A Study On The Leakage Characteristics Of Tip Seal Mechanism In The Scroll Compressor

The tip seal as a sealing mechanism to reduce the leakage flow rate through the axial clearance has been widely used in scroll compressors, but the leakage characteristics of tip seal is not well clarified due to the complicated leakage path. This paper discusses the analytical results for four theoretical models used to predict the leakage flow rate and the relationship between the changes of tip clearance and leakage flow rate under various pressure ratios. In the experiment, it was found that the tip clearance increased due to reduction of the tip sealing force that pressed the tip seal against the opposite plate under small pressure difference between upstream and downstream. Among the four models, the predicted result for the nozzle flow model was most similar to experimental result. The equivalent clearance corresponding to the measured leakage flow rate was obtained by using the nozzle model. Additionally, the behavior of tip seal forced in the tip seal groove by pressure difference was measured with a laser displacement sensor to clarify the leakage phenomenon. NOMENCLATURE Hg : height of wrap tip groove Ps : suction pressure Ht : height of tip seal Pup : upstream pressure Lg : width of wrap tip groove ∆P : differential pressure Lt : thickness of tip seal ri : inner diameter of circular wrap m& : leakage mass flow rate ro : outer diameter of circular wrap Pback : groove pressure under tip seal Tup : upstream temperature Pc : compression pressure δa : clearance between tip seal and glass plate Pd : discharge pressure δs : clearance between wrap tip and glass plate Pdn : downstream pressure e : pressure ratio of Pdn to Pup INTRODUCTION In order to attain the high efficiency of scroll compressors widely used for the air-conditioning and heat pump systems, it is greatly important to minimize the leakage flow through the narrow clearances due to the pressure difference between compression chambers. Especially, when the alternative refrigerant having highpressure properties such as R410A or CO2 is applied to the scroll compressors instead of R22, the compressor efficiency decreases due to the increase of the leakage flow caused by large pressure difference between the compression chambers. Up to now, many kinds of the models to predict the leakage flow through the narrow clearances of the compressors have been proposed, and the experimental approaches to validate the analytical models have been tried [1-9]. The scroll compressors having superior compression mechanism to any other compressors have two kinds of leakage flows. One is the radial leakage flow through the axial clearance between the scroll wrap tip and the base plate of opposite scroll, the other is the tangential leakage flow through the radial clearance between the flanks of orbiting and fixed scroll wraps. In order to reduce these leakages in scroll compressors, the sealing mechanisms to control the narrow clearances have been used. Especially, it is more important to control the axial clearance for the high efficiency because the sealing length of the axial clearance is several times longer than that of the radial clearance corresponding to wrap height. Among the sealing mechanisms to control the axial clearance, the tip seal, which is placed in the groove machined on the tip of scroll wraps, has been the most widely used due to the advantages of simple structure and good sealing ability. In the previous investigations for clarifying the leakage characteristics of tip seal mechanism, Inaba et al. [10] investigated the leakage losses about three leakage paths in the case of inserting the tip seal into a groove tightly or loosely, and they also developed the assembling techniques to insert the tip seal sufficiently close to the bottom surface of groove. On the other hand, Hirano et al. [11] clarified the sealing pressure forcing the tip seal against the base plate of opposite scroll by measuring the groove pressure under the tip seal along the scroll wrap length and measured the tip seal motion with eddy current sensors. Ancel et al. [12] investigated the dynamic behavior of two different types of tip seals, which are the multi-blade and the monobloc tip seals. Youn et al. [13] developed the experimental apparatus to measure the leakage flow using tip seal under the actual operating conditions. However, it is not enough to clarify the leakage characteristics of tip seal because of the complicated leakage path. In this study, the leakage characteristics of scroll compressors having tip seal as the sealing mechanism for the axial clearance is investigated theoretically and experimentally. The calculation using four kinds of analytical models for the estimation of the leakage flow rate is conducted. The experimental apparatus having a simplified circular wrap is set up to measure the leakage flow rate, the axial clearance and the differential pressure between upstream pressure and groove pressure under the tip seal with the various pressure ratio conditions at the same time. The calculated results are compared with the experimental ones. Moreover, the behavior of tip seal in the tip seal groove is measured directly with a laser displacement sensor to clarify the leakage phenomenon.