluid flow between two vertical porous walls under the influence of nonuniform heat generation/absorption. Variable viscosity, variable thermal conductivity, Joule heating, thermal radiation, and slip effects are considered. Blood is taken as base fluid (third grade), iron oxide or magnetite and titanium dioxide are as nanoparticles. The nondimensional quantities are utilized to attain the dimensionless flow governing nonlinear partial differential equations (PDEs) and then the perturbation technique is employed to obtain the set of nonlinear ordinary differential equations (ODEs) from PDEs. The Runge-Kutta 4th order technique along with shooting procedure is utilized to get solutions for the set of ODEs. The variations in different physical quantities on velocity, temperature, heat transfer rate, entropy generation, and Bejan number are depicted via graphs. The velocity enhances for the higher values of Grashof number. Accelerating space and temperature-dependent heat source/sink coefficient accelerates the temperature. The presence of variable viscosity and thermal conductivity intensifies the temperature when compared to their absence. A rise in radiation parameter accelerates the entropy and Bejan number. An increment in Eckert number and radiation parameter increases the heat transfer rate. Intensifying Hartmann number reduces the skin friction.

Application of Legendre wavelet collocation method to the analysis of poro-thermoelastic coupling with variable thermal conductivity

Application of Legendre wavelet collocation method to the analysis of poro-thermoelastic coupling with variable thermal conductivity

Significance of binary chemical reaction with activation energy in magneto-bioconvection flow of a Powell Eyring nanofluid past an inclined stretching sheet by considering temperature-dependent viscosity and thermal conductivity

ABSTRACT In this paper, an unsteady magneto-bioconvection flow of Powell Eyring nanofluid over an inclined stretching sheet in the presence of variable viscosity and thermal conductivity with multiple slip effects has been investigated numerically. The binary chemical reaction with activation energy, viscous dissipation and nonlinear thermal radiation effects are included in this study. The fluid viscosity and the thermal conductivity are assumed to be a linear function of temperature. The basic governing equations are solved numerically by the Runge-Kutta Felhberg method after using similarity transformation. The impact of bioconvection and important physical parameters on profiles of velocity and temperature of nanofluid and nanoparticles concentration, the density of motile microorganisms are analyzed graphically. Findings reveal that the velocity profile decreased by increasing the Powell Eyring fluid parameter values, whereas the temperature distribution depicts the opposite trend. The temperature profile increased with an increment of thermal conductivity parameter, whereas the opposite trends are found for Eckert number. The trend of the graph of temperature profile and nanoparticle concentration profile declined with an increment of the temperature slip parameter. Also, the nanoparticle concentration profile and density of microorganism profile decreased with an increment of activation energy parameter and chemical reaction parameter. The scope of the present investigation can be related to the advanced nanomechanical bioconvection energy conversion devices and bio-nanocoolant systems, among other relevant things.

Evaluation of bioconvection for sinusoidally moving Jeffrey nanoparticles in view of temperature dependent thermal conductivity and Cattaneo-Christov heat diffusion model

With impressive thermal outcomes, the nanofluids present multidisciplinary applications in the cooling processes, thermal systems, extrusion processes, heat storage devices and many more. The aim of current research is to inspect thermal impact of Jeffrey fluid with tiny particles under the assumptions of variable thermal conductivity. The problem is supported with applications of chemical reaction, activation energy and magnetic force. For heat and mass transfer phenomenon, Cattaneo-Christov diffusion theories have been implemented. The formulated model is solved by using the homotopy analysis method (HAM) with excellent accuracy. The graphical analysis is performed with specified range of parameters like 0.2⩽H⩽0.8, 0.1⩽ϖ⩽1.7, 0.0⩽N⩽1.5, 0.0⩽Π⩽3.1, 0.3⩽γ⩽0.6, 0.6⩽Ψ⩽3.2, 0.5⩽Ω⩽2.0, 0.0⩽Σ⩽1.5, 0.2⩽Nt⩽1.7, 1.0⩽Pr⩽1.9, 0.5⩽Sc⩽1.4,0.3⩽β⩽1.5, 0.1⩽ε⩽1.0, 0.2⩽Nb⩽1.7. The assessment of flow parameters is graphically evaluated. It is observed that both velocity profiles periodically enhance for Deborah number while temperature, microorganisms and concentration distributions decelerate. The greater estimates of variable thermal conductivity and heat generation improve the temperature distribution while conflicting scenario ensures for thermic relaxation constant.

Numerical study of the melting process of spherical phase change material with variable thermal conductivity

Numerical study of the melting process of spherical phase change material with variable thermal conductivity

New approach in the reuse of modified ground tire rubber as thermal and acoustic insulation to be used in civil engineering

The concern for the amount of end-of-life tires generated each year has arisen from constant research directed to their valorisation. Herein we propose a new material, which is constituted by GTR with a binder, as acoustic and also as a thermal insulator for civil engineering. The insulator can also include the fibre mat present in the tire, seldomly considered as a recyclable sub-product. To provide insight into the insulating behaviour of these materials, four mathematical models have been tested and compared with the experimental results of thermal conductivity. The Lewis-Nielsen modelization presented good accuracy with deviations of less than 3%. A statistical analysis has also been conducted on the experimental data showing that the parameter with more effect on thermal conductivity is thickness (differences up to 43%) being particle size, less important (ca 6%). In acoustic properties, different effects can be observed depending on the frequency range, being the density the most relevant. From the mathematical, statistical and experimental analysis can be deduced that good insulation properties would be achieved in materials with: low density, porous; including mat and thick. The effect of these parameters causes variations of thermal conductivity from 0.189 to 0.117 W/m·K and in sound absorption coefficient from 0.06 to 0.6.

Impact of variable thermal conductivity on flow of trihybrid nanofluid over a stretching surface.

The present article describes the impact of variable thermal conductivity on the flow of ternary hybrid nanofluid with cylindrical shape nanoparticles over a stretching surface. Three nanoparticles combine in base fluid polymer. The assumption made will be used to model an equations. Modeled equations are in the form of a system of partial differential equations are difficult to solve can be converted to system of an ordinary differential equations, through resemblance substitutions, and will be solved numerically. Numerical scheme of Runge-Kutta order four is coupled with the shooting method to solve the resulting equations. The graphs in the study illustrate how physical quantities, such as magnetic field, injection/suction, nanoparticles volume fraction, and variable thermal conductivity, affected the velocity, skin friction, temperature, and local Nusselt number. The velocity profiles deflate as the volume fraction rises. While the temperature rises with an increase in the volume fraction of nanoparticles for both injection and suction, the velocity profiles also decline as the injection and suction parameter increases. Furthermore, as the magnetic field increases, the temperature profile rises while the velocity profile falls. The temperature curves increase as thermal conductivity increases. Finally, as the magnetic field is strengthened, the Nusselt number and skin friction decrease. The combination of mathematical modeling, numerical solution techniques, and the analysis of physical quantities contributes to the advancement of knowledge in this ternary hybrid nanofluid.