Driver Gas Research Articles

Suppression of methane/air explosion by water mist with potassium halide additives driven by CO2

Abstract To enhance the explosion suppression effects of water mist, various potassium halide additives were tested in a confined vessel filled with a 10% mixture of methane/air. Air and CO2 (0.7 MPa) were used as driver gases. The results revealed that halide additives exhibit considerable suppression effects on explosion overpressure. A 30% KI mist decreased the explosion overpressure by 27.46% compared with the suppression by pure water mist under the same conditions. When CO2 is used as the driver gas, it will dissolve in water under high pressure. The synergistic effect of a CO2 solution with an effective additive afforded significant suppression. Under the same conditions, the overpressures suppressed by a mist of 30% KI + 0.7 MPa CO2 solution decreased by 33.53% compared with those suppressed by pure water mist driven by air. The synergistic suppression effect is much better than that of a 0.7 MPa CO2 solution mist or 30% KI mist alone. The multicomponent additives can be considered when suppressing methane/air explosions with pressure-formed water mist.

Ignition and detonation onset behind incident shock wave in the shock tube

The paper analyses in details and describes the process of ignition kernel formation and subsequent detonation onset behind the shock wave propagating in the shock tube. To get the overall pattern of the process a series of one- and two-dimensional calculations are carried out with the use of a dissipation-free numerical technique. It is shown that one of the leading roles in the process of ignition kernel formation belongs to the non-steady flow dynamics establishing behind the shock wave. The development of the boundary layer determines both the temperature re-distribution in the near-wall region and the conditions for gas-dynamic acceleration of the flow. With an account of thermal runaway, the most intensively heated region corresponds to the area between the inner margin of the boundary layer and the contact surface separating driver gas and the test mixture. After localized ignition takes place the forming reaction wave propagates out from the ignition epicenter. Reaction wave propagates behind the outrunning compression wave through the already reacting mixture. Shock-induced compression of the test mixture provides conditions for the self-sustained acceleration of the combustion wave, and finally, the detonation onset takes place.

Time-Resolved On-Axis Spectroscopic Stagnation Temperature Measurements in Shock Tunnel Flows

This paper demonstrates time-resolved stagnation temperature measurements in a shock tunnel at a frequency of 25 kHz using emission spectroscopy in air and nitrogen test conditions. The two most important parameters for determining the flow conditions generated in a shock tunnel experiment are the stagnation pressure and temperature of the flow just upstream of the supersonic nozzle. While the pressure can be measured using a wall-mounted transducer with relative ease, the measurement of the temperature requires a optical technique such as time resolved emission spectroscopy. Knowledge of the transient stagnation temperature behavior is critical to all subsequent expansion tube flow processes. The driver gas emission spectrum data at the post-shock condition shows continuum and atomic line radiation. The continuum radiation can be described by a black body radiator with the individual spectra showing sufficient continuum information for accurately fitting Planck functions. Atomic line radiation was excluded by skipping those data from the measured spectra. The fitting routine shows clear differences in determined temperatures including and neglecting atomic line radiation. These measurements allow for the exact determination of the shock tunnel flow conditions in combination with pressure transducer data. The flow condition used in the experiment corresponds to a nominal Mach-10 condition at an altitude of 65 km, however, the technique is not limited to this condition and can be used for a large range of flow conditions.

Convective heat-transfer rate and surface pressure distribution over a cone model at hypersonic speeds

In this article, experiments are carried out in a hypersonic shock tunnel with helium as driver gas and air as the test gas to obtain the convective heating rate and surface pressure distribution on a cone model placed at hypersonic speed. Test is performed in hypersonic shock tunnel for a flow Mach number of 6.5 at two different angles of attack, 0° and 5°. The sputtered thin film platinum sensors are used to measure the heat flux on a cone model. The measured heat-transfer rate compares well with theoretically estimated values using reference enthalpy method and computational fluid dynamics (CFD) simulation. The measured surface pressure compares well with CFD.

