Application of gaseous laser-induced fluorescence in low-temperature convective heat transfer research

Abstract

Laser-induced fluorescence (LIF) thermography has emerged as a technique for measurement of two-dimensional temperature fields with minimal intrusion. This technique has been applied in the past to convective heat transfer problems involving liquids and can provide valuable local heat transfer information in spatially non-uniform convective flows. It has also been used in high-temperature gaseous flows, including combustion chambers and shock tubes. However, LIF has scarcely been used in low-temperature convective heat transfer applications involving gases, where its sensitivity is often limited. Low temperature here refers to the range from room temperature to 100–200 °C, where most other gaseous LIF studies are classified high temperature with maxima exceeding 250 °C. This study investigates the use of LIF thermography for low-temperature gaseous convective flows, with temperatures near ambient conditions. It demonstrates the utility of LIF thermography for a wider range of low-temperature engineering applications. The fluorescence was excited with a 266 nm laser sheet using a custom-built apparatus. The relationship between toluene fluorescence intensity and temperature was validated in the temperature range 20–60 °C, which is substantially lower than in previous studies to date. The same setup was used to measure the temperature field that develops during free and forced convection around a heated cylinder. The thermographic performance of anisole, which has been used in relatively few LIF studies to date, was also investigated. Sample images of toluene fluorescence intensity $$I\left( {x,y} \right)$$ surrounding a heated cylinder for (a) $$\dot{Q} = 2.15$$ W ( $${\text{Ra}} = 15,200$$ ) and (b) $$\dot{Q} = 0$$ W and $$T\left( {x,y} \right) = T_{{{\text{ref}}}} = 21$$ °C. The dashed circle marks the position of the cylinder surface. Dividing image (a) by image (b) gives the (c) the normalized fluorescence intensity $$I^{*} \left( {x,y} \right) = I\left( {x,y,T} \right)/I\left( {x,y,T_{{{\text{ref}}}} } \right)$$ , from which the (d) temperature field $$T\left( {x,y} \right)$$ can be determined.