Secondary Flow Pressure Research Articles

Experimental investigation and numerical analysis on high-efficiency shock vectoring control serpentine nozzles

The high-efficiency Shock Vectoring Control Serpentine Nozzle (SVCSN) takes into account both thrust vectoring and infrared stealth, and significantly improves the comprehensive performance of the aero-engines through an additional auxiliary duct. In this paper, the schlieren photographs at the exit of the high-efficiency SVCSN and the wall static pressure distributions were obtained by experiments, and the numerical results were used to enrich the thrust vectoring characteristics. The effects of the auxiliary injection were analyzed first to reveal the advantages of the high-efficiency SVCSN compared to the conventional SVCSN. Then, the aerodynamic parameters and the structural parameters of the high-efficiency SVCSN were investigated, including the Nozzle Pressure Ratio (NPR), the Secondary flow Pressure Ratio (SPR), the secondary flow relative area and the secondary flow injection angle. Finally, the coupling performance of the high-efficiency SVCSN is studied by using the approximate modeling technology. Results show that the auxiliary injection increases the range between the two shock legs of the “λ” shock wave induced by the secondary flow, then causes the separation zone and high-pressure boss of the down wall to expand upstream, and finally results in a prominent increase in the thrust vectoring performance. The thrust vectoring angle and Vectoring Efficiency (VE) of the high-efficiency SVCSN are about 61.6% and 75.7%, respectively, higher than those of the conventional SVCSN at NPR = 6. The effects of the NPR and the SPR on the thrust vectoring performance of the high-efficiency SVCSN are coupled with each other. A larger NPR matched with a smaller SPR shows better thrust vectoring performance. The maximum fluctuations in thrust vectoring angle and VE caused by the NPR and SPR are about 22% and 64%. The VE decreases monotonously with the increase of the secondary flow relative area. Smaller secondary flow injection angle shows better thrust vector performance, and the thrust vectoring angle and VE of the secondary flow injection angle of 90° are about 20% higher than those of the secondary flow injection angle of 110° at NPR = 6. Therefore, the secondary flow relative area of 0.06 and the secondary flow injection angle of 90° are recommended.

Research on performance of jet refrigeration cycle driven by waste heat of refrigerated vehicle

Nowadays, refrigerated food storage and transport vehicles are very common. However, with regard to the engine of vehicles, a significant amount of heat is discharged into the environment in the form of exhaust gas and jacket water. If the cooling system is used to convert this waste heat into cooling capacity, the thermal efficiency of the engine will be improved to a certain extent. Therefore, this paper uses two waste heat recovery jet refrigeration systems to recover heat from the exhaust gas and jacket water of the refrigerated vehicle. One is a conventional jet refrigeration system that uses only engine exhaust as the heat source, and the other is a system that adds a preheater upstream of the generator based on the conventional system, using engine jacket water to preheat the working fluid. This paper presents the results of comparing the impact of R141b/R123, R141b/R245fa, R142b/R134a, and R142b/R152a working fluids on system performance, and considers the effects of operating temperature and secondary flow pressure drop in the ejector receiving chamber on the system performance. The research results indicate that of the four selected working fluids, the R141b/R245fa (0.4/0.6) blend has the best overall performance. After the addition of a preheater, the system using R142b/R134a (0.6/0.4) hybrid working fluid achieves the maximum cooling capacity of 16.0994 kW and the maximum thermal efficiency of 5.45%, and the exergy loss in the generator is 1 kW lower than without the preheater.

Investigation on key geometries optimization and effect of variable operating conditions of a transcritical R744 two-phase ejector

There was no literature that involved the combined investigation on the key geometries optimization and the effect of variable operating conditions (temperature, pressure and dryness fraction of primary and secondary flows) for a R744 two-phase ejector. In this paper, CFD simulation on geometries optimization and effect of variable operating conditions of a transcritical R744 two-phase ejector was carried out for the first time. Firstly, each of three geometries of area ratio (AR), nozzle exit position (NXP) and constant-area mixing chamber length (L2) was optimized to determine their relative sensitivity on ejector performance. Then, two or three geometries under different optimization orders were optimized. Finally, the effects of temperature, pressure and dryness fraction of primary and secondary flows as well as back pressure on ejector performance were analyzed based on the optimal ejector geometries. The results showed that: (1) the relative sensitivity of entrainment ratio (ER) to AR is the highest with value of 44.56%, followed by NXP and L2; (2) optimizing two or three of these parameters under different optimization sequences can achieve certain optimization effect with ER improvement within 5.96%–33.91%, but the result depends on the optimization order; (3) the effect of dryness fraction of the primary flow on the ejector performance is greater than that of temperature; (4) the change of secondary flow pressure has a greater impact on the ejector ER as compared to the dryness fraction of the secondary flow; (5) not only increasing the primary and secondary flow pressure but also reducing the back pressure is conducive to improving the ejector performance.

