Study on Transonic Tone in an Axisymmetric Supersonic Nozzle

This paper describes experimental and numerical works to investigate noise phenomenon in supersonic flow dis- charged 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 con- sidering the propagating perturbation. The results of the model corresponded to the results of this experiment well.     

Supersonic flows over cavities

The characteristics of supersonic cold flows over cavities were investigated experimentally and numerically, and the effects of cavities of different sizes on supersonic flow field were analyzed. The results indicate that the ratio of length to depth L/D within the range of 5–9 has little relevance to integral structures of cavity flow. The bevel angle of the rear wall does not alter the overall structure of the cavity flow within the range of 30°–60°, but it can exert obvious effect on the evolvement of shear layer and vortexes in cavities. __________ Translated from Journal of National University of Defense Technology, 2007, 29(3): 1–5 [译自: 国防科技大学学报]     

Effect of plate position and size for self-induced flow oscillation of underexpanded supersonic jet impinging on perpendicular plate

When the underexpanded supersonic jet impinges on the obstacle, it is well known that the self-induced flow oscillation occurs at the specific condition of the pressure ratio in the flowfield, the position of an obstacle and so on. This oscillation is related with the noise problems of aeronautical and other industrial engineering so that the characteristic and the mechanism of self-induced flow oscillation have to be cleared to control the various noise problems. But, it seems that the characteristics of t...     

Experimental study of unsteady supersonic flow interacting with porous cavity and jets or rods

In this study, an experiment was performed to clarify the flow field, in which the jets were normally injected into a main supersonic flow surrounded by a porous cavity, and this report figures out interaction between starting shock wave and porous cavity. In the experiment, a porous cavity is attached to a main duct and jets and rods are inserted to the main duct on the porous cavity. To reveal this flow field, the thermal tuft probe was adopted to ex- perimentally investigate the flow in the cavity. In the experiments, the effect of the porous cavity with jets or rods on the flow field is studied by means of visualization of schlieren method with a high speed camera and measurement of cavity flow with thermal tuft probe. As a results, frequency analysis of output of the thermal tuft probe revealed that some clear dominant frequencies were confirmed when the starting shock wave existed around the porous cavity in all cases of jets and rods arrangements. Moreover, visualization of schlieren method with a high speed camera clarified that a starting shock wave had the same dominant frequencies as that of the flow fluctuation in the cavity only around the cavity.     

Heat transfer enhancement of mixed convection in a square cavity with inlet and outlet ports due to oscillation of incoming flow

A numerical study of unsteady laminar mixed convection flow in a square cavity with ventilation ports due to an oscillating velocity at the inlet port is performed. It is found that after certain time duration, a periodic variation in the fluid flow and temperature field in the cavity are created. It is observed that the heat transfer is enhanced for all the Strouhal numbers investigated in comparison to its steady state case. To realize the optimizing Strouhal number to reach the best performance of the system, the total Nusselt number and the coefficient of pressure drop in a cycle of the oscillation is evaluated with respect to the Strouhal number. It is found that for a region of the Strouhal number between 0.5 and 1, the performance of the system will be desirable with considering both the maximum heat transfer rate and minimum pressure drop in the cavity.     

Numerical study on detonation reflections over concave and convex double wedges

The solver DCRFoam is used to analyze the characteristics of detonation reflection over concave and convex double wedges. Seven reflection processes are observed depending on the wedge angles (θ₁ and θ₂), the critical angle of a single surface (θ_c,single), and the angle difference (Δθ). For concave wedges with small θ₁ and Δθ, a Mach reflection is established along the first surface and the leading edge of the second surface. At the end of the second surface, either a Mach reflection or a regular reflection could be observed depending on the θ₂. For concave wedges with large θ₁ and Δθ, the Mach stem formed over the first surface regularly reflects at the leading edge of the second surface. Then the incident wave regularly reflects over the tail of the second surface. For concave wedges, the actual critical angle of the second surface is found to be larger than the θ_c,single. While the second surfaces of convex double wedges are found to have the same as a single surface. The detonation reflection processes over convex wedges are much simpler.     

