Quantitative thermal imaging, the measurement of temperature by use of thermal imaging devices, is reviewed here from a metrological perspective with a focus on measurement confidence and system application to fields such as condition monitoring and healthcare diagnostics. Thermal imaging has seen greatly increased application for the measurement of temperature following dramatic improvements in practicality and price. Selected thermal imaging systems are reviewed here by providing some example measurements outputs from devices, highlighting their outcomes on measurement confidence and impact on practical use, such as in condition monitoring and healthcare diagnostics.

Accurate temperature measurements are critical in manufacturing, affecting both product quality and energy consumption. At elevated temperatures, non-contact thermometers are often the only option. However, such instruments require prior knowledge of the surface emissivity, which is often unknown or difficult to determine, leading to large errors. Here we present a novel imaging luminescence thermometer based on the intensity ratio technique using magnesium fluorogermanate phosphor, with the potential to overcome this limitation. We describe measurements performed on a number of engineering alloys undergoing heat treatment at temperatures of up to 750 °C and compare these measurements against a traditional contact thermocouple and thermal imager system. Agreement between the luminescence and embedded thermocouple temperatures was found to be better than 45 °C at all temperatures. However, the thermal imager measurement on the bare metal samples, with the instrument emissivity set to 1.0, showed differences of up to 500 °C at 750 °C, a factor of 10 larger. In an effort to improve the thermal imager accuracy, its instrument emissivity was adjusted until its temperature agreed with that of the thermocouple. When measuring on the bare metal, the effective emissivity was strongly sample dependent, with mean values ranging from 0.205 to 0.784. Since the phosphor derived temperatures exhibited substantially smaller errors compared to the thermal imager, it is suggested that this method can be used to compliment the thermal imaging technique, by providing a robust mechanism for adjustment of the instrument emissivity until agreement between the thermal imager and phosphor thermometer is obtained.

In this laboratory-based study, a plate was designed, manufactured and then characterised thermally and dimensionally using a thermal imager. This plate comprised a range of known scratch, dent, thinning and pitting artefacts as mimics of possible surface anomalies, as well as an arrangement of higher emissivity targets. The thermal and dimensional characterisation of this plate facilitated surface temperature determination. This was verified through thermal models and successful defect identification of the scratch and pitting artefacts at temperatures from 30 °C to 170 °C. These laboratory measurements demonstrated the feasibility of deploying in-situ thermal imaging to the thermal and dimensional characterisation of special nuclear material containers. Surface temperature determination demonstrated uncertainties from 1.0 °C to 6.8 °C (k=2). The principle challenges inhibiting successful deployment are a lack of suitable emissivity data and a robust defect identification algorithm suited to both static and transient datasets.

Recent advancements in the electronics industry led to smaller and more powerful systems. Therefore, efficient heat dissipation for cooling of microelectronics systems, integrated circuit chips, power semiconductor devices such as IGBTs and laser diodes is required. In such systems, heat fluxes in the order of MW/m2 need to be removed from small spaces, while maintaining the temperature below a certain design limit (Karayiannis and Mahmoud in Appl Therm Eng 115:1372–1397, 2017 [1]). Flow boiling in microchannels is one of the most promising methods for achieving these high cooling demands because it can dissipate large heat fluxes over a small surface area by utilizing the latent heat of the coolant (Wang in Appl Therm Eng 110:369–381, 2017 [2]). Literature review indicates that there are still disagreements on the prevailing flow patterns, heat transfer rates, and pressure drop trends; see Mahmoud and Karayiannis (Encyclopedia of Two-Phase Heat Transfer and Flow IV, 2018, pp 233–301 [3]). The main objective of the present work was to investigate the flow boiling flow patterns, heat transfer coefficient, and pressure drop characteristics in a square metallic microchannel at different inlet subcooling, mass flux and heat flux conditions, using water as the working fluid.

In this study the local two-phase heat transfer coefficients were obtained using wall temperature measurements and fluid saturation temperatures in a rectangular high-aspect-ratio (a = W-ch/H-ch = 22) polydimethylsiloxane (PDMS) microchannel with a hydraulic diameter of 192 mu m. The experiments used FC-72 liquid with a mass flux of 7.37 kg m(-2)s(-1) and various heat fluxes ranging between 3.34 and 61.95 kW m(-2). Fluid saturation temperatures were determined by interpolating pressure measurements obtained with integrated silicon ceramic based pressure sensors located near the inlet and outlet of the microchannel. The hydrodynamic and flow boiling characteristics of the microchannel were monitored using high frequency and high spatial resolution infrared thermography, with heat transfer coefficients obtained as a function of axial position, lateral position and time and at the inlet, middle and outlet sections of the microchannel. This enabled the effect of heat flux on local temperature, flow boiling heat transfer coefficient distribution and two-phase pressure drop to be determined. These results suggest that the two-phase heat transfer coefficient does not increase monotonically with the heat flux, and actually decreases under certain conditions. This work identifies that the heat transfer coefficient depends on the range of the heat flux and is correlated with vapour-liquid dynamics and liquid film thinning resulting in suspected dryout, observed simultaneously with the temperature measurements. The simultaneous application of high speed thermography and flow visualization has enabled uniquely detailed information to be obtained that is useful to a more detailed understanding of two-phase flow and bubble dynamics.

Infrared (IR) thermography was combined with simultaneous high speed flow visualisation and pressure measurements from integrated pressure sensors inside the microchannel, in order to produce two-dimensional (2D) high spatial and temporal resolution two-phase heat transfer coefficient (HTC) maps across the full domain of a polydimethylsiloxane (PDMS) high aspect ratio (a = 22) microchannel (D-h = 192 mu m). High spatial and temporal resolution two-dimensional wall temperature measurements and pressure data were obtained for a range of mass fluxes (G = 7.37-298 kg m(-2) s(-1)) and heat fluxes (q = 13.64-179.2 kW m(-2)) using FC-72 as the working liquid. The 3D plots of HTC provided fine details of local variations during bubble nucleation, confinement, elongated bubble, slug flow and annular flow regime. The optical images from the channel top revealed the local flow regimes and were correlated with simultaneous thermal images obtained from the channel base. The 3D plots of the 2D two-phase heat transfer coefficient with time across the microchannel domain were correlated with vapour-liquid dynamics and liquid film thinning (from the contrast of the optical images) which caused suspected dryout. The correlation between the synchronised at the same frame rate high-resolution thermal and optical images will assist in a better understanding of the heat transfer mechanisms during two-phase flow boiling in microchannels. This work intends to give a better insight into heat transfer coefficient spatial variation during flow instabilities with two-dimensional heat transfer coefficient plots as a function of time during a cycle of liquid-vapour alternations. Crown Copyright (C) 2017 Published by Elsevier Ltd. All rights reserved.

The precise characterisation of boiling in microchannels is essential for the optimisation of applications requiring two phase cooling. In this paper polydimethylsiloxane (PDMS) is employed to make microchannels for characterising microboiling. In particular the material properties of PDMS facilitate rapid prototyping and its optical transparency provides the capability to directly view any fluid flow. The production of microchannels is complicated by the need to integrate custom made sensors. This paper presents a PDMS microfluidic device with integrated commercial pressure sensors, which have been used to perform a detailed characterisation of microboiling phenomena. The proposed approach of integrating commercial pressure sensors into the channel also has potential applications in a range of other microsystems.