Numerical simulation study on the optimization of the central ejector intake tube

A CFD numerical simulation study was conducted for the primary flow non-axial inlet L/Y intake tube ejector. The effects of different configurations of the inlet piping on the secondary flow of the ejector were analyzed in terms of total pressure loss, separation zone volume, and velocity uniformity. The results showed that: the volume of the separation zone decreases with the increase of secondary flow total pressure. The L intake tube causes a larger separation zone under the same working condition. When the total pressure of secondary flow is 20kPa, the volume of the separation zone caused by the L intake tube reaches 15 times that of the Y intake tube, and the total pressure loss is 2.3 times that of the Y intake tube. A smaller separation zone has less effect on the flow field structure, resulting in better velocity uniformity in the mixing chamber inlet section of the Y intake tube ejector.

On the flow of liquid crystals through 90° bends

During the processing of nematic soft solids through process flow elements (pipe bends, elbows, etc.), the constitutive behavior makes its presence felt via processing (with rheology driven effects increasing pressure drop) and the final product microstructure. This paper explores the flow and microstructure configurations of nematic liquid crystals in a pressure driven flow through 90° pipe bends with different types of wall anchoring. The governing equations of the Leslie–Ericksen theory are solved numerically in a newly developed OpenFOAM solver. We show that the bend curvature deforms the nematic axis distribution; the distortion can be driven either by elastic or hydrodynamic effects. The interaction between the nematic microstructure and flow field generates non-zero normal stresses (in the radial, azimuthal, and streamwise directions), which produce a secondary flow and increase pressure losses. The strength of the secondary flow depends on the type of wall anchoring and Ericksen number; in configurations with homeotropic anchoring, decreasing the Ericksen number increases the relative strength of the secondary flow (with respect to the mean flow velocity). Conversely, homogeneous (planar) anchoring reduces normal stresses, thus weakening the secondary flow strength. We show that as the fluid enters/leaves the bend, there is a perturbation in the transverse velocity caused by streamwise stress gradients. The perturbation magnitude depends on material properties and can be of different values at the bend exit and entrance. Finally, we show that the spatial development of the nematic field downstream of the bend exit is controlled by both material properties and the Ericksen number.

Numerical investigation into effects of ejector geometry and operating conditions on hydrogen recirculation ratio in 80 kW PEM fuel cell system

Ejectors play a crucial role in improving the fuel utilization efficiency of proton- PEMFC systems by recirculating the excess hydrogen discharged from the anode outlet side of the stack. This study has constructed a full MATLAB/Simulink model of an 80 kW PEMFC system consisting of an anode hydrogen gas supply system, a cathode oxygen gas supply system, a fuel cell stack, a terminal exhaust and drainage system, and an ejector-based anodic gas recirculation system. The model has been used to examine the effects of the ejector geometry and PEMFC operating conditions on the hydrogen recirculation ratio and hydrogen stoichiometric ratio. The results show that as the primary flow pressure increases, the primary mass flow rate increases, but the recirculation ratio and secondary mass flow decrease. By contrast, as the secondary pressure increases, the primary mass flow rate remains unchanged, but the secondary mass flow rate and recirculation ratio increase. For a constant primary flow pressure, the recirculation ratio increases with an increasing primary flow temperature. Conversely, for a constant secondary flow pressure, the recirculation ratio reduces slightly with an increasing secondary flow temperature. For a hydrogen stoichiometric ratio of 1.5, the optimal value of the mixing chamber diameter (D2) to nozzle throat diameter (Dt) is found to be D2/Dt = 2.2.

