Xiaohan Jia

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.

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.

Numerical study and experimental validation of a Roots blower with backflow design

ABSTRACT A three-dimensional computational fluid dynamics (CFD) model of a Roots blower with a backflow design was established to analyse the effects of the backflow on the Roots blowers performance. A prototype of Roots blower with a backflow design was manufactured to validate the CFD model through the pressure distribution and the mass flow rate. The results showed that the proposed CFD model agreed well with the experimental data. The effects of the sizes and directions of the backflow passage on the Roots blowers performance were then investigated using the validated CFD model. It was found that under a properly sized backflow passage, the pressure pulsation and the shaft power can be decreased by 80% and13%, respectively; However, the mass flow rate was reduced by 12% under the same size of backflow passage. Although the direction of the backflow passage affected the shaft power, it had no effect on the mass flow rate or pressure pulsation. The shaft power consumption of a Roots blower with a vertical backflow was 4% lower than a horizontal backflow.

Attenuation of Gas Pulsation in the Valve Chamber of a Reciprocating Compressor Using the Helmholtz Resonator

This paper presents testing and analysis results associated with a new control method based on the Helmholtz resonator to suppress the pressure pulsations in the valve chamber and cylinder nozzle of a reciprocating compressor. The characteristic response of the designed Helmholtz resonator was analyzed and its attenuation characteristics on the gas pulsation were investigated. A three-dimensional acoustic model of the gas pulsation was established by means of the finite element method (FEM) for a compressor discharge piping system with and without the resonator. The gas column natural frequencies of the piping system and the pressure wave profiles were predicted using the presented model and validated by comparing the simulated results with the experimental data. The results showed that the pressure pulsating amplitude in the valve chamber was reduced by 40.4% when the resonator was installed. If the resonance frequency of the resonator shifted from the cylinder nozzle characteristic frequency by a range of ±13%, the reduction in the pressure fluctuations within the valve chamber was about 24%. The best attenuation effectiveness on the valve chamber, a reduction of 47%, was obtained when two resonators were installed on the valve covers of both the head and crank ends. Two new frequencies of 40.4 Hz and 66.9 Hz appeared to replace the original cylinder nozzle characteristic frequency of 53.9 Hz with the Helmholtz resonator installation, and the corresponding resonance region was transferred from the valve chamber to the resonator.

Numerical Simulation and Experimental Validation of the Vibration Modes for a Processing Reciprocating Compressor

The low-order vibration modes of a reciprocating compressor were studied by means of numerical simulation and experimental validation. A shell element model, a beam element model, and two solid element models were established to investigate the effects of bolted joints and element types on low-order vibration modes of the compressor. Three typical cases were compared to check the effect of locations of moving parts on the vibration modes of the compressor. A forced modal test with the MRIT (Multiple References Impact Test) technique was conducted to validate the simulation results. Among four numerical models, the solid element model with the bolt-pretension method showed the best accuracy compared with experimental data but the worst computational efficiency. The shell element model is recommended to predict the low-order vibration modes of the compressor with regard to effectiveness and usefulness. The sparsely distributed bolted joints with a small bonded region on the contact surface were key bolted joints that had greater impacts on the low-order vibration modes of the compressor than the densely distributed bolted joints. The positions of the moving parts had little effect on the low-order vibration modes of the compressor.

Numerical Simulation and Experimental Validation of Failure Caused by Vibration of a Fan

This paper presents the root cause analysis of an unexpected fracture occurred on the blades of a motor fan used in a natural gas reciprocating compressor unit. A finite element model was established to investigate the natural frequencies and modal shapes of the fan, and a modal test was performed to verify the numerical results. It was indicated that the numerical results agreed well with experimental data. The third order natural frequency was close to the six times excitation frequency, and the corresponding modal shape was the combination of bending and torsional vibration, which consequently contributed to low-order resonance and fracture failure of the fan. The torsional moment obtained by a torsional vibration analysis of the compressor shaft system was exerted on the numerical model of the fan to evaluate the dynamic stress response of the fan. The results showed that the stress concentration regions on the numerical model were consistent with the location of fractures on the fan. Based on the numerical simulation and experimental validation, some recommendations were given to improve the reliability of the motor fan.

An investigation into oil—gas two-phase leakage flow through micro gaps in oil-injected compressors

Abstract This article presents the investigation on the oil—gas two-phase leakage flow through the micro gaps in oil-injected compressors and provides a new way of investigating the internal leakage process in the compressors. The oil—gas leakage rates were measured through the micro gaps of various gap sizes, the volume ratios of oil to gas, and pressure differences/ratios; and the flow patterns reflecting the flow characteristics were observed by using a high-speed video. The experimental results showed that the leakage flowrate was significantly related to the flow patterns in the gap, which were similar to those found in the existing literature and agreed well with the predicted ones by the Weber number. The gas leakage flowrate through the gap increased rapidly with the increased pressure ratio until the pressure ratio reached the critical pressure ratio, which ranged from 1.8 to 2.7. At the critical pressure ratio, the flow pattern transition from churn flow to annular flow occurred, resulting in gas leakage driven by a different sealing mechanism. As the volume ratio of oil to gas increased by 0.5 per cent, the gas leakage flowrate decreased by 77 per cent.