Experimental and numerical investigation of circularly lobed nozzle with/without central plug

The experimental rig was established for the pumping performance investigation of a circularly lobed nozzle with a central plug, and of the same nozzle without a central plug for comparison. Seven different cylindrical mixers were designed to match the circularly lobed nozzle. Results show that the central plug increases the pumping ratio of the secondary mass flow rate to the primary mass flow rate about 60-70% over the same lobed nozzle without a central plug, and the total pressure prior to flow's entry into circularly lobed nozzle increased about 0.8-1.0% over the same lobed nozzle without a central plug. In order to know the mixing process between the primary and the secondary flows, in addition, a numerical procedure was developed to solve the incompressible Navier-Stokes equations in a curvilinear nonorthogonal coordinate. The numerical results show that there are great differences in the thermal mixing process, of the case with a central plug in which the low temperature contour lines in the exit cross-section of cylindrical mixer can reach the central line of the mixer, from the case without a central plug in which the elevated temperature contour lines in the exit cross-section of cylindrical mixer still occupy the central zone of the mixer. This was the fundamental reason why the thermal mixing efficiency of the lobed nozzle with a central plug was much more than that of without a central plug. Lastly, good agreements of temperature and axial velocity component in the exit cross-section of the mixer had been observed between the numerical results and the measured ones.     

Experimental and numerical research on high pumping performance mechanism of lobed exhauster-ejector mixer

The lobed nozzle exhauster-ejector mixer has a lot of potential applications in industry. The experimental setup is established to explore its high pumping performance mechanism. Experimental results show that the pumping performance is dominantly dependent on the primary flowing fluid “attachment” on the inner wall of the mixer. If the primary flowing fluid attaches the inner wall of the mixer in the exit section, the pumping ratio of the entrained secondary mass flow rate to the primary mass flow rate is definitely high. In order to find the high pumping performance mechanism from flow field, the N–S equations are solved. The numerical results show that the flow field to give high or the highest pumping ratio is that there is a distinct thin layer (tear layer) of the secondary flowing fluid with less velocity on the inner wall of the mixer. The distinct thin layer much decreases the friction loss between the primary high-speed flow and the solid wall. The necessary conditions for establishing the distinct thin layer are that the cross-area ratio is not too high or too low and the expand angle of the conical mixer is slightly less than the initial diffusion angle of the primary eject flow. According to the experimental and numerical results, the conical mixer with the cross-area ratio of 2.5 and the expand angle of 14.4° gives the highest pumping ratio of 0.528. The highest pumping ratio 0.598 for cylindrical mixers is experimentally in the case of the cross-area ratio of 3.8. The reason for the highest pumping ratio difference between the conical mixer and the cylindrical mixer is that in the cylindrical mixer the primary flowing fluid properly attaches the inner wall of the mixer in the outlet section and there is a larger space with negative pressure downstream of the lobed nozzle for entraining the secondary flowing fluid.     

Numerical investigation of lobe spacing ratio on performance of forced mixer nozzle

Geometric models of a lobed mixer nozzle with variation of lobe spacing ratios are created and the corresponding flow fields are simulated using a steady Reynolds Averaged Navier–Stokes (RANS) equation with a realizable k‐? turbulence model and standard wall function. According to the numerical simulation results, the spacing ratio has a large influence on the shape of streamwise vortices, but a relatively small effect on the streamwise vorticity field at the lobe exit. Therefore, at the initial part of mixing, a change of spacing ratio has little effect on the thermal mixing efficiency and the total pressure recovery coefficient. In addition, the thermal mixing efficiency increases sharply at the initial part, and near the nozzle exit it approaches some constant. The development of the total pressure recovery coefficient is totally reverse to the thermal mixing efficiency. It falls rapidly at the beginning. Then the rate of descent gradually slows down, and approaches some constant. At the nozzle exit cross‐section, the thermal mixing efficiency ascends and the total pressure recovery efficiency reduces as the spacing ratio increases. Besides, the thrust coefficient has little relationship to the spacing ratio, with a variation smaller than 0.01 when the spacing ratio increases from 4.87 to 0.497. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20363     

