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Simultaneous mapping of single bubble dynamics and heat transfer rates for SiO <sub/>2</sub> /water nanofluids under nucleate pool boiling regime
摘要： Dependence of single vapor bubble dynamics and heat transfer rates on varying concentration of SiO2 nanoparticles for a range of subcooled conditions (0–9 ?C) has been experimentally studied under nucleate pool boiling configuration. Non-invasive measurements have been carried out using rainbow schlieren deflectometry. Results on bubble dynamics showed that the bubble diameter and aspect ratio decrease with increasing subcooling levels as well as concentration of nanofluids. The frequency of bubble oscillations was found to increase first and then decrease with increasing subcooling levels while it decreases monotonically with increasing nanofluid concentration. Bubble departure frequency increased significantly for nanofluids, while it decreased with increasing subcooling levels. Condensation effects at the bubble interface were reflected in the form of redistribution of colors around it. Schlieren images clearly revealed a spread in the spatial extent of the thermal boundary layer region caused by the suspended nanoparticles around the vapor bubble as well as near the heated substrate. This phenomenon has been considered as one of the factors that tends to alter the condensation effects and, in turn, affects the bubble dynamics. Quantitative analysis of schlieren images revealed that the natural convective heat flux increases with increasing subcooling levels, while it decreases with increasing nanoparticle concentration. Deterioration in the natural convection phenomenon in the presence of suspended nanoparticles has been attributed to the reduced strength of thermal gradients adjacent to the heater substrate. On the other hand, evaporative heat flux was observed to decrease with increasing subcooling levels and increase with increasing concentration of nanofluids.
关键词： nucleate pool boiling,bubble dynamics,schlieren deflectometry,subcooling,heat transfer,nanofluids
更新于2025-09-23 15:23:52
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Rainbow schlieren-based direct visualization of thermal gradients around single vapor bubble during nucleate boiling phenomena of water
摘要： Real time non-intrusive diagnostics of thermal gradients around a single vapor bubble in isolated nucleate pool boiling with water as the working fluid have been presented. Direct visualization of transient evolution of temperature gradients in the vicinity of the heated substrate and around the single bubble as it undergoes various sub-processes has been performed using rainbow schlieren deflectometry. Boiling experiments have been conducted under saturated conditions. Results have been presented in the form of two-dimensional rainbow schlieren images wherein the degree of color re-distribution gives a direct measure of the strength of thermal gradients. Through the real time experimental images, various sub-processes/phenomena associated with boiling heat transfer such as development of superheat layer in the vicinity of the heated substrate, inception and further growth of the vapor bubble followed by scavenging of the superheat layer as the vapor bubble departs into the bulk fluid have been discussed. The profiles of hue distribution near the triple contact line brought out the presence of near stagnant fluid zone in which the heat transfer phenomenon was seen to be primarily diffusion-dominated. Beyond this narrow region, significant bulk fluid movement was observed on the basis of the spatial distributions of hue values recorded in the form of real time schlieren images. The experiments further revealed an instantaneous localized bending of the superheat layer as the bubble leaves the heater surface and subsequent shedding of the wake vortices from the edges of the departing bubble as it moves upwards in the bulk fluid.
关键词： Whole field temperature distribution,Heat transfer rates,Nucleate pool boiling,Rainbow schlieren deflectometry
更新于2025-09-23 15:22:29
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Structured-illumination Makyoh-topography: optimum grid position and its constraints
摘要： Complementing conventional Makyoh topography with structured illumination using a sparse square grid, the large-scale surface shape can be calculated with a deflectometry approach, while the sample’s morphology can still be imaged. However, the grid’s image must be sharp not to mask the Makyoh image of the sample morphology. In this paper, the instrumental conditions for the grid sharpness are established. The two main types of Makyoh set-ups (lens and mirror based) are analysed. It is shown that the lens-based set-ups allow the position of the grid to be sharp on the Makyoh image. However, for mirror-based set-ups this is not possible because of geometrical instrumental constraints. The calculations are corroborated with experiments.
关键词： Makyoh topography,surface defects,structured illumination,deflectometry,geometrical optics,flatness testing
更新于2025-09-23 15:21:01
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Line–planes deflectometry
摘要： The key issue in phase measurement deflectometry is the solution of incident light which is used to determine the surface normal. To restore the 3D mirror surface information, line–planes deflectometry is proposed to determine the surface normal from the reflected light and the incident light planes containing incident light. To this end, a point light source is used to replace the continuous surface light source of traditional deflectometry. Accordingly, the incident light is calculated by the intersection of incident light planes which are determined by two or more projected lights of a point light source. Finally, the precise mirror surface is restored through the radial basis function interpolation from the gradient which is transformed from the normal information. To verify the proposed method, a line–planes deflectometry system is developed with two projectors and one camera. The system can be applied for two measurement modes concerning different measurement requirements: the ‘line–plane model’ and the ‘line–planes model’. The ‘line–plane model’ can be inferred with a measurement error of less than 0.25 mm to measure objects with low accuracy and non-connectivity. In contrast, the ‘line–planes model’ can be used to measure objects with high accuracy and good area connectivity, and the measurement error is about 1 μm.
关键词： point light source,radial basis function interpolation,line–planes deflectometry,incident light plane,incident light
更新于2025-09-09 09:28:46