Fluorescence Thermometry Research Articles

The temperature of an evaporating water droplet with a monolayer coating: comparison of experiment and theory

An analytic solution is presented for the steady state temperature of an evaporating water droplet with a monolayer coating. The solution is based on the flux matching arguments of Fuchs where the continuum flux is matched to the kinetic flux one mean free path from the droplet surface. Theoretical temperatures are compared to experimental values measured with fluorescence thermometry for evaporating water droplets coated with hexadecanol and octadecanol monolayers.

Planar Liquid and Gas Visualization

AbstractPlanar Laser Induced Fluorescence (PLIF) diagnostics based on the fluorescent intensity, lifetime, and/or quenching properties of organic molecules have been developed for a variety of applications: (1) Exciplex‐Based Vapor/Liquid Visualization systems provide spectrally‐separated emissions from the vapor and liquid phases and thus allow the separate visualization of the vapor and liquid phases of an evaporating fuel spray; (2) Exciplex Fluorescence Thermometry systems allow the measurement of liquid phase temperatures over the range 20–400 °C. Such systems have been used to measure the temperature of hydrocarbon droplets falling through heated nitrogen; (3) Streamlines by Oxygen Quenching, in combination with droplet‐slicing‐imaging, has allowed the visualization of internal circulation patterns within sub‐millimeter droplets hydrocarbon droplets; and (4) Fluorescence Lifetime Imaging, a technique which uses two PLIF images, one obtained a few nanoseconds after the other, has been applied to the imaging of the fuel/oxygen equivalence ratio in isothermal methane jets and to the imaging of temperature in oxygen‐free jets. This paper reviews the photophysical and diagnostic principles upon which the current systems are based and addresses their applications and limitations.

Fluorescence Thermometry

The fluorescence of rare-earth-doped ceramic phosphors depends on temperature. Particularly, the fluorescence lifetime is decreased by temperature. This dependence makes the fluorescence suitable for measuring temperature. This paper describes the physics of the fluorescence of these ceramics, noting the works of Forster, Dexter, Inokuti, Hirayama, and others. Next, it outlines the several advantages of fluorescence thermometry. These advantages include (a) measurement of temperature by transfer to measurement standards for time, (b) remote sensing for surface thermometry, (c) high speed of measurement relative to many physical and mechanical phenomena, (d) narrow-spectrum optical sensing suitable for hostile electrical and luminous environments, and (e) the transfer of calibration standards for precise thermometry. The paper presents engineering considerations for realizing these advantages. It presents parameter-estimation techniques that allow measurement of the temperature-dependent fluorescence parameters. It describes instrumentation techniques that transfer the measurement of temperature to measurement of units of time, with instrument calibration by atomic standards. It also discusses other measurement and instrumentation details.

Exciplex Fluorescence Thermometry of Falling Hexadecane Droplets

Exciplex fluorescence thermometry has been used to measure the temperature of 283 micron hexadecane droplets falling through a quiescent, oxygen-free, approximately 500°C ambient. After a period of negligible change, the derived droplet temperatures exhibit a sharp rise of about 100°C followed by a gentle increase to approximately 200°C. The derived temperatures, although averaged over most of the volume of the droplet, still provide some evidence of internal processes in the droplet due to the partially selective optical sampling of the droplet volume, in which fluorescence from the region between 0.50 and 0.75 of the droplet radius contributes disproportionately. At longer times, the droplet is presumed to be approximately homogeneous, and the exciplex fluorescence thermometry measurements provide accurate, interpretable temperatures for the freely falling droplets.

Temperature-jump relaxation kinetics at liquid/solid interfaces: fluorescence thermometry of porous silica heated by a Joule discharge

Abstract : Temperature-jump relaxation techniques are adapted to measuring the kinetics of reversible reactions at interfaces of liquid phases/solid phases. In the present work, we determine whether fast temperature changes can be created within a packed bed of silica gel using Joule discharge heating. Fast heating of the surface of porous silica depends on the solution within the pores carrying current through the particle, since heat flow from the surrounding solution into the interior of the silica particle would generally be slow. To characterize the rate of heating of the silica surface, fluorescence thermometry was employed to measure the temperature rise at a liquid/solid interface on a microsecond time- scale. 9,10 dichloroanthracene (9,10 DCA) was found to be a suitable probe molecule for fast fluorescence measurement of temperature changes at alkylated silica surfaces. For 10 micrometers silica particles, we observe an exponential relaxation of the temperature-dependent fluorescence intensity with a decay time that corresponds to the rate of energy deposition in the sample. These results indicate that ionic current flows through the silica, uniformly heating the surface area of smaller particle-size silica, uniformly heating the surface area of smaller particle-size silica. Larger particle silica shows a slower thermal relaxation indicating non-conductive domains which are heated on a slower time- scale by thermal conduction.

