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Effect of the header pressure drop induced flow maldistribution on the microchannel evaporator performance / Hanfei Tuo in International journal of refrigeration, Vol. 36 N° 8 (Décembre 2013) 

Public ISBD [article]  in International journal of refrigeration > Vol. 36 N° 8 (Décembre 2013) . - pp. 2176–2186 Titre : Effect of the header pressure drop induced flow maldistribution on the microchannel evaporator performance Titre original : Effet de la mauvaise distribution de l'écoulement engendrée par la chute de pression dans le collecteur sur la performance d'un évaporateur à microcanaux Type de document : texte imprimé Auteurs : Hanfei Tuo, Auteur ; Pega Hrnjak, Auteur Année de publication : 2014 Article en page(s) : pp. 2176–2186 Note générale : Refrigeration Langues : Anglais (eng) Mots-clés : Microchannel;  evaporator;  air-conditioning  system;  flow  distribution;  header Résumé : This paper presents an experimental and numerical investigation of the flow maldistribution caused by the pressure drop in headers and its impact on the performance of a microchannel evaporator with horizontal headers and vertically oriented tubes. Experimental results show that the flash gas bypass method almost eliminates the quality induced maldistribution. However, refrigerant flow maldistribution caused by the header pressure drop still exists. This is mainly because the pressure drop along the headers results in uneven pressure difference and therefore non-uniform liquid refrigerant mass flow rate across each microchannel tube. A microchannel evaporator model validated by experimental results is employed to quantify header pressure drop induced flow maldistribution. Parametric analysis reveals that such maldistribution impact is significantly reduced by enlarging the outlet header size, increasing heat exchanger aspect ratio, or reducing the microchannel size while other parameters are kept constant. When ratio of outlet header to the total evaporator pressure drop is less than 30%, the cooling capacity reduction is limited below 3%. En ligne : http://www.sciencedirect.com/science/article/pii/S0140700713001497 [article] Effect of the header pressure drop induced flow maldistribution on the microchannel evaporator performance = Effet de la mauvaise distribution de l'écoulement engendrée par la chute de pression dans le collecteur sur la performance d'un évaporateur à microcanaux [texte imprimé] / Hanfei Tuo, Auteur ; Pega Hrnjak, Auteur . - 2014 . - pp. 2176–2186. Refrigeration Langues : Anglais (eng) in International journal of refrigeration > Vol. 36 N° 8 (Décembre 2013) . - pp. 2176–2186 Mots-clés : Microchannel;  evaporator;  air-conditioning  system;  flow  distribution;  header Résumé : This paper presents an experimental and numerical investigation of the flow maldistribution caused by the pressure drop in headers and its impact on the performance of a microchannel evaporator with horizontal headers and vertically oriented tubes. Experimental results show that the flash gas bypass method almost eliminates the quality induced maldistribution. However, refrigerant flow maldistribution caused by the header pressure drop still exists. This is mainly because the pressure drop along the headers results in uneven pressure difference and therefore non-uniform liquid refrigerant mass flow rate across each microchannel tube. A microchannel evaporator model validated by experimental results is employed to quantify header pressure drop induced flow maldistribution. Parametric analysis reveals that such maldistribution impact is significantly reduced by enlarging the outlet header size, increasing heat exchanger aspect ratio, or reducing the microchannel size while other parameters are kept constant. When ratio of outlet header to the total evaporator pressure drop is less than 30%, the cooling capacity reduction is limited below 3%. En ligne : http://www.sciencedirect.com/science/article/pii/S0140700713001497

Heat rejection from R744 flow under uniform temperature cooling in a horizontal smooth tube around the critical point / Chieko Kondou in International journal of refrigeration, Vol. 34 N° 3 (Mai 2011) 

