Simultaneous High-speed Schlieren Research Articles

Fluid-Structure Interactions of a Fin over a Compliant Panel in Supersonic Flow

The fluid-structure interactions of a blunt fin mounted over a compliant panel in Mach 2 flow were investigated. The test geometry consisted of a thin, flexible panel fixed on all edges with a blunt fin with a hemispherically rounded leading edge mounted in a manner that the leading edge of the fin overhung the panel. The blunt fin was also pivoted to several angles of attack with respect to the oncoming flow. This allowed for the study of the dynamic system created by the interactions between the compliant panel and the blunt fin geometries. Both rigid and compliant panel models were used in order to provide a comparison between the flow with and without the dynamics of the compliant panel. High-speed schlieren and surface oil flow visualizations showed that the time-averaged flow remained almost entirely unchanged between the rigid and compliant panels. The instantaneous flow fields, however, showed a highly dynamic system where the compliant panel induced oscillatory shock waves that were both stationary and freely moving. Simultaneous high-speed schlieren visualization and stereo digital-image correlation showed a strong linkage between the motion of the shock waves and the deformation of the panel. This was demonstrated through frequency analysis and modal decomposition of the panel deformation and shock motions.

High spatiotemporal resolution optical measurements of two-stage ignition and combustion in Engine Combustion Network Spray D flames

This study explores flame structures and combustion dynamics in high-pressure n-dodecane fuel sprays, focusing on the formation and consumption of formaldehyde (CH2O) during autoignition and the development of poly-aromatic hydrocarbons (PAH) as soot precursors. These processes are crucial for optimizing combustion efficiency and reducing emissions. However, traditional approaches, which rely on single-shot measurements or ensemble-averaged visualizations, often overlook critical early-stage processes during low-temperature ignition. To overcome these challenges, we employed an innovative high-speed planar laser-induced fluorescence (PLIF) technique at 50 kHz using a pulse-burst Nd:YAG laser system with an excitation wavelength of 355 nm. This approach, applied for the first time to Engine Combustion Network (ECN) Spray D flames, provides unprecedented insights into the combustion processes at varying ambient temperatures and oxygen concentrations. Additionally, simultaneous high-speed schlieren imaging at 100 kHz was used to visualize spray penetration, first-stage ignition, and thermal expansion zones. Our findings reveal that, similar to Spray A flames, CH2O forms in cold, fuel-rich zones well upstream of the combustion zone. However, in Spray D flames, the schlieren signal softening observed in the jet's head does not lead to complete disappearance, and the CH2O signal is absent from the full head of the spray. During the second-stage ignition, CH2O consumption accelerates due to high-temperature reactions, leading to a significant reduction in its signal. Unlike the mushroom-shaped structure seen in Spray A flames, Spray D flames exhibit a quasi-steady PAH phase structure, with lean peripheral mixtures insufficient for soot precursor formation. Notably, reducing ambient oxygen concentration to 13 % while maintaining or increasing temperature prolongs the presence of CH2O, highlighting its influence on ignition dynamics and oxidation processes in dodecane spray flames. This study provides new insights into the combustion mechanisms of high-pressure sprays and offers valuable data for developing next-generation combustion technologies, including models, aimed at improving efficiency and reducing emissions.

Development of Polyamine/Alkanolamine-based Hypergolics with hydrogen Peroxide: A new route to N-methylimidazole with MDEA as a promising green fuel

A novel non-ionic liquid fuel based on a mixture of N-methylimidazole (MIm) and N-methyldiethanolamine (MDEA) called PAHyp 2 which reacts hypergolically with hydrogen peroxide is presented in this work. To study the hypergolic behaviors, a drop test setup able of capturing simultaneous high-speed schlieren, infrared and visible imaging was used. While infrared images capture the temperature history of the ignition process, schlieren images offer advantages of capturing the pressure waves as well as providing better visualization of the morphological description of the liquid dynamics. PAHyp 2 with hydrogen peroxide has specific impulse 6% lower than hydrazine-based fuels but because of its higher density it requires smaller propellant tanks for a given mission. In order to meet the requirements of good chemical stability and low viscosity of<30 cP, the proportion of MIm should range between 40 and 50% (or 60 to 50% of MDEA) and the amount of catalyst should be no more than 3 wt%. FTIR spectra showed no chemical degradation. By adding 3 wt% catalyst, the shortest ignition delay time of 8.8 ms was recorded. Based on these preliminary results, we believe that just like the other polyamiane/akanolamine-based fuels recently reported by our group, this new recipe is also a promising candidate to replace conventional toxic propellants and as such should be further studied.

