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Recent Progress in Shock Tube and Laser Absorption Technique for the Combustion Chemistry Study

Junjie Chen*, Deguang Xu

School of Mechanical and Power Engineering, Henan Polytechnic University, Jiaozuo, China

Abstract: Recent progress in shock tube and laser absorption technique for the combustion chemistry study are reviewed. The principles of shock tube operation are discussed, and then the overview of shock tube diagnostic methods and experiments is covered. Recent shock tube developments include the use of driver inserts to counteract the small pressure gradient seen in conventional reflected shock wave experiments and the use of a constrained-reaction-volume strategy to enable the implementation of near-constant-pressure gasdynamic test conditions during energetic processes. Recent laser absorption developments include the use of a CO₂ laser absorption sensor to accurately monitor temperature during shock wave experiments, the use of multi-wavelength laser absorption strategies to simultaneously monitor multiple species time-histories, and the used of isotopic labeling strategies to identify individual reaction sites during the measurement of elementary reaction rate constants. The improved ability to accurately constrain the test conditions in shock tube experiments, combined with non-intrusive, species-sensitive and quantitative laser absorption diagnostics, is enabling experimenters to provide a new generation of high-quality experimental kinetics targets for combustion chemistry model validation and refinement. The paper concludes with a brief discussion of newly emerging laser-diagnostic techniques and a summary of future research directions.

Keywords: laser absorption, laser-diagnostic technique, shock tube, isotopic labeling, combustion chemistry

Received December 02, 2014; Revised December 11, 2014; Accepted December 21, 2014

Citation: Junjie Chen, Deguang Xu, Recent Progress in Shock Tube and Laser Absorption Technique for the Combustion Chemistry Study. Journal of Optoelectronics Engineering, 2 (2). 33-41. January 2014. doi:10.12691/joe-2-2-3

1. Introduction

The shock tube is an instrument used to replicate and direct blast waves at a sensor or a model in order to simulate actual explosions and their effects, usually on a smaller scale. Shock tubes (and related impulse facilities such as shock tunnels, expansion tubes, and expansion tunnels) can also be used to study aerodynamic flow under a wide range of temperatures and pressures that are difficult to obtain in other types of testing facilities. Shock tubes are also used to investigate compressible flow phenomena and gas phase combustion reactions.

There is a growing role for shock tubes in studies of combustion chemistry, enabled particularly by the addition of laser-based species diagnostics and recent work to improve shock tube performance. In particular, the availability of solid-state tunable lasers developed originally for the telecommunications industry has recently opened up the infrared spectrum to laser absorption diagnostics and allowed sensitive access to many new species [1]. As well, recently-implemented improvements in the operation of shock tubes now can provide near-constant-volume gasdynamic conditions (for low energy release processes) or near-constant-pressure gasdynamic conditions (particularly useful for large energy release processes). Both strategies allow the combustion modeler to implement high-fidelity modeling of the reactive gasdynamics processes as part of their analysis of reflected shock wave experiments. The improved ability to accurately constrain the test gas conditions in these ways, combined with non-intrusive, species-sensitive and quantitative laser absorption diagnostics, is enabling experimenters to provide higher-quality experimental kinetics targets for combustion chemistry model validation and refinement. In this paper we will overview recent advances in shock tube and laser methodologies for chemical kinetics [2].

