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Dropwise condensation underneath chemically textured surfaces / Basant Singh Sikarwar 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. [021501/1-15] Titre : Dropwise condensation underneath chemically textured surfaces : simulation and experiments Type de document : texte imprimé Auteurs : Basant Singh Sikarwar, Auteur ; Nirmal Kumar Battoo, Auteur ; Sameer Khandekar, Auteur Année de publication : 2011 Article en page(s) : pp. [021501/1-15] Note générale : Physique Langues : Anglais (eng) Mots-clés : Dropwise  condensation  Chemical  texturing  Pendant  droplets  Modeling  and  simulation  Imaging Index. décimale : 536 Chaleur. Thermodynamique Résumé : Experimental observations of dropwise condensation of water vapor on a chemically textured surface of glass and its detailed computer simulation are presented. Experiments are focused on the pendant mode of dropwise condensation on the underside of horizontal and inclined glass substrates. Chemical texturing of glass is achieved by silanation using octyl-decyl-tri-chloro-silane (C18H37C13Si) in a chemical vapor deposition process. The mathematical model is built in such a way that it captures all the major physical processes taking place during condensation. These include growth due to direct condensation, droplet coalescence, sliding, fall-off, and renucleation of droplets. The effects arising from lyophobicity, namely, the contact angle variation and its hysteresis, inclination of the substrate, and saturation temperature at which the condensation is carried out, have been incorporated. The importance of higher order effects neglected in the simulation is discussed. The results of model simulation are compared with the experimental data. After validation, a parametric study is carried out for cases not covered by the experimental regime, i.e., various fluids, substrate inclination angle, saturation temperature, and contact angle hysteresis. Major conclusions arrived at in the study are the following: The area of droplet coverage decreases with an increase in both static contact angle of the droplet and substrate inclination. As the substrate inclination increases, the time instant of commencement of sliding of the droplet is advanced. The critical angle of inclination required for the inception of droplet sliding varies inversely with the droplet volume. For a given static contact angle, the fall-off time of the droplet from the substrate is a linear function of the saturation temperature. For a given fluid, the drop size distribution is well represented by a power law. Average heat transfer coefficient is satisfactorily predicted by the developed model. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] Dropwise condensation underneath chemically textured surfaces : simulation and experiments [texte imprimé] / Basant Singh Sikarwar, Auteur ; Nirmal Kumar Battoo, Auteur ; Sameer Khandekar, Auteur . - 2011 . - pp. [021501/1-15]. Physique Langues : Anglais (eng) in Journal of heat transfer > Vol. 133 N° 2 (Fevrier 2011) . - pp. [021501/1-15] Mots-clés : Dropwise  condensation  Chemical  texturing  Pendant  droplets  Modeling  and  simulation  Imaging Index. décimale : 536 Chaleur. Thermodynamique Résumé : Experimental observations of dropwise condensation of water vapor on a chemically textured surface of glass and its detailed computer simulation are presented. Experiments are focused on the pendant mode of dropwise condensation on the underside of horizontal and inclined glass substrates. Chemical texturing of glass is achieved by silanation using octyl-decyl-tri-chloro-silane (C18H37C13Si) in a chemical vapor deposition process. The mathematical model is built in such a way that it captures all the major physical processes taking place during condensation. These include growth due to direct condensation, droplet coalescence, sliding, fall-off, and renucleation of droplets. The effects arising from lyophobicity, namely, the contact angle variation and its hysteresis, inclination of the substrate, and saturation temperature at which the condensation is carried out, have been incorporated. The importance of higher order effects neglected in the simulation is discussed. The results of model simulation are compared with the experimental data. After validation, a parametric study is carried out for cases not covered by the experimental regime, i.e., various fluids, substrate inclination angle, saturation temperature, and contact angle hysteresis. Major conclusions arrived at in the study are the following: The area of droplet coverage decreases with an increase in both static contact angle of the droplet and substrate inclination. As the substrate inclination increases, the time instant of commencement of sliding of the droplet is advanced. The critical angle of inclination required for the inception of droplet sliding varies inversely with the droplet volume. For a given static contact angle, the fall-off time of the droplet from the substrate is a linear function of the saturation temperature. For a given fluid, the drop size distribution is well represented by a power law. Average heat transfer coefficient is satisfactorily predicted by the developed model. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]