Noncontact fluorescence thermometry of acoustically levitated waterdrops

Noncontact thermometry based on the fluorescence excitation spectrum of aqueous Eu(3+) (EDTA) near 579 nm allows us to measure the temperature of an evaporating drop of water. The results for drop diameters in the 500-microm-3-mm range confirm theoretical temperature predictions for steady state evaporation. Calibration of the excitation spectrum in a constant temperature cell indicates that +/-1.0 degrees C resolution is possible for temperatures below 20 degrees C. The spectrum depends only on the solution temperature when Eu(3+) (EDTA) concentrations are below 1 x 10(-3) M and when the solution pH is between 4.0 and 10.0. Excitation spectra from levitated waterdrops contain additional noise which degrades the temperature resolution to +/-1.2 degrees C. With this technique we are able to follow the temperature change in an evaporating drop of water as a monolayer of 1-octadecanol forms on the surface and retards the evaporation.

Temperature Measurements of Falling Droplets

The temperature of 225-μm-dia decane droplets, which have fallen 100 mm through a hot quiescent, oxygen-free environment, has been measured using exciplex fluorescence thermometry. The droplets were doped with pyrene, and the relative intensities of pyrene monomer and excimer emissions were used to determine the droplet temperatures. The droplet temperature increases approximately 0.4°C per °C increase in the ambient temperature up to an ambient temperature of 200°C. Less than 10 percent evaporation was observed for the droplets at the highest ambient temperatures.

Steady-state temperature of an evaporating water droplet with a monolayer coating

A mathematical model is derived for the steady-state temperature of an evaporating water droplet with a monolayer coating. The model uses the flux-matching arguments of Fuchs to describe the concentration and temperature fields outside the evaporating droplet. Assuming the mean molecular speed, the gas-phase diffusion coefficient, and the gas thermal conductivity to be temperature independent, a closed-form expression is derived for the steady-state temperature of the droplet as a function of the monolayer accommodation coefficient. Comparison between model predictions and recent fluorescence thermometry experiments shows good agreement.

EXCIPLEX method for remote probing of a fuel droplet temperature.

EXCIPLEX-based fluorescence thermometry was developed for remote probing of fuel droplet temperature. The fuel mixtures tested were n-heptane, n-decane and n-hexadecane which contained 1% naphthalene and 2.5% tetramethyl-p-phenylene diamine (TMPD). Also examined was unleaded regular gasoline into which no naphtalene or TMPD was dissolved. The fuel mixture suspended at the tip of a quartz fiber was allowed to evaporate in a stream of air or gaseous nitrogen, and its temperature was monitored using a fine thermocouple. The droplet was irradiated with a nitrogen laser and fluorescence emission spectra were taken by means of a light collection system which consisted of a convex lens, a light stop, a double monochromator and a photomultiplier. The results showed that remote probing of the droplet temperature could be achieved by measuring the ratio of fluorescence emission intensities at two different wavelengths.

Fluorescence thermometry of shocked water

We describe a fluorescence probe thermometer for measuring shock heating directly in transparent solvents. As a demonstration of this scheme, the temperature of shocked water was measured in a previously inaccessible pressure regime, 0–9 kbar, by observing the fluorescence enhancement from a fluorescein dye additive following 527-nm excitation and laser driven shock loading. A ring-up shock temperature increase of 33 °C was observed at 8.4 kbar with an accuracy of ±4 °C.

Atomic Fluorescence Study of High Temperature Aerodynamic Levitation

ABSTRACTUltraviolet laser induced atomic fluorescence has been used to characterize supersonic jet aerodynamic levitation experiments. The levitated specimen was a 0.4 cm sapphire sphere that was separately heated at temperatures up to 2327K by an infrared laser. The supersonic jet expansion and thermal gradients in the specimen wake were studied by measuring spatial variations in the concentration of atomic Hg added to the levitating argon gas stream. Further applications of atomic fluorescence in containerless experiments, such as ideal gas fluorescence thermometry and containerless process control are discussed.