Effects of working conditions on the performance of an ammonia ejector used in an ocean thermal energy conversion system

AbstractOcean thermal energy has attracted much attention for its huge potential resources. It's proved that an ocean thermal energy conversion (OTEC) cycle system using ammonia ejectors is beneficial to improve the cycle efficiency. Thus, the performance of an ammonia ejector is studied numerically in this paper. Effects of working conditions which occurred inside the ammonia ejector, like primary flow pressure (1.3‐2.1 MPa), secondary flow pressure (0.45‐0.65 MPa), and back pressure (0.5‐1.05 MPa), on the mixing process and flow phenomenon, are discussed in detail. The results demonstrate that the entrainment ratio of ammonia ejector ranging from 0.35‐0.65 increases with decreasing primary flow pressure at the critical working mode, while the reverse is true for the critical back pressure. Two small vortexes are formed near the ejector wall due to the boundary layer separation when the ejector works at the subcritical or critical mode. These vortexes reduce the effective area and flow rate of the secondary flow. The Ma contours are nearly the same in the nozzle and mixing zone for the critical and subcritical mode, but distributions of Ma vary dramatically in the diffuser corresponding to the variation of secondary series of oblique shock.

Numerical and analytical predictions of nuclear steam generator secondary side flow field during blowdown due to a feedwater line break

For the structural integrity evaluation of pressurized water reactor (PWR) steam generator (SG) tubes subjected to transient hydraulic loading, determination of the tube-to-tube gap velocity and static pressure distributions along the tubes is prerequisite. This paper addresses both computational fluid dynamics (CFD) and analytical approaches for predicting the tube-to-tube gap velocity and static pressure distributions during blowdown following a feedwater line break (FWLB) accident at a PWR SG. First of all, a comparative study on CFD calculations of the transient velocity and pressure distributions in the SG secondary sides for two different models having 30 or no tubes is performed. The result shows that the velocities of sub-cooled water flowing between any adjacent two tubes of a tubed SG model during blowdown can be roughly estimated by applying the specified SG secondary side porosity to those of the no-tubed SG model. Secondly, simplified analytical approximate solutions for the steady two-dimensional SG secondary flow velocity and pressure distributions under a given discharge flowrate are derived using a line sink model. The simplified analytical solutions are validated by comparing them to the CFD calculations.

Rarefied flow separation in microchannel with bends

Abstract

Numerical studies on the performance of ammonia ejectors used in ocean thermal energy conversion system

Ocean thermal energy conversion (OTEC) has attracted lots of attention for its potential in clean energy production. In order to further improve system efficiencies, a new type of OTEC system using two ammonia ejectors is proposed. Effects of working pressures and geometrical parameters on vapor-vapor ammonia ejectors are studied numerically. The results show that performance characteristics of ammonia ejectors are mostly in subcritical mode for the working conditions of OTEC, which is different from other refrigeration applications. Entrainment ratios of ammonia ejectors range from 0.44 to 1.68 based on the working conditions in the present work. Increases of area ratio and decreases of nozzle exit position are beneficial to the improvement of ejector entrainment ratios, while the performances of ejectors decrease with increasing primary flow pressure and with decreasing secondary flow pressure. The thermal efficiency of the new OTEC system decreases with increasing primary and secondary flow pressures, but increases with increasing entrainment ratios. Due to the enhancement of turbine output work and reduction of pumping power brought by the two ejectors, the highest thermal efficiency for the proposed OTEC system with ejectors is 3.95% which is about 10.33% higher than a normal OTEC system without ejectors.

Experimental study on the primary flow expansion characteristics in transcritical CO2 two-phase ejectors with different primary nozzle diverging angles

This paper presented an experimental study to explore the primary flow expansion characteristics in the transcritical CO2 two-phase ejectors for various primary nozzle diverging angles (NDAs) and secondary flow pressure. The phase change of the primary flow was visualized, and the effect of the primary flow expansion state on the entrainment ratio was evaluated. The results disclosed that the primary flow changed from the under-expanded state to the over-expanded state with increasing the NDA, and the primary flow expansion state was insensitive to the secondary flow pressure. The visualization images showed that the phase change position moved towards the nozzle exit visibly when the NDA was 0.0°, and the effect of secondary flow pressure on the phase change position was insignificant. Besides, the results revealed that the over-expanded state of primary flow had a negative effect on the entrainment ratio, and the entrainment ratio obtained the maximum when the NDA was 2.0°. The present work is indispensable to realize a comprehensive understanding of the primary flow expansion mechanism inside the transcritical CO2 ejector, and the insights gained from this study may be of assistance to optimize the ejector structure and improve the ejector performance.

