Accurate knowledge of the heat flux characteristics provided by optical heat sources of long heating time nondestructive infrared thermography techniques is essential to determine the adequate application of such techniques; however, detailed characterizations are scarce. Therefore, a thermal and statistical characterization of a halogen lamp was developed. A highly repeatable experimental procedure was used to characterize the heat flux generated at an ideal inspection sample top surface. The characteristics studied were: lamp distance, bulb color, lamp orientation, heat quality, and heating time. The heat flux was determined by using the readings of temperature and heat flux from the sample back, and a finite differences lumped capacitance thermal model. Detailed studies using three sensors determined that the heat flux was nonuniform (13% maximum variation). Therefore, a full quantitative characterization of the lamp was developed by using the average of such sensor readings, determining that: this halogen lamp can provide consistent top heat fluxes (although not uniformly distributed) adequate for nondestructive testing infrared thermography, the lamp distance and bulb color affected the amount of heat provided as well as the heat flux uniformity, and lamp orientation did not affect the mean top heat fluxes. This research approach can be used to determine an approximation of the lamp time-averaged heat fluxes for any material with similar top surface optical characteristics. Moreover, the technical data provided are useful to determine the adequacy of heating time, lamp distance, lamp orientation, and bulb color for long heating time nondestructive testing infrared thermography.

Matrix cooling has opened new possibilities for enhancing the convective heat transfer coefficients without compromising upon the structural rigidity and the life of the gas turbine blade at elevated temperatures. However, the dense structure of the matrix significantly increases the flow resistance and imposes the limitation to its usage. Recently, a matrix with a gap on the sidewalls called open matrix has been proposed by few researchers to reduce the associated pressure penalties. This detailed experimental investigation aims to study the open matrix channel flow and presents the effects of varying sidewall gaps on heat transfer characteristics and friction factor in the open matrixes having rib angle 45 deg for three different subchannel aspect ratios 1.2, 0.8, and 0.4. Liquid crystal thermography has been utilized to discern the detailed heat transfer characteristics. The results have been evaluated in terms of augmentation Nusselt number, friction factor ratio, and overall thermal performance factor over the Reynolds numbers 5800–14,000. The closed matrixes provided the highest augmentation in Nusselt number, and the gaps on the sidewall have shown an overall reduction in augmentation Nusselt number in most cases. However, the suitable sidewall gaps show the effective reduction in pressure penalties for the smaller subchannel aspect ratios. The highest augmentation Nusselt number among the open matrixes has been found as 3.83 with a reduced friction factor ratio for the matrix with a 4-mm gap in subchannel aspect ratio = 0.8 (i.e., 4 subchannels) at Re = 8100.

High-performance computing systems are needed in advanced computing services such as machine learning and artificial intelligence. Consequently, the increase in electron chip density results in high heat fluxes and requires good thermal management to maintain the servers. Spray cooling using liquid offers higher heat transfer rates and is efficient when implemented in electronics cooling. Detailed studies of fundamental mechanisms involved in spray cooling, such as single droplet and multiple droplet interactions, are required to enhance the process's knowledge. The present work focuses on studying a train of two FC-72 droplets impinging over a heated surface. Experimental investigation using high-speed photography and infrared thermography is conducted. Simultaneously, numerical simulations using opensource CFD package, OpenFOAM are carried out, emphasizing the significance of contact angle hysteresis. The surface temperature is chosen as a parameter, and different boiling regimes along with Dynamic Leidenfrost point (DLP) for the present impact conditions are identified. Spreading hydrodynamics and heat transfer characteristics of these consecutively impinging droplets till the Leidenfrost temperature, are studied and compared.

