The CRC Handbook of Mechanical Engineering. Chapter 4. Heat and Mass Transfer (776127), страница 47
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Spectroscopic gastemperature measurements can be accurate to +/–3 or 4% of reading, but require a significant investmentin effort as well as equipment (on the order of 1 to 2 years and $100,000 to $200,000). Several techniquesbased on Raman scattering have been used in combustion systems. Planar-lasar-induced fluorescencehas shown considerable promise as one of the newer methods.Infrared emission from a surface is described by two laws: the Stefan Boltzmann law describing thetotal emitted radiation as a function of temperature, and Planck’s law describing its distribution as afunction of temperature. These laws form the basis for all radiation-based surface-temperature detectors.Early radiometers focused the total infrared energy on a thermopile bolometer and used the temperaturerise across its calibrated heat loss path to measure the incident energy flux.
Solid-state photon detectorshave replaced thermopile bolometers as the detector of choice. Such a detector will respond to anyphoton having energy above a certain level (specific to the detector). Since the energy of a photon isinversely proportional to its wavelength, detectors respond to all wavelengths below some value. Moderndetectors use band-pass filters to limit the wavelength band of photons admitted to the detector and relyon Planck’s law to infer the temperature from the energy flux:Eb,l =where Eb,l =T=C1 =C2 =C l-5e1C2 lT-1(4.6.7)radiated power at the wavelength l, W/m2temperature, K3.743 ´ 108, Wmm4/m21.4387 ´ 104, mmKCommercial radiation temperature detectors use different wave bands for different temperature ranges,with different detectors for each band.
The emissivity of the surface must be known, as a function oftemperature, in the wavelength band used by the detector.© 1999 by CRC Press LLCHeat and Mass Transfer4-193Radiation detectors are vulnerable to interference from four sources: low signal-to-noise ratio at lowtemperatures (below a few hundred degrees C); radiation from the surroundings reflecting into thedetector (also usually more important at low temperatures); low spatial resolution (also more evident atlow temperatures); uncertainty in the emissivity of the surface (at all temperatures); and absorption ofradiation into water vapor and CO2 in the line of sight (at any temperature).A fiber-optic blackbody temperature detector system is offered by the Luxtron Corporation forstandards room and field service above 300°C.
The unit consists of a blackbody capsule fiber-opticallycoupled to a filtered, band-limited photon detector. Accuracy of 0.01 to 0.1°C is claimed, depending ontemperature level.A fluoroptic temperature-measuring system is also offered by the same company, for use only at lowertemperatures (–200 to +450°C). This system uses an ultraviolet-stimulated phosphor on the end of anoptical fiber as its sensor. The fluorescent signal from the phosphor decays with time, and its “timeconstant” is a function of temperature. Accuracy of +/–0.5°C is claimed for measurements within +/–50°Cof a calibration point, or +/–1°C within 100°C.Temperature-Sensitive Paints, Crayons, and BadgesTemperature-sensitive paints, crayons, and badges are available from several suppliers (Omega Engineering, Inc., Stamford, CT, and others in Germany and Japan).
Each undergoes an irreversible change(e.g., a change in color or a change from solid to liquid) at one specified temperature. With a range ofpaints, temperatures from ambient to about 1500°C can be covered. The accuracy generally quoted isabout +/–1% of level, although melting standards are available to +/–0.5°C.The phase-change materials melt at well-defined temperatures, yielding easily discernible evidencethat their event temperature has been exceeded. When more than one phase-change paint is applied tothe same specimen, there can be interference if the melt from the low-melting paint touches the highmelting material. Color change materials do not interfere, but are more difficult to interpret. Thecalibration of high-temperature paints (both phase change and color change) may shift when they areused on heavily oxidized materials, due to alloying of the oxide with the paint.
