Influence Of Oil On Flow Characteristics Through Capillary Tube In Refrigeration Cycle

A capillary tube is widely used as an expansion device for small refrigeration cycles. Recently, immiscible oil is sometimes used in the refrigeration cycle to improve severe lubricating conditions and to prevent blockage of the capillary tube caused by the hydrolysis of ester oil. This paper discusses the influence of mixing of the immiscible oil with the refrigerant on the flow characteristics through the capillary tube. The combination of mineral oil and HFC134a is used, and mass flow rate and temperature and pressure distributions are measured experimentally under several conditions of subcooled degree and oil concentration. Even in the case of the immiscible combination, the oil droplet is pretty small and it mixes homogeneously in a liquid phase in the capillary tube. The mass flow rate of refrigerant decreases with increasing the oil concentration. The mixing of oil seems to delay inception of flash slightly, although the further increase of oil concentration does not affect the inception. Theoretical model is developed based on the same concept of a miscible oil-refrigerant combination, and calculated results are compared with experimental ones. The theoretical model for the miscible combination is applicable to the immiscible one, and the mass flow rate is estimated within an error of ±5 %. INTRODUCTION A capillary tube is commonly used as an expansion device for domestic refrigerators and small air conditioning units because of its low cost and simple structure. In contrast with the simplicity, the flow through the capillary tube is quite complex, and many studies have been done both experimentally and theoretically to clarify the flow characteristics and to estimate the flow rate through the capillary tube. Although most of the studies discuss the flow characteristics of pure refrigerant, small amount of refrigeration oil is discharged from a compressor into the refrigeration cycle and circulates in the cycle with the refrigerant in a practical cycle. The oil flowing with the refrigerant increases viscosity of the refrigerant-oil mixture, and affects the flow characteristics and the mass flow rate through the capillary tube. Some papers reported the influence of oil on the performance of capillary tube. Bolstad and Whitacre reported that the oil increased the mass flow rate through the capillary tube. Wijaya tested the influence of oil circulation and observed no significant difference between data obtained with and without an oil separator. Huerta studied the oil influence experimentally and theoretically and showed that the presence of oil reduces the mass flow rate in the capillary tube. The authors also investigated the oil influence both experimentally and theoretically by using a combination of mineral oil and HCFC22 and concluded that the mass flow rate through the capillary tube decreases by the mixing of oil because the viscosity of liquid phase increases. A mathematical model developed in the previous study took account of the mixing of oil and estimated the mass flow rate within an error of ±5 %. Recently, immiscible oil is sometimes used in the refrigeration cycle using HFC refrigerants in order to improve severe lubricating conditions and to prevent blockage of the capillary tube caused by the hydrolysis of ester oil that is miscible with the HFCs. When the immiscible oil mingles with the refrigerant circulating in the cycle and the refrigerant-oil mixture flows through the capillary tube, the flow pattern, the flow characteristics and the mass flow rate may be different from the case that the pure refrigerant flows through the capillary tube. In this study, the influence of mixing of the immiscible oil with the refrigerant on the flow characteristics through the capillary tube is investigated by using the combination of HFC134a and mineral oil. The mass flow rate of refrigerant through the capillary tube is measured with changing the oil concentration quantitatively. Pressure and temperature distributions along the capillary tube are also measured under adiabatic condition. A sight glass and a glass capillary tube are used to observe the flow pattern before and inside the tube respectively. The mathematical model developed for the miscible combination is used and calculated results are compared with experimental ones to evaluate the applicability of the model to the immiscible combination. EXPERIMENT Figure 1 shows a schematic diagram of the experimental setup. The experimental refrigeration cycle consists of a compressor, a condenser, a subcooler, a capillary tube and an evaporator. The refrigerant is HFC134a and the refrigeration oil is naphthenic type mineral oil (ISOVG32), which is immiscible with the refrigerant. The compressor is a rolling piton type rotary compressor and its case has an additional volume on the top of the compressor for oil separation. The refrigerant delivered from the compressor condenses at the condenser. Then, it enters the capillary tube after its subcooled degree is controlled by the subcooler and an electric heater. The refrigeration oil stored in the compressor casing is fed through a needle valve to a liquid line at an outlet of the condenser. The mass flow rates of the refrigerant and the oil are measured respectively by positive displacement type flowmeters. The capillary tube is a copper one of 1.04 mm I.D., 0.5 μm inside surface roughness and 0.8 m long. The inner diameter is obtained by a preliminary test using water. The pressure gradient and the flow rate under laminar flow condition are measured and these are substituted in Hagen-Poiseuille equation to calculate the inner diameter. The capillary tube is insulated by a urethane foam and twelve thermocouples are soldered on the outer surface of the tube to measure the temperature distribution along the tube. Eight pressure taps of 0.3 mm diameter are machined on the capillary tube wall and Bourdon type pressure gauges are connected to each pressure tap. Since the pressure and temperature distributions become steep as the flow goes downstream, interval of the pressure and temperature measuring point is arranged smaller in the portion of downstream. Besides the copper tube, a glass capillary tube whose diameter is 1.0 mm is used for visualization of the flow. Fluorescence dye is added to the oil so that the oil phase becomes clear by applying ultra-violet light. A glass pipe (8mm I.D.) as sight glass is installed in the liquid line just before the capillary tube to observe the flow pattern in the liquid line. In the experiment, the mass flow rate and the temperature and pressure distributions are measured under several conditions of the subcooled degree and the oil concentration. Test conditions are shown in Table 1. THEORETICAL ANALYSIS In this study, a mathematical model is developed based on the same idea for the miscible combination of refrigerant and oil. This paper focuses on the influence of the immiscible oil on the flow characteristics through the capillary tube and examines an applicability of the model to the immiscible combination. Discussion about what models are the best is, therefore, beyond the purpose of this study, and we employ correlations with which calculated results agree reasonably with experimental ones under the condition of no oil mixing. Figure 2 illustrates the flow model of the capillary tube. It is adiabatic and consists of a converging nozzle, single-phase region and two-phase region in the capillary tube having a constant cross section. The outline of the mathematical model is described in the following. Entrance The condition of the refrigerant entering the capillary tube is generally subcooled liquid and the pressure decreases when the velocity increases at the entrance of the tube. Under the assumption of an incompressible flow through the converging nozzle, the relationship between the pressure and the velocity across the entrance nozzle and mass continuity are given as follows. ( ) 2 1 2 2 2 t t i i V P V P ζ ρ ρ + + = +