Hikmet Ş. Aybar

A Review of Thermal Conductivity Models for Nanofluids

Nanofluids, as new heat transfer fluids, are at the center of attention of researchers, while their measured thermal conductivities are more than for conventional heat transfer fluids. Unfortunately, conventional theoretical and empirical models cannot explain the enhancement of the thermal conductivity of nanofluids. Therefore, it is important to understand the fundamental mechanisms as well as the important parameters that influence the heat transfer in nanofluids. Nanofluids’ thermal conductivity enhancement consists of four major mechanisms: Brownian motion of the nanoparticle, nanolayer, clustering, and the nature of heat transport in the nanoparticles. Important factors that affect the thermal conductivity modeling of nanofluids are particle volume fraction, temperature, particles size, pH, and the size and property of nanolayer. In this paper, each mechanism is explained and proposed models are critically reviewed. It is concluded that there is a lack of a reliable hybrid model that includes all mechanisms and influenced parameters for thermal conductivity of nanofluids. Furthermore, more work needs to be conducted on the nature of heat transfer in nanofluids. A reliable database and experimental data are also needed on the properties of nanoparticles.

Performance analysis of single and double basin-inclined solar water distillation systems with and without black-fleece wick

AbstractThe aim of this study is to design and to test the double basin-inclined solar water distillation (DB-ISWD) system and compare it with the single basin-inclined solar water distillation (SB-ISWD) system under the northern Cyprus environmental conditions. The SB-ISWD and the DB-ISWD systems have the ability to produce both freshwater and hot water at the same time. In the SB-ISWD system, the feeding water falls down on the bare plate (BP) through the distribution pipe. The DB-ISWD system has two sections: the lower section and the upper section. In the lower section, the feeding water falls down on the BP through distribution pipe. In the upper section, the feeding water falls down on the transparent glass through distribution pipe. These systems are tested by two variants: BP and with black-fleece wick. It is observed that both systems show higher production rate of freshwater when the black-fleece wick is used. Also, the hot water production by both systems is hot enough for domestic appliances. ...

Unsteady Darcian Natural Convection within Porous Media of Square Enclosure at Various Rayleigh Numbers

Transient natural convection within a 2D square cavity filled with a porous medium is numerically investigated. The left wall is suddenly heated to a constant temperature Th, while the right wall is suddenly cooled to a constant temperature Tc. Both the horizontal walls are insulated. The Finite Volume numerical method is used to solve the dimensionless governing equations. The results are obtained for the initial transient state assuaging to the steady state, and for Rayleigh number values of 102–104. It is indicated that the average Nusselt number showing an undershoot during the transient period and that the time needed to reach the steady state is longer for low Rayleigh number and shorter for high Rayleigh number.

Mathematical modeling of transpiration cooling in cylindrical domain

This research reports on the development of a mathematical modeling to simulate the transpiration cooling in a rocket thrust chamber. The transpiration cooling is a process used to protect the thrust chamber wall of a liquid rocket engine so that the coolant can mitigate the large amount of heat transfer by getting preheated prior to entering the thrust chamber. The mathematical model for the porous layer is developed in cylindrical coordinate system, and the results are validated by the experimental results available in open literature. Also, a numerical method is developed to solve the equations of hot gases which are flowing through the nozzle in order to provide the hot boundary condition of the porous layer. The flow equations within the porous medium are solved separately to analyze the important parameters affecting the transpiration cooling process. A parametric study is performed to investigate the effect of coolant mass flow rate, hot face boundary condition, gas type, and porous layer thickness. Moreover, the internal convective heat transfer between solid structure and the coolant flow is studied. The porous solver code is then coupled with the gas solver code to examine the cooling process more realistically. The results show the variation of gas properties through the nozzle and the effect of transpiration cooling on them. The paper concludes with a demonstration of a sample method of controlling the nozzle wall temperature.

Influence of Insulated Rectangular Block on Natural Convection Heat Transfer within Porous Enclosure

Numerical study has been performed to investigate the influence of rectangular block on mean Nusselt number of hot wall in a square cavity of porous media. Top and bottom walls are insulated and the right and left walls are cold and hot, respectively, at constant temperature which are heated differentially assuaging a steady condition. 4 distinct cases are introduced regarding the block position: The block at the bottom/top edge with variable distance from the hot wall and the block at the middle edge of bottom/top with variable height. Mean Nusselt numbers are normalized with the simple case of no insulated block. Results show a significant dependence of mean Nusselt on the block position. It also affects the dimensionless time to reach a steady status..

Experimental Investigation of Pressure Drop in One Phase Flow, Particle-Liquid Two-Phase Flow, and Porous Media Consisting of the Same Particle Size

Many researches have performed some studies about pressure gradient in the one phase flow, particle-liquid two-phase flow and porous media. However, there is no any report about interaction among them. In this study, this interaction idea is developed by using the same particle size for particle-liquid two-phase flow and porous media. For the experimental study, an apparatus is designed, and at the first step one phase water pressure gradient is investigated, next in the further steps, little by little spherical particles are added to the cycle till accumulation of the particles did not allow any movement to the particles (i.e. porous media occur), and after that well pack porous media is investigated. The results confirm the relation between pressure gradient over mass flow rate in the one phase flow, particle-liquid two-phase flow and porous media obeys as a parabolic curve.

Simplification of Ensemble Averaged Two-Phase Flow with Heat and Mass Transfer Equations for Boiling in a Channel

Generation of vapor and predication of its behavior is an important problem in many industries. In this study, the three dimensional governing equations for turbulence two-phase flow are derived using ensemble averaging two fluid model. The governing equations are simplified by a heuristic approach based on boiling data, and the equations are used to obtain the parameters for each phase along the channel. A computer program is written for the simplified one-dimensional equations, and the results are compared with experimental data.