Characterization of liquid spray impact onto walls and films

• Main Author: Kalantari, Davood
• Format: Others
• Language: English; en
• Published: 2007
• Online Access: http://tuprints.ulb.tu-darmstadt.de/781/1/Dissertation_Kalantari_2006_Lebenslauf.pdf; Kalantari, Davood <http://tuprints.ulb.tu-darmstadt.de/view/person/Kalantari=3ADavood=3A=3A.html> : Characterization of liquid spray impact onto walls and films. [Online-Edition] Technische Universität, Darmstadt [Ph.D. Thesis], (2007)

The present work is concerned with the characterization of liquid spray impact onto walls and films. Empirical studies on spray impact rely almost solely on the phase Doppler instrument for obtaining quantitative data about drop size and velocity distributions. The thesis begins therefore with a careful examination of applying the phase Doppler instrument to new-wall measurements beneath a spray. This includes consideration of the influence of the measurement volume height above the rigid wall, the input laser power and the spatial location of the detection volume on the measured characteristics of the impinging and secondary spray. This knowledge is then used for all subsequent measurements made within the framework of this investigation. After a qualitative and quantitative characterization of the resulting secondary spray and the accumulated wall film, a set of empirical models is presented for prediction of the characteristics of the secondary spray generated due to a liquid spray impact onto a rigid wall. In the models, characterization of the secondary spray has been formulated in terms of correlations for the velocity and trajectory of secondary droplets and the mass and number ratio of the secondary spray. The novel aspect of the model is that the correlations are based on the mean statistics over many events in the spray and not on the outcome of single drop impact experiments. Another interesting feature of the experiments is the rather large range of oblique impact angles captured, due to the different drop trajectories exiting from the spray. A phase Doppler instrument has been used to measure drop size and two components of velocity directly above the target. A high-speed CCD camera has been used to measure the average film thickness formed due to spray impact. In a second step, a theoretical model to predict the average film thickness formed due to a liquid spray impinging onto a flat and rigid wall is presented. This model takes into account the characteristics of the impinging spray, e.g. flux density of impacting droplets, hydrodynamic pressure of the impinging spray and viscosity of the impacting liquid droplets. It considers the mass and momentum balance of the film, including viscosity effect and neglecting the Laplace pressure. A second simplified model for predicting the average film thickness as a function of mean Reynolds number and flux density of the impacting droplets and the average drop diameter is presented based on dimensional analysis. Both theoretical derivations for the average film thickness show good agreement with most of the measurements. This thesis also provides an experimental comparison of the splashing phenomenon for single drops and for drops in a spray, followed by a derived theoretical model. Such a comparison can be very valuable for future modelling of spray impact. The last section of this thesis presents an experimental study for different aspects of liquid spray impact onto a deep liquid layer under well controlled experimental conditions; deformation of the air-liquid film interface due to the hydrodynamic pressure exerted by the impacting drops, the generation of a secondary spray, and the air bubble entrainment into the liquid film. A high-speed CCD camera has been used to measure the deformation of the air-liquid film interface and the distribution of the air bubbles inside the deep liquid film. Two different configurations of a phase Doppler instrument have been used to measure drop size and two components of velocity directly above the film as well as the size and two components of velocity of the air bubbles inside the deep pool.