Chemical kinetics has come a long way since Arrhenius found out the empirical relation between the rate constant and temperature. Today, physical chemists do research in state-to-state kinetics in which the rates are measured for reactions with a reactant prepared in a single quantum state transforming in to the product in a single quantum state. State-to-state kinetics is very valuable for determining the potential energy hyper-surface that controls the reaction and for fundamental understanding in reaction dynamics. However, the need for temperature dependent rate constant k(T) in practical systems such as combustion, explosion, detonation etc. is as pertinent today as ever. Chemists have always been looking for improving the performance of a fuel, propellant, or a fire-retardant. Ethanol has been used as a fuel additive for over 20 years in many countries and recently, Government of India also issued a directive to add ethanol to petroleum products used as a fuel in automobiles. Also, now it is known that the effective fire-retardant methyl bromide (halon) and aerosol propellants (freons) have to be replaced as they can cause stratospheric ozone depletion. Naturally, as we learn more about the nature of the chemicals used in these commercial applications, there will be a need to develop alternatives. It will be important to study these chemicals in the laboratory under conditions pertinent to the real situation. In this regard shock tubes offer unique features to enable such studies in the laboratory. Shock tube is by no means a modern technique. It has been used for the last several decades with applications in chemistry, physics, material sciences, and aerodynamics and biology, as well. International biennial symposia are held on the development and applications of shock tube technology and the proceedings from the symposia are useful sources of references. Last year a comprehensive three volume handbook has been produced. There is a vast body of literature available about not only the design and performance of the shock tubes but also the nature of the shock wave and its interaction with the test gas. In India, shock tube facilities have been established in a few laboratories in IITs and IISc. However, so far 'chemists' have not exploited the unique capabilities of the shock tube in India. Realizing the importance of a fruitful collaboration between Aerospace Engineering and Chemistry Departments, we have established a high temperature chemical kinetics laboratory with two dedicated shock tubes. This paper describes the details of the newly developed shock tube facilities, highlighting the important differences between our design and the existing ones along with some typical data obtained using these facilities.
The pulsed nozzle Fourier transform microwave spectrometer combines the supersonic expansion technique with the cavity Fourier transform microwave spectrometer. It has several components: i) a Fabry-Perot cavity made of two highly polished Aluminium mirrors (surface roughness better than microns) one of which is movable; ii) supersonic nozzle source for producing a cold jet/beam of molecules; iii) high vacuum chamber pumped by a 20" diffusion pump housing both i and ii; iv) microwave electrical circuit for polarizing the molecules and for detecting the molecular emission. The complete design of the spectrometer is described below in two parts as mechanical and electrical.
INTRODUCTION
CHEMICAL (SINGLE PULSE) SHOCK TUBE: CST-1.
(a) Mechanical design
The chemical shock tube, CST-1, is an Aluminium tube of 50.8 mm diameter. It has 1.3 m long driver section and 2.6 m long driven section separated by an aluminium diaphragm. The length of the driver and/or driven section could be varied by adding small segments. A schematic diagram of the CST-1 is shown in Fig.1. Two homemade platinum thin film thermal sensors, mounted 30.4 cm apart towards the end portion of the driven section are used to measure the shock velocity. The outputs from the two sensors trigger a counter (HP 5314A) to start and stop counting. The output from one of the sensors is also used as the trigger source for the digital oscilloscope (Tektronix TDS 210) which collects the pressure signal from a piezoelectric transducer (Kistler 601A) mounted at the end of the driven section. Thus the shock velocity could be independently measured using the scope and cross- verified with that measured from the timer. Helium is used as the driver gas.
The sample is loaded into the driven section as a dilute mixture in Argon at a pressure, P1 and temperature T1. The diaphragm is ruptured by increasing the pressure P4in the driver side, which creates a normal shock wave travelling through the sample gas in the driven side. At the same time, an expansion fan travels into the driver section in the opposite direction. The temperature and pressure of the sample gas is raised to T2 and P2 by the passage of the primary shock wave. The heated gas travels behind the normal shock wave at a slower velocity than the shock wave velocity. The contact surface separates the shocked test gas from the driver gas. The spacing between the contact surface and the shock front (i.e. the test gas region) increases along the length of the driven section. The primary shock is reflected at the end flange and the reflected shock wave further raises the temperature and pressure of the test gas to T5 and P5, respectively. The sample is kept at this temperature until the expansion fan passes through the contact surface, resulting in sudden drop in temperature and pressure. Cooling rate of 1 * 106 K s-1 is achieved easily. Thus, the reflected shock produces a high temperature pulse with a rise time of about 1 ms and pulse length which can be varied from 0.5 to 1.5 ms. Hence, such shock tubes are known as single pulse shock tubes (SPST). If the reflected shock wave meets the contact surface before the arrival of the expansion fan, it can lead to complicated interactions between the two depending on the impedance of the two regions. In the single pulse shock tube, this is avoided by adding two attachments, which are discussed next.
Firstly a dump tank is used for producing an expansion fan as well as for 'swallowing' the un-reacted sample, if any. In the original version of SPST, the dump tank was placed at the end of the driver section but later on it was placed near the diaphragm in the driven section. The operation of the dump tank was never fully understood but the pressure trace measured near the end flange showed that the heating pulse was well defined and the cooling rate was good in presence of the dump tank. Another attachment that is common to the SPSTs is a ball valve near the end section. The ball valve is essential to have a well-defined pulse length or dwell time, as it is commonly referred to in SPST literature.