Heat Transfer Enhancement Using Nanofluid Flow in Microchannels

Simulation of Heat and Mass Transfer

Davood Domairry Ganji, Amir Malvandi

• Publisher: William Andrew
• Year: 2016
• Print ISBN: 9780323431392
• eBook ISBN: 9780323431781
• Topic: Physical Sciences
• Subtopic: Thermodynamics

Heat Transfer Enhancement Using Nanofluid Flow in Microchannels: Simulation of Heat and Mass Transfer focuses on the numerical simulation of passive techniques, and also covers the applications of external forces on heat transfer enhancement of nanofluids in microchannels.Economic and environmental incentives have increased efforts to reduce energy consumption. Heat transfer enhancement, augmentation, or intensification are the terms that many scientists employ in their efforts in energy consumption reduction. These can be divided into (a) active techniques which require external forces such as magnetic force, and (b) passive techniques which do not require external forces, including geometry refinement and fluid additives.- Gives readers the knowledge they need to be able to simulate nanofluids in a wide range of microchannels and optimise their heat transfer characteristics- Contains real-life examples, mathematical procedures, numerical algorithms, and codes to allow readers to easily reproduce the methodologies covered, and to understand how they can be applied in practice- Presents novel applications for heat exchange systems, such as entropy generation minimization and figures of merit, allowing readers to optimize the techniques they use- Focuses on the numerical simulation of passive techniques, and also covers the applications of external forces on heat transfer enhancement of nanofluids in microchannels

1

Introduction to Heat Transfer Enhancement

Abstract

The aim of this chapter is to study the different mechanisms of heat transfer enhancement. Heat transfer enhancement techniques are divided into two main groups, namely passive (those which do not require direct application of external power) and active (require external power) techniques. Each method is described and explained with its advantages/disadvantages to provide basic information for the reader.

Keywords

Active techniques; Channel; Efficient heat transport; Energy; Heat transfer enhancement; Passive techniques

1.1. Why Enhancing Heat Transfer Rate Is Crucial?

For the past century, reducing energy consumption has attracted considerable attention from engineers and scientists. Whether it is called energy saving, energy conservation, or energy efficiency, many efforts have led to creating more professional equipment. In heat exchange equipment, other factors like space limitations and economic incentives have accelerated this concept further. Firstly, the focus was on boosting the heat transfer coefficient by improving the thermal properties. This has enabled us to solve other significant problems such as lowering the maximum temperature of surfaces and reducing the mass and size of heat exchangers. A complete review of these attempts has been conducted [1,2]. Then, other concepts like Exergy Analysis [3,4], Entropy Generation Minimization [5,6], and Field Synergy Principle [7] were developed to consider the overall performance of the system including increasing the heat transfer rate and reducing the initial and capital costs of the heat transfer system.

In general, the goal is simple: reducing heat exchanger size with improved heat transfer performance. It should be noted that the improved heat transfer performance is referred to such enhancement in heat transfer rate without any undesirable problems like additional pressure loss.

1.2. Heat Transfer Enhancement Classification

According to Bergles [1], heat transfer enhancement methods can be classified into two main groups: passive and active techniques. Passive techniques are those which do not require direct application of external power, while active techniques require external power. Except for extended surfaces like fins, which enhance the effective heat transfer surface area, the passive techniques improve the heat transfer coefficients by disturbing or altering the existing flow behavior which is usually accompanied by a rise in the pressure drop. In the case of active techniques, on the other hand, the addition of external power essentially facilitates the desired flow modification and the concomitant improvement in the rate of heat transfer.

It is worth mentioning that the effectiveness of both types of techniques is completely dependent on the mode of heat transfer (such as single- or two-phase flow, free or forced convection, and boiling) and type and process application of the heat exchanger. Fig. 1.1 shows different methods of enhancing the heat transfer rate.

Figure 1.1 Different methods of heat transfer enhancement.

Using more than one technique of those pertained to in Fig. 1.1 simultaneously is useful and can enhance the heat transfer rate to a large extend. This method, the compound technique, has attracted considerable attention from researchers recently.

1.2.1. Passive Techniques

By changing the flow treatment, the passive techniques increase the heat transfer rate which usually increases the pressure drop. These methods do not need external power and usually utilize geometry refinement and fluid additives. As shown in Fig. 1.1, the various ways of passive heat transfer enhancement are as follows.

1.2.1.1. Treated Surfaces

This technique is usually used for boiling and condensing, which involves surface coating and fine-scale alteration of the surface finish. This technique found its application in various industries, such as microelectronics, biotechnology, and microelectromechanical systems. With the recent progress in surface engineering, many efforts have been directed toward improved surface wettability, like hydrophobic, hydrophilic, and superhydrophobic (see Fig. 1.2) [8] surfaces. These surfaces, in addition, enhance the convective heat transfer rate since they result in nonadherence of fluid to solid at the boundaries, known as “slip” boundary conditions [9–13]. Slippage of liquids near the walls of microscale channels has been encountered as a result of the interaction between a coated solid wall (hydrophobic, hydrophilic, or superhydrophobic materials) and the adjacent fluid particle. In fact, because of the repellent nature of the hydrophobic and superhydrophobic surfaces, the fluid molecules which are close to the surface do not follow the solid boundary, resulting in an overall velocity slip. More discussion on the slip effects can be found in open literature (eg, [14,15]).

1.2.1.2. Rough Surfaces

Rough surfaces are usually employed to enhance the turbulence. Although they do not increase the surface area, they increase the heat transfer coefficients by disturbing the viscous sublayer, particularly in single-phase flow. For instance, a dimpled surface, as shown in Fig. 1.3, increases the heat transfer rate, while having negligible effect on the pressure drop.

Figure 1.2 Water on a superhydrophobic surface. http://itg.beckman.illinois.edu/communications/iotw/2008-04-29.

Figure 1.3 Dimple surface (A) and its effect on the fluid flow with computational fluid dynamics simulation (B).

1.2.1.3. Extended Surfaces

This technique is used to increase the heat transfer area and is a commonplace method in increasing the heat transfer rate in many heat exchangers. Furthermore, they can create swirl turbulence and secondary flow. Extended surfaces, or fins, have a variety of applications such as air conditioning systems, compressors, and cooling of electronic components. They are usually placed in the side with the highest thermal resistance to increase the heat transmission. Up to now, many kinds of fins are introduced by researchers in various geometrical shapes such as rectangular, trapezoidal, spins, and triangular. Some typical examples of fins are illustrated in Fig. 1.4. Suitable fins are those that induce small pressure gradients in the flow.

Recently, finding a close-form solution for heat transfer coefficient and surface temperature of the fins has attracted considerable attention. Many researchers have tried to find appropriate analytical solutions for these problems which can be found in open literature [16,17].

Figure 1.4 Some types of fins.

Figure 1.5 Wire matrix tabulators that offer the greatest benefit for laminar and transitional flow regimes. http://www.allturbulators.com/products/product-and-design-philosophy.html.

1.2.1.4. Displacing Devices

Such devices insert into the channels to promote the energy transp...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• 1. Introduction to Heat Transfer Enhancement
• 2. Heat Transfer and Pressure Drop in Channels
• 3. Preparation and Theoretical Modeling of Nanofluids
• 4. Simulation of Nanofluid Flow in Channels
• 5. External Forces Effect on Intensification of Heat Transfer
• Index