Encyclopedia of Thermal Packaging

The following sections are included:

• Foreword to the Encyclopedia of Thermal Packaging

• PREFACE

• CONTENTS

Microchannel heat transport offers great promise in achieving efficient heat dissipation from high-heat-flux components. The rationale for the use of microchannels, and their application to thermal management of electronic components, is discussed in this chapter.

Prediction of heat transfer and pressure drop in single-phase flow of a liquid or gas through microchannels is discussed, along with an assessment of the applicability of conventional theory developed for larger-scale channels. Methods for device-level and system-level optimization for thermal performance in cooling applications are described. The importance of manifold design is elucidated. A novel approach to hot-spot thermal management using microchannel heat sinks is demonstrated.

Boiling in microchannel heat sinks is extremely attractive for high-performance electronics cooling due to the high heat transfer rates that can be achieved. Since the fluid latent heat of vaporization is exploited, lower flow rates are needed in microchannels undergoing boiling than with single-phase liquid to achieve the same amount of cooling, resulting in lower pumping powers and more efficient and compact systems. The very high heat transfer rates achievable are coupled with the ability to keep the wall temperature relatively uniform, contributing to more reliable operation and longer lifetime for the electronics being cooled. Since the heat transfer coefficient increases with increasing heat flux over typical ranges, two-phase microchannel heat sinks also have the intrinsic ability to accommodate the thermal management of hot spots on the electronics. In this chapter, fundamentals of boiling in microchannel heat sinks are discussed along with the main distinctions from boiling in conventional-sized channels. Conditions under which the transition from macroscale to microscale boiling occurs are evaluated, and transition criteria are developed to identify the range of operating and geometric conditions that define microchannel heat sinks. Flow regime maps for microchannel boiling that can facilitate systematic design of microchannel coolers are presented.

Over the past decade, extensive experimental and analytical investigations have led to a good understanding of boiling heat transfer mechanisms in microchannels and their deviation from the behavior in larger channels. The influence of flow patterns and channel geometry on the heat transfer has been investigated experimentally. Empirical correlations have been proposed in various studies to predict thermal performance of microchannel heat sinks undergoing boiling. Although empirical correlations are generally easy to use, their range of applicability is limited. Regime-based models that are developed based on the flow physics, on the other hand, can provide a more robust prediction of the thermal performance of microchannels. However, such models are scarce and difficult to formulate due to the complexity of the boiling phenomenon at the microscale. In this chapter, heat transfer during saturated boiling in microchannels and the effects of geometry and flow parameters are discussed. Empirical prediction and physics-based modeling of thermal performance and pressure drop are reviewed.

Two-phase pressure drop in microchannels is discussed along with its variation with microchannel dimensions and flow properties. Prediction of pressure drop using empirical correlations and physics-based models is explained.

Microchannel heat sinks are amongst the most effective solutions for handling high levels of heat dissipation in space-constrained electronics; however, their high pumping requirements and the limits on available space precludes the use of conventional pumps. Therefore, significant research efforts have been undertaken in recent years towards the investigation and development of microscale pumping technologies. In this chapter, the potential and capabilities of various micropumping strategies in the literature are discussed and their working principle, the state of the art, and unresolved issues reviewed. Also, novel approaches for integrated micropumping are presented. A major part of this chapter is based on the state-of-the-art review of the literature by Iverson and Garimella in which recent advances in microscale pumping technologies were reviewed and evaluated.

Although microchannel heat sinks have received much attention over the last decade for electronics cooling applications, their implementation in practical applications has lagged due to the challenges and complexities that exist especially when the microchannels operate in the two-phase regime. One of the major concerns in practical implementation of microchannels is unstable flow in microchannels which can lead to low critical heat fluxes. In this chapter, the common reasons for occurrence of instabilities in microchannels are discussed and remedies are proposed.

This chapter focuses on measurement techniques for single-phase and two-phase flow in microchannels. Experimental measurements and flow visualizations are crucial in understanding the fundamental mechanisms of flow and heat transfer in microchannels. Furthermore, theoretical and numerical models need to be validated using accurate and reliable experimental measurements. The small length scales present in microchannels and the desired micron-scale spatial resolution in measurements poses challenges for the measurement of flow and temperature fields using conventional techniques. Moreover, quantities commonly measured in two-phase flow are different from those in single-phase flow; hence different measurement methods need to be developed and utilized in each system based on the existing phases.

A bulk of the experimental work on microchannel flows has utilized conventional measurement techniques such as thermocouples for local temperatures, and the measurement of overall flow rates and pressure drops. More recently, sophisticated, non-intrusive visualization and optical techniques and microfabricated sensors have become available for localized measurements of velocity, shear stress, temperature, and heat flux in microchannels with small spatial resolutions, resulting in more precise local measurements.

The following sections are included:

• Author Index

• Subject Index

• About the Authors

• About the Editor-in-Chief