Accurate quantification of local heat transfer coefficient (HTC) is imperative for design and development of heat exchangers for high heat flux dissipation applications. Liquid crystal and infrared thermography (IRT) are typically employed to measure detailed surface temperatures, where local HTC values are calculated by employing suitable conduction models, e.g., one-dimensional (1D) semi-infinite conduction model on a material with the low thermal conductivity and low thermal diffusivity. Often times, this assumption of 1D heat diffusion and ignoring its associated lateral conduction effects leads to significant errors in HTC determination. Prior studies have identified this problem and quantified the associated errors in HTC determination for some representative cooling concepts, by accounting for lateral heat diffusion. In this paper, we have presented a procedure for solution of three-dimensional (3D) transient conduction equation using alternating direction implicit (ADI) method and an error minimization routine to find accurate HTCs at relatively lower computational cost. Representative cases of a single jet and an array jet impingement under maximum crossflow condition have been considered here, for IRT and liquid crystal thermography, respectively. Results indicate that the globally averaged HTC obtained using the 3D model was consistently higher than the conventional 1D model by 7–14%, with deviation levels reaching as high as 20% near the stagnation region. Proposed methodology was computationally efficient and is recommended for studies aimed toward local HTC determination.

In order to give more sights into the melting (and solidification) heat transfer processes of nano-enhanced phase change material (NePCM) with invisible phase interfaces, a novel indirect method for tracking the phase interface by thermochromic liquid crystal (TLC) thermography is proposed. As an example case to demonstrate the applicability of the proposed method, the classical problem of melting heat transfer in a differentially heated rectangular cavity was revisited in the presence of NePCM of various loadings. A narrowband TLC was selected and calibrated carefully to build the hue–temperature relationship prior to being applied in the melting experiments. For validation purpose, the case of an unloaded NePCM, with a clear visible phase interface, was tested via combined direct and indirect observations. It was shown that this TLC method can easily and accurately capture the dynamic motions of the phase interface during melting. Based on the shape evolutions of the phase interface, it was concluded that for the NePCM sample with a higher loading (and hence a much greater viscosity), heat conduction becomes the dominant mode of heat transfer during melting as a result of the significantly deteriorated natural convection effect. This gives an intuitive confirmation of the hypothesis made in previous studies that were conducted using volume-average-based indirect methods.

In this work, fluid flow and heat transfer characteristics of three-dimensional (3D) wall jets exiting from a circular and square opening are presented based on experimental investigations. Two hydraulic diameters, namely, 2.5 and 7.5 mm and a Reynolds number range of 5000–20,000 have been considered. Mean velocity and turbulence intensity distribution in the walljet are quantified using a hot wire anemometry. Measurements are done both along the streamwise and spanwise directions. Transient infrared thermography is used for mapping the temperatures over the surface, and the heat transfer coefficients are estimated using a semi-infinite approximation methodology. Results show that, for circular jets, the effect of the jet diameter on the local and the spanwise-averaged Nusselt number is most pronounced near the jet exit. Further, it is also observed that circular jets have an edge over square jets. A correlation with a high correlation coefficient of 0.95 has been developed for spanwise average Nusselt number as a function of the Reynolds number and the dimensionless streamwise distance.

The thermal performance and operating modi of a flat-plate closed-loop pulsating heat pipe (PHP) are experimentally observed. The PHP is manufactured through computer numerical controlled milling and vacuum brazing of stainless steel 316 L. Next to a plain closed-loop PHP, also one that promotes fluid circulation through passive Tesla-type valves was developed. Each channel has a 2 × 2 mm 2 square cross section, and in total, 12 parallel channels fit within the 50 × 200 mm 2 effective area. During the experimental investigation, the power input was increased from 20 W to 100 W, while cooling was performed using a thermo-electric cooler (TEC) and thermostat bath. Three working fluids were assessed: water, methanol, and ammonia. The PHP was charged with a 40% filling ratio. Thermal resistances were obtained for different inclination angles. It was observed that the PHP operates well in vertical evaporator-down orientation but not horizontally. Moreover, experiments show that the minimum operating orientation is between 15 and 30 deg. Two operating modi are observed, namely, the thermosyphon modus, without excessive fluctuations, and the pulsating modus, in which both the temperature and pressure responses oscillate frequently and violently. Overall thermal resistances were determined as low as 0.15 K/W (ammonia) up to 0.28 and 0.48 K/W (water and methanol, respectively) at a power input of 100 W in the vertical evaporator-down orientation. Infrared thermography was used to visualize the working fluid behavior within the PHPs. Infrared observations correlated well with temperature and pressure measurements. The experimental results demonstrated that the developed flat-plate PHP design, suitable for high-volume production, is a promising candidate for electronics cooling applications.