2维,3维可压缩流体分支流场的数值模拟

本文提出了一种分析质量、动量和能量传输问题的ＴＧ有限元法。该法保留了空间的３阶精度，隐含了流线迎风的耗散作用，可很好地用于高Ｒｅｙｎｏｌｄｓ和高Ｐｅｃｌｅｃｔ数的流动中，采用了压力修正法，使计算格式同时适应可压缩流体和不可压缩流体。为了适应现有计算条件，将内存和速度进行了均衡选择，提出了相应的算法和贮存方式。最后给出了２维、３维分支流场的计算结果。     

Characteristic and mechanism of pressure fluctuation caused by self-induced oscillation of supersonic impinging jet

When the underexpanded supersonic jet impinges on the obstacle, it is well known that the self-induced flow oscillation occurs. This oscillation depends on the pressure ratio in the flowfield, the position of an obstacle and is related with the noise problems of aeronautical and other industrial engineering. The characteristic and the mechanism of self-induced flow oscillation, have to be clarified to control various noise problems. But, it seems that the characteristics of the oscillated flowfield and the mechanism of an oscillation have to be more cleared to control the oscillation. This paper aims to clarify the effect of the pressure ratio and the obstacle position and the mechanism of self-induced flow oscillation by numerical analysis and experiment, when the underexpanded supersonic jet impinges on the cylindrical body. From the result of this study, it is clear that occurrence of the self-induced flow oscillation depends on the pressure balance in the flowfield.     

Steady and unsteady flow characteristics of dual cavity in strut injection scramjet combustor

This study aims to understand the flow performance of a dual wall-mounted cavities in a strut-injector mounted scramjet combustor for steady-state and transient reacting conditions. Conventionally, two-dimensional Reynolds Averaged Navier-Stokes approach is adopted to compute the steady flow, whereas the current research employs Delayed Detached Eddy Simulation for predicting the unsteady flow characteristics as well. The calculated flow patterns, density, pressure, and temperature fields of dual cavities are compared with shadowgraph and wall pressure measurements from DLR experiments. The dual cavities position substantiates to explore the interplay between wave propagation and shear layer mixing characteristics. Employing a dual cavities arrangement accelerates toward the complete combustion relative to the baseline model. The combustion zone widens in the lateral direction as the dual cavities shift the shock train downstream of the strut injector owing to intense shock shear layer interactions. These cavities' existence significantly modifies the dominating frequencies and affects the strength of the diverging section's coherent flow structures.     

Effect of water-based Al2O3 nanofluids on heat transfer and pressure drop in periodic mixed convection inside a square ventilated cavity

A numerical study of unsteady mixed convection flows through an alumina-water nanofluid in a square cavity with inlet and outlet ports due to incoming flow oscillation is performed. It is found that an oscillating velocity at the inlet port cased to creating a periodic variation in the fluid flow and temperature field in the cavity after a certain time duration. The influence of the nanoparticle on the flow and temperature fields has been plotted and discussed. The effect of the oscillation frequency is concealed in a dimensionless number which is the Strouhal number. It is observed that the heat transfer is enhanced for all the Strouhal and Richardson numbers investigated by adding the nanoparticle to the base fluid. It is also found that the performance of the nanoparticle on the enhancement of the heat transfer at higher Richardson numbers is less than that of lower Richardson numbers.     

Two-phase flow and maldistribution in gas channels of a polymer electrolyte fuel cell

Liquid water transport in a polymer electrolyte fuel cell (PEFC) is a major issue for automotive applications. Mist flow with tiny droplets suspended in gas has been commonly assumed for channel flow while two-phase flow has been modeled in other cell components. However, experimental studies have found that two-phase flow in the channels has a profound effect on PEFC performance, stability and durability. Therefore, a complete two-phase flow model is developed in this work for PEFC including two-phase flow in both anode and cathode channels. The model is validated against experimental data of the wetted area ratio and pressure drop in the cathode side. Due to the intrusion of soft gas diffusion layer (GDL) material in the channels, flow resistance is higher in some channels than in others. The resulting flow maldistribution among PEFC channels is of great concern because non-uniform distributions of fuel and oxidizer result in non-uniform reaction rates and thus adversely affect PEFC performance and durability. The two-phase flow maldistribution among the parallel channels in an operating PEFC is explored in detail.     

