Experimental studies of a steam jet refrigeration cycle: Effect of the primary nozzle geometries to system performance

This paper describes an experimental investigation of a steam jet refrigeration. A 1 kW cooling capacity experimental refrigerator was constructed and tested. The system was tested with various operating temperatures and various primary nozzles. The boiler saturation temperature ranked from 110 to 150 °C. The evaporator temperature was fixed at 7.5 °C. Eight primary nozzles with difference geometries were used. Six nozzles have throat diameters ranked from 1.4 to 2.6 mm with exit Mach number of 4.0. Two remained nozzles have equal throat diameter of 1.4 mm but difference exit Mach number, 3.0 and 5.5. The experimental results show that the geometry of the primary nozzle has strong effects to the ejector performance and therefore the system COP.     

Enhancement of a steam-jet refrigerator using a novel application of the petal nozzle

This study applies a petal nozzle to enhance the performance of a steam-jet refrigeration system. The behavior and characteristics of a petal nozzle are also investigated by testing it under various operating conditions, i.e., generator temperature, evaporator temperature and condenser temperature. In addition, the effect of area ratio (AR of constant area section of ejector to nozzle throat) on ejector performance is studied by testing ejectors with various values of AR. For comparison, a conventional conical nozzle with the same Mach number as that of petal nozzle is also used. Experimental results demonstrate that the compression ratio and the entrainment ratio can be enhanced if the petal nozzle in an ejector with a larger AR value is used. Moreover, for the ejector with a petal nozzle, an optimum AR value exists under which a maximum compression ratio can be found.     

Near field shock structure of dual co-axial jets

Experimental results on the shock structure of dual co-axial jets are presented. The effects of the geometric parameters of the inner nozzle, jet static pressure ratio (ratio of the exit plane static pressures of the inner and outer nozzles) and the ratio of outer to inner nozzle throat area on the shock structure were studied. A superimposed outer and inner jet structure was observed in the schlieren photographs. The inner flow is compressed by the outer flow resulting in the formation of a Mach disc and an exit shock. A parameter incorporating the effect of Mach number of the inner nozzle and jet static pressure ratio was found to correlate the observations regarding the Mach disc location.     

On nitrogen condensation in hypersonic nozzle flows: numerical method and parametric study

A numerical method for calculating two-dimensional planar and axisymmetric hypersonic nozzle flows with nitrogen condensation is developed. The classical nucleation theory with an empirical correction function and the modified Gyarmathy model are used to describe the nucleation rate and the droplet growth, respectively. The conservation of the liquid phase is described by a finite number of moments of the size distribution function. The moment equations are then combined with the Euler equations and are solved by the finite-volume method. The numerical method is first validated by comparing its prediction with experimental results from the literature. The effects of nitrogen condensation on hypersonic nozzle flows are then numerically examined. The parameters at the nozzle exit under the conditions of condensation and no-condensation are evaluated. For the condensation case, the static pressure, the static temperature, and the amount of condensed fluid at the nozzle exit decrease with the increase of the total temperature. Compared with the no-condensation case, both the static pressure and temperature at the nozzle exit increase, and the Mach number decreases due to the nitrogen condensation. It is also indicated that preheating the nitrogen gas is necessary to avoid the nitrogen condensation even for a hypersonic nozzle with a Mach number of 5 operating at room temperatures.     

引射器位置对脉冲爆震发动机性能影响

以氢气和氧气混合物为例,通过对加装收敛扩张型引射器,采用数值计算方法对脉冲爆震发动机性能进行模拟,获得了爆震管出口相对引射器喉部处于上游、中间和下游三种引射器位置对脉冲爆震发动机性能的影响规律.计算结果表明,爆震管出口相对引射器喉部处于下游时的增推效果最好,处于中间时次之,上游时效果最差.将二次流率与增推比进行比较,发现引射器的冲量增推比随引射的二次流率增加而增大.     

