Dr. Carlos Dorao

Norwegian University of Science and Technology, Noruega

Heat transfer during convective flow boiling

The unprecedented demand for air conditioning and refrigeration is unleashing extraordinary technological challenges where our limited knowledge about how heat is transferred plays a major role. During flow boiling inside pipes two clear regimes are distinguished, namely nucleate boiling and convective flow boiling. At high heat fluxes, nucleate boiling is dominant, and bubbles produced at the wall are attributed the control of the heat transfer. At low heat fluxes, e.g. related to conventional refrigeration applications, convective flow boiling is dominant, and the heat transfer coefficient is observed to be directly dependent on the mass flux, i.e. the mass flow of refrigerant per cross area of the pipe, and the thermodynamic quality, i.e. the ratio of the mass vapor flow to the total mass flow. During the last 80 years, research has been motivated for predicting the heat transfer and unveiling the dominant physical mechanism transferring heat between the surface and the fluid but with limited success. This fact has limited the development of accurate models and a large number of models  has been developed during the past decades with an increasing number of different dimensionless groups and adjusted parameters that can reach more than a dozen.

This work will focus on analytical and experimental studies focused on identifying the dominant mechanisms controlling heat transfer from the wall to the liquid phase during flow boiling. It will be shown that during convective flow boiling, the heat transfer coefficient is equivalent to the one for single-phase flow when the influence of the vapor velocity is considered.