A Second-Generation Aerosol Shock Tube for Combustion Research

Abstract

An improved, second-generation aerosol shock tube has been developed for the study of chemical kinetics of low-vapor pressure fuels. The improvements enable a wider range of fuel concentrations, enhanced spatial uniformity near the endwall of the shock tube, and limit the aerosol filling of the driven section to the region adjacent to the endwall. The range of fuel loading and the extent of spatial uniformity were confirmed using a line-of-sight forward scattering laser diagnostic as well as gas phase direct absorption measurements of the fuel. The shock tube aerosol filling technique uses a separate holding tank to prepare the mixture and a slightly under-pressure dump tank to carefully pull the aerosol mixture into the tube in a plugflow. As measured over the entire tube, the typical non-uniformity observed in post-evaporated aerosol concentration had a 1% coefficient of variance for an n-dodecane aerosol over the range of equivalence ratio 0.1 to 3.0. Similar performance occurs when using jet fuel, diesel fuel, or other real fuels as the liquid. Preliminary n-dodecane experiments show a significant improvement in the precision of ignition delay time measurements over previous results using the first-generation aerosol shock tube fill procedure. Introduction Shock tubes are near-ideal devices to study high temperature combustion chemistry. Properly operated, they can provide a near-constant-volume reactor volume with uniform temperature and pressure conditions that allows simple optical access for line-of-sight laser diagnostics. They are used extensively to measure ignition delay times, species concentration time-histories and elementary reaction rates that are needed in the development of chemical reaction mechanisms that describe the pyrolysis and oxidation of practical fuels and their surrogates [1]. A shock tube consists of a close-ended tube with two sections, driver and driven, divided by a thin diaphragm. During a shock wave experiment, the driver section is filled up to a high pressure (typically with helium) until the diaphragm bursts and an incident shock wave formed which propagates into the driven section filled with test gas mixture. The incident shock wave heats and compresses the test gas mixture moving it toward the endwall. Once the incident shock wave reaches the endwall, it reflects and this reflected shock wave further compresses and heat the test gas mixture. The test gas behind the reflected shock wave is stationary and heated to the final temperature and pressure conditions. Optical diagnostics can be applied to 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposi 4 7 January 2010, Orlando, Florida AIAA 2010-196 Copyright © 2010 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. 2 image and measure the different species that are formed or removed during the high temperature reactions. Although the shock tube is very versatile and can be applied to many different fuels, these devices, until recently, were generally limited to experimental mixtures of gaseous fuels. Low vapor pressure fuels like n-dodecane, and real fuels such as gasoline, jet fuel, and diesel fuel that are liquids at room temperature, were difficult to introduce into test gas mixtures. Preheating and evaporation of low-vapor-pressure fuels was necessary. This preheating had the potential of introducing uncertainties associated with condensation, fractional distillation, and premature decomposition and oxidation, all of which can severely affect the quality of the ultimate reaction kinetics data obtained. Recently Davidson et al. demonstrated a method which avoids these problems by filling the shock tube with fuel in an aerosol form, with relatively small droplets (d ~ 3 μm) [2]. In this method, the incident shock wave heats the aerosol in the test gas mixture to intermediate temperatures (500-800K), which then rapidly evaporates the very small droplets and creates a very uniform gas phase mixture containing the desired quantities of fuel. This uniform mixture is then further heated and compressed by the reflected shock wave, typically designed to trigger chemical reactions. The ability of this aerosol shock tube technique to produce high quality kinetic data is highly dependent on the initial uniformity of the aerosol in the driven section of the shock tube. In Davidson et al. [2] the aerosol shock tube was loaded with fuel from the endwall through a series of poppet valves that mixed the aerosol into the tube by turbulence. The loading method is also detailed in Hanson et al. [3]. This method, however, can be improved upon. The current study describes a second generation aerosol shock tube which provides improved spatial uniformity and higher fuel loadings. As well, the shock tube has also been modified to limit the extent that the aerosol fills the shock tube length.