2. Shock Tube Technique

2.1. Principles of Shock Tube Operation

Figure 1. Schematic diagram of the shock tube

A shock tube (Figure 1) is its simplest form is a long tube with closed ends separated into two sections by a diaphragm. The test gas mixture is placed in the driven (low-pressure) end of the tube and the driver end of the tube is over-pressured with gas, often helium or a helium mixture, until the diaphragm bursts. A shock wave quickly forms and moves rapidly ahead of the driver gas expanding into the driven section. Conservation of mass, momentum and energy relations, along with the equation of state of the test gas and the measured speed of the shock wave, enable calculation of the step change in temperature and pressure of the moving test gas behind the wave. Upon reflection of this incident shock wave from the closed end wall of the driven section, the test gas is further compressed and heated to its initial pre-reaction temperature and pressure, and is effectively stagnated. Because of the near-one-dimensionality of the shock tube flow and the near-planar nature of the shock wave it is possible with a precise measurement of the incident shock speed to accurately determine (with <±1% uncertainty) the temperature and pressure behind the reflected shock [2]. Because of the effective step change in shock conditions and the effective stagnation of the test gas mixture behind the reflected shock wave, the test gas mixture can potentially be treated as a zero-dimensional reactor (excluding a very thin boundary layer) and modeled using either a constant-volume or constant-pressure constraint. However, important deviations from the idealized constraints are often present unless proper attention is given to experimental design.

One operational drawback of shock tube operation is the single-shot nature of the facility. This disadvantage can be avoided by the use of diaphragmless shock tubes. Recently, Tranter et al. have replaced the traditional diaphragm with a fast-acting valve [3]. This method has allowed repeated shocks at near-similar conditions and for certain diagnostics systems (e.g. time-of-flight/mass-spectrometry (TOF/MS)) has significantly improved the quality of measurements. Tranter and Lynch also have extended this concept to a miniature high-repetition-rate shock tube [4]. This facility provides excellent reproducibility of high-pressure, high-temperature reflected shock conditions that enable the pioneering use of diagnostics based on synchrotron light sources, where many experiments need to be averaged to obtain adequate signal-to-noise levels. Using this facility, they have achieved repetition rates of 4 Hz, pressure up to 100 atm and reflected shock test times of the order of 100 μs [4]. Similar repetition rates in a small diameter (2 mm) diaphragmless shock tube have been achieved by Shiozaki et al. [5]. Experimental work with fast-acting valves has also been applied in larger shock tube facilities. Recent work by Heufer et al. has achieved opening times of order 1 ms for these valves [6]. Such systems provide the opportunity for a significant reduction in shock tube impurity loading that can occur with the pyrolysis of burst diaphragm particles.

2.2. Capabilities of Shock Tubes

A wide range of test gas conditions can be achieved in shock tubes. Typical reflected shock conditions for combustion chemistry studies cover temperatures from 600 to 3000 K and, depending on the construction and wall thickness of the shock tube, can cover pressures from sub-atmospheric to as high as 1000 atm as demonstrated by Tranter and co-workers [7]. Different configurations of shock tubes can be used for specific purposes. Large diameter shock tubes, typically 10 cm inside diameter and above, generally exhibit relatively small influences of boundary layers and provide test conditions that are closely predicted by the shock jump equations. Small diameter shock tubes (e.g. 5 cm diameter or less), however, can have boundary layers that are a significant fraction of the internal tube diameter, in which case the one-dimensional shock equations provide less accurate determinations of the test gas conditions; a discussion of this can be found in Michael and Sutherland [8]. Although kinetics studies can be conducted behind incident shock waves, most current work is performed using reflected shock waves. Variations in the lengths of the two sections, driver and driven, can be used to vary the test time, defined as the time available when test conditions are approximately uniform. In a conventional shock tube, with a 15 cm internal diameter and a driven L/D of 70-100 and a driver L/D of 20-50, reflected shock wave experiments with nitrogen or argon as the carrier gas and helium as the driver gas have usable test times of 1-3 ms, often limited by non-ideal wave processes that occur when the reflected shock wave meets the moving driver gas-driven gas interface. Proper selection of driver gas composition, known as tailoring, can be used to obviate these non-ideal processes and thereby extend the reflected-shock test times to about 10 ms. Petersen and co-workers have explored the use of different driver gases that satisfy the tailoring condition while reducing the sound speed and thereby delaying the ultimate end of test time due to the arrival of the rarefraction wave of driver gases [9]. Using tailored driver gas mixtures of carbon dioxide-helium and/or propane-helium, they were able to achieve test times of 15 ms for a driver of length 3.5 m and a driven section of length 10.7 m at a reflected shock temperature of 891 K.

