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Shock Tube Investigations of High Temperature Reaction Kinetics

 

Combustion systems are characterized by a complicated interaction of flow and transport processes and a large number of elementary chemical reactions. The purpose of chemical kinetics is to unravel the underlying complex reaction mechanisms and to investigate the products and rates of selected reactions as function of temperature and pressure. In particular, the formation and the reactions of short-lived reactive intermediates (atoms and radicals) have to be well known. These intermediates maintain the combustion process and determine the end product distribution.

 

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Fig. 1: Top: schematic setup of a shock tube. Bottom: pressure/temperature-time profile during a shock tube experiment and typical concentration-time profiles of a) thermal or b) photolytic generation of the detected species.

The shock tube technique has been proven to be a very powerful method for investigating gas phase reactions at high temperatures. With shock tubes, the experimental pressure (0.1-1000 bar) and temperature (500-15000 K) can be easily varied over a wide range. Essentially, a shock tube apparatus (Fig. 1) is just a several meters long tube, which is divided by a diaphragm into a driver (high pressure) and a driven (low pressure) section. The test gas mixture, typically a highly diluted mixture of reactants in argon, is found in the driven section. The driver section is filled with helium or hydrogen until the diaphragm bursts. A shock wave is formed, which propagates downwards the tube at supersonic speed and heats and compresses the test gas within less than 1 μs (incident shock wave). The shock wave is reflected at the end wall and the preheated testgas is heated and compressed again (reflected shock wave). The resulting pressure- and temperature-time-profile is shown in the lower left part of Fig. 1. Typically, the constant conditions behind the reflected wave last for roughly 1 ms - long enough for studying chemical reactions, which are mostly fast at high temperatures. Experimental pressures and temperatures behind the shock waves are calculated from the measured shock wave velocity and the initial conditions using a 1-dimensional shock tube code. Small, independent corrections account for boundary layer perturbations.

In many cases, the species of interest are generated thermally by the decomposition of suitable precursor molecules (Fig. 1a), but often a photolytic production is also feasible (Fig. 1b). Finally, real-time detection of the concentration-time-profiles is accomplished through optical windows by means of a variety of sensitive spectroscopic absorption or emission techniques. We often use the highly sensitive laser absorption based frequency modulation (FM) spectroscopy for quantitative detection of small radicals behind shock waves. With FM spectroscopy the radicals methylene (1CH2 ) and formyl (HCO), which are both of considerable importance in combustion chemistry, could be detected behind shock waves for the first time.

 

Kiel Shock Tube

 

Our hydrogen driven shock tube, which is designed for the investigation of elementary gas phase reactions of small radicals (e.g., NH2, HCO, SiH2, 1CH2, NCN, CN), can be operated at temperatures of 700 K < T < 3500 K and pressures of 0.75 bar < p < 3.5 bar. Radicals are detected by means of FM spectroscopy, UV absorption, and (very soon) IR absorption.


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    Shock tube:

  • 8 m long, 8 cm inner diameter
  • electro-polished stainless steel
  • shock velocity measured by four piezo-electric pressure sensors
  • pmin < 10-7 mbar, oil free turbomolecular drag / diaphragm backing pump combination
    Sample preparation:
  • 18 l stainless steel tank
  • oil free vacuum (turbomolecular drag / diaphragm backing pump)
  • flow system with mass flow controllers

    Detection systems:

  • Nd:YVO4 laser pumped cw ring-dye laser system
  • FM spectroscopy setup
  • UV absorption setup
  • IR absorption setup
  • excimer laser for "side-on" photolysis



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Last Updated on Wednesday, 04 January 2017 10:47
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