In internal cooling passages in a turbine blade, rib structures are widely applied to augment convective heat transfer by the coolant passing through over the ribbed surfaces. This study concentrates on perforated 90 deg ribs with inclined holes in a cooling duct with rectangular cross section, aiming at improving the perforated holes with additional secondary flows caused by inclined hole arrangements. Two sets of perforated ribs are used in the experiments with the inclined angle of the holes changing from 0 deg to 45 deg and the cross section are, respectively, circular and square. Steady-state liquid crystal thermography (LCT) is applied to measure the ribbed surface temperature and obtain corresponding convective heat transfer coefficients (HTCs). Two turbulence models, i.e., the k – ω shear stress transportation (SST) model and the detached eddy simulation (DES) model, are used in the numerical studies to simulate the flow fields. All the inclined cases have slightly larger overall averaged Nusselt number (Nu) than with straight cases. The enhancement ratio is approximately 1.85–4.94%. The averaged Nu in the half portion against the inclined direction is enlarged for the inclined hole cases. The inclined hole cases usually have smaller averaged Nu in the half portion along the inclined direction. For the straight hole case and small inclined angle case, the penetrated flows mix with the mainstream flows at the perforated regions. When the inclined angle is larger, the penetrated flows are pushed to the inclined direction and mixing with the approaching flows occurs just at the side of the inclined direction.

Pulse thermography (PT) is a nondestructive testing method in which an energy pulse is applied to a surface while the surface temperature evolution is measured to detect sub surface defects and estimate their depth. This nondestructive test method was developed on the assumption of instantaneous surface heating, but recent work has shown that relatively long pulses can be used to accurately determine defect depth in polymers. This paper examines the impact of varying input pulse length on the accuracy of defect depth quantification as a function of the material properties. Simulations using both thermoplastics and metals show that measurement error is dependent on a nondimensionalized pulse length. The simulation results agree with experimental results for three-dimensional (3D) printed acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) components. Analysis and experiments show that defects can be accurately detected with minor modification to the standard methods as long as the pulse ends before the characteristic defect signal is detected.

This investigation reports the combined effect of synthetic jet and a surface-mounted rib on heat transfer in a square cross-section channel flow. The rib height to hydraulic diameter ratio is equal to 0.1625. The Reynolds number of the channel has been set equal to 5500. The synthetic jet actuator has been operated at different actuation voltages with different amplitude modulation frequencies. At actuation voltage of 55 V, the maximum overall heat transfer is enhanced by 132.6% compared with smooth duct flow.

A new experimental method based on infrared thermography (IRT) is developed to study deformation-induced anisotropic thermal conductivity in polymers. An analytic solution for the temperature field of samples heated by a point source is utilized with a robust fitting procedure allowing for quantitative measurement of two components of the normalized thermal conductivity tensor of uniaxially stretched samples. In order to validate the method, we compare measurements on a cross-linked polybutadiene network with those obtained from a previously developed technique based on forced Rayleigh scattering (FRS). We find excellent agreement between the two techniques. Uncertainty in the measurements using IRT method is estimated to be about 2–5%. The accuracy of the method and its potential application to nontransparent materials make it a good alternative to extend current research on anisotropic thermal transport in polymeric materials.

