Hydrodynamics and Heat Transfer in Gas-Driven Liquid Film Flows

In many technical applications such as cooling systems used in chemical industry and fuel injection systems of modern gas turbines, thin liquid films driven by gravity or turbulent gas stream can be found. Since the thermo-hydrodynamic process in such liquid-gas flow configuration is rather complex, the transport mechanisms are not well understood. However, numerical simulations or theoretical models rely on these mechanisms. Experimental investigations are necessary in order to delineate this complex thermo-hydrodynamic phenomenon and to provide validation data to the theoreticians. The main objective of this work is to study the hydrodynamics and convective heat transfer in gravity and gas-driven thin liquid films on uniformly heated walls. To achieve this, an experimental set-up has been designed and measurements were performed in a flow channel. The liquid film was annularly applied on a vertically mounted heated tube. In the arranged two-phase flow domain, both fluids were thermally and hydrodynamically developing. The Reynolds numbers of liquid and gas flows were varied between 80 - 800 and 10000 - 100000, respectively. The wall heat flux was kept constant at 15 W/cm2. The gas velocity profile in the flow channel was measured with hot-wire anemometry to determine the shear stresses on the dry wall surface. The effect of surface topography of the wall was investigated. The hydrodynamics and heat transfer of gas-driven liquid films was studied on micro-structures heated tubes. The dynamics of the liquid film flow was recorded by high-speed shadowgraphy technique. Using the high-speed images, the wave amplitudes and wave frequencies were determined. A high-speed infrared camera was used to qualitatively visualize the film rupture on micro-structured surfaces. The wall temperature distribution in streamwise direction was measured using thermocouples embedded inside the heated wall. Correlations for Nusselt number at unstructured surfaces have been proposed. This study reveals that the action of shear stress at a thin liquid layer flowing along an unstructured wall has a remarkable influence on the stability of the liquid-gas interface. Disturbances at the liquid film surface appear as the shear stress reaches a critical value. Measurements at various axial locations show that the fluctuations grow in the flow direction. The rate of growth is determined by the gas and liquid mass flow rates. With the increase in liquid Reynolds number, the liquid free-surface deformation is suppressed and the temporal film thickness fluctuations in the flow direction either decrease slightly or remain constant. A significant enhancement in heat transfer happens when the shear stress at the liquid-gas interface increases. However, there exists a threshold level of shear stress, only beyond which this is true. This exists as identified by comparing the experimentally determined heat transfer coefficients with the solutions of the classical Graetz-Nusselt model. Furthermore, the Nusselt numbers are compared with the Nusselt numbers of laminar, hydrodynamically and thermally developed falling films, falling films which develop thermally and those which are in the transition regime from laminar to turbulent flow, and with a turbulence model used from the literature. The comparison shows, that above gas Reynolds numbers larger than 70000, the heat transfer coefficient is following the trend predicted by the turbulence model. Micro-structures have significant influence on the waviness of gas-driven liquid films. With increasing shear stress and liquid mass flow rate, the film waviness increases. Especially micro-structures embossed with obstacles normal to the flow direction lead to large wave amplitudes and high wave frequencies at low shear stress compared to the unstructured surface and the surfaces, incorporating structures oriented parallel to the flow direction. At low liquid mass flow rates and high shear stress, the area of local film rupture increases. Furthermore, micro-structures significantly enhance the heat transfer compared to the unstructured surface. Especially micro-structures combined by longitudinal and horizontal geometries are very effective in heat transfer enhancement at low shear stress and comparably low liquid mass flow rates.