PDF《Compressors》

2 International Compressor Engineering Conference at Purdue, July 14-17,

2008 L δ C High pressure chamber Low pressure chamber O-ring Test piece δa:Axial clearance LLeak passage length Leakage flow Low pressure chamber Test piece δr:Radial clearance High pressure chamber Leakage flow δ r O-ring Release valve Release valve A A A A Frame Frame (a) axial clearance model (b) radial clearance model Figure

1 Test models for CO2-gas leakage in scroll compressors. leakage flow is characterized in terms of a friction factor. As a result, it was shown that the effect of surface roughness on the friction factor becomes remarkable when the leakage clearance is less than about

10 μm. In the present study, very simple experiments are conducted with leakage channel surfaces with known relative roughness to identify more precisely the relationship between surface roughness and friction factor. Initially pressurized CO2 gas in a large vessel is released to the atmosphere through axial and radial clearances, where the pressure decay due to gas leakage is measured for a variety of initial pressures, up to 3.1 MPa. The Darcy-Weisbach equation for incompressible, viscous fluid flow is used to calculate the leakage flow rate through the axial and radial clearances in terms of an unknown friction factor. Subsequently, the measured pressure decay is carefully simulated by assuming a polytropic process and provides an independent measure of mass discharge, from which the empirical friction factor is determined and subsequently plotted on a Moody diagram. The empirical friction factor is then correlated with its corresponding surface roughness, thus identifying the surface roughness effect. Finally, the empirically determined friction factor for various roughnesses is incorporated into computer simulations of a scroll compressor to calculate the volumetric, mechanical, compression and resultant overall efficiencies, thereby identifying the role surface roughness plays in the performance of a scroll compressor. 2. EXPERIMENTAL SET-UP Experiments on leakage flows through axial and radial clearances in scroll compressors have been conducted using the two representative models shown in Figure 1, under the assumption that surface roughness has a significant effect on the leakage flow, with clearances of

10 μm and smaller. Tests were conducted with average surface roughness ε, ranging from 0.2 μm to 0.8 μm. Figure 1a shows a plane view of the axial clearance model, where the leakage path has a streamwise length of L=4.0 mm and a depth of 15.0 mm. The leakage clearance height δa was carefully adjusted to

10 μm using thickness gauges with a width of 2.5 mm. Thus, the net leakage depth becomes 10.0 mm. In addition, an O-ring was attached between the test piece and the thrust plate, and a liquid gasket was also used on the contact surface to entirely eliminate any unintended leakage. Figure 1b shows the model for the radial clearance, where the involute curves of orbiting and fixed scrolls are represented by two circular arcs with different radii. The radii were fixed at 14.4 mm and 11.8 mm, respectively, and the net leakage depth of the radial clearance was fixed at 10.0 mm. The high pressure chamber on the left side of test section is directly connected to a supply tank with a volume of

860 cm3 . The low pressure chamber on the right hand side is opened to atmospheric pressure through a discharge valve. With the discharge valve closed, both the high pressure and low pressure chambers are initially pressurized at a specified pressure. Then the low pressure chamber is suddenly vented to the atmosphere by opening the discharge valve. The pressure in the high pressure chamber decreases due to refrigerant leakage through the test section, and 1429, Page