A numerical and experimental investigation is undertaken for developing laminar flow in a duct with one opaque, uniformly heated wall and one transparent wall. In the numerical model, mixed convection, radiative exchange, as well as two-dimensional conduction in the substrate are considered. Experiments are conducted in a high-aspect-ratio rectangular channel using infrared thermography to validate the numerical model and visualize the temperature field on a heated surface. An extended parametric study using the validated model is also carried out to assess the impact of channel height, and thermal conductivity and thickness of the substrate. For a channel height of H = 6 mm and a heating power of q s = 257 W / m 2 , as Re increases from 150 to 940 the fraction of heat transfer by convection from the heated surface rises from 65% to 79%. At Re = 150 , as H increases from 6 mm to 25 mm, radiation from the heated surface increases from 35% to 70% of the total heating power. The influence of substrate conductivity and thickness on local flux distributions is limited to regions near the channel inlet and outlet. Over the entire parametric space considered, radiation loss from the interior duct surfaces to the inlet and outlet apertures is less than 2% of the total heat input and thus unimportant.

Surface-tension forces can drive fluid motion within thin liquid layers with a free surface. Spatial variations in the temperature of the free surface create surface tractions that drive cellular motions. The cells are most commonly hexagonal in shape and they scale on the thickness of the fluid layer. This investigation documents the formation of cells in the liquid film in the presence of a uniform-heat-flux lower boundary condition. Liquid crystal thermography was used to image the cells and measure the temperature distribution on the lower surface of the liquid layer. A 1.1 mm deep pool of silicone oil was supported on a 50 μ m thick electrically heated metal foil. The oil was retained inside an independently heated acrylic ring mounted on the top surface of the foil and a dry-ice cooling plate served as the low-temperature sink above the free surface of the oil. Color images of hexagonal convection cells were captured using liquid crystal thermography and a digital image acquisition and processing system. The temperature distribution inside a typical cell was measured using thermographic image analysis. Experimental issues, such as the use of an independently heated retaining ring to control the height of the liquid film and the utility of a flux-based Marangoni number are discussed.

This paper presents results from a study aimed at developing a novel thermochromic liquid crystal (TLC) temperature measurement system that uses light transmission instead of light reflection to measure surface temperature fields. In previous work, we reported on the effect of temperature on light transmission through TLCs as measured with a spectrophotometer [ Roth, T. B., and Anderson, A. M., 2005, “Light Transmission Characteristics of Thermochromic Liquid Crystals,” Proceedings of IMECE2005, Orlando, FL, Paper No. IMECE2005-81812; Roth, T. B., and Anderson, A. M., 2007, “The Effects of Film Thickness, Light Polarization and Light Intensity on the Light Transmission Characteristics of Thermochromic Liquid Crystals,” ASME J. Heat Transfer, 129(3), pp. 372–378 ]. Here we report on results obtained using a charge coupled device (CCD) camera and polychromatic light setup that is similar to the type of equipment used in TLC reflection thermography. We tested three different light sources, a white electroluminescent light, a green electroluminescent light, and a halogen fiber optic light, using both direct and remote lighting techniques. We found that the green signal (as detected by the CCD camera) of the green electroluminescent light makes the best temperature sensor, because under remote lighting conditions it showed a 500% linear signal increase as the temperature of the R25C10W TLCs was raised from 30 ° to 48 ° C . We further found that the angle of the CCD camera relative to the light did not significantly affect the results for angles up to 10 deg for remote lighting and 15 deg for direct lighting. The effect of light intensity variation was not significant for intensities up to 40% of the original level when normalized on the intensity at 19 ° C (a temperature outside the active range of the TLCs). The use of light transmission results in a larger range of temperature over which the TLCs can be calibrated and offers opportunities for more uniform lighting conditions, which may help overcome some of the problems associated with light reflection.

