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Experimental investigation of the flow of R-134a through adiabatic and diabatic capillary tubes.

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INTRODUCTION

The capillary tube is a commonly used expansion device in low-capacity vapor-compression refrigeration systems (e.g., household refrigerators, window-type air conditioners). Capillary tubes can be classified into two groups on the basis of flow conditions--adiabatic and diabatic capillary tubes. In an adiabatic arrangement, the refrigerant expands from the high-pressure side (condenser) to the low-pressure side (evaporator) with no heat transfer, with surroundings as shown in Figure 1a, A P-h diagram is also drawn in Figure 1a, in which the process of expansion is depicted by the process 3-4, with enthalpy remaining constant in the single-phase liquid region, whereas there is a fall in enthalpy in the two-phase liquid-vapor region. It is because of this fact that a part of total enthalpy gets converted into kinetic energy in the two-phase region. Conversely, in the diabatic arrangement, the expansion is accompanied by the heat exchange between the capillary tube and the compressor suction-line, as shown in Figure 1b. Similarly, also in this case, the P-h diagram shows that there is a continuous fall of enthalpy throughout the process of expansion on account of heat transfer from the capillary tube to the suction line. The refrigerant usually enters the capillary tube in a subcooled state, and the pressure drop causes the flow through the capillary tube divided into two distinct regions--the single-phase liquid region in the initial part, and the two-phase liquid-vapor region in the remaining part. In the case of the diabatic capillary tube, and the compressor-suction line being colder, the heat transfer from the capillary tube to the suction line causes the liquid length to increase, thereby causing the refrigerant mass flow rate through the capillary tube to increase. Consequently, the refrigerant enters the evaporator with low vapor quality, which ultimately results in an increased refrigerating effect. On the other hand, on receiving the heat from the capillary tube, the saturated vapor inside the suction line entering the heat exchanger gets superheated, thus diminishing the chances of liquid refrigerant entering the compressor.

[FIGURE 1 OMITTED]

Adiabatic capillary tubes have been investigated by a number of researchers (1-15). The studies of the flow of refrigerants through straight capillary tubes were pioneered by Bolstad and Jordan (1). Mikol (3) conducted an extensive study on the flow of refrigerants R-12 and R-22 through an adiabatic straight capillary tube. The glass capillary tube was 1.244 mm (0.049 in.) in diameter and 1.83 m (6 ft) in length, and a delay of vaporization was reported. After vaporization, homogeneous two-phase flow was also observed by Mikol (3). A number of correlations for the prediction of mass flow rate of various refrigerants through adiabatic capillary tubes have been proposed. Bansal and Rupasinghe (5) presented an empirical correlation for sizing both adiabatic and diabatic capillary tubes as well. The refrigerant mass flow rate predicted by the proposed model was found to be in the error band of [+ or -]8% for both types of capillary tubes. An experimental investigation for flow refrigerants such as R-12, R-134a, and R-600a was conducted by Melo et al. (6). They proposed separate correlations for each of the above-mentioned refrigerants. Also, a combined correlation for all of these refrigerants was also developed to predict the refrigerant mass flow rate. The proposed correlation was then compared with the Wolf et al. (7) correlation. Choi et al. (9) also developed a generalized empirical mass flow rate correlation for the adiabatic capillary tubes, pulling the experimental data of Melo et al. (6), Wolf et al. (7), and Fiorelli et al. (10) together. Further, Choi et al. (11) proposed another modified generalized mass flow rate correlation using R-22, R-407C, and R-290 experimental data of previous researchers. Jabaraj et al. (12) proposed a mass flow rate correlation for the flow of R-22 and M20 (R-407C/R-600a/R-290) through the adiabatic capillary tube. Most recently, Khan et al. (13) proposed a correlation for predicting the mass flow rate of R-134a through adiabatic spiral capillary tube. Apart from experimental research, numerical models are also available to predict the performance of adiabatic capillary tubes. Khan et al. (14-15) have also proposed mathematical models for adiabatic spiral and helical capillary tubes.

Compared to adiabatic capillary tubes, limited literature is available on diabatic capillary tubes (16-25). Pate and Tree (16) studied the diabatic flow of R-12 through the capillary tube with air flowing in the suction line in a counterflow direction, forming an open loop. They did not observe metastable flow in diabatic arrangement, whereas, the metastability was observed during adiabatic flow. Melo et al. (17) conducted the experiments on the concentric diabatic capillary tubes. They proposed separate empirical correlations using a factorial design of experiment technique for the determination of refrigerant mass flow and the suction line outlet temperature. Sinpiboon and Wongwises (18) developed a simple mathematical model for the refrigerant flow through a lateral diabatic capillary tube. The linear quality model of Pate and Tree (19) was used in the analysis of the heat exchange region of the diabatic capillary tube. Xu and Bansal (20) developed a numerical model by dividing the flow domain into...

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