Effect of hub clearance size and shape of a cantilevered stator on the performance of a small-scale transonic axial compressor

To deeply understand the hub leakage flow and its influence on the aerodynamic performance and flow behaviors of a small-scale transonic axial compressor, variations of the performance and the flow field of the compressor with different hub clearance sizes and clearance shapes were numerically analyzed. The results indicate that the hub clearance size has remarkable impacts on the overall performance of the compressor. With the increase of the hub clearance, the intensity of the hub leakage flow increases, resulting in more intense flow blockage near the stator hub, which reduces the compressor efficiency. However, the flow field near the blade mid-span is modified due to the more convergent flow as the reduced effective flow area caused by the passage blockage, and the flow separation range is narrowed, thus the flow stability of the compressor is enhanced. On this basis, two kinds of non-uniform clearance cases of expanding clearance and shrinking clearance with the same circumferential leakage area as the design clearance were investigated. The occurrence position of the double leakage flow which is closely connected with the flow loss and blockage is shifted backward by the expanding clearance, the flow capacity near the stator hub is enhanced, and the unsteady fluctuation intensity of the flow field is attenuated but fluctuation frequency remains. Similarly, the modification of the stator blade root flow field may result in the reduction of stall margin. The effect of the shrinking clearance on compressor performance is opposite to that of the expanding clearance, which reduces the peak efficiency and delays the stall inception.

Vulnerability Analysis of Urban Rail Transit Network by Considering the Station Track Layout and Passenger Behavior

The vulnerability of an urban rail transit (URT) network is an index that reflects its ability to cope with risks. However, existing URT network vulnerability studies have paid less attention to station track layout and passenger choice behavior, both of which significantly affect the consequences of a disruption incident. In the present study, we first analyze an actual scenario of URT section disruption and passenger behavior during an incident. Then, we propose two section vulnerability indexes that quantitatively evaluate the effect of a URT section disruption from two aspects: detour delay and loss in passenger flow. To make the application scenario of this method more realistic, the track layout and depot location are taken into account. By considering the relationship between train routing and the sections, a concept of “dominant section” is put forward to make the calculation of the vulnerability indexes more efficient and can be used for a simultaneous multi-section-disruption scenario. Finally, a case study of the Beijing Subway network is provided. The results show that disruptions in only a few critical sections can significantly affect the URT network passenger flow. Disruption of only 3% of the sections can lead to 80% passenger-flow loss, which reflects the high vulnerability of URT networks. The method proposed in this paper can provide support for the evaluation of URT network performance.

Numerical investigation on the compressor for gas-entraining diffuser EGR

Exhaust Gas Recirculation (EGR) is an effective way to reduce nitrogen oxide (NO x) emissions, and the EGR application increases the engine backpressure to some extent. In this paper, a new EGR method named gas-entraining diffuser EGR was proposed to reduce pumping loss. It introduces the exhaust gas into the compressor diffuser inlet where the static pressure is the lowest without blades fouled by exhaust gas. As a result, lower pressure at the turbine upstream can achieve EGR. Then, a newly designed induced structure not only introduces exhaust gas into the compressor diffuser but also reduces the energy loss caused by EGR application. Furthermore, the performance of compressor with different induced angles of the induced structure was investigated using simulation method. Results showed that the compressor’s adiabatic efficiency was the best when the induced angle was 20°. Regarding the induced angle of 20°, the adiabatic efficiency drop of compressor was in the range of 0.8%–12%. Approximately 10% of the adiabatic efficiency drop was caused by the induced structure, the other was mainly from the flow loss and mixing loss in diffuser system. The induced structure mainly affected the static pressure difference between induced structure inlet and impeller outlet ([Formula: see text]). When the impeller mass flow was 0.23 kg/s, [Formula: see text] was 11.21, 13.95, 15.59, 17.18 kPa respectively with corresponding induced angles of 20°, 30°, 40°, 50°. The primary energy loss leading to the adiabatic efficiency drop of compressor with induced structure occurred in diffuser system. It was caused by the mixing process of induced gas and impeller exit gas, and the enhanced effect from the shroud side’s impeller jet-wake and volute tongue.

