Experimental Study on Physical Properties of CuO - PVE Nano-oil and its Mixture with Refrigerant

Refrigeration oil is generally used in refrigerant compressor for lubrication. Recently, a nano-oil, which is a suspension of base lubrication oil and nano-order particles, is proposed to pursue further improvement of refrigeration cycle efficiency. There are many advantages by using nano-oil as lubrication oil in refrigerant compressor; (a) improvement of thermophysical property of lubrication oil, (b) bearing effect at sliding part, and (c) sealing effect of leakage of refrigerant and so on. For actual application of nano-oil to refrigeration system, it is important to investigate the effect of addition of nano-particles to the physical properties of refrigeration oil and its mixture of refrigerant at first. In this study, we measured not only thermal conductivity, but also other physical properties such as viscosity and dielectric constant of nano-oil itself. In addition to nano-oil, such physical properties of refrigerant / nano-oil mixture were also measured and the effect of nano-particles was discussed. Nano-oil was prepared by dispersing CuO nano-particles into refrigeration oil with 0.5, 1.0 and 3.0 vol% of the volume fraction. We chose Polyvinyl ether (PVE) as a refrigeration oil in this study. Temperature of nano-oil was ranged from 30 to 100 °C. The results show that thermal conductivity, viscosity and dielectric constant of the nano-oil are larger than those of the base oil. The increase of these properties becomes large with increasing volume fraction of nano-particles. On the other hand, the physical properties of refrigerant / nano-oil mixture were measured in a pressure vessel. R410A was used as a refrigerant in this study. The temperature was kept at 30 °C. The concentration of refrigerant in nano-oil was measured by the sampling method. The results show that thermal conductivity, viscosity and dielectric constant of R410A / PVE-CuO nano-oil are larger than those of R410A / PAG for the small concentration of refrigerant. The tendency that the increasing level of these properties becomes small with increasing refrigerant concentration was observed

Quality Measurement of Two Phase Flow with Plug Flow

In order to control refrigeration cycles employing an injection system properly, it is important to detect quality of gas–liquid two-phase refrigerant in a two-phase region of the cycle. Although there are some techniques such as using a capacitance sensor or a X-ray beam scanning to measure a cross-sectional void fraction of the gas-liquid two-phase flow in a pipe, the measurement of the quality or flow rate of each phase is quite difficult since the liquid phase and gas phase of two-phase flow flowing in a pipe have different velocities in most cases. Meanwhile, the flow through a narrow tube becomes plug flow and the velocities of gas plug and liquid plug are almost the same. Therefore, the void fraction or quality of two-phase flow with plug flow in the narrow tube can be measured by detecting each plug length. Authors have examined the quality measurement of two-phase flow in the refrigeration cycle based on the plug flow characteristics. In previous studies, it was confirmed that the quality can be measured with an accuracy of about ±10% when the flow regime is plug flow in the narrow tube. However, the quality range where the flow regime becomes the plug flow is limited to the quality less than 0.1. In this study, multiple narrow tubes are installed with a gas bypass line to extend the quality range to be measured. Consequently, the measurable range of quality up to 0.8 was achieved with an accuracy of ±10%

Evaluation of Wettability of Solid Surface with Oil/Refrigerant Mixture

A part of lubrication oil stored in an oil sump of a refrigeration compressor becomes fine oil mist in a compressor shell. Although some of the oil mist is separated from refrigerant in the compressor shell, a certain amount of the oil mist is discharged with refrigerant into a refrigeration cycle. The oil circulated with refrigerant in the cycle degrades the heat transfer efficiency of heat exchangers, increases pressure drop in lines and the heat exchangers and sometimes reduces reliability of the compressor when an oil return from the cycle to the compressor is insufficient. It is therefore important to reduce the oil discharge from the compressor to the cycle. In order to decrease the oil amount discharged from the compressor, development of a superior oil separator should be done and many researches were focused on it. More basically, since the separation efficiency of the oil separator strongly depends on droplet size of the oil mist, factors associated with the droplet size such as generation mechanism of the oil mist in the compressor, the droplet behavior on solid surface in the compressor and surface tension of the oil in which the refrigerant is dissolved should be clarified. However, the surface tension of the oil mixed with refrigerant and wettability of solid surface with the oil/refrigerant mixture which influences the droplet behavior have not been reported sufficiently. In this study, the surface tension of the PAG oil/CO2 mixture is measured by a pendant drop method and the wettability of the solid surface is evaluated as the fundamental study for discussing the generation mechanism of the oil mist in the compressor. The measurements of the surface tension of the oil/refrigerant mixture and contact angle of the mixture on the solid surface are achieved under high pressure and temperature condition. It is found that the surface tension of oil/refrigerant mixture decreases steeply with increasing the refrigerant concentration in the oil. The contact angle on solid surface decreases with the refrigerant concentration due to the reduction of surface tension. The contact angle on Aluminum surface is smaller than that on the PTFE surface and decreases with elapsed time. With using Ohnesorge number which is a ratio of viscous force to inertia force and surface tension force to evaluate the oil droplet behavior on the metal surface, Ohnesorge number is estimated to be very small under discharge condition in the compressor and it suggest that the oil droplet of oil/refrigerant mixture will spread immediately on the metal surface

Seal Mechanism of Tip Seal in Scroll Compressor

Scroll compressors are widely used in room air conditioning cycles, package air conditioning cycles, refrigeration, water heater and automobile air conditioning cycles as well as air compressors, helium compressors and vacuum pump. There are two main leakage paths in a compression chamber formed by a fixed scroll and an orbiting scroll. One is the leakage path through a radial clearance between the wraps of fixed and orbiting scroll. The leakage through the radial clearance is prevented by pressing the orbiting scroll radially against the fixed scroll by a mechanism such as a compliance mechanism. Oil inside the compression chamber also has the sealing effect and reduces the leakage through the radial clearance. Another leakage path is an axial clearance which is the clearance between a tip of the scroll wrap and a base plate. A tip seal is often used to prevent the leakage through the axial clearance. Although there have been many studies on the tip seal, the seal mechanism of the tip seal is not thoroughly clarified yet, and the influence of design parameters on efficiency of the tip seal is unclear. In addition, the relationship between the sealing effect and a frictional loss of the tip seal is also not validated well. In this study, a test apparatus which can evaluate the sealing effect and the frictional loss of the tip seal simultaneously is developed. The influence of several design parameters on the sealing effect and the frictional loss is examined with the test apparatus. By measuring pressure distribution around the tip seal in a tip seal groove, the sealing mechanism of the tip seal is clarified and a design guideline of the tip seal groove is obtained