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The physical mechanism of heat transfer augmentation in stagnating flows subject to freestream turbulence / Andrew R. Gifford in Journal of heat transfer, Vol. 133 N° 2 (Fevrier 2011) 

Public ISBD [article]  in Journal of heat transfer > Vol. 133 N° 2 (Fevrier 2011) . - pp. [021901/1-11] Titre : The physical mechanism of heat transfer augmentation in stagnating flows subject to freestream turbulence Type de document : texte imprimé Auteurs : Andrew R. Gifford, Auteur ; Thomas E. Diller, Auteur ; Pavlos P. Vlachos, Auteur Année de publication : 2011 Article en page(s) : pp. [021901/1-11] Note générale : Physique Langues : Anglais (eng) Mots-clés : Coherent  structures  Digital  particle  image  velocimetry  Heat  flux  sensor  Heat  transfer  augmentation  Hiemenz  flow  Mechanistic  model  Turbulence Index. décimale : 536 Chaleur. Thermodynamique Résumé : Experiments have been performed in a water tunnel facility to examine the physical mechanism of heat transfer augmentation by freestream turbulence in classical Hiemenz flow. A unique experimental approach to studying the problem is developed and demonstrated herein. Time-resolved digital particle image velocimetry (TRDPIV) and a new variety of thin-film heat flux sensor called the heat flux array (HFA) are used simultaneously to measure the spatiotemporal influence of coherent structures on the heat transfer coefficient as they approach and interact with the stagnation surface. Laminar flow and heat transfer at low levels of freestream turbulence ([overline Tu[sub x]]=0.5–1.0%) are examined to provide baseline flow characteristics and heat transfer coefficients. Similar experiments using a turbulence grid are performed to examine the effects of turbulence with mean streamwise turbulence intensity of [overline Tu[sub x]]=5.0% and an integral length scale of [overline Lambda[sub x]]=3.25 cm. At a Reynolds number of [overline Re[sub D]]=[overline U[sub [infinity]]]D/upsilon=21,000, an average increase in the mean heat transfer coefficient of 64% above the laminar level was observed. Experimental studies confirm that coherent structures play a dominant role in the augmentation of heat transfer in the stagnation region. Calculation and examination of the transient physical properties for coherent structures (i.e., circulation, area averaged vorticity, integral length scale, and proximity to the surface) shows that freestream turbulence is stretched and vorticity is amplified as it is convected toward the stagnation surface. The resulting stagnation flow is dominated by dynamic, counter-rotating vortex pairs. Heat transfer augmentation occurs when the rotational motion of coherent structures sweeps cooler freestream fluid into the laminar momentum and thermal boundary layers into close proximity of the heated stagnation surface. Evidence in support of this mechanism is provided through validation of a new mechanistic model, which incorporates the transient physical properties of tracked coherent structures. The model performs well in capturing the essential dynamics of the interaction and in the prediction of the experimentally measured transient and time-averaged turbulent heat transfer coefficients. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] The physical mechanism of heat transfer augmentation in stagnating flows subject to freestream turbulence [texte imprimé] / Andrew R. Gifford, Auteur ; Thomas E. Diller, Auteur ; Pavlos P. Vlachos, Auteur . - 2011 . - pp. [021901/1-11]. Physique Langues : Anglais (eng) in Journal of heat transfer > Vol. 133 N° 2 (Fevrier 2011) . - pp. [021901/1-11] Mots-clés : Coherent  structures  Digital  particle  image  velocimetry  Heat  flux  sensor  Heat  transfer  augmentation  Hiemenz  flow  Mechanistic  model  Turbulence Index. décimale : 536 Chaleur. Thermodynamique Résumé : Experiments have been performed in a water tunnel facility to examine the physical mechanism of heat transfer augmentation by freestream turbulence in classical Hiemenz flow. A unique experimental approach to studying the problem is developed and demonstrated herein. Time-resolved digital particle image velocimetry (TRDPIV) and a new variety of thin-film heat flux sensor called the heat flux array (HFA) are used simultaneously to measure the spatiotemporal influence of coherent structures on the heat transfer coefficient as they approach and interact with the stagnation surface. Laminar flow and heat transfer at low levels of freestream turbulence ([overline Tu[sub x]]=0.5–1.0%) are examined to provide baseline flow characteristics and heat transfer coefficients. Similar experiments using a turbulence grid are performed to examine the effects of turbulence with mean streamwise turbulence intensity of [overline Tu[sub x]]=5.0% and an integral length scale of [overline Lambda[sub x]]=3.25 cm. At a Reynolds number of [overline Re[sub D]]=[overline U[sub [infinity]]]D/upsilon=21,000, an average increase in the mean heat transfer coefficient of 64% above the laminar level was observed. Experimental studies confirm that coherent structures play a dominant role in the augmentation of heat transfer in the stagnation region. Calculation and examination of the transient physical properties for coherent structures (i.e., circulation, area averaged vorticity, integral length scale, and proximity to the surface) shows that freestream turbulence is stretched and vorticity is amplified as it is convected toward the stagnation surface. The resulting stagnation flow is dominated by dynamic, counter-rotating vortex pairs. Heat transfer augmentation occurs when the rotational motion of coherent structures sweeps cooler freestream fluid into the laminar momentum and thermal boundary layers into close proximity of the heated stagnation surface. Evidence in support of this mechanism is provided through validation of a new mechanistic model, which incorporates the transient physical properties of tracked coherent structures. The model performs well in capturing the essential dynamics of the interaction and in the prediction of the experimentally measured transient and time-averaged turbulent heat transfer coefficients. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]