新型可调式喷射器性能的计算与分析

提出在喷射器喷嘴内插入喷针来调节喷射器工作参数的方案,建立了可调武喷射器性能计算模型,分析了喷嘴截面积变化对喷射系数、气体压力、气体流量等参数的影响。结果表明,通过对喷射器喉口面积的调节,可以实现把出口流量控制在一个稳定的区域内,从而减小喷射器入口参数对出口参数以至整个系统的影响。可调式喷嘴可拓宽喷射器的有效工作范围。     

Numerical simulation of hydrothermal behavior in a concentric curved annular tube

This study performs a numerical investigation of the steady‐state fully developed laminar flow and forced convection heat transfer characteristics in a concentric curved annular tube with two different curvature angles, a 90°‐bend annular tube and a U‐bend annular tube. A wide range of aspect ratios (r* = 0.1, 0.25, 0.5, and 0.75) and three curvature ratios (δ_c = 0.1, 0.2, and 0.5) were adopted in this study. The governing equations consisting of continuity, momentum, and energy equations are solved by considering the outer wall to be insulated (adiabatic), and a constant temperature is applied at the inner wall by using the finite‐volume method (FVM) to investigate the hydrothermal performance for these two different bend angles.Features of axial velocity contours, temperature patterns, and secondary flow streamlines at different cross‐sectional locations along the angular coordinate of curved annulus are observed with a Dean number range of (De = 32‐632). Additionally, the circumferential friction factor and averaged Nusselt number are obtained along the concentric curved annulus flow direction. The numerical results indicate that the normalized average Nusselt number and Performance Evaluation Criteria (PEC) increase with increasing De and curvature ratio for both curvature angles of concentric curved annular tube. Moreover, the normalized average Nusselt number, normalized friction factor‐Reynolds number product, and PEC increase with decreasing the aspect ratio because the annular gap between the surfaces of the inner and outer tubes (the boundaries of annulus) increases with decreasing aspect ratio. The hydrothermal performance of the concentric curved annular tube is higher than that of the straight annular tube attributed to the formation of secondary flows (Dean's vortices) in a cross‐sectional direction and the impact of the inner tube wall boundary. The value of PEC for both curvature angles of the curved annular tube at aspect ratio = 0.1 and De = 632 is approximately two‐fold of the straight annular tube under the same conditions while at aspect ratio = 0.75, it increases by nearly 80%.     

Two-phase flow pattern in small diameter tubes with the presence of horizontal return bend

This study provides a qualitatively visual observation of the two-phase flow patterns for air-water mixtures inside 6.9, 4.95, and 3 mm smooth diameter tubes with the presence of horizontal return bend. The influence of the return bend on the two-phase flow patterns are investigated. For D=6.9 mm and at a mass flux of 50 kg m⁻² s⁻¹ having a quality less than 0.1, no influence on the flow patterns is seen at a larger curvature ratio of 7.1. However, were the curvature ratio reduced to 3, the flow pattern in the recovery region is temporarily turned from stratified flow into annular flow. The temporary flow pattern transition phenomenon from stratified flow to annular flow is not so pronounced with the decrease of tube diameter. It is likely that this phenomenon is related to the influence of surface tension and the reduction of developing length of the swirl flow. Based on the present flow visualization, three flow pattern maps are proposed to describe the effect of return bend on the transition of two-phase flow pattern.     

Optimal r/b ratio of bend channel in centrifugal compressor

A numerical investigation on the flow in a bend channel by coupling the impellerwith the vaneless diffuser in a centrifugal compressor with different r/b ratios (bend radius r to bend channel width b) is presented. The jet-wake effect of the impeller outlet is considered and flow pattern in the bend channel and the performance of the centrifugal compressor stage are investigated. The results indicate that there is an optimal r/b ratio for increasing the stage efficiency to the highest for a specific compressor stage. The change in r/b ratio significantly affects the flow angle of the bend channel outlet. The prime reason for the total pressure loss in the bend channel is the wall friction in the bend channel. __________ Translated from Journal of Engineering Thermophysics, 2008, 29(2): 233–236 [译自: 工程热物理学报]     

