Mass Transfer Measurement by Holographic Interferometry for a Turbulent Jet Impinging on a Flat Surface at Moderate Reynolds Numbers
Creators: name: Nebrensky, Henry
name: Macleod, Norman
Publisher: name: figshare
Year: 2019
Subjects: subject: 91505 Heat and Mass Transfer Operations
subject: FOS: Other engineering and technologies
subject: FOS: Other engineering and technologies
subject: 20501 Classical and Physical Optics
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: AbstractThe swollen polymer method is an established technique for the measurement of both local and averaged mass transfer coefficients, but in the past attempts to determine local transfer coefficients by using this method in conjunction with live fringe holographic interferometry have been hampered by spurious fringe movements. By using a prism to gain optical access to the polymer surface from behind it is possible to eliminate these movements and get quantitative measurements of the local transfer coefficient solely from the observation of live fringes.<br>As an illustration of this technique, measurements are reported of the convective mass transfer coefficients due to a free jet impinging on a flat surface from a distance of 2.1 tube diameters at Reynolds numbers of 8100 to 18700.<br>BackgroundThe use of mass-transfer analogues, notably the naphthalene-sublimation technique, has a long history in the field of heat-transfer measurement. In the swollen-polymer method (Macleod and Todd, 1973) the test surface is coated with an elastomer that is then soaked in a suitable solvent; when exposed to the flow the solvent is removed resulting in microscopic changes to the local surface profile. The swollen-polymer method has two discrete advantages over solid-sublimation techniques: the test piece can be "recharged" simply by re-soaking in a liquid bath instead of re-casting a solid artefact; and the surface is defined by the receding edge of the persistent polymer matrix, and so does not de-correlate due to the micro-crystalline structure of the solid, which allows the ready application of optical profilometric techniques for measuring the local convective transfer coefficients across the surface, starting with the use of frozen-fringe holographic interferometry (Kapur and Macleod, 1974) which makes a fixed comparison of the surface before and after the flow.Frozen-fringe measurements have an inherent limitation, in that the pattern alone contains no information about the ordering of the fringes. It is often necessary to make a separate fiducial measurement to know the change at, say, the edge of the field to correctly assign the orders of the fringes in view, and there may be assumptions about the nature of the deformation of the surface. For example the fringe pattern of fig.3 no. 4 in this work could readily be interpreted as a monotonically decreasing distribution of mass transfer, similar to that seen by Scholtz and Trass (1963), rather than one including a secondary minimum and maximum at the radial locations labelled B and C in fig.3 similar to those seen by Baughn <i>et al.</i> (1991). The ambiguity can be avoided by making live-fringe measurements: fringes will move away from regions of high mass-transfer towards areas of low mass-transfer. Local maxima in the mass-transfer coefficient show up as sources of fringes and local minima show up as areas where fringes converge and vanish; these can be seen in fig.3 and MASSTRAN.MPG. However, live-fringe measurements are much harder to make for a range of reasons, including the effects due to the fluid motion as discussed in the paper. <br><br>MaterialThis material was presented as paper 32.2 at <i>LADOAN 7: The 7<sup>th</sup> International Symposium on the Applications of Laser Techniques to Fluid Mechanics</i>, The Calouste Gulbenkian Foundation, Lisbon, Portugal, 11<sup>th</sup>-14<sup>th</sup> July 1994.<br>LADOAN7.PDF is a scan of a photocopy of the camera-ready copy sent in for the on-site proceedings. Note the correction to eq. 7: as printed, the solvent density was omitted.<br>L7-FIG3.JPG is a scan of a better quality photocopy of figure 3.<br>MASSTRAN.MPG is an animated equivalent of fig. 3. It is in MPEG format which unfortunately specifies an unrealistic frame-rate; reducing the replay speed to four or five frames/second should give a more realistic impression of the fringe movement including the multiple minima and maxima. The source images were live fringe patterns photographed on to 35mm film, developed and printed, and scanned as a series of still images that were then combined into a movie using CMPEG for MS-DOS.<br><br>Back in 1994 "slides" for presentations in large venues were still literally 35mm slides, and any sort of animation would require special arrangements for showing movies. Since the novelty of the paper lies in the demonstration of high-quality live, i.e. moving, fringes, I arrived in Lisbon optimistically carrying a VHS tape and - a long shot for that time - a digital animation (see above) and appropriate replay software on a 3½-inch floppy disk. It turned out that the venue did actually have one of the then new-fangled LCD projection panels that could sit on an overhead projector and display the output from a PC, so we were able to set up to display the animation from a computer at the end of the talk. Naturally this created a great deal of interest, rather more than in the content of the talk itself, unfortunately! <br>ReferencesJ.W Baughn, A.E. Hechanova and X. Yan: "An Experimental Study of Entrainment Effects on the Heat Transfer from a Flat Surface to a Heated Circular Impinging Jet" <i>ASME Journal of Heat Transfer</i> <b>113</b>(4) pp.1023-1025. DOI: 10.1115/1.2911197 (1991)<br><br>N. Gholizadeh: "<i>Interferometric Measurement of Local Mass Transfer Rates to an Impinging Jet by Non-Photographic Holography</i>" PhD Thesis, University of Edinburgh, UK. HANDLE: 1842/14897 (1992)<br>A.J. Harper and N. Macleod: "Hot Spots in Heat Transfer" <i>Physics Bulletin</i> <b>29</b>(1) p.13. DOI: 10.1088/0031-9112/29/1/017 (1978)<br>A.L. Larez: "<i>The Measurement of Convective Mass Transfer Coefficients by Holographic Interferometry</i>" PhD Thesis, University of Edinburgh, UK. HANDLE: 1842/11029 (1982)<br><br>N. Macleod and R.B. Todd: "Experimental Determination of Wall-Fluid Mass-Transfer Coefficients using Plasticised Polymer Surface Coatings" <i>International Journal of Heat and Mass Transfer</i> <b>16</b>(2) pp.485-504. DOI: 10.1016/0017-9310(73)90074-4 (1973)<br>D.N. Kapur and N. Macleod: "Determination of Local Mass Transfer Coefficients by Holographic Interferometry - I" <i>International Journal of Heat and Mass Transfer</i> <b>17</b>(10) pp.1151-1162. DOI: 10.1016/0017-9310(74)90117-3 (1974)<br>M.T. Scholtz and O. Trass: "Mass Transfer in the Laminar Radial Wall Jet" <i>A.I.Ch.E Journal</i> <b>9</b>(4) pp.548-554. DOI: 10.1002/aic.690090426 (1963)<br><b></b><i></i><sub></sub><sup></sup><br>