A new shock tube configuration for studying dust-lifting during the initiation of a coal dust explosion

The traditional defence against propagating coal dust explosions is the application of dry stone dust. This proven and effective safety measure is strictly regulated based on extensive international experience. While new products, such as foamed stone dust, offer significant practical benefits, no benchmark tests currently exist to certify their dust lifting performance in comparison to dry stone dust. This paper reviews the coal dust explosion mechanism, and argues that benchmark testing should focus on dust lifting during the initial development of the explosion, prior to arrival of the flame. In a practical context, this requires the generation of shock waves with Mach numbers ranging from 1.05 to 1.4, and test times of the order of 10’s to 100’s of milliseconds. These proposed test times are significantly longer than previous laboratory studies, however, for certification purposes, it is argued that the dust lifting behaviour should be examined over the full timescales of an actual explosion scenario. These conditions can be accurately targeted using a shock tube at length scales of approximately 50 m. It is further proposed that useful test time can be maximised if an appropriately sized orifice plate is fitted to the tube exit, an arrangement which also offers practical advantages for testing. The paper demonstrates this operating capability with proof-of-concept experiments using The University of Queensland’s X3 impulse facility

Expansion fan spectroscopy of ionising super orbital flows

This paper reports on the use of spectral techniques for probing properties of a radiating flow through a rapid expansion. Experiments were conducted to measure relative excited state populations in ionising super-orbital expanding flows. The X2 expansion tube, located at The University of Queensland, was used to generate super-orbital conditions and provide a sample case of high speed gas moving through a shock layer and into an expansion fan. Spectrometers were used to monitor the electronic energy levels of argon by measuring electromagnetic radiation at visible and infra-red wavelengths. This configuration was found to be capable of measuring temperature integrated over a two-dimensional plane in space. Excitation temperatures were found to increase from 7500K to 10000K between the shock and expansion fan. Numerical simulations of the flow showed a similar trend although simulated emperatures were lower than measured values. Interesting phenomena were also measured showing that the excitational temperature before the shock and in the expansion fan may be influenced by radiation from the post shock pre-expansion fan region

Stress wave force balance sting design for magnetohydrodynamic drag force measurements in expansion tubes

This paper discusses the existence of noise observed in measurements taken using a stress wave force balance for magnetohydrodynamic drag measurements in an ionised argon expansion tube test flow. There is compelling evidence to suggest that the noise appearing in the strain gauge signal is a result of electrical interference generated by the ionised test flow. Since the magnetohydrodynamic drag force will generate a signal on a similar order of magnitude to this electrical noise, extracting accurate drag estimations is impractical. This paper analyses a set of new sting designs which aim to increase the signal to noise ratio by reducing axial stiffness of the bar. The materials tested include brass, aluminium, nylon and poly-carbonate. Results indicate that the signal to noise ratio can be improved by a factor of 90 when comparing the signal from the original solid brass design to a new hollow poly-carbonate design

Calculating shock arrival in expansion tubes and shock tunnels using Bayesian changepoint analysis

To understand the flow conditions generated in expansion tubes and shock tunnels, shock speeds are generally calculated based on shock arrival times at high-frequency wall-mounted pressure transducers. These calculations require that the shock arrival times are obtained accurately. This can be non-trivial for expansion tubes especially because pressure rises may be small and shock speeds high. Inaccurate shock arrival times can be a significant source of uncertainty. To help address this problem, this paper investigates two separate but complimentary techniques. Principally, it proposes using a Bayesian changepoint detection method to automatically calculate shock arrival, potentially reducing error and simplifying the shock arrival finding process. To compliment this, a technique for filtering the raw data without losing the shock arrival time is also presented and investigated. To test the validity of the proposed techniques, tests are performed using both a theoretical step change with different levels of noise and real experimental data. It was found that with conditions added to ensure that a real shock arrival time was found, the Bayesian changepoint analysis method was able to automatically find the shock arrival time, even for noisy signals

Simulation of high mach number scramjet flow conditions using the X2 expansion tube

Expansion tubes are the only type of ground test facility currently able to simulate high Mach number scramjet test ows. These access-to-space ow conditions are characterised by total pressures of the order of gigapascals. The University of Queensland's X2 expansion tube facility has recently been used to generate scramjet ow conditions between Mach 10-14, with total pressures up to 10 GPa. Flow conditions were relevant to a 96 kPa dynamic pressure ascent trajectory. For ground testing of sub-scale scramjet-powered vehicles, pressure-length (p-L) scaling is used in order to maintain similarity for various ight parameters, such as Reynolds number, total enthalpy, and binary reaction rates. X2 was congured to achieve test ow static pressures considerably higher than the true ight values, while maintaining true ight velocities and temperatures, thereby demonstrating signicant potential for p-L scaling, which is typically necessary since most model scramjet engines need to be tested at sub-scale. This paper details the combined analytical and numerical process used to develop new ow conditions in X2. Experimental results are presented for four new ow conditions, and axisymmetric CFD analysis is used to fully characterise test ow properties

Performance considerations for expansion tube operation with a shock-heated secondary driver

A shock-heated secondary driver is a modification typically applied to an expansion tube which involves placing a volume of helium between the primary diaphragm and the test gas. This modification is normally used to either increase the driven shock strength through the test gas for high-enthalpy conditions, or to prevent transmission of primary driver flow disturbances to the test gas for low-enthalpy conditions. In comparison to the basic expansion tube, a secondary driver provides an additional configuration parameter, adds mechanical and operational complexity, and its effect on downstream flow processes is not trivial. This paper reports on a study examining operation of a shock-heated secondary driver across the entire operating envelope of a free-piston-driven expansion tube, using air as the test gas. For high-enthalpy conditions it is confirmed that the secondary driver can provide a performance increase, and it is further shown how this device can be used to fine tune the flow condition even when the free-piston driver configuration is held constant. For low-enthalpy flow conditions, wave processes through the driven tube are too closely coupled, and the secondary driver no longer significantly influences the magnitude of the final test gas flow properties. It is found that these secondary driver operating characteristics depend principally on the initial density ratio between the secondary driver helium gas and the downstream test gas