Hermetik Pistonlu Bir Kompresörde Birleşik (konjuge) Isı Geçişinin Sayısal Olarak İncelenmesi
Hermetik Pistonlu Bir Kompresörde Birleşik (konjuge) Isı Geçişinin Sayısal Olarak İncelenmesi
Dosyalar
Tarih
2014-01-31
Yazarlar
Dincer, Mehmet Onur
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Institute of Science and Technology
Institute of Science and Technology
Özet
Hermetik pistonlu kompresörlerin verimini termodinamik, mekanik ve elektriksel kayıplar belirlemektedir. Günümüz teknolojisi ile ekonomik faktörler de göz önüne alındığında mekanik ve elektriksel kayıpların limitlerine yakın olduğunu, termodinamik kayıpların ise iyileştirmelere açık olduğunu söylemek mümkündür. Kompresörlerde verim artırma ve yeni tasarım çalışmalarının yürütülebilmesi ve kompresör bileşenleri için uygun malzemelerin belirlenebilmesi doğrultusunda kompresör içi sıcaklık dağılımının ve bileşenler arasındaki ısı transferi mekanizmalarının detaylı olarak bilinmesi gerekmektedir. Kompresör termal haritasının çıkarılması amacıyla gerçekleştirilen deneysel sıcaklık ölçüm çalışmaları, çok sayıda termokuplın kompresör içerisine yerleşimi nedeniyle zorluklar taşımaktadır. Komponentlerin malzemelerinde veya tasarımlarında yapılacak değişikliklerin sıcaklık dağılımına etkilerinin görülmesi adına bu çalışmaların tekrarlanması da gerekmektedir. Bu tez çalışmasında tasarıma ait tüm detaylar dikkate alınarak oluşturulan kompresör modelinde birleşik (konjuge) ısı geçişi analizi yapılarak detay sıcaklık ölçümlerinin sayısal olarak incelenmesi hedeflenmiştir. Analizler için gerekli sınır şartlarını temin etmek ve sayısal sonuçlarla karşılaştırmak amacıyla kompresör detay sıcaklık ölçümleri ve indikatör diyagramının (pV) çıkarılmasını içeren deneysel çalışmalar gerçekleştirilmiştir. Sayısal analizlerde öncelikle soğutkan hattı deaktif yapılarak yalnız enerji denklemi çözdürülmüş, sonuçlar doğrultusunda katı model yüzeylerindeki ısı taşınım katsayıları optimize edilmiştir. Bu kısımda fiziksel olarak akışkanın temas ettiği kısımlar sabit sıcaklık şartında kabul edilmiş ve çeşitli noktalarda deneysel olarak ölçülen iç gaz sıcaklıkları sayısal modele girilerek komponentlerdeki ısı taşınım katsayılarının mertebeleri belirlenmiştir. İkinci kısımda soğutkan hattı aktif duruma getirilip ilk analiz sonucu elde edilen ısı taşınım katsayıları da kullanılarak sürekli rejimde gövde, silindir kafası ve valf tablası malzeme farklılıklarının ve motor veriminin sıcaklık dağılımına etkisi incelenmiş, sıcaklık dağılımındaki değişimlerin kompresör performansına olası etkileri yorumlanmıştır. Malzeme farklılıklarının inceleneceği komponentler, kompresör termodinamik verimini olumsuz yönde etkileyen; emme hattındaki soğutkanın ısınmasını önleme kriterine göre seçilmiştir. Üçüncü kısımda, soğutkan hattındaki akışın sürekli rejim analizlerinin tamamlanmasının ardından, kompresörün tüm katı bileşenleri başlangıç koşulu olarak ortam sıcaklığına (26oC) getirilmiş ve kompresörün termal rejime giriş sürecini incelemek amacıyla zamana bağlı CFD analizleri gerçekleştirilmiştir. Ayrıca soğutkan hattında zamana bağlı CFD analizleri ile özellikle emme susturucusu içerisindeki akış hareketleri ve ısı geçişi mekanizmaları incelenmiştir. Son kısımda ise kompresörün egzoz patikasında yalıtım uygulamalarının etkinliği lokal egzoz hattı CFD analizleri ile araştırılmıştır.
