Flow boiling in horizontal flattened tubes: Part I – Two-phase frictional pressure drop results and model Jesús Moreno Quibén a,b , Lixin Cheng a,c , Ricardo J. da Silva Lima a , John R. Thome a, * a Laboratory of Heat and Mass Transfer (LTCM), Faculty of Engineering (STI), École Polytechnique Fédérale de Lausanne (EPFL), Station 9, CH-1015 Lausanne, Switzerland b Wolverine LDA, Apartado 21-4740 Esposende, Portugal c School of Engineering, University of Aberdeen, King’s College, Aberdeen, AB24 3UE Scotland, UK article info Article history: Received 26 November 2008 Accepted by 17 December 2008 Keywords: Flow boiling Two-phase flow Diabatic Frictional pressure drop Flattened tube Flow patterns Phenomenological Experiment Model R22 R410A abstract Experiments of diabatic two-phase pressure drops in flow boiling were conducted in four horizontal flat- tened smooth copper tubes with two different heights of 2 and 3 mm. The equivalent diameters of the flat tubes are 8.6, 7.17, 6.25, and 5.3 mm. The working fluids are R22 and R410A, respectively. The test con- ditions are: mass velocities from 150 to 500 kg/m 2 s, heat fluxes from 6 to 40 kW/m 2 and saturation tem- perature of 5 °C (reduced pressures p r are 0.12 for R22 and 0.19 for R410A). The experimental results of two-phase pressure drops are presented and analyzed. Furthermore, the predicted two-phase frictional pressure drops by the flow pattern based two-phase pressure drop model of Moreno Quibén and Thome [J. Moreno Quibén, J.R. Thome, Flow pattern based two-phase frictional pressure drop model for horizon- tal tubes, Part I: Diabatic and adiabatic experimental study, Int. J. Heat Fluid Flow 28 (2007) 1049–1059; J. Moreno Quibén, J.R. Thome, Flow pattern based two-phase frictional pressure drop model for horizontal tubes, Part II: New phenomenological model, Int. J. Heat Fluid Flow 28 (2007) 1060–1072] using the equivalent diameters were compared to the experimental data. The model, however, underpredicts the flattened tube two-phase frictional pressure drop data. Therefore, correction to the annular flow friction factor was proposed for the flattened tubes and now the method predicts 83.7% of the flattened tube pres- sure drop data within ±30%. The model is applicable to the flattened tubes in the test condition range in the present study. Extension of the model to other conditions should be verified with experimental data. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Flattened tube heat exchangers have a potential use in a wide range of industrial applications: air-conditioning, heat pump and refrigeration systems, automotive radiators, and fuel cell engines, etc. Compared to a circular tube, a flattened tube has a higher sur- face-to-cross-sectional flow area ratio, which may be used to en- hance the heat transfer rate and increase the compactness of heat exchangers. For example, flattened heat transfer tubes can greatly reduce the refrigerant charge in direct-expansion evapora- tors and condensers and thus provide more compact heat exchan- ger design. Furthermore, potential advantages of flattened tube profiles are reduced air-side pressure drop and increased air-side heat transfer. Flattened heat transfer tubes in the present study re- fer to plain round tubes that have been extruded flat on top and bottom and remain round at the two ends as shown by the photo in Fig. 1. So far, there are very limited studies on two-phase flow and heat transfer in flattened tubes in the literature. Wilson et al. [1] investigated refrigerant charge, two-phase pressure drop and heat transfer during condensation of refrigerants R134a and R410A in several flattened tubes. Their results show significant reduction in refrigerant charge as a tube is flattened. They also indicate enhancement of condensation heat transfer and an increase of pressure drop in the flattened tubes. Krishnaswamy et al. [2] inves- tigated condensation heat transfer of steam-air mixtures in a hor- izontal flattened tube. They also proposed a simple heat transfer model. Their model predicts their data satisfactorily. Koyama et al. [3] conducted experiments on two-phase pressure drop and heat transfer of condensation of refrigerant R134a in multi-port ex- truded flattened tubes with hydraulic diameters of 1.114 and 0.807 mm. They concluded that to establish a prediction method of the pressure drop and heat transfer characteristics of pure refrig- erant condensing in a small diameter tube, more experimental data for small diameter tubes should be investigated by considering the following terms: (1) flow patterns, (2) the effect of tube diameter, and (3) the interaction effect among the vapor shear stress and the gravitational acceleration and the surface tension. As for flow boil- ing in flattened tubes, however, there is no study available in the 0017-9310/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2008.12.032 * Corresponding author. Tel.: +41 21 693 5981; fax: +41 21 693 5960. E-mail addresses: jmoreno@wolverine.com.pt (J.M. Quibén), lixincheng@hot- mail.com (L. Cheng), ricardo.lima@epfl.ch (R.J. da Silva Lima), john.thome@epfl.ch (J.R. Thome). International Journal of Heat and Mass Transfer 52 (2009) 3634–3644 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt