Arvind Pattamatta

Understanding the droplet-hot wall interaction is crucial in industrial applications such as spray cooling, desalination and refrigeration, and IC engines. The present study focusses on simulations of two consecutively impinging concentric droplets on a heated surface in ambient conditions. A numerical model with fluid-solid coupling is implemented in opensource CFD software OpenFOAM considering contact line evaporation and dynamic contact line motion. The implemented numerical solver is assessed by validating it against the sessile droplet evaporation case available in the literature, followed by in-house experimental results of a single droplet impact. Consequently, the validated solver is used to conduct the simulations of consecutive droplet impingement on a hot surface, considering the droplet flow rate (i.e., the time interval between the two droplets) and surface thickness as parameters. The droplet flow rate is chosen in the order of 102−103 droplets per second (time interval is in the range of 3 - 100 milliseconds), and two variants of the surface thickness of 0.025 and 2 mm are used. The spread and heat transfer dynamics of each droplet are calculated in terms of spread factor, dimensionless input and evaporation heat transfers, respectively, and compared at all parametric conditions. The study reveals that the selected droplet flow rate affects the spread dynamics of the two droplets, resulting in various droplet heat transfer patterns. Moreover, it is observed that a surface with higher thickness results in more droplet heat transfer due to large thermal inertia. The numerical results are in good agreement with theoretical calculations of maximum spread factor and corresponding droplet heat transfer.

The present study aims at the numerical investigation of drop-on-drop impingement over heated surfaces. Drop-on-drop impact has been found to be one of the basic processes in spray cooling applications. A two-phase solver implemented in open source CFD toolbox OpenFOAM, is used with VOF interface tracking technique which considers contact line evaporation and dynamic contact line motion. This numerical model is validated using the experimental data available in literature for single drop-hot wall interactions. The hydrodynamic behaviour (in terms of spread factor) and evaporation dynamics (in terms of input and evaporation heat transfers) of drop-on-drop impingement is compared to a single drop impact over a heated surface under similar conditions. It was found that the spread factor is higher for drop-on-drop impingement than the single droplet impact. However, high input and evaporation heat transfers were observed for the case of single droplet due to the high spread surface area-to-volume ratio. In addition, a parametric study is carried out to study the effect of some of the influencing parameters on the drop-on-drop impingement, choosing Weber number (We), Bond number (Bo), Jakob number (Ja) and Radius ratio (R∗) as parameters. Results showed that all these parameters substantially affect the spread and evaporation dynamics of the drop-on-drop collision over a heated surface. Also based on the conservation of energy principle, an analytical model is developed to find the maximum spread factor. In order to quantify the simulation heat transfer effects, a correlation for input heat transfer available in the literature is used to review the numerical findings. Both theoretical and simulation results are in good agreement with a maximum deviation of 10% in spread factor and 20% in input heat transfer prediction.

Extensive studies of two concentric droplets consecutively impinging over a thin heated foil surface are carried out to compare the spread and heat transfer dynamics of a single drop, and drop-on-drop configurations using high speed imaging and infrared thermography. Millimeter-sized deionized water droplets (2.80 ± 0.04 mm) are impinged upon a heated Inconel surface (thickness of 25 μm) from a fixed height corresponding to a Weber number (We) of 50 ± 2 and Reynolds number (Re) of 3180 ± 90 with a flow rate of 20 droplets per minute. Surface temperature is chosen as a parameter, and is varied from 22 °C (non-heated) to 175 °C. Temperature and heat flux distributions associated with droplet-surface interactions are obtained, and the outcomes of the process are measured in terms of spread diameter, droplet input heat transfer, dynamic contact angle, and surface mean temperature. A decline in the droplet heat transfer for drop-on-drop impingement is observed for all temperatures investigated in the present work. This is attributed to the surface pre-cooling by the initial droplet and also to the reduced surface area-to-volume ratio i.e., increased spreading film thickness. High heat transfer rates are observed around the three-phase contact line region, especially during the receding phase of the droplet, for both configurations, confirming the significance of contact line evaporation in droplet-hot wall interactions. Theoretical models predicting the maximum spread factor and corresponding input heat transfer into the droplet are identified from the literature, and found to be in good agreement with present experimental observations.