Ignition of ultra-lean premixed hydrogen/air by an impinging hot jet

The ignition characteristics of a hot turbulent jet impinging on a flat plate surrounded by an ultra-lean premixed H2/air was studied both experimentally and numerically. The hot turbulent jet was generated by burning a small quantity of stoichiometric H2/air mixture in a separate small volume called the pre-chamber. The higher pressure resulting from pre-chamber combustion pushed the combustion products into the main chamber through a small nozzle (0.75–4.5 mm in diameter) in the form of a hot turbulent jet, which then impinged on a flat plate. Six different plates with varying impinging heights and angles were used. Two important parameters controlling the impinging characteristics of the jet, the ratio of the impinging distance to the nozzle diameter, H/D and the impinging angle, θ were examined. Simultaneous high-speed Schlieren and OH∗ chemiluminescence imaging were applied to visualize the jet penetration/impinging and ignition process inside the main combustion chamber. Results illustrate the existence of two distinct types of ignition mechanisms. If the impinging distance is short and the hot turbulent jet hits the plate with high enough momentum, the temperature increases around the stagnation point and ignition starts from this impinging region. However, if the impinging distance is long, the hot turbulent jet mixes with the ambient unburned H2/air in the main chamber and ignites the mixture at the upstream from the plate. For such type of ignition, the impinging plate has a minimum role on main chamber ignition. Employing the stagnation point ignition, a leaner limit of H2/air in the main chamber was achieved. Numerical modeling of the turbulent hot jet impingement process was carried out to explain the impinging jet ignition mechanism. It was found that H/D ratio was the controlling parameter between the two ignition mechanisms. The limiting H/D ratio was found to be 21.6, below which ignition occurred via jet impingement. Unlike the H/D ratio, the impinging angle did not affect the ignition mechanism; however, it affected the main chamber burn time.

Ignition of ultra-lean premixed H2/air using multiple hot turbulent jets generated by pre-chamber combustion

A detailed investigation on the ignition characteristics of ultra-lean premixed H2/air mixtures by multiple hot turbulent jets in a dual combustion chamber system was carried out. Simultaneous high-speed Schlieren and OH∗ chemiluminescence imaging were applied to visualize the jet penetration and ignition processes inside the main combustion chamber. The focus was on the effects of the spark location and fuel/air equivalence ratio within the pre-chamber on the ignition pattern of the main-chamber mixture. The results show that multiple jets resulted in similar lean flammability limit of H2/air as the single jets, given that the total nozzle areas are the same. However, the ignition probability improved significantly near the lean flammability limit using multi-jets compared to single jets. Additionally, depending on the spark location and pre-chamber equivalence ratio, either the side jet, the middle jet, or all the jets can initiate ignition in the main chamber. The ignition characteristics of straight and angled multi-jets were compared, and angled jets produced shorter main chamber burn time. Numerical modeling of the flame propagation process within the pre-chamber was carried out to explain the behavior of individual jets in a multi-jet system. It was found that depending on the pre-chamber spark position, the flame shape inside the pre-chamber changes drastically. The effective L/D ratio governs the flame dynamics in the pre-chamber and subsequently determines which jet (side, middle, or all) will first ignite the main chamber.

Simultaneous schlieren photography and soot foil in the study of detonation phenomena