Numerical investigation of the impact of tailored driver gases and driver inserts on shock tube flows

Presented is a numerical study of the effects of imperfect driver gas tailoring and the use of a driver section insert on the flow field within the shock tube. Two-dimensional axisymmetric simulations of a shock tube are conducted with pure argon as the driven gas and four $$\hbox {He/N}_2$$ mixtures representing different tailoring conditions. The second part of the study investigates the use of a profiled driver insert with two $$\hbox {He/N}_2$$ driver gas mixtures to minimize the pressure rise in the test section such as that resulting from over-tailored driver mixtures. The chemical kinetic implications of imperfectly tailored driver gas mixtures on the interpretation of shock tube experiments are explored through simulations of propanol ignition with pressure and temperature changes prescribed based on the simulation results. The simulations show that imperfect tailoring toward under-tailored mixtures is more detrimental to kinetic studies in the post-reflected shock region than over-tailoring. The least compromising situation in test conditions is observed for a mildly over-tailored condition with 25% of nitrogen as opposed to the tailored value of 20%. The numerical results further show that the combination of the driver insert and slightly over-tailored driver mixture leads to longer test times. The study seeks to address problems of available test times and accuracy of shock tube reactor conditions that are crucial to the use of shock tube data for validation of chemical kinetic models.

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

An experimental investigation towards calibration of a shock tube and stagnation heat flux determination

This paper aims at comprehensive investigations towards design and installation of a moderate size shock tube (7 m long having an inner diameter of 55 mm, 20 mm thickness) and its experimental calibration through comparative parametric study. Necessary instrumentations are incorporated for measuring shock speeds and pressure rise across primary as well as reflected shock, obtained using nitrogen/helium as driver gas. The experimental evidence shows reasonable agreement between theoretical (one-dimensional shock tube relations) results. Further, an E-type coaxial surface junction thermocouple (CSJT) has been prepared in the laboratory and oil-bath calibration experiment gives its 'sensitivity'. Measurement of instantaneous surface temperature rise and subsequent stagnation heat flux are obtained using CSJT mounted on a specially-designed end-flange. The thermocouple noted a maximum rise of temperature of 7,800 K, marked as a characteristic constant of the sensor; since it is found to be independent of the magnitude of the step change in temperature.

X3 expansion tube driver gas spectroscopy and temperature measurements

The University of Queensland’s X3 facility is a large, free-piston driven expansion tube used for super-orbital and high Mach number scramjet aerothermodynamic studies. During recent development of new scramjet test flow conditions, experimentally measured shock speeds were found to be significantly lower than that predicted by initial driver performance calculations. These calculations were based on ideal, isentropic compression of the driver gas and indicated that loss mechanisms, not accounted for in the preliminary analysis, were significant. The critical determinant of shock speed is peak driver gas sound speed, which for a given gas composition depends on the peak driver gas temperature. This temperature may be inaccurately estimated if an incorrect fill temperature is assumed, or if heat losses during driver gas compression are significant but not accounted for. For this study, the ideal predicted peak temperature was 3750 K, without accounting for losses. However, a much lower driver temperature of 2400 K is suggested based on measured experimental shock speeds. This study aimed to measure initial and peak driver gas temperatures for a representative X3 operating condition. Examination of the transient temperatures of the driver gas and compression tube steel wall during the initial fill process showed that once the filling process was complete, the steady-state driver gas temperature closely matched the tube wall temperature. Therefore, while assuming the gas is initially at the ambient laboratory temperature is not a significant source of error, it can be entirely mitigated by simply monitoring tube wall temperature. Optical emission spectroscopy was used to determine the driver gas spectra after diaphragm rupture; the driver gas emission spectrum exhibited a significant continuum radiation component, with prominent spectral lines attributed to contamination of the gas. A graybody approximation of the continuum suggested a peak driver gas temperature of 3200 K; uncertainty associated with the blackbody curve fit is ±100 K. However, work is required to quantify additional sources of uncertainty due to the graybody assumption and the presence of contaminant particles in the driver gas; these are potentially significant. The estimate of the driver gas temperature suggests that driver heat losses are not the dominant contributor to the lower-than-expected shock speeds for X3. Since both the driver temperature and pressure have been measured, investigation of total pressure losses during driver gas expansion across the diaphragm and driver-to-driven tube area change (currently not accounted for) is recommended for future studies as the likely mechanism for the observed performance gap.

