Conventional Shock Tube Research Articles

Characterization of a large caliber explosively driven shock tube.

Research on evaluating weapon systems, building structures, and personnel protection has attracted considerable attention due to the high incidence of blast accidents. The explosively driven shock tube is an affordable and replicable method for investigating high pressure blast waves and extreme shock environments. A newly constructed large caliber explosively driven shock tube with an inner diameter of 2.5m and a length of 18m has been documented and characterized in this paper. It is capable of providing a peak pressure of at least 5.49 MPa in the test section with 160kg of TNT charges. The tube can produce an overpressure that is significantly higher than conventional shock tubes, which expands the capability to simulate a high overpressure blast load. A two-dimensional axisymmetric simulation model has been developed, validated, and calibrated for the characterization of the flow field inside the shock tube. The influence of the charge mass on the overpressure, arrival time, and positive impulse was discussed, and the planarity of the shock wave was also quantitatively characterized. To aid in designing further shock experiments and applications, a physics-based prediction model was developed using the dimensional analysis.

Measurements of ignition delay time of gas-to-liquid (GTL) fuel blends

The ignition delay time (IDT) is a fundamental combustion parameter of any fuel, which has a direct impact on its combustion and exhaust emissions in diesel engines. In this research, a thorough experimental study was conducted to investigate the ignition delay of Gas-to-Liquid (GTL) fuel and its 50%–50 % blends with diesel under a wide range of operating conditions using a conventional shock tube apparatus. Simultaneously, a comprehensive chemical kinetic model, based on the well-established POLIMI mechanism, was developed to simulate the complex combustion chemistry of diesel, GTL, and their blends. Several loading pressures in the driver section ranging from 6 to 14 bar that results in reflected pressure of 8–16 bar, and equivalence ratios ranging from 0.5 to 1.3 as well as different initial temperatures have been examined. Calibration of the shock tube indicated that a driver gas tailoring technique was necessary to be used, which was conducted by adding a small ratio of either argon or nitrogen to helium to get an extended test time by 32 % with a good rapture pressure. IDT measurements showed that GTL has the shorter ignition delay time than that of diesel fuel for all conditions. The findings show that IDT measurements for GTL fuel has a shorter ignition delay time than that of diesel fuel for all conditions. Under stoichiometric conditions (ɸ = 1.0), IDT of the blended fuel was about 3 ms, very close to that of GTL at 975 K. However, at a temperature above 1000 K, the IDT of the three fuels were almost identical and ranging between 1 and 2 ms. At a temperature of 1100 K, the IDT of the blended fuel was 5.5 ms, almost a middle value between those of the pure diesel, 0.522 ms, and GTL fuel, 0.495 ms. The shortest IDT of the blended fuel was about 0.4 ms with the highest value of the reflected pressure of 14 bar, while the longest IDT was around 10 ms with the rich mixture at ɸ = 1.3. The study can add to the existing literature illustrating the behavior of GTL-diesel blend can affect IDT.

Methane and n-hexane ignition in a newly developed diaphragmless shock tube

Shock tubes have been routinely used to generate reliable chemical kinetic data for gas-phase chemistry. The conventional diaphragm-rupture mode for shock tube operation presents many challenges that may ultimately affect the quality of chemical kinetics data. Numerous diaphragmless concepts have been developed to overcome the drawbacks of using diaphragms. Most of these diaphragmless designs require significant alterations in the driver section of the shock tube and, in some cases, fail to match the performance of the diaphragm-mode of operation. In the present work, an existing diaphragm-type shock tube is retrofitted with a fast-acting valve, and the performance of the diaphragmless shock tube is evaluated for investigating the ignition of methane and n-hexane. The diaphragmless shock tube reported here presents many advantages, such as eliminating the use of diaphragms, avoiding substantial manual effort during experiments, automating the shock tube facility, having good control over driver conditions, and obtaining good repeatability for reliable gas-phase chemical kinetic studies. Ignition delay time measurements have been performed in the diaphragmless shock tube for three methane mixtures and two n-hexane mixtures at P5 = 10–20 bar and T5 = 738–1537 K. The results obtained for fuel-rich, fuel-lean, and oxygen-rich (undiluted) mixtures show very good agreement with previously reported experimental data and literature kinetic models (AramcoMech 3.0 [1] for methane and Zhang et al. mechanism [2] for n-hexane). The study presents an easy and simple method to upgrade conventional shock tubes to a diaphragmless mode of operation and opens new possibilities for reliable chemical kinetics investigations.

