Bir buzdolabı buharlaştırıcısının teorik ve deneysel incelenmesi
A Theoretical and experimental study on a domestic refrigerator evaporator
- Tez No: 46109
- Danışmanlar: DOÇ.DR. TANER DERBENTLİ
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1995
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 114
Özet
ÖZET Buharlaştırıcı, buhar sıkıştırmalı çevrim esasına göre çalışan bir buzdolabında, soğutma sistemi devre elemanlarından biri olup, buzdolabı kabinini soğutmaya yarar. Kanatlı borulu türden bir ısı değiştiricisi olan buharlaştırıcıda, soğutucu akışkan boruların içinden akarken, soğutulmak istenen ortam havası, boru ve kanatların dışından bir fan yardımıyla zorlanmak suretiyle akmaktadır. Buzdolabı soğutma devresinin tasarımı aşamasında, belirli şartlar altında istenen soğutma yükünü gerçekleştirecek uygun bir buharlaştırıcının seçimi çok önemlidir. Bu çalışmada, levha kanatlı borulu türden bir buzdolabı buharlaştırıcısında ısı ve momentum geçişi teorik ve deneysel olarak incelenmiştir. Buharlaştırıcının buzdolabı üzerindeki davranışını simüle eden bir deney düzeneği tasarlanarak kurulmuş ve değişik kanat aralığına sahip 4 farklı buharlaştırıcı üzerinde yapılan deneyler sonucunda, hava tarafı ısı taşınım katsayısı ve hava tarafı sürtünme faktörünü veren bağıntılar elde edilmiştir. Çalışmanın temelinde, geometrisi belirli olan bir buharlaştırıcının performans büyüklüklerinin belirlenmesi yatmaktadır. Bu amaçla, elde edilen deney sonuçlarından ve yapılan teorik çalışmalardan yararlanarak buharlaştırıcının matematiksel modeli oluşturulmuş; buharlaştırıcının performans büyüklüklerini hesaplama yöntemi ortaya konmuştur. Ayrıca bu hesap yöntemi bir bilgisayar programı olarak ifade edilmiştir. Bu bilgisayar programı, Arçelik A.Ş. Araştırma Geliştirme Merkezi' nde oluşturulan ve buzdolabı soğutma devresinin sistem simülasyonunu yapmaya yarayan bir paket programın alt modülü olarak çalışmaktadır. Bu nedenle program, gerek paket program ile birlikte soğutma devresinin tasarımında gerekse, kendi başına buharlaştırıcı tasarımında kullanılmaktadır. Bilgisayar programı, değişik buharlaştırıcı tasarım alternatiflerinin değerlendirilme olanağını sağlamıştır. Parametrik çalışmalar yaparak, istenen kapasiteyi sağlayacak bir buharlaştırıcının seçimi, program yardımıyla kısa sürede gerçekleştirilebilir hale gelmiştir.
Özet (Çeviri)
SUMMARY A THEORETICAL AND EXPERIMENTAL STUDY ON A DOMESTIC REFRIGERATOR EVAPORATOR In this study, heat and momentum transfer in a plate finned - tube refrigerator evaporator was examined both theoretically and experimentally. Heat transfer coefficient and friction factor for the four finned tube evaporators having different fin densities were obtained experimentally. A computational procedure was developed for the sizing of the refrigerator evaporator. In the design of an evaporator heat exchanger, it is necessary for the design engineer to know how the performance characteristics of the evaporator, such as overall heat transfer coefficient, heat transfer capacity, pressure drops of the cold and hot fluids and the outlet conditions of the fluids at the evaporator exit, change with different exchanger geometries. The heat transfer phenomenon between the two fluids, refrigerant and air, strongly depends on many parameters such as the exchanger geometry, flow rate, flow regime and the fluid type. Although numerical methods can be applied to such problems, because of the complexity of the problem and the large number of parameters, the heat and momentum transfer chracteristics of the heat exchangers are usually obtained experimentally. Therefore, designers need correlations to predict the heat transfer coefficients and friction factors as a function of flow rate, the fluid type and the exchanger geometry. In the literature, there are many correlations.which give the heat transfer coefficient and friction factor for plate finned - tube heat exchangers having flat, continuous fins and having four or less tubes in the air flow direction. The air velocities are relatively higher, the air flow is horizontal and uniformly distributed through the coil. The air inlet temperature is uniformly distributed at the exchanger inlet. In contrast to the previous plate finned - tube heat exchangers studied in the literature, for the refrigerator evaporator coil considered in this study, the air flow is vertical, nonuniforms distributed, the tube row number is greater than 4 and the fins have slots on their surfaces. An evaporator test facility was designed and constructed to nearly simulate the evaporator air flow conditions in the refrigerator. The hot water was circulated through the evaporator tubes, while the air was forced through the coil tubes and fins in cross flow fashion. The air flows in three different air ducts before reaching the evaporator inlet. The wind tunnel facility was constructed in such