Heat Transfer Engineering, 31(4):331–334, 2010 Copyright C  Taylor and Francis Group, LLC ISSN: 0145-7632 print / 1521-0537 online DOI: 10.1080/01457630903312098 Experimental Investigations of Two-Phase Steam Flows in Horizontal Channels of Small Diameter MICHAL LUKASZUK Mechanical Engineering Department, Bialystok Technical University, Bia lystok, Poland This article describes experimental investigations of two-phase ﬂow regimes for steam–water ﬂowing in horizontal ducts of small diameters. In this respect two-phase ﬂow patterns are determined on a test stand based on visualization of real ﬂows. Measurements were performed for a round tube with an internal diameter of 2.8 mm under mass ﬂux ranging from 160 kg/s-m 2 to 1600 kg/s-m 2 at saturation temperatures between 373 K and 403 K. Registered steam quality varied from 0.02 to 0.27. Experimental setup, methodology, and recorded two-phase ﬂow patterns referenced to annular, intermittent, and transient ﬂows are presented. The results obtained have been compared with annular to wavy transition criteria described on the ﬂow map by Soliman. INTRODUCTION Two-phase ﬂows inside channels are encountered in an in- creasing amount of engineering equipment used in the power and process industries. In the last couple of years such ﬂows in channels of small diameter received signiﬁcance attention due to the high heat transfer coefﬁcients. This is an area of grow- ing importance [1] because increasing trends in miniaturization technologies applied for cooling of sophisticated electronic in- struments and for air-cooled compact heat exchangers. Two- phase ﬂow patterns strongly inﬂuence the heat and momentum transfer process; therefore, it is very important for designers to predict what ﬂow pattern is expected based on the ﬂow rate, tube diameter, ﬂuid properties, and vapor quality. There are nu- merous ﬂow regime maps for predicting ﬂow patterns. Most of them are based on experimental observations and some oth- ers are theoretically based [2]. One of them is a map to pre- dicting ﬂow regime transition developed by Soliman [3, 4]. In these works he distinguished between three ﬂow regimes: mist, The author gratefully acknowledges ﬁnancial support for this work from the Ministry of Science and Higher Education of Poland under grant S/WM/3/08 of Bialystok Technical University. Address correspondence to Mr. Micha l Lukaszuk, Mechanical Engineering Department, Bialystok Technical University, ul. Wiejska 45C, 15-351 Bia lystok, Poland. E-mail: lukaszm@pb.edu.pl annular, and wavy ﬂow. In his approach the wavy ﬂow regime includes commonly called stratiﬁed, intermittent (slug, plug), and wavy regimes. There he developed two ﬂow regime tran- sition criteria: mist to annular and annular to wavy transitions. Soliman postulated that the wave to annular transition can be determined based on a balance between inertia and gravitational forces for the liquid ﬁlm. Based on comparisons with experi- mental data [5–8] with ﬂuids: • R-12 (D = 4.8–15.9 mm, T sat ∼ = 303 K). • R-113 (D = 4.8–15.9 mm, T sat ∼ = 333 K). • Steam (D = 13.4 mm, T sat ∼ = 383 K). Soliman [3] concluded that wavy ﬂows are observed for Fr so < 7 and annular ﬂows are observed for Fr so > 7. Dobson [9] and Dobson et al. [10, 11] reported that Fr so = 7 can serve as good indicator of the transition from wavy to wavy annular ﬂow, although a symmetric annular ﬂow was not observed until around Fr so = 18 [12]. Soliman’s investigations concern channels classiﬁed as con- ventional (D > 3 mm) [3]. However, experimental data pre- sented in this paper are obtained for the two-phase ﬂow in a circular channel of small diameter: minichannels (D = 2.8 mm) [3]. The aim of this work is to extend Soliman’s approach for ﬂow regime predictions under steam–water ﬂows in minichan- nels. 331