CFD Modelling of Flow Characteristics in Micro Shock Tubes

The use of micro shock tubes has become common in many instruments requiring a high velocity and temperature flow field, for example in micro-propulsion systems and drug delivery devices for medical systems. A shock tube has closed ends, and the flow is generated by the rupture of a diaphragm separating a driver gas at high pressure from a driven gas at relatively low pressure. The rupture results in the movement of a shock wave and contact discontinuity into the low-pressure gas, and an expansion wave into the high pressure gas. The characteristics of the resulting unsteady flow for micro shock tubes are not well known as the physics of such tubes includes additional phenomena such as rarefaction and complex viscous effects at low Reynolds numbers. In the present study, computational fluid dynamics (CFD) calculations are made for unsteady compressible flow within a micro shock tube using the van-Leer MUSCL scheme and the two-layer k-e turbulence model. Novel results have been obtained and discussed of the effects of using different diaphragm pressure ratios, shock tube diameters and wall boundary conditions, namely no slip and slip walls.

Test Gas Slug Characterization for Large-Scale Shock Tube Facility

An experimental methodology for characterizing the available test gas slug is presented for a gas-driven shock tube. The method employs simple models for theoretical and experimental estimates of the available test gas slug at the time of interaction of the contact surface with the reflected shock wave, avoiding any direct measurement of driver gas in the tube. The measure of this interaction length is estimated against the primary shock Mach number over the entire operating range of a large-diameter shock tube. The trends of driver gas mixing show a strong relation to the theoretical predictions. The trends also suggest that mixing of the contact surface gases in the large shock tube is independent of driver gas composition and could depend largely on diaphragm opening times. The same analysis is also used on a smaller-diameter shock tube, in which resulting trends are readily explained by the known behavior of the contact surface from previous studies. The larger tube data also indicate a laminar boundary layer growth proportionality which can be used to reduce data further. This leads to available test gas slug quantification as a function of the produced shock Mach number alone. The experimental measurement requirements for this method are that of a regular shock tube calibration setup with wall-mounted pressure sensors, which makes it very convenient to implement in any shock tube for preliminary analysis.

Validation of Laboratory Animal and Surrogate Human Models in Primary Blast Injury Studies.

Blast-induced neurotrauma has affected more than 300,000 service members. It is important to understand the effect of single and repeated shock-blast wave exposures on the neuropsychological behavior of soldiers, to offer them better protection, diagnostics, and treatment. Preclinical animal models and helmet design studies on human surrogate models have relied on the use of compression gas-driven shock tubes. Traditional shock tubes are so simple that if not carefully designed and operated, the test results can easily introduce detrimental artifacts clouding the conclusions. In this work, we present live-fire test results of an instrumented human surrogate head-neck model and compare with the data obtained in a carefully designed shock tube. We present various features incorporated in the shock tube design that led to better fidelity between live-fire and laboratory shock-blast conditions. The effect of specimen placement, choice of driver gas, pressure and volume of driver, end-plate conditions, and measurement techniques all determine the successful replication of live-fire loading conditions. These parameters become more important when conducting animal testing as the totality of loading will dictate the injury severity and type which ultimately will determine the mechanisms of blast-induced neurotrauma and hence their prevention and treatment strategies.

