7 research outputs found
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Investigation of turbulent flow characteristics within screw compressor
The material presented in this paper is part of a research project dedicated to investigate the fluid mean velocity distribution and the corresponding turbulence fluctuations at various cross-sections across the working and discharge chambers of a screw compressor, in order to characterise the flow development through the working chamber and its discharge port at different phase angles. The axial mean flow and the corresponding turbulent fluctuation were measured inside the machine, both upstream and downstream of the discharge port, with high spatial and temporal resolution using laser Doppler Velocimetry (LDV) at a rotational speed of 1000 rpm and a pressure ratio of 1.0
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Turbulent Flow Measurements near the Discharge Port of a Screw Compressor
Mean flow and turbulence characteristics have been measured within the male and female rotors close to the discharge port of a double screw compressor at different radial positions, two axial positions from the exit port, Hp, and two radial planes, αp. Cycle-resolved axial and tangential mean flow measurements and their corresponding turbulent velocity fluctuations were made over a time window of 1° using a laser Doppler velocimetry, LDV, system. Measurements were performed through two transparent windows near the inlet of the discharge port inside the male and female working chambers. The results revealed a highly complex 3-D flow within the male and female working chambers, in particular, near the discharge port with two distinct flow zones 1 and 2 before and after the opening of the port, respectively. The flow in zone 1 was controlled by the rotor motion while in zone 2 was greatly influenced by the discharge process. In zone 2, both components of mean velocities were subjected to a sudden increase in velocity forming strong axial and tangential jet flows due to rapid change in pressure across the port as the flow is exposed into the discharge port. It was found that the flow structures have been affected considerably by the position of the discharge port, radial planes and radial positions. Axial and tangential RMS velocity distributions within both rotors were found to be relatively high and less affected by the flow changes of zones 1 and 2 with almost uniform distribution. The measured magnitudes of axial and tangential RMS velocities suggest it would be reasonable to assume the local turbulence to be isotropic for the modelling purposes. To authors’ knowledge, the results are unique, original and in great details not only to describe the flow structure, but also, they can be used in CFD codes to establish a reliable model of the flow and pressure distribution within twin screw machines
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Axial Flow Characteristics within a Screw Compressor
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° 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, Vp . 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
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Flow measurements in the discharge port of a screw compressor
The angle-resolved mean velocity and turbulence characteristics of axial air flow within the rotors and discharge chambers of a screw compressor have been measured, using a laser Doppler velocimeter with high spatial and temporal resolution. The measurements were made through special transparent windows fixed in the compressor casing and in the pipe immediately above the discharge port. Results were obtained at a speed of 1000 r/min, a discharge pressure 1 bar, and a temperature of 57 °C.
The flow interaction between the rotors and the discharge chamber was established as well as the spatial variation of the axial mean velocity and turbulence velocity fluctuation. It was shown that the discharge flow was complex, strongly time-dependent, and controlled by several mechanisms. In general, the axial velocity, on entering the working chamber downstream of the discharge port exit was higher than that immediately upstream with large variation in mean and root mean square velocities immediately after the opening of the discharge port, then flow becomes more uniform. The high velocity values and large fluctuation are mainly controlled by the pressure gradient across the port at the very beginning of the discharge process, after that, as the port opens wider, uniform flow is influenced mainly by the rotor action.
These measurements will be used as input data for more reliable optimization of compressor design and to validate a computational fluid dynamics model of fluid flow within twin screw compressors, already developed in-house
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Turbulent flow development within the discharge cavity of a screw compressor
Spatial flow field velocities within the discharge cavity of an optical screw compressor have been measured using LDV and PIV techniques. Angle-resolved velocities were obtained over a time window of 1° at a speed of 1000 rpm, pressure ratio of 1 and temperature of 55°C. Comparison between the LDV and PIV results showed very good agreement and provided a good level of confidence in the presented data. Overall, the flow field results revealed the presence of a complex, turbulent, 3D and vortical flow structure within the discharge cavity. LDV measurements at the exit of the discharge port showed that the inflow into the cavity has two distinct flow features that includes undulated velocity profiles with high gradient during the opening of the port, and uniform jet-like flows during the rest of the time. The energy necessary to create that jet-like flow was from the built-in pressure in the rotors. Turbulence fluctuations were high and followed the mean flow variations with values up to 35% of the mean values during the undulating flow. PIV spatial mean flow measurements showed a uniform axial flow close to bottom of cavity that has been transformed to a stable solid body vortex at the top of the cavity. These measurements within the discharge cavity are made for the first time and they are unique and in great detail that can be used for validation of CFD codes and optimisation of compressors to improve their efficiency for different system applications