The use of schlieren photography has been essential in unravelling the complex nature of high-speed combustion phenomena, but its line-of-sight integration makes it difficult to decisively determine the nature of multi-dimensional combustion wave propagation. Conventional schlieren alone makes it impossible to determine in what plane across the channel an observed structure may exist. To overcome this, a technique of simultaneous high-speed schlieren photography and soot foils was demonstrated that can be applied to the study of detonation phenomena. Using a kerosene lamp, soot was deposited on a glass substrate resulting in a semi-transparent sheet through which schlieren source light could pass. In order to demonstrate the technique, experiments were carried out in mixtures of stoichiometric hydrogen–oxygen at initial pressures between 10 and 15 kPa. Compared to schlieren imaging obtained without a sooted foil, high-speed video results show schlieren images with a small reduction of contrast with density gradients remaining clear. Areas of high temperature cause soot lofted from the foil to incandescence strongly, resulting in the ability to track hot spots and flame location. Post-processing adjustments were demonstrated to make up for camera sensitivity limitations to enable viewing of schlieren density gradients. High-resolution glass soot foils were produced that enable direct coupling of schlieren video to triple-point trajectories seen on the soot foils, allowing for the study of three-dimensional propagation mechanisms of detonation waves.

Simultaneous OH-PLIF and schlieren imaging of flame acceleration in an obstacle-laden channel

Flame acceleration in stoichiometric H2/O2 at 12 and 25 kPa initial pressure in an obstacle-laden square cross-section channel was studied experimentally using planar laser-induced fluorescence imaging of hydroxyl radicals (OH-PLIF) and simultaneous high-speed schlieren imaging. Results were obtained resolving the explosion front structure as it develops immediately after ignition as a slow-flame to the eventual formation of a shock-flame complex in the fast-flame regime. The images provide a novel level of detail and allow for the determination of the effects of turbulence-flame and shock-flame interaction. In the slow-flame regime, vortex shedding off obstacle edges occurs over long time-scales, vortices are convected downstream and turbulent combustion takes place in the obstacle wakes. The fast-flame regime is marked by the presence of compression waves (and shock waves) which interact with the flame and cause macroscopic deformation of the flame and small-scale wrinkling due to Richtmyer-Meshkov instability. A quasi-steady fast-flame is characterized by the close proximity of the precursor shock and the turbulent flame. The flow-field that governs the flame shape is established impulsively by the precursor shock. Shock-flame interactions lead to flame front perturbations on both small and large scales. The OH-PLIF technique makes it possible to discern the flame front from other density interfaces that appear in the complex fast-flame structure observed in schlieren images and also eliminates the line-of-sight integration limitation.

Flame propagation across an obstacle: OH-PLIF and 2-D simulations with detailed chemistry

Flame propagation across a single obstacle inside a closed square channel is studied experimentally and numerically using a stoichiometric H2/O2 mixture at initial conditions 15 kPa and 300 K. The 50% blockage obstacle consists of a pair of fence-type obstacles mounted on the top and bottom walls of the channel. Direct optical visualization was performed using single-image measurement of the planar laser-induced fluorescence of the OH radical (OH-PLIF) and simultaneous high-speed schlieren video to study the flame topology and the flame tip velocity along the channel streamwise axis, respectively. The OH-PLIF images provide a novel level of detail and permit a thorough evaluation of the simulation accuracy. The flame tip accelerates to a peak velocity of 590 m/s just downstream of the obstacle followed by a deceleration and subsequent re-acceleration. The unburnt gas flow ahead of the flame is subsonic at all times. The flame does not show any signs of diffusive-thermal instability. Vortex–flame interactions in the recirculation zones downstream of the obstacle wrinkle the flame. The numerical simulations, based on solving the 2-D compressible reactive Navier–Stokes equations with detailed chemistry, predict the flame tip velocity accurately. However, differences in flame topology are observed, specifically, wrinkling is over-estimated. The over-prediction of flame wrinkling suggests a lower dissipation rate in the numerical simulations than in reality, which could be a consequence of neglecting the third channel dimension. Conditional means of the fuel consumption rate are similar to the consumption rates of 1-D unstretched laminar flames at all times. The increase in pressure during flame propagation causes an increase in fuel consumption rate which needs to be accounted for in simplified modeling approaches.