Recommended practiceis to calibrate the paints on specimens of the application material. The event temperature which willcause transformation depends on the time at temperature: short exposure to a high temperature oftenhas the same effect as long exposure to a lower temperature.The paints and crayons are nonmetallic and, therefore, tend to have higher emissivities for thermalradiation than metals. They should be used only over small areas of metallic surfaces, compared withthe metal thickness, or else their different emissivities may lead to a shift in the operating temperatureof the parts.The principal disadvantages of the paints and crayons are that they require visual interpretation, whichcan be highly subjective, and they are one-shot, irreversible indicators which respond to the highesttemperature encountered during the test cycle.
They cannot record whether the peak was reached duringnormal operation or during soak-back.Liquid crystals can be divided into three groups, depending on their molecular arrangements: (1)smectic, (2) nematic, and (3) cholesteric. Most of the temperature-sensitive liquid crystals now in useare cholesteric: made from esters of cholesterol. Their molecules are arranged in planar layers ofmolecules with their long axes parallel and in the plane of the layer. The molecules in each layer arerotated with respect to those in its neighboring layers by about 15 min of arc in a continuous, helicalpattern along an axis normal to the layers.The colors reflected from cholesteric liquid crystals are thought to be due to Bragg diffraction fromthe aligned layers.
The “wrap angle” between adjacent layers increases with temperature; hence, thecolor of the liquid crystal shifts toward short wavelengths (toward blue) as the temperature is raised.The color can also be affected by electric fields, magnetic fields, pressure, shear stress, and some chemicalvapors.Warm cholesterics are colorless liquids and they pass through a series of bright colors as they areheated through their “color-play” temperature band. The first color to appear is a deep red, followed by© 1999 by CRC Press LLC4-194Section 4yellow, green, blue, and violet. Further heating yields a colorless liquid again.
This cycle is reversibleand repeatable, and the color–temperature relationship can be calibrated.Liquid crystals selectively reflect only a small fraction of the incident light; hence, to enhance thebrightness of the color image, they must be backed up with black paint or a nonreflecting surface.A typical calibration is shown in Figure 4.6.8 for liquid crystals painted over black paint on analuminum calibration strip. The upper part of Figure 4.6.8 describes the color variation, while the lowerpart shows the imposed linear temperature distribution. The hot end is blue, the cold end is red.
Colorplay intervals range from 0.5 to 10.0°C. Liquid crystals whose color-play interval is on the order of 0.5to 2.0°C are often referred to as narrow-band materials, while those whose interval extends to 5.0 to10°C are called wide band. Narrow-band images are easy to interpret by eye. Wide-band images showonly subtle variations of color for small changes in temperature, and accurate work requires digital imagehandling or multiple images taken with different filters.FIGURE 4.6.8 Schematic representation of a calibration strip.Several different narrow-band liquid crystals can be mixed together to make a single, multi-eventpaint covering a wide range of temperatures, provided their color-play intervals do not overlap.
Such amixture yields a set of color-play bands, one for each component.Calibration. Liquid crystals are sold by event temperature and color-play bandwidth, with a nominalaccuracy of +/–1°C on the event temperature. In many applications, especially if the image is to bevisually interpreted, no further calibration is needed.The accuracy attainable with a liquid crystal is related to the width of the color-play interval.
Withnarrow-band material (a color-play interval of about 1.0°C), visual interpretation can be done with anuncertainty of 0.25 to 0.5°C. With digital image interpretation, spectrally controlled lighting and appropriate corrections for reflected light interference, the uncertainty can be held below 0.25°C.Early users reported that the perceived color of a liquid crystal depended on both the lighting angleand the viewing angle. This dependence can be eliminated by using a light source along the line of sight(coaxial viewing and illumination).Multiple-Event Paints. Several narrow-band paints can be mixed together to make a single paint withall the characteristics of each component, if their color-play intervals do not overlap. Each componentretains its original calibration and acts independently of the other components.Figure 4.6.9 shows the image from a five-event paint used to map the adiabatic wall temperatureisotherms around a heated block in mixed convection.
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