Numerical analysis of nanofluid behavior in long cavities

Master's Thesis

Energy Conversion Trend

Abstract:

Increasing heat transfer as well as increasing energy efficiency due to the limitation of natural resources and reducing costs has always been one of the most basic concerns of engineers and researchers. This is especially important in fluids due to the small thermal conductivity coefficient. One of the most important ways to achieve this, which has received a lot of attention in recent years, is the addition of solid particles with high thermal conductivity in nano dimensions. The natural displacement flow inside the cavity, which is the only driving factor in that buoyancy force, is one of the important phenomena that has been widely studied in the science of heat transfer, due to the variety of applications in engineering and industry. The purpose of this research is to investigate the effect of nano particles on heat transfer and fluid flow, as well as the effect of particle diameter on it in a rectangular cavity with different aspect ratios (0.1, 0.2, 0.25, 0.5, 0.75, 1=L/H). In this research, two basic fluids, water and ethylene glycol, and three types of solid nanoparticles, copper (Cu), titanium oxide (TiO3), and aluminum oxide (Al2O3) have been used for four different volume ratios (?=0.025, 0.05, and 0.1). The smooth flow is considered within the limits of Bozinsk assumption and the results are presented for three Rayleigh numbers 105, 106 and 107. Simple algorithm is used to model the flow and the results are presented for incompressible flow. In this way, by using the written numerical program, it is possible to model the heat transfer in slow fluid flow using Bouzinsk's assumption. The results have shown that nanoparticles suspended in the fluid increase the heat transfer rate in each Rayleigh number and aspect ratio. Also, the results have shown that the maximum Nusselt number and the average Nusselt number increase with the increase in the volume ratio of nano particles. Also, the highest average Nusselt value has been observed for copper (Cu) nanoparticles. The comparison of the results obtained from the flow solution with previous researchers shows that these results are acceptable.

Key words: heat transfer, nanofluid, incompressible, cavity, aspect ratio

Chapter One

Introduction

The purpose of this research is to simulate the natural displacement flow of nanofluid. Based on this and in order to get to know more about the features of this research, there is a need to better understand the proposed concepts such as natural displacement, nanofluid properties and nanofluid flow. This chapter introduces each of the above concepts separately and briefly presents their features and complexities. 1-1- Natural displacement One of the most important issues in fluid mechanics is the movement of fluids in nature and industry that engineers deal with every day. Some currents resulting from natural displacement [1] are caused by Archimedes' force. In the topic of heat transfer, the adjective "natural" is assigned to flows that are the result of mass density difference, while when the flow occurs due to pressure gradient or velocity boundary conditions, forced displacement [2] is a more appropriate term. Some writers and researchers make a mistake between natural movement inside (in a closed area) and outside (around objects). The behavior patterns of these two are different and the second one is also called free movement [3]. Density difference is caused by phase difference, concentration difference or temperature. Steam bubbles in water are an example of the first case. Archimedes' law states that the net upward force acting on the bubble is equal to the acceleration of gravity multiplied by the difference between the mass displaced by the water and the mass of the bubble's vapor, and this buoyant force causes the bubble to rise. Diffusion movements are an example of the second state in which nature tries to equalize the solution concentration in order to maximize entropy. The problem in front is an example for the third situation, which will be investigated from now on.. As part of the industrial and engineering applications and practical examples of this flow, the following can be mentioned: air movement and ventilation inside buildings and structures, liquid storage tanks, solar cell structure, cooling of electronic equipment, heat transfer during the growth of crystals and flow between the walls of a nuclear reactor. We know that when part of the fluid is hotter than the other part, it expands and Its density decreases. This is why thermal eddies are created in the atmosphere and oceans, or balloons filled with hot air rise.  Natural displacements are divided into two categories, each of which is characterized by specific behavior patterns. The first category, which is called "heating from the bottom surface" [4], is caused by heating a bottom plate on which a colder fluid is flowing. The main characteristic of this category is the existence of large and coherent structures in the fluid such as plumes [5], thermal cells [6] and Rayleigh-Bernard cells [7]. The second category is known as "heating from the sides" [8], and the warm vertical plate is the simplest example of this category. The main characteristic of this category is the extreme gradients of temperature and velocity in the boundary layers.

Today, fluid mechanics research in this regard is limited to two fields of study. The first field of study is the experimental measurement of flow data and the second is the numerical simulation of mathematical equations governing the flow. Studying in each of these fields has its own problems. Experimental work suffers from uncertainties in the boundary conditions as well as the problem of the actual size of the model and is usually more expensive than the numerical method. However, to prove the correctness of the numerical method and obtain the assumptions and experimental constants, the experimental method is always necessary. But if a numerical model for a certain state is confirmed with the help of experimental data, the results of that model can be cited for similar states as well, without requiring the cost of experimental work for those states, and this is the strength of numerical simulation. Conductors are very important in ultra-fast computers, car engines and many factories. All cooling and heating systems are designed based on heat transfer. Due to this, the development of effective heat transfer techniques is very necessary due to the limitation of natural resources and the desire to reduce costs. Generally, air cooling systems are used more and are more reliable. But when there is a need for high heat flux [9] and fast heat transfer, liquids such as water, ethylene glycol and other suitable liquids that have thermal limitations are used. Common fluids used for heat transfer have a low coefficient of thermal conductivity, while metals have thermal conductivity higher than three times of such fluids. Therefore, the use of solid metal particles and their combination with such fluids to increase the coefficient of thermal conductivity and as a result increase the thermal efficiency seems very desirable. Maxwell in 1881 [10] [1] for the first time raised the issue of increasing solid particles to fluid and provided interfaces for the coefficient of thermal conductivity of a mixture of pure fluid and solid particles. For years, the use of fluid suspension and very small solid particles in micro dimensions has been the focus of researchers. But these fluids with suspended solid particles in the range of micrometer [11] have many problems such as sedimentation, impurity, corrosion and increased pressure drop. until first Masuda et al. [2] and then Choi [3] proposed the idea of ??nanofluid [12] for the first time and created a great revolution in the field of heat transfer in fluids. Also, corrosion, impurity and pressure drop problems were reduced to a large extent due to the small size of the particles, and on the other hand, the stability of some fluids against sedimentation was significantly improved. Nanotechnology generally represents the method of moving individual atoms and arranging them as desired. For this reason, the size and working dimensions of this collection are very small, of course, the nano prefix indicates the limits of this technology. Nanofluid is very fine solid particles with dimensions between 1 and 100 nm [13] suspended in a base fluid. Normally, nanoparticles are made of metals such as copper, aluminum, potassium, silica and their oxides, and the base fluids are mainly fluids with low conductivity such as water, ethylene glycol, and fluids of this category that are used as heat transfer conductors in the industry.