Experimental Studies on Flow Boiling in Inclined Tubes: z - In the Regions Encountered in zyx Solar Collectors zy M. K. MADDI and D. P. RAO* Department of Chemical Engineering, Indian Institute of Technology, Kanpur, Kanpur-208016, India Flow boiling in inclined tubes is encountered in solar collectors, but there is no literature on inclined flow boiling on which to base their design. Experimental studies have been carried out on heat transfer, pressure drop and flow maps for flow boiling of water in a circular tube. The angle of inclination was varied from 0” to 90”. The inclination was found to influence the transport processes in the bubbly and the intermittent flow regimes. The correlation of Chen (1966) has been extended to correlate the heat-transfer data to account for the angle of inclination. The Baroczy (1965) and the Lockhart-Martinelli ( 1949) correlations were adapted to correlate the frictional pressure gradient in the inclined flow boiling. The observed flow patterns were presented as flow maps. Les Ccoulements bouillonants dans les tubes inclinCs se rencontre dans les collecteurs solaires, mais il n’existe pas de textes scientifiques sur cette matitre, sur lesquels on pourrait s’appuyer pour leur conception. Des etudes expirimentales ont CtC menCes sur le transfert de chaleur, la perte de charge et les cartes d’Ccoulement pour des Ccoulements d’eau en Cbullition dans un tube circulaire. L’angle d’inclinaison a CtC van6 de 0 h 90°C. On a trouvC que I’inclinaison influenqait les processus de transport en rkgime d’kcoulement bouillonnant et intermittent. La corrClation de Chen (1966) a CtC Ctendue pour corrkler les donnCes de transfert de matitre afin de prendre en compte I’angle d’inclinaison. Les corrtla- tions de Baroczy (1965) et de Lockhart-Martinelli (1949) ont CtC adapt6es pour corrkler le gradient de pression dCi h la friction dans les Ccoulements bouillonants inclinCs. Les modtles d’bcoulement observCs sont prCsentCs h I’aide de cartes d’Ccoulements. Keywords: flow boiling, two-phase flow, solar collectors low boiling is encountered in the solar devices used F for process-steam and power generation, refrigeration, water pumping and in some cases of water heating as well. Solar collector arrays are kept at an inclination to maximize the interception of solar radiation. Thus, the flow boiling in inclined tubes naturally arises in some of the solar devices. Extensive literature is available on flow boiling in vertical and horizontal tubes, dealing with the heat transfer, pressure drop, void fraction and flow patterns (e.g. Hsu and Graham, 1976; Butterworth and Hewitt, 1977; Kakac and Mayinger, 1977; Collier, 1981; Kandlikar, 1990). However, studies on boiling heat transfer in inclined tubes have not been reported in the literature. The pressure drops in two-phase flow in inclined pipes using the air-water system have been reported by Gregory (1975), Collier (1981), Spedding et al. (1982) and more recently by Kokal and Stanislav (1989), but there are no reports on pressure drop in the inclined flow boiling. The flow maps for the inclined flow boiling are also not available. The transport processes in two-phase flow are expected to depend on the geometrical configurations assumed by the liquid and vapour phases in the tube. The angle of inclination is expected to have considerable influence on the configurationsand, hence, on the transport processes. The design of solar collector arrays, in which flow boiling takes place, requires an understanding of the transport processes. The aim of the present experimental study is to obtain pressure-drop and heat-transfer data and to propose correlations which are of use in the design of solar collectors. Experimental A schematic diagram of the boiling loop is given in Figure 1. The main section of the setup consisted of three pre-evaporators, a test section and a glass section for observing the flow patterns. Though there were two test sections, T1 and T2, only T2 was used for the heat-transfer *Author to zyxwvutsrq whom correspondence should be addressed. zyxwvuts To Vent * Figure 1 - Schematic diagram of flow boiling loop. A, Feed water tank; B, Reflux condenser; C, Centrifugal pump; D, Gear pump; El, E2, and E3, Pre-evaporators; F, Rotameter; zy G, Aluminium plate; H, Ball-bearing mounting; HE, Kettle heater; I, Glass section; J, Condenser; L, Expansion bellows; M, Flexible tube; T1 and T2, Test sections; P, Pressure tap; TC, thermocouple; PI, Pressure gauge; V, Control valve; PH, Water preheater tank. measurements. All these sections were carefully joined to form a hydraulically smooth tube. A brass bellow was provided at the end of the glass section to protect it from breakage due to thermal and mechanical shocks. The assembly was rigidly fixed on a thick aluminium plate. The plate was mounted on a ball bearing which was fixed to a rigid frame. The test section could be set at any angle by turning the aluminium plate to the desired angle and clamping it. THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 73, FEBRUARY, 1995 73