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Subcooled Flow Boiling Critical Heat Flux (CHF) and Its Application to Fusion Energy Components. Part II. A Review of Microconvective, Experimental, and Correlational Aspects

Ronald D. Boyd

Fusion Science and Technology

Volume 7 / Number 1 / January 1985 / Pages 31-52


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Microconvective, instability, experimental, and correlational aspects of subcooled flow boiling critical heat flux (CHF) are summarized. The present understanding of CHF in subcooled flow boiling is reviewed and research directions that will permit the accommodation of higher heat fluxes are outlined. This survey (Parts I and II), which contains a representative coverage of the literature over the last 30 years, is concerned only with CHF in the subcooled flow boiling regime, and unless otherwise noted, all references to CHF are confined to that regime. The following are considered major areas that pose momentary constraints to the high-heat flux removal from fusion components:

  1. There is a scarcity of CHF data between 0.1 and 2.0 kW/cm2 for large length-to-diameter ratios (between 50 and 600) for both single- and multiple-channel components.
  2. Existing CHF correlations for subcooled flow boiling can have large errors and are usually confined to narrow ranges of parameters.
  3. The mechanistic understanding has advanced little beyond that which is included in Gunther's (early 1950s), Jiji and Clark's (mid 1960s), and Mattson's (early 1970s) flow visualizations.
  4. The CHF models emphasize hydrodynamic effects and minimize coupled thermal and mass transfer effects.
  5. Little attention has been devoted to investigating thermal-hydraulic instabilities associated with parallel channels, subchannels, interconnected channels, boiling transition, thermal relaxation, and condensation-induced instabilities.
  6. Insufficient quality control has been exercised in measuring CHF.
In all cases, optimum values of CHF are desired. This implies that the experimental direction will be one of steepest ascent through a complicated CHF/parameter space.

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