Exit Position Research Articles

Optimization of motive nozzle position in a modified two-phase ejector expansion household refrigeration cycle using an artificial neural network

Two-phase ejectors are highly useful devices because they can recover and use the energy lost in the expansion process of the refrigeration cycle. However, in a small-sized household refrigeration cycle, a modified ejector cycle is required due to the low operating pressure and mass flow rate. In this study, a dual evaporator ejector cycle (DEEC), which is advantageous for household refrigeration, was applied. In the DEEC, unlike the conventional ejector cycle, the motive nozzle exit position (NXP) considerably affects the cycle performance. The objective of this study was to optimize the NXP of a DEEC for a household refrigeration cycle with low-pressure refrigerants via an artificial neural network (ANN) model. Using the developed model, the DEEC pressure lifting ratio and coefficient of performance (COP) were analyzed under various operating and ejector geometry conditions. Moreover, the optimal NXP of a small-sized household DEEC was proposed using the developed correlation to achieve maximum performance under different operating conditions. The COP of the DEEC with the optimized NXP is 2.3% and 8.4% higher than those of the DEEC with the conventional NXP and the baseline cycle, respectively. These results validate the ANN model used for optimization and serve as design guidelines for obtaining optimized NXPs and cycle performance with increased energy efficiency.

Adaptable control approach on a new HFO-R1234yf heat driven jet cooling system

The ejector heat driven refrigeration systems are extensively under study nowadays. The current major challenge with such system is how to keep the ultimate ejector performance with the expected variations in the operating conditions. This work takes one step forward towards a full control technique on the single phase ejector cooling cycle using the promising refrigerant HFO-R1234yf utilizing the nozzle exit position (NXP). This is to accommodate all the operating scenarios seeking a continuous ejector critical operation. A prototype of a nominal cooling load of 20 kW was utilized to formulate the control correlations. The impact of the entrained and the motive streams conditions on the ejector performance were investigated and justified based on validated CFD simulations. NXP was found a practical and significant optimization parameter. In addition, a dimensionless form of the NXP was introduced for the sake of extending its effect over a wide range of cooling loads for future studies. Generally, it is recommended to operate at the optimum NXP. The only obstacle is that the condenser pressure has to be equal to the critical backpressure , or less, in order for the critical mode to be guaranteed. Correlations of the entrainment ratio , dimensionless NXP, and the optimum dimensionless NXP as a function of condenser and boiler pressures were derived using regression analysis to keep the required ultimate performance. Moreover, a control and throttling valve with temperature sensor were introduced to bypass some fraction of the ejector outlet flow to the evaporator inlet for the capacity control purposes.

Experimental investigation on the influence of ejector geometry on the pull-down performance of an ejector-enhanced auto-cascade low-temperature freezer

• Effects of ejector geometry on EARC based low-temperature freezer were explored. • D t and L m are the critical parameters influencing the pull-down performances. • Malfunction of pressure lift at start-up phase was prone to occur at small D m . • NXP seriously affects ejector performance instead of the pull-down rate Employing an ejector to recover the expansion work of the auto-cascade refrigeration cycle is a feasible method to improve the system performance. The system operation characteristics are closely relevant to the ejector geometry parameters. In order to obtain the critical structure parameters influencing the freezer's pull-down performance, experimental research was conducted on an ejector-enhanced auto-cascade refrigeration cycle applied in a low-temperature freezer. The impacts of the ejector nozzle throat, mixing chamber diameter and length, and the nozzle exit position on the system's pull-down and steady operation characteristics were explored. The experimental results illustrated that the cooling rate and the attainable freezing temperature were mainly influenced by the nozzle throat and mixing chamber length instead of the nozzle exit position and mixing chamber diameter. The nozzle throat diameter of 0.52 mm and the mixing chamber length of 25 mm was optimal concerning the fastest cool-down rate and lowest freezing temperature of -61.2 °C. At the early phase of the pull-down process, a small mixing chamber diameter would cause the ejector malfunction of the pressure lift. There was a worst nozzle exit position of slowing down the pull-down speed, rising the freezing temperature, and reducing the system COP and exergy efficiency at the given operations. The ejector yielded the maximum pressure lift ratio of 1.196 and the entrainment ratio of 0.523 at the optimal ejector geometries. This work would be helpful to guide the ejector structure optimization for the ejector-enhanced auto-cascade low-temperature freezers.