Measurement of heat transfer distribution is frequently required in engineering. However, some heat transfer techniques are not able to measure accurately on two-dimensional curved surfaces. In this field, periodic-transient measurement methods are advantageous. This paper describes the development of a periodic-transient technique for high-resolution heat transfer measurement and its application to multiple air-jet cooling of a concave solar receiver window. In contrast to other measurement techniques, the periodic-transient technique requires neither homogenous heating nor quantitative measurement of surface or fluid temperatures. The heat transfer coefficient is determined by periodically heating the substrate and evaluating the phase shift between the heat flux penetrating the substrate and the resulting temperature response. Equations for a hollow-sphere and flat-plate substrates are derived. The curved window surface is periodically heated by a simple device with standard light bulbs. A procedure for taking the transient heating characteristic into consideration is described. The distribution of surface temperature fluctuation is measured nonintrusively by thermography. For the sample application of air-jet cooling, a detailed uncertainty estimation is presented. The relative measurement uncertainty of the local, convective heat transfer coefficient ranges from − 2.4 % to + 14.1 % for h = 10 W ∕ ( m 2 K ) and from − 2.3 % to + 9.7 % for h = 200 W ∕ ( m 2 K ) . The uncertainty of the spatially averaged heat transfer coefficient lies between + 2.0 % and + 9.8 % for h m = 10 W ∕ ( m 2 K ) and between + 0.7 % and + 6.7 % for h m = 200 W ∕ ( m 2 K ) . The periodic-transient method described complements established techniques for high-resolution heat transfer measurements on two-dimensional curved surfaces.

Pulsed thermography is an effective technique for quantitative prediction of defect depth within a specimen. Several methods have been reported in the literature. In this paper, using an analysis based on a theoretical one-dimensional solution of pulsed thermography, we analyzed four representative methods. We show that all of the methods are accurate and converge to the theoretical solution under ideal conditions. Three methods can be directly used to predict defect depth. However, because defect features that appear on the surface during a pulsed thermography test are always affected by three-dimensional heat conduction within the test specimen, the performance and accuracy of these methods differs for defects of various sizes and depths. This difference is demonstrated and evaluated from a set of pulsed thermography data obtained from a specimen with several flat-bottom holes as simulated defects.

A theoretical model of a Thermochromic Liquid Crystal (TLC) imaging system was developed to aid in understanding the results of experiments on spectral effects and to investigate the various factors affecting the hue-temperature calibration of TLC’s. The factors in the model include the spectral distribution of the illumination source and UV filter, surface reflection of both the TLC and background, and the sensing device (camera) spectral characteristics and gain settings. It was found that typical hue-temperature calibration curves could not be entirely explained by a TLC reflectivity model with either a monochromatic spike or a narrow bandwidth reflectivity, which is often assumed. Experimental results could be explained, however, by a model that reflects over a relatively large band of wavelengths. The spectral characteristics of the five illumination sources (those for which experiments were performed) were considered. Background reflection, which commonly accounts for 30%–50% of the reflected light, was found to significantly attenuate the hue-temperature calibration curves toward the background hue value. The effect of the illumination source on the hue-temperature calibration curves is demonstrated and several experimentally observed phenomena are explained by the results of the theoretical calculations, specifically the spectral reflective properties of the liquid crystals and the transmissivity of the R, G, and B filters in the image capture camera.

Experiments have been performed to examine the spectral effects of the illumination source on the hue-temperature characteristics of thermochromic liquid crystals (TLCs) used in a liquid-crystal thermography system. Five illumination sources were compared in this study. It was found that “full spectrum” sources, which have a relatively uniform radiant intensity across the visible spectrum, tend to have the lowest temperature uncertainties and the broadest useful ranges, which are desirable calibration attributes. Radiation in the infrared, which leads to (usually undesirable) heating of a test surface, and in the ultraviolet, which can damage TLCs, are discussed for the various light sources. Experimental observations of the effect that UV damage has on liquid crystal calibrations are also provided. The use of a new method called background subtraction and the use of white balancing are investigated as methods of improving the calibration characteristics of TLCs. The uncertainty in temperature associated with different illumination sources and both background subtraction and white balancing is determined and discussed. It is shown that these methods can reduce the uncertainty in some cases.