On ignition mechanisms of premixed CH4/air and H2/air using a hot turbulent jet generated by pre-chamber combustion

An experiment was developed to investigate the ignition mechanisms of premixed CH4/air and H2/air mixtures using a turbulent hot jet generated by pre-chamber combustion. Simultaneous high-speed Schlieren and OH∗ chemiluminescence imaging were applied to visualize the jet penetration and ignition process inside the main combustion chamber. Results illustrate the existence of two ignition mechanisms: jet ignition and flame ignition. The former produced a jet comprising of only hot combustion products from pre-chamber combustion. The latter produced a jet full of wrinkled turbulent flames and active radicals. A parametric study was performed to understand the effects of pressure, temperature, equivalence ratio along with geometric factors such as orifice diameter and spark position on the ignition mechanisms and probability. A global Damköhler number was defined to remove parametric dependency. The limiting Damköhler number, below which ignition probability is nearly zero, was found to be 140 for CH4/air and 40 for H2/air. Lastly, the ignition outcomes were plotted on the turbulent premixed combustion regime diagram. All non-ignition cases fell within the broken reaction zone regime, whereas flame and jet ignition mostly fell within the thin reaction zone regime. These results can provide useful guidelines for future pre-chamber design and optimization.

Simultaneous High Speed Schlieren and Direct Imaging of Ignition Process with Digitally Enhanced Visualisation

Abstract In this paper, a combined method of schlieren and digital colour image processing is developed to study the effect of ignition location on the propagation of propane diffusion flames. The early weak blue flame is visualised through selective digital imaging enhancement techniques, which have revealed that a typical orange diffusion flame is often formed inside a blue flame pocket at the beginning of the ignition when ignited at the centreline of the fuel jet. When the ignition location towards the out edge of the fuel/air mixing boundary, the orange flame shifted and gradually broke the blue flame pocket. In the meantime, the flame was observed to propagate faster. The ratio of orange flame areas rose and the blue flame area fell accordingly.

Incipient thermal choking and stable shock-train formation in the heat-release region of a scramjet combustor. Part I: Shock-tunnel experiments

A series of experiments is performed in the High Enthalpy Shock Tunnel Göttingen to investigate the response of the HyShot II scramjet combustor to equivalence ratios close to the critical value at which the onset of thermal choking occurs. This critical equivalence ratio is first identified (for simulated Mach-8, 27-km altitude conditions) as 0.38–0.39. Subsequent experiments cover the range between this value and 0.50. As the HyShot II combustor has a constant-area cross section, the expected behavior in this equivalence-ratio range is the formation and upstream propagation of an unsteady shock train that would eventually lead to inlet unstart. Instead, however, the shock train is observed to become lodged in the heat-release zone of the combustion chamber, with a quasi-stable position that shifts upstream with increasing equivalence ratio; this behavior is distinct from the steady isolator shock trains seen in traditional dual-mode scramjet configurations. The shock-train development in the present experiments is characterized through fast-response surface pressure and heat-flux measurements, as well as simultaneous high-speed schlieren and OH∗ chemiluminescence visualization. Possible explanations for this unexpected behavior are proposed in terms of the increased heat-transfer to the combustor walls and a possible increased uniformity of heat release across the combustor cross-section caused by the presence of the shock train.

Micro vortex generator control of axisymmetric high-speed laminar boundary layer separation

Interest in the development of micro vortex generators (MVGs) to control high-speed flow separation has grown in the last decade. In contrast to conventional vortex generators, MVGs are fully submerged in the boundary layer and have the potential of inducing surface flow mixing with marginal drag penalty when suitably designed. Also, they do not result in undesired reduced mass flow such as with suction methods. The flow mechanisms at the location of MVGs are not yet fully understood, and optimal designs are difficult to establish given that both numerical predictions and experiments are particularly challenged for short element heights, yet optimal MVGs are generally expected to be at least shorter than half the local boundary layer thickness. The present work aims at investigating experimentally the fundamental flow physics concerning an individual MVG element (of ‘canonical’ or simplified geometry) at a range of near-wall heights. A fully laminar base flow is considered so as to isolate the effect of incoming turbulence as well as the more complex physics that may occur when specific and/or multiple elements are used. Tests were performed in a gun tunnel at a freestream Mach number of 8.9 and Reynolds number of $$47.4\times 10^{6}$$ /m, and the basic test model consisted of a blunt-nosed cylinder which produced an axisymmetric laminar boundary layer with an edge Mach number of 3.4 and Reynolds number of $$3.2\times 10^{6}$$ /m at the MVG location. A laminar shock-wave/boundary layer interaction with separation was induced by a flare located further downstream on the model. Measurements consisted of time-resolved surface heat transfer obtained in the axial direction immediately downstream of the MVG and along the interaction, together with simultaneous high-speed schlieren imaging. The height ( $$h$$ ) of the MVG element used in a ‘diamond’ configuration (square planform with one vertex facing the flow) was adjusted between tests ranging from $$h/\delta $$ = 0.03 to 0.58, where the local undisturbed boundary layer thickness was $$\delta $$ = 1.75 mm. The effect of planform geometry was further assessed by performing tests with the MVG used in a ‘square’ configuration (one edge normal to the incoming flow). Results show that MVG height drives the intensification of heat transfer fluctuations in the wake of the element. The optimal MVG operating conditions, where downstream boundary layer separation is avoided and minimal flow interference is produced, based here on heat transfer unsteadiness, are $$h/\delta \approx 0.3$$ with a ‘diamond’ arrangement.

