Heat flux gauges

Figure 16.12) the heat flux field map was determined by taking several measurements using a heat flux gauge. 

Recent research has shown that phosphor-based TSPs could be used for high temperature thermometry applications. Fluorescence from YAGiEu and YAG Ce could be detected through several thin YSZ samples. The average prompt fluorescence decay time for YAGiCe was measured to be 62.7 zb 2.9 ns, which is close to the accepted value of 65 ns. This result can be used directly to develop an operational high temperature heat flux gauge. 

The determination of the heat flux through a surface is important in a variety of scientific and engineering applications. Noel, Turley, and Tobin, and Turley et al. pursued development of several thermal phosphor-based heat flux gauges.A standard expression relates the heat flux, q, to the insulator thickness, J, thermal conductivity, k, and temperature difference, AT, across an insulating barrier ... 

Fig. 7 (A) Phosphor-based heat flux gauge on turbine blade and (B) fluorescing gauge with matching triangular phosphor layers.

Fig. 8 Experimental arrangement phosphor-based heat flux gauge.

Turley, W.D. Borella, H.M. Noel, B.W. Beasley, A. Sartory, W.K. Cates, M.R. The Design and Characterization of a Prototype Optical Heat-Flux Gauge Los Alamos National Laboratory Technical Report No. LA-11408-MS, Jan 1989. 

FIGURE 1633 Sandwich-type heat flux gauge. 

Circular Foil Heat Flux Gauge. The instruments (often called Gardon heat flux gauges [114]) shown in Fig. 16.34 are also based on Fourier s law. A copper heat sink is installed in the wall of the measuring site with a thin constantan disc mounted over it. A small copper wire is attached to the center of the constantan disc. Another copper wire attached to the copper heat sink completes a thermocouple circuit. Heat flow to the constantan disc is conducted radially outward to the copper heat sink, creating a temperature difference between the center and edge of the disc. The copper and constantan (other materials could be used) act as a thermocouple pair to measure this temperature difference. 

However, Eq. 16.72 may involve some errors when a long-period calibration is performed on a rectangular thin-film heat flux gauge, shown in Fig. 16.35. Thus, a three-dimensional heat diffusion equation should be used. The boundary and initial conditions for the rectangular heat flux gauge are expressed as ... 

FIGURE 16.35 Geometry for determining the temperature response of a thin-film heat flux gauge attached on an infinitely long surface [120]. 

By solving the three-dimensional heat diffusion equation on the surface at x = 0, the average temperature for the rectangular heat flux gauge with an area of Aab [120] is... 

Standard Method for Measurement of Heat Flux Using a Copper-Constantan Circular Foil Heat Flux Gauge, ASTM Std. E-511, Annual Standard of ASTM, Pt. 41,1981. 

N. R. Keltner and M. W. Wildin, Transient Response of Circular Foil Heat-Flux Gauges to Radiative Fluxes, Rev. Sci. Instrum., 46, pp. 1161-1166,1975-... 

While total heat flux includes conduction, convection, and radiation, in most industrial applications the total heat flux is primarily radiation plus convection. Total heat flux has been measured using both steady-state and transient methods. Three different steady-state methods have been used. The first is to calculate the flux from an energy balance on an uncooled solid. The second is to measure the sensible energy gain of the coolant, for a cooled solid. The third is to directly measure the flux with a calibrated heat flux gauge imbedded in a cooled target. 

It was concluded that radiation was not important for their experimental conditions. You [46] measured flame radiation to the surroundings, using commercial heat flux gauges. The radiant flux to a plane surface was also indirectly measured. For these measurements, commercial heat flux gauges were coated with either gold or black foil, to measure the convective or total heat flux, respectively. The radiant flux was determined by subtracting the convective flux from the total. Thermal radiation was... 

Heat flux gauges are often used as radiometers by putting a glass window in front of them to prevent convective heat transfer to the gauge. A variety of materials may be used for the infrared detectors, each having a specific spectral characteristic as shown in Figure 6.19 [81]. 

Heat flux gauges need to be properly calibrated to ensure the measurements are accurate. Gauges... 

Another method of determining the convective heat flux by direct measurement involves adding a radiant shield between the heat source and the heat flux gauge to exclude radiation from being received by the gauge. The problem is that the shield often disturbs the flow and the measured flux is not representative of the actual convective flow. Anofher problem is that the surface temperature under the shield will be lower than it would be without the shield, so the measurement would need to be done quickly before fhe wall femperature under the shield drops too much. In addition, although radiation from fhe primary source (burners) may be shielded, the rest of fhe furnace walls may still contribute radiation to the gauge. 

A third technique that could be used is to make a perfectly reflective heat flux gauge so that any radiation received by the gauge is reflected away and not measured by the gauge. Then (in theory) the gauge would only measure convection, assuming no other heat transfer mechanisms were important. Although in theory this sounds plausible, the reality is that it is very difficult to have a perfect reflector in an industrial combustion environment. 

A fourth technique that could be used is to use a heat flux gauge embedded in the wall where convection is to be measured and then to turn off fhe radianf source and then quickly take measurements with the gauge. The radiation from fhe hot furnace walls can be calculated... 

You [10] used a high emissivity (0.96) paint to maximize the amount of radiant heat absorbed by the target. Three interrelated studies [105-107] used polished copper surfaces to reduce the effects of flame radiation. In some studies the heat flux gauge surface was coated or treated. 

Incident radiant heat flux measurements were performed using two instruments a hemispherical ellipsoidal radiometer and a circular foil heat flux gauge radiometer [51]. The former probe having a longer time response (30-60 seconds) and the latter probe capable of higher frequency (up to 250 Hz) measurements. 

Instrument Description Time-resolved and time-averaged incident heat flux data were obtained with a water-cooled, circular foil, heat flux gauge radiometer. A schematic of the instrument is shown in Figure 32.7. This instrument consisted of a... 

For the data acquired with the foil heat flux gauge, the amplified oufpuf from fhe detector was between 0 and... 

Data acquired using the circular foil heat flux gauge was reduced according to the previously stated linear calibration relation provided by the manufacturer, and incident radiant heat flux was calculated directly from the recorded foil gauge voltage output. 

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