A shock train inside a diverging duct is analyzed at different pressure levels and Mach numbers. Nonreactive pressurized cold gas is used as fluid. The structure and pressure recovery inside the shock train is analyzed by means of wall pressure measurements, Schlieren images and total pressure probes. During the course of the experiments, the total pressure of the flow, the back pressure level and the Mach number upstream of the compression region have been varied. It is shown that the Reynolds number has some small effect on the shock position and length of the shock train. However, more dominant is the effect of the confinement level and Mach number. The results are compared with analytical and empirical models from the literature. It was found that the empirical pseudo-shock model from Billig and the analytical mass averaging model from Matsuo are suitable to compute the pressure gradient along the shock train and total pressure loss, respectively.

/pdf/behavior-of-shock-trains-in-a-diverging-duct-1abiqulb0z.pdf

Behavior of shock trains in a diverging duct

The interaction of a shock train with a normal suction slot is presented. It was found that when the pressure in the suction slot is smaller or equal to the static pressure of the incoming supersonic flow, the pressure gradient across the primary shock is sufficient to push some part of the near wall boundary layer through the suction slot. Due to the suction stabilized primary shock foot, the back pressure of the shock train can be increased until the shock train gradually changes into a single normal shock. During the experiments, the total pressure and therewith the Reynolds number of the flow were varied. The structure and pressure recovery within the shock train is analysed by means of Schlieren images and wall pressure measurements. Because the boundary layer is most important for the formation of a shock train, it has been measured by a Pitot probe. Additionally, computational fluid dynamics is used to investigate the shock boundary-layer interaction. Based on the experimental and numerical results, a simplified flow model is derived which explains the phenomenology of the transition of a shock train into a single shock and derives distinct criteria to maintain a suction enhanced normal shock. This flow model also yields the required suction mass flow in order to obtain a single normal shock in a viscous nozzle flow. Furthermore, it allows computation of the total pressure losses across a normal shock under the influence of boundary-layer suction.

Behaviour of a shock train under the influence of boundary-layer suction by a normal slot

The stability of agglomerates is not only an important material parameter of powders but also of interest for estimating the particle size upon accidental release into the atmosphere. This is especially important when the size of primary particles is well below the agglomerate size, which is usually the case when the size of primary particles is below 100 nm. During production or airborne transportation in pipes, high particle concentrations lead to particle coagulation and the formation of agglomerates in a size range of up to some micrometers. Binding between the primary particles in the agglomerates is usually due to van der Waals forces. In the case of a leak in a pressurized vessel (e.g. reactor, transport pipe, etc.), these agglomerates can be emitted and shear forces within the leak can cause agglomerates to breakup. In order to simulate such shear forces and study their effect on agglomerate stability within the airborne state, a method was developed where agglomerate powders can be aerosolized and passed through an orifice under various differential pressure conditions. First results show that a higher differential pressure across the orifice causes a stronger fragmentation of the agglomerates, which furthermore seems to be material dependent.

Investigation of airborne nanopowder agglomerate stability in an orifice under various differential pressure conditions

Detailed numerical and experimental investigations of pseudo-shock systems in a Laval nozzle with parallel side walls are carried out. The location of the pseudo-shock system is defined in this system of two choked Laval nozzles by the ratio of the critical cross sections $${{A}_{2}^*/{A}_{1}^*}$$
 , the stagnation pressure loss across the shock system and viscous losses. The wall pressure distributions and high-speed schlieren videos recorded in the experiments are compared to the results of a steady and an unsteady numerical simulation. For the steady case, good agreement is found between the calculated and measured shock structure and pressure distribution along the primary nozzle wall, except for a remaining slight deviation in the shock position. For the unsteady case, in which asymmetric shock configurations are observed, deviations of the results with respect to the stochastic wall attachment of the shock system are given which indicate the necessity of further investigations on that topic.

Experimental and numerical analysis of the structure of pseudo-shock systems in laval nozzles with parallel side walls

In this article, the influence of the water vapor concentration on structural changes of SiO2 aerosol nanoparticle agglomerates during tempering was studied. The presence of water vapor in the carrier gas was shown to strongly accelerate the kinetics of sintering. While dry sintering at temperatures between 1100°C and 1500°C generated aggregates only, the addition of water to the process yields individual, completely coalesced nanoparticles at a temperature of 1300°C. Furthermore, depending on the water vapor concentration and temperature of the process, evaporation and condensation processes could be observed, leading to bimodal size distributions. The results prove the significant role of the water concentration in high temperature synthesis of silica and may be used to improve the control over morphology and specific surface area in these processes.

https://www.tandfonline.com/doi/pdf/10.1080/02786826.2011.590555

The Effect of Water Vapor on the Particle Structure and Size of Silica Nanoparticles During Sintering

A novel method for the production of oxide nanoparticles from gas-phase organic precursors in a high-velocity flow apparatus is described. In contrast to conventional methods of reaction initiation by flames, plasmas or heat conduction, the gas mixture is instantaneously heated by a stationary shock wave of an overexpanded supersonic nozzle flow above a critical initiation temperature. Following the reaction initiation, different molecular and intermolecular processes lead to the generation and growth of nanoparticles. After an adjustable reaction time that depends on the gas velocity and the reactor length the particle growth by surface growth and coagulation is interrupted by fast expansion and, therefore, cooling of the gas in a convergent-divergent nozzle flow. The total enthalpy of the flow is finally reduced by injecting water into the flow behind the nozzle exit. The aim of this gas-dynamic nanoparticle synthesis strategy is the generation of non-agglomerated single particles with a narrow size distribution. In this paper, a process is described which aims at establishing flow conditions for the heating and quenching process that are as uniform as possible in order to generate material with well-defined properties. This synthesis path is in contrast to today’s industrial production facilities where the heating of the precursor gas is typically achieved by a low velocity flow, e.g. through a flame in connection with low cooling rates at the end of the particle growth zone. This results in inhomogeneous particle formation conditions along the individual flow paths leading to agglomerated nanoparticles with strongly varying primary particle sizes and agglomerate sizes and structures. This paper focuses on the design strategy of the planned facility that is based on numerical and experimental investigations of the underlying fluid-dynamic and chemical-kinetic requirements. This joint project of five German universities and the German Aerospace Center is funded by DFG (Deutsche Forschungsgemeinschaft) and is performed in close cooperation with Degussa AG, Germany. First experiments with this facility are planned for mid of 2007.

Gas-Phase Synthesis of Non-Agglomerated Nanoparticles by Fast Gasdynamic Heating and Cooling

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Gas-Phase Synthesis of Non-Agglomerated Nanoparticles by Fast Gasdynamic Heating and Cooling