# Film boiling around a Finite Size Cylindrical Specim--A Transient Conjugate Heat Transfer Approach
###### Abstract
It should be noted that only the energy jump mass transfer model, among the three presented models, stems from basic principles, i.e., it does not require any empirical input to estimate the mass transfer across the interface. On the other hand, it requires an initial vapor cell to induce mass transfer, thus being incompatible for bulk boiling simulations. An exception to this general rule was found by [PERSON] et al. [8], wherein the local cell size and gradient of the volume field were involved in the Lee mass transfer model. A study of the pool boiling process in which a film boiling regime was achieved as a consequence of exceeding the critical heat flux (CHF) limit was carried out by [PERSON] and [PERSON] [9]. The authors used the aforementioned approach [5] for handling multiphase flow with a phase change in an Eulerian framework, that is, using a single set of governing equations with a color function as an interface indicator, but adapting it to turbulent flow modeling using a large eddy simulation (LES) approach. More recently, the simulation of the dispersed flow film boiling (DFFB) regime using the level set method (LSM) in conjunction with direct numerical simulation (DNS) of turbulence was presented by [PERSON] and [PERSON] [10]. This adiabatic flow study was carried out using \"in-house\" computational software PHASTA ([[https://docs.cci.rpi.edu/software/PHASTA/](https://docs.cci.rpi.edu/software/PHASTA/)]([https://docs.cci.rpi.edu/software/PHASTA/](https://docs.cci.rpi.edu/software/PHASTA/))), based on the stabilized finite element method, with conditions at the inlet obtained using single-phase computation. More recent studies involved the [PERSON] model's application in modeling of film boiling around a horizontal cylinder, such as that presented by [PERSON] et al. [11]. In summary, fundamental studies have revealed the main issues regarding mass transfer modeling, and demonstrated the main mass transfer models; among them, the energy balance model has been identified as the most prominent, since it does not rely on empirical constants and stems from pure energy balance at the interface between the fluids. The two main issues in successful resolution of film boiling during immersion quenching using this model are:  
1. Reconstruction of the temperature gradient at the vapor-liquid interface in the mass transfer model. 2. Adequate turbulence modeling. The necessary prerequisite to this end (first item) is the existence of the identifiable interface between the phases, such that mass transfer may take place at the interface. This is due to the gradient of volume fraction incorporated in the computation of interfacial area density. Furthermore, an additional remark must be pointed out regarding turbulence modeling (second item), since the turbulent heat flux is defined by consideration of the effective thermal conductivity and has to be computed accurately in order to estimate the transient temperature distribution in a solid object correctly. Hence, more elaborate turbulence modeling has to be carried out, since there is a complex interaction between the vapor and liquid phases in the process. However, these approaches are mostly assigned to one-fluid formulations, such as VOF or LSM, wherein a single set of governing equations is solved for both phases, and hence individual phase modeling, instead of their mixture, becomes difficult to handle. Therefore, in this paper, a novel method for the estimation of mass transfer across the interface in a thermally driven phase change process is presented. The method is implemented within a two-fluid VOF formulation (available in the commercial CFD software ANSYS Fluent). It consolidates the benefits of the two multiscale multiphase modeling approaches--low-resolution meshes and large time steps from the two-fluid formulation--whilst preserving the sharp interface treatment using a geometrical reconstruction algorithm, that is, a property of the pure VOF method, generally used for DNS of interface motion.