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Effects of insulated and isothermal baffles on pseudosteady-state natural convection inside spherical containers / Yuping Duan in Journal of heat transfer, Vol. 132 N° 6 (Juin 2010) 

Public ISBD [article]  in Journal of heat transfer > Vol. 132 N° 6 (Juin 2010) . - pp. [062502-1/10] Titre : Effects of insulated and isothermal baffles on pseudosteady-state natural convection inside spherical containers Type de document : texte imprimé Auteurs : Yuping Duan, Auteur ; S. F. Hosseinizadeh, Auteur ; J. M. Khodadadi, Auteur Article en page(s) : pp. [062502-1/10] Note générale : Physique Langues : Anglais (eng) Mots-clés : Boundary  layer  turbulence  Finite  volume  methods  Flow  simulation  Fluid  oscillations  Iterative  methods  Natural  convection  Stratified  flow  Vortices Index. décimale : 536 Chaleur. Thermodynamique Résumé : The effects of insulated and isothermal thin baffles on pseudosteady-state natural convection within spherical containers were studied computationally. The computations are based on an iterative, finite-volume numerical procedure using primitive dependent variables. Natural convection effect is modeled via the Boussinesq approximation. Parametric studies were performed for a Prandtl number of 0.7. For Rayleigh numbers of 104, 105, 106, and 107, baffles with three lengths positioned at five different locations were investigated (120 cases). The fluid that is heated adjacent to the sphere rises replacing the colder fluid, which sinks downward through the stratified stable thermal layer. For high Ra number cases, the hot fluid at the bottom of the sphere is also observed to rise along the symmetry axis and encounter the sinking colder fluid, thus causing oscillations in the temperature and flow fields. Due to flow obstruction (blockage or confinement) effect of baffles and also because of the extra heating afforded by the isothermal baffle, multi-cell recirculating vortices are observed. This additional heat is directly linked to creation of another recirculating vortex next to the baffle. In effect, hot fluid is directed into the center of the sphere disrupting thermal stratified layers. For the majority of the baffles investigated, the Nusselt numbers were generally lower than the reference cases with no baffle. The extent of heat transfer modification depends on Ra, length, and location of the extended surface. With an insulated baffle, the lowest amount of absorbed heat corresponds to a baffle positioned horizontally. Placing a baffle near the top of the sphere for high Ra number cases can lead to heat transfer enhancement that is linked to disturbance of the thermal boundary layer. With isothermal baffles, heat transfer enhancement is achieved for a baffle placed near the bottom of the sphere due to interaction of the counterclockwise rotating vortex and the stratified layer. For some high Ra cases, strong fluctuations of the flow and thermal fields indicating departure from the pseudosteady-state were observed. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] Effects of insulated and isothermal baffles on pseudosteady-state natural convection inside spherical containers [texte imprimé] / Yuping Duan, Auteur ; S. F. Hosseinizadeh, Auteur ; J. M. Khodadadi, Auteur . - pp. [062502-1/10]. Physique Langues : Anglais (eng) in Journal of heat transfer > Vol. 132 N° 6 (Juin 2010) . - pp. [062502-1/10] Mots-clés : Boundary  layer  turbulence  Finite  volume  methods  Flow  simulation  Fluid  oscillations  Iterative  methods  Natural  convection  Stratified  flow  Vortices Index. décimale : 536 Chaleur. Thermodynamique Résumé : The effects of insulated and isothermal thin baffles on pseudosteady-state natural convection within spherical containers were studied computationally. The computations are based on an iterative, finite-volume numerical procedure using primitive dependent variables. Natural convection effect is modeled via the Boussinesq approximation. Parametric studies were performed for a Prandtl number of 0.7. For Rayleigh numbers of 104, 105, 106, and 107, baffles with three lengths positioned at five different locations were investigated (120 cases). The fluid that is heated adjacent to the sphere rises replacing the colder fluid, which sinks downward through the stratified stable thermal layer. For high Ra number cases, the hot fluid at the bottom of the sphere is also observed to rise along the symmetry axis and encounter the sinking colder fluid, thus causing oscillations in the temperature and flow fields. Due to flow obstruction (blockage or confinement) effect of baffles and also because of the extra heating afforded by the isothermal baffle, multi-cell recirculating vortices are observed. This additional heat is directly linked to creation of another recirculating vortex next to the baffle. In effect, hot fluid is directed into the center of the sphere disrupting thermal stratified layers. For the majority of the baffles investigated, the Nusselt numbers were generally lower than the reference cases with no baffle. The extent of heat transfer modification depends on Ra, length, and location of the extended surface. With an insulated baffle, the lowest amount of absorbed heat corresponds to a baffle positioned horizontally. Placing a baffle near the top of the sphere for high Ra number cases can lead to heat transfer enhancement that is linked to disturbance of the thermal boundary layer. With isothermal baffles, heat transfer enhancement is achieved for a baffle placed near the bottom of the sphere due to interaction of the counterclockwise rotating vortex and the stratified layer. For some high Ra cases, strong fluctuations of the flow and thermal fields indicating departure from the pseudosteady-state were observed. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]

A Theoretical and Experimental Investigation of unidirectional freezing of nanoparticle-enhanced phase change materials / Liwu Fan in Journal of heat transfer, Vol 134 N° 9 (Septembre 2012) 