Measurements of propane–O2–Ar laminar flame speeds at temperatures exceeding 1000 K in a shock tube

Laminar flame speed (SL) measurements of stoichiometric propane in an oxygen-argon oxidizer were performed in a shock tube at unburned-gas temperatures of 296–1234 K and near-atmospheric pressures. Non-intrusive laser-induced breakdown is used to ignite expanding flames following the reflected-shock passage. Flame propagation is recorded using schlieren imaging in a recently implemented side-wall imaging flame test section (SWIFT). In a refined approach to account for flame distortion and the slight residual motion of the post-reflected-shock gas, an area-averaged formulation of the linear-curvature model (the AA-LC model) is derived for use extrapolating flame data to zero stretch. Measured SL values extracted using the AA-LC model closely agree with previous experimental measurements performed in a conventional kinetics shock tube (CKST) using much smaller flame kernels, providing evidence of the earlier data having been ignition affected. Below the chemistry-affected limit of 1050 K, experimental SL values fall in the range of values simulated with the detailed AramcoMech 3.0 kinetic mechanism and propane-specific mechanisms from NUIG and San Diego but exhibit a stronger temperature dependence than predicted by the mechanisms. Over the wide temperature range of the present data, the ubiquitous power-law form of empirical fit is shown to be inadequate for capturing the SL temperature dependence; a non-Arrhenius form is shown to perform favorably. The uncertainties of flame speed measurements performed in the SWIFT average 3.0% and 4.4% for experiments performed under static and post-reflected-shock conditions, respectively, a reduction from the 5.8% average uncertainty of CKST experiments. This work represents a significant step forward in the development of experimental capabilities for high-temperature flame speed measurements. The present results illustrate the potential value of the shock-tube flame speed method to provide measurements useful for informing kinetic model tuning and validation at conditions for which experimental data were not previously obtainable.

Experiments on the single-mode Richtmyer–Meshkov instability with reshock at high energy densities

The hydrodynamic instability growth of a reshocked single-mode interface between high energy density fluids is studied. A laser-driven shock wave is used to drive an initially solid, sinusoidal interface between a dense plastic (1.43 g/cc) and a light foam (≈ 0.110 g/cc). After the interface has grown to a nonlinear state where the amplitude is of order of the wavelength, it is reshocked. The reshock compresses the nonlinear perturbation, which then grows at about twice the rate. While the pre-reshock growth rate is sensitive to the initial amplitude and wavelength of the perturbation, the post-reshock growth rate is comparatively insensitive to the initial condition. Qualitatively, we observe that the perturbations are less coherent after reshock, consistent with the idea that having a reshock accelerates the transition to turbulence. We find that some memory of the initial condition remains, even after reshock at late time: it appears if the initial perturbations have large enough wavelengths, and the flow structure of size comparable to the initial wavelength persists through reshock. Our results agree with design simulations and are consistent with the phenomenology of reshock studies in conventional gaseous shock tubes.

