Xueyuan Peng

Study of a Rotary Vane Expander for the Transcritical CO2 Cycle—Part I: Experimental Investigation

This paper presents the experimental investigation of a double acting rotary vane expander for work recovery in the transcritical CO2 cycle and focuses on the design improvements for leakage and friction within the expander. The bench tests were carried out to investigate the leakage and friction distributions within the expander. The test results showed that the end gaps caused the major leakage within the improved expander prototype, while the friction losses associated with the vanes—especially due to the springs in the slots—were dominant and accounted for about 70% of the total friction losses. By comparing the pressure-rotation angle diagrams of the improved prototype with the original one, the effects of adding springs in the slots and arranging sealing vanes at the sealing arc on the thermodynamic processes were analyzed. It was shown that the tight contact between the vanes and cylinder wall owing to the springs had a significant improvement on the thermodynamic processes in that the cycle duration resumed being normal and the expander demonstrated a reasonable expansion process. By putting springs in the vane slots and arranging the sealing vane in the cylinder at the sealing arc, the volumetric efficiency increased from 17% to 30%, and the isentropic efficiency improved from 9% to 23%, resulting in a maximum coefficient of performance COP improvement of 14.2% compared with the throttling cycle under the same test conditions.

Study of a Rotary Vane Expander for the Transcritical CO2 Cycle—Part II: Theoretical Modeling

A mathematic model focusing on expander thermodynamics and vane dynamics was developed to investigate the major factors influencing the efficiencies of the rotary vane expander. Several factors were taken into account, including the leakage through various leakage paths, friction associated with the vanes, and flow through the inlet/outlet ports. The model was validated by comparing the calculated thermodynamic processes and vane movement with the experimental data, which showed the deviation was less than 10%. The predicted results from the model indicated that the expander would have an optimum pressure ratio of about 2.2. Although the volumetric efficiency increased with the rotational speed, the optimal rotational speed of 2300 rpm was obtained at inlet and outlet pressures 8 MPa (1160 psi) and 4 MPa (580 psi) and an inlet temperature of 40°C. The leakage through both the end gaps and the sealing arc had a significant influence on the expander efficiency, accounting for 35% and 28%, respectively, of the losses in the volumetric efficiency. Among the major geometric parameters of the expander, larger eccentricity and vane incline angle had a positive effect on the expander efficiencies, and the increase in the vane width had a negative effect, while the effect of the ratio of length to radius seemed to be insignificant.

Simulation of discharge pressure pulsation within twin screw compressors

Abstract 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.

Attenuation of Gas Pulsation in a Reciprocating Compressor Piping System by Using a Volume-Choke-Volume Filter

This paper presents an investigation of the use of a volume-choke-volume low-pass filter to achieve gas pulsation attenuation in a reciprocating compressor piping system, with a focus on its frequency response characteristics and influence on the actual attenuation effects. A three-dimensional acoustic model of the gas pulsation was established for a compressor discharge piping system with and without the volume-choke-volume filter, based on which the gas column natural frequencies of the piping system and the pressure wave profiles were predicted by means of the finite element method. The model was validated by comparing the predicted results with the experimental data. The results showed that the characteristic frequency of the filter was sensitive to both diameter and length of the choke but independent of the parameters of the piping beyond the filter. It is worth noting that the characteristic frequency of the filter constituted one order of the gas column natural frequencies of the piping system with the filter. The pressure pulsation levels in the piping system downstream of the filter could be significantly attenuated especially for the pulsation components at frequencies above the filters characteristic frequency. The measured peak-to-peak pressure pulsation at the outlet of the filter was approximately 61.7% lower than that of the surge bottle with the same volume.

Modeling the valve dynamics in a reciprocating compressor based on two-dimensional computational fluid dynamic numerical simulation

This article presents a numerical simulation of the thermodynamic process in the cylinder and dynamics of the self-acting valves for an air reciprocating compressor. The finite-volume method was employed to solve the compressible turbulent flow in the cylinder and through the valves. A single degree-of-freedom model was adopted to simulate the valve dynamics. The piston’s motion was defined by a user’s preset function, and the fluid-structure interaction between the gas flow and the valve dynamics was solved in a strictly coupled fashion. The technique of non-conformal interfaces was adopted to allow fluxes between adjacent zones with different mesh node locations. Based on the analysis of the characteristics of the flow through valves and the valve movement, sensitivity analysis on the valve impact velocity and the angle of inclination indicates that the valve impact velocity was more sensitive to the variation of the rotational speed and the valve lift while severe inclining motion occurs when the valves are installed in the radial direction. Instantaneous gas flow and force coefficients with the variation of valve displacement are also obtained according to the mass-flow rate across the valve and the gas force acting on the ring plate.

