INVESTIGATION OF NATURAL CONVECTION IN VERTICAL CHANNELS BY SCHLIEREN AND HOLOGRAPHIC INTERFEROMETRY (original) (raw)
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European Journal of Physics, 2006
In this work, natural convection heat transfer in vertical channels is experimentally investigated by applying different optical techniques, namely holographic interferometry and schlieren. Both these techniques are based on the temperature dependence of the air refractive index but they detect different optical quantities and their use involves different instrumentation and optical components. Optical methods, non-intrusive in nature, are particularly suitable for the visualization of flow and thermal fields as witnessed by their increasing use in a range of scientific and engineering disciplines; for this reason, the introduction of these experimental tools into a laboratory course can be of high value. Physics and engineering students can get familiarized with optical techniques, grasp the basics of thermal phenomena, usually elusive, which can be more easily understood if they are made visible, and begin to master digital image analysis, a key skill in laboratory activities. A didactic description of holographic interferometry and schlieren is provided and experimental results obtained for vertical, smooth and rib-roughened channels with asymmetrical heating are presented. A comparison between distributions of the local heat transfer coefficient (or its dimensionless counterpart, the Nusselt number) revealed good agreement between the results separately obtained by the two techniques, thus proving their suitability for investigating free convection heat transfer in channels.
Investigation of natural convection in channels by optical techniques
2002
The aim of this work is to investigate natural convection heat transfer in a vertical channel by applying different optical, indexof-refraction techniques. The channel, consisting of two parallel vertical plates at a given spacing value, was asymmetrically heated at uniform wall temperature. Local heat transfer coefficients and isotherm patterns were obtained by using the schlieren technique and the holographic interferometry technique, separately. Experiments were conducted at DITEC, University of Genova and at the Energetic Dept., University of L'Aquila on the same test section.
Optical diagnostics of the process of free liquid convection
Optics and Spectroscopy, 2015
A technique for complex investigation of the process of free liquid convection under the action of a heat source is presented, which is based on the use of digital holographic interferometry. The mode of studying an object, which is a flat liquid layer in a glass cell, has been developed. Experimental results have been obtained in the form of space-time temperature distributions for water, glycerol, and a mixture thereof. A dominant mechanism of heat transfer in different stages of heating the liquids under study has been revealed. A mathematical model of the process of free convection is constructed and the space-time temperature distribution is calculated for the case in which water is used as a test object. The results of mathematical simulation correctly describe the experimentally observed character of changes in the thermal field.
Heat transfer study of free convection through a horizontal open ended axisymmetric cavity using holographic interferometry, 2015
In this work, experimental study using holographic interferometry was performed to investigate heat transfer in an open ended cavity between two horizontal parallel circular plates. The lower plate was upward-facing and isothermally heated whereas the upper plate was maintained at ambient temperature. Air was used as the heat transfer medium. To achieve accurate reconstruction of the axisymmetric temperature field, wavelet transformation and a proposed matrix form of the modified Fourier-Hankel were used for the phase extraction and the Abel inversion respectively. Isotherms were mapped in the cavity for several values of the plates spacing and the heated plate temperature. We provide an experimental correlation of the Nusselt number to the Rayleigh number (Ra) based on the heated plate-to-ambient temperature difference and the cavity height. Ra and aspect ratio ranges covered in this work extends from 7 Â 10 3 to 4.5 Â 10 5 and 0.35 to 1 respectively.
Flow visualization of Bénard convection using holographic interferometry
Applied Optics, 1982
An application of holographic interferometry to Rayleigh-Bénard flow is reported. A fluid seeded with fine alumina particles is illuminated with sheetlike light, and the light scattered from the plane is recorded in a hologram. The reconstructed images give 2-D full-field velocity information within the volume of moving liquid at any instant in time. The convective rolls are regular and oriented perpendicular to the long side of the cell. The vertical velocity distribution closely approximates a sinusoidal function of the horizontal distance and its maximum is nearly proportional to the half-power of the reduced temperature.
Measurement of local convective heat transfer coefficients using three-dimensional interferometry
International Journal of Heat and Mass Transfer, 1991
The reconstruction of the three-dimensional temperature distribution above a horizontal heated disk in air is used to determine the local heat transfer coefficients. Holographic interferometry is used to obtain instantaneous, three-dimensional information on the local index of refraction distribution. Several reconstruction procedures are tested under the constraints of limited field of view and limited fringe order data. These techniques are used to accurately measure the instantaneous local temperature distribution. Subsequently, the surface temperature gradients are specified by curve fitting procedures and the local heat transfer coefficients determined. The surface averaged Nusselt number agrees very well with previous studies in the literature. Real time holographic interferometry is also used to suggest some of the dynamic characteristics of the convective heat transfer process. Results show local instabilities in the form of distributed bursting of thermal energy from the surface. Also, at sufficiently high surface temperatures the entire temperature field shows evidence of oscillation or pulsation which indicates a global heat release mechanism.
Use of Holographic Interferometry for Monitoring Heat Transfer
MM Science Journal, 2022
In the paper is described possibility of holographic interferometry in research of heat transfer above samples. Experiments with utilization of this method enable to explain many of actions going in environment and in phase interface between material and this environment in transport of heat. The presented experiments are aimed at verifying the possibilities of safe thermal loading of wood using with metal protective plate.
Temperature measurement in laminar free convective flow using digital holography
Applied Optics, 2009
A method for measurement of temperature in laminar free convection flow of water is presented using digital holographic interferometry. The method is relatively simple and fast because the method uses lensless Fourier transform digital holography, for which the reconstruction algorithm is simple and fast, and also the method does not require use of any extra experimental efforts as in phase shifting. The quantitative unwrapped phase difference is calculated experimentally from two digital holograms recorded in two different states of water-one in the quiescent state, the other in the laminar free convection. Unknown temperature in laminar free convection is measured quantitatively using a known value of temperature in the quiescent state from the unwrapped phase difference, where the equation by Tilton and Taylor describing the variation of refractive index of water with temperature is used to connect the phase with temperature. Experiments are also performed to visualize the turbulent free convection flow.
Applied optics, 2014
In this paper, the local convective heat transfer coefficient (h) is measured along the surface of an electrically heated vertical wire using digital holographic interferometry (DHI). Experiments are conducted on wires of different diameters. The experimentally measured values are within the range as given in the literature. DHI is expected to provide a more accurate local convective heat transfer coefficient (h) as the value of the temperature gradient required for the calculation of "h" can be obtained more accurately than by other existing optical interferometric techniques without the use of a phase shifting technique. This is because in digital holography phase measurement accuracy is expected to be higher.