Public ISBD [article]  in Journal of heat transfer > Vol 134 N° 9 (Septembre 2012) . - 09 p. Titre : A Theoretical and Experimental Investigation of unidirectional freezing of nanoparticle-enhanced phase change materials Type de document : texte imprimé Auteurs : Liwu Fan, Auteur ; J. M. Khodadadi, Auteur Année de publication : 2012 Article en page(s) : 09 p. Note générale : heat transfer Langues : Anglais (eng) Mots-clés : colloids;  freezing;  nanoparticles;  nanostructures;  PCM;  phase  change;  solidifaction;  Stefan  model;  suspensions;  thermal  conductivity Index. décimale : 536 Chaleur. Thermodynamique Résumé : Highly-conductive nanostructures may be dispersed into phase change materials (PCM) to improve their effective thermal conductivity, thus leading to colloidal systems that are referred to as nanostructure-enhanced PCM (NePCM). Results of a theoretical and experimental investigation on freezing of NePCM in comparison to the base PCM are presented. A one-dimensional Stefan model was developed to study the unidirectional freezing of NePCM in a finite slab. Only the thermal energy equation was considered and the presence of static dispersed nanoparticles was modeled using effective media relations. A combination of analytical and integral methods was used to solve this moving boundary problem. The elapsed time to form a given thickness of frozen layer was therefore predicted numerically. A cooled-from-bottom unidirectional freezing experimental setup was designed, constructed, and tested. Thermocouple readings were recorded at several equally spaced locations along the freezing direction in order to monitor the progress of the freezing front. As an example, cyclohexane (C6H12) and copper oxide (CuO) nanoparticles were chosen to prepare the NePCM samples. The effective thermophysical and transport properties of these samples for various particle loadings (0.5/3.8, 1/7.5, and 2/14.7 vol. %/wt. %) were determined using the mixture and Maxwell models. Due to utilization of the Maxwell model for thermal conductivity of both phases, the numerical predictions showed that the freezing time is shortened linearly with increasing particle loading, whereas nonmonotonic expediting was observed experimentally. The maximum expediting was found to be nearly 8.23% for the 0.5 vol. % sample. In the absence of a nanoparticle transport model, the mismatch of the cold plate boundary conditions, lack of accurate thermophysical properties, especially in the solid phase of NePCM samples and precipitation issues with 2 vol. % samples were addressed by improving the experimental setup. Through adopting a copper cold plate, utilizing measured thermal conductivity data for both phases and using 1, 2, and 4 wt. % samples, good agreement between the experimental and numerical results were realized. Specifically, adoption of measured thermal conductivity values for the solid phase in the Stefan model that were originally underestimated proved to be a major cause of harmony between the experiments and predictions. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000009 [...] [article] A Theoretical and Experimental Investigation of unidirectional freezing of nanoparticle-enhanced phase change materials [texte imprimé] / Liwu Fan, Auteur ; J. M. Khodadadi, Auteur . - 2012 . - 09 p. heat transfer Langues : Anglais (eng) in Journal of heat transfer > Vol 134 N° 9 (Septembre 2012) . - 09 p. Mots-clés : colloids;  freezing;  nanoparticles;  nanostructures;  PCM;  phase  change;  solidifaction;  Stefan  model;  suspensions;  thermal  conductivity Index. décimale : 536 Chaleur. Thermodynamique Résumé : Highly-conductive nanostructures may be dispersed into phase change materials (PCM) to improve their effective thermal conductivity, thus leading to colloidal systems that are referred to as nanostructure-enhanced PCM (NePCM). Results of a theoretical and experimental investigation on freezing of NePCM in comparison to the base PCM are presented. A one-dimensional Stefan model was developed to study the unidirectional freezing of NePCM in a finite slab. Only the thermal energy equation was considered and the presence of static dispersed nanoparticles was modeled using effective media relations. A combination of analytical and integral methods was used to solve this moving boundary problem. The elapsed time to form a given thickness of frozen layer was therefore predicted numerically. A cooled-from-bottom unidirectional freezing experimental setup was designed, constructed, and tested. Thermocouple readings were recorded at several equally spaced locations along the freezing direction in order to monitor the progress of the freezing front. As an example, cyclohexane (C6H12) and copper oxide (CuO) nanoparticles were chosen to prepare the NePCM samples. The effective thermophysical and transport properties of these samples for various particle loadings (0.5/3.8, 1/7.5, and 2/14.7 vol. %/wt. %) were determined using the mixture and Maxwell models. Due to utilization of the Maxwell model for thermal conductivity of both phases, the numerical predictions showed that the freezing time is shortened linearly with increasing particle loading, whereas nonmonotonic expediting was observed experimentally. The maximum expediting was found to be nearly 8.23% for the 0.5 vol. % sample. In the absence of a nanoparticle transport model, the mismatch of the cold plate boundary conditions, lack of accurate thermophysical properties, especially in the solid phase of NePCM samples and precipitation issues with 2 vol. % samples were addressed by improving the experimental setup. Through adopting a copper cold plate, utilizing measured thermal conductivity data for both phases and using 1, 2, and 4 wt. % samples, good agreement between the experimental and numerical results were realized. Specifically, adoption of measured thermal conductivity values for the solid phase in the Stefan model that were originally underestimated proved to be a major cause of harmony between the experiments and predictions. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000009 [...]