Figure 2. Shock tube X-t diagram for an un-tailored shock wave experiment. Brown, high-pressure driver gas (T₄, P₄); dark blue, low-pressure test gas mixture (T₁, P₁); blue, test gas mixture at incident shock conditions (T₂, P₂); green and yellow and red in expansion wave regime, expanded driver gas (T₃, P₃); turquoise triangle, test gas mixture at reflected shock conditions (T₅, P₅) [9]

Figure 2 and Figure 3 are X-t diagrams that show the idealized wave structure during shock wave experiments. In Figure 2 the test time is determined as the time between the arrival of the reflected shock wave at the test location and the arrival of the weak wave produced by the interaction of the reflected shock wave and the driven gas-driver gas interface. In Figure 3 the test time is terminated when an expansion wave finally reaches the measurement location.

Figure 3. Shock tube X-t diagram for a tailored shock wave experiment. The turquoise area in the upper right hand corner represents the extended test times of the uniform reflected shock conditions (T₅, P₅) enabled because of driver gas tailoring [9]

To investigate low-temperature regimes of combustion reactions, longer test times are generally required. This is of particular importance in studies of the negative-temperature-coefficient (NTC) ignition regime seen in diesel and other practical engine fuels. To extend the test time in a shock tube, two requirements are thus generally needed: a longer driver section to delay the return of the expansion wave from the driver end wall, and a tailored driver gas mixture that matches the “impedance” at the driver gas/driven gas contact surface so that the reflected shock wave passes without disturbance through this contact surface. When optimized, shock tubes can have low-temperature test times in excess of 50 ms, thereby enabling overlap with rapid compression machines (see Figure 4), and indeed test times of up to 100 ms have been recently demonstrated for T5 = 524 K [10].

Figure 4. Typical operation regimes of Stanford shock tubes. Blue area: high-quality test time in conventional configuration; red area: with driver modification and tailored driver gases; green area: conventional rapid compression machine. Representative ignition delay times for n-dodecane/air mixtures at 10 atm, ϕ = 1 are shown in black [10]

The ability of shock tubes and RCMs to overlap test regimes will enable the trends in ignition delay time behavior to be characterized over a wider temperature range. For example, a recent RCM study of n-butylbenzene/air and n-propylbenzene/air ignition by Curran and co-workers [11] was able to extend the temperature and pressure range of a shock tube study of the same fuels [12] from 980 to 720 K and from 30 to 50 atm and provide data on the NTC behavior of these fuels. Consistent measurements from RCM and shock tubes in this overlapping test regime also provide a unique verification of the operating capabilities of both devices under their extreme operating conditions. It should be noted that it is also possible to operate a shock tube with a significantly shorter driver section or attached dump tank as demonstrated by Tsang, who has used this technique to study a wide range of high-temperature reactions [13]. This configuration, because of the shorter test time, enables the experimenter to quench the elevated test gas conditions with a large dT/dt (typically ∼1000 K/ms) generated by an arriving expansion wave from the driver end wall or the dump tank. These “single-pulse” shock tubes can be used in conjunction with post-shock gas sampling to measure, e.g. with gas chromatography, the distribution of stable intermediate components at fixed time after the initial reflected shock heating as has been demonstrated by Brezinsky and co-workers [14].

There is a critical need to understand the formation of large hydrocarbons, soot precursors and soot at high pressure in rich combustion. The complexity and wide variety of these molecules and structures requires a technique such as gas chromatography/mass spectrometric (GC/MS) analysis that can distinguish among these different product constituents. Tranter, Brezinsky and Fulle developed a high-pressure (to 1000 atm) single-pulse shock tube for chemical kinetic investigations that enables this type of study [7]. Using this facility for example, measurements of the major pyrolysis products of benzene (C₂H₂ and C₄H₂) have been elucidated at high pressures and detailed kinetics models have been developed to predict these product distributions [15].