Numerical study for the influences of nozzle exit position, mixing, and diffuser section lengths on performance of CRMC ejector

Numerical study for the influences of nozzle exit position, mixing, and diffuser section lengths on performance of CRMC ejector

Numerical studies on ejector in proton exchange membrane fuel cell system with anodic gas state parameters as design boundary

A comprehensive entrainment performance evaluation system of the ejector was built including four indexes. An ejector's Computational Fluid Dynamics (CFD) model was established, and the sensitivity analysis of the entrainment performance to four key geometry parameters of the ejector, namely, the nozzle diameter (Dn), the primary nozzle exit position (NXP), the mixing tube diameter (Dm), and the secondary flow inlet diameter (Ds) was performed. Based on the quantified boundary conditions obtained by the Simulink simulation, the ejector structure was optimized with a new method. It is found that the total recirculation ratio increases but the hydrogen recirculation ratio decreases with the increase of the relative humidity of the secondary flow. The hydrogen recirculation ratio shows a unidirectional increase tendency with the increase of Ds and the decrease of Dn and NXP. The hydrogen recirculation ratio increases firstly and then decreases with the increase of Dm. High hydrogen recirculation ratio with low primary hydrogen flow rate, corresponding to low current operation point of fuel cell system and low sensitiveness to the changing relative humidity are usually incompatible. The hydrogen recirculation ratio with low primary flow rate degrades significantly when Dn increases and Dm is larger than a certain value. The ejector with smaller Ds shows lower sensitiveness to the changes of relative humidity, while the hydrogen recirculation ratio with low primary hydrogen flow rate is not affected badly. When matching with a specific system, it is necessary to balance the ejection performance at low current density operating points and the sensitiveness to the changes of relative humidity in combination with the anodic gas state at each operating point, so as to find the optimal structural parameters. The optimization sequence of structural parameters should follow: Dn is selected firstly, then NXP and Ds are optimized, and finally Dm is chosen.

Optimization on key geometries of a highly coupled two-stage ejector

In this paper, an ejector-based multi-evaporator refrigeration system is proposed, in which, the refrigerant mixture coming from the exit of the first-stage enters into the second-stage primary nozzle and entrains its secondary flow refrigerant, thus, the two stages of the ejector are highly coupled. In this case, the primary nozzle throat diameter of the second-stage (PNTD2) plays a key role in affecting and improving both-stage performance. By using Computational Fluid Dynamics (CFD) simulation, key geometries of both stages such as: area ratio (AR), nozzle exit position (NXP), and the angle of constant-pressure mixing chamber (θ) are optimized with varied PNTD2. When PNTD2 increases from 4.1 mm to 5.3 mm, the simulation results show that: (1) Optimum AR of first-stage increases from 2.9 to 5.9, optimum NXP of first-stage rises from 8 mm to 14 mm, and optimum θ of first-stage improves from 7.5° to 15.5°, however, optimal AR of second-stage decreases from 10.5 to 6.9, optimal θ of second-stage decreases from 15.7° to 13.7°, and optimal NXP of second-stage keeps a constant value of 28 mm; (2) AR is the most sensitive one to influence each stage performance, which is followed by NXP, and the least one is θ; and (3) AR and NXP give an evident performance influence to the local stage but a negligible influence to the other stage. The novelty of this work is to investigate the effect of key geometries on a highly coupled two-stage ejector.