Development of the free piston expander for work recovery in transcritical CO2 refrigeration cycle

Abstract Replacing the throttle valve with an expander is beneficial to the performance improvement on the transcritical CO2 refrigeration cycle. The expander is hereby developed for this purpose in this paper. With the required working conditions, performance potential, reliability, etc. considered, the free piston machine is selected as the most suitable concept after a comparison among some candidates. The single acting structure is adopted, in which an auxiliary compressor is arranged on a common shaft to utilize the work recovered by the expander. A slider-based scheme is applied to control the expander inlet/outlet, and an adjustable clearance volume is designed to modulate the operating ranges of the low/high pressures. A design model is established to determine the geometric parameters of the expander-compressor unit. A prototype expander is manufactured and preliminary test is carried out on a test rig with both air and CO2 as the working fluid. The experimental results show that the expander-compressor unit can work in some ranges of the low/high pressures, which proves that this type of expander concept as well as the inlet/outlet control method is feasible. A serious pressure pulsation is observed at the expander inlet, resulting in considerably large pressure drop during the filling process of the expander. The work to improve the expander is therefore being done, with the attenuation of the pressure pulsation focused on.

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

Abstract 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.

Experimental investigation of the fault diagnosis of typical faults in reciprocating compressor valves

The failure of suction/discharge valves is the most common cause of unscheduled compressor shutdowns; therefore, the in-time fault diagnosis of valves is crucial to the reliable operation of reciprocating compressors. Major valve faults include leakage, valve flutter, delayed closing, and improper lift. To determine the features for diagnosing these typical valve faults, this paper presents an experimental study of the fault diagnosis of reciprocating compressor valves with acoustic emission technology and simulated valve motion. The measured AE signals and simulated valve motions of normal and failed valves are studied. The results of the fault diagnosis indicate that an earlier occurrence of the suction process can diagnose suction valve leakage and that an earlier occurrence of the discharge process can be used for detecting discharge valve leakage. The leakage also causes an increase in the amplitude of the continuous acoustic emission signal. Valve faults resulting from improper valve lift can be diagnosed by the amplitude of the burst acoustic emission signal. The number of burst acoustic emission signals and the shape of the simulated valve motion can be used to monitor the valve flutter conditions. The location where the valve closes can diagnose a valve-delayed closing fault.

A new generatrix of the cavity profile of a diaphragm compressor

The small flowrate and the diaphragm’s short life are two shortcomings of the diaphragm compressor. This paper presents a new generatrix of the cavity profile of a diaphragm compressor to increase cavity volume and decrease diaphragms radial stress. To verify the design theory, the radial stresses on the oil side of the diaphragm in the cavities with the new and traditional generatrices were tested, and the experimental radial stresses agreed with the theoretical values. As the most important evaluation criteria of the cavity profile, the volumes of the cavities with different generatrices and the radial stress distribution of the diaphragm within were investigated under various design conditions. The results showed that the volume of the cavity with the new generatrix was about 6.5% larger than that with the traditional generatrix under the same design condition. Otherwise, with the same cavity volume and radius, the maximal radial stress of the diaphragm in the cavity with the new generatrix decreased by 10.3% stably, compared to that in the cavity with a traditional generatrix. Likewise, in the diaphragm’s centric region where the additional stress caused by the discharge holes occurred, the maximal radial stress of the diaphragm in the cavity with the new generatrix decreased about 11.5%.

Influence of an Orifice Plate on Gas Pulsation in a Reciprocating Compressor Piping System

This paper presents an investigation of the influence of the orifice plate parameters and installation positions on the attenuation of gas pulsation in a reciprocating compressor piping system. The acoustic wave theory and transfer matrix approach were applied to establish the simulation model, in which the valve chamber was assumed to be the pipe–volume–pipe element. Based on the model, the effects of the size and installation positions of the orifice plate on the gas column natural frequencies and pressure pulsation amplitudes were analyzed for the discharge piping system of a two-stage reciprocating air compressor. A test rig was built to validate the simulation results. The gas column natural frequencies and pressure pulsation amplitudes at different locations of the piping system were measured to verify the model. A favorable agreement was noted, with a maximum error of 2.1% for the natural frequencies and 6.3% for the pulsating amplitudes. The influence of the orifice plate on the gas column natural frequency varied according to its position and parameters. The results showed that all orders of natural frequencies decreased slightly as the inner diameter of the orifice plate decreased when the orifice plate was installed downstream of the vessel. However, the distribution of the gas column natural frequency changed when the orifice plate was installed upstream of the vessel. The pressure fluctuations in the piping system could be attenuated substantially by placing an orifice plate of reasonable parameter downstream of the vessel, within a distance of 0.4 m. The degree to which the orifice plate could attenuate the gas pulsation varied under different operating conditions. However, its attenuation effect was more sensitive to the compressor speed than to the discharge pressure.