While shock tubes have commonly been used to study gas-phase chemistry, the vapor pressure of many practical fuels and their surrogates is so low that they occur only as liquids or butters at room temperature. In practice, the gas-phase kinetics of these fuels can be accessed by heating the entire mixing assembly, gas handling and shock tube to the point where these fuels are entirely in the gas phase. In a recent study by Oehlschlaeger and co-workers [16] the heated shock tube approach was used to study the ignition delays of diesel fuel and bio-derived diesel fuels. In this study the shock tube and mixing vessel were heated to 200 °C, and gas chromatography/mass spectrometry measurements of the pre-shock gases were used to ensure that the fuel was not partially decomposed during preparation.

However, there are complications with this heating approach; fuels can decompose before they are shock heated, they can react and oxidize in the mixture preparation tank, or they can fractionate. A new approach has recently been introduced at Stanford, known as an aerosol shock tube, which provides an alternative method to study the evaporation and gas-phase kinetics of low-vapor-pressure fuels [17] and [18]. In the aerosol shock tube, a spatially-uniform distribution of fuel aerosol droplets and oxidizer/buffer is drawn into the shock tube test section adjacent to the shock tube end wall. When the incident shock wave passes this fuel mixture, it rapidly evaporates the micron-sized droplets and the evaporated fuel quickly diffuses to create a uniform gas-phase mixture of the low-vapor-pressure fuel and oxidizer/diluent. The reflected shock wave further compresses this test gas mixture, as in conventional shock tube operation, enabling the same types of kinetics experiments that are performed in conventional shock tubes. The advantage of this method is that the evaporation process and the chemical reaction process of the fuel can be temporally separated and each process can be studied independently.

One novel application of the aerosol shock tube was to explore the impact of shock wave heating on bio-aerosols or spores. In these experiments, spores were carried into the shock tube in micron-sized water droplets that evaporate behind the incident shock wave, leaving the bare spore exposed to further heating by the reflected shock wave [19].

Finally, recent work has also focused on studying the kinetics of sublimating dry particles. These particles can be dispersed in the flow using a flat plate method described by Petersen et al [20].

2.3. Diagnostic Methods

After approximately 50 years of modern chemical kinetics studies in shock tubes, a wide variety of diagnostic methods have been developed to probe the high-temperature test gas behind the reflected shock wave. Fundamental to all shock tube studies is the measurement of pressure time-histories. This diagnostic provides critically needed information on the thermodynamic state of the test gas during the experiment, especially as, until just recently, there have been no simple yet highly quantitative methods to measure temperature accurately and directly in the heated test gas. Emission methods are also used as a standard diagnostic tool to probe the progress of chemical change in shock tube experiments. Emission from excited OH and CH (near 306 and 431 nm, respectively), in particular, has classically been used to identify the time of the rapid formation of radicals that is coincident with the onset of ignition as described, for example, by Hall and Petersen [21]. Infrared emission has also been used to provide quantitative information on the formation and removal rates of species such as CO, CO₂, H₂O, NO, and others [22] and [23]. In situ time-resolved sampling methods are also available. With sufficiently fast sampling systems using orifices at the shock tube end wall, mass-spectroscopic measurements of stable and transient species can be made during reflected shock wave experiments. Recent work by Schulz and co-workers has demonstrated high-repetition-rate time-of-flight mass spectrometry that is capable of providing spectra measurements every 10 μs and thus providing multiple species time-histories [24].

A variety of other methods have also been used successfully. Laser schlieren densitometry has been used very successfully to study reaction kinetics immediately behind the incident shock wave, particularly by Kiefer and co-workers and also by Tranter and co-workers [25] and [26]. Optical absorption methods are particularly attractive for quantitative monitoring of species time-histories, especially for reactive radicals and many stable species. These optical methods can be divided into two types: lamp systems including atomic resonance absorption spectroscopy (ARAS) and laser absorption. ARAS is used, typically, to follow H and O atom time-histories, where very low, sub-ppm-ppm, levels of these atoms are formed. Spectroscopically-clean facilities are needed for these experiments as have been demonstrated successfully by Michael and co-workers, Schulz and co-workers, and Frank and Just [27], [28] and [29].

