Session: 11-08-02: Multiphase Flows and Applications II

Paper Number: 167312

Dynamics of Dripping From Tubular Surfaces 

The dynamics of droplet dripping from tubular surfaces are fundamental to various industrial applications, including membrane distillation, liquid transport, and filtration processes. This study experimentally investigates the interplay between gravitational and interfacial forces in droplet formation, spreading, and detachment from hydrophobic membranes and glass plates. Droplet behavior was examined on three distinct surface types: a porous polypropylene (PP) cylindrical surface, a dense PP cylindrical surface, and a hydrophobic flat glass plate. The porous PP membrane, a commercial tubular membrane (Microdyn®), is widely used in membrane distillation due to its ease of cleaning and ability to treat highly concentrated brines. This membrane has a porosity of 85% with an average pore size of 0.2 μm, an inner diameter of 5.6 mm, and an outer diameter of 8.5 mm. In contrast, the dense PP cylindrical surfaces consisted of solid rods with diameters of 1/8, 1/4, and 1/2 inch, exhibiting a hardness in the range of Rockwell R80-R90. The hydrophilic glass plate was modified with a FluroPel 1604V fluoropolymer coating, and baked at 180°C, to render it hydrophobic. Droplet images before and after coating confirmed the modification.

Droplet formation and detachment dynamics were captured using a high-speed camera. The maximum Bond number observed before necking initiated was 3.5, indicating the point at which gravitational forces overcame interfacial forces, independent of liquid flow rate. The dripping cone geometry was characterized by its cone angle and base diameter, while the detached droplet size increased with decreasing membrane diameter. The test liquids included distilled water, with a density of 998 kg/m³, a surface tension of 72.8 mN/m, and a viscosity of 0.009 Pa·s at room temperature.

The experimental procedure involved depositing controlled volumes of water onto both tubular and flat surfaces. For the dripping test condition, a rotameter was used to regulate the flow rate. Each test was conducted under controlled conditions, with relative humidity maintained at 50% and room temperature at 20°C. After each test, the cylindrical surface was dried using compressed air before repeating the experiment to ensure consistency. Droplet images were processed using ImageJ software, with contact angles and droplet spreading analyzed. The resolution of the base measurement was 14 μm, and contact angles were measured with a precision of ±2.5 degrees. Repeated trials ensured measurement uncertainty was minimized to within 10% at a 95% confidence interval.

This study provides comprehensive insights into the fundamental mechanisms governing droplet dripping from tubular surfaces, contributing to the understanding of fluid transport phenomena in industrial and environmental applications.

Presenting Author: Khaled Sallam Oklahoma State University

Presenting Author Biography: Dr. Khaled Sallam is the associate head of the School of Mechanical and Aerospace Engineering at Oklahoma State University. He is an expert in the thermal-fluid sciences, with a particular focus on atomization and spray processes, high-speed propulsion, and energy conversion. He received his Ph.D. in Aerospace Engineering from the University of Michigan, Ann Arbor, in 2002. Throughout his career, Dr. Sallam has authored more than 75 journal articles and conference papers. His research contributions in atomization and spray systems have been recognized by the Institute for Liquid Atomization and Spray Systems (ILASS) – North and South America, where he received the W.R. Marshall Award, as well as through a Summer Faculty Fellowship at the Air Force Research Laboratory at Wright-Patterson Air Force Base. Dr. Sallam has demonstrated leadership within the engineering community, serving as Chair of the ASME Mid-Continent Section. He is an Associate Fellow of the American Institute of Aeronautics and Astronautics (AIAA).

Authors:

Bipin Kafle Oklahoma State University
Khaled Sallam Oklahoma State University