Efficiency of hermetic reciprocating compressors, which are used in household refrigerators, is subjected to thermodynamic, mechanical and electrical losses. Heat transfer analyses reveal reference points that involve improvement of the compressors. A proper understanding of heat transfer and temperature distribution of various components in compressors helps in determining the parts geometry and material selection. However, the temperature of each compressor component is affected by several phenomena that act simultaneously, making the temperature distribution harder to predict. This study presents detailed CFD analyses of a hermetic reciprocating compressor for several cases and experiments which support numerical analyses. Temperature distribution of the compressor which is subjected to CFD analyses is investigated experimentally. Compressor, which is designed for R600a refrigerant and has a cooling capacity of 170W at ASHRAE conditions, is instrumented with several T-type thin thermocouples. Temperature values are collected while the compressor is operating on a fully automated calorimeter system with conditions of -23.3°C and +54.4°C evaporation and condensation temperatures respectively. Subcooling, superheating and ambient temperatures were 32.2°C. Time dependent pressure values at suction and discharge lines are also investigated experimentally. CAD model of the hermetic reciprocating compressor is prepared by using commercial software (NX 7.5). Mesh structure is created with ANSYS meshing software. Finer mesh elements are used at required locations in order to improve mesh quality. A commercially available CFD code (ANSYS Fluent v14.0) is used for the analyses. The numerical model is formed with both refrigerant (fluid) domain and solid domain which comprises compressor components. In the first case, the refrigerant domain was deactivated, so the pure heat conduction analysis was performed for the solid components in order to optimize heat transfer coefficients by comparing temperature values with experimental data. Refrigerant domain was reactivated for the second case in which conjugated heat transfer problem was to be solved supported by heat transfer coefficients obtained in the first case. Heat generation during the compression process inside the cylinder causes heat transfer between the crankcase, valve plate and cylinder head. It is seen that suction muffler gains heat from these aforementioned components which are at higher temperatures. Effects of using different materials; for some key components such as the crankcase, cylinder head and the valve plate; on temperature distribution inside the compressor were investigated. Material of the crankcase was changed to aluminum at first place. This case lead to a homogenous temperature distribution on the crankcase and lower temperature values around the cylinder region were obtained. The other modification is to change the material of the valve plate to an insulative. This effort concluded in reducing heat conduction from cylinder region to the suction muffler in means of constituting a resistance region to heat transfer. The last modification is performed for plastic originated cylinder head which resulted in significant change on the suction chamber wall temperature. In virtue of low thermal conductivity of the material suction muffler was remained at lower temperatures. Transient analysis was performed in order to understand flow behavior and heat transfer mechanisms causing superheating within the suction muffler. Time dependent pressure profiles, which were obtained experimentally, are used as the boundary conditions at the suction chamber, discharge chamber and cylinder. It is observed that temperature of the refrigerant inside the suction muffler rises during the compression and discharge processes caused by heat transferred from cylinder head, valve plate and suction reed valve. Especially the suction chamber region of the suction muffler was being heated. Refrigerant at higher temperature enters the cylinder at the former phase of the suction process. Density of the refrigerant lowers due to rising temperature which reduces mass flow rate. This situation causes reduction in volumetric efficiency and cooling capacity. In the last case, the local insulation approaches in the discharge line were examined with steady state CFD analyses, searching the most effective locations to insulate in an effort to reduce the heat transfer to the internal ambient of the compressor. Motivation of insulating particular regions is to reduce total heat transfer from whole exhaust region to the inner gas of the compressor. It is clear that inner gas is one of the common heat sources for suction gas inside the suction muffler, which causes decrease in volumetric efficiency. CFD analyses exposed that, using a plastic originated material inserted inside the resonator hole and first exhaust muffler does not deduce significant effect on heat transfer rate. The reason of this situation can be explained as; reducing heat transfer from the refrigerant at the former sections of the exhaust path leads other sections to higher temperatures. Reducing heat transfer on some sections, while increasing on others, lowers impact of the insulation. On the other hand; insulating the end region of the exhaust path may decrease the total heat transfer rate up to %12. It is concluded that, end regions of the exhaust path are the most important locations for effective insulation. The next plan for the numerical model of the reciprocating compressor is to attach inner gas and oil to the solution domain in order to obtain temperature distribution of compressors inner ambient. Effect of radiation heat transfer is also to be investigated.
