Effect of air temperature variation on the performance of wet vapour organic rankine cycle systems

A multi-variable optimization program has been developed to investigate the performance of Wet Organic Rankine Cycles (WORC) for generating power from low temperature liquid dominated brines. This cycle model contains a detailed thermodynamic model of a twin-screw expander, and the methods used to match the operation of the expander to the requirements of the cycle are described. Optimum operating conditions are calculated for a particular design point, which specifies the required size of heat exchangers and the port geometry and operating speed of the expander. Performance at off-design conditions can then be optimized within these constraints. This allows a rigorous investigation of the effect of air temperature variation on performance of WORC systems. The capability of the cycle model has been demonstrated for the case of power generation from a brine heat source at 120°C, assuming typical air temperature conditions for Nevada, USA. There are two main findings from the paper. Firstly, optimization of the WORC system using the annual average air temperature of 10.5°C achieves maximum power output with 75% dry working fluid at the inlet to the expander. Secondly, analysis of the off-design performance of the system shows that positive net power output is possible for air temperatures up to around 40°C. The estimated average power output over the course of a year was only 3.4% smaller than the power generated at the average annual temperature of 10.5°C. This confirms that a single calculation of WORC system performance using the average temperature for the region gives a good estimate of the expected average annual power output of the system. For the resource conditions assumed, screw driven WORC systems can be built with net power outputs of the order of 600kW, using standard size machines

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

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

Operational characteristics of internally geared positive displacement screw machines

Cylindrical helical gearing profiles can allow an externally lobed inner gear to rotate inside an internally lobed outer gear while maintaining continuous lines of contact between the gears. A series of separate working chambers are formed between the rotors, and this paper investigates the use of ported end plates to control the period during which fluid is allowed to enter or leave these working chambers. A simple method of defining the rotor profiles is considered, allowing the geometry of such internally geared screw machines to be characterised. The performance of the machine is then investigated for particular applications. The key considerations in this preliminary study are the effect of geometry on the forces and torques that act on both the inner and outer rotors. The results suggest that low rotor contact forces are possible, which is necessary to ensure high efficiency and low wear in practical machines

Analysis of rolling bearing power loss models for twin screw oil injected compressor

The mechanical losses inside a screw compressor limit the performance of the compressor in terms of efficiency. These losses arise due to relative motion between elements inside the screw compressor. The estimation of mechanical losses predicted in the literature is around 10-15% of the total shaft power. One of the elements which contribute significantly to these losses is rolling element bearings. There are numerous mathematical models available which predict power losses in the rolling bearings. The objective of this paper is to study different models to predict power loss for rolling bearings and to predict the power losses for the bearings used for oil injected, twin screw compressor. A comparison between different power loss models for different operating conditions of compressor is also presented in this paper and results of analysis are compared with available experimental observations. The analysis helps to determine suitable power loss model for different operating conditions and more realistic predictions of the power losses. This allows designers for more accurate estimation of the performance of screw compressors

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

Internally geared screw machines with ported end plates

It is possible to design cylindrical helical gearing profiles such that an externally lobed inner gear rotates inside an internally lobed outer gear while maintaining continuous lines of contact between the gears. The continuous contact between the inner and outer rotors (analogous to the main and gate rotors in a conventional screw machine) creates a series of separate working chambers. In this type of machine the rotors have parallel axes of rotation, and if both rotors are free to rotate about their own axes, these axes can be fixed in space. The use of ported end plates is proposed to control the period during which fluid is allowed to enter or leave the working chambers of the internally geared screw machine. As with conventional screw machines, these internally geared rotors can then be used to achieve compression or expansion of a trapped mass of fluid, and the machine geometry can be designed in order to optimise performance for particular applications. This paper describes the geometrical analysis of some simple rotor profiles and explores the effect on rotor torques for particular applications of this novel screw configuration