The thermal imaging technique was applied in this work to measure the transient temperature fields during melting of a phase change material (PCM) in a metal foam. A paraffin wax was used as the PCM that was filled in an open-celled copper foam. Melting of a paraffin wax in the presence of copper foam was studied in a rectangular cavity that was heated from one lateral side wall, while the top surface was exposed to an infrared (IR) camera. A thermocouple (TC) was also employed to validate the accuracy of temperature measurements by IR thermal imaging. The relative deviation of measured temperature by the TC and IR camera was found to be under 2% in steady state and under 4% during the entire course of melting. The resolution of IR thermal imaging with the aid of a macro lens allowed for temperature measurements at pore-scale of the copper foam. Local thermal imaging was captured through a minor window on the top plate of the container. Three points (Sp1–3) inside a selected individual pore were marked to quantify the temperature variations of melting process within metal foam/PCM at pore-scale. The average temperature differences between Sp1 and Sp2, Sp3 were found to be about 1 °C over the entire course of melting, and the maximum value was up to nearly 10 °C around the melting point. These preliminary results clearly highlighted the effect of metal ligaments on the temperature distributions at pore-scale.

The effects of droplet train impingement on spreading-splashing transition and surface heat transfer were investigated experimentally and numerically. Experimentally, a single stream of HFE-7100 droplet train was generated using a piezo-electric droplet generator with the ability to adjust parameters such as droplet impingement frequency, droplet diameter and droplet impingement velocity. A thin layer of Indium Tin Oxide (ITO) was coated on a translucent sapphire substrate, which was used as heating element. High-speed and infrared imaging techniques were employed to characterize the hydrodynamics and heat transfer of droplet train impingement. Numerically, the high frequency droplet train impingement process was simulated using ANSYS-Fluent with the Volume of Fluid (VOF) method [1]. The heat transfer process was simulated by applying constant heat flux conditions on the droplet receiving surface. Droplet-induced spreading-splashing transition behavior was investigated by increasing the droplet Weber number while holding flow rate constant. High speed crown propagation images showed that at low-Weber number ( We < 400), droplet impingements resulted in smooth spreading of the droplet-induced crown. However, within the transitional droplet Weber number range ( We = 400–500), fingering and splashing (i.e. emergence of secondary droplets) could be observed at the crown’s rim. At high droplet Weber number ( We > 800), breakup of the crown was observed during the crown propagation process in which the liquid film behaved chaotically. Droplet-induced spreading-splashing transition phenomena were also investigated numerically. Reasonable agreement was reached between the experimental and numerical results in terms of crown morphology at different droplet Weber number values. The effects of spreading-splashing transition on surface heat transfer were also investigated at fixed flow rate conditions. Time-averaged Infrared (IR) temperature measurements indicate that heat flux-surface temperature curves are linear at low surface temperatures and before the onset of dry-out, which indicate that single phase forced convection is the primary heat transfer mechanism under those conditions. Numerical heat transfer simulations were performed within the single phase forced convection regime only. Instantaneous numerical results reveal that droplet-induced crown propagation effectively convect heat radially outward within the droplet impingement zone. Under high heat flux conditions, a sharp increase in surface temperature was observed experimentally when dry-out appeared on the heater surface. It was also found that strong splashing ( We > 800) is unfavorable for heat transfer at high surface temperature due to the onset of instabilities seen in the liquid film, which leads to dry-out conditions. In summary, the results indicate that droplet Weber number is a significant factor in the spreading-splashing transition and surface heat transfer.

The increased availability of thermal imaging cameras has led to a growing interest in the application of infrared imaging techniques to the detection and identification of subsurface structures. These imaging techniques are based on the following principle: when a surface is heated or cooled, variations in the thermal properties of a structure located underneath the surface result in identifiable temperature contours on it. These contours are characteristic of the structure’s shape, depth, and its thermal properties. We study the use of the transient thermal response of skin layers to determine to which extent the surface temperature distribution reflects the properties of subsurface structures, such as lesions. A numerical model using the finite element method is described to obtain this response and key results are reported in the paper. A sensitivity study is conducted first to better understand the thermal response of the system and the role of various system and model parameters. We explore the extent to which we are able to draw conclusions regarding the size, depth and nature of subsurface structures and accuracy of these conclusions based on the surface temperature response alone. This work validates the idea of examining the transient thermal response and using thermal imaging as a solution for lesion identification. A sensitivity study of surface temperature distribution to variations of thermophysical properties, blood perfusion rate, and thicknesses of skin layers is performed. It is observed that variations in these parameters have little impact on the surface temperature distribution. The work reported in the paper is a portion of a comprehensive research effort involving experiments on a phantom model as well as measurements on patients. Future work will focus on comparing the results of our 2D numerical modeling efforts with the experimental results using a skin tissue-mimicking phantom. Knowledge gained from the modeling and experimental efforts will be utilized in characterizing lesions in patient studies. The focus of this paper is the computational sensitivity analysis.

Experimental measurements and numerical simulations are conducted to investigate the effect of mist on the spray heat transfer and fluid dynamics in the cooling of a low-carbon steel cylindrical surface heated to the nucleate boiling temperature range. Multiple tests are performed to investigate the effect of the droplet size, and liquid-to-air loading on the spray heat transfer along the circumference of the annulus cylinder. Infrared imaging is used to capture the effect of the spray flow conditions on the droplets transportation process around the heated cylinder. A computational fluid dynamics model is also developed to simulate the spray transportation process, droplets impaction and evaporation over the cylinder surface. The model takes into consideration droplet-to-surface interaction and water-film accumulation on the surface. Simulation results show the wetting of the cylinder depends on the droplet size and liquid-to-air loading. The smaller the droplets, the closer to the surface they remain. For high liquid-to-air loadings and high air velocities, droplet wetting of the cylinder back surface dramatically increases due to the increase in the flow turbulence. An overall good agreement is observed between the experimental measurements, numerical simulations and the thermal images. The results of this study shall lead to a better understanding of the multiphase heat transfer enhancement that plays an important role in the design of heat exchangers using multiphase cooling.