Shock Tube Ignition Delay Data Affected by Localized Ignition Phenomena

ABSTRACTShock tubes have conventionally been used for measuring high-temperature ignition delay times of approximately O(1 ms). In the last decade or so, the operating regime of shock tubes has been extended to lower temperatures by accessing longer observation times. Such measurements may potentially be affected by some non-ideal phenomena. The purpose of this work is to measure long ignition delay times for fuels exhibiting negative temperature coefficient and to assess the impact of shock tube non-idealities on ignition delay data. Ignition delay times of n-heptane and n-hexane were measured over the temperature range of 650–1250 K and pressures near 1.5 atm. Driver gas tailoring and long length of shock tube driver section were utilized to measure ignition delay times as long as 32 ms. Measured ignition delay times agree with chemical kinetic models at high (>1100 K) and low (<700 K) temperatures. In the intermediate temperature range (700–1100 K), however, significant discrepancies are observed between the measurements and homogeneous ignition delay simulations. It is postulated, based on experimental observations, that localized ignition kernels could affect the ignition delay times at the intermediate temperatures, which lead to compression (and heating) of the bulk gas and result in expediting the overall ignition event. The postulate is validated through simple representative computational fluid dynamic simulations of post-shock gas mixtures, which exhibit ignition advancement via a hot spot. The results of the current work show that ignition delay times measured by shock tubes may be affected by non-ideal phenomena for certain conditions of temperature, pressure, and fuel reactivity. Care must, therefore, be exercised in using such data for chemical kinetic model development and validation.

Optimum operating conditions for existing natural gas compression station

There has been continuous evolution of NG treatment processes to meet the ever– increasing demands of electricity generation and various manufacturing sectors. Our focal point is the booster compression station of an existing gas plant which consists of two centrifugal compressors. The objective of the presented work is to debottleneck both compressor trains to increase its capacity and to study the effect of the proposed modifications on the downstream dehydration package that using adsorption by silica gel fixed beds. The presented modifications are adding two slip steams to the gas/gas heat exchangers, the first one to be installed on tube side and the second one on shell side, each slip stream will accommodate 160 MMscf/d. These will give the compression station an important advantage as the capacity of each train will increase from 450 to 610 MMscf/d per train that means we can got a gain of 320 MMscf/d per both trains to be added to the national gas grid at Egypt. The adsorption performance changed after adding both slip streams where the operating parameters of the inlet gas have changed such as temperature and pressure. The simulations for both cases existing and modifying compression station were conducted using Aspen- HYSYS simulation program. The simulation results shows that fuel gas consumption of both compressors driver gas turbines has increased from 6880 to 10000 Kw that equal 45.0% equivalent to 2.25 MMscf/d (1.125 MMscf/d per each) compared to compressed gas capacity increment by 35.5 % that equal 320 MMscf/d. Adsorption calculation for hydration beds shows that pressure drop per each bed increased from 0.66 to 1.19 bar but still within limit (Max 1.25 bar) and adsorbed water for each bed has increased from 1260 to 2850 lb H2O that equal 1590 lb H2O or 126 % while the bed should hold 6500 lb H2O.

Expansion of capabilities of the short-duration wind tunnel with an opposing pressure multiplier

A method for raising the maximum settling-chamber pressure in a short-duration wind tunnel equipped with pressure multipliers arranged in opposition to each other for stabilization of test gas parameters is proposed. For this purpose, a wind-tunnel design with an additional third pressure multiplier attached to the body of the second pressure multiplier was developed. The rod of the additional multiplier contacts the large-area piston stage of the second multiplier, and the pre-piston space being connected to the receiver. The inclusion of an additional pressure multiplier in the wind-tunnel design at the maximum attainable driver-gas pressure of 150−170 bar, defined by the standard industrial pressure of air used for filling wind-tunnel receivers with the driver gas, allows a two-fold increase in the maximum settling-chamber pressure, from 1100 to 2000−2200 bar. For raising the maximum settling-chamber pressure above 2000–2200 bar, the use of one additional pressure multiplier proved to be insufficient because, in the latter case, its becomes necessary to simultaneously raise the driver-gas pressure over 150−170 bar.

