Phenomenon of White Mist in Pipelines Rapidly Filling with Water with Entrapped Air Pockets Ling Zhou, Ph.D. 1 ; Deyou Liu 2 ; Bryan Karney, M.ASCE 3 ; and Pei Wang 4 Abstract: The phenomenon of white mist in a rapidly filling pipeline containing an entrapped air pocket is numerically and experimentally investigated. The air-water flow patterns, pressure, and temperature histories are synchronously recorded to illustrate their interrelations. The white mist phenomenon is particularly observed during fast transients, especially during the first compression of the air pocket. Comparisons between calculations and experiments indicate that the white mist primarily reflects a condensation process. More specifically, the air temper- ature increases because of rapid compression of an entrapped air pocket, and the high temperature could cause water to adhere to vapor at the pipe surface. For fast transients, the first compression causes a near-adiabatic air compression, but heat exchange effects become more significant in the subsequent compression and expansion cycles. As the initial air length decreases, the maximum pressure first increases and then declines, with the most dangerous air length occurring when about 3.4% is initially occupied by air. The ratio of the maximum pressure to the driving pressure increases approximately linearly with respect to the upstream pressure. A local-interpolation elastic-water model is developed by considering air-temperature change and its validity is confirmed by comparing the model and experimental results. DOI: 10.1061/(ASCE)HY.1943-7900.0000765. © 2013 American Society of Civil Engineers. CE Database subject headings: Water pipelines; Transient flow; Air-water interactions. Author keywords: Water pipeline system; Transient flow; Entrapped air pocket; White mist; Air-water interface. Introduction Air pockets are frequently entrapped in water pipelines. When a pump starts or an upstream valve is opened abruptly, a strong and potentially destructive pressure increase can be created as the entrapped air pocket is compressed (Martin 1976; Zhou et al. 2004; Wylie and Streeter 1993). The writers have devoted substantial attention to entrapped air pockets in water-filling pipeline systems. Liu et al. (2011) devel- oped a rigid-plug elastic-water model that effectively avoids the interpolation error of the method of characteristics (MOC) and reduces its complexity when tracking the air-water interface. Zhou et al. (2011, 2013b) experimentally investigated the effect of the initial air volume on the maximum pressure in a dead-end pipeline with one entrapped air pocket and also proposed the virtual plug elastic-water column model by neglecting the liquid inertia and energy loss of a short water column near the air-water interface. Zhou et al. (2013a) investigated the transient pressure associated with a rapidly filling pipeline containing two entrapped air pockets experimentally and numerically. Zhou and Liu (2013) experimen- tally investigated the effect of tail water on a pressure surge during rapid filling of a partially full pipe having a dead end; the phenom- enon of white mist was first observed by the writers in these exper- iments. When the valve is instantaneously opened, the white mist sometimes arose during the first compression of an entrapped air pocket; after that, the white mist is quickly scoured because of the flow turbulence. Moreover, it is difficult to monitor air temper- ature in a horizontal pipeline because turbulent water can easily contact the temperature transducer. In this paper, to explore the phenomenon a horizontal-vertical pipeline is constructed with the transparent glass; the vertical assists in temperature measurements by providing a better air-water separation. A high-speed, high- resolution digital video camera is also used to record the air-water flow during the filling process. The phenomenon of white mist was also observed in the experiments of a rapidly filling horizontal- vertical pipeline. The observations show that the phenomenon should be associated with air behavior. The phenomenon of white mist has not been reported in the pub- lished literature, although the subject on entrapped air pockets in water pipeline systems has been widely discussed. Many research- ers (Chaudhry 1987; Fox 1989; Swaffield and Boldy 1993; Thorley 2004) suggest approaches to modeling the air behavior, depending on the transient characteristics. Generally, fast transients have insufficient time to allow thermal transfers and the air behavior can be considered to be adiabatic. When the filling process is slow, the isothermal assumption is reasonable. Other researchers (Evans and Crawford 1954; Parmakian 1955; Ruus and Karney 1997) modeled the entrapped air pocket as the intermediate polytropic case. Graze (1996) reported that the compression period is close to the adiabatic condition because of reduced heat transfer, whereas the expansion period shows unstable flow by the possibility of large heat transfer. However, Graze’ s method has proved difficult to implement. Martin (1976), Izquierdo et al. (1999), and Abreu et al. (1991) assumed different air processes to simulate the entrapped air 1 Lecturer, College of Water Conservancy and Hydropower Engineering, Hohai Univ., 1 Xikang Rd., Nanjing 210098, China (corresponding author). E-mail: zlhhu@163.com 2 Professor, College of Water Conservancy and Hydropower Engi- neering, Hohai Univ., 1 Xikang Rd., Nanjing 210098, China. E-mail: Liudyhhuc@163.com 3 Professor, Dept. of Civil Engineering, Univ. of Toronto, 35 St. George St., Toronto, ON, Canada M5S 1A4. E-mail: karney@ecf.utoronto.ca 4 Ph.D. Candidate, College of Water Conservancy and Hydropower Engineering, Hohai Univ., 1 Xikang Rd., Nanjing 210098, China. E-mail: franciswp2012@163.com Note. This manuscript was submitted on March 12, 2012; approved on April 11, 2013; published online on April 13, 2013. Discussion period open until March 1, 2014; separate discussions must be submitted for individual papers. This paper is part of the Journal of Hydraulic Engineering, Vol. 139, No. 10, October 1, 2013. © ASCE, ISSN 0733-9429/2013/ 10-1041-1051/$25.00. JOURNAL OF HYDRAULIC ENGINEERING © ASCE / OCTOBER 2013 / 1041 J. Hydraul. Eng. 2013.139:1041-1051. Downloaded from ascelibrary.org by UNIVERSITY OF TORONTO on 06/03/14. Copyright ASCE. For personal use only; all rights reserved.