Uncertainty Analysis of Infrared Thermography in Convective Heat Transfer

Infrared thermography is the preferred choice in many industrial processes for thermal diagnostics, condition monitoring, and non-destructive testing. However, the inherent uncertainty of surface emissivity affects the accuracy of temperature measurement by infrared thermography. In this paper a comprehensive experimental investigation was conducted to assess the uncertainty of infrared thermography in convective heat transfer. Four convective heat transfer conditions, including natural and forced convection on a flat plate, were studied. A composite test plate was constructed with an embedded heater and thermocouples. The thermocouples were used as references to compare with measurements by the infrared camera. The results indicate that the uncertainty of temperature measurement is about 4°F (2.7 % of the wall-to-ambient temperature difference) with the largest uncertainty being contributed by calibration of the infrared camera. The uncertainty of the heat transfer coefficient is 4.2% which is largely contributed by wall temperature measurement. NOMENCLATURE A Plate surface area. K Thermal conductivity of air Kc Thermal conductivity of copper Kp Thermal conductivity of Plexiglas Nu Nusselt number, Equation (5) PF Power factor Pr Prandlt number Q Total heater power Rax Local Rayleigh number, Equation (10) Rex Local Reynolds number Rl Shunt resistance Ta Non-uniformity of temperature within mapping area TTC Mean temperature measured by thermocouple TIR Mean temperature measured by infrared camera TwTC Wall temperature measured by thermocouple TambTC Ambient temperature measured by thermocouple TIso TC Isothermal box temperature measured by thermocouple TBPTC Boiling point temperature measured by thermocouple TIPTC Ice point temperature measured by thermocouple TwIR Wall temperature measured by infrared camera VL Load voltage drop VR Shunt resistance voltage drop Greek Letters σ Stefan-Boltzmann constant ε Emissivity ρ Density Subscripts: o Imprecision uncertainty u Unsteadiness uncertainty c Calibration uncertainty INTRODUCTION Infrared thermography provides a non-intrusive and remote monitoring capability for evaluating the thermal signature of an object or process to establish its performance or operating condition. The accuracy of infrared thermography is not easy to be assessed because it is impractical to measure the emissivity of every object in the field of view and it is also difficult to determine the emissivity of a single object since surface conditions of objects (especially metals) change with time due to surface oxidation or deposition of foreign objects. The objective of this paper is to investigate the uncertainty of emissivity variations on temperature measurement using an infrared camera and its impact on calculating heat transfer coefficient. EXPERIMENTAL PROGRAM The experimental setup includes a wind tunnel, a composite test wall with embedded thermocouples, a thermocouple measurement system, and an infrared camera system. There are four experimental settings, which are natural convection on a vertical flat plate, natural convection on an opened and partially enclosed horizontal flat plate, and forced convection on a vertical flat plate. The forced convection is conducted at the exit of the wind tunnel.