An Accelerated Shooting and Bouncing Ray Method Based on GPU and Virtual Ray Tube for Fast RCS Prediction

An accelerated shooting and bouncing ray (SBR) method is presented in this letter to improve the efficiency of radar cross section prediction for electrically large and complex objects. The SBR method is inefficient because it needs to trace a large number of ray tubes. Therefore, its efficiency can be improved by reducing the amount of ray tubes and increasing the speed of ray tracing. In this letter, we use rays instead of ray tubes to reduce the amount of rays that need to be traced, and construct a virtual ray tube at the ray exit position for field integration. Moreover, ray tracing and integration operations are executed on the graphics processing unit to further improve the efficiency of SBR method. Several examples are designed to prove the effectiveness of the proposed method.

The effects of swirl number on the combustion characteristics in an internally-staged combustor

The three-dimensional turbulent swirling flame in an internally-staged combustor is numerically investigated. Four cases over a range of swirl intensities are explored by the Flamelet Generated Manifold model in this paper. Special attention is paid to analyzing the variation of the flow field, temperature, major species concentrations and emissions. The results clearly show the effects of swirl number on the size of the center recirculation zone, fuel distribution and combustion characteristics. When the third premixed stage swirl number increases from 0.6 to 1.2, the axial length of the center recirculation zone decreases by 3.7%, while the radial length increases by 6.9%. The characteristics of the flow field play an important role in the spatial distribution of the fuel, which further affects the temperature distribution in the combustor. The backflow effect is enhanced, resulting in a greater concentration of fuel at the outlet of the swirler. After the maximum temperature is reached at the exit position of the pilot stage, the temperature decreases compared to the peak temperature downstream as the proportion of premixed combustion mode increases. This creates a high concentration region of OH at the outlet of the pilot stage injector and the heat release region is squeezed upstream. At the same time, the volume of the high-temperature region downstream of the pilot stage is reduced. In addition, as the swirl number of the third premixed stage increases from 0.6 to 1.2, the emissions of NO and CO decrease by 28.7% and 75%, respectively.

Experimental investigation on the dynamic characteristic of the direct expansion solar assisted ejector-compression heat pump cycle for water heater

A continuing research on the direct expansion solar assisted ejector-compression heat pump cycle for water heater is presented in this paper, aiming to reveal the system dynamic characteristic by experimental method. The performance advantage of the presented system is verified at first. According to the test results, under the considered condition, the presented system can outperform the traditional ejector enhanced heat pump system and the air-source heat pump system by 24.1% and 29% in the heating coefficient of performance aspect, 24.2% and 30.5% in the average heating capacity aspect. After that, the main parameters affecting the dynamic behavior of the presented system are examined, including the ejector nozzle exit position, ejector mixing chamber diameter and solar radiation intensity. According to the test results, the system achieves the maximum heating coefficient of performance and heating capacity under mixing chamber diameter of 4 mm and nozzle exit position of 6 mm. Note that the ejector malfunction phenomenon appears when the mixing chamber diameter is reduced to 3 mm, due to the blocked flow channel of the ejector secondary flow. Additionally, the experiment finds that enhancing the solar radiation can obviously improve the ejector pressure lift ratio and help the system achieve superior heating performance.

Performance evaluation of a variable geometry ejector applied in a multi-effect thermal vapor compression desalination system

Nowadays, desalination is the primary alternative to increase water supply for domestic, agriculture and industrial use. Energy demanding desalination could be a sustainable solution in places where renewable energy resources are abundant. Multi-effect thermal vapour compression desalination is widely applied; however, its integration with solar energy requires more research effort. One of the key components of these systems is the steam ejector. The present study focuses on the geometry modification of an ejector, designed for a small-scale multi-effect thermal vapour compression unit, by means of computational fluid dynamics simulations. Its performance is evaluated considering a range of operating temperatures of the primary flow (i.e. 120 °C–180 °C) that would be suitable for running the system using low-grade solar thermal energy. Results show that for relatively high primary inlet temperatures, the primary jet undergoes a considerable expansion after leaving the nozzle. Consequently, the effective area for the secondary fluid becomes reduced, leading to a poor ejector performance and the entrainment ratio decreases from 1.2 to 0.3. In order to improve the ejector performance, variable geometry features are tested by allowing two geometry components to vary: the nozzle exit position and the area ratio (rA). The results show that nozzle exit position influences the ejector performance only about 16% within the considered range. The influence of the area ratio on the entrainment ratio is however very significant. The results indicate that the improvement of the entrainment ratio for high primary inlet temperatures could be as high as 400% when compared to a fixed geometry design.

