| Popis: |
Understanding the mechanism of bubble nucleation and growth is critical for optimizing the boiling heat transfer process. The microlayer region formed under the bubble departure is known to be a major contributor to phase change heat transfer but its evolution, spatio-temporal stability, and impact on macroscale bubble dynamics are still relatively unknown. In the thin film microlayer region, the vapor pressure is balanced by a combination of capillary, disjoining, and liquid pressures. Thermally driven evaporation in the curved thin film is suppressed by disjoining pressure at the nanoscale leading to a peak in evaporation flux in the microlayer region. Using a lubrication approximation coupled with conservation laws and the augmented Young-Laplace equation, 3rd order nonlinear film evolution equation is obtained. The evolution equation is solved numerically using the ODE 45 method in MATLAB. A variable wall temperature boundary condition is employed at the solid-liquid interface and is balanced by evaporative heat loss at the liquid-vapor interface. Contrary to most models, the solution begins in the thicker region of the film and proceeds in the direction of reducing film thickness. The solution method is devoid of tuning parameters or guessed inputs. Experimentally measured film thickness and its derivatives are used as inputs in the thicker region and the model is evaluated to obtain a film profile down to the nanoscale and a corresponding local evaporative flux. The modeling results compare favorably with spatio-temporal experimental measurements of the microlayer. By comparing the film profiles at multiple time steps, the accommodation coefficient could be estimated and its influence on the evaporative flux distribution is discussed. |
