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Experimental and numerical investigation of R-134a flow through a lateral type diabatic capillary tube.

Article Excerpt
INTRODUCTION

The capillary tubes are drawn copper tubes with internal diameters of 0.5-2.0 mm and lengths of 2-6 m. They are used in low-capacity refrigeration systems, such as household refrigerators and window-type air conditioners. The capillary tubes offer a number of advantages: they are simple in construction, low-cost, and require no maintenance. Further, a system employing a capillary tube requires a low-starting-torque motor, as during off-cycle the pressure difference across the capillary tube equalizes. Capillary tubes are available in a range of sizes and their proper selection is a necessity for satisfactory performance of a system. Figure 1a shows the vapor compression system employing a diabatic capillary tube. In a diabatic capillary tube, the capillary tube is bonded with the cold compressor suction line to form a counter flow heat exchanger. The thermal contact between the capillary tube and the compressor suction line can be attained by bonding the capillary tube with the compressor suction line by means of a solder or brazing joint, as shown in Figure 1b. The heat transfer from the capillary to suction-line results in a higher refrigerating effect and, thus, a better performance is achieved. On the other hand, after receiving heat from the capillary tube, the low-temperature saturated refrigerant vapors in the suction line are superheated, which reduces the chance of liquid refrigerant entering the compressor.

[FIGURE 1 OMITTED]

Capillary tubes of different geometries (e.g., straight, helical, and spiral shapes) and flow conditions (e.g., adiabatic and diabatic arrangements) are used extensively in the refrigeration industry. It is a known fact that coiling of capillary tubes results in system compactness, whereas use of diabatic capillary tubes enhances the refrigerating effect of a refrigeration system. An exhaustive review of the literature reveals that most of the work on capillary tubes focuses on adiabatic capillary tubes with a straight geometry. Only a few researchers have investigated diabatic capillary tubes with a straight geometry.

A majority of the experimental research on diabatic capillary tubes was conducted on lateral arrangement. The experimental work on diabatic capillary tubes was pioneered by Staeblar (1948). The capacity balance characteristics to determine the length of a diabatic capillary tube for R-12 and R-22 were presented. It was concluded that the effect of changes in evaporator pressure on the refrigerant mass flow rate was insignificant. Pate and Tree (1984b) studied the diabatic flow of R-12 through the capillary tube with air flowing in the suction line in counter flow direction, forming an open loop. They did not observe metastable flow in a diabatic arrangement, although the metastability was observed during adiabatic flow. Melo et al. (2002) conducted experiments on the concentric diabatic capillary tubes with R-600a as a working fluid. Based on their experimental results, they proposed separate empirical correlations using a factorial design of experiments technique for the determination of refrigerant mass flow and the suction-line outlet temperature. The research work on coiled capillary tubes was pioneered by Kuehl and Goldschmidt (1990). It was found that coiling reduces the mass flow rate through the capillary tube by as much as 5% only. However, some researchers also reported a higher drop in refrigerant mass flow rate in helically coiled capillary tubes. For instance, Kim et al. (2002) found that the refrigerant mass flow rate through a helically coiled capillary tube was reduced by 9%. Zhou and Zhang (2006) also found that the mass flow rate through a helical capillary tube was reduced by 10%. Park et al. (2007) reported a slightly higher reduction in mass flow rates for the coiled capillary tubes, i.e., 5%-16%, compared to straight capillary tubes. Khan et al. (2008a) carried out an experimental study on adiabatic spiral capillary tubes and proposed a correlation for the prediction mass flow rate of R-134a.

Various investigators also developed numerical models for the flow refrigerant through coiled capillary tubes. A numerical model was developed by Zhou and Zhang (2006) for adiabatic helical capillary tubes and validated with their own experimental data. Khan et al. (2008b) developed a numerical model for adiabatic helical capillary tubes based on homogeneous two-phase flow theory. They compared the results of their numerical model with the experimental data of Zhou and Zhang (2006) and Kim et al. (2002). Further, they compared their model with that of Zhou and Zhang (2006). The predictions of the Khan et al. (2008b) model were found to be very close to those of the Zhou and Zhang (2006) model. In addition, Khan et al. (2007) also developed a numerical model for adiabatic spiral capillary tubes and compared the performance of spiral capillary tubes with adiabatic straight capillary tubes.