Tailoring the Blast Exposure Conditions in the Shock Tube for Generating Pure, Primary Shock Waves: The End Plate Facilitates Elimination of Secondary Loading of the Specimen.

The end plate mounted at the mouth of the shock tube is a versatile and effective implement to control and mitigate the end effects. We have performed a series of measurements of incident shock wave velocities and overpressures followed by quantification of impulse values (integral of pressure in time domain) for four different end plate configurations (0.625, 2, 4 inches, and an open end). Shock wave characteristics were monitored by high response rate pressure sensors allocated in six positions along the length of 6 meters long 229 mm square cross section shock tube. Tests were performed at three shock wave intensities, which was controlled by varying the Mylar membrane thickness (0.02, 0.04 and 0.06 inch). The end reflector plate installed at the exit of the shock tube allows precise control over the intensity of reflected waves penetrating into the shock tube. At the optimized distance of the tube to end plate gap the secondary waves were entirely eliminated from the test section, which was confirmed by pressure sensor at T4 location. This is pronounced finding for implementation of pure primary blast wave animal model. These data also suggest only deep in the shock tube experimental conditions allow exposure to a single shock wave free of artifacts. Our results provide detailed insight into spatiotemporal dynamics of shock waves with Friedlander waveform generated using helium as a driver gas and propagating in the air inside medium sized tube. Diffusion of driver gas (helium) inside the shock tube was responsible for velocity increase of reflected shock waves. Numerical simulations combined with experimental data suggest the shock wave attenuation mechanism is simply the expansion of the internal pressure. In the absence of any other postulated shock wave decay mechanisms, which were not implemented in the model the agreement between theory and experimental data is excellent.

Development of a novel miniature detonation-driven shock tube assembly that uses in situ generated oxyhydrogen mixture.

A novel concept to generate miniature shockwaves in a safe, repeatable, and controllable manner in laboratory confinements using an in situ oxyhydrogen generator has been proposed and demonstrated. This method proves to be more advantageous than existing methods because there is flexibility to vary strength of the shockwave, there is no need for storage of high pressure gases, and there is minimal waste disposal. The required amount of oxyhydrogen mixture is generated using alkaline electrolysis that produces hydrogen and oxygen gases in stoichiometric quantity. The rate of oxyhydrogen mixture production for the newly designed oxyhydrogen generator is found to be around 8 ml/s experimentally. The oxyhydrogen generator is connected to the driver section of a specially designed 10 mm square miniature shock tube assembly. A numerical code that uses CANTERA software package is used to predict the properties of the driver gas in the miniature shock tube. This prediction along with the 1-D shock tube theory is used to calculate the properties of the generated shockwave and matches reasonably well with the experimentally obtained values for oxyhydrogen mixture fill pressures less than 2.5 bars. The miniature shock tube employs a modified tri-clover clamp assembly to facilitate quick changing of diaphragm and replaces the more cumbersome nut and bolt system of fastening components. The versatile nature of oxyhydrogen detonation-driven miniature shock tube opens up new horizons for shockwave-assisted interdisciplinary applications.

A gasdynamic gun driven by gaseous detonation.

A gasdynamic gun driven by gaseous detonation was developed to address the disadvantages of the insufficient driving capability of high-pressure gas and the constraints of gunpowder. The performance of this gasdynamic gun was investigated through experiments and numerical simulations. Much more powerful launching capability was achieved by this gun relative to a conventional high-pressure gas gun, owing to the use of the chemical energy of the driver gas. To achieve the same launching condition, the initial pressure required for this gun was an order of magnitude lower than that for a gun driven by high-pressure H2. Because of the presence of the detonation, however, a more complex internal ballistic process of this gun was observed. Acceleration of projectiles for this gun was accompanied by a series of impulse loads, in contrast with the smooth acceleration for a conventional one, which indicates that this gun should be used conditionally. The practical feasibility of this gun was verified by experiments. The experiments demonstrated the convenience of taking advantage of the techniques developed for detonation-driven shock tubes and tunnels.

