Framework for a Unified Model for Nucleate and Transition Pool Boiling

Abstract

An area and time-averaged model for saturated pool boiling heat fluxes has been developed. In the model, which is valid in the upper end of nucleate boiling and in transition boiling, the existence of stationary vapor stems at the wall is assumed. The energy from the wall is conducted into the liquid macro/micro thermal layer surrounding the stems and is utilized in evaporation at the stationary liquid–vapor interface. The heat transfer rate into the thermal layer and the temperature distribution in it are determined by solving a two-dimensional steady-state conduction equation. The evaporation rate is given by the kinetic theory. The heater surface area over which the vapor stems exist is taken to be dry. Employing experimentally observed void fractions, not only the nucleate and transition boiling heat fluxes but also the maximum and minimum heat fluxes are predicted from the model. The maximum heat fluxes obtained from the model are valid only for surfaces that are not well wetted and includes the contact angle as one of the parameters.

A Comprehensive Model for Liquid Film Boiling in Internal Combustion Engines

“…Furthermore, Dhir and Liaw [21] found that during pool boiling, α dry (T w ≈ Τ Ν ) varies between 0.68 and 0.8 when contact angles vary between 27 and 90 degrees respectively.…”

Do surfaces with mixed hydrophilic and hydrophobic areas enhance pool boiling?

“…First, nucleation sites in appropriate number and dimensions need to be provided such as cavities, rough areas or hydrophobic islands 2 . As of today, the performance of boiling surfaces has been increased by using wicking structures to prevent dry out 3 , by increasing the surface area with fins or fluidized bed [3][4][5][6] , and by enhancing the wettability of the surface [5][6][7][8][9] . The latter strategy is justified by experiments of Wang and Dhir 10 , showing that the CHF was increased by enhancing surface wettability.…”

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“…Such modification generally increases surface wettability, which causes an increased CHF through the enhanced liquid spreading over the heated area. 14,15 Another effect of the surface modification is to increase the number of microscale cavities, which serve as the starting sites for heterogeneous nucleation of liquid for bubble formation. 1,2 The relevant sizes of such cavities are generally in the range of tens to hundreds of micrometers.…”