Built-in Volume Ratio Research Articles

Experimental study and CFD approach for scroll type expander used in low-temperature organic Rankine cycle

This study focuses on experimental test of scroll type expanders in low-temperature organic Rankine cycle (ORC) system. In this circuit, lubricant has been mixed with R245fa as working fluid. In this experiment, two scroll expanders with different built-in volume ratio have been experimentally tested. Main test parameters considered are the pressure difference and the rotational speed of the expanders. It is found that the expander performance could be significantly improved when bigger built-in volume ratio is used. However the internal leakage and friction loss are vital factors to influence expander performance. The maximum shaft power output by expander of 1.77 kW and deliver electricity by generator of 1.375 kW. A Computational Fluid Dynamics approach (CFD) has been employed for preliminary investigation on the thermal-hydraulic behavior of the scroll type expanders. The simulation result shows that unbalance pressure distribution were occurred in the expander by means of top scroll wrap has been modified.

Research on operating characteristics of single slide valve capacity control mechanism of the single screw refrigeration compressor

In the single screw refrigeration compressor (SSRC), the single slide valve capacity control mechanism (SVCCM) is the functional component used to achieve capacity control under different load conditions with cooperation of screw rotor and housing. In this paper, a new calculation model considering internal compression and leakage is presented to research the capacity control process and built-in volume ratio control process. By this model, the actual capacity and built-in volume ratio at different slide valve work locations can be determined using the calculated by-pass area and discharge area. The influences of structural parameters of the slide valve to the control characteristics of the capacity control process and the thermodynamic properties of the compressor are analyzed. Through the comparison of the capacity control process and the built-in volume ratio control process, the axial displacements of the slide valve required in the capacity control and in the built-in volume ratio control were obtained. All of this work can provide the basis for the later optimization design for the slide valve.

Characterization and modeling of a scroll expander with air and ammonia as working fluid

This paper presents the experimental characterization and modeling of a scroll expander. The expander used here is a scroll compressor modified to work as an expander. It is characterized in two experimental setups using air and ammonia as working fluids. The paper studies how the main operating variables (supply pressure and temperature, pressure ratio, rotational speed and lubrication) influence the performance of the scroll expander. A semi-empirical model is proposed to determine the scroll expander performance. This model uses some semi-empirical parameters (such as built-in volume ratio, leakage area and mechanical losses), obtained through experimentation, to calculate the mechanical power, supply mass flow rate and exhaust temperature. Using this semi-empirical model, the deviations in the calculated mechanical power, exhaust temperature and supply mass flow rate are ±9%, ±4 K and ±5% Hz.

Optimization of Screw Expanders for Power Recovery From Low-Grade Heat Sources

AbstractIn systems for recovering power from low-grade heat sources, using twin screw machines to expand wet vapors, the power output depends on the conflicting requirements of high heat recovery and high working fluid temperatures, the dryness fraction of the fluid entering the expander, the rotor diameter and profile, the speed, and the built-in volume ratio. To obtain the best combination of these, for maximum power output per unit flow of a given heating medium, optimization methods and their associated numerical procedures can be used. This article focuses on the optimization of the geometry of a twin screw machine for the expansion of wet steam, with the aim of maximizing power output. Representative machine operating conditions are specified, and the methods used for the design of the machine are described. The predicted performance is compared with experimental data for the optimized machine, and close agreement is found between the predicted and measured results for power output, mass flow rate, ...

자동차 엔진 폐열 회수 동력시스템에서 용적형 팽창기의 설계 팽창비 최적화

본 연구에서는 가솔린 엔진 자동차의 엔진 폐열 회수를 위한 이중 회로 랭킨 사이클 시스템에서 용적형 팽창기의 설계 팽창비에 따른 성능 해석이 수행되었다. 자동차 엔진 폐열 이용 랭킨 사이클 시스템에 사용되는 용적형 팽창기는 운전 조건에 따라 설계 팽창비가 운전 압력비보다 낮은 저팽창 조건과 설계 팽창비가 운전 압력비보다 높은 과팽창 조건으로 운전되므로 탈설계 조건에서 성능 예측이 중요하다. 또한 용적형 팽창기는 자동차 적용시 팽창효율 뿐만 아니라 부피와 무게를 최소화하는 것이 매우 중요하므로 이를 고려한 설계 팽창비의 최적화가 요구된다. 본 연구에서는 용적형 팽창기의 탈설계 조건에서 성능 예측을 통해 팽창효율과 팽창기의 용적을 동시에 고려하여 설계 팽창비를 목표 운전 압력비보다 낮도록 하여 저팽창 운전을 하도록 설계 하는 것이 유리함을 제시하였다. The effect of built-in volume ratio of expander on the performance of a two-loop Rankine cycle system for engine waste heat recovery of vehicle has been investigated. In the case of positive displacement expander in the various operating condition of the vehicle, it can operate in both under-expansion and over-expansion conditions. Therefore, the analysis of off-design performance for the expander is very important. Furthermore, the volume and weight of the expander as well as the efficiency must be considered in the optimization of the expander. This study shows that the built-in volume ratio of expander causing under-expansion at a target condition is more desirable considering the off-design performance and size of the expander, based on the simple modeling of off-design operation of the expander.

