International Journal of Thermal Sciences 47 (2008) 423–430 www.elsevier.com/locate/ijts Performance evaluation of a non-adiabatic capillary tube in a transcritical CO 2 heat pump cycle Neeraj Agrawal, Souvik Bhattacharyya * Department of Mechanical Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721302, India Received 14 November 2006; received in revised form 5 March 2007; accepted 5 March 2007 Available online 12 April 2007 Abstract A non-adiabatic capillary tube in a transcritical CO 2 heat pump cycle has been simulated to investigate the effect of parameters such as gas cooler and evaporator temperature, capillary tube diameter and heat exchanger length on various performance indicators. The homogeneous flow model is employed to simulate two-phase flow in the non-adiabatic capillary tube. Fundamental equations of mass, energy and momentum are solved simultaneously through an iterative process. Single and two-phase heat transfer coefficients are calculated by employing appropriate empirical correlations. Subcritical and supercritical thermodynamic and transport properties of CO 2 are calculated employing an in-house precision property code. Lowering evaporator temperature is found to be more effective for heat transfer from the capillary tube compared to the gas cooler temperature. Heat transfer rate variation with respect to gas cooler temperature in case of CO 2 is distinctly different compared to conventional refrigerants due to its transcritical nature and is influenced by initial quality, mass flow rate of the refrigerant and the prevailing temperature difference at the gas cooler. Increase in gas cooler temperature causes the heat transfer rate to first increase and then to decrease. Lowering evaporator and gas cooler temperature increases the cooling capacity. Throttling effect decreases rapidly as internal tube diameter becomes larger leading to higher mass flow rate of the refrigerant. Shorter inlet adiabatic capillary length with larger heat exchanger length is better for heat transfer. This study is an attempt to allay the scepticism prevailing in the parlance of CO 2 based transcritical systems overemphasising the need for a throttle valve to control the optimum discharge pressure.  2007 Elsevier Masson SAS. All rights reserved. Keywords: Capillary tube; CO 2 ; Non-adiabatic; Capillary tube-suction line heat exchanger 1. Introduction Capillary tubes are a type of refrigerant flow control de- vice and are widely accepted as an expansion device in small vapour compression refrigerating and air conditioning systems due to its simplicity, low initial cost and low starting torque of the compressor. Flow inside the capillary tube is a flash- ing process where it undergoes phase change and is complex in nature. It is a common practice to solder the capillary tube to the outer surface of the suction line (Fig. 1) to improve the system performance, and to avoid liquid entry into the compres- sor by superheating the suction vapour. Such a configuration of capillary tube and suction line form a counter-flow heat ex- * Corresponding author. Tel.: +91 3222 282904. E-mail address: souvik@mech.iitkgp.ernet.in (S. Bhattacharyya). changer commonly known as capillary tube-suction line heat exchanger (CL-SLHX). Under such conditions, the capillary tube is considered non-adiabatic and has the unusual charac- teristic of flashing, while simultaneously being cooled by heat transfer to a wall where large frictional effects enhance flashing, while heat transfer to the suction line retards flashing. These complexities of the non-adiabatic two phase flow make it chal- lenging to analyse. Natural refrigerants have become the preferred choice to re- place conventional refrigerants in view of their benign nature and CO 2 appears to be leading the pack. In addition to its en- vironmental benefits, CO 2 has attractive thermo-physical and safety characteristics compared to the currently used refriger- ants. A capillary tube employing CO 2 as the refrigerant is al- together a different phenomenon because of the transcritical 1290-0729/$ – see front matter  2007 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ijthermalsci.2007.03.002