Two-Phase Flow Visualization of Evaporating Liquid Fuels at Atmospheric Pressure

Two-phase flow visualization of fuel sprays is important for the design of better engines because it determines the efficiency and emissions of the combustion process. Simultaneous two-phase flow imaging using techniques such as planar laser-induced fluorescence (PLIF) has been a challenge due to the large variation in LIF signals from the gas and liquid phases. After laser excitation, the liquid signal initially overwhelms the gas phase signal due to its higher number density. However, the liquid signal quenches dramatically due to quenching effects that dominate the liquid LIF signal. By applying the novel concept of temporal filtering, separation of liquid and vapor signal can be achieved using different time delayed camera systems. The optical measurement provides a non-intrusive means of obtaining the liquid and vapor distributions in a spray. The experiment is performed using an ultraviolet beam from a burst-mode Nd:YAG laser in combination with two intensified cameras that are timed to maximize either the liquid or vapor phase signal. The setup is complemented by a drop generator and vaporizer flow system to allow studies of aviation fuels such as Jet-A or JP10, as well as reciprocating engine fuels such as diesel or toluene (as a surrogate for gasoline)

Cryogenic Hydrogen Oxygen Propulsion System for Planetary Science Missions

A Cryogenic Hydrogen Oxygen Propulsion System (CHOPS) that uses liquid hydrogen (LH2) and liquid oxygen (LO2) propellants can dramatically enhance NASA's ability to explore the solar system due to their superior specific impulse (Isp) capability. Although these cryogenic propellants can be challenging to manage and store, they allow significant mass advantages over traditional hypergolic propulsion systems and are therefore enabling for many planetary science missions. New cryogenic storage techniques such as subcooling, advanced insulation, low thermal conductivity structures allow for the long term storage and use of cryogenic propellants for solar system exploration and hence allow NASA to deliver more payloads to targets of interest, launch on smaller and less expensive launch vehicles, or both

Tracer-free liquid–vapor imaging using lifetime-filtered planar laser-induced fluorescence

The separation of liquid phase and vapor phase laser-induced fluorescence (LIF) signals using tracer species suffers from uncertainties in tracer–fuel coevaporation, as well as a disparity in liquid and vapor signals. This work demonstrates the use of a simple technique, referred to as lifetime-filtered LIF, to help separate the liquid and vapor signals of fuel sprays in oxygen-free environments without the use of added tracers. This is demonstrated for a common aviation fuel, Jet-A, using prompt detection of the liquid phase and time-delayed detection of the vapor phase. A scaled liquid signal subtraction algorithm is also demonstrated for removing vapor phase signal contamination caused by the largest droplets

Liquid-Vapor Imaging in Fuel Sprays Using Lifetime-Filtered Planar Laser-Induced Fluorescence

Many performance parameters in combustion systems are heavily dependent on the fuel injection process. Liquid-vapor imaging in spray applications has proven to be difficult because of the dominance of the liquid phase signal due to its higher number density. This work addresses this issue by employing temporal filtering in Planar Laser-Induced Fluorescence (PLIF) imaging. Temporal filtering takes advantage of the fact that although the vapor signal tends to be weaker, its fluorescence signal persists longer than the liquid signal under the same conditions. An experimental setup was designed with the capability to supply a two-phase flow in a controlled environment at atmospheric pressure. Fuels such as Jet-A, JP-10, and toluene were tested to determine their respective fluorescence lifetimes as well as their absorption and emission spectra. The effect of temporal filtering was then demonstrated for each material by characterizing the fluorescence decay profile for liquid and vapor under identical flow conditions and imaging system settings. A droplet subtraction method was also demonstrated using Jet-A in which a two-camera setup was utilized to subtract the liquid signal contribution from an image, leaving only a vapor measurement. Temporal filtering was also applied to exciplex tracers using various concentrations of N,N-Diethylmethylamine (DEMA) and fluorobenzene in hexane. The emission spectrum was determined for this combination in the liquid and vapor phases. The liquid, vapor, and cross-talk signals were quantified with increasing time delay after the laser pulse to determine the optimal timing for imaging. Finally, a demonstration of this technique was completed showing complete separation of the liquid and vapor signals

Tracer-free liquid–vapor imaging using lifetime-filtered planar laser-induced fluorescence

The separation of liquid phase and vapor phase laser-induced fluorescence (LIF) signals using tracer species suffers from uncertainties in tracer–fuel coevaporation, as well as a disparity in liquid and vapor signals. This work demonstrates the use of a simple technique, referred to as lifetime-filtered LIF, to help separate the liquid and vapor signals of fuel sprays in oxygen-free environments without the use of added tracers. This is demonstrated for a common aviation fuel, Jet-A, using prompt detection of the liquid phase and time-delayed detection of the vapor phase. A scaled liquid signal subtraction algorithm is also demonstrated for removing vapor phase signal contamination caused by the largest droplets.This article is published as Douglawi, Alber, Anthony McMaster, Megan E. Paciaroni, James B. Michael, Benjamin R. Halls, James R. Gord, and Terrence R. Meyer. "Tracer-free liquid–vapor imaging using lifetime-filtered planar laser-induced fluorescence." Optics Letters 44, no. 8 (2019): 2101-2104. DOI: 10.1364/OL.44.002101.</p