Public ISBD [article]  in International journal of refrigeration > Vol. 34 N° 3 (Mai 2011) . - pp. 719-731 Titre : Heat rejection from R744 flow under uniform temperature cooling in a horizontal smooth tube around the critical point Titre original : Rejet de chaleur lors de l'écoulement du R744 à l'intérieur d'un tube lisse horizontal sous des conditions de refroidissement à température uniforme proche du point critique Type de document : texte imprimé Auteurs : Chieko Kondou, Auteur ; Pega Hrnjak, Auteur Année de publication : 2011 Article en page(s) : pp. 719-731 Note générale : Génie Mécanique Langues : Anglais (eng) Mots-clés : Carbon  dioxide  Heat  transfer  Horizontal  tube  Critical  point Index. décimale : 621.5 Energie pneumatique. Machinerie et outils. Réfrigération Résumé : Heat rejection from CO2 flow near the critical point, where commercial refrigerators spend most of their operating hours, was investigated in this study. Experimental results on the heat transfer coefficient and pressure drop of mass flux from 100 to 240 kg m−2 s−1 at pressures from 5 to 7.5 MPa in a horizontal smooth tube of 6.1 mm inner diameter are provided and compared with correlations. The heat rejection process below critical pressure was categorized into superheat, two-phase, and subcool zones in which the bulk-mean temperature was superheated, saturated, and subcooled, respectively. The results indicated that the heat transfer coefficient in superheat zone is significantly higher than correlations proposed for single-phase turbulent flow, and the condensation was identified from the tube wall temperature below saturation temperature. This superheat zone accounts for a significant portion of the heat rejected in the subcritical cycle and affects on condenser sizing. DEWEY : 621.5 ISSN : 0140-7007 En ligne : http://www.sciencedirect.com/science/article/pii/S014070071000263X [article] Heat rejection from R744 flow under uniform temperature cooling in a horizontal smooth tube around the critical point = Rejet de chaleur lors de l'écoulement du R744 à l'intérieur d'un tube lisse horizontal sous des conditions de refroidissement à température uniforme proche du point critique [texte imprimé] / Chieko Kondou, Auteur ; Pega Hrnjak, Auteur . - 2011 . - pp. 719-731. Génie Mécanique Langues : Anglais (eng) in International journal of refrigeration > Vol. 34 N° 3 (Mai 2011) . - pp. 719-731 Mots-clés : Carbon  dioxide  Heat  transfer  Horizontal  tube  Critical  point Index. décimale : 621.5 Energie pneumatique. Machinerie et outils. Réfrigération Résumé : Heat rejection from CO2 flow near the critical point, where commercial refrigerators spend most of their operating hours, was investigated in this study. Experimental results on the heat transfer coefficient and pressure drop of mass flux from 100 to 240 kg m−2 s−1 at pressures from 5 to 7.5 MPa in a horizontal smooth tube of 6.1 mm inner diameter are provided and compared with correlations. The heat rejection process below critical pressure was categorized into superheat, two-phase, and subcool zones in which the bulk-mean temperature was superheated, saturated, and subcooled, respectively. The results indicated that the heat transfer coefficient in superheat zone is significantly higher than correlations proposed for single-phase turbulent flow, and the condensation was identified from the tube wall temperature below saturation temperature. This superheat zone accounts for a significant portion of the heat rejected in the subcritical cycle and affects on condenser sizing. DEWEY : 621.5 ISSN : 0140-7007 En ligne : http://www.sciencedirect.com/science/article/pii/S014070071000263X

New approach to improve performance by venting periodic reverse vapor flow in microchannel evaporator / Hanfei Tuo in International journal of refrigeration, Vol. 36 N° 8 (Décembre 2013) 

Public ISBD [article]  in International journal of refrigeration > Vol. 36 N° 8 (Décembre 2013) . - pp. 2187–2195 Titre : New approach to improve performance by venting periodic reverse vapor flow in microchannel evaporator Titre original : Nouvelle approche pour améliorer la performance en évacuant l'écoulement inverse périodique de vapeur à l'intérieur d'un évaporateur à microcanaux Type de document : texte imprimé Auteurs : Hanfei Tuo, Auteur ; Pega Hrnjak, Auteur Année de publication : 2014 Article en page(s) : pp. 2187–2195 Note générale : Refrigeration Langues : Anglais (eng) Mots-clés : Vapour;  periodic  reverse  flow;  boiling;  fluctuation;  venting;  evaporator;  microchannel Résumé : This paper presents a proposal for a venting reverse vapor in flash gas removal A/C system in order to improve refrigerant distribution and reduce pressure drop in microchannel evaporator and thus increase system efficiency. Introduction to the reverse vapor flow observed in parallel flow microchannel evaporator was presented in earlier IJR paper by the authors. An experimental comparison of the A/C system with new approach to an FGB system revealed that vapor venting provided a 5% increase of cooling capacity and 3% of COP when operated at identical test conditions, while the maximum COP improvement was approximately 10%–12% when capacity is matched by reduction of compressor speed. The improvement compared to direct expansion system was significantly higher. En ligne : http://www.sciencedirect.com/science/article/pii/S0140700713001473 [article] New approach to improve performance by venting periodic reverse vapor flow in microchannel evaporator = Nouvelle approche pour améliorer la performance en évacuant l'écoulement inverse périodique de vapeur à l'intérieur d'un évaporateur à microcanaux [texte imprimé] / Hanfei Tuo, Auteur ; Pega Hrnjak, Auteur . - 2014 . - pp. 2187–2195. Refrigeration Langues : Anglais (eng) in International journal of refrigeration > Vol. 36 N° 8 (Décembre 2013) . - pp. 2187–2195 Mots-clés : Vapour;  periodic  reverse  flow;  boiling;  fluctuation;  venting;  evaporator;  microchannel Résumé : This paper presents a proposal for a venting reverse vapor in flash gas removal A/C system in order to improve refrigerant distribution and reduce pressure drop in microchannel evaporator and thus increase system efficiency. Introduction to the reverse vapor flow observed in parallel flow microchannel evaporator was presented in earlier IJR paper by the authors. An experimental comparison of the A/C system with new approach to an FGB system revealed that vapor venting provided a 5% increase of cooling capacity and 3% of COP when operated at identical test conditions, while the maximum COP improvement was approximately 10%–12% when capacity is matched by reduction of compressor speed. The improvement compared to direct expansion system was significantly higher. En ligne : http://www.sciencedirect.com/science/article/pii/S0140700713001473

