Critical heat flux induced by flow instability in boiling channels - a review

OZAWA Mamoru¹, UMEKAWA Hisashi¹, MATSUMOTO Ryosuke¹, and
AMI Takeyuki¹

1. Department of Mechanical Engineering, Kansai University Suita, Osaka 564-8680, Japan

　(ozawa@kansai-u.ac.jp, umekawa@kansai-u.ac.jp, matumoto@kansai-u.ac.jp, t_ami@kansai-u.ac.jp )

Abstract: Recent advanced technologies in boiler manufacturing bring about highly efficient water-tube boilers for gas firing or oil firing. These boilers have been designed mainly based on the critical heat flux criteria obtained under steady state. This is, however, not enough. In reality, physical burnout often takes place even after these 4 or 5 decades-long research history. Why? This may be mainly caused by a lack of knowledge on transient or unsteady behavior of two-phase flow in boiler tubes. In addition, there exists a certain discontinuity or gap between the laboratory test and industrial practices, and also discontinuity of knowledge and experience between senor and younger generations is much more serious. This paper describes various aspects of CHF and related phenomena with reference to practical applications in fossil-fuel-fired water-tube boilers. Starting from the various examples of accidents, important issues to be solved are discussed. Then the CHF problems under oscillatory flow condition are described. CHF - i.e. dryout in the present case, is mainly controlled by two-phase flow dynamics and is suitably simulated with a simple lumped-parameter modeling of boiling channel, while many other problems still remains unsolved. And finally a high potential of newly developed approach is demonstrated by realizing inherent void fraction fluctuation and flow pattern map of two-phase flow.

Keyword: boiling channel, critical heat flux, flow instability, density wave oscillation, quenching