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Description
This study presents experimental two-phase frictional data for refrigerant-oil mixtures in two tubes with inside diameters D = 2.50 and 6.34 mm. Two-phase pressure gradients across a test section are measured for inlet quality of 0.1-0.8, mass flux of 200-400 kg/[m.sup.2] x s, and heat flux of 3-14 kW/[m.sup.2]. The results show frictional performance of refrigerants in small tubes is more sensitive to oil presence than that in larger tubes. The maximum penalty factor is up to 1.2 for the 6.34 mm tube, while it is up to 1.4 or more for the 2.50 mm tube at the same conditions. But it is also found that frictional pressure drop in smaller tubes may be reduced due to oil presence at some combinations of quality and oil concentration. For D = 6.34 mm, the pressure gradient increases continuously with quality for all circulating oil concentrations. In contrast, for D = 2.50 mm, the pressure gradient reaches peak values at quality [congruent to] 0.75 for circulating oil concentrations [less than or equal to]3%. No peaks occur at all for higher oil concentrations.
A modified Zurcher et al. (1998) correlation and Choi et al. (2002) correlation can give fair agreement with the present data of D = 6.34 mm but fails for the data of D = 2.50 mm. A proposed correlation based on refrigerant-oil mixture properties shows satisfactory predictability to all the present data. More than 95% of the experimental data are within [+ or -]10% and [+ or -]20% deviations for 6.34 mm and 2.50 mm tubes, respectively. The correlation has the benefit of incorporating physical behavior and gaining insights on in-tube flow boiling of refrigerant-oil mixtures in spite of containing fluid-specific coefficients.
INTRODUCTION
Compact heat exchangers for refrigeration and air-conditioning systems are beneficial to reduce cost, charge inventory and leakage of refrigerant, and improve energy efficiency and safety. Using small-diameter tubes is one way to decrease the size of heat exchangers. In order to achieve good designs, it is necessary to know the pressure drop of working fluids inside small channels. Inevitably, some amount of oil circulates with the refrigerant and has a significant impact on refrigerant evaporation frictional performance because oil changes refrigerant thermal and transport properties, such as saturated temperature, density, viscosity, etc.
Prior investigations, such as Ticky et al. (1986), Schlager et al. (1990), Eckels et al. (1998a, 1998b), and Zurcher et al. (1997, 1998), showed that the effect of oil circulation always increased refrigerant pressure drop. The tube diameters in these studies are larger than 8 mm, and the experimental data, in most of them, ignored the effect of quality. Zurcher et al. (1997) had quality changes in 0.048 to 0.065 increments. According to Mehendale et al. (2002), a diameter range of 2.50~6.34 mm spans compact to meso tube sizes--smaller than conventional tubes and therefore called "small tubes." Two-phase frictional performance of oil-free refrigerants in small tubes has been reported by Zhang and Webb (2001), Tran et al. (2002), and Chen et al. (2002). Coleman and Garimella (1999) investigated the flow pattern of two-phase flow in small tubes. Their results showed that the flow mechanism in small tubes was different from that in large tubes. However, there are few papers on the effect of oil in such tubes.
Although much information is available in the open literature, more research is still needed because of the complexity of the refrigerant/oil phenomenon. The main objective of this study is to analyze the effects of oil on local frictional performance of refrigerant flow boiling inside small-diameter tubes. To accomplish this objective, a special experimental apparatus was designed and constructed. This apparatus allows for control and measurement of oil concentration in refrigerants online.
EXPERIMENTAL APPARATUS
The experimental apparatus, shown in Figure 1, consists of a refrigerant main loop, a refrigerant bypass loop, and a lubricant oil loop. These three loops share some of the same components.
In the refrigerant main loop, a portion of liquid refrigerant from a condenser passes through a refrigerant mass flowmeter and an electronic expansion valve (EEV) to a partially evaporated outlet state. Then it flows through a preheater to achieve the set quality at inlet to a test section. The refrigerant enters the test section at a known mass flux and vapor quality and is evaporated by an electric heating tape wrapped around the outside of the test tube. The two-phase mixture leaves the test section and enters an after-heater, an electric heater, where it is fully evaporated and superheated. The superheated vapor flows through three oil separators in parallel, joins with the refrigerant vapor from the bypass loop, and enters the suction inlet of a variable-frequency compressor.
[FIGURE 1 OMITTED]
The refrigerant bypass loop is used to vary both refrigerant flow rate (to the main loop) and evaporation pressure. In this loop, the remaining portion of liquid refrigerant from the condenser enters an EEV and leaves partially evaporated. It... |
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