Evaluation of Physicothermal Properties of Solar Thermic Fluids Dispersed with Multiwalled Carbon Nanotubes and Prediction of Data Using Artificial Neural Networks

A review of multiwalled carbon nanotubes as solar thermic fluids and their thermophysical properties is done in this article. The basic fluids were ethylene glycol and water in ratios of 100 : 0, 90 : 10, and 80 : 20. To investigate how surface modification impacts thermophysical properties, three base fluids were combined with surfactant-assisted MWCNTs and oxidized MWCNTs in weight fractions of 0.125, 0.25, and 0.5 percent, respectively. It takes two months to check whether the dispersion stays constant. Thermal conductivity and viscosity measurement were done using heated discs and Anton Paar viscometers. Using oxidized MWCNTs to disperse, the base fluids increased thermal conductivity by 15% to 24%. Surfactant-assisted MWCNTs in nanofluids perform worse than oxidized MWCNTs. The dynamic viscosity of nanofluids is higher than that of basic fluids between 50 and 70°C. During a mathematical computation, all of the MWCNT weight fractions and ethylene glycol volume percentages are included. The correlation may be a good fit for the experimental data within limits. The characteristics are forecasted using feed-forward backpropagation. In this research, buried layer neurons and factors are examined.

Application of differential transformation method for an annular fin with variable thermal conductivity

The thermal analysis of the annular fin is performed by applying the differential transformation method. The thermal conductivity of the annular fin has been considered as a function of temperature. The effects of non-dimensional parameters, namely thermal conductivity and thermo-geometric fin parameters on the fin efficiency and temperature distribution are determined. Obtained results from the differential transformation method are also compared with the exact analytical results and the results of the finite difference method in the constant thermal conductivity condition. It has been concluded that the differential transformation method provides accurate results in the solution of nonlinear problems.

Effects of Viscous Dissipation Parameter on Magneto-hydrodynamic Free Convective Flow through a Porous Media of Parallel Plates with a Non-uniform Heat Source for an Unsteady Case

The magnetic hydrodynamic free convective flow past an infinite stretching porous sheet at constant density for electrically conducting fluid with viscous dissipation was numerically studied. The study revolved around an unsteady two-dimensional free convective laminar flow through a porous medium with the interaction of magnetic area standard to the stream. The graphs represented the effects of material parameters on the temperature and velocity profiles across the fluid boundary layer. The solutions of partial differential equations obtained numerically using an implicit finite difference method for various values of (nu), numbers (0.5 to 0.7) at a constant thermal conductivity (kappa=0.1). The velocity and temperature of MHD flow increased with an increase in viscous dissipation and vice versa.

Mechanical and Tribological Properties of Spark Plasma Sintered SiC–TiB2 and SiC–TiB2–TaC Composites: Effects of Sintering Temperatures (2000 °C and 2100 °C)

SiC–TiB2 (10 wt. %) and SiC–TiB2 (10 wt. %)–TaC (5 wt. %) composites are consolidated using spark plasma sintering (SPS) technique at different sintering temperatures (2000 °C and 2100 °C) for 15 min soaking time under 35 MPa pressure. The effects of sintering temperature on densification and mechanical properties of composites have been investigated in detail. SiC–TiB2 and SiC–TiB2–TaC composites sintered at 2100 °C showed high Vickers hardness value, i.e., 27.20 ± 1.23 GPa and 26.40 ± 0.80 GPa, respectively, under 1 kgf (9.81 N) load. Poor fracture toughness {2.28 MPa(m)1/2 at 1 kgf (9.81 N) load} of monolithic silicon carbide (SiC) sintered at 2100 °C is improved with addition of titanium diboride (TiB2) and tantalum carbide (TaC) as secondary phases. Scratch resistance of SiC–TiB2 and SiC–TiB2–TaC composites show coefficient of friction value below 0.40 and 0.50 under 5 N and 10 N loads, respectively. SiC–TiB2 and SiC–TiB2–TaC composites show constant thermal conductivity response above 810 °C and 603 °C in the range of 48.70–47.15 W/m K and 60.35–60.41 W/m K, respectively.

Thermodynamic Extremum Principles for Nonequilibrium Stationary State in Heat Conduction

Minimum entropy production principle (MEPP) is an important variational principle for the evolution of systems to nonequilibrium stationary state. However, its restricted validity in the domain of Onsager's linear theory requires an inverse temperature square-dependent thermal conductivity for heat conduction problems. A previous derivative principle of MEPP still limits to constant thermal conductivity case. Therefore, the present work aims to generalize the MEPP to remove these nonphysical limitations. A new dissipation potential is proposed, the minimum of which thus corresponds to the stationary state with no restriction on thermal conductivity. We give both rigorous theoretical verification of the new extremum principle and systematic numerical demonstration through 1D transient heat conduction with different kinds of temperature dependence of the thermal conductivity. The results show that the new principle remains always valid while MEPP and its derivative principle fail beyond their scopes of validity. The present work promotes a clear understanding of the existing thermodynamic extremum principles and proposes a new one for stationary state in nonlinear heat transport.

A NOTE ON THE CONSTANT THERMAL CONDUCTIVITY HYPOTHESIS AND ITS CONSEQUENCE FOR CONDUCTION HEAT TRANSFER PROBLEMS

This work discuss the usual constant conductivity assumption and its consequences when a given material presents a strong dependence between the temperature and the thermal conductivity. The discussion is carried out considering a sphere of silicon with a given heat generation concentrated in a vicinity of its centre, giving rise to high temperature gradients. This particular case is enough to show that the constant thermal conductivity hypothesis may give rise to very large errors and must be avoided. In order to surpass the mathematical complexity, the Kirchhoff transformation is used for constructing the solution of the problem. In addition, an equation correlating thermal conductivity and the temperature is proposed.

Thermochemical ablation of carbon/carbon composites with non-linear thermal conductivity

Carbon/carbon composites have been typically used to protect a rocket nozzle from high temperature oxidizing gas. Based on the Fourier?s law of heat conduction and the oxidizing ablation mechanism, the ablation model with non-linear thermal conductivity for a rocket nozzle is established in order to simulate the one-dimensional thermochemical ablation rate on the surface and the temperature distributions by using a written computer code. As the presented results indicate, the thermochemical ablation rate of a solid rocket nozzle calculated by using actual thermal conductivity, which is a function of temperature, is higher than that by a constant thermal conductivity, so the effect of thermal conductivity on the ablation rate of a solid rocket nozzle made of carbon/carbon composites cannot be neglected.