Experimental Research on Noise Load Characteristics of Supersonic Jets in Rectangular Nozzles with Different Mach Numbers

Aiming at the jet noise problem of the rectangular supersonic nozzle, rectangular nozzle test models with Mach numbers of 1.35 and 1.65 were designed and processed. Using the jet noise test platform in a full anechoic chamber, the nozzle supersonic jet noise characteristic test was carried out. The test results show that the surface noise load of the model gradually increases along the nozzle axis; the unsteady flow field structure at the nozzle exit position has complex evolution, with strong turbulent pulsation and high noise load; the directivity trend of the far-field noise total pressure level of the two nozzles is basically the same, and the directivity is obvious in the upstream and 90° direction of the nozzles; from the far-field sound pressure spectrum, it can be seen that the far-field noise is composed of broadband noise and discrete frequency noise. The amplitude of the second order frequency varies significantly from 70° to 90° of upstream, and the fourth harmonic frequency of the whistling mode is captured in the direction of 90°.

Complete characterization of sub-Coulomb-barrier tunnelling with phase-of-phase attoclock

Laser-induced electron tunnelling—which triggers a broad range of ultrafast phenomena such as the generation of attosecond light pulses, photoelectron diffraction and holography—has laid the foundation for strong-field physics and attosecond science. Using the attoclock constructed by single-colour elliptically polarized laser fields, previous experiments have measured the tunnelling rates, exit positions, exit velocities and delay times for some specific electron trajectories, which are mostly created at the field peak instant, that is, when the laser electric field and the formed potential barrier are stationary in terms of the derivative versus time. From the view of wave-particle dualism, the electron phase under a classically forbidden, tunnelling barrier has not been measured, which is at the heart of quantum tunnelling physics. Here we present a robust measurement of tunnelling dynamics including the electron sub-barrier phase and amplitude. We combine the attoclock technique with two-colour phase-of-phase (POP) spectroscopy to accurately calibrate the angular streaking relation and to probe the non-stationary tunnelling dynamics by manipulating a rapidly changing potential barrier. This POP attoclock directly links the measured phase of the two-colour relative phase with the ionization instant for the photoelectron with any final momentum on the detector, allowing us to reconstruct the imaginary tunnelling time and the accumulated phase under the barrier. The POP attoclock provides a general time-resolved approach to accessing the underlying quantum dynamics in intense light–matter interactions. Electron non-stationary tunnelling dynamics is probed by the attoclock with the two-colour phase-of-phase photoelectron spectroscopy. Contrary to the case of static tunnelling, angle-to-time mapping in attoclock is found to be not angularly uniform.

Vertical Position of the Central Retinal Vessel in the Optic Disc and Its Association With the Site of Visual Field Defects in Glaucoma

