Intro1

.    Research in the High Pressure Shock Tube Laboratory focuses on the experimental examination and the modeling simulation of the kinetics of combustion chemistry. The chemistry of interest is that which takes place during the pyrolysis and oxidation of hydrocarbon fuels. The fuels are either aircraft engine or ground transportation fuels which are introduced into the high pressure shock for experimental examination of reaction products and into the low pressure shock tube when complementary studies are required. Descriptions of these two versatile devices follow.

HPST1

Fig. 1. High pressure shock tube set-up with direct connection to GC (right foreground).

.    The high-pressure single-pulse shock tube, a “tried and true” experimental device, is shown in Fig. 1. It is rated, nominally, over its maximum operating range, at 15 to 1000 bar, 900 to 2500K, 0.5 to 3 msec. The high-pressure tube operates by using high pressure helium driver gas to create a shock wave, upon the breaking of a metal diaphragm, that propagates down the driven section of the tube which contains the test gas (reactant mixture) of predominantly argon (greater than 99%) into which the reactant or reactants are mixed. The shock wave creates a near instantaneous temperature and pressure rise when the shock wave is reflected off the driven section end wall of the tube. The reactant mixture (ultrahigh purity argon + reactants) undergoes pyrolysis or oxidation at the post-reflected shock conditions of a known temperature, pressure, and reaction time.

Fig. 2. Top, total ion chromatogram of all the species with mass greater than 40 obtained from the pyrolysis of methylcyclohexane. Bottom, mass fragmentation pattern corresponding to the peak eluting at time 33.199 min.

.    A gas sample transfer line is connected to the end wall of the driven section of the shock tube. After the reflected shock has passed through the reactant mixture, the line opens for the proper sample time to collect a gas sample that has reacted for the known reaction time, temperature, and pressure and then delivers this sample to a series of analytic instruments. This instrumentation as pictured in Fig. 1 consists of one gas chromatograph (GC) with flame ionization (FID) and thermal conductivity (TCD) detectors, plus another gas chromatograph interfaced to a mass spectrometer, GC/MS. The availability of the powerful combination of GC/MS makes it possible to obtain total ion chromatograms, top Fig. 2, for comparison with FID chromatograms as well as mass fragmentation patterns for identification of individual chromatogram peaks, bottom Fig. 2.

.    Results obtained from the single pulse shock tube appear as species concentrations, in mole fraction, against temperature for a fixed reaction pressure (P5) and time. The reaction temperature for each shock is calculated using an external chemical thermometer which correlates the measured shock velocity (uncertainty ≤ 1%) with the gas temperature within the reaction zone. These shock velocities have been calibrated, external to the experimental sets, by measuring the extent of the unimolecular decomposition of 1,1,1-trifluoroethane which has a well-established rate constant. Previous studies into the effects of the quenching time and pressure fluctuations found these effects to be insignificant with respect to the reaction temperature. Therefore, the uncertainty in the defined reaction temperature is estimated to be no more than ±2% across the temperature range spanning these experimental sets.

.    In the photo shown in Fig. 1, the transfer lines and the shock tube are all wrapped in yellow insulation. This insulation ensures that the transfer lines and shock tube are maintained at 100°C in order to avoid condensation of large molecular weight species. Avoidance of condensation and loss of species is also ensured by using silica lined transfer lines and by having a direct connection between the sampling port (exit) of the shock tube and the entrance to the analytic instruments. Successful analytical results having a greater than 90% carbon recovery, suggest that any molecules, even large molecular weight species as seen in the chromatogram in Fig. 3, can very effectively be identified and quantified by our sampling system without much loss, Fig. 4.

Fig. 3. Typical chromatographic signal for phenyl radical (from phenyl iodide) pyrolysis using FID detector with DB-17ms column.

Fig. 4. Experimental carbon balance: (a) phenyl iodide decomposition and (b) phenyl + acetylene reaction.

 

 

 

 

 

 

 

LPST1

Fig. 5. UIC low pressure single pulse shock tube with direct connection to gas chromatograph (right bottom).

.    This tube functions as a single pulse shock tube in the same manner as the high pressure tube, i.e. one that will allow chemical analysis of withdrawn gas phase samples. This second shock tube can operate from 15 bar down to at least 0.1 atm. The low pressure shock tube is shown in Fig. 5 where in the foreground the direct connection to the gas chromatograph can be seen. The added value of this tube is that it can explore a complementary lower pressure range and has already been very successfully used in pyrolysis studies. The lower pressure range of this tube, along with the range accessible using the high pressure tube, provides a wide dynamic pressure range for examination of hypotheses about pressure dependent chemistry. Also in the photo in the bottom right corner is the LECO two dimensional (2D) gas chromatograph providing what is called GCxGC analysis in contrast to one dimensional (1D) gas chromatography. The potential importance of having a GCxGC capability is demonstrated in Fig. 6 below. On the left a 1D chromatogram of JP-8 is shown. JP-8 like almost all jet fuels (except JP-10) is a mixture of an enormous number of species. These species show up in the 1D chromatogram as many peaks. Nevertheless, these peaks do not capture all the separate species since there are many other species that are co-eluting (exiting the column at the same time) which only get resolved using a second gas chromatographic column as in GCxGC. Therefore, although each peak on the 1D plot on the left of Fig. 6 groups together all the different co-eluting species for a given retention time and thus obscures the phenomenon of co-elution, the 2D plot on the right of Fig. 6 shows the multiple species that in a sense lie behind the 1D plot. Therefore, if complex fuels need detailed composition characterization, then both 1D and 2D GC capabilities, as available in the UIC Single Pulse Shock Tube Facility, are essential.

Fig. 6. 1D (left) and a 2D (right) GCxGC chromatogram of JP-8 obtained at UIC.