Optimization Study of a Coanda Ejector

The Coanda effect has long been employed in the aerospace applications to improve the performances of variousdevices.This effect is the ability of a flow to follow a curved contour without separation and has well been util-ized in ejectors where a high speed jet of fluid emerges from a nozzle in the ejector body, follows a curved sur-face and drags the secondary flow into the ejector.In Coanda ejectors,the secondary flow is dragged in the ejec-tor due to the primary flow momentum. The transfer of momentum from the primary flow to the secondary flowtakes place through turbulent mixing and viscous effects.The secondary flow is then dragged by turbulent shearforce of the ejector while being mixed with the primary flow by the persistence of a large turbulent intensitythroughout the ejector.The performance of a Coanda ejector is studied mainly based on how well it drags thesecondary flow and the amount of mixing between the two flows at the ejector exit.The aim of the present studyis to investigate the influence of various geometric parameters and pressure ratios on the Coanda ejector per-formance.The effect of various factors,such as,the pressure ratio, primary nozzle and ejector configurations onthe system performance has been evaluated based on a performance parameter defined elsewhere.The perform-ance of the Coanda ejector strongly depends on the primary nozzle configuration and the pressure ratio.The mix-ing layer growth plays a major role in optimizing the performance of the Coanda ejector as it decides the ratio ofsecondary mass flow rate to primary mass flow rate and the mixing length.     

Investigation of Vapor-liquid Ejector with Organic Working Fluids

The vapor-liquid ejector is a simply flow device and driven by thermal energy. In this paper, a modified mathematical model of the vapor-liquid ejector is proposed, and the validation shows good agreements with the experimental data. A study is carried out with six organic working fluids, namely R1233 zd(E), R1336 mzz(Z), R236 ea, R245 ca, R245 fa and R365 mfc. The influences of the entrainment ratio, the area ratio, the superheating at the vapor nozzle inlet, the subcooling at the liquid nozzle inlet, and the pressures at these inlets on the pressure lifting are parametrically investigated. An increase in the subcooling leads to the great increasing of pressure lifting and the superheating has slight effect on the pressure lifting, whereas others have the opposite tendency. The studies of the pressures and temperatures at the typical locations inside the vapor-liquid ejector are further conducted by using R1336 mzz(Z). The results show that the above parameters have great influence on these pressures and temperatures inside except that the pressures are insignificantly impacted by the superheating, and the temperatures are negligibly affected by the area ratio. R1336 mzz(Z) is recommended as a good working fluid for the vapor-liquid ejector.     

Numerical investigation of flow mixture enhancement and infrared radiation shield by lobed forced mixer

For the exhaust system of turbo-fan engine, the numerical calculation is used to investigate the different mixer configurations. The method using forward–backward ray-tracing and narrow-band model to predict the radiation from engine exhaust has been developed. The scheme has been validated in accuracy for spectral radiation intensity predictions, and some useful results of practical importance have been obtained to establish its ability for infrared signature analysis of exhaust system. A series of computations on exhaust system model have been conducted to obtain the effects of mixer configuration on the aerodynamic performances and infrared radiation intensity. The results indicate that the lobed forced mixer can increase the mixing efficiency by 65%, decrease the thrust coefficient by 3% only, but the infrared radiation of plum can reduce about 40% relative to confluent mixer at zero degree aspect (normal to nozzle outlet), especially. That means the lobed forced mixer has the remarkable ability to shelter the infrared radiation from the inside enclosure of nozzle.     

An experimental study of air-solid two-phase flow in a 90° bend using LDV system

The measurements of the mean streamwise and radial velocities, the associated turbulence and the relative particle densities were made in an air-solid two-phase flow in a square sectioned (30mm×30mm) 90° vertical to horizontal bend using laser Doppler velocimetry. The radius ratio of the bend was 2.0. Glass beads of 100μm in diameter were employed to form the solid phase. The measurements of air and solid phases were performed separately at the same bulk velocity 19.34m/s, corresponding to a Reynolds number of 3.87×10⁴. The mass ratio of solid to air was 1.6%. The results indicate that the particle trajectories are very close to straight lines. The streamwise velocity profiles for the gas and the solids cross over near the outer wall with the solids having the higher speed. At θ=30° and 45°, particle-wall collisions happen mostly in the region from θ=30° to θ=75°, and cause a sudden change in solid velocity. The particles tend to move towards the outer wall in 90° bend. The particle concentration near the outer wall is much higher than that near the inner wall in the bend, and there are few particles in the inside of the bend. The bend leads to apparent phase separation: at θ=45°, the solids concentrate in the half of the duct near the outer wall. After θ=60° the second peak concentration appears, and goes gradually towards the inner wall.     