Experimental generation of spherical converging shock waves

Spherical converging shock waves with incoming Mach number 1.18 are generated in a conical test chamber fitted to a conventional square-section shock tube. A wave cutter is employed to ensure a proper shock transition from the square to a cylindrical straight pipe installed upstream of the convergent section. The incident planar wave is transformed into a spherical shock by the use of an ellipsoidal membrane acting as a gas lens separating two gases with different densities. The transmitted shock wave propagates within the conical section, and accelerates towards the apex. The trajectory and the shape of the shock wave are both characterized by planar Mie scattering. The spherical shock follows the similarity solution proposed by Guderley (1942), whose trajectory agrees very well with the numerical simulation of the Chester-Chisnell-Whitham (CCW) theory for the geometry considered. The procedure demonstrated here provides a potential method for future investigations of shock focusing and Richtmyer–Meshkov instability in spherical geometry.

Diaphragmless shock tube for primary dynamic calibration of pressure meters

In conventional shock tubes with a diaphragm many effects related to the burst of the diaphragm can influence the shock formation and thus prevent an ideal pressure step change predicted by the shock tube measurement model being generated. This paper presents a newly developed diaphragmless shock tube, in which a diaphragm is replaced with a quick-acting pneumatic valve. The developed shock tube has a capability to generate pressure steps calculable from its measurement model with a relative expanded uncertainty of less than 0.025, which can be used as the input signal in primary calibrations of pressure meters.

Thermal-pyrolysis induced over-driven flame and its potential role in the negative-temperature dependence of iso-octane flame speed at elevated temperatures

Recently, high-temperature flame speed measurement up to 900 K has been successfully realized at Stanford University using a conventional shock tube, assisted by laser ignition and chemiluminescence image systems. For a near stoichiometric iso-octane/O2/N2/He mixture at atmospheric pressure, their results show that while flame speed increases with increasing unburnt temperature in the low and high-temperature range, it decreases with increasing unburnt temperature in the intermediate temperature regime (750–800 K). The measurement time for the flame initiation and propagation is orders of magnitude shorter than the first-stage ignition delay time, implying that the unburnt mixture upstream of the flame is largely chemically frozen. Therefore, such non-monotonic behavior in the flame speed is unlikely to be induced by autoignition chemistry. To understand this interesting phenomenon, 1-D transient simulation of spherical flame initiation and propagation with different ignition energy has been performed and compared with the experiment data. It is shown that for the relatively low ignition energy 7.5 mJ, an over-driven flame regime characterized by flame speeds and stretch rates well above the steady-state values occurs in the intermediate temperature range. The chemical structure of an over-driven flame involves substantial fuel thermal decomposition in the preheat zone, followed by C1-C4 oxidation in the reaction zone. It is found that the non-monotonicity in the observed flame speed is strongly correlated with the occurrence of over-driven flames in simulations. These results demonstrate a new flame structure which can potentially explain the observed non-monotonic flame speed for iso-octane at high temperatures. Higher ignition energy or smaller ignition kernel is suggested in future experiments to mitigate such over-driven effects.

Characterization of a newly developed diaphragmless shock tube for the primary dynamic calibration of pressure meters

In conventional shock tubes with a diaphragm, many effects related to the burst of the diaphragm can influence shock formation, thus preventing an ideal pressure step change, as predicted by the shock tube measurement model being generated. This paper presents a newly developed diaphragmless shock tube with a quick-acting pneumatic valve, which demonstrates several advantages over conventional shock tubes with a diaphragm. The tests of the developed diaphragmless shock tube were performed using nitrogen at the atmospheric initial driven pressure, and at initial driver gauge pressures from 1 MPa to 7 MPa. The results show that the construction of the developed shock tube effectively reduces the generation of secondary shock waves and mechanical vibrations, which are often generated during the operation of shock tubes, thereby increasing the uncertainty of the shock tube calibration method. The uncertainty analysis shows that the developed diphragmless shock tube can act as a primary time-varying pressure calibration standard, generating pressure steps with a magnitude calculable from its measurement model to characterize the dynamic performance of pressure transducers with a relative expanded uncertainty of less than 0.025.