Strategies for obtaining long constant-pressure test times in shock tubes

Several techniques have been developed for obtaining long, constant-pressure test times in reflected shock wave experiments in a shock tube, including the use of driver inserts, driver gas tailoring, helium gas diaphragm interfaces, driver extensions, and staged driver gas filling. These techniques are detailed here, including discussion on the most recent strategy, staged driver gas filling. Experiments indicate that this staged filling strategy increases available test time by roughly 20 % relative to single-stage filling of tailored driver gas mixtures, while simultaneously reducing the helium required per shock by up to 85 %. This filling scheme involves firstly mixing a tailored helium–nitrogen mixture in the driver section as in conventional driver filling and, secondly, backfilling a low-speed-of-sound gas such as nitrogen or carbon dioxide from a port close to the end cap of the driver section. Using this staged driver gas filling, in addition to the other techniques listed above, post-reflected shock test times of up to 0.102 s (102 ms) at 524 K and 1.6 atm have been obtained. Spectroscopically based temperature measurements in non-reactive mixtures have confirmed that temperature and pressure conditions remain constant throughout the length of these long test duration trials. Finally, these strategies have been used to measure low-temperature n-heptane ignition delay times.

Free-piston driver performance characterisation using experimental shock speeds through helium

Tuned free-piston driver operation involves configuring the driver to produce a relatively steady blast of driver gas over the critical time scales of the experiment. For the purposes of flow condition development and parametric studies, it is useful to establish some average working values of the driver pressure and temperature for a given driver operating condition. However, in practise, these averaged values need to produce sufficiently accurate estimates of performance. In this study, two tuned driver conditions in the X2 expansion tube have been used to generate shock waves through a helium test gas. The measured shock speeds have then been used to calculate the effective driver gas pressure and temperature after diaphragm rupture. Since the driver gas is typically helium, or a mixture of helium and argon, and the test gas is also helium, ideal gas assumptions can be made without significant loss of accuracy. The technique is applicable to tuned free-piston drivers with a simple area change, as well as those using orifice plates. It is shown that this technique can be quickly used to establish average working driver gas properties which produce very good estimates of actual driven shock speed, across a wide range of operating conditions. The use of orifice plates to control piston dynamics at high driver gas sound speeds is also discussed in the paper, and a simple technique for calculating the restriction required to modify an established safe condition for use with lighter gases, such as pure helium, is presented.

気体デトネーション駆動型ガス銃を用いた飛行体加速実験

The gaseous detonation driven gas gun was developed for accelerating the projectile to a supersonic speed. The gas gun was simply consisted of two straight stainless-steel tubes. The one was the detonation tube and the other was the launch tube. The detonation tube was 50 mm inside diameter with 2180 or 4280 mm long, and the launch tube was 5 mm inside diameter with 1040 mm long. Chapman-Jouguet detonation wave was initiated in the detonation tube, and the projectile was accelerated in the launch tube via combustion products behind the detonation wave. The spherical projectile of 4.76 mm diameter was made of high-density polyethylene with 52 mg mass. The driver mixture was stoichiometric hydrogen-oxygen premixed gas with initial pressure ranging from 120 to 450 kPa. The gas gun was successfully operated, and the maximum projectile velocity of 1400 m/s was obtained for the conditions that the detonation tube was 4280 mm long and the initial pressure of the driver gas was 450 kPa. The results of the longer detonation tube demonstrated that the projectile velocity was 1.15 - 1.25 times higher than the case of shorter detonation tube. This velocity change of the projectile could be explained by the pressure increase at the inlet of the launch tube by using longer detonation tube. The reason of the pressure increase has a possibility that the length of Taylor wave behind the detonation wave becomes longer for the case of longer detonation tube.