Numerical simulation of condensation in boiling (bubbly) flow

In the framework of safety studies of Pressurized Water Reactors, boiling bubbly flow and Departure from Nucleate Boiling are now investigated at the CFD scale in the NEPTUNECFD code developed in the framework of the NEPTUNE project, financially supported by CEA (Commissariat a l'energie Atomique), EDF, IRSN (Institut de Radioprotection et de Surete Nucleaire) and AREVA-NP. The prediction of subcooled convective boiling flows throughout PWR assemblies relies on both prediction of nucleation of bubbles in the wall region and of their life time in the core flow where subcooled liquid may induce condensation. In the perspective of validation of CFD simulations of convective nucleate boiling flows, there is a need in assessment of both boiling and condensation independently. This study extends the validation of the NEPTUNECFD code to liquid-vapour bubbly flows by focusing on the subcooled flows with condensation but in the absence of wall heat transfer. Experimental data incoming from the TESS experimental program are used to validate the models for (i) bubble migration from wall to core, and (ii) interfacial heat transfers that result in bubbles condensation. In a first part, the experimental data are presented and analysed. The experimental device consists in a two-part vertical cylindrical pipe of diameter close to hydraulic diameter of a typical sub-channel of a PWR assembly. In the lower upstream part, subcooled liquid is injected and the wall heating induces nucleate boiling regime. Vapor volumetric fraction increases and near-wall liquid is heated up. In the upper downstream part of the pipe, the wall is adiabatic and measurements are made at different axial and radial positions: a thermocouple measures the liquid temperature whereas a two-tip optical probe provides information about vapor volumetric fraction, vapor bubbles size distribution, and bubbles velocities. The fluid used is R-134A cooling fluid. Thanks to dimensional analysis, the flow and fluid properties allow then to simulate with a low pressure the industrial configuration in terms of mass flux, pressure, and wall heat-flux to critical heat flux ratio. The experimental data provide a detailed map of the vapor flow characteristics above the wall-heated part. An interpretation of these results is provided in the present study to compare the condensation rate with classical correlations, as well as to characterize the migration of bubbles and to identify the possible coalescence of bubbles. Experimental uncertainties are discussed. In a second part, numerical results allow to analyse the existing modelling for vapor-liquid pipe bubbly flow in NEPTUNECFD code. Only the non-heated part of the pipe is simulated, in order to consider the results independently from the modelling of nucleate boiling. Experimental profiles allow determining boundary conditions for the vapor flow as well as for the liquid temperature at the inlet of the simulation domain. We analyze the sensitivity of the results to some numerical and modelling issues. In particular, we apply to some extent the Best Practice Guidelines by investigating the mesh influence, in terms of both geometry and cell size. We study a way to build boundary conditions for the liquid flow, considering several levels of turbulence and show the need for a fine determination of these data, in order not to perturb the two-phase flow analysis. Several modelling issues concerning bubbles migration, bubbles size evolution, and turbulent dispersion are discussed. Finally conclusions are drawn on the condensation based on the comparison of experimental data and available correlations