Optimum Nusselt number for simultaneously developing internal flow under conjugate conditions in a square microchannel / Manoj Kumar Moharana in Journal of heat transfer, Vol. 134 N° 7 (Juillet 2012) 

Public ISBD [article]  in Journal of heat transfer > Vol. 134 N° 7 (Juillet 2012) . - 10 p. Titre : Optimum Nusselt number for simultaneously developing internal flow under conjugate conditions in a square microchannel Type de document : texte imprimé Auteurs : Manoj Kumar Moharana, Auteur ; Piyush Kumar Singh, Auteur ; Sameer Khandekar, Auteur Année de publication : 2012 Article en page(s) : 10 p. Note générale : heat transfer Langues : Anglais (eng) Mots-clés : microchannel;  axial  heat  conduction;  conjugate  heat  transfer;  thermally  developing  flow;  optimum  Nusselt  number Index. décimale : 536 Chaleur. Thermodynamique Résumé : A numerical study has been carried out to understand and highlight the effects of axial wall conduction in a conjugate heat transfer situation involving simultaneously developing laminar flow and heat transfer in a square microchannel with constant flux boundary condition imposed on bottom of the substrate wall. All the remaining walls of the substrate exposed to the surroundings are kept adiabatic. Simulations have been carried out for a wide range of substrate wall to fluid conductivity ratio (ksf ~ 0.17–703), substrate thickness to channel depth (deltasf ~ 1–24), and flow rate (Re ~ 100–1000). These parametric variations cover the typical range of applications encountered in microfluids/microscale heat transfer domains. The results show that the conductivity ratio, ksf is the key factor in affecting the extent of axial conduction on the heat transport characteristics at the fluid–solid interface. Higher ksf leads to severe axial back conduction, thus decreasing the average Nusselt number ([overline Nu]). Very low ksf leads to a situation which is qualitatively similar to the case of zero-thickness substrate with constant heat flux applied to only one side, all the three remaining sides being kept adiabatic; this again leads to lower the average Nusselt number ([overline Nu]). Between these two asymptotic limits of ksf, it is shown that, all other parameters remaining the same (deltasf and Re), there exists an optimum value of ksf which maximizes the average Nusselt number ([overline Nu]). Such a phenomenon also exists for the case of circular microtubes. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000007 [...] [article] Optimum Nusselt number for simultaneously developing internal flow under conjugate conditions in a square microchannel [texte imprimé] / Manoj Kumar Moharana, Auteur ; Piyush Kumar Singh, Auteur ; Sameer Khandekar, Auteur . - 2012 . - 10 p. heat transfer Langues : Anglais (eng) in Journal of heat transfer > Vol. 134 N° 7 (Juillet 2012) . - 10 p. Mots-clés : microchannel;  axial  heat  conduction;  conjugate  heat  transfer;  thermally  developing  flow;  optimum  Nusselt  number Index. décimale : 536 Chaleur. Thermodynamique Résumé : A numerical study has been carried out to understand and highlight the effects of axial wall conduction in a conjugate heat transfer situation involving simultaneously developing laminar flow and heat transfer in a square microchannel with constant flux boundary condition imposed on bottom of the substrate wall. All the remaining walls of the substrate exposed to the surroundings are kept adiabatic. Simulations have been carried out for a wide range of substrate wall to fluid conductivity ratio (ksf ~ 0.17–703), substrate thickness to channel depth (deltasf ~ 1–24), and flow rate (Re ~ 100–1000). These parametric variations cover the typical range of applications encountered in microfluids/microscale heat transfer domains. The results show that the conductivity ratio, ksf is the key factor in affecting the extent of axial conduction on the heat transport characteristics at the fluid–solid interface. Higher ksf leads to severe axial back conduction, thus decreasing the average Nusselt number ([overline Nu]). Very low ksf leads to a situation which is qualitatively similar to the case of zero-thickness substrate with constant heat flux applied to only one side, all the three remaining sides being kept adiabatic; this again leads to lower the average Nusselt number ([overline Nu]). Between these two asymptotic limits of ksf, it is shown that, all other parameters remaining the same (deltasf and Re), there exists an optimum value of ksf which maximizes the average Nusselt number ([overline Nu]). Such a phenomenon also exists for the case of circular microtubes. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000007 [...]