International Journal of Refrigeration 29 (2006) 539–546 www.elsevier.com/locate/ijrefrig An experimental study on heat transfer and pressure drop characteristics of carbon dioxide during gas cooling process in a horizontal tube
Chang-Hyo Sona,*, Seung-Jun Parkb a Department of Refrigeration and Air-Conditioning Engineering, College of Engineering, Pukyong National University, San 100,
Yongdang-dong, 608-739 Nam-gu, Pusan, South Korea b Korea Air-conditioning and Refrigeration Research Association, #601 Gyeonggi Technopark, 1271-11 Sa 1-dong, Sangnok-gu, Ansan,
426-901 Gyeonggi-do, South Korea
Received 25 May 2005; received in revised form 5 October 2005; accepted 13 October 2005
Available online 20 December 2005

Abstract
The heat transfer coefficient and pressure drop during gas cooling process of CO2 (R744) in a horizontal tube were investigated experimentally. The experiments are conducted without oil in the refrigerant loop. The main components of the refrigerant loop are a receiver, a variable-speed pump, a mass flow meter, a pre-heater and a gas cooler (test section). The water loop consists of a variable speed pump, an isothermal tank, and a flow meter. The refrigerant, circulated by the variable-speed pump, condenses in the inner tube while water flows in the annulus. The gas cooler of tube diameter is 6000 mm in length, and it is divided into 12 subsections.
The pressure drop of CO2 in the gas cooler shows a relatively good agreement with those predicted by Blasius’s correlation. The local heat transfer coefficient of CO2 agrees well with the correlation by Bringer–Smith. However, at the region near Pseudocritical temperature, the experiments indicate higher values than the Bringer–Smith correlation. Based on the experimental data presented in this paper, a new correlation to predict the heat transfer coefficient of supercritical CO2 during in-tube cooling has been developed. The majority of the experimental values are within 18% of the values predicted by the new correlation. q 2005 Elsevier Ltd and IIR. All rights reserved.
Keywords: Carbon dioxide; Heat transfer; Pressure drop; Flow; Horizontal tube; Experiment; Heat transfer coefficient

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Dioxyde de carbone: etude experimentale sur les caracteristiques de transfert de chaleur et de chute de pression lors du refroidissement dans un tube horizontal
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Mots cles : Dioxyde de carbone ; Transfert de chaleur ; Chute de pression ; Ecoulement ; Tube horizontal ; Experimentation ; Coefficient de transfert de chaleur

* Corresponding author. Tel.: C82 51 621 6802; fax: C82 51 611 6368.
E-mail address: sonch@mail1.pknu.ac.kr (C.-H. Son).

0140-7007/$35.00 q 2005 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2005.10.010 540

C.-H. Son, S.-J. Park / International Journal of Refrigeration 29 (2006) 539–546

Nomenclature
Cp
d
G
h ifg m
Q
T x z

specific heat at constant pressure (kJ kgK1 KK1) diameter (m) mass flux (kg mK2 sK1) heat transfer coefficient (k WmK2 KK1) latent heat (kJ kgK1) mass flow rate (kg sK1) heat capacity (kW) temperature (K) quality length of test section (m)

Subscripts avg average b bulk
1. Introduction
Chlorine-containing CFC and HCFC refrigerants used in refrigeration and air conditioning application are now being phased out due to their ozone-depleting effect. The fluorocarbon chemical industry is offering chloride-free
HFC fluids as replacement refrigerants, but these fluids still have considerable global warming impact. A number of researchers [1–4] studying for alternative refrigerants have been interested in development of new candidate and natural refrigerants. Among these natural refrigerants, the CO2(R744) has recently been rediscovered as a possible vapor compression working fluid mainly due to its unique combination of ecological and personal safety. CO2 is a

cal cw exp gc i in loc out pc re sub w calculated
cooling