喷射制冷系统关键部件的数值分析

建立喷射制冷系统中可调喷嘴喷射器的数学模型,采用数值模拟方法对可调式喷射器与固定结构喷射器的流场进行对比分析,并计算调节锥在不同位置的可调式喷射器内部流场的变化。结果显示,可调式喷射器在喷嘴出口处的速度提高3.5%,真空度提高65.3%,喷射系数提高47.6%;调节锥进入喷嘴可达到更低的轴线压力,喷射器出口轴线流速降低8.9%。     

波瓣形混合器加力燃烧室冷态流场特性分析

以某波瓣形混合器加力燃烧室三维模型为研究对象,采用CFD方法对模型进行数值分析,建模时重点对波瓣形混合器和环形火焰稳定器进行网格加密。以飞行试验工况点获取的低压涡轮后总温、总压结合Gasturb软件计算出加力燃烧室出口的总温、总压作为整体流场CFD计算的边界条件进行计算。计算结果表明：经过波瓣形混合器后,内、外涵气流能够在较短轴向距离充分混合,总压恢复系数最大为0.984;在接近出口处热混合效率达到0.516;气流经过环形火焰稳定器后,形成明显的低压回流区,且在出口处截面静压分布均匀,因此该加力燃烧室模型具有非加力状态流阻小的特点。通过对该型加力燃烧室流场特性分析,为加力燃烧室加力接通前内部参数获取提供技术支撑。     

Transonic instability in entrance part of mixing chamber of high-speed ejector

Introduction The research of flow structure in the entrance part of the mixing chamber of two-dimensional supersonic ejector[1,2] shows, how this structure depends both on stagnation pressure ratio of streams p01/p02[3] and on back pressure ratio pb/p02 [4]. It was found out that the structure of shock waves is not stable, but it oscillates less or more. For the high back pressure ratio a terminal shock wave is in the mixing chamber and due to this shock wave the mixing processes change quali…     

斯特林发动机小空间燃气引射特性仿真研究

采用FLUENT软件对应用于斯特林发动机小空间燃烧室的不同类型的引射器的引射特性进行了计算研究.研究结果表明:在一次流体总质量流量和喷嘴总流通面积相同的条件下,多孔式喷嘴引射器的速度、温度及浓度分布均匀性明显优于中心、环形式喷嘴引射器,喷孔数量越多均匀性越好;多孔式喷嘴引射器的引射系数明显大于中心、环形式喷嘴引射器,喷孔数量越多引射系数越大.     

CFD modeling for slurry flow through a horizontal pipe bend at different Prandtl number

The present work shows the slurry flow characteristics of bottom ash particulates having density 2219 kg/m³ at different Prandtl number through horizontal pipe bend. The simulation is carried out by adopting Eulerian two-phase model in conjunction with RNG k-ε turbulence model using available commercial software ANSYS Fluent. The transportation of solid particulates has the settling behaviour in the slurry pipeline and that leads to the sedimentation and blockage of the pipeline resulting more power and pressure drop in the pipeline. Therefore, it is important to know the transport capability of the solid particulates at different Prandtl fluids to minimise the pressure loss. The fluid properties at four Prandtl numbers i.e., 1.34, 2.14, 3.42 and 5.83 are used to carry the bottom ash concentration ranging from 40 to 60% (by weight) at mean flow-velocity ranging from 1 to 5 ms^?1. The obtained computational results for pressure drop are validated with the published data in the literature and found in good agreement. The findings show that the pressure drop rises with escalation in flow velocity and Prandtl number for chosen efflux concentration range. The bottom ash particulates flowing at higher Prandtl fluid experiences less pressure drop through bend cross section in comparison to bottom ash particulates flowing at low Prandtl fluid. Finally, the contours of granular pressure, granular temperature and wall shear stress are predicted and discussed in details through the bend cross section to understand the complex slurry flow for chosen Prandtl numbers.     