Optimization Analysis and Simulation of the EAST Cryogenic System

This paper presents the optimization analysis and simulation of the main Claude cycle of the EAST cryogenic system. A simulation program of this cycle was developed on the basis of the characteristics of the components and verified with the experimental results. The built-in volume ratio of the No. 7 compressor was tested with the purpose of finding the optimum internal pressure ratio to comply with the external pressure ratio so as to increase the isothermal efficiency and cycle efficiency. In addition, the UA (U, heat transfer coefficient; A, heat transfer area) values of the heat exchangers, efficiencies of turbines, mass flow fraction of the turbine stream were analyzed in order to improve the capacity or efficiency of the cycle. Finally, in view of the estimated heat loads of EAST in future, an upgraded operational cycle of 1500 W/4.5 K +1000 W/3.5 K +5 g/s is proposed according to the simulation.

Optimization of a scroll compressor for liquid flooding

In two companion papers, simulation models for the working processes of liquid-flooded scroll compressors and expanders have been developed and validated against experimental data. In this study, analytic models are presented for the modes of irreversibility generation in the liquid-flooded scroll compressor including built-in volume ratio maladjustment, pressure drop and leakage. A thermodynamic model is used to derive the ideal volume ratio for a liquid-flooded compressor, which is higher than that of dry compression. An optimum set of built-in volume ratio and scroll base circle radius is found which maximizes the overall isentropic efficiency by minimizing the leakage and pressure drop irreversibilities. The irreversibility generation models are used to optimize a scroll compressor for the Liquid-Flooded Ericsson cycle application. The model predicts scroll compressor overall isentropic efficiency of over 80% (based on the shaft power) at an oil mass fraction of 88%.

A study on scroll compressor conversion into expander for Rankine cycles

In this paper, we investigate using a refrigeration scroll compressor as expander for power generation applications with a Rankine cycle. The methodology employed here has three steps: In the first step, a scroll compressor is selected from a refrigeration manufacturer catalog. Based on catalog data and our simplified model, the specific parameters of the compressor such as the built-in volume ratio and leakage coefficient are determined through mathematical regression. In the second step, the parameters and the efficiency of the Rankine cycle are determined, which use the selected scroll machine in reverse, namely as expander, without any geometrical modifications. The range of temperatures and pressures are kept the same as that characterizing the compressor operation. A simplified expander model is used to predict the efficiency of the prime mover and of the Rankine cycle. A range of working fluids are considered and compared. The expander does not operate optimally when converted from a compressor without any modifications. In the third phase, the geometry of the expander is modified with respect to the rolling angle only in order to obtain the appropriate built-in volume ratio which assures better efficiency of the Rankine heat engine. This paper also presents a parametric study in terms of geometry, working fluid and operating conditions. Copyright , Oxford University Press.

Simulation of discharge pressure pulsation within twin screw compressors

Even in the absence of self-acting valves, flow through the discharge port of a twin screw compressor is oscillatory in nature. This unsteady but periodic flow variation at the discharge port excites the pressure pulsation, which is the main source of noise and vibration. In this paper, a mathematic model based on the one-dimensional unsteady gas flow equations is established to describe the discharge pressure pulsation, which considers the effects of friction and heat transfer between the gas and the pipe. The boundary conditions of the discharge pressure pulsation model are given in detail. The two-step Lax—Wendroff scheme is applied to solve the one-dimensional unsteady gas flow equations. In order to verify the theoretical analysis, the discharge pressure pulsations under various working conditions are measured and compared. It is shown that the model established in this paper is a useful tool to obtain a better understanding of the behaviour of the pressure pulsation in discharge pipe. It is found that the most important factor that affects the discharge pressure pulsation is the pressure difference between the pressure upstream of the discharge port in the compression chamber and the pressure in the discharge line some way downstream. The minimum fluctuation occurs when the discharge pressure is equal to the pressure corresponding to the built-in volume ratio. For a constant discharge pressure, the pressure pulsation amplitude increases with the rotational speed.

