aerial infrared thermography

Download Aerial Infrared Thermography

Post on 01-Jan-2017

215 views

Category:

Documents

3 download

Embed Size (px)

TRANSCRIPT

  • ARTICLE IN PRESSOptics and Lasers in Engineering 44 (2006) 2612810143-8166/$ -

    doi:10.1016/j

    CorrespoE-mail adInfrared thermography: An optical method inheat transfer and fluid flow visualisation

    T. Astarita, G. Cardone, G.M. Carlomagno

    Universita degli studi di Napoli Federico II, Dipartimento di Energetica Termofluidodinamica Applicata

    e Condizionamenti Ambientali, DETEC, P.le Tecchio 80, 80125 Napoli, Italy

    Available online 23 May 2005Abstract

    This paper deals with the evolution of infrared thermography into a powerful optical

    method to measure wall convective heat fluxes as well as to investigate the surface flow field

    behaviour over complex geometries. The most common heat-flux sensors, which are normally

    used for the measurements of convective heat transfer coefficients, are critically reviewed.

    Since the infrared scanning radiometer leads to the detection of numerous surface

    temperatures, its use allows taking into account the effects due to tangential conduction

    along the sensor; different operating methods together with their implementations are

    discussed. Finally, the capability of infrared thermography to deal with three complex fluid

    flow configurations is analysed.

    r 2005 Elsevier Ltd. All rights reserved.

    Keywords: Heat-flux sensors; Convective heat transfer; Surface flow visualisation; Infrared thermography1. Introduction

    Usually, measuring convective heat fluxes requires both a sensor (with itscorresponding thermal model) and some temperature measurements. In the ordinarytechniques [16], where temperature is measured by thermocouples, resistancesee front matter r 2005 Elsevier Ltd. All rights reserved.

    .optlaseng.2005.04.006

    nding author. Tel.: +39081 768 3389; fax: +39081 239 0364.

    dress: astarita@unina.it (T. Astarita).

  • ARTICLE IN PRESS

    T. Astarita et al. / Optics and Lasers in Engineering 44 (2006) 261281262temperature detectors or pyrometers, each transducer yields either the heat flux at asingle point, or the space-averaged one; hence, in terms of spatial resolution, thesensor itself can be considered as zero-dimensional. This constraint makesexperimental measurements particularly troublesome whenever temperature, and/or heat flux, fields exhibit spatial variations.

    As long as the fluid is transparent to the employed infrared band, the infraredscanning radiometer (IRSR) constitutes a true two-dimensional temperaturetransducer since it allows the performance of accurate measurement of surfacetemperature maps even in the presence of relatively high spatial temperaturevariations. Correspondingly, the heat-flux sensor may become two-dimensional aswell. In particular, infrared thermography can be fruitfully employed to measureconvective heat fluxes, in both steady and transient techniques [79]. Within thiscontext, IRSR can be intrinsically considered as a thin-film sensor [5] because itgenerally measures skin temperatures. The thermal map obtained by means ofcurrently available computerised thermographic systems is formed through a largeamount of pixels (20300K and more) so that IRSR can be practically regarded as atwo-dimensional array of thin films. However, unlike standard thin films, which havea response time of the order of microseconds, the typical response time of IRSR is ofthe order of 101103 s.

    The use of IRSR as a temperature transducer in convective heat transfermeasurement appears, from several points of view, advantageous if compared tostandard transducers. In fact, as already mentioned, IRSR is fully two-dimensional;it permits the evaluation of errors due to tangential conduction and radiation, and itis non-intrusive. For example, the last characteristic allows to get rid of theconduction errors through the thermocouple or resistance temperature detectorwires.2. Heat-flux sensors

    Heat-flux sensors generally consist of plane slabs with a known thermal behaviour,whose temperature is to be measured at fixed points [16]. The equation for heatconduction in solids applied to the proper sensor model yields the relationship bywhich measured temperature is correlated to the heat transfer rate.

    The most commonly used heat-flux sensors are the so-called one-dimensional ones,where the heat flux to be measured is assumed to be normal to the sensing elementsurface, i.e. the temperature gradient components that are parallel to the slab planeare neglected. In practice, the slab surfaces can also be curved, but their curvaturecan be ignored if the layer affected by the input heat flux is relatively small ascompared to the local radius of curvature of the slab.

    In the following, first ideal one-dimensional sensors are considered and then,whenever possible, the use of some of them will be extended to the multi-dimensionalcase. The term ideal means that thermophysical properties of the sensor material areassumed to be independent of temperature and that the influence of the temperaturesensing element is not considered.