a way that an axial fan mounted in the downstream of the evaporator provides the total air flow while another fan ( booster fan ) mounted in the upstream of the evaporator facilitates the adjustment of flow split between the incoming air ducts. XVIIThe total airflow rate was measured in a wind tunnel constructed at the evaporator downstream while the split ratio in the coming ducts was measured by another tunnel mounted before the incoming air ducts at the evaporator upstream.. The air flow rates at the tunnels were measured by separate orifice plate - micromanometer systems. Fans were drived by direct current type electric motors. The booster fan' s motor was operated in both directions to supply the required flow rate at each duct, by adjusting the pressure level at the tunnel. The measured parameters were inlet and outlet temperatures of the air and the water. For this purpose type T thermocouples were used. The mass flow rate of the water was measured by a coriolis type flowmeter. The air side pressure loss of the coil was measured by a micromanometer. The pressure taps were mounted at the coil inlet and outlet. After steady state conditions were reached, temperatures and pressures were recorded by data loggers. The inlet flow conditions at both the air and the water side were kept constant during the test. Heat exchangers with four different fin spacings and fin patterns were tested and air flow rate was changed from 10 l/s to 35 l/s during each test. Water mass flow rate was 2000 g/min and the change in the water temperature was 2-3 °C at most while the change in the air temperature was 17-21 °C. Nearly pure counter flow conditions were obtained during the tests. The coils tested had 8mm outside tube diameter while the tubes were in staggered arrangement. The tubes were merely forced into the fins.which have slits on their surfaces, during assembly, i.e., there was no metallurgical bonding. Tube and fin material were aluminium. In the banks of tubes with plate fins, the total heat transfer can be written as follows : Q = UAATlm,ç where UA is the overall conductivity based on the outside total heat transfer area. 1 t 1 UA = 1/(- - +- 22-+ ' -) h,A, kwApm h0(Ato+TifAt) The fin efficiency, ti, used in the above equation was obtained from a Finite Element Analysis ( FEA ) for the evaporator slit fin geometry. The fin CAD geometry and the computational grid were created using a commercial software. After the boundary conditions were set, the conduction and convection heat transfer analysis was done and the fin efficiency of the fin was obtained at the given boundary condition set. The fin efficiency was formulated as a function of fin geometry and convective heat transfer coefficient using the different boundary condition values. XVIIIThe water side convective heat transfer coefficient was calculated using the Dittus-Boelter correlation. Nu = 0.023 Re08 Pr03 Considering a temperature distribution in a cross flow test coil as shown belove, Temp. Dist. the logarithmic mean temperature difference can be written as follows, R = P = The heat transfer rate was separately calculated at both the air side and the water side using the enthalpy change and the flow rate for the each test coil. The difference between the heat transfer rates calculated at the air side and the water side was within 4 %. The air side heat transfer coefficient was calculated from the heat transfer equation above and was correlated in terms of the following dimensionless equation. b r>.1/3 c Nu = aRe°Pr s XIXUsing the Colburn analogy, the above equation can be written as follows; Nu RePr 1/3 = a Re^s b-1 c A multiple linear regression analysis of the experimental data permitted the determination of the coefficients a, b and c. The air side friction factor coefficient was calculated from the pressure drop equation, G Pa 2Pi / \ (w) -e- i VPo J rnln "m and it was correlated by using a multiple linear regression analysis, e-1_f f = d Re^s the constant coefficients, d,e and f calculated from the regression analysis. Experimental results are presented graphically in section 6. A strong dependence of the heat transfer coefficients on the finning factor - s - was observed. The convective heat transfer coefficient increases as the fin spacing increases. It was observed that the convective heat transfer coefficient of a bare tube bundle was greater than that of a finned tube heat exchanger. It was also noticed that the friction factor increases for larger values of the fin spacing of the heat exchanger. A mathematical model of the evaporator was developed in the second part of the study. A lumped evaporator model which was based on the effectiveness - NTU method was used. The model assumed that there was no frosting or condensation on the outside surfaces of the evaporator. The model requires the geometric parameters such as tube diameter, tube - pass