Periodical reverse flow and boiling fluctuations in a microchannel evaporator of an air-conditioning system / Hanfei Tuo in International journal of refrigeration, Vol. 36 N° 4 (Juin 2013) 

Public ISBD [article]  in International journal of refrigeration > Vol. 36 N° 4 (Juin 2013) . - pp. 1263–1275 Titre : Periodical reverse flow and boiling fluctuations in a microchannel evaporator of an air-conditioning system Titre original : Ecoulement inverse périodique et fluctuations de l'ébullition dans l'évaporateur à microcanaux d'un système de conditionnement d'air Type de document : texte imprimé Auteurs : Hanfei Tuo, Auteur ; Pega Hrnjak, Auteur Année de publication : 2013 Article en page(s) : pp. 1263–1275 Note générale : Refrigeration Langues : Anglais (eng) Mots-clés : Periodic  reverse  flow;  Flow  visualization;  Boiling  fluctuation;  Microchannel  evaporator Résumé : This paper presents the phenomenon of periodic reverse flow and associated boiling fluctuation found in experiments with a parallel microchannel evaporator used in an R134a air conditioning system. A simultaneous flow visualizations and measurements confirmed the periodic flow reversal. It caused synchronized oscillations of the evaporator inlet pressure and the pressure drop. The magnitude and frequency of oscillations increased with heat flux. Three potential impacts of flow reversal on evaporator performance are identified: 1) moderate liquid maldistribution; 2) reduced heat transfer coefficient; 3) increased refrigerant side pressure drop. Finally, to mitigate impacts of periodic reverse flow, a solution is proposed: to vent and bypass backflow vapor accumulated in the inlet header. En ligne : http://www.sciencedirect.com/science/article/pii/S0140700713000236 [article] Periodical reverse flow and boiling fluctuations in a microchannel evaporator of an air-conditioning system = Ecoulement inverse périodique et fluctuations de l'ébullition dans l'évaporateur à microcanaux d'un système de conditionnement d'air [texte imprimé] / Hanfei Tuo, Auteur ; Pega Hrnjak, Auteur . - 2013 . - pp. 1263–1275. Refrigeration Langues : Anglais (eng) in International journal of refrigeration > Vol. 36 N° 4 (Juin 2013) . - pp. 1263–1275 Mots-clés : Periodic  reverse  flow;  Flow  visualization;  Boiling  fluctuation;  Microchannel  evaporator Résumé : This paper presents the phenomenon of periodic reverse flow and associated boiling fluctuation found in experiments with a parallel microchannel evaporator used in an R134a air conditioning system. A simultaneous flow visualizations and measurements confirmed the periodic flow reversal. It caused synchronized oscillations of the evaporator inlet pressure and the pressure drop. The magnitude and frequency of oscillations increased with heat flux. Three potential impacts of flow reversal on evaporator performance are identified: 1) moderate liquid maldistribution; 2) reduced heat transfer coefficient; 3) increased refrigerant side pressure drop. Finally, to mitigate impacts of periodic reverse flow, a solution is proposed: to vent and bypass backflow vapor accumulated in the inlet header. En ligne : http://www.sciencedirect.com/science/article/pii/S0140700713000236