Recently, the greatest advances in shock tube diagnostic methods have been in the area of laser absorption, and it is this diagnostic aspect we will emphasize in this paper. New laser absorption strategies, based mostly on commercially-available laser sources, have provided diagnostic methods for sensitive and quantitative measurements of time-histories for a wide range of fuel, intermediate and product species. In addition, infrared measurements of the stable combustion product CO₂ have recently enabled a straightforward and quantitative method to measure T(t), providing a powerful way to quantify shock tube performance and validate new constant-P, constrained reaction volume strategies [30].

2.4. Types of Experiments

Shock tubes are normally used to acquire combustion kinetics data in three types of experiments: ignition delay time measurements, species time-history measurements, and elementary reaction rate constant measurements. A very good overview of earlier experimental work, both methods and kinetics results, can be found in the Chemical and Combustion Kinetics Section of Volume 3 of the Handbook of Shock Waves [31].

Ignition delay time experiments provide a global measurement of the behavior of a combustion system [32] and [33]. These measurements take advantage of the effectively instantaneous jump in test conditions behind the reflected shock wave to initiate a near-constant-volume or near-constant-pressure experiment. The ignition delay time in these experiments is defined as the time from initial heating to the time of rapid radical formation and simultaneous energy release, a time interval that normally can be accurately determined. Two primary concerns face the experimenter for these experiments: accurately determining the fuel loading, and accurately determining the reaction temperature and pressure time-histories. Recent advances in shock tube design, including the use of tailoring and driver inserts to achieve near-constant temperature and pressures at long test time, and the use of laser absorption measurements to measure fuel concentration and test gas temperature, have proven critical to resolving these two primary concerns.

Species time-history experiments provide a temporal record of the concentration of one or more species during the progress of the combustion process. These time-histories can provide strong constraints on the reaction pathways and rate constants of detailed reaction mechanisms used to model the chemistry of these processes. As described later in this paper, recently developed multi-wavelength laser absorption diagnostics can now provide constraints on the reactions of fuel, transient and stable intermediates and product species, simultaneously.

Finally, the primary input parameters for detailed reaction mechanisms are the rate constants for individual elementary reactions. These rate constants can be measured accurately in experiments designed such that an individual species time-history shows strong sensitivity to a single reaction rate constant. Shock tube/laser absorption experiments have provided some highly accurate examples of these types of experiments. A good example is the recent work by Hong et al. to provide an improved determination of the rate constant for the important reaction H + O₂→O + OH [34].

3. Recent Progress

In the next sections we briefly describe several recent developments in shock tube techniques in chemical kinetics studies. These include: 1) the use of driver inserts to provide extremely uniform test conditions behind reflected shock waves, 2) the use of a constrained-reaction-volume strategy to achieve near-constant-pressure test conditions, 3) the use of CO₂ laser absorption to measure T(t), 4) the use of new laser systems to provide diagnostics for a wide range of important combustion reactants, intermediates and product species, 5) the use of isotopic labeling to identify reaction pathways, and 6) newly emerging laser-diagnostic methods.

3.1. Driver Inserts

Critical in the comparison of measurements of shock tube ignition delay times and simulations using detailed reaction mechanisms is the accurate and explicit characterization of the actual test conditions. As a key example, pressure profiles behind reflected shock waves in conventionally-operated shock tubes generally exhibit small, but significant, gradual rises, typically in the 1-3%/ms regime (or higher in small diameter shock tubes). These pressure changes are due to boundary layer formation and incident shock attenuation. Coincident with these pressure rises are temperature rises that are isentropically related to the pressure rise. Thus, at long test times significant changes in the temperature of the reflected shock test gas may be present, even in non-reactive experiments. The most obvious evidence of the impact of the temporal variations in temperatures is in ignition delay time measurements where small increases in temperature can significantly shorten the measured ignition delay time.

