喷管不足膨胀冲击喷流的数值研究

针对喷管不足膨胀冲击喷流具有复杂波系结构的特点，应用高分辨率的ＴＶＤ格式建立了求解轴对称Ｎ－Ｓ方程的数值研究方法．对一定压比的冲击喷流在不同的喷管和平板间距下的流场进行了数值计算，得到了冲击喷流的复杂波系．对冲击喷流中盘形激波的形状、冲击平板的压力分布以及二者的关系进行了研究分析，揭示了冲击喷流产生分离泡的机理．

狭槽式空气喷流冲击于旋转圆柱体侧面的换热特性

利用实验方法探讨旋转圆柱体受狭槽式空气喷流的换热特性,圆柱体的高度与直径为定值,变动参数为(1)喷流Reynolds数(Rej)、(2)旋转Reynolds数(Rer)、(3)圆柱体直径(D)与喷嘴宽度(w)的比例(D/w)、(4)相对喷流冲击距离(L/w,L为喷嘴距圆柱体的最近距离)等。实验结果显示平均Nusselt数(Nu)随Rej与Rer增大而提升,而D/w增加会使Nu减小,且D/w对Nu的影响将随L/w增大而衰退,且存在一临界L/w值能产生最高的Nu,且临界L/w值将随D/w增大而增大,最后,提出合理而准确的经验公式。

HIGH EFFICIENCY COOLING TECHNOLOGY BASED ON GREEN MANUFACTURING

Green manufacturing （GM） and high efficiency machining technology are inevitable trends in the field of advanced manufacturing of the 21st century. To ensure green and high-efficiency machining, a new high efficiency cooling technology-cryogenic pneumatic mist jet impinging cooling （CPMJI） technology is presented. For obtaining the best cooling effect, a little quantity of coolant is carried by high speed cryogenic air （-20 C ） and reaches the machining zone in the form of mist jet to enhance heat transfer. Experimental results indicate that under the conditions of 40 m/s in the jet impinging speed and 10 mm in the jet impinging distance, the critical heat flux（CHF） nearly reaches 6× 10^7 W/m^2, more than six times of the CHF of the grinding burn with a value of （8～10）×10^6 W/m^2.

Simulation of Droplet-gas Flow in the Effervescent Atomization Spray with an Impinging Plate

Abstract A comprehensive three-dimensional model of droplet-gas flow was presented to study the evolution of spray in the effervescent atomization spray with an impinging plate. For gas phase, the N-S equation with the κ-ε turbulence model was solved, considering two-way coupling interaction between droplets and gas phase. Dispersed droplet phase is modeled as Lagrangian entities, accounting for the physics of droplet generation from primary and secondary breakup, droplet collision and coalescence, droplet momentum and heat transfer. The mean size and sta- tistical distribution of atomized droplets at various nozzle-to-plate distances were calculated. Some simulation resuits were compared well with experimental data. The results show that the existence of the impinging plate has a pronounced influence on the droplet mean size, size distribution and the droplet spatial distribution. The air-to-liquid ratio has obvious effects on the droplet size and distribution.

Effect of three-electrode plasma synthetic jet actuator on shock wave control

A three-electrode high-energy plasma synthetic jet(PSJ) actuator was used for shock wave control. This actuator is an enhanced version of the two-electrode actuator as a high-voltage trigger electrode is added to increase the cavity volume and the input energy while retaining a relatively low disruptive voltage. The electrical properties were studied using current-voltage measurements, and the energy consumption was calculated. To assess the jet strength, the penetration of PSJ was compared with empirical values, and the results show that the momentum flux ratio of PSJ for a capacitance of 0.96, 1.6, and 3 μF was approximately equal to 0.6, 1.0, and 1.3, respectively. The interaction of PSJ with shock waves was acquired using high-speed shadowgraph imaging. The shock was generated by a 25° compression ramp in Mach 2 flow, and PSJ actuator was placed up-stream of the compression ramp. Under the action of PSJ, the strength of the shock was notably weakened, and the near-wall part of the shock was entirely eliminated. The results show the good control effect of the three-electrode high-energy PSJ in high-speed flow.

