Visualization of foam formation from vertically free-falling impinging water jet
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R E G UL A R P A P E R
Michael Chukwuemeka Ekwonu • Kyung Chun Kim
Visualization of foam formation from vertically free-falling impinging water jet
Received: 3 March 2020 / Revised: 4 June 2020 / Accepted: 6 August 2020 The Visualization Society of Japan 2020
Abstract The phenomenon of foam formation by air entrainment of impinging water jet is reported in this paper. An effort has been made to understand the physics of the phenomenon through visual and photographic observations. The formation of an air sheath and downward motion of the impinging jet are reported. Experimental visualization of the mechanism of air entrainment by foam formation at low jet flowrates of 8:33 106 1:83 106 m3 =s for jet lengths of 13, 18, 23 and 25 cm, respectively, was performed with a vertically free-falling jet. Foam formation via air entrainment has been classified into four types: no entrainment, incipient entrainment, intermittent entrainment and continuous entrainment with increases in jet impinging velocity with fixed jet length. Foam formation depends on the impinging jet velocity, nozzle design, impinging jet length and physical property of the jet such as the surface tension which constitutes the major controlling factors for foam formation via air entrainment of impinging jet. Keywords Visualization Foam formation Vertical impinging jet Air entrainment
1 Introduction Most thermal and nuclear power plants are located on the coast and utilize seawater to cool the power generating equipment. The resulting effluent cooling water is then discharged back into the sea via the power plant outfall (Oh et al. 2008, 2010). The outfall of these power plants results in formation of foams, and residents around these coastal areas have complained of environmental damage such as odor, spoiled scenery and effects on fishing and surfing (Kim et al. 2014). Although sea foam may not be directly harmful to humans, environmentalists feared it could impact negatively on the environment. Bird scientists found that sea foam removes water proofing of bird feather, thereby creating flying difficulty for birds (Kim et al. 2014). Sea foams are composed of mixture of decomposed organic material, e.g., phytoplankton, algae, bacteria, fungi, protozoans, etc. And these microorganisms constitute the dissolved organic matter (DOM), which reduces the surface tension of sea water and thus creates the enabling environment for formation of sea foam via air entrainment and stabilization of foam bubbles. Air entrainment due to plunging liquid jets can be observed in nature and in many industrial processes (Roy et al. 2013). It is responsible for the bulk of mass transfer that occurs at the free surface of oceans, rivers and streams, providing the transport of oxygen and carbon dioxide, and also very important in hydrodynamics because of the breakup mechanism of the entrained air into bubbles, thereby providing a source of noise, surfactant scavenging and cavitation nuclei in the boundary layer and wake of ships (Kiger and Duncan 2012).
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