Figure 5. Representative ignition delay time data: propane/O₂/argon. Red line: conventional shock tube, dP/dt = 3%/ms; blue line: with the addition of driver inserts, dP/dt ∼0%/ms. The initial temperature, T₅ (0), in both cases is 1039 K [37]

Based on the early work by Dumitrescu [35], it is possible to generate test conditions with near-constant pressure by slightly degrading the performance of the shock tube using inserts in the driver section [36]. These inserts act as a blockage in the driver, typically either an axial cone or a circumferential cylinder, providing a reduction in the shock tube driver area and acting as a source of expansion waves that exit the driver during the blowdown process. The driver inserts can be designed such that the timing and magnitude of the expansion waves, which travel through the driver gas to the driven gas, degrade or reduce the pressure in the reflected shock test gas region in such a way that a near-constant pressure is achieved at the measurement location, typically 1-2 cm from the end wall. Representative P(t) data without and with the insert are shown in Figure 5 [37]. The impact on ignition delay is apparent. A similar insert strategy was also reported by Stotz et al [38].

3.2. Constrained Reaction Volume Strategy

One difficulty in modeling energetic ignition events in shock tubes is that because of the open-endedness of the test gas volume behind the reflected shock, the gasdynamics constraint on the test gas is not truly constant volume throughout the entire ignition process. Computational work, for example by Melguizo-Galvilanes and Bauwens confirm the inadequacy of the constant-volume assumption in modeling ignition delay times [39]. At low fuel concentrations, typically below 0.1% fuel, pressure and volume changes due to ignition are small enough that a constant-volume or constant-pressure description of the process may be acceptably correct. At higher fuel concentrations, however, there is a larger energy release during fuel oxidation, and consequently the test gas pressure increases, causing the volume to expand rapidly at ignition. Thus, while the constant-volume, or constant-pressure model of the process is often approximately correct up to the point just before ignition, the proper gasdynamics constraint is not easily defined through the exponential growth part of the ignition process and beyond, and this has prohibited complete and accurate modeling throughout the combustion process for comparison with the species time-history data that are now becoming available.

Figure 6. Schematic of conventional and constrained-reaction-volume filling strategies [40]

A solution to this difficulty is the recently reported constrained-reaction-volume (CRV) strategy [40]. This strategy can provide near-constant-pressure gasdynamics conditions throughout an entire highly-energetic ignition process (or throughout an endothermic pyrolysis process). The strategy can best be explained by comparing the reactive gas volume in a conventional shock wave experiment and the reactive gas volume in the CRV configuration. The two configurations are shown schematically in Figure 6. In a conventional shock tube experiment, the entire driven section is filled with test gas. The incident shock wave compresses and heats this test gas, reducing it in length by the density ratio (ρ₂/ρ₁) across the incident shock wave. This test gas slug is again compressed and heated behind the reflected shock wave, reducing it again in length by the density ratio (ρ₅/ρ₂) across the reflected shock. In a shock tube with a driven length of 10 m, and at representative combustion conditions, this results in a reactive gas slug of approximately 2 m in length. As the test gas mixture is heated by the reflected shock wave, it normally begins to ignite first in the thin volume closest to the shock tube end wall that was heated first. Then, successive thin slices of the volume, at increasing axial distance from the end wall, begin to ignite. This cascading effect generates a larger and larger region of energy release and increased pressure that can quickly develop into a blast wave. The effect on the local test conditions can be seen in the measured pressure in Figure 5.

3.3. Temperature Measurement using CO₂ Laser Absorption

As noted above, there has not previously been a simple method to achieve highly-precise fast time-response measurement of the temperature in shock-heated test gases. Temperature measurements in shock tubes have been performed using multi-line scanning laser absorption methods, with OH radicals [41], but these were only able to offer 50 K resolution and 3 kHz sampling frequencies. Recently, high-speed, high-precision measurements of temperature have been performed using tunable continuous-wave (CW) diode-laser absorption of CO₂ near 2.7 μm [30]. These measurements used wavelength modulation spectroscopy with second-harmonic detection (WMS-2f) and inferred the temperature from the ratio of 2f signals for two individual CO₂ absorption transitions. By taking advantage of the large CO₂ absorption strength near 2.7 mm and the WMS-2f strategy, reflected shock temperatures could be measured to within 0.5-1.0% with a sensor bandwidth of 50 kHz. An alternative cross-band strategy for inferring temperature from relative absorption in the 2.7 and 4.3 μm bands of CO has been recently reported by Spearrin et al. [42].

