Home | Business News | Browse by Publication | H | HVAC & R Research

Experimental study of the flow of R-134a through an adiabatic helically coiled capillary tube.

Article Excerpt
INTRODUCTION

Capillary tubes are drawn copper tubes that have internal diameters ranging from 0.5 to 2.0 mm (0.0197 to 0.0787 in.) and lengths ranging from 2 to 6 m (6.56 to 19.69 ft). The low cost, zero maintenance, and low starting torque of the compressor motor are some of the advantages of using capillary tubes. Capillary tubes are used in low-capacity refrigeration equipment such as household refrigerators and window air conditioners. The flow of refrigerant through the capillary tube is adiabatic in nature. Usually, the refrigerant enters the capillary tube in a subcooled liquid state. As the refrigerant makes progress through the capillary, the pressure falls almost linearly, due to friction, while the temperature of the expanding refrigerant remains constant as long as it is in the liquid state. As the pressure falls below the vapor pressure of the refrigerant, corresponding to capillary inlet temperature, a part of liquid refrigerant flashes into vapor. In a adiabatic helically coiled capillary tube, shown in Figure 1a, the pressure drop is further increased because of the onset of secondary flows in the cross section of the coiled capillary tube. The use of helically coiled capillary tubes results in a smaller tube length to achieve the same pressure drop when used in place of straight capillary tubes. Furthermore, the coiling of the capillary results in a compact design. In helical tubes, a secondary flow, perpendicular to the axis, is induced by the curvature of the tube (Figure 1b). The secondary flow, also known as the Dean effect, affects heat, mass, and momentum transfer in coiled tubes.

[FIGURE 1 OMITTED]

Capillary tubes have been investigated both experimentally and numerically for many years. Bolstad and Jordan (1948) pioneered the use of the capillary tube. Mikol (1963) conducted another comprehensive experimental study to investigate the various phenomena associated with refrigerant flowing inside an adiabatic capillary tube. Koizumi and Yokoyama (1980) conducted experiments on glass capillary tubes for flow visualization. They found that the flow inside the capillary tube is a homogeneous two-phase flow. A number of semi-empirical correlations for the prediction of mass flow rates of various refrigerants for a given size of capillary tube and given inlet and outlet conditions have appeared in the literature. For instance, Wolf et al. (1995) proposed a correlation for the prediction of the mass flow rate through adiabatic capillary tubes for the flow of refrigerant R-134a. Another important study regarding the flow of newer refrigerants inside capillary tubes was carried out by Melo et al. (1999). They proposed separate correlations for each refrigerant, as well as a combined correlation for all of the three refrigerants flowing through an adiabatic straight capillary tube. Choi et al. (2003) modified the dimensionless parameters and proposed a correlation for the prediction of the mass flow rate through the capillary tube. Fiorelli et al. (2006) carried out an experimental analysis of refrigerant mixtures R-407C and R-410A through adiabatic capillary tubes. The geometric parameters, such as capillary tube diameter and the tube's length, were also incorporated in the study. Jaba-raj et al. (2006) also investigated the flow characteristics of the newer eco-friendly refrigerant mixture R-407C/R-600a/R-290 (termed M20) inside an adiabatic capillary tube and developed a nondimensional correlation to predict the mass flow rate.

Scanning the literature has revealed that most of the research on adiabatic capillary tubes has been carried out using straight capillary tube geometry. However, some work has also been reported on the flow of R-22 through adiabatic helically coiled capillary tubes. Kuehl and Goldschmidt (1990) explored the effect of coiling on the refrigerant mass flow rate and have concluded that irrespective of the length of the capillary tube coiled, the mass flow rate is reduced by no more than 5%. Another important correlation for the mass flow rate of R-22, R-407C, and R-410A through a helical capillary tube was presented by Kim et al. (2002). It was found that the mass flow rate is reduced by almost 9% because of coiling. Zhou and Zhang (2006) studied the flow characteristics of refrigerant R-22 inside an adiabatic helical capillary tube. They studied the effect of inlet subcooling, capillary tube diameter, capillary tube length, and coil diameter on the mass flow rate of refrigerant R-22. They also developed a mathematical model and validated it with the experimental study. Khan et al. (2008) also proposed a mathematical model for the flow through adiabatic helical capillary tubes. The predictions of both the model proposed by Khan et al. (2008) and Zhou and Zhang's (2006) model are close to each other. In addition, the Khan et al. (2008) model is simpler than Zhou and Zhang's (2006) model. The Khan et al. (2008) model can predict the effect of coil pitch in addition to the effect of coil diameter on the refrigerant mass flow rate.

Studies on...

View this article FREE - Now for a Limited Time, try Goliath Business News
Free for 3 Days!