Investigation of high frequency noise generation in the near-nozzle region of a jet using large eddy simulation

This paper reports on the simulation of the near-nozzle region of an isothermal Mach 0.6 jet at a Reynolds number of 100,000 exhausting from a round nozzle geometry. The flow inside the nozzle and the free jet outside the nozzle are computed simultaneously by a high-order accurate, multi-block, large eddy simulation (LES) code with overset grid capability. The total number of grid points at which the governing equations are solved is about 50 million. The main emphasis of the simulation is to capture the high frequency noise generation that takes place in the shear layers of the jet within the first few diameters downstream of the nozzle exit. Although we have attempted to generate fully turbulent boundary layers inside the nozzle by means of a special turbulent inflow generation procedure, an analysis of the simulation results supports the fact that the state of the nozzle exit boundary layer should be characterized as transitional rather than fully turbulent. This is believed to be most likely due to imperfections in the inflow generation method. Details of the computational methodology are presented together with an analysis of the simulation results. A comparison of the far field noise spectrum in the sideline direction with experimental data at similar flow conditions is also carried out. Additional noise generation due to vortex pairing in the region immediately downstream of the nozzle exit is also observed. In a second simulation, the effect of the nozzle exit boundary layer thickness on the vortex pairing Strouhal frequency (based on nozzle diameter) and its harmonics is demonstrated. The limitations and deficiencies of the present study are identified and discussed. We hope that the lessons learned in this study will help guide future research activities towards resolving the pending issues identified in this work.

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Presented as AIAA Paper 2006-2499 at 12th AIAA/CEAS Aeroacoustics Conference, 8–10 May 2006, Cambridge, MA, USA.     

Influence of nozzle geometry on the near-field structure of a highly underexpanded sonic jet

Detailed near-field structures of highly underexpanded sonic free jets have been investigated with the help of computational fluid dynamics. Two-dimensional, axisymmetric Euler equations have been chosen to predict the underexpanded jets, and the third-order total variation diminishing finite-difference scheme has been applied to solve the system of governing equations numerically. Several different nozzles have been employed to investigate the influence of the nozzle geometry on the near-field structures of highly underexpanded sonic free jets. The results obtained show that the distance from the nozzle exit to the Mach disk is an increasing function of the jet–pressure ratio, which also significantly influences the shape of the jet boundary. The diameter of the Mach disk increases with the jet–pressure ratio, and it is further significantly influenced by the nozzle geometry, unlike the distance of the Mach disk from the nozzle exit. However, such a dependence on the nozzle geometry is no longer found when an effective-diameter concept is taken into account for the flow from a sharp-edged orifice. A good correlation in the diameters of the Mach disk is obtained, so that the near-field structure of highly underexpanded sonic free jets is a unique function of the pressure ratio, regardless of the nozzle geometry.     

Static pressure measurement along the centerline of an induced flow ejector

Conclusion The static pressure measuring system described in this paper is simple and does not disturb the flow perceptibly. Compared to wall pressure taps, the present measuring system has the advantage of being able to measure the static pressure at any section of the nozzle and to provide a continuous distribution, if necessary. An arrangement using a large number of wall pressure taps can be avoided.In addition, this system allows the internal static pressure of the flow to be measured, which is particularly interesting in the case of two coflowing streams inside an induced flow ejector. The primary jet can be investigated downstream of the primary nozzle exit plane. The static pressure at this exit plane, which is of great importance in numerical simulation of flows in ejectors, can be directly measured by means of this pressure probe (Lu 1986). This system also permits the detection of shock waves in the supersonic stream and gives accurate information about their location, their strength and the lengths of the disturbed areas. Hence, this system is useful for shock structure investigation, especially when flow visualization techniques are impossible to apply (non-transparent walls).However, the use of this measuring system is restricted to relatively short ejectors in order to limit the length of the capillary tube (less than 1.50 m in these experiments) and to minimize its oscillations in the flow.     

Experimental Investigation of Nozzle Exit Reflector Effect on Supersonic Jet

The present study describes an experimental work to investigate the effect of a nozzle exit reflector on a supersonic jet that is discharged from a convergent–divergent nozzle with a design Mach number of 2.0. An annular reflector is installed at the nozzle exit and its diameter is varied. A high-quality spark schlieren optical system is used to visualize detailed jet structures with and without the reflector. Impact pressure measurement using a pitot probe is also carried out to quantify the reflector’s effect on the supersonic jet which is in the range from an over-expanded to a moderately under-expanded state. The results obtained show that for over-expanded jets, the reflector substantially increases the jet spreading rate and reduces the supersonic length of the jet, compared with moderately under-expanded jets. The reflector’s effect appears more significant in imperfectly expanded jets that have strong shock cell structures, but is negligible in correctly expanded jet.     