Experimental and Three-Dimensional Numerical Study on Under-Expanded Impinging Jets

In this paper, under-expanded impinging jets issued from converging circular and rectangular nozzles were investigated. The ratio of the distance between the nozzle exit and the plate L to the diameter D was set at 2, 3, 4 for the circular nozzle and 2, 3 for the rectangular nozzle. Two-dimensional temperature and static pressure distributions on the impinging plate were measured using an infrared camera and a semi-conductor pressure sensor and flow fields were visualized by means of schlieren method. Three-dimensional numerical calculations were also conducted by solving the three-dimensional compressible Euler equations and compared to the experimental results. As a result, it is found that the numerical calculations for the circular and the rectangular nozzles are in good agreement with the experiments. In the experiments, it is found that the stagnation temperature on the plate depends on the pressure in the settling chamber and the distance between the nozzle exit and the plate. The temperature and pressure distributions in the experiments illustrate that even in the case of the circular nozzle, the distributions on the impinging plate are non-axisymmetric, which is confirmed by the three dimensional calculations. The calculation for the rectangular nozzle indicates that two circulating regions occur immediately upstream of the plate.

Three-Dimensional Rainbow Schlieren Measurements in Underexpanded Sonic Jets from Axisymmetric Convergent Nozzles

The rainbow schlieren deflectometry has been combined with the computed tomography to obtain three-dimensional density fields of shock containing free jets and we call the method the schlieren CT. Experiments on the schlieren CT have been performed at a nozzle pressure ratio of 4.0 by using an axisymmetric convergent nozzle with an inner diameter of 10 mm at the exit where the nozzle was operated at an underexpanded condition. Multidirectional rainbow schlieren pictures of an underexpanded sonic jet can be acquired by rotating the nozzle about its longitudinal axis in equal angular intervals and the three-dimensional density fields are reconstructed by the schlieren CT. The validity of the schlieren CT is verified by a comparison with the density fields reconstructed by the Abel inversion method. As a result, it is found that excellent quantitative agreement is reached between the three-dimensional jet density fields reconstructed from both methods.

Shock Wave-Boundary Layer Interaction in Forced Shock Oscillations

The flow in transonic diffusers as well as in supersonic air intakes becomes often unsteady due to shock wave boundary layer interaction. The oscillations may be induced by natural separation unsteadiness or may be forced by boundary conditions. Significant improvement of CFD tools, increase of computer resources as well as development of experimental methods have again.drawn the attention of researchers to this topic. To investigate the problem forced oscillations of transonic turbulent flow in asymmetric two-dimensional Laval nozzle were considered. A viscous, perfect gas flow, was numerically simulated using the Reynolds-averaged compressible Navier-Stokes solver SPARC, employing a two-equation, eddy viscosity, turbulence closure in the URANS approach.For time-dependent and stationary flow simulations, Mach numbers upstream of the shock between 1.2 and 1.4 were considered. Comparison of computed and experimental data for steady states generally gave acceptable agreement. In the case of forced oscillations, a harmonic pressure variation was prescribed at the exit plane resulting in shock wave motion. Excitation frequencies between 0 Hz and 1024 Hz were investigated at the same pressure amplitude.The main result of the work carried out is the relation between the amplitude of the shock wave motion and the excitation frequency in the investigated range. Increasing excitation frequency resulted in decreasing amplitude of the shock movement. At high frequencies a natural mode of shock oscillation (of small amplitude) was observed which is not sensitive to forced excitement.

Numerical and experimental investigations for an air cannon optimization

Air cannon is a kind of de-clogging device which produces impulse force by instantly releasing the compressed air deposited in a pressure vessel. Air cannons are widely used in the transport pipes of warehouses, docks, furnaces and coal mines. In this paper, the theoretical analysis with isentropic flow hypothesis is firstly conducted on a simplified mode/ to deduce the theo- retical maximum of impulse force. And numerical study is carried out to predict the steady and unsteady impulse forces via simulating the whole exhausting process of the air cannon. The results demonstrate that the impulse force can be improved via increasing the piston sleeve inlet length and increasing the nozzle diameter. Laval nozzle can also increase the impulse force of the air without increasing the air mass flow. The optimization of the air cannon is then conducted on the basis of the theoretical and numerical analyses. Experimental measurements indicate that the computations well simulate the working process of the air cannon and the impulse force of the optimized design is 50% higher than the original model. For the cases with working pressure of 0.8 MPa, the optimized design is 60% higher than the original one.

Study on Transonic Tone in an Axisymmetric Supersonic Nozzle

This paper describes experimental and numerical works to investigate noise phenomenon in supersonic flow discharged from a convergent-divergent nozzle.The noise phenomenon of flow is generated by an emission of 'transonic tones'.The results obtained show that the frequency of a transonic tone,that differs from the frequency of a screech tone due to the shock-cell structures in a jet and originates in the shock wave in the nozzle,increases in proportion to the nozzle pressure ratio.The high-order transonic tone has the directivity in the direction of the flow.As for the transonic tone's frequency,the separated zone was calculated by using a simple flow model considering the propagating perturbation.The results of the model corresponded to the results of this experiment well.