The Evaluation of k-ε and k-ω Turbulence Models in Modelling Flows and Performance of S-shaped Diffuser

Diffusing S-shaped ducts are critical components in modern vehicle, primarily employed in directing the airflow to the engine. It links the air box and the engine in a very restricted place. The air flow through an S-duct is complex in nature, which perhaps includes boundary layer separation, secondary flow, and total pressure loss effects that influences the engine performance. In this work, the flow and performance of S-shaped duct was predicted and analysed using computational fluid dynamics. The main objective is to evaluate the performance of the realizable k-e and k-ω SST models qualitatively and quantitatively in modelling flow of a highly bend duct where a high stress distorted flow may have developed in proximity the duct wall leading to stall. CFD computations were performed for the flow entering the diffuser at Reynolds number of 80,000 cases. The results obtained suggested that the k-ω SST model reasonably predicts the flow characteristics qualitatively and quantitatively. The realizable k-e turbulence model however poorly captures the actual magnitudes of the calculated flow features. The growth of the stream-wise velocity profile was calculated at three stream-wise stations and point out a smooth down the interior profile of the divergent section. An extreme flow distortion and a shift of the region of flow with the highest velocity were developed toward the outer wall of the first bend of the diffuser. A significant pressure recovery potential with no flow separation arise over the diffuser tube range was predicted well by the simulations.

Comprehensive experimental study on a transcritical CO2 ejector-expansion refrigeration system

A comprehensive experimental study on the ejector and cooling performances of a transcritical CO2 ejector-expansion system is presented. The ejector performance of the entrainment ratio, pressure lift ratio, and efficiency were investigated under various primary flow pressure, secondary flow pressure, and back pressure conditions. A new coefficient of mass balance, β, was introduced based on the liquid mass balance in and/or out of the vapor–liquid separator, which was able to estimate the deviation between current operating state and a steady state. The experiment results show that the coefficient of performance (COP) ratio of the ejector-expansion refrigeration system to the basic system decreases from 18.9% to −11% with the coefficient of mass balance increasing from −0.1 to 0.1. Both the entrainment ratio and COP decrease with an increase in the coefficient of mass balance.

Experimental and Numerical Analysis on the Internal Flow of Supersonic Ejector Under Different Working Modes

ABSTRACTSupersonic ejectors involve very complex phenomena such as interaction between supersonic and subsonic flows, shock trains, instabilities, which strongly influences the performance of supersonic ejector. In this study, the static pressure distribution along the ejector wall and Mach number distribution along the axis are used to investigate the internal flow field of supersonic ejector. Results indicate that when the back pressure is much less than the critical back pressure, there are two series of shock trains, and the change of the back pressure will not affect the flow field before the effective area section, so the entrainment ratio would remain constant. The second shock train moves further upstream and is combined with the first shock train to form a single shock train as the back pressure rises. When the back pressure is greater than the critical back pressure, the position of the shock train, the static pressure at its upstream and the entrainment ratio, will be affected. The “effective area section” in the mixing tube is obtained. The effective area section position moves downstream with the increase of the primary flow pressure, while it moves upstream with the increase of the secondary flow pressure. The entrainment ratio shows inversely proportional relationship with the effective section position. Besides, the first shock train length increases with the increase of primary flow pressure or secondary flow pressure. The critical back pressure represents direct proportional relationship to the first shock train length.

비속도에 따른 원심펌프의 내부유동 특성 연구

In this paper, a numerical investigation of a centrifugal pump for different specific speeds has been performed to study the flow characteristics inside the passage of an impeller. Three-dimensional steady Reynolds-averaged Navier-Stokes equations were solved by a finite volume-based solver, and the shear stress transport model was adopted as a turbulence closure. Numerical results for the performance parameters showed good agreements with experimental data. Jet-wake flow, secondary flow, three-dimensional streamlines, and total pressure loss distributions in the centrifugal pumps, were investigated to find a correlation between the flow characteristics and specific speed. It was observed that flow structure in the centrifugal pump varied remarkably with the specific speed of the pump. Also, it is noted that understanding of the jet-wake and secondary flow inside the passage of the impeller, is highly important to grasp the mechanisms of total pressure losses of the centrifugal pump.