Figure 7. Reflected-shock temperatures measured using CO₂ laser absorption during an ethylene ignition experiment. Predicted temperature profiles using two recent detailed mechanisms, JetSurF 2.0 and GRI-Mech 2.0 are also shown. Initial reflected shock conditions: 1135 K and 2.3 atm [42]

The CO₂ laser absorption T(t) diagnostic can be used to generate temperature time-history profiles that can be used as kinetics targets in the validation and refinement of detailed reaction mechanisms. Figure 7 shows the representative temperature measurement using this diagnostic during an ethylene oxidation experiment. Though both the JetSurF and the GRI-Mech mechanisms capture the long-time post-ignition temperature plateau seen in this experiment, only the JetSurF mechanism accurately captures the changing temperature profile during the entire ignition event. This new and accurate capability to directly measure temperature during complex reaction process will enable modelers to validate and refine the ability of their individual detailed reaction mechanisms to capture the heat release rate, an important combustion observable and relevant engine or burner design criterion.

3.4. Multi-species Laser Absorption Diagnostics

Laser absorption diagnostics provide a sensitive, quantitative, and non-intrusive method to measure species concentration time-histories in shock tube experiments [43]. Much of the initial development of these diagnostic methods has been done in our laboratory over the past 35 years. The extension of this method to multi-wavelength, multi-species detection has enabled the development of kinetics databases comprised of species-time-histories for reactants (fuel), transient radical species, (e.g. OH and CH₃) [44,45], stable intermediates (e.g. C₂H₄ and CH₄) [46,47], and combustion products (CO, CO₂ and H₂O) [48,49]. These databases can be used to strongly constrain the hypothetical reaction pathways and rate constants used in detailed sub-mechanisms and full reaction mechanisms proposed for fuel pyrolysis and oxidation. These diagnostics can also be used in pseudo-first-order rate measurements of elementary reactions where the kinetics of individual species are dominated by single reactions.

The list of species accessible with laser absorption is continually expanding. Several recent studies are of note. Oehlschlaeger and co-workers have developed a carbon monoxide and thermometry sensor based on mid-IR (4.6 mm) quantum-cascade laser wavelength-modulation absorption spectroscopy (WMS) with second-harmonic detection, providing increased sensitivity [50,51].

Petersen and co-workers have begun to investigate the spectroscopy of H₂O₂ near 7.7 microns using a lead salt diode laser [52]. This approach has also been pursued by Farooq and co-workers [53]. In their studies, high-sensitivity measurements of the H₂O₂ decomposition rate were performed and are found to be in very good agreement with earlier work. Camou et al. have recently measured CO near 4.5 microns using a DFB QC laser [54].

3.5. Isotopic Methods

Laser absorption can also be used to measure elementary reaction rate constants behind reflected shock waves. Some of the simplest and most direct measurements are pseudo-first-order determinations of OH + stable species rate constants. In these studies, OH is generated by the rapid high-temperature decomposition of tert-butyl-hydroperoxide (TBHP) to OH, CH₃ and acetone, which in the 900-1400 K range is effectively instantaneous, as was originally suggested by Bott and Cohen [55]. The small quantities (∼20 ppm) of OH used in these experiments combined with the excess of stable reactant (typically 200-2000 ppm) provide a pseudo-first-order reaction environment where the measured OH time-history shows major sensitivity to only the target reaction.

3.6. New Emerging Methods

The rapid development of solid-state or semiconductor laser technologies is spawning new approaches to laser absorption measurements for shock tubes. We mention three very recent and highly promising examples here.

The first new approach is the work of Farooq and co-workers with high-repetition-rate pulsed quantum cascade (QC) lasers near 7.6 microns [56]. This method takes advantage of the down-chirp phenomenon that occurs when long modulation pulses are applied to QC lasers, enabling more rapid spectral scans of the QC laser wavelength multiple transitions than has been demonstrated previously. Proper selection of spectral features and laser modulation parameters enabled the measurement of both H₂O concentration and temperature behind reflected shock waves with MHz sampling rates.