A pyrolysis study of allylic hydrocarbon fuels

AbstractThe pyrolysis of selected C3–C5 allylic hydrocarbons has been studied using a single‐pulse shock tube. A new single‐pulse shock tube has been designed and constructed by recommissioning an existing conventional shock tube. This facility enables the investigation of high‐temperature chemical kinetics with an emphasis on combustion chemistry. The modifications performed on the existing shock tube are described, and the details of the sampling system to analyze the species concentration using a gas chromatography‐mass spectrometry‐flame ionization detection (GC‐MS with a flame ionization detector) system are also provided. This facility is characterized and validated by performing cyclohexene pyrolysis experiments. Furthermore, the performance of the shock tube is demonstrated by reproducing previous literature measurements on the pyrolysis of isobutene. Postvalidation, this setup is used to study the pyrolysis of trans‐2‐butene and 2‐methyl‐2‐butene (2M2B). A newly developed mechanism, NUIGMech1.0, is used to simulate the experimental data of propene, isobutene, 2‐butene, and 2M2B, allylic hydrocarbon fuels. A description using two different kinetic simulation approaches is provided using our isobutene experiments as a reference. We found no significant differences between the two methods. Additionally, the contribution of different reaction classes on fuel consumption is detailed and the influence of geometry on fuel consumption and first aromatic ring: benzene is discussed.

Analysis of nitrogen recombination activity on silicon dioxide with stagnation heat-transfer

Investigation of surface recombination activity of nitrogen on certain materials is crucial in the design of hypersonic re-entry vehicles. In the present work, an experimental study using a conventional shock tube facility has been conducted to investigate the phenomena of nitrogen recombination. Stagnation heat-transfer measurements were conducted behind reflected shock waves in a dissociated shock-tube flow for a mixture of 20% nitrogen and 80% argon. A platinum thin-film gauge was used to measure the stagnation heat-transfer in a shock heated gas mixture under a well-defined test condition. The surface of the heat-transfer gauge was coated with silicon dioxide (SiO2), the principal material used in thermal protection systems (TPS) for the hypersonic vehicles. Spectroscopic analyses were performed for in-depth characterization of the initial surface condition of the test specimen. Recombination activity behavior of nitrogen was evaluated by combining the heat-transfer data with catalytic heat transfer theories. The nitrogen recombination efficiency of silicon dioxide is found to be from 0.0026 to 0.035 at surface roughness factors of 1.01 to 1.10. Compared to the oxygen recombination activity, the nitrogen recombination activity showed a more than five times higher sensitivity to the roughness factor of the SiO2-coated surface.

On using converging shock waves for pressure amplification in shock tubes

While conventional shock tubes have a distinct advantage in dynamic calibration methods due to their inherent capability to generate pressure pulses of desired amplitude and fast rise time, they are limited to the lower levels of the pressure amplitude realization range (≤7 MPa). With the increasing need for a traceable dynamic calibration standard across wider pressure ranges, a novel technique using converging shock waves is demonstrated in this work that pushes the upper limit of the standard shock tube into the medium-high pressure range. The experiments are conducted in the shock tube facility equipped with a test section that smoothly transforms the incident plane shock into a spherical shock wave that converges, accelerates and thereby amplifies its strength many folds. The experimentally recorded pressure traces are compared with numerical simulations performed by an in-house code. Using this technique, pressure pulses with peak amplitudes in the range of 30–40 MPa, with 3.4% uncertainty based on numerical reference profile, were realized in the test section with nominal usage of resources.

Impulse Loading of Plates using a Diverging Shock Tube

Impulsive blast loading studies typically require facilities with sub-millisecond decay time simulation capability. Using a conventional shock tube for such studies, however, is not viable due to a finite shock formation distance that limits the size of the shortest, functional driven tube. To overcome this, a diverging (conical) shock tube has been proposed in this work, motivations for which are grounded in simple shock tube theory. Using such a shock tube, for the first time, blast pulses having decay times of around 0.7 ms have been achieved repeatably (± 5.5%). Subsequently, blast loading experiments (equated to 30 − 40 g TNT surface explosion at 0.5 m stand-off distance) carried out on metal plates showed that this device can replicate the deflection patterns of an impulsive blast loading which could not be obtained so far using conventional shock tubes.

