Increase In Thermal Conductivity Ratio Research Articles

Three-dimensional simulation of conjugate heat transfer using the hybrid lattice Boltzmann-finite difference method

This paper presents a three-dimensional numerical study of natural convection and conduction thermal phenomena using MRT-lattice Boltzmann and finite difference approaches. Conduction is examined using a simple conjugate heat transfer technique, which is predicated on the assumption of continuity of heat flux and temperature at the solid/fluid interfaces, instead of utilizing the conventional method based on the solution of the conduction equation. Several parameters such as Rayleigh number and thermal conductivity related to common construction materials are also investigated to explore their effects on the evolution of heat transfer. The obtained results show that augmentation of the Rayleigh number increases the quality of heat transfer by around 35–293% for various values of thermal conductivity ratio (solid/fluid). While the increase in thermal conductivity ratio enhances the heat transfer rate by about 212–685%, where the maximum rate of heat exchange is still the same for the thermal conductivity ratio ≥1000. Entropy generation is also examined in this paper to evaluate the efficiency of our studied system. The performed investigations indicate that the rate of entropy generation due to heat transfer, friction, and total entropy generation varies significantly with increasing Rayleigh number and thermal conductivity of the solid.

Effect of oxide nanoparticles on the thermal, rheological and tribological behaviours of refrigerant compressor oil: An experimental investigation

Oxide based nanolubricants are prepared by dispersing TiO2, SiO2 and Al2O3 nanoparticles into synthetic refrigerant compressor oil (Polyalkylene glycol) using two-step method. Thermal conductivity of nanolubricants for 0.07 to 0.6% volume fractions in the temperature range 20 °C–50 °C and rheological properties in the range 20 °C–80 °C have been investigated. The tribological characterization is performed with a four ball tribo-tester. The results show that as the volume fraction increases thermal conductivity and viscosity of nanolubricants increase. The maximum increase in thermal conductivity ratio for TiO2, SiO2 and Al2O3 nanolubricants are 1.38, 1.31 and 1.48, respectively and corresponding increase in viscosity ratios are 5.64, 10.34 and 9.71, respectively. Rheological studies show that, unlike pure lubricant, nanolubricants exhibit non-Newtonian shear thinning behaviour. The tribological test results disclose that, lubricant appended with TiO2 and SiO2 have excellent friction reduction and anti-wear properties compared to pure lubricant.

The Effect of Slight Deformation on Thermocapillary-Driven Droplet Coalescence and Growth

The collision efficiency of two slightly deformable drops in thermocapillary motion at small Reynolds and Marangoni numbers is determined by a trajectory analysis involving methodology from matched asymptotic expansions. The outer solution for two spherical drops which are nearly touching provides the contact force driving the inner solution. Accurate calculation of the contact force and near-contact motion is aided by new solutions for the mobility functions parallel and normal to the drops' line of centers that are valid at very small separations. Governed by a system of integro-differential equations coupling the flow inside the drops and that within the small gap, the inner solution allows demarcation of the regions of drop coalescence and separation. Apart from the driving force, the thin-film equations are unchanged to leading order from the buoyancy-driven case, since no additional singularity is introduced into the tangential stress by the presence of the finite temperature gradient. The interplay of small deformation, as measured by the capillary number (Ca), and attractive van der Waals forces controls the apparent contact motion. Results for the collision efficiency are mapped out for a range of five dimensionless parameters: Ca, size ratio, drop-to-medium viscosity ratio, drop-to-medium thermal conductivity ratio (k̂), and a dimensionless Hamaker parameter. Since the only effect on the inner solution of an increase in the thermal conductivity ratio is an increase in the amount of time the drops spend in close approach, it is possible for the collision efficiency of two slightly deformable drops with higher k̂ to be greater than that for two similar drops with lower k̂. This behavior differs from that of spherical drops, where an increase in thermal conductivity ratio always leads to a decrease in the collision efficiency, as a result of greater hydrodynamic interaction between the spherical drops due to the temperature gradient. In addition, collision efficiencies are provided for a model system of ethyl salicylate (ES) drops in diethylene glycol (DEG). The collision efficiency decreases rapidly with increasing drop size above a critical value, due to the increasing role of deformation in retarding the drainage of the thin film between two drops in close approach. Population dynamics simulations are performed for homogeneous suspensions of the ES/DEG system, showing that slight deformation limits droplet growth due to coalescence in dilute dispersions.