Cavity ignition and flameholding of ethylene–air and hydrogen–air flows by a repetitively pulsed nanosecond discharge

Repetitive nanosecond pulse plasma assisted ignition and flameholding of premixed and non-premixed ethylene–air and hydrogen–air flows are studied in a cavity flow at a pressure of 0.2 atm and flow velocities of up to 100 m/s. Ignition occurs via formation of multiple filaments in the fuel–air plasma, although air plasma remains diffuse until the fuel is added. After ignition occurs in the cavity, with ignition delay time of a few milliseconds, the plasma becomes diffuse and the flame couples out to the main flow. The use of a short cavity (length-to-depth ratio L/D = 1) results in repetitive ignition and flame blow-off, caused by slow mixing between the main flow and the cavity. Increasing the length-to-depth ratio to L/D = 3, as well as choking inlet air and fuel flows resulted in stable flameholding and nearly complete combustion in both premixed and non-premixed ethylene–air and hydrogen–air flows at u = 35–100 m/s. Air plasma temperature before fuel is added ranges from 70 °C to 200 °C. When the nanosecond pulse discharge is operated in repetitive burst mode, continuous ethylene–air flame is maintained only at a high duty cycle, which increases with the flow velocity. In hydrogen–air, the flame remains stable after the plasma is turned off. Nanosecond pulse discharge ignition of ethylene–air is compared with ignition by DC arc discharge of approximately the same power. DC arc discharge results in sporadic ignition and flame blow-off, much lower burned fuel fraction, and significantly lower flow velocity at which ignition can be achieved. Kinetic modeling is used to identify the reduced mechanism of plasma chemical oxidation and ignition of hydrogen, and to demonstrate the mechanism of energy release low-temperature reactions of radicals generated in the plasma (primarily O and H atoms).     

MHD effect on flow structures and heat transfer characteristics of liquid metal-gas annular flow in a vertical pipe

The magnetohydrodynamic (MHD) effect on the flow structures and heat transfer characteristics was studied numerically for a liquid metal-gas annular flow under a transverse magnetic field. The side layers, in which the velocity was increased, appeared near the eastern and western sidewalls in an annular MHD flow as in a single-phase liquid metal MHD flow. Temperature distribution in the liquid film, and the Nusselt number distribution in the angular direction were influenced by the flow structures with the side layers. Consequently heat transfer rate was higher at the eastern/western sidewalls than that at the southern/northern walls. The pressure drop in the MHD annular flow is of the same order of magnitude as in the single-phase MHD pipe flow under similar liquid metal flow condition.     

Two-phase flow in GDL and reactant channels of a proton exchange membrane fuel cell

Understanding the effect of two-phase flow in the components of proton exchange membrane fuel cells (PEMFCs) is crucial to water management and subsequently to their performance. The local water saturation in the gas diffusion layer (GDL) and reactant channels influences the hydration of the membrane which has a direct effect on the PEMFC performance. Mass transport resistance includes contributions from both the GDL and reactant channels, as well as the interface between the aforementioned components. Droplet–channel wall interaction, water area coverage ratio on the GDL, oxygen transport resistance at the GDL–channel interface, and two-phase pressure drop in the channels are interlinked. This study explores each factor individually and presents a comprehensive perspective on our current understanding of the two-phase transport characteristics in the PEMFC reactant channels.     

Estimation of heat transfer coefficient and friction factor in the transition flow with low volume concentration of Al2O3 nanofluid flowing in a circular tube and with twisted tape insert

Experiments to evaluate heat transfer coefficient and friction factor for flow in a tube and with twisted tape inserts in the transition range of flow with Al₂O₃ nanofluid are conducted. The results showed considerable enhancement of convective heat transfer with Al₂O₃ nanofluids compared to flow with water. It is observed that the equation of Gleninski applicable in transitional flow range for single-phase fluids showed considerable deviation when compared with values obtained with nanofluid. The heat transfer coefficient of nanofluid flowing in a tube with 0.1% volume concentration is 23.7% higher when compared with water at number of 9000. Heat transfer coefficient and pressure drop with nanofluid has been experimentally determined with tapes of different twist ratios and found to deviate with values obtained from equations developed for single-phase flow. A regression equation is developed to estimate the Nusselt number valid for both water and nanofluid flowing in the transition flow Reynolds number range in circular plain tube and with tape inserts. The maximum friction factor with twisted tape at 0.1% nanofluid volume concentration is 1.21 times that of water flowing in a plain tube.