Two-phase flow visualization and pressure drop measurements of HFO-1234yf and R-134a refrigerants in horizontal return bends Miguel Padilla, Rémi Revellin ⇑ , Jocelyn Bonjour Université de Lyon INSA-Lyon, CETHIL, CNRS, UMR5008, F-69621, Villeurbanne, France Université Lyon 1, F-69622, France article info Article history: Received 16 May 2011 Received in revised form 9 January 2012 Accepted 10 January 2012 Available online 26 January 2012 Keywords: Two-phase flow Pressure drop Refrigerant Return bend abstract Two-phase flow regimes of HFO-1234yf and R-134a in a horizontal 6.7 mm inner diameter return bend are investigated. The different flow regimes observed are: slug, intermittent and annular flows. The perturba- tion lengths up- and downstream of the singularity are experimental investigated and their contribution to the pressure drop is identified. It is shown that the upstream perturbation length is less than 10D and that the downstream perturbation length is less than 20D for the conditions tested in this study. In addition, this paper presents 574 pressure drop data points measured for two-phase flow of HFO-1234yf and R-134a in horizontal return bends of inner diameters (D) from 7.90 to 10.85 mm and curvature ratios (2R/D) from 3.68 to 4.05. The experimental pressure drop data in return bends is compared to four well-known prediction methods available in the literature. The whole present pressure drop in return bends experimental database for the next generation HFO-1234yf and for the R-134a refrigerants is presented in the appendix. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction Return bends are extensively used in compact refrigeration sys- tems such as air conditioners or heat pumps. Their effect on the hydrodynamic performance become more important due to the fact that nowadays, there is a growing interest in refrigerant charge reduction. The change in the design of evaporator and condenser coils is necessary in order to obtain more compact heat exchangers. Two-phase pressure drops in return bends in refrigeration sys- tems have been experimentally investigated by several authors in the open literature. These studies highlight the impact of the large number of independent parameters governing the flow among which geometric configuration of the tube such as the curvature ratio, mass and volume fractions of the individual phases, pressure, fluid properties, mass velocity, orientation of the return bend (i.e horizontal, vertical or inclined) and flow patterns. To cater to the needs of diverse engineering applications, such as refrigeration, two-phase pressure drops in return bends have also been experi- mentally investigated by several authors. Since the 1960s, several refrigerants have been studied, among which CFCs, HCFCs and HFCs [12,15,5]. Most recent researches have been carried out for R-134a and R-410A [2,14,11]. Some of the most representative cor- relations for frictional pressure drop prediction in return bends have been presented by Chisholm [3] and Idelshik [6], Chen et al. [1], Domanski and Hermes [4] and Padilla et al. [9]. The latter presented 325 pressure drop data points measured in return bends for R-12, R-134a and R-410A, which were collected from the liter- ature. These methods are presented in Tables 1 and 2. In order to extend the existing database available in the litera- ture towards different tube diameters and curvature ratios for two- phase flow in horizontal return bends, Padilla et al. [11] presented 238 pressure drop data points measured for two-phase flow of R-410A in horizontal return bends of diameter (D) from 7.9 to 10.85 mm and curvature ratios (2R/D) from 3.68 to 4.05. This study showed the influence of mass velocity, saturation temperature and inner diameter on the return bend pressure drop. In addition, there is a growing interest in refrigeration, heat pump and air condition- ing industries to protect the environment from refrigerants with high Global Warming Potential (GWP). This has led to a demand for new environmentally friendly refrigerants. In this sense, the HydroFluoroOlefin (HFO) 1234yf, with a GWP of 4, is presented as a solution able to provide efficient and effective cooling with a near drop-in replacement for the current refrigerant R-134a. Among its interesting characteristics, HFO-1234yf has an atmo- spheric lifetime of only 11 days, compared to 13 years for R-134a. The aim of this article is to present original experimental two-phase flow regime visualizations and two-phase pressure drop data points in horizontal return bends for the next generation HFO- 1234yf refrigerant. This permits to extend the existing database available in the literature and also to better understand the physical phenomena involved. In addition, this article aims at discussing the perturbation length encountered up- and downstream of the return bend. 0894-1777/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermflusci.2012.01.013 ⇑ Corresponding author. Tel.: +33 4 72 43 72 31; fax: +33 4 72 43 88 10. E-mail address: remi.revellin@insa-lyon.fr (R. Revellin). Experimental Thermal and Fluid Science 39 (2012) 98–111 Contents lists available at SciVerse ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs