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Axial flow characteristics within a screw compressor.

Article Excerpt
Angle-resolved axial mean flow and turbulence characteristics were measured inside the working chamber of the male rotor of a screw compressor with high spatial and temporal resolution using laser Doppler velocimetry at two rotor speeds, 750 and 1000 rpm. Measurements were performed through a transparent window near the discharge port to allow the application of various laser techniques. The results showed that an angular resolution up to 2[degrees] could fully describe the flow variation inside the chamber. The cyclic flow variation between different working chambers was found to be similar in both the mean and turbulence velocities. The effect of the discharge port opening on the axial mean and root mean square velocities was found to be significant near the leading edge of the rotors, causing a steep increase in mean and root mean square velocities of the order of 4.2 times the pitch velocity, [V.sub.p] . This effect is less pronounced on the flow near the root of the rotor; large fluctuations and instability in the mean flow was caused by rapid flow expansion during the port opening. The obtained data will be used to validate a computational fluid dynamics model of the fluid flow within twin screw compressors, which could allow reliable optimization of various compressor designs.

INTRODUCTION

The use of screw compressors in the HVAC industry is widespread, as they have replaced the traditional reciprocating compressor in a large range of applications, particularly in air compression and refrigerant compression as well as engine supercharging in the automotive industry. Improvements in screw compressors are continually sought in order to increase their performance and reduce energy consumption, noise generation, and manufacturing costs. Screw compressor rotors are contained in a casing where their meshing lobes form a series of working chambers within which the compression process takes place. As the rotors turn, air is admitted through the space between the rotor lobes and the suction port. Further rotation of the rotor leads to cutoff of the suction port, and the trapped volume is pushed forward axially and circumferentially toward the discharge port by the action of the screw rotors, during which the trapped volume in each passage is reduced and its pressure is increased. This process continues until the working volume between the rotors is exposed to the discharge port, allowing gas with high pressure to flow out (Stosic 1998). The performance of such compressors depends on the flow field characteristics of the gas trapped between the chambers of the screw rotors and the housing. It is thus essential to have a good understanding of the gas motion within the compressor by quantifying the velocity field in the suction, discharge, and working chambers, as well as the flow through the clearance gaps, in order to complete the picture about the sequence of processes that occur within the compressor. These velocities are presently estimated today by means of mathematical models of different levels of accuracy and, to the authors' best knowledge, no velocity measurements within a screw compressor have been published and no attempt to do so has been reported.

The material presented in this paper is the initial phase of a long-term research project attempting to measure the mean velocity distribution and the corresponding turbulence fluctuations at various cross sections of the compressor's working volume within the interlobe space at different phase angles. This is expected to reveal how major features of the heat and fluid flow within the compressor are affected by the rotor and lobe geometry. In addition, the major integral properties of the compressor, including the suction and discharge flow velocities, pressures, and temperatures, as well as the torque/power, will be measured with various instruments and compared with predicted values of the same properties derived from an existing computational fluid dynamics (CFD) model (Kovacevic et al. 2000, 2002); the latter is under development and can assist in the design of future screw compressors. Potential benefits to be derived from such a project are the ability to design rotors and other components with correct allowance for distortion and increased compressor efficiencies due to minimization of clearances.

As described above, the flow in screw compressors is complex, three-dimensional, and strongly time dependent, i.e., it is similar to those in the cylinders of gasoline and diesel engines (Arcoumanis and Whitelaw 1987; Kampanis et al. 2001), centrifugal pumps (Liu et al. 1990), or in related and mixed-flow turbines (Zaidi and Elder 1993; Arcoumanis et al. 1997, 1998) and mixing reactors agitated by turbines (Hockey and Nouri 1996; Distelhoff et al. 1997). This implies that the measuring instrumentation must be robust to withstand the unsteady aerodynamic forces, have high spatial and temporal resolution, and most importantly must not disturb the flow. Only point optical diagnostics like laser Doppler velocimetry (LDV) can fulfill these requirements, as was demonstrated by previous research in similar flows. Recent relevant experimental work by Schleer and Abtahi (2006) made use of an LDV system to measure the flow characteristics within a vaneless parallel diffuser downstream of the impeller. The spatially and temporally resolved data showed a complex and highly vortical flow structure within the diffuser that depended on the relative tip clearance.

The first phase of the proposed method of research is the characterization of the axial component of the fluid velocity and turbulence fluctuations at a range of preselected measurement points inside the working chamber from the root of the rotor to its...

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