Unsteady Behaviors of a Hypersonic Inlet Caused by Throttling in Shock Tunnel

A two-dimensional hypersonic inlet/isolator model that exhibits self-starting characteristics is tested with different exit throttling ratios at a freestream Mach number of 5.9 in a shock tunnel. Various flow characteristics are observed and measured by applying simultaneous high-speed Schlieren imaging and surface pressure measurements. The results indicate that the backpressure generated by the throttling device can be tolerated, and the inlet can maintain the starting mode at low throttling ratios, whereas unstart flows are initiated from the near-choke throttling ratios, and a shock wave oscillation appears. The Schlieren movie demonstrates that the upstream-propagating shocks in the duct play important roles during the oscillation cycles and that the formations of the upstream-propagating shocks are related to the downstream-propagating compression waves/shock waves that encounter the throttling section. The frequency of the shock wave oscillation increases with increasing exit throttling ratios, primarily because of the acceleration of the upstream propagation.

Experimental Investigation of the Unstart Process of a Generic Hypersonic Inlet

To provide information for the detection, prediction, and control of the inlet unstart, the entire process from a started status to an unstarted status of a generic two-dimensional hypersonic inlet is studied experimentally at Mach 5. The movement of a flow plug at the exit of the duct is used to gradually increase the throttling to simulate the unstart process caused by the excessive heat release in the combustor. Simultaneous high-speed schlieren imaging and dynamic surface pressure measurements are used to record the unsteady flow structures and surface pressures of the unstart process. According to the internal and external flows, the unstart process can be divided into four stages, namely, shock train in the combustor, shock train in the isolator, separation bubble in the throat, and unstart. The transient flow patterns of each stage are substantially different and the corresponding dynamics pressures also have prominent time-frequency features, which make the detection and prediction of the inlet unstart based on dynamic pressures possible. Since the sensor placed at the end of the top surface of the inlet contract part, marked by C1, obtains the most abundant time-frequency characteristics which can be used to discern the different stages of the inlet unstart process, the location where it stays is regarded as the first choice for sensor placement to construct a practical inlet-status-monitoring system.

On the structure of stationary flames

Flame acceleration in stoichiometric H2/O2 at 12 and 25 kPa initial pressure in an obstacle-laden square cross-section channel was studied experimentally using planar laser-induced fluorescence imaging of hydroxyl radicals (OH-PLIF) and simultaneous high-speed schlieren imaging. Results were obtained resolving the explosion front structure as it develops immediately after ignition as a slow-flame to the eventual formation of a shock-flame complex in the fast-flame regime. The images provide a novel level of detail and allow for the determination of the effects of turbulence-flame and shock-flame interaction. In the slow-flame regime, vortex shedding off obstacle edges occurs over long time-scales, vortices are convected downstream and turbulent combustion takes place in the obstacle wakes. The fast-flame regime is marked by the presence of compression waves (and shock waves) which interact with the flame and cause macroscopic deformation of the flame and small-scale wrinkling due to Richtmyer-Meshkov instability. A quasi-steady fast-flame is characterized by the close proximity of the precursor shock and the turbulent flame. The flow-field that governs the flame shape is established impulsively by the precursor shock. Shock-flame interactions lead to flame front perturbations on both small and large scales. The OH-PLIF technique makes it possible to discern the flame front from other density interfaces that appear in the complex fast-flame structure observed in schlieren images and also eliminates the line-of-sight integration limitation.