  • ARTICLE IN PRESS

    T. Astarita et al. / Optics and Lasers in Engineering 44 (2006) 261281 263The most commonly used one-dimensional sensor models are:(1) Thin-film sensor: A very thin resistance thermometer (film) classically measuresthe surface temperature of a thermally thicker slab to which is bonded. Heat fluxis inferred from the theory of heat conduction in a semi-infinite wall. The surfacefilm must be very thin so as to have negligible heat capacity and thermalresistance as compared to the slab ones. To use this sensor with infraredthermography, the heat exchanging surface must be necessarily viewed by IRSR.(2) Thick-film sensor: The slab is used as a calorimeter; heat flux is obtained from thetime rate of change of the mean slab temperature. This temperature is usuallymeasured by using the slab as a resistance thermometer.(3) Wall calorimeter or thin-skin sensor: The slab is made thermally thin (so that itstemperature can be assumed to be constant across its thickness) and, as in thecase of the thick-film sensor, is used as a calorimeter. Heat flux is typicallyinferred from the time rate of change of the slab temperature which is usuallymeasured by a thermocouple. To use this sensor with infrared thermography,either one of the slab surfaces can be generally viewed by IRSR.(4) Gradient sensor: In this sensor the temperature difference across the slabthickness is measured. By considering a steady-state heat transfer process, heatflux is computed by means of the temperature gradient across the slab. Thetemperature difference is usually measured by thermopiles made of very thin-ribbon thermocouples, or by two thin-film resistance thermometers.(5) Heated-thin-foil sensor: This method consists of steadily heating a thermally thinmetallic foil, or a printed circuit board, by Joule effect and by measuring the heattransfer coefficient from an overall energy balance. Also, in this case, due to thethinness of the foil, either one of the slab surfaces can be viewed by IRSR.Strictly speaking, there is another type of one-dimensional sensor, the circularGardon gauge, in which the heat flux normal to the sensor surface is related to aradial temperature difference, in the direction parallel to the gauge plane [1]. Thissensor is practically of no use in infrared thermography.

    Recently, another type of heat-flux sensor based on a three-dimensional unsteadyinverse model and IRSR surface temperature measurements has been also developed[10] but for sake of simplicity it will not be herein described.

    Application of IRSR to both the thick-film and the gradient sensors is not verypractical, so these sensors will not be herein described. The heated-thin-foil sensorrepresents a quasi-steady technique that will be discussed in the next paragraph; thethin-film and the wall calorimeter sensors constitute transient techniques that will betreated in the following one.3. The heated-thin-foil steady-state technique

    Within the class of steady-state techniques to measure convective heat fluxesbetween a fluid stream and a surface, a method, where the application of IRSR seems

  • ARTICLE IN PRESS

    Fig. 1. Heated-thin-foil sensor.

    T. Astarita et al. / Optics and Lasers in Engineering 44 (2006) 261281264to be very effective, is the heated-thin-foil technique. The sensor is made of a thinmetallic foil which is heated by Joule effect (see the sketch of Fig. 1a). The mainlimitation of this technique is that, for practical reasons, the exchanging surfaceshould have a cylindrical, or conical, geometry.

    In the following, it is initially supposed that the sensor is one-dimensional and thatthe surface not exposed to flow is adiabatic. By making a very simple (one-dimensional) steady-state energy balance, it is found

    Qj Qr Qc, (1)

    where Qj is the imposed constant Joule heating per unit area, Qr is the radiative heatflux to ambient, and Qc is the convective heat flux to fluid.

    The radiative heat flux can be evaluated by

    Qr sT4w T4amb, (2)

    where s is the StefanBoltzmann constant, is the total emissivity coefficient, and Twand Tamb are the temperature of the wall and of the experimental ambient,respectively. When standard techniques are used to measure the wall temperature, itis possible to have a very low wall emissivity coefficient so as to ignore the radiative

  • ARTICLE IN PRESS

    T. Astarita et al. / Optics and Lasers in Engineering 44 (2006) 261281 265heat flux to ambient. Obviously, this is not the case when measuring temperatures bymeans of IRSR.

    The convective heat flux can be expressed according to Newton law:

    Qc hTw T r, (3)

    where h is the convective heat transfer coefficient and Tr is a reference temperature.The reference temperature depends on the stream experimental conditions. Forexample, for hi

Recommended

View more >