The interaction of a cylindrical shock wave segment with a converging–diverging duct

In this study, we present the results of an investigation of the propagation of cylindrical shock wave segments in converging–diverging channels. Three planar-symmetric channels were used: two formed from a pair of walls with circular wall profiles (radii 150 mm and 225 mm) and a third following a third-order-polynomial profile. In contrast to the circular walls, which are convex, the polynomial wall has both convex and concave curved sections. A plane shock was generated using a conventional shock tube after which a 165-mm-radius cylindrical shock segment was formed by allowing the plane shock to pass through a circular arc-shaped annular space. This shock was then allowed to propagate in the converging–diverging channel formed from the two walls described earlier. The resulting shock evolution was captured using a z-type schlieren technique. Whitham’s geometric shock dynamics (GSD) and computational fluid dynamics (CFD) were used to create numerical models of the shock’s propagation. Comparisons between the three methods (experiment, CFD, and GSD) were made. In general, qualitative agreements between the three methods were observed (with slight discrepancies). For example, a shock with an initial Mach number of 1.37 interacting with a 150-mm-radius wall exhibited high curvature at the shock’s central position towards the channel’s throat (as observed in experiment), an observation which was not replicated by either CFD or GSD. Quantitatively, there were significant differences (before accounting for experimental errors). On comparing centreline shock Mach numbers between the three methods, CFD results were closer to experimental results, while GSD results were consistently higher but within the experimental data error bounds. However, the general trend was the same in all three, i.e., the shock strengthens and weakens in the converging and diverging sections, respectively.

Shock tube study of normal heptane first-stage ignition near 3.5 atm

Shock tube ignition delay times and species time history measurements for Primary Reference Fuels (PRFs) such as normal heptane provide targets for the validation of combustion models, which in turn are used to develop more fuel-efficient engines that have smaller environmental footprints. However, a review of the literature has revealed that most of the shock tube ignition delay time and species measurement data for normal heptane have been obtained at elevated pressures, rather than at relatively low pressures where many other important experimental techniques such as jet-stirred reactors and flow reactors can provide corroborating results. One central problem preventing previous shock tube studies from examining first-stage ignition at these lower pressures was that ignition times were too long under these conditions to be measured within the available shock tube test times. To address this issue, recent advances in shock tube techniques for achieving long uniform test times have been applied in order to measure low-pressure first-stage ignition times of normal heptane together with normal heptane fuel time-history records at times up to about 30 ms. These measurements were performed in the Negative Temperature Coefficient (NTC) region (T=664−792 K) in lean mixtures (21%O2/Ar, equivalence ratio ϕ=0.5) at pressures of roughly P=3.5 atm using both the conventional and Constrained Reaction Volume (CRV) shock tube filling strategies. The data have been used to evaluate the performance of several combustion models, have been compared with other higher-pressure shock tube first-stage ignition times in n-heptane/20–21%O2 mixtures found in the literature, and have been fit using a two-zone Arrhenius model for first-stage ignition delay times. The results showed that some combustion models yield ignition delay time predictions that differ from the experimental results by as much as an order of magnitude, that under these conditions n-heptane first-stage ignition times scale by roughly P−0.71 but are insensitive to ϕ, and that the fraction of fuel remaining after first-stage ignition increases with increasing initial experimental temperature.