Characteristics of static pre-cyclonic steam ejector

The thermal vapor recompression (TVR) technology can partially recover condensation latent heat of the secondary steam in the multi-effect evaporation (MEE) process. And the two-dimension of x and r momentum exchange in the key device-steam ejector, makes induction efficiency relatively low and mixing length long. In this article, a novel three-dimensional momentum exchanging ejector called static pre-cyclonic steam ejector (SCSE) is first proposed. Its performances are mainly evaluated by means of computational fluid dynamics(CFD) at different dynamic pressure ratios (DPRs) and preliminarily validated by experiments. The SCSE can not only reduce the axial dimension of conventional ejector, but also show excellent performance at a low boosted pressure occasions. The SCSE with DPR = 4/1 improves the entrainment ratio(ER) by 10.8% compared to traditional ejector. For cases with secondary flow pressure disturbances, the application of the SCSE is limited because of an adverse impact on the operation stability. For cases with outlet flow fluctuates, the SCSE with larger DPR has higher peak ER and narrower choked-band-width at respective break-down supercharging ratios (BSRs), and there are no obvious influences on the operation stability for SCSE.

Simulation on the performance of ejector in a parallel hybrid ejector-based refrigerator-freezer cooling cycle

In this paper, the simulation is carried out to investigate the performance of ejector used in a parallel hybrid ejector-based refrigerator-freezer cooling cycle. The geometric parameters such as primary nozzle diameter, area ratio and constant-pressure mixing chamber length are studied firstly to achieve a good performance of the fixed area ratio ejector under a certain condition. Based on the obtained ejector geometry, an adjustable ejector with a spindle is used in the system to meet the requirements of variable cooling loads. The results show that the effect of blocking percentage of primary nozzle with a spindle on the ejector performance is considerable. Moreover, the adjustable ejector works well with relatively high entrainment ratio when the back pressure increment percentage as compared to the secondary flow pressure is no less than 16.15%.

Thermodynamic modelling and parameter determination of ejector for ejection refrigeration systems

This paper presents a detailed thermodynamic modelling method of an ejector for ejection refrigeration system. In this model, the primary flow in the ejector was assumed to fan out from the nozzle without mixing with the secondary flow in a certain downstream distance, so that a hypothetical throat was formed where the secondary flow reached the sound speed. However, the area of this hypothetical throat remained unknown. Therefore, based on several sets of experimental results, the present study developed empirical correlations of the hypothetical throat area to aid further modelling. The ratio of the hypothetical throat area to the mixing area was correlated with two dimensionless variables: one was the ratio of nozzle throat area to the mixing area, and the other one was the primary and secondary flow pressure ratio. The model has been validated by the measured primary mass flow rates and the critical back pressures.

Thermodynamic investigation of a booster-assisted ejector refrigeration system

In order to improve performance of ejector refrigeration system, a booster is added before an ejector to enhance secondary flow pressure, which is called a booster assisted refrigeration system. Based on mass, momentum and energy conservation, a 1D model of ejector for optimal performance prediction was presented and validated with experimental data. A detailed study of working characteristics of the booster assisted ejector refrigeration system was carried out and compared against conventional ejector refrigeration system and compressor based refrigeration system, on the basis of first and second laws of thermodynamics. Effects of booster outlet pressure on COPth based on thermal energy and COPw based on work input, and also on entrainment ratio and area ratio of ejector were studied. The exergy destruction rates were also computed and analyzed for components of the booster-assisted ejector refrigeration system. Ways to reduce exergy destruction were discussed.

LDV에 의한 정사각 단면 180° 곡덕트에서 난류진동유동의 유동특성

In the present study the characteristics of turbulent oscillatory flows in a square-sectional curved duct were investigated experimentally. A series of experiments for air flow were conducted to measure axial velocity profiles, secondary flow velocity profiles and pressure distributions. The measurements were made by a Laser Doppler Velocimeter (LDV) system with a data acquisition and processing system which includes Rotating Machinery Resolve (RMR) and PHASE software. The results from the experiment are summarized as follows. (1) The maximum velocity moved toward the outer wall from the region of a bend angle of . The velocity distribution had a positive value extended over the total phase in the region of a bend angle of . (2) Secondary flows were generally proportional to the velocity of the main flow. The intensity of the secondary flow was about 25% as much as that in the axial direction. (3) Pressure distributions were effects of the oscillatory Dean number and respective region.