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Wall Boiling Models

By Gladys Murray,2014-06-23 15:25
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Wall Boiling Models

17.5.16. Wall Boiling Models

    17.5.16.1. Overview

    The term “subcooled boiling” is used to describe the physical situation where the wall temperature is high enough to cause boiling to occur at the wall even though the bulk volume averaged liquid temperature is less than the saturation value. In such cases, the energy is transferred directly from the wall to the liquid. Part of this energy will cause the temperature of the liquid to increase and part will generate vapor. Interphase heat transfer will also cause the average liquid temperature to increase, however, the saturated vapor will condense. Additionally, some of the energy may be transferred directly from the wall to the vapor. These basic mechanisms are the foundations of the so called Rensselaer Polytechnic Institute (RPI) models.

    In ANSYS FLUENT, the wall boiling models are developed in the context of the Eulerian multiphase model. The multiphase flows are governed by the conservation equations for phase continuity (Equation 17119), momentum

    (Equation 17120), and energy (Equation 17126). The wall boiling

    phenomenon is modeled by the RPI nucleate boiling model of Kurual and Podowski [190] and an extended formulation for the departed nucleate boiling regime (DNB) by Lavieville et al [200].

    The wall boiling models are compatible with three different wall boundaries: isothermal wall, specified heat flux, and specified heat transfer coefficient (coupled wall boundary).

    Specific submodels have been considered to account for the interfacial transfers of momentum, mass, and heat, as well as turbulence models in boiling flows, as described below.

    To learn how to set up the boiling model, please refer to Including the

    Boiling Model.

    17.5.16.2. RPI Model

    According to the basic RPI model, the total heat flux from the wall to the liquid is partitioned into three components, namely the convective heat flux, the quenching heat flux, and the evaporative heat flux:

    (17274)

    The heated wall surface is subdivided into area , which is covered by nucleating bubbles and a portion , which is covered by the fluid.

    ; The convective heat flux is expressed as

    (17275)

    where is the single phase heat transfer coefficient, and and

     are the wall and liquid temperatures, respectively.

    ; The quenching heat flux models the cyclic averaged transient

    energy transfer related to liquid filling the wall vicinity after

    bubble detachment, and is expressed as

    (17276)

    Where is the conductivity, is the periodic time, and is the diffusivity.

    ; The evaporative flux is given by

    (17277)

    Where is the volume of the bubble based on the bubble departure diameter, is the active nucleate site density, is the vapor density, and is the latent heat of evaporation, and is the bubble departure frequency. These equations need closure for the following parameters:

    ; Area of Influence

    Its definition is based on the departure diameter and the nucleate site density:

    (17278)

    Note that in order to avoid numerical instabilities due to unbound empirical correlations for the nucleate site density, the area of influence has to be restricted. The area of influence is limited as follows:

    (17279)

    The value of the empirical constant is usually set to 4, however it has been found that this value is not universal and may vary between 1.8