Uncertainty analysis of numerical inversions of temperature logs from boreholes under injection conditions

Conventional methods to estimate the static formation temperature (SFT) require borehole temperature data measured during thermal recovery periods. This can be both economically and technically prohibitive under real operational conditions, especially for high-temperature boreholes. This study investigates the use of temperature logs obtained under injection conditions to determine SFT through inverse modelling. An adaptive sampling approach based on machine-learning techniques is applied to explore the model space efficiently by iteratively proposing samples based on the results of previous runs. Synthetic case studies are conducted with rigorous evaluation of factors affecting the quality of SFT estimates for deep hot wells. The results show that using temperature data measured at higher flow rates or after longer injection times could lead to less-reliable results. Furthermore, the estimation error exhibits an almost linear dependency on the standard error of the measured borehole temperatures. In addition, potential flow loss zones in the borehole would lead to increased uncertainties in the SFT estimates. Consequently, any prior knowledge about the amount of flow loss could improve the estimation accuracy considerably. For formations with thermal gradients varying with depth, prior information on the depth of the gradient change is necessary to avoid spurious results. The inversion scheme presented is demonstrated as an efficient tool for quantifying uncertainty in the interpretation of borehole data. Although only temperature data are considered in this work, other types of data such as flow and transport measurements can also be included in this method for geophysical and rock physics studies.

Effects of experimental multi-season drought on abundance, richness, and beta diversity patterns in perennially flowing stream insect communities

Freshwater systems are projected to experience increased hydrologic extremes under climate change. To determine how small streams may be impacted by shifts in flow regimes, we experimentally simulated flow loss over the span of three summers in nine 50 m naturally fed stream channels. The aquatic insect community of these streams was sampled before, during, and after experimental drought treatments as well as following one unforeseen flood event. Abundance, richness, and beta diversity were measured as indicators of biotic effects of altered flow regimes. Abundance declined in proportion to flow loss. In contrast, we observed a threshold response in richness where richness did not decrease except in channels where losses of surface flow occurred and disconnected pools remained. The flood reset this pattern, but communities continued their prior trajectories shortly thereafter. Beta diversity partitions suggested no strong compositional shifts, and that the effect of drought was largely experienced uniformly across taxa until flow cessation. Pools served as a refuge, maintaining stable abundance gradients and higher richness longer than riffles. Upon flow resumption, abundance and richness returned to pre-treatment levels within one year. Our results suggest that many taxa present were resistant to drought conditions until loss in surface flow occurred.

Integrated Multiscale Appearance Features and Motion Information Prediction Network for Anomaly Detection

The rise of video-prediction algorithms has largely promoted the development of anomaly detection in video surveillance for smart cities and public security. However, most current methods relied on single-scale information to extract appearance (spatial) features and lacked motion (temporal) continuity between video frames. This can cause a loss of partial spatiotemporal information that has great potential to predict future frames, affecting the accuracy of abnormality detection. Thus, we propose a novel prediction network to improve the performance of anomaly detection. Due to the objects of various scales in each video, we use different receptive fields to extract detailed appearance features by the hybrid dilated convolution (HDC) module. Meanwhile, the deeper bidirectional convolutional long short-term memory (DB-ConvLSTM) module can remember the motion information between consecutive frames. Furthermore, we use RGB difference loss to replace optical flow loss as temporal constraint, which greatly reduces the time for optical flow extraction. Compared with the state-of-the-art methods in the anomaly-detection task, experiments prove that our method can more accurately detect abnormalities in various video surveillance scenes.

Numerical Study On The Internal Irreversible Flow Loss of Single Screw Expanders

As a type of positive displacement expander, single screw expander (SSEs) can be widely applied in the energy storage systems and waste heat recovery field. The irreversible losses (such as leakage, flow, heat transfer, intake and exhaust pressure loss…) have great influence on the expander performance. However, irreversible flow loss in the expander is nearly impossible to investigate experimentally and theoretically. In this paper, a three-dimensional computational fluid dynamics (CFD) study of SSE using mesh deformation approach was presented. The CFD model was validated by the experimental results. Field distribution of pressure, temperature and velocity of SSE were carried out. An energy loss factor based on entropy production principle was used to measure the irreversible flow (including leakage) loss. The energy loss caused by direct dissipation and turbulent fluctuation dissipation was compared. The energy loss of different region was investigated. Results show that energy loss of the turbulent dissipation is far more than that of direct dissipation. The energy loss factor decreases from 0.547 to 0.221 when the rotation speed changes from 2000rpm to 4000rpm. The shaft efficiency increases from 39.8% to 52.1% with the internal volume ratio from 3 to 5.

