Prediction of nucleate boiling mechanisms and burnout conditions, when heat transfer coefficient sharply drops and the heating surface destruction could occur are one of the crucial topics in thermal design and safety analyses of various thermal equipment. Although these phenomena have been intensively investigated for decades, various influencing factors and complexity of coupled thermal and fluid dynamic processes have not yet been fully understood. The integral approach towards prediction of nucleate boiling and burnout conditions requires modelling and numerical simulation of micro level phenomena of bubble rise and departure at a numbers of nucleation sites, as well as macroscopic two-phase mixture behaviour on the heating surface. In this paper multidimensional numerical simulation of the atmospheric saturated pool boiling is performed under high heat fluxes, near to and at the occurrence of burnout conditions. Micro level phenomena on the heating surface are modelled with the key parameters of vapour generation on the heating wall, such as bubble nucleation site density, bubble residence time on the heating wall and certain level of randomness in the location of bubble nucleation. Heat flux is non-uniformly distributed on the heating surface with peaks at the nucleation sites. The nucleation sites are determined by a random function, while the mean number of nucleation sites is prescribed according to the material characteristics and roughness of the heating surface. The applied numerical grids are able to represent the nucleation sites on the heating wall for both fresh (polished) and aged (rough) heaters at the atmospheric pool boiling conditions. The macro level phenomena are modelled with two-fluid model of liquid-vapour flow. The interfacial drag is modelled with appropriate closure laws. The applied modelling and numerical methods enable full representation of the two-phase mixture behaviour on the heating surface with inclusion of the swell level prediction. In this way the integral conditions of nucleate pool boiling with the possibility of burnout are simulated and the critical heat flux conditions are predicted. The result of the three-dimensional numerical simulations and analyses are presented as the extension of the previously published two-dimensional numerical results. Here presented three-dimensional investigation is performed in order to take into account more realistically spatial effects of vapour generation and two-phase flow, such as phase dispersion within the two-phase mixture, than it was able with previously performed two-dimensional investigation. Results are presented for short time period after the initiation of heat supply and vapour generation on the heating surface, as well as for quasi steady-state conditions after several seconds from pool boiling initiation. A replenishment of the heating surface with water and partial surface wetting for lower heat fluxes is shown, while heating surface dry-out is observed for high heat fluxes. The influence of the density of nucleation sites and the bubble residence time on the wall on the pool boiling dynamics is investigated. Also, the influence of the heat flux intensity on the pool boiling dynamics is analysed. Numerical simulations show that decrease of the density of nucleation sites and increase of bubble residence time on the heating surface (characteristics pertinent to fresh-polished heaters) lead to the reduction of critical heat flux values. Obtained results are in excellent agreement with the recent experimental investigations of the upward facing burnout conditions on the horizontal heated plate. Details of the developed numerical procedure are presented. The introduced method of random spatial and temporal generation of the vapour at the heated wall is a new approach. It enables the macroscopic representation of the population of microscopic vapour bubbles, which are generated at nucleation sites on the heater wall, and which burst through liquid micro-layer in thermal-hydraulic conditions close to the burnout. The applied numerical and modelling method has shown robustness by allowing stable calculations for wide ranges of applied modelling boiling parameters (density of nucleation sites and bubble residence time).
Calculation of temperature and velocity fields in the furnace chambers of tube ovens