Performance improvement of the vapour compression refrigeration cycle by a two‐phase constant area ejector

The performance of a vapour compression system that uses an ejector as an expansion device was investigated. In the analysis, a two‐phase constant area ejector flow model was used. R134a was selected as the refrigerant. According to the obtained results, for any operating temperature there are different optimum values of pressure drop in the suction chamber, ejector area ratio, ejector outlet pressure and cooling coefficient of performance (COP). As the difference between condenser and evaporator temperatures increases, the improvement ratio in COP rises whereas ejector area ratio drops. The minimum COP improvement ratio in the investigated field was 10.1%, while its maximum was 22.34%. Even in the case of an off‐design operation, the performance of a system with ejector is higher than that of the basic system. Copyright © 2008 John Wiley & Sons, Ltd.     

Optimal technical analysis of vacuum ejector for passive hydrogen recovery

In this study, the numerical analysis and experimental measurements are conducted on the internal flow field and temperature distribution of ejectors with different throat diameters. The computational fluid dynamics (CFD) is used to simulate different ejectors and investigate the effects of Mach number, pressure, and temperature distributions. The hydrogen Entrainment Ratio (ER) of ejectors is also measured for proton exchange membrane fuel cell applications. The experimental measurements and simulations of the hydrogen Entrainment Ratio of the ejectors showed that the recovery efficiencies are 59%, 53%, and 33% for the pipe diameters of 0.5, 0.7, and 1.0 mm at the inlet pressures of 340 kPa, respectively. In different area ratios, the larger area ratio of the nozzle leads to greater difference between the diameter of the throat and the diameter of the throat outlet. This causes a smaller recovery rate. In the internal flow field of the ejector, higher recovery rate can be achieved by using the closer location of the positive shock wave to the nozzle outlet.     

Performance testing of a novel ejector refrigerator for various controlled conditions

The paper presents the experimental results of a novel ejector refrigerator that was designed to be suitable for an air‐conditioning application using vacuum tube solar collectors for vapour generation. The primary flow of the ejector is controlled using a spindle in order to provide fine tuning for ejector operation as heat input changes with solar radiation. Water, the most environmentally friendly substance is used as the working fluid. The performance of the ejector was tested for a range of controlled primary flows, boiler temperatures, condensation capacities using different primary nozzles with different lengths. The effect of the operating conditions and nozzle length on the performance of the ejector was analyzed. It was found that in the tested boiler temperature range of 84–96°C the maximum cooling capacity (4.01 kW) of the ejector with short nozzle is much higher than that of the ejector with long nozzle (2.9 kW) on the spindle position of 21 mm. However, the ejector with long nozzle has increased COP when the boiler temperature is below 88°C and has higher critical back pressure. Copyright © 2010 John Wiley & Sons, Ltd.     

Numerical investigation of mixing length on performance of lobed forced mixer nozzles

Geometric models of a lobed mixer nozzle with variation of mixing length are created and corresponding flow fields are simulated using a steady Reynolds Averaged Navier–Stokes (RANS) equation with realizable k–? turbulence model. The numerical simulation results show that the mixing length of the nozzle has a relatively great influence on the development of streamwise vortices and the effective range for streamwise vortices to intensify mixing is about 0.5D distance from the lobe trailing edge. The change of mixing length has little effect on the thermal mixing efficiency. As for the total pressure recovery coefficient, the shorter the mixing length, the sharper the total pressure recovery coefficient curve. On the nozzle exit section, the thermal mixing efficiency increases at first and then slightly declines, and the total pressure recovery efficiency changes little as the mixing length increases. In addition, the thrust coefficient has some small relationship with the mixing length. The thrust coefficient increases in waves as the mixing length increases and the difference between the maximum and the minimum values is slightly less than 1% when the mixing length increases from 0.4D to D. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library wileyonlinelibrary.com/journal/htj . DOI 10.1002/htj.20343