Genetic features (basic mechanism) and puzzles of nucleate boiling

Prof. V.V. Yagov

Department of thermal physics

Professor, Doctor of Technical Science

Moscow Power Engineering Institute (Technical University)

Summary

At high reduced pressures extremely high nucleate boiling heat transfer coefficients (HTC) were measured. A single mechanism, which presents a consistent explanation of such HTCs, is very high intensity of liquid evaporation at the periphery of dry spots (nucleation sites) at the heated wall. Due to very small size the nucleation sites can be considered as point heat sinks. Between them convective heat transfer occurs, which in its turn is governed by the inherent mechanisms of boiling. The above two mechanisms comprise a total heat flux from the heated wall in nucleate boiling. The predicting equation, which determines heat flux in boiling via the wall superheat and liquid properties, has been developed with accuracy to two universal numerical factors fitted to the experimental data. Although the equation developed is found to be in good agreement with numerous experimental data for different liquids and in the wide range of reduced pressures and heat fluxes there exists a problem in nucleate boiling, which has not been understood to the full even qualitatively. This problem is the dependence of nucleation site density on the physical properties of the liquid and on the controlling parameters. Obtaining theoretical equation for nucleation site density remains the most significant challenge in nucleate boiling theory.

About the Speaker

Prof. V. V. Yagov is famous specialist in the field of boiling heat transfer and two-phase flow. He has developed an approximate theory of nucleate boiling heat transfer for pure liquids and for binary mixtures, original models for pool and flow boiling crisis, for bubble growth rate and departure. In 2004-2006 under his leadership the project “Heat transfer mechanisms and conditions of thermohydraulic pattern transitions in two-phase flow through fuel microelement bed” supported by the RFBR has been successfully fulfilled.