Leakage flow characteristic of radial inflow turbine adopted in CAES system: Review on progress

Compressed Air Energy Storage (CAES) System is an important power output component of the energy storage technology. Radial inflow turbine is the main power output device in CAES system, it is operated at extraordinary operation condition (inlet pressure ≥ 75 bar and inlet temperature < 500 K) which is different from gas turbine and other turbomachinery. Therefore, clearance existing in the CAES radial inflow turbine will result in special leakage flow characteristic and higher flow loss, which decreases the aerodynamic performance and the economic efficiency of the CAES system. However, most of researches for CAES radial inflow turbine mainly focus on the performance prediction of CAES system with one-dimensional model, the detailed leakage flow loss mechanism based on three-dimensional analysis, which significantly influences the flow structure and efficiency, are still needed to be further conducted. In present study, the progress on leakage flow characteristic in the CAES radial inflow turbine is reviewed. The effects of tip clearance, case-shroud clearance and back cavity of rotors are summarized, the leakage flow mechanism and loss reduction method are also analyzed and discussed. Suggestions for the future work on leakage flow of CAES radial inflow turbine are also proposed. The present review can provide a guide for new design and optimization of the radial inflow turbine adopted in CAES system.

Effect of Gas Volume Fraction on the Energy Loss Characteristics of Multiphase Pumps at Each Cavitation Stage

In the process of conveying a medium, when the inlet pressure is low, the cavitation phenomenon easily occurs in the pump, especially in the gas–liquid two-phase working condition. The occurrence of the cavitation phenomenon has a great impact on the performance of the multiphase pump. In this paper, the SST (sheard stress transport) k-ω turbulence model and ZGB (Zwart–Gerber–Belamri) cavitation model were used to simulate the helical axial flow multiphase pump (hereinafter referred to as the multiphase pump), and the experimental verification was carried out. The effect of gas volume fraction (GVF) on the energy loss characteristics in each cavitation stage of the multiphase pump is analyzed in detail. The study shows that the critical cavitation coefficient of the multiphase pump gradually decreases with the increase in GVF, which depresses the evolution of cavitation, and the cavitation performance of the multiphase hump is improved. The ratio of total loss and friction loss to total flow loss in the impeller fluid domain gradually increases with the development of cavitation, and the pressurization performance of the multiphase pump gradually decreases with the development of cavitation. The results of the study can provide theoretical guidance for the improvement of the performance of the multiphase pump.

Secondary Flow Loss Reduction Method by Use of Endwall Contouring in Gas Turbine Cascade Using Optimization Method

High efficiency is strongly demanded for gas turbines to reduce CO2 emissions. In order to improve the efficiency of gas turbines, the turbine inlet temperature is being raised higher. In that case, the turbine blade loading is higher and secondary flow loss becomes a major source of aerodynamic losses due to the interaction between the horseshoe vortex and the strong endwall cross flow. One of the authors have optimized a boundary layer fence which is a partial vane to prevent cross-flow from pressure-side to suction-side between blade to blade. However, it was also found that installing the fence leads to increase another loss due to tip vortex, wake and viscosity. Therefore, in this paper, we focused on the endwall contouring and the positive effect findings from the boundary layer fence were used to study its optimal shape. Firstly, the relationship between the location of the endwall contouring and the internal flow within the turbine cascade was investigated. Two patterns of contouring were made, one is only convex and another is just concave, and the secondary flow behavior of the turbine cascade was investigated respectively. Secondly, the shape was designed and the loss reduction effect was investigated by using optimization method. The optimized shape was manufactured by 3D-printer and experiment was conducted using cascade wind tunnel. The total pressure distributions were measured and compared with CFD results. Furthermore, flow near the endwall and the internal flow of the turbine cascade was experimentally visualized. The internal flow in the case of a flat wall (without contouring), with a fence, and with optimized endwall contouring were compared by experiment and CFD to extract the each feature.