Jianqiang Deng

Thermodynamic study on a new transcritical CO2 ejector expansion refrigeration system with two-stage evaporation and vapor feedback

By the stability analysis of the basic transcritical CO2 ejector expansion refrigeration cycle (EERC), the paper proposed a new system which introduces another evaporator downstream the ejector to increase the gas quality into the separator and a vapor feedback valve to decrease the exceed gas into the compressor. The two new components stand for two different cycles: two-stage evaporation cycle and vapor feedback cycle. The theoretical analysis of the new system is carried out based on the first and second laws of thermodynamics to show the effect of the parameters on the system performance, such as entrainment ratio, high-side pressure, outlet temperature of gas cooler, etc. The results by the first law show that, compared with basic EERC the new system can be used in wider range of working conditions, and the COP of the two-stage evaporation cycle is 28.6% higher and the vapor feedback cycle is lower slightly. By exergy analysis at optimum high-side pressure, it is found that the exergy destruction of ejector is the greatest part. The simulation results also give the working ranges of the two cycles, which can help to analyze the system control. Hence, the improvement in the system is a promising method to reduce the restrain in basic EERC system but more study is still needed.

CO2 Leakage Identification in Geosequestration Based on Real Time Correlation Analysis Between Atmospheric O2 and CO2

Abstract The paper describes a method for monitoring CO2 leakage in geological carbon dioxide sequestration. A real time monitoring parameter, apparent leakage flux (ALF), is presented to monitor abnormal CO2 leakage, which can be calculated by atmospheric CO2 and O2 data. The computation shows that all ALF values are close to zero-line without the leakage. With a step change or linear perturbation of concentration to the initial CO2 concentration data with no leakage, ALF will deviate from background line. Perturbation tests prove that ALF method is sensitive to linear perturbation but insensitive to step change of concentration. An improved method is proposed based on real time analysis of surplus CO2 concentration in least square regression process, called apparent leakage flux from surplus analysis (ALFs), which is sensitive to both step perturbation and linear perturbations of concentration. ALF is capable of detecting concentration increase when the leakage occurs while ALFs is useful in all periods of leakage. Both ALF and ALFs are potential approaches to monitor CO2 leakage in geosequestration project.

Dynamic simulation on operating modes of ejector in transcritical CO2 ejector expansion refrigeration cycles

This article proposes an ejector model based on the real properties of CO2, which includes the critical mode and sub-critical mode (the critical mode means the primary and suction flows are both choked, and the sub-critical mode refers to only the primary flow choking). Moreover, a dynamic model of the transcritical CO2 ejector expansion refrigeration cycle is developed to simulate system responses at different ejector operational modes. The prediction results by the ejector model and system model are compared with available experimental data, respectively. Furthermore, the dynamic responses of the ejector expansion refrigeration cycle based on the entire ejector model are compared with those predicted upon the ejector model only with critical mode. The present results show the entrainment ratio provided by the ejector model coincides well with the experimental data, and most data lie within ±10% error. The system model predicts the gas cooler pressure and evaporator pressure with errors of 1.8% and 4.2% for the measured results, respectively. Moreover, the pressure and mass flow rates of the system based on the ejector model only with critical mode are higher than that by the entire ejector model. The proposed model is useful to predict performances accurately and conduct dynamic analysis reasonably.

Sedimentation of an elliptical particle in periodic oscillatory pressure driven flow

The sedimentation of a heavy elliptical particle in a two-dimensional channel filled with Newtonian fluid under oscillatory pressure driven flow has been numerically investigated by using the finite element arbitrary Lagrangian–Eulerian method. The effects of particle Reynolds number, initial position, blockage ratio, as well as oscillation frequency and amplitude on the flow patterns during sedimentation have been studied. The results show that there exists an equilibrium position for high frequency flow, and the position of the heavier particle is closer to the centerline. As rotation contributes to non-uniform pressure on particle surface, the further initial position and lower amplitude lead to the larger scale zigzag migration; however, the maximum lateral displacements of these low frequency zigzag motions are nearly the same due to the consistent lubrication limit. Moreover, our simulation results indicate that there are five distinct modes of settling in oscillatory flow: horizontal with offset, oscillating, tumbling throughout channel, tumbling at one side and the special resonance phenomenon. The resonance induced by the wall is shown to have a close association with the harmonious change of drag and lift on particle surface, and be sensitive to the oscillation in the wake and the periodic discharge of vorticity from behind the body.

Integration of CFD and RTD analysis in flow pattern and mixing behavior of rotary pressure exchanger with extended angle

AbstractIn this study, the computational fluid dynamics (CFD) approach combined with the residence time distribution (RTD) analysis was implemented to examine the mixing performance and flow pattern of rotary pressure exchanger (RPE). Based on oscillatory Reynolds number, a flow regime classification was established for RPE. A concept of extended angle of RPE was proposed, and then, its effects on mixing behavior were evaluated by CFD simulation in laminar model. Meanwhile, flow pattern in RPE was quantified by RTD study, and was well captured in the flow field analysis. In addition, the effects of operating conditions on the mixing and flow pattern were discussed. According to the results, it was shown that the extended angle of RPE is beneficial for mixing control, and a minimum volumetric mixing rate was achieved when the extended angle is ±30° compared with other configurations. In different operating conditions, the mixing rate was minimized at an oscillatory Reynolds number of about 178. Moreover, t...

Three-dimensional Voronoï analysis of preferential concentration of spheroidal particles in wall turbulence

Three-dimensional Voronoi analysis is performed to quantify both global and local aspects of clustering of inertial spheroidal particles in wall turbulence using data sets from a direct numerical simulation coupled with a Lagrangian point-particle approach. We consider oblate and prolate spheroids and characterize their inertia and shape by means of the Stokes number St and aspect ratio λ, respectively. It is observed that particles tend to drift toward the wall, and this tendency is most prominent for St = 30. Although inertia dominates over shape on the particle clustering, intermediate asphericity (λ = 0.33 and 3) is found to promote spheroids’ flux to the wall for St ≤ 30, while heavy spheroids (St = 100) with greater departure from spheres (λ = 0.1 and 10) distribute more evenly across the channel. The tendency of inertial spheroids to concentrate locally in preferred turbulence structures decreases with the distance from the walls. Owing to the particles’ preferential distribution in lower-than-mean fluid velocity regions, the local clustering of spheroidal particles decreases with the increasing asphericity. Particles with large inertia (St ≥ 30), especially spheres and prolate spheroids, are more likely to cluster in the viscous sublayer.Three-dimensional Voronoi analysis is performed to quantify both global and local aspects of clustering of inertial spheroidal particles in wall turbulence using data sets from a direct numerical simulation coupled with a Lagrangian point-particle approach. We consider oblate and prolate spheroids and characterize their inertia and shape by means of the Stokes number St and aspect ratio λ, respectively. It is observed that particles tend to drift toward the wall, and this tendency is most prominent for St = 30. Although inertia dominates over shape on the particle clustering, intermediate asphericity (λ = 0.33 and 3) is found to promote spheroids’ flux to the wall for St ≤ 30, while heavy spheroids (St = 100) with greater departure from spheres (λ = 0.1 and 10) distribute more evenly across the channel. The tendency of inertial spheroids to concentrate locally in preferred turbulence structures decreases with the distance from the walls. Owing to the particles’ preferential distribution in lower-than-mean...