Enhancement of Heat Transfer in Swirled Boiling Flows

While there are many ways of creating swirl flows in tubes and thus enhancing heat transfer, twisted-tape (TT) inserts are the most practical and widely used. This paper reviews the status of twisted tapes for flow of water with boiling. The general characteristics of TT swirl flow and the mechanisms of heat transfer are described. Subcooled boiling is especially useful for the accommodation of high heat fluxes. Incipient boiling and fully developed boiling are considered. There is considerable discussion of critical heat flux (CHF), since that defines the upper limit of heat flux. The importance of pressure drop on the stability of parallel boiling channels is emphasized. One-sided heating, as found in certain applications, is discussed. The problem of TTs applied to saturated boiling is discussed in general. A CHF correlation based on world data for various types of swirl situations and for both subcooled and saturated boiling, developed recently at MPEI, is presented. Various other aspects of TTs are mentioned. This paper presents much Western literature, and is intended to compliment the paper by E. D. Fedorovich, which has a distinctly Russian perspective.