The second new method, also introduced by Farooq and co-workers, is high-repetition-rate cavity ringdown (CRD) spectroscopy, conducted using a pulsed QC laser [44]. This method utilizes high-reflectivity mirrors to create a large effective path length for absorption, and infers the absorber number density for each laser pulse form the ringdown (decay) time of the laser radiation escaping the cavity.

The third new approach is a modified form of cavity-enhanced absorption spectroscopy (CEAS) using a CW tunable diode laser source. Recent experimental work by Sun et al. [57,58] has demonstrated that this CEAS method is able to provide significantly longer effective path lengths (101-103 times the tube diameter) for shock tube/laser absorption experiments, without significant degradation of temporal resolution. These longer effective path lengths provide significantly increased diagnostic sensitivity and will enable access to kinetically important species that, because of low absorption coefficients at high temperatures, have not been accessible using conventional single-pass absorption schemes. Equally important is the potential to measure strongly absorbing species at very low concentrations (e.g. ppb levels), thus enabling the study of simple kinetics systems with low concentration reactants and significantly-reduced secondary chemistry.

4. Conclusions and Future Directions

Recent advances in shock tube/laser absorption methodologies are revolutionizing the use of shock tubes for chemical kinetics studies. New operational protocols in shock tube/laser absorption experiments can provide well-characterized near-constant-pressure test conditions, even during energetic reaction events, for test times of 50 ms and longer. Laser absorption diagnostics can provide quantitative high-bandwidth measurements of time-histories during these experiments for a wide range of species. With these methods, a range of new kinetics target data can be acquired. We have outlined several of these strategies below.

Longer constant-pressure test times will allow for the validation and refinement of negative-temperature-coefficient (NTC) ignition processes. The existing shock tube NTC ignition delay time database for practical fuels and their surrogate components does not include detailed characterization of the test conditions, and there is a critical need for this characterization for long-test-time data if they are to be used as reliable modeling targets. As well, little, if any, species time-history studies of these fuels have been performed in the NTC regime, and these data are also critically needed to test and refine detailed reactions mechanisms.

The CO₂ laser absorption diagnostic for T(t) provides unique capability to test the ability of detailed reaction models to accurately capture heat release rates. Previous modeling studies have relied on the assumption that satisfactory agreement of the model simulations with ignition delay times or flame speeds resulted in accurate heat release rates, as there were no other simple and accurate methods available to test this property of the model. The T(t) time-histories kinetic targets should be considered as important as individual species time-history data for mechanism testing and refinement.

Recently advocated, top-down kinetics modeling approaches will benefit from accurate measurements of concentration distributions of rapidly-forming intermediate species. These distributions can also be used as inputs to guide development of reduced detailed reaction mechanisms. Currently, concentration measurements are possible for a wide range of species such as fuel, CH₃, C₂H₄, CH₄, OH, H₂O, CO, CO₂, and others. For those species that cannot be measured spectroscopically (e.g. homonuclear diatomics molecules such as H₂, or species that are difficult to access spectrally such as allene) another approach can be used that takes advantage of the new ability to measure T(t) in a constant pressure/constant enthalpy test environment. Accurate measurements of the change in T at constant pressure can be related to changes in the bond energy, i.e. the chemical contribution to the enthalpy change. This quantity can provide, along with atom balances and available measured species concentrations, multiple constraints on the product distribution, thereby allowing the accurate inference of several unmeasured species concentrations.

Finally, the continued development of new laser systems that provide access to wider wavelength coverage will allow for the measurement of an even larger array of species. Recent work in our laboratory has focused on the development of diagnostics to monitor acetylene near 3.0 μm, and iso-butene and other alkenes and alkynes near 11 μm. This work builds on the past 40 years of laser development and offers to provide simple and direct methods to characterize the chemistry of important combustion systems.

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