A visual study of R-404A/oil flow through adiabatic capillary tubes Samuel F. Yana Motta a, *, Jose´ A.R. Parise b , Sergio Leal Braga b a Honeywell International Inc.- Buffalo Research Laboratory, 20 Peabody Street, Buffalo, NY, 14210, USA b Pontifical Catholic University of Rio de Janeiro, Brazil Received 10 May 2000; received in revised form 27 February 2001; accepted 10 May 2001 Abstract The present study explores the potential of using visualization techniques to investigate refrigerant/oil flow through adiabatic capillary tubes. A literature review shows that these techniques have been used before for capillary tube investigations, but none of these studies focused on the refrigerant/oil phenomena. Therefore, the main objective is to investigate the flow of a refrigerant/oil mixture through a glass capillary tube, with special emphasis on the behavior of the vaporization point. The test fluids are R-404A (a near azeotropic blend) and a polyolester-type oil. Experimental data cover oil concentrations ranging from 5.6 to 6.9% (by mass), degrees of subcooling ranging from 6.2 to 21.5  C (11.2 F to 38.7 F), and a condensing pressure of 1825 kPa (250 psig). The results show trends of mass flow rate, and give some useful insights about the location of the vaporization point for various oil concentrations and operating conditions. # 2002 Elsevier Science Ltd and IIR. All rights reserved. Keywords: Refrigerant; R404A; Lubricant; Mixture; Flow; Capillary tube Etude visuelle de l’e´coulement du R404A dans des tubes capillaires adiabatiques Mots cle ´s : Frigorige`ne ; R404A ; Lubrifiant ; Me´lange ; E ´ coulement ; Capillaire 1. Introduction Flow visualization is an extremely useful tool for research on two-phase flows and processes of heat transfer with phase change [1]. Some researchers have used the visualization of refrigerant flow, through expansion devices of fixed area, with different objectives (e.g. two-phase flow characterization and metastable phenomenon verification). In the following, we present a literature review of former studies regarding visualiza- tion of two-phase flow through expansion devices and refrigerant/oil flow through capillary tubes. 1.1. Visualization of flow through expansion devices Pascua [2] reported the occurrence of metastable flow in short tube orifices. His results showed that the flow rate of R-12 for saturated inlet conditions was the same as that for small degrees of inlet subcooling, indicating 0140-7007/02/$22.00 # 2002 Elsevier Science Ltd and IIR. All rights reserved. PII: S0140-7007(01)00057-3 International Journal of Refrigeration 25 (2002) 586–596 www.elsevier.com/locate/ijrefrig * Corresponding author. Formerly a graduate student at Catholic University, now a Sr. Staff Engineer at Honeywell Inc. Tel.: +1-716-827-1485; fax: +1-716-827-6275. E-mail address: samuel.yanamotta@honeywell.com (S.F.Y. Motta).