Shock wave configurations and flow structures in non-axisymmetric underexpanded sonic jets

An experimental and numerical study of underexpanded free sonic jet flows issuing from rectangular, elliptical and slot nozzles has been undertaken. Aspect ratios (AR) of 1, 2, and 4 are described at pressure ratios (exit plane pressure to ambient pressure), of 2 and 3. There is good qualitative agreement between the experimental observations and the numerical predictions. In the case of rectangular jets, a complex system of shock waves forming the incident shock system is identified. This shock wave system originates at the corners of the nozzle exits, and proceeds downstream. Mach reflections are found to occur on the incident shock wave surface as well as the presence of a Mach disk terminating the first jet cell. This Mach disk has the shape of a square, a hexagon, or an octagon depending on the nozzle shape. For slot and elliptical jets, the formation of the incident shock wave was not observed along the minor axis plane of the nozzle for AR > 2. The incident shock wave was observed to originate downstream of the nozzle exit in the major axis plane. This wave system undergoes a transition to Mach reflection as it propagates downstream of the nozzle exit. In all cases tested, the shape of the jet boundary is significantly distorted. In rectangular jets, the narrowing of the jet boundary along the diagonal axis of the nozzle exit is observed, and in the case of the elliptical and slot jets axis switching is noted.     

Computation of the base pressure in ejector nozzles of different length with zero coefficient of ejection

A simple method is proposed for the analysis of the base pressure in ejector nozzles with arbitrary cowling length by using an empirical criterion of attachment in a broad range of variation of the Mach number of the main nozzle.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 164–166, November–December, 1976.     

UV-mediated coalescence and mixing of inkjet printed drops

In the present study, the characteristics of supersonic rectangular microjets are investigated experimentally using molecular tagging velocimetry. The jets are discharged from a convergent–divergent rectangular nozzle whose exit height is 500 μm. The jet Mach number is set to 2.0 for all tested jets, and the Reynolds number Re is altered from 154 to 5,560 by changing the stagnation pressure. The experimental results reveal that jet velocity decays principally due to abrupt jet spreading caused by jet instability for relatively high Reynolds numbers (Re > ~450). The results also reveal that the jet rapidly decelerates to a subsonic speed near the nozzle exit for a low Reynolds number (Re = 154), although the jet does not spread abruptly; i.e., a transition in velocity decay processes occurs as the Reynolds number decreases. A supersonic core length is estimated from the streamwise distribution of the centerline velocity, and the length is then normalized by the nozzle exit height and plotted against the Reynolds number. As a result, it is found that the normalized supersonic core length attains a maximum value at a certain Reynolds number near which the transition in the velocity decay process occurs.     

Two-way coupled simulation of a flow laden with metallic particulates in overexpanded TIC nozzle

A simulation of non-reacting dilute gas–solid flow in a truncated ideal contour nozzle with consideration of external stream interactions is performed. The Eulerian–Lagrangian approach involving a two-way momentum and thermal coupling between gas and particles phases is also adopted. Of interests are to investigate the effects of particles diameter and mass flow fraction on the flow pattern, Mach number, pressure and temperature contours and their distributions along the nozzle centerline and wall. The main goal is to determine the separation point quantitatively when the particles characteristics change. Particles sample trajectories are illustrated throughout the flow field and a qualitative discussion on the way that physical properties of the nozzle exit flow and particles trajectories oscillate is prepared. The existence of solid particulates delays the separation prominently in the cases studied. The bigger particles and the higher particles mass flow fractions respectively advance and delay the separation occurrence. The particles trajectories oscillate when they expose to the crisscrossing (or diamond-shape) shock waves generated outside the nozzle to approach the exit jet conditions to the ambient. The simulation code is validated and verified, respectively, against a one-phase 2D convergent–divergent nozzle flow and a two-phase Jet Propulsion Laboratory nozzle flow, and acceptable agreements are achieved.     