The purpose of this study was to investigate the association between the vertical position of the central retinal vessel (CRV) within the optic nerve head (ONH) and the site of visual field defects (VFDs) in glaucoma. Cross-sectional study. The vertical position of the CRV was identified in 134 glaucoma eyes and 61 normal eyes at the point at which CRV exited the lamina cribrosa (LC) onto the ONH surface, by using spectral-domain optical coherence tomography (exit position). The position was also identified at the entry point into the LC from the retrolaminar ONH region (entry position), which was little influenced by glaucomatous LC deformation, therefore close to the original position before the glaucoma development. Positions were compared among glaucoma eyes with different sites of VFDs, and between glaucoma and normal eyes. In glaucoma eyes, the entry position of the CRV was in the superior ONH region in 63.0% of eyes with superior VFDs and in the inferior ONH region in 97.8% of eyes with inferior VFDs (P < .0001). The exit position exhibited a similar percentage. The vertical CRV positions were not significantly different between glaucoma and normal eyes, both at the entry and exit positions. Eyes with CRVs in the superior ONH region were significantly more likely to form VFDs in the superior hemifields and vice versa. The vertical position of the CRV was little altered by the development of glaucoma. The original position of the CRV before the development of glaucoma may influence regional susceptibility to glaucomatous stress and may be useful in predicting initial sites of VFDs.

مدل سازی عددی و بهینه سازی هندسی اجکتور تک‌فاز مافوق صوت

In the present study, the performance of the 2D single-phase supersonic ejector with working fluid of air is simulated in ANSYS CFX. The aim is to investigate the velocity field, pressure distribution, primary nozzle flow regime, and entertainment ratio in different operational conditions. The primary pressure inlet with bar and the secondary inlet as an opening with bar at different outlet pressures are simulated. The turbulence model is used. The sidewalls are considered symmetry boundary conditions and the no-slip condition is applied to the ejector walls. Then, the ejector geometry is optimized using Multi-Objective Genetic Algorithm (MOGA) in order to reach greater efficiency. Optimization is performed considering geometric parameters including primary nozzle exit diameter, nozzle exit position, diameter, and length of the constant area section. Sensitivity analysis results show that the diameter of constant area section has major effect on the entertainment and pressure ratios as two objective functions. The nozzle exit position and external diameter of primary nozzle are the second and third dominant parameters that respectively influence the performance of the ejector, while the effect of constant area section length is negligible. Results indicate that by increasing the pressure ratio, the shock train moves upstream and at the design point, the last oblique shock is located in the exit of the constant area section, letting the remaining pressure recovery be done in the subsonic diffuser which reduces the pressure losses and increases the efficiency. Above the critical pressure ratio, due to the movement of the shock train to upstream weakening the shock strength, the suction pressure increases. Then, the pressure difference is reduced, leading to the lower secondary mass flow suction. The optimized ejector in the double choking condition has a % higher entertainment ratio and its operational range is enhanced by percent in comparison to the original geometry.

Learning Multiple‐Scattering Solutions for Sphere‐Tracing of Volumetric Subsurface Effects

AbstractAccurate subsurface scattering solutions require the integration of optical material properties along many complicated light paths. We present a method that learns a simple geometric approximation of random paths in a homogeneous volume with translucent material. The generated representation allows determining the absorption along the path as well as a direct lighting contribution, which is representative of all scatter events along the path. A sequence of conditional variational auto‐encoders (CVAEs) is trained to model the statistical distribution of the photon paths inside a spherical region in the presence of multiple scattering events. A first CVAE learns how to sample the number of scatter events, occurring on a ray path inside the sphere, which effectively determines the probability of this ray to be absorbed. Conditioned on this, a second model predicts the exit position and direction of the light particle. Finally, a third model generates a representative sample of photon position and direction along the path, which is used to approximate the contribution of direct illumination due to in‐scattering. To accelerate the tracing of the light path through the volumetric medium toward the solid boundary, we employ a sphere‐tracing strategy that considers the light absorption and can perform a statistically accurate next‐event estimation. We demonstrate efficient learning using shallow networks of only three layers and no more than 16 nodes. In combination with a GPU shader that evaluates the CVAEs’ predictions, performance gains can be demonstrated for a variety of different scenarios. We analyze the approximation error that is introduced by the data‐driven scattering simulation and shed light on the major sources of error.

