Forced convection heat transfer of non-Newtonian nanofluid under turbulent flow in microtubes

Dissertation for M.Sc degree

Mechanical Engineering - Energy Conversion Orientation

Abstract:

In this research, the turbulent flow of a Non-Newtonian nanofluid is simulated in a microchannel with a circular cross section. First, the types of classification of microchannels, the methods of making microchannels, as well as the advantages and challenges of using microchannels are stated. In the following, various models are described in describing the behavior of non-Newtonian fluids and then the concept of nanofluid, how to produce nanoparticles and preparation of nanofluid, various models for expressing the thermophysical properties of nanofluids such as density, specific heat coefficient, thermal conductivity coefficient and dynamic viscosity are explained. Also, suitable models have been selected for use in this research. Using CFX software, the equations of conservation of mass, conservation of momentum and conservation of energy have been solved for the turbulent flow of non-Newtonian fluid of aqueous solution of 0.5 wt% carboxymethyl cellulose and also for nanofluid containing copper oxide particles in said non-Newtonian fluid. Velocity, pressure and temperature fields of nanofluids have been obtained and calculated by analyzing the results of displacement heat transfer coefficient and Nusselt number of nanofluids. Also, the effects of nanoparticle volume fraction or concentration, Reynolds number and nanoparticle diameter have been investigated on the results, which indicate an increase in displacement heat transfer coefficient and Nusselt number using non-Newtonian nanofluid compared to the base non-Newtonian fluid. There is a direct relationship between this increase with nanoparticle volume fraction and Reynolds number. Also, as the diameter of nanoparticles decreases, the displacement heat transfer coefficient increases.

Keywords: microchannel, nanofluid, non-Newtonian, displacement heat transfer, turbulent flow, Nusselt number

Introduction

In the last few decades, in order to save energy and raw materials and considering economic and environmental issues, many efforts have been made to build high efficiency heat exchange devices. The main goal is to reduce the size of the heat exchanger needed for a certain heat load and increase the capacity of existing heat exchangers. Global demand for efficient, reliable and economical heat exchange devices, especially in process industries, electricity generation, cooling and air conditioning systems, heat exchangers, vehicles, etc. is increasing rapidly. If the principles related to the methods of increasing heat transfer and the design of heat transfer devices with a large surface are well known, it will be possible to save energy and reduce environmental pollution. There are several methods to increase heat transfer, which are divided into two general categories.

Passive methods[1] that do not require the application of external force.

Active methods[2] that require external power.

Passive methods include the use of wide surfaces, compact heat exchangers, ducts with non-circular sections, increasing heat transfer. Vortices[3], changing the rheological property of the fluid, microchannels, coating and polishing the surface, the use of moving devices inside the fluid channel, the use of flow rotating devices, creating interruptions and breaks in the flow, spiral tubes, additives to liquids and gases. Active methods include mechanical stirring, surface scraping, rotating surfaces, surface oscillation, fluid oscillation, use of electric field, injection and suction. In this study, passive methods including microchannels, changing the fluid's rheological properties and additives to liquids will be used to increase heat transfer. Robotic systems, microelectromechanical systems and microreactors are used. As the size of the channel decreases, the assumption of continuous flow loses its accuracy, but for a certain value of the channel size, it is possible to apply the Navier-Stokes equations by modifying the boundary conditions.

1-2 changing the rheological property of the fluid

One ??of the most effective ways to increase heat transfer is to change the rheological property of the fluid. By adding special materials to different fluids, their rheological properties can be changed from Newtonian to quasi-elastic or viscoelastic. Changing the rheological property of the fluid is one of the most important ways to increase heat transfer, because at the same time as the heat transfer increases, the friction coefficient and as a result the pressure drop decreases. Increasing the heat conductivity coefficient is the main idea in improving the heat transfer characteristics of fluids. Since the thermal conductivity coefficient of solid particles is usually much higher than that of fluids, the addition of these solid particles is expected to increase the thermal conductivity coefficient of the base fluid.

Increasing the thermal conductivity coefficient of liquids as a result of adding millimeter and micrometer sized particles has been known for more than 100 years [2]. However, it is not possible to use these particles due to practical problems such as rapid sedimentation of particles, severe wear, increased pressure drop, and the impossibility of using them in very small channels. Recent advances in material technology have enabled the production of nanometer-sized particles (nanomaterials) that can overcome these problems. By spreading these nanomaterials in the fluid, a new type of fluid is created, which is called nanofluid [5]. rtl;"> 

Microchannels

2-1 Abstract

The growing demand for miniaturization of products in all industrial sectors, with global competition for more reliability, more speed and products It has been accompanied by cost-effectiveness and has led to new challenges for the design and operation of thermal management systems. The rapid increase in the number of transistors on the chip, with increased capability or power and as a result higher heat flux, is one of these great challenges in the electronics industry. Microchannel heat exchanger and mass exchanger technologies are finding new applications in various industries as a promising solution for changing technologies. In this way, we design and launch the next generation of high performance thermal management systems. In this chapter, we will deal with the principles of microchannels. We start by introducing the history, technical background, classification, advantages and disadvantages of microchannels. The manufacturing method (conventional technology and modern technology) for microchannels is considered together. Finally, the relationship of pressure drop and heat transfer coefficient for single-phase flow will be presented for a variety of internal flow conditions.

2-2 History of microchannels

Many works for single-phase heat transfer in microchannels by Tuckerman [6] and Pease [7] [3] for cooling integrated circuits on a very large scale. (VLSI) [8] was done. In the early years, Tuckerman and Pease [3] provided the first explanation for the microchannel heat well concept and predicted that single-phase forced displacement cooling in microchannels could remove 1000 W/m2 of heat. Forced channel displacement and liquid injection have been used for faster, larger-scale cooling in industry for decades. Microchannel heat transfer, compared to normal air and liquid of cold systems, has a high heat transfer coefficient, along with a high potential for heat transfer coefficient and moderate pressure drop. Microchannel heat transfer has become a popular and interesting phenomenon for researchers. For example, it has been proven to remove heat flux of 500 W/m2 for high-power microchannel heat well cooling with laser diode arrangement.