Mitsuhiro Fukuta

An investigation of the performance of a scroll compressor under liquid refrigerant injection

Abstract In this study, fundamental and practical influence of liquid refrigerant injection on the performance of a refrigerant scroll compressor has been investigated experimentally and theoretically. In the theoretical analysis, a compression model of vapor/liquid mixture is developed by taking account of heat transfer from the cylinder wall to suction, compression and injection refrigerant. An experiment has been done under the condition of keeping the oil temperature constant in order to investigate the fundamental influence of the liquid refrigerant injection on the compressor performance, and the results were compared with the theoretical ones. It was found that the injection basically increases the compression power and decreases the compressor efficiency, though the situation depends on the condition of the heat transfer to the injection refrigerant. And furthermore, the performance of the liquid refrigerant injection compressor under practical operating condition without controlling the oil temperature has been investigated. Under this condition, the compressor showed recovery and slight improvement of performance due to the decrease of the oil and cylinder temperatures by the injection. In addition, influence of the refrigerant injection on the oil viscosity and refrigerant solubility in the oil, which relate mechanical loss and reliability of the compressor, have been discussed.

Performance of compression/absorption hybrid refrigeration cycle with propane/mineral oil combination

This paper discusses the feasibility of a vapor compression/absorption hybrid refrigeration cycle for energy saving and utilization of waste heat. The cycle employs propane as a natural refrigerant and a refrigeration oil as an absorbent. A prototype of the cycle is constructed, in which a compressor and an absorption unit are combined in series. The performance of the cycle is examined both theoretically and experimentally. Although the solubility of the propane with the oil is not enough as a working pair in the absorption unit, the theoretical calculation shows that the hybrid cycle has a potential to achieve a higher performance in comparison with a simple vapor compression cycle by using the waste heat. In the experiment, the prototype cycle is operated successfully and it is found that an improvement of an absorber is necessary to achieve the good performance close to the theoretical one. The application of an AHE (absorber heat exchanger) can reduce the heat input to a generator. Further examinations on some other combinations of refrigerant/refrigeration oil and additives are desirable.

Performance of Vane-Type CO2 Expander and Characteristics of Transcritical Expansion Process

Carbon dioxide (CO2) is one candidate for alternatives to hydrofluorocarbon refrigerants for refrigeration or heat-pump cycles. Since the inherent COP of an air-cooled CO2 cycle is lower than that of HFCs, the use of an expander as an expansion device is recommended to improve the cycle performance of the CO2 cycle. The performance of the expander greatly influences the improvement of the cycle performance. Moreover, an expansion process is complicated, since the expansion process occurring in the expander operated under a typical condition proceeds from supercritical to subcritical. Therefore, it is important to clarify both the expander performance and the expansion process in the expander. In this study, a small vane-type expander is developed, and the performance of the expander is measured. The prototype vane expander has a large leakage loss, and the performance is improved by changing the way of supplying a vane back pressure. Additionally, the expansion process in the expander is examined experimentally and analytically. The transcritical expansion process is also investigated in detail by using a simple piston-cylinder expansion mechanism with a glass window. It is shown that an inflection point of the pressure change under the transcritical expansion process appears with a certain time delay in comparison with an ideal case when the inlet temperature is less than about 40°C. A cluster structure of a CO2 molecule remains for a long time after entering the subcritical region.

Foaming characteristics of an oil-refrigerant mixture

Abstract This paper reports an investigation of the foaming of an oil-refrigerant mixture which may occur in refrigeration compressors. Characteristics of foaming caused by blade rotation, vapour blow and both blade rotation anf vapour blow were experimentally investigated in a hermetic casing. The foaming becomes violent with increased rotational speed of the blade and increased flow-rate of the bloqing vapour. The amount of foam caused by blade rotation and vapour blow together is always larger than that by blade rotation alone but is not always larger than that by vapour blow alone. The effects of the mixing ratio and temperature of the mixture on foaming are correlated with pressure in such a way that the foaming is relatively violent when the pressure is close to the saturation point.

Influences of miscible and immiscible oils on flow characteristics through capillary tube—part I: experimental study

A capillary tube is widely used as an expansion device for small refrigeration cycles. In a practical refrigeration cycle, some amount of refrigeration oil is discharged from a compressor and refrigerant/oil mixture flows through the capillary tube. This study investigated experimentally the influence of mixing of the refrigeration oil with the refrigerant on the flow through the capillary tube. The experiments are carried out with not only a miscible combination of refrigerant and oil but also an immiscible combination. In both cases, the mass flow rate through the capillary tube and temperature and pressure distributions along the tube are measured under several conditions of subcooled degree and oil concentration. In the case of miscible combination, the mass flow rate of refrigerant decreases with increasing the oil concentration because the viscosity of liquid phase increases by the mixing of viscous oil. Even in the case of the immiscible combination, the oil droplet is so small that it mixes homogeneously in the liquid phase in the capillary tube and the refrigerant mass flow rate decreases by the mixing of immiscible oil. There is no significant influence of the oil concentration on the underpressure, which means pressure difference between saturation pressure and flash inception pressure, in both miscible and immiscible combinations.

Characteristics of CO2 transcritical expansion process

In order to improve the performance of the CO2 refrigeration cycle, the use of an expander as an expansion device has been examined. The CO2 expansion process in the expander is a transcritical expansion process in which the CO2 condition changes from supercritical to two-phase. In this study, the transcritical expansion process is investigated in detail using a single piston expander with glass windows. The expansion process is observed with a high-speed camera, and the heat transfer coefficient within the expansion chamber is examined based on the enthalpy change during the process. It is shown that the transcritical expansion process is accompanied by a non-equilibrium condition (delay of flash) when an initial temperature of the expansion process is low. When the expansion process passes across the saturation gas line or the saturation liquid line near the critical point, a blackout phenomenon occurs by generation of fine mist of the liquid phase. In addition, the heat transfer coefficient for the heat transfer from the expander wall to CO2 inside the expansion chamber is found to be about 2–7 kW/(m2K) (352–1233 Btu/(hft2°F)) in the two-phase region.

Influences of Miscible and Immiscible Oils on Flow Characteristics Through Capillary Tube—Validation of Calculation Model

A capillary tube is widely used as an expansion device for small refrigeration cycles. This series of papers discusses the influence of mixing refrigeration oil with the refrigerant on the flow through the capillary tube. In the first report, the mass flow rate and temperature and pressure distributions are examined experimentally when miscible or immiscible oil is mixed with refrigerant. In this study, a theoretical model is developed to estimate the influence of oil on the properties of refrigerant/oil mixture and on the mass flow rate. The theoretical model for the miscible combination is also applicable to the immiscible one, since the immiscible oil mixes homogeneously in the liquid phase in the capillary tube. The mass flow rates in both cases of miscible and immiscible combinations are calculated within an error of ±5% in comparison to measured data. In the immiscible combination, however, the model tends to underestimate the mass flow rate slightly as the oil concentration becomes larger than 0.1.