Two-phase pressure drops in a helically coiled steam generator Lorenzo Santini * , Andrea Cioncolini 1 , Carlo Lombardi, Marco Ricotti Nuclear Engineering Department, Politecnico di Milano, Via Ponzio 34/3, 20133 Milano, Italy Received 26 February 2007 Available online 21 April 2008 Abstract An experimental investigation regarding two-phase diabatic pressure drops inside a helically coiled heat exchanger have been carried out at SIET thermo-hydraulics labs in Piacenza (Italy). The experimental campaign is part of a wide program of study of the IRIS inno- vative reactor steam generator. The test section consists of an AISI 316 stainless steel tube, 32 m length, 12.53 mm inner diameter, curved in helical shape with a bend radius of 0.5 m and a helix pitch of 0.8 m, resulting in a total height of the steam generator tube of 8 m. The explored operating conditions for two-phase flow experiences range from 192 to 824 kg/m 2 s for the mass flux, from 0 to 1 for the quality, from 1.1 to 6.3 MPa for the pressure, from 50 to 200 kW/m 2 for the heat fluxes. A frictional two-phase pressure drops correlation, based on an energy balance of the two-phase mixture and including the 940 experimental points, is proposed. Comparison with existing cor- relations shows the difficulty in predicting two-phase pressure drops in helical coil steam generators. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Coils; Helically coiled pipes; Steam generator; Two-phase flow; Pressure drops 1. Introduction Heat exchangers are one of the most common techno- logical device applied in power, chemical and food process- ing industries. Many options are available for obtaining compactness and efficiency in exchanging thermal power. In the field of tubular heat exchangers one possible way for reducing the space occupied by the exchanger is by bending tube axis in helicoidal shape. This option is partic- ularly suitable when construction simplicity is needed and when the geometry of the place in which the exchanger has to be housed is the cylindrical one. Many advantages derive from this disposition, such as an excellent behaviour in presence of severe thermal expansions; in fact the helical shape allows the exchanger to behave as a spring, thus accommodating the stresses due to the expansions. More- over the exchanger has the possibility of working locally in the cross-flow disposition and globally in counter-flow (the so called cross-counter principle applied in many shell and tube exchangers), thus combining all the positive aspects of the two arrangement. Helically coiled heat exchangers in the multi-start disposition has no internal baffle leakage problems and are little sensitive to flow mal- distributions [1]. However, some difficulties could rise in the manufacturing process and in the bundle fabrication phase, thus increasing costs. In the past many industrial applications of helically coiled tube-bundle heat exchangers have been realized: nat- ural gas liquefaction apparatus [1], solar energy concentra- tor receivers [2] and many steam generators for nuclear power plants (e.g. Otto Hahn nuclear ship, SuperPhoenix fast reactor, AGR , Fort St. Vrain HTGR, THTR-300 etc.). Up to now there are several projects in nuclear indus- try for electricity production involving helically coiled steam generators [3]. The presence of two new geometrical variables, such as coil diameter and coil pitch, renders single-phase thermo- hydraulics phenomena in coiled ducts more complex than 0017-9310/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2008.02.034 * Corresponding author. Tel.: +39 02 2399 6330; fax: +39 02 2399 6309. E-mail address: lorenzo.santini@polimi.it (L. Santini). 1 Present address: Heat and Mass Transfer Laboratory, EPFL-E ´ cole Polytechnique Fe ´de ´rale de Lausanne, EPFL-STI-IGM-LTCM, Mail 9, CH-1015 Lausanne, Switzerland. www.elsevier.com/locate/ijhmt Available online at www.sciencedirect.com International Journal of Heat and Mass Transfer 51 (2008) 4926–4939