Analysis of the working process in an oil-flooded screw compressor by means of an indicator diagram

This paper presents the results of an experimental investigation of the thermodynamic processes in an oil-flooded screw compressor. The pressure within the working chamber is measured with a small pressure sensor, embedded in the female rotor on the discharge side. The results so obtained were transformed into an indicator diagram. Based on the indicator diagrams at various operating conditions, the working process is analysed. Owing to oil restricting back-flow of the gas through the discharge port, constant-volume compression is not evident even at substantial under-compression conditions, thus making the compressor maintain a high efficiency over a wide range of pressure ratios. However, the additional power consumption resulting from over-compression is comparatively large at pressure ratios lower than that for which the compressor was designed. So a compressor with fixed volume ratio should be designed with the built-in volume ratio low enough to avoid this effect. At the end of the discharge process, the pressure rises steeply due to increased resistance to oil flow, and therefore, the design of a flow-guiding slot to assist oil discharge is recommended. Higher rotating speeds increase the pressure slightly, but the losses associated with this are compensated by the increased volumetric efficiency. Thus a screw compressor maintains its isentropic efficiency over a wide range of speeds. Larger oil:gas ratio presents a higher pressure level in the indicator diagram, offsetting the improved sealing effect, and test results show that the appropriate oil:gas mass ratio range is between 8:1 and 20:1.

Numerical Simulation and Performance Analysis of Twin Screw Air Compressors

A theoretical model is proposed in this paper in order to study the performance of oil‐less and oil‐injected twin screw air compressors. Based on this model, a computer simulation program is developed and the effects of different design parameters including rotor profile, geometric clearance, oil‐injected angle, oil temperature, oil flow rate, built‐in volume ratio and other operation conditions on the performance of twin screw air compressors are investigated. The simulation program gives us output variables such as specific power, compression ratio, compression efficiency, volumetric efficiency, and discharge temperature. Some of the above results are then compared with experimentally measured data and good agreement is found between the simulation results and the measured data.

Optimum operating pressure ratios for scroll compressors.

A scroll compressor has a built-in volume ratio, and ideally it should be operated under a built-in pressure ratio condition that is a function of the built-in volume ratio. but in actual operation, the compressor shows its maximum efficiency at a pressure ratio that differs from the built-in pressure ratio. In this paper, theoretical performances are presented by analyses accounting of flow resistance at the opening of scroll warps and leakage through wrap clearances. The flow resistance at the discharge opening causes an over compression loss in a built-in pressure operation but reduces a undercompression loss in a high-pressure operation, which indicates that the optimum operating pressure ratio shifts to a pressure ratio higher than the built-in pressure ratio. Leakage accelerated the pressure rise during a compression process so that the optimum pressure ratio becomes lower than it would be in the absence of leakage. Increase in the compressor speed moderates the under compression loss and leads to a higher optimum operating pressure ratio.

Monoscrew - A newly developed refrigerating compressor

A description is given of a new positive displacement compressor design, Monoscrew, in which the compression is achieved in the grooves of a screw shaped rotor, sealed off by the teeth of two gear wheels. Special attention is given to the significance of the built-in volume ratio and its influence on the operating efficiency, and also t̂o the capacity control and the oil system of this machine.

The Thermodynamic Performance of Oil-Flooded Helium Screw Compressors

In recent years the cryogenic community has become interested in the application of hermetic screw compressors for helium refrigeration/liquefaction systems. In the past the screw compressor had not been an attractive compressor for cryogenic systems because of the problems associated with both dry and oil-flooded screw compressors. However, oil separation technology has evolved to a point where oil can successfully be separated from helium gas. The screw compressor has exhibited excellent reliability and performance characteristics for various refrigeration gases, and preliminary test results reveal that the same is true for helium gas. The present paper presents the thermodynamic performace characteristics of 30-kW (40-HP) and 113-kW (150-HP) oil-flooded hermetic screw compressors with various built-in volume ratios operating with helium gas at pressure ratios between 2.0 and 18.0, and for suction pressures of 0.5 atm, 1.0 atm, and 2.0 atm. The thermodynamic parameters of interest are the volumetric efficiency and the isentropic efficiency. It is found that the volumetric efficiency decreases with pressure ratio; however, depending on the built-in volume ratio of the compressor, the maximum value of the isentropic efficiency occurs in a narrow range of values for the pressure ratio at which the optimum isentropic efficiency occurs. The results of the tests indicate that oil-flooded screw compressors may successfully be used for helium service.