CFD analysis of operating condition effects on optimum nozzle exit position of a supersonic ejector using the refrigerant R134a

CFD analysis of operating condition effects on optimum nozzle exit position of a supersonic ejector using the refrigerant R134a

Development of converging-diverging multi-jet nozzles for molten smelt shattering in kraft recovery boilers

The effective shattering of molten smelt is highly desired in recovery boiler systems. Ideally, shatter jet nozzle designs should: i) generate high shattering energy; ii) create a wide coverage; and iii) minimize steam consumption. This study proposes a novel converging-diverging multi-jet nozzle design to achieve these goals. A laboratory setup was established, and the nozzle performance was evaluated by generating jet pressure profiles from the measurement of a pitot tube array. The results show that the shatter jet strength is greater with a large throat diameter, high inlet pressure, and a short distance between the nozzle exit and impingement position. Increasing the number of orifices generates a wider jet coverage, and the distance between the orifices should be limited to avoid the formation of a low-pressure region between the orifices. The study also demonstrates that an optimized converging-diverging multi-jet nozzle significantly outperformed a conventional shatter jet nozzle by achieving higher energy and wider coverage while consuming less steam.

Applicability Evaluation of Several Spatial Clustering Methods in Spatiotemporal Data Mining of Floating Car Trajectory

Spatial analysis is an important means of mining floating car trajectory information, and clustering method and density analysis are common methods among them. The choice of the clustering method affects the accuracy and time efficiency of the analysis results. Therefore, clarifying the principles and characteristics of each method is the primary prerequisite for problem solving. Taking four representative spatial analysis methods—KMeans, Density-Based Spatial Clustering of Applications with Noise (DBSCAN), Clustering by Fast Search and Find of Density Peaks (CFSFDP), and Kernel Density Estimation (KDE)—as examples, combined with the hotspot spatiotemporal mining problem of taxi trajectory, through quantitative analysis and experimental verification, it is found that DBSCAN and KDE algorithms have strong hotspot discovery capabilities, but the heat regions’ shape of DBSCAN is found to be relatively more robust. DBSCAN and CFSFDP can achieve high spatial accuracy in calculating the entrance and exit position of a Point of Interest (POI). KDE and DBSCAN are more suitable for the classification of heat index. When the dataset scale is similar, KMeans has the highest operating efficiency, while CFSFDP and KDE are inferior. This paper resolves to a certain extent the lack of scientific basis for selecting spatial analysis methods in current research. The conclusions drawn in this paper can provide technical support and act as a reference for the selection of methods to solve the taxi trajectory mining problem.

A methodology for capturing tire wear particles: Computational particle fluid dynamics modelling and experimental verification

An approach that forces TWPs to enter and retain in a tunnel to prevent their release into the environment was proposed and investigated by a novel solid-gas two-phase aerodynamic car-wheel-TWP model and verified by a newly-designed equipment. Capture efficiency were investigated by changing the entrance and exit positions of the tunnels. The results show that the maximum capture ratio exceeds 50% and there is good agreement on the capture ratios obtained from the simulations and the experiments. The capture efficiency is mostly determined by the pressure difference between the tunnel ends. The maximum capture ratio appears at 60 km/h. The TWPs captured by the rear tunnel exceeds 60% in total capture amount. Health-concerned TWPs with sizes smaller than 10 μm may be the main source that are captured by the tunnels. This aerodynamic model and capturing approach can provide a methodology and guidelines for relieving the pollution induced by TWPs.

Research on general layout parameters of I-SIZE child safety seat in frontal impact

In order to explore the influence of the general layout parameters of the child restraint system on the evaluation index of child injury in a frontal impact, and to explore the general layout plan that can minimize the child’s injury, this paper selects an I-Size child safety seat that meets the ECE R44 regulations as the research object, in accordance with the ECE R129 regulations, uses the Q3 child dummy to perform a frontal impact sled test on it, and record the test results. On this basis, a simulation concept model of the child safety seat is established, and the parameter combination of the seat back angle and cushion angle and the parameters of the five-point seat belt exit position are studied, and the overall layout design scheme with the best child protection performance is obtained.