Effectiveness of secondary tabs for supersonic mixing

The effect of vortex generators, in the form of small tabs projecting into the flow at the nozzle exit, aided by secondary tabs on either side, on the mixing characteristics of an axisymmetric jet at Mach number 1.7 is investigated. Experimental studies on the basic features of the jet from a nozzle with secondary tabs are conducted to assess the free jet characteristics as well as the momentum and thermal mixing behavior. The secondary tabs were found to increase the jet spread and distort the jet cross-section and were found to cause substantial enhancement of mixing of supersonic jets. Jet structure is observed using flow-visualization techniques. LLS images are employed to obtain cross-sectional views of the jet with the introduction of secondary tabs. The ability of secondary tabs to eliminate the screech noise of the supersonic jet is also observed. Received: 3 February 2000/Accepted: 8 February 2001     

Calculation of spatial flow past an inclined flat plate with a detached shock wave

The results of a numerical analysis of a supersonic underexpanded jet impinging on an inclined flat plate are presented. The effects of the angle between the plate and the jet symmetry axis, the distance from the nozzle exit section, the exit Mach number, and the off-design conditions on the distribution of the gasdynamic parameters in the jet flowfield and on the plate surface are demonstrated. Specific features of the compressed layer and obstacle surface flows are revealed. The three-dimensional flow is simulated using the large particle method on the basis of the nonstationary Euler equations written in the cylindrical coordinate system. The calculated results are compared with experimental data. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 31–35, January–February, 1997.     

Numerical and experimental study on parameters distribution inside a two-phase ejector

The two-phase flow process in an ejector was numerically and experimentally studied using R141b as a working fluid. A modified one-dimensional gas–liquid ejector model was proposed to remedy the defect in the traditional one. Gas–liquid boundary layer regions were discussed and used to close the model. Mac Cormack method is used to discrete controlling equations of gas–liquid two-phase flow in the ejector. The radial distribution of velocity and temperature, the variation of void fraction, the axial velocity variation and the influence of primary steam pressure on the mixing process were predicted with the numerical model. An experimental rig was set up to validate the model by comparing the experimental pressure distribution in the ejector with the calculating one.     

Effect of impinging plate geometry on the self-excitation of subsonic impinging jets

In the generation of discrete tones by subsonic impinging jets, there exists a difference of opinion as how the feedback is achieved, i.e., the path of the feedback acoustic waves is whether inside the jet or outside the jet? The only available model (Tam and Ahuja model) for the prediction of an average subsonic jet impingement tone frequency assumes that the upstream part of the feedback loop is closed by an upstream propagating neutral wave of the jet. But, there is no information about the plate geometry in the model. The present study aims at understanding the effect of the plate geometry (size and co-axial hole in the plate) on the self-excitation process of subsonic impinging jets and the path of the acoustic feedback to the nozzle exit. The present results show that there is no effect of plate diameter on the frequency of the self-excitation. A new type of tones is generated for plates with co-axial hole (hole diameter is equal to nozzle exit diameter) for Mach numbers 0.9 and 0.95, in addition to the axisymmetric and helical mode tones observed for plates without co-axial hole. The stability results show that the Strouhal number of the least dispersive upstream propagating neutral waves match with the average Strouhal number of the new tones observed in the present experiments. The present study extends the validity of the model of Tam and Ahuja to a plate with co-axial hole (annular plate) and by doing so, we indirectly confirmed that the major acoustic feedback path to the nozzle exit is inside the jet.     

模拟火箭引射混合特性的实验研究

 专门设计了可用于研究箭基组合循环发动机(RBCC)在起动阶段 (Ma=0)所使用的引射火箭性能的实验装置. 作为初步试验，研究了不 同工况的引射热喷流(一次流)和被引射空气(二次流)之间混合的演变、 发展过程，找出不同来流条件下影响引射性能的主要参数，为最终探 明引射火箭的最佳工作条件打下基础，同时根据试验结果提出了促进 一、二次流混合的可行方案，便于下一步深入研究.