Nuclear Engineering and Design 240 (2010) 1795–1802 Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes Freezing technique for measuring and predicting the size of droplets in a horizontal annular flow Eo Hwak Lee a , Hee Cheon NO a,∗ , Seung Hun Yoo a , Kyung Won Lee b , Chul-Hwa Song c a Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea b Korea Institute of Nuclear Safety, 19 Guseong-dong, Yuseong-gu, Daejeon 305-338, Republic of Korea c Korea Atomic Energy Research Institute, Yuseong-gu P.O. Box 105, Daejeon 305-600, Republic of Korea article info Article history: Received 30 September 2009 Received in revised form 9 February 2010 Accepted 23 February 2010 abstract A freezing technique for measuring the size of droplets was developed to obtain the droplet size distri- bution in a horizontal annular flow in a pipe with a 37.1 mm diameter. Droplets are frozen by using an extremely low temperature nitrogen gas with liquid film extraction. They are then photographed with a microscope and a CCD camera and measured by means of an image process. The results are compared with various experimental data. The droplet sizes measured by the freezing technique are comparable with those measured by other methods at a high air superficial velocity (of 50 m/s). However, because of the film extraction problem, the droplet sizes measured at a low air superficial velocity of less than 40 m/s are higher than those measured by other methods. A suggested method of predicting the Sauter mean diameter is based on the maximum droplet size correlation for the experimental data, with and without liquid film extraction. The average droplet size is remarkably smaller downstream of the liquid film extractor because large droplets from the liquid film are excluded. © 2010 Elsevier B.V. All rights reserved. 1. Introduction A number of experimental studies in recent decades have used various methods to measure the size of droplets in a horizon- tal annular flow as shown in Table 1. However, few studies have attempted direct measurement through the freezing of droplets in a two-phase air–water flow (Azzopardi, 1997). Azzopardi (1997) pointed out that the literature on droplets and the associated topic of waves in annular gas–liquid flows is extensive. Yet, in spite of the extensive research, some fundamental questions remain unan- swered and large discrepancies exist in the parametric range of the published researches. A couple of correlations achieved good results in terms of predicting the size of droplets but there are still some questions regarding their application. In an annular flow, gas flowing over a liquid film creates the initial droplets. The deformation and breakup of the wave crests are governed by aerodynamic, viscous and surface tension forces. Under certain conditions, hydrodynamic force dominates over the other forces and it leads to an extreme unstable interface of the wave, which results in the breakup of the wave crest into several droplets (Ishii and Grolmes, 1975). In general, we can consider the five basic types of entrainment mechanisms as discussed by Ishii and Grolmes (1975): namely, the shearing off of the tops of large ∗ Corresponding author. Tel.: +82 42 350 3817; fax: +82 42 350 3810. E-mail address: hcno@kaist.ac.kr (H.C. NO). amplitude roll waves from wave crests by a turbulent gas flow; the undercutting of a liquid film by a gas flow; the bursting of gas bubbles; the impingement of liquid droplets or a mass to the film interface; and the entrainment associated with the flooding phenomenon (Fig. 1). Azzopardi (1979) and Tayali et al. (1990) thoroughly reviewed the techniques for measuring the size of droplets: a charge removal method (Tatterson et al., 1977); a needle bridging method (Wicks and Dukler, 1966); an immersion method (Ueda, 1979); a laser grating technique (Lopes and Dukler, 1985); photographic meth- ods (Cousins and Hewitt, 1968; Andreussi et al., 1978; Hay et al., 1996); laser diffraction techniques (Azzopardi et al., 1980; Jepson et al., 1989); and a phase Doppler anemometry technique (Azzopardi and Teixeira, 1994). Though Azzopardi (1979) recommended opti- cal techniques for the study of annular flow, there are several droplet size studies for annular flow with the other droplet size measurement methods, such as photographic methods (Pogson et al., 1970; Lindsted, 1977; Hay et al., 1996) and an immersion method (Hurlburt and Hanratty, 2002). A few studies on droplet size have been reported for horizontal annular flows with laser diffraction (Ribeiro et al., 1995; Azzopardi et al., 1996; Simmons and Hanratty, 2001) and with immersion sampling (Hurlburt and Hanratty, 2002). LDT: Laser diffraction techniques have the advantage of large sample sizes and precise statistics, but the measurement requires a curve-fitting process which is used to convert the scattered light intensity signals into droplet size distribution results based on well- 0029-5493/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2010.02.035