G. Cardone

A survey on infrared thermography for convective heat transfer measurements

During the past several years infrared thermography has evolved into a powerful investigative means of thermo-fluid-dynamic analysis to measure convective heat fluxes as well as to investigate the surface flow field behaviour over complicated body shapes. The basic concepts that govern this innovative measurement technique together with some particular aspects linked to its use are herein reviewed. Different operating methods together with their implementations are also discussed. Finally, the capability of infrared thermography to deal with several simple, or complex, fluid flow configurations is analysed.

Thermofluidynamic analysis of the flow in a sharp 180° turn channel

Abstract The aim of the present study is to obtain surface flow visualisation, as well as local and spanwise averaged heat transfer measurements near a 180° sharp turn in a rectangular channel. The channel aspect ratio (width to height ratio) varies from 1 to 5 and the ratio between the width of the channel and that of the partition wall is always equal to 5. Heat transfer measurements are performed by means of the heated-thin-foil technique, which practically corresponds to a constant heat flux boundary condition, and by using infrared (IR) thermography. Two different heating conditions, in particular heating from one side (asymmetrical), or from two sides (symmetrical), are implemented. The convective heat transfer coefficient is evaluated from the measured temperature maps and the local bulk temperature of the flow which is obtained by making a one-dimensional balance along the channel. Results are presented in terms of local, or averaged, Nusselt number which is normalised with the classical Dittus and Boelter correlation. The fluid used during the test is air and the Reynolds number, based on the flow average velocity and channel hydraulic diameter, is varied between 16,000 and 60,000.

Heat Transfer Measurements on a Rotating Disk

Heat transfer to a rotating disk is measured for a wide range of Reynolds number values in the laminar, transitional and turbulent flow regimes. Measurements are performed by making use of the heated-thin-foil technique and by gauging temperature maps with an infrared scanning radiometer. The use of the IR radiometer is advantageous on account of its relatively good spatial resolution and thermal sensitivity and because it allows one to perform measurements down to very low local Reynolds numbers. Data is obtained on three disks, having an external diameter varying from 150mm to 450mm; the smallest disk is used only to measure the adiabatic wall temperature and can rotate up to 21,O00rpm. Heat transfer results are presented in terms of Nusselt and Reynolds numbers based on the local radius and show a substantial agreement with previous experimental and theoretical analyses. Transition to turbulent flow is found at about Re=250,000. A discussion about the role played by the adiabatic wall temperature is also included.

Viscous interaction phenomena in hypersonic wedge flow

We deal with an experimental investigation carried out to study some aspects of shock/boundary-layer interaction in nominally two-dimensional hypersonic wedge flow, i.e., over flat plate/rump configurations. These flow conditions are two dimensional only geometrically because some spanwise periodic variations of the heat flux over the ramp in the reattaching flow region are observed. The measurements, basically made by means of a computerized infrared (IR) imaging system, have been performed in a blowdown wind tunnel at Mach number equal to 7.14 and unit Reynolds number ranging from 7.6 x 10 6 to 24 X 10 6 /m. The influence of the leading-edge shape (bluntness and geometry), flat plate length, and ramp angle on the separation region, average heat transfer at reattachment, and wavelength of the heat transfer variations has been analyzed. IR results are compared to other experimental data as well as to semiempirical correlations for the heat flux peak.

Wall heat transfer in static and rotating 180° turn channels by quantitative infrared thermography

Abstract The aim of the present study is to develop a new experimental methodology that allows one to perform accurate measurements of the local heat transfer distribution before, in, and after a 180° sharp turn in static and rotating channels. Preliminary measurements of convective heat transfer coefficients are performed by means of infrared thermography applied to the steady state ‘heated-thin-foil’ technique. Some preliminary results in terms of Nusselt number Nu distributions and profile, as well as averaged Nu profiles along the channel axis, are presented. Results prove that infrared thermography is capable of measuring heat flux coefficients and detecting particular phenomena linked to the fluid flow configuration such as location of separation bubbles, influence of the channel aspect ratio as well as the influence of the channel rotation.

Experimental analysis of surface flow on a delta wing by infrared thermography

IR thermography has been employed for heat transfer measurements and surface flow visualizations on a 65-deg delta wing model.

Modulation transfer function cascade model for a sampled IR imaging system

The performance of the infrared scanning radiometer (IRSR) is strongly stressed in convective heat transfer applications where high spatial frequencies in the signal that describes the thermal image are present. The need to characterize more deeply the system spatial resolution has led to the formulation of a cascade model for the evaluation of the actual modulation transfer function of a sampled IR imaging system. The model can yield both the aliasing band and the averaged modulation response for a general sampling subsystem. For a line scan imaging system, which is the case of a typical IRSR, a rule of thumb that states whether the combined sampling-imaging system is either imaging-dependent or sampling-dependent is proposed. The model is tested by comparing it with other noncascade models as well as by ad hoc measurements performed on a commercial digitized IRSR.

Goertler instability of a hypersonic boundary layer

The Goertler instability of a hypersonic boundary layer and its influence on the wall heat transfer are experimentally analyzed. Measurements, made in a wind tunnel by means of a computerized infrared (IR) imaging system, refer to the flow over two-dimensional concave walls. Wall temperature maps (that are interpreted as surface flow visualizations) and spanwise heat transfer fluctuations are presented. Measured vortices wavelengths are correlated to non-dimensional parameters and compared with numerical predictions from the literature.

Convective Heat Transfer and Infrared Thermography

Abstract: Infrared (IR) thermography, because of its two‐dimensional and non‐intrusive nature, can be exploited in industrial applications as well as in research. This paper deals with measurement of convective heat transfer coefficients (h) in three complex fluid flow configurations that concern the main aspects of both internal and external cooling of turbine engine components: (1) flow in ribbed, or smooth, channels connected by a 180° sharp turn, (2) a jet in cross‐flow, and (3) a jet impinging on a wall. The aim of this study was to acquire detailed measurements of h distribution in complex flow configurations related to both internal and external cooling of turbine components. The heated thin foil technique, which involves the detection of surface temperature by means of an IR scanning radiometer, was exploited to measure h. Particle image velocimetry was also used in one of the configurations to precisely determine the velocity field.

Görtler-type vortices in hypersonic flows: the ramp problem

Abstract This paper deals with the wall heat flux measurements in the presence of thermal striations that can occur in the reattaching flow after a Mach 7.1 shock wave-boundary layer interaction on a 15° flap. Two different experimental techniques were used: thin skin with thermocouples and thin film with infrared thermography. Special care was taken to correct the lateral conduction effects. They can be important because of the small wavelengths of the spanwise striations. The influence of the Reynolds number was assessed by varying the unit Reynolds number and the length of the flat plate while keeping the same leading edge geometry. The effect of a sinusoidal perturbation at the leading edge also was studied. Finally, Navier Stokes laminar computations were performed to obtain the proper level of the laminar heat flux and to analyze the effect of the leading edge thickness. The calculations confirm the strong influence of the leading edge on the interaction phenomena.