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Boiling and Evaporation
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A B C D E F G H I J K L M N
Nahme-Griffith number, Nakashima, CY Nanoparticles, for heat transfer augmentation, Naphthalene: Napthenes: National practice, in mechanical design, guide to, Natural convection: Natural draft cooling towers: Natural frequency of tube vibration in heat exchangers, Navier-Stokes equation, Neon: Neopentane: Net free area, in double-pipe heat exchangers, Netherlands, guide to national mechanical design practice, Networks, of heat exchangers, pinch analysis method for design of, Neumann boundary conditions, finite difference method, Nickel, thermal and mechanical properties Nickel alloys, Nickel steels, Niessen, R, Nitric oxide: Nitriles: Nitrobenzene: Nitro derivatives: Nitroethane: Nitrogen: Nitrogen dioxide: Nitrogen peroxide: Nitromethane: m-Nitrotoluene: Nitrous oxide Noise: Nonadecane: Nonadecene: Nonane: Nonene: Nonanol: Nonaqueous fluids, critical heat flux in, Non-circular microchannels: Noncondensables: Nondestructive testing, of heat exchangers Nongray media, interaction phenomena with, Nonmetallic materials: Non-Newtonian flow: Nonparticipating media, radiation interaction in, Nonuniform heat flux, critical heat flux with, Non-wetting surfaces, in condensation augmentation, North, C, No-tubes-in-window shells, calculation of heat transfer and pressure drop in, Nozzles: Nowell, D G, Nucleate boiling:
augmentation of, in axial flow reboilers, in evaporators, in forced convective boiling of binary and multicomponent mixtures, in forced convective heat transfer in vertical tubes, in horizontal tubes, in kettle reboilers, in microchannels,
Boiling and Evaporation
outside tubes and tube bundles in crossflow, in pool boiling of binary and multicomponent mixtures, in pool boiling systems,
Nuclear industry, fouling problems in, Nucleation: Nucleation sites: Nuclei, formation in supersaturated vapor, Number of transfer units (NTU): Numerical methods: Nusselt: Nusselt-Graetz problem, in laminar heat transfer in ducts, Nusselt number:
O P Q R S T U V W X Y Z
N Nucleate boiling: in microchannels,
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Boiling and Evaporation

DOI 10.1615/hedhme.a.000228

2.13.4 Boiling and evaporation

Sujoy Kumar Saha and Gian Piero Celata

A. Introduction

Microcooling elements in micro-thermal-mechanical systems (MTMS) are a modern high-end technology. Microchannel heat sinks are extensively used in high-heat flux systems, e.g., microelectronic, optical and microfluidic devices, high-performance supercomputers, electric vehicles, advanced military avionics, power and laser devices, very-high temperature gas-cooled reactors and liquid metal fast reactors, radiator panels of spacecraft, thermal control of satellites, etc. In pharmaceutical and chemical industries, by integrating heat exchangers, mixers, and reactors as a block of microcomponents, it is possible to achieve a significant decrease in the system size. Heat exchangers based on microscale channels are used in automotive fuel cells where the heat dissipation system should be compact and contain a minimum number of components. The heat dissipation rate is extremely high in these devices. Phase-change processes such as flow boiling are very important in such devices due to possible higher heat transfer coefficients and much lower temperature gradients along the heat exchanger. In addition, two-phase microchannel heat sinks are one of the strongest candidates for heat removal devices in high-heat flux environments by virtue of their large surface area-to-volume ratio, compact dimensions, and low flow rate requirements. Flow boiling in microchannels has drawn considerable attention from researchers worldwide. Microchannels are used in MEMS and high-end microprocessor cooling applications. Microchannels are used in applications involving high operational pressures (up to 600 bar) and temperatures (1,000 °C). There is a scale effect that becomes evident when the hydraulic diameter is less than the capillary length, i.e., [(σ/g)(ρl - ρv)]0.5.

In microchannels, the hydraulic diameter of the channels is smaller than the capillary length. Identifying the threshold, or the transition band, beyond which a two-phase flow may be considered "micro" is an open issue. The channel classification has been given by Kandlikar and Grande (2003). This classification should be used as a mere guide indicating the size range, rather than rigid demarcations based on specific criteria. Relevant literature in this direction would be Kawaji and Chung (2003). The confinement number (Co), as the distinguishing parameter, gives only a rough idea,

\[\label{eq1} \mbox{Co}=\frac 1D\sqrt{\dfrac{\sigma}{(\rho_l-\rho_v)\,g}} \tag{1}\]

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