An Approach to Thermocouple Measurements That Reduces Uncertainties in High-Temperature Environments

Abstract

Obtaining accurate temperature measurements with thermocouples in flame environments is challenging due to the effects of radiative heat losses, as these losses are difficult to quantify. Efforts to minimize radiative losses by, for example, suction pyrometry, often result in a significant sacrifice in spatial resolution. In this work, a new experimental methodology is presented that both minimizes the temperature correction and allows the remaining correction to be accurately quantified. The approach is based on increasing and controlling the convective heat transfer to the thermocouple junction, and is accomplished by spinning the thermocouple at high speed. The rotation yields a large and known convective velocity over the thermocouple. Heat transfer can then be modeled for the thermocouple, and a functional relationship between temperature and rotational speed can be found. Fitting this model to the data allows for an accurate temperature correction. To test the feasibility of the rotating thermocouple (RTC) technique for temperature measurement in high temperature gases, experiments were conducted over a range of rotational speeds in a controlled flame where the temperature was known. The measured thermocouple temperatures as a function of rotational speed closely match the theoretical temperatures, yielding a straightforward approach to highly accurate gas temperature measurement. The results also demonstrate limited perturbation to the flow field, even at high rotational speeds. Finally, a method of deconvolution is described that significantly enhances the spatial resolution of the technique, approaching that of a stationary thermocouple. Area (m 2 ) D Diameter of bead (m) ℎ Convective heat transfer coefficient (W/m 2 -K) RTC Rotating thermocouple L Length (m) ܳ ሶ Heat flux (W) r Radius (m) T Temperature (°C) V Velocity (m/s) ω Rotational speed (revolutions/min)

Diffuse-reflection-based single-ended laser absorption sensor for H2O temperature and concentration in kerosene-fuelled combustor

“…Real-field, time-resolved, in situ measurement results of temperature and H 2 O concentration in the kerosene-fuelled combustor versus varied level of equivalence ratio is shown in figure 6. While laser measurements were made through the centre flame zone, thermocouple measurements 72 mm downstream of the laser beam path are also plotted for comparison by considering the heat-transfer correction and reading error [30][31][32].…”

Heat Transfer Characteristic for Premixed Flame Jet from Swirl Chamber

“…Also, this thermocouple was able to adjust the measurement positions on the flame jet region with the X-Y transverse system. For accuracy measurements, the bead thermocouple was considered about the convective and radiative heat losses to surrounding that evaluated with a heat balance correlation of the bead, these suggested by Krishnan et al, [21]. The temperature measurement positions of the swirling flame jet structure in this work were evaluated in the range of 2<Z/Dh<10 and -2<r/Dh<2, as shown in Figure 4.…”

Quantifying the Effects of Combustion Gases’ Radiation on Surface Temperature Measurements Using Two-Color Pyrometry

“…Radiation thermometry is of great industrial importance and is a widely used method for measuring surface temperature in the severe environment of high-temperature industrial processes where resistance thermometers, thermocouples, or other contact-type sensors are found to be inappropriate. , A significant application of radiometric temperature measurements is the measurement of the surface temperature of parts in gas turbines and furnaces where the combustion gas environment is often at a temperature higher than that of the target surface . The main problem in this application is the large error arising from the absorption and emission of intervening combustion gases.…”

“…1,6 A significant application of radiometric temperature measurements is the measurement of the surface temperature of parts in gas turbines and furnaces where the combustion gas environment is often at a temperature higher than that of the target surface. 7 The main problem in this application is the large error arising from the absorption and emission of intervening combustion gases. The radiation from the target surface must spread some distances through the hot combustion gas layer to the thermometer.…”