Boiling phenomena mechanisms

Mechanisms of CHF in Fool Boiling. The mechanism of the critical heat flux phenomenon in pool boiling has been the subject of widespread interest and controversy. Recent reviews relating to mechanisms are presented by Katto [157], Dhir [87], and Bergles [158]. The postulated mechanisms can be approximately classified into four types as follows ... 

In this section, the phenomenon of BLEVE is discussed according to theories proposed by Reid (1976), Board (1975), and Venart (1990). Reid (1979, 1980) based a theory about the BLEVE mechanism on the phenomenon of superheated liquids. When heat is transferred to a liquid, the temperature of the liquid rises. When the boiling point is reached, the liquid starts to form vapor bubbles at active sites. These active sites occur at interfaces with solids, including vessel walls. 

The main difficulty is to establish the dependence of the heat transfer coefficient on vapour quality in relation to different mechanisms controlling flow boiling. Some correlations do not take into account the two mechanisms. Others account for convective and nucleate boiling. To the present author s knowledge, none take into account the influence of channel size. The aim here is to summarise recent work on flow boiling, to describe an experiment on the phenomenon in minichannels and to compare the results with classical correlations. 

Correlations for Boiling Heat Transfer Nucleate boiling is a complex phenomenon. Several mechanisms have been proposed to explain the boiling process, such as latent heat transport, microconvection, vapor-liquid exchange, wake flow, enhanced convection, and microlayer evaporation, details of which can be found in Ginoux (1978). 

More recent research studies on the heat transfer deterioration have revealed the following characteristics. Generally, the heat transfer deterioration phenomenon occurs only around the critical point (for water, the critical point is at 374.2°C and 22.1 MPa) or the pseudocritical temperature. The mechanisms of the heat transfer deterioration differ from those of the boiling crises of the subcritical pressure. Compared with the boiling crisis, the temperature rise of the heated surface wall is milder. The post deterioration heat transfer rate can be predicted by numerical analyses based on turbulence models and the occurrence of the heat transfer deterioration can be suppressed by promoting the turbulence. 

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