Efficiency of hermetic reciprocating compressors, which are used in household refrigerators, is subjected to thermodynamic, mechanical and electrical losses. Heat transfer analyses reveal reference points that involve improvement of the compressors. A proper understanding of heat transfer and temperature distribution of various components in compressors helps in determining the parts geometry and material selection. However, the temperature of each compressor component is affected by several phenomena that act simultaneously, making the temperature distribution harder to predict. This study presents detailed CFD analyses of a hermetic reciprocating compressor for several cases and experiments which support numerical analyses. Temperature distribution of the compressor which is subjected to CFD analyses is investigated experimentally. Compressor, which is designed for R600a refrigerant and has a cooling capacity of 170W at ASHRAE conditions, is instrumented with several T-type thin thermocouples. Temperature values are collected while the compressor is operating on a fully automated calorimeter system with conditions of -23.3°C and +54.4°C evaporation and condensation temperatures respectively. Subcooling, superheating and ambient temperatures were 32.2°C. Time dependent pressure values at suction and discharge lines are also investigated experimentally. CAD model of the hermetic reciprocating compressor is prepared by using commercial software (NX 7.5). Mesh structure is created with ANSYS meshing software. Finer mesh elements are used at required locations in order to improve mesh quality. A commercially available CFD code (ANSYS Fluent v14.0) is used for the analyses. The numerical model is formed with both refrigerant (fluid) domain and solid domain which comprises compressor components. In the first case, the refrigerant domain was deactivated, so the pure heat conduction analysis was performed for the solid components in order to optimize heat transfer coefficients by comparing temperature values with experimental data. Refrigerant domain was reactivated for the second case in which conjugated heat transfer problem was to be solved supported by heat transfer coefficients obtained in the first case. Heat generation during the compression process inside the cylinder causes heat transfer between the crankcase, valve plate and cylinder head. It is seen that suction muffler gains heat from these aforementioned components which are at higher temperatures. Effects of using different materials; for some key components such as the crankcase, cylinder head and the valve plate; on temperature distribution inside the compressor were investigated. Material of the crankcase was changed to aluminum at first place. This case lead to a homogenous temperature distribution on the crankcase and lower temperature values around the cylinder region were obtained. The other modification is to change the material of the valve plate to an insulative. This effort concluded in reducing heat conduction from cylinder region to the suction muffler in means of constituting a resistance region to heat transfer. The last modification is performed for plastic originated cylinder head which resulted in significant change on the suction chamber wall temperature. In virtue of low thermal conductivity of the material suction muffler was remained at lower temperatures. Transient analysis was performed in order to understand flow behavior and heat transfer mechanisms causing superheating within the suction muffler. Time dependent pressure profiles, which were obtained experimentally, are used as the boundary conditions at the suction chamber, discharge chamber and cylinder. It is observed that temperature of the refrigerant inside the suction muffler rises during the compression and discharge processes caused by heat transferred from cylinder head, valve plate and suction reed valve. Especially the suction chamber region of the suction muffler was being heated. Refrigerant at higher temperature enters the cylinder at the former phase of the suction process. Density of the refrigerant lowers due to rising temperature which reduces mass flow rate. This situation causes reduction in volumetric efficiency and cooling capacity. In the last case, the local insulation approaches in the discharge line were examined with steady state CFD analyses, searching the most effective locations to insulate in an effort to reduce the heat transfer to the internal ambient of the compressor. Motivation of insulating particular regions is to reduce total heat transfer from whole exhaust region to the inner gas of the compressor. It is clear that inner gas is one of the common heat sources for suction gas inside the suction muffler, which causes decrease in volumetric efficiency. CFD analyses exposed that, using a plastic originated material inserted inside the resonator hole and first exhaust muffler does not deduce significant effect on heat transfer rate. The reason of this situation can be explained as; reducing heat transfer from the refrigerant at the former sections of the exhaust path leads other sections to higher temperatures. Reducing heat transfer on some sections, while increasing on others, lowers impact of the insulation. On the other hand; insulating the end region of the exhaust path may decrease the total heat transfer rate up to %12. It is concluded that, end regions of the exhaust path are the most important locations for effective insulation. The next plan for the numerical model of the reciprocating compressor is to attach inner gas and oil to the solution domain in order to obtain temperature distribution of compressors inner ambient. Effect of radiation heat transfer is also to be investigated.
Açıklama
Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2014
Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2014
Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2014
Anahtar kelimeler
hermetik,
kompresör,
ısı transferi,
CFD,
hermetic,
reciprocating,
compressor,
heat transfer,
CFD