A number of numerical investigations were made for the flow through diabatic capillary tubes. Pate and Tree (1984a) proposed a linear quality model for the flow of R-12 through diabatic capillary tubes operating in an open loop--i.e., cold air in the suction line and refrigerant R-12 in the capillary tube. The proposed numerical model was validated with their own experimental data. Sinpiboon and Wongwises (2002) developed a simple mathematical model for the refrigerant flow through a lateral diabatic capillary tube. The linear quality model of Pate and Tree (1984a) in the analysis of the heat exchange region of the diabatic capillary tube was used. Xu and Bansal (2002) developed a numerical model by dividing the flow domain into numerous control volumes along the length of capillary tubes. They observed that when the effect of heat transfer was stronger than the pressure drop effect, the refrigerant was condensed within the heat exchanger region; whereas, if the pressure drop effect was stronger, the refrigerant flashed within the heat exchange region. Bansal and Xu (2003) also conducted a parametric study on the diabatic flow of R-134a through a capillary tube. Bansal and Yang (2005) proposed a model for the flow of refrigerant through a diabatic capillary tube. It was observed that the rate of heat transfer from the capillary tube to suction decreases by 8%-10% if the inlet diabatic arrangement was considered, compared to adiabatic inlet conditions. The control volume formulation for the flow through a diabatic capillary tube was adopted by a number of researchers. Escanes et al. (1995) developed a numerical simulation model based on the control volume formulation on 4.0 m of capillary tube length assuming that the first 1.3 m length of capillary tube is diabatic and rest of the tube is adiabatic. The solution was carried out using an implicit step-by-step numerical scheme. The calculation of mass flow rate for both critical and noncritical flow was made iteratively by means of the Newton-Raphson algorithm. A reasonably low computational cost resulted. Valladares et al. (2002a) developed a numerical simulation similar to the simulation model by Escanes et al. (1995). This model was based on the finite volume formulation of the governing equations. As a sequel to the numerical simulation, Valladares et al. (2002b) validated their simulation model with the experimental data of previous researchers. Parametric studies for the concentric capillary-tube suction-line heat exchangers were also presented. Valladares (2004) presented a review of, and, most recently (Valladares 2007a, 2007b), extended the numerical work of Valladares et al. (2002a; 2002b) on diabatic capillary tubes considering separated flow model and metastable regions. He validated his renewed model with the existing experimental data.

Consequently, there is a need to conduct more experimental and numerical research on the diabatic capillary tubes to evaluate their performance for different input conditions. Therefore, the present work carries out experimental as well as numerical investigations on diabatic capillary tubes. In fact, in the present experimental investigation, a novel configuration of capillary tubes is proposed that includes the advantages of coiling in helical form as well as those of a diabatic flow arrangement. The experimental part of this paper may be perceived as an extension of our own experimental work (Khan et al. 2008c) on the flow of R-134a through an adiabatic helically coiled capillary tube. Further, an attempt has also been made to develop a numerical model for a diabatic straight capillary tube. The developed numerical model was compared with previous and present experimental data.

EXPERIMENTAL SETUP AND PROCEDURE

The experimental setup shown in Figure 2 has been designed to carry out experiments on the adiabatic and diabatic flows of R-134a through a helical capillary tube. The refrigerant was expanded from the high-pressure condenser side to low-pressure evaporator side in the helical test section (1). The test section was a capillary tube brazed on a 6.35 mm compressor-suction line forming a counter-flow heat exchanger. The evaporator (2) consisted of a copper coil submerged in a water tank. An electric heater (3) was fitted in the evaporator tank to provide heating load to the evaporator. The heating load was controlled by a variac (4). An agitator (5) was also fitted in the tank to maintain the uniform bulk temperature of water. The refrigerant vapors, after producing a refrigerating effect inside the evaporator (2), passed through the liquid accumulator...

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