Boiling in Micro-Channels
The subject of Chap. 6 is boiling in micro-channels. Several aspects of boiling are also considered for conventional size channels and comparison with micro-channels was carried out. Significant differences of ONB in micro-channels have been discussed com
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Boiling in Micro-Channels
The subject of Chap. 6 is boiling in micro-channels. Several aspects of boiling are also considered for conventional size channels and comparison with micro-channels was carried out. Significant differences of ONB in micro-channels have been discussed compared to conventional channels. Effect of dissolved gases on boiling in water and surfactant solution was revealed. Attention was paid on pressure drop and heat transfer, critical heat flux and instabilities during flow boiling in microchannels.
6.1 Onset of Nucleate Boiling in Conventional Size Channels In this section the general approach used to describe the parameters corresponding to the onset of nucleate boiling (ONB) in a single channel are considered, and models and experiments are compared. The mode of ONB in parallel micro-channels is presented in Sect. 6.2. We will also look at the effect of dissolved gases on ONB during flow boiling of water and surfactant solutions in micro-channels. Section 6.3 deals with the dynamics of vapor bubble in a single micro-channel and in parallel micro-channels. Pressure drop in two-phase flow boiling in parallel micro-channels is considered in Sect. 6.4. In this section we look at a new correlation incorporating the effects of both channel size and coolant mass velocity, which shows better accuracy than prior correlations. In Sect. 6.4 heat transfer correlations in two-phase flow boiling in parallel micro-channels are examined and correlations based on the Martinelli parameter are considered. Parameters that affect the explosive boiling are discussed in Sect. 6.5. Existing critical heat flux (CHF) correlations for flow boiling of water with available databases taken from small-diameter channels are also evaluated.
L.P. Yarin, Fluid Flow, Heat Transfer and Boiling in Micro-Channels © Springer 2009
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6 Boiling in Micro-Channels
6.1.1 Models for Prediction of Incipient Boiling Heat Flux and Wall Superheat The onset of bubble nucleation usually requires that the temperature on the heated surface exceeds the saturation temperature of the liquid corresponding to a given pressure. There has been a great deal of analysis on bubble nucleation and a number of semi-empirical models of boiling incipience have been proposed during the past few decades (Carey 1992; Dhir 1998). Two general approaches were used to describe the parameters corresponding to the onset of nucleate boiling. The first is based on analysis of the behavior of a single bubble on a rough surface where the temperature of the surrounding liquid exceeds the saturation temperature. The second approach considers vapor bubbles generated in liquid (homogeneous nucleation), on a surface or at a corner (heterogeneous nucleation) based on the classical kinetics of nucleation. The first approach developed by Hsu (1962) is widely used to determine ONB in conventional size channels and in micro-channels (Sato and Matsumura 1964; Davis and Anderson 1966; Celata et al. 1997; Qu and Mudawar 2002; Ghiaasiaan and Chedester 2002; Li and Cheng 2
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