Comparative measurements of natural convection heat transfer in channels by holographic interferometry and schlieren

Abstract

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.

Natural convection heat transfer from a horizontal cylinder using holographic interferometry

“…HI can be used to obtain wavefront distortion superposed on a holographic plate between two different object waves, one with and one without heat transfer. In this case, less precise optical components are allowed …”

“…In many optical methods, the temperature of a fluid is measured from the phase difference in the light rays due to the change in the refractive index. Methods, such as Mach-Zehnder interferometry (MZI), 14,15 lateral shearing interferometry (LSI), 16,17 and holographic interferometry (HI), [18][19][20] have been applied. Owing to the highly accurate, nonintrusive, and full-field measurement, optical methods are preferred for flow visualization and heat transfer analyses.…”

“…In this case, less precise optical components are allowed. 19 In double-exposure HI, the variations in the flow field between two exposures can be directly observed from the fringe pattern. Herraez and Belda 20 measured the natural convection heat transfer from horizontal cylinders with different diameters but of equal length using double-exposure HI.…”

Experimental investigation of natural convection in a vertical rib-roughened channel with asymmetric heating

“…Their results showed that the stepped surface helped improve the heat transfer until 23.2% corresponds to an optimum step pitch-to-height ratio equal to 16, while all of the ribbed surfaces decrease the heat transfer performance. Tanda [21,22]; Ambrosini and Tanda [23] analyzed experimentally the natural convection inside a vertical channel formed with a ribbed wall and an opposing adiabatic smooth wall. The natural convection from a grooved plate with different sizes of the grooves has been studied experimentally and numerically by Kwak and Song [24].…”

Sparse approximations of phase and amplitude for wave field reconstruction from noisy data

“…Digital holography has a huge number of practical applications, such as microscopy imaging, 1-6 information encryption 7, 8 object recognition, 9 data storage, 10 terahertz imaging, 11,12 measurements of thermal fields, [13][14][15][16][17] monitoring of a singlet oxygen in solutions, 18 measuring statistics of hydrodynamics facilities, [19][20][21][22] testing vibrations, 23 investigations of strained surfaces, 24,25 and objects displacements. 26 In digital holography, there are specific problems, such as zero-order and conjugate images, sampling, sensitivity to vibration of optical components.…”