Hand-operated shock tube and hypersonic shock tunnel

This manuscript outlines the invention of a miniaturized version of the conventional shock tube and hypersonic shock tunnel that is small enough to be mounted on a table top The shock tube named as Reddy tube works solely with the effect of human force and the driver gas is pressurized by manual action of a plunger rod Test can be conducted within a shock Mach number range of ndash The extension of the Reddy tube into a table top single pulsed hypersonic shock tunnel named as Reddy tunnel proves to be an important input into the field as it can be established as a fraction of the cost of a full fledged facility thereby providing access to such experiments to virtually any person in the experimental aerodynamic community

A combined laser absorption and gas chromatography sampling diagnostic for speciation in a shock tube

The first implementation of a combined laser absorption diagnostic/gas chromatography (GC) sampling system for the measurement of combustion-relevant species in a conventional shock tube configuration is reported, with ethylene pyrolysis as an example application. A heated, endwall sampling system is used to extract a post-shock sample for GC analysis. Analysis of the gas sample yields a measurement of the ultimate mole fraction values of multiple species (currently ethylene, acetylene, hydrogen, and methane) at the end of the reflected shock test time. A 10.532-µm laser absorption diagnostic is simultaneously used to measure time-resolved ethylene. A method to accurately model sampled speciation results using published kinetic models is discussed. A method for extending laser measurements into the expansion fan region for direct comparison with sampled GC results has also been developed. The combined optical and sampled-gas measurement techniques were used to study ethylene pyrolysis (1.0% mole fraction ethylene/argon) at approximately 5 atm, over a range of temperatures (1200–2000 K). The ethylene mole fraction measurements obtained using both techniques show close agreement.

Note: A contraction channel design for planar shock wave enhancement.

A two-dimensional contraction channel with a theoretically designed concave-oblique-convex wall profile is proposed to obtain a smooth planar-to-planar shock transition with shock intensity amplification that can easily overcome the limitations of a conventional shock tube. The concave segment of the wall profile, which is carefully determined based on shock dynamics theory, transforms the shock shape from an initial plane into a cylindrical arc. Then the level of shock enhancement is mainly contributed by the cylindrical shock convergence within the following oblique segment, after which the cylindrical shock is again "bent" back into a planar shape through the third section of the shock dynamically designed convex segment. A typical example is presented with a combination of experimental and numerical methods, where the shape of transmitted shock is almost planar and the post-shock flow has no obvious reflected waves. A quantitative investigation shows that the difference between the designed and experimental transmitted shock intensities is merely 1.4%. Thanks to its advantage that the wall profile design is insensitive to initial shock strength variations and high-temperature gas effects, this method exhibits attractive potential as an efficient approach to a certain, controllable, extreme condition of a strong shock wave with relatively uniform flow behind.

Nonlinear growth of the converging Richtmyer-Meshkov instability in a conventional shock tube

The cylindrical Richtmyer-Meshkov (RM) instability is studied in a shock tube. The growth rate of this instability does not saturate in the nonlinear regime as it does in the planar geometry. Numerical and theoretical studies show that a Rayleigh-Taylor instability superimposes on the RM one and that Bell-Plesset effects enhance the growth of the instability.

Shock wave attenuation by geotextile encapsulated sand barrier systems

This paper presents a laboratory scale experimental technique to study the performance of the encapsulated sand barrier systems in mitigating shock waves. The geotextile encapsulated sand barrier systems are made of cubical wire mesh formwork lined with geotextile and form a thick protective barrier when filled with granular materials. In the present study, dry sand particles of size varying from microns to few millimeters (fine and coarse) are used as infill granular material. Spherical shaped glass beads are also used as the infill material to study the influence of shape of the infill particle on the attenuation behavior. The process of shock wave attenuation by the sand barrier, with and without the geotextile facing formwork is examined. The experiments are performed using a conventional shock tube, where shock waves with incident Mach number in the range of 1.29–1.70 are generated. The experimental results show that the presence of geotextile layer has contributed significantly towards shock wave attenuation. The geotextile also plays an important role as a regulator, which is able to deliver gradual pressure rise at the downstream end of the barrier.