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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 [Fi r s [11 8 Lin e * 27 . 2 —— No r m * PgE [11 8 CHAPTER 16 Heat Pipes JAY M. OCHTERBECK Department of Mechanical Engineering Clemson University Clemson, South Carolina 16.1 Introduction 16.1.1 Heat pipe basics 16.1.2 Wick structures 16.1.3 Classi±cation by type of control 16.1.4 Capillary action 16.2 Transport limitations 16.2.1 Introduction 16.2.2 Capillary limit 16.2.3 Boiling limit 16.2.4 Entrainment limit 16.2.5 Viscous limit 16.2.6 Sonic limit 16.2.7 Condenser limit 16.3 Heat pipe thermal resistance 16.4 Figures of merit 16.5 Transient operation 16.5.1 Continuum vapor and liquid-saturated wick 16.5.2 Wick depriming and rewetting 16.5.3 Freeze–thaw issues 16.5.4 Supercritical startup 16.6 Special types of heat pipes 16.6.1 Variable conductance heat pipes 16.6.2 Micro and miniature heat pipes 16.6.3 Pulsating heat pipes 16.6.4 Loop heat pipes and capillary pumped loops Nomenclature References 1181
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1182 HEAT PIPES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 [118 2 Lin e 5.7 p —— Nor m PgE n [118 2 16.1 INTRODUCTION 16.1.1 Heat Pipe Basics Capillary-driven two-phase systems offer signiFcant advantages over traditional single-phase systems. With the typically increased thermal capacity associated with the phase change of a working fluid, considerably smaller mass flow rates are re- quired to transport equivalent amounts than in single-phase liquid or gas systems for a given temperature range. Moreover, heat transfer coefFcients of two-phase sys- tems are much greater than in single-phase flows and result in enhanced heat transfer. Lower mass flow rates and enhanced thermal characteristics provide the beneFts of smaller system size (and weight) while providing increased performance. The thermal capacity of a single-phase system depends on the temperature change of the work- ing fluid; thus, a large temperature gradient or a high mass flow rate is required to transfer a large amount of heat. However, a two-phase system can provide essentially isothermal operation regardless of variations in the heat load. Additionally, single- phase systems require the use of mechanical pumps and fans to circulate the working fluid, while capillary-driven two-phase systems have no external power requirements, which make such systems more reliable and free of vibration. The best known capillary-driven two-phase system is the heat pipe, where a sche- matic of a conventional heat pipe is shown in ±ig. 16.1. The concept of the heat pipe was Frst presented by Gaugler (1944) and Trefethen (1962), but was not widely pub- licized until an independent development by Grover et al. (1964) at the Los Alamos ScientiFc Laboratories. Heat pipes are passive devices that transport heat from a heat source (evaporator) to a heat sink (condenser) over relatively long distances via the latent heat of vaporization of a working fluid. As shown, a heat pipe generally has three sections: an evaporator section, an adiabatic (or transport) section, and a con- denser section. The major components of a heat pipe are a sealed container, a wick
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This note was uploaded on 12/03/2010 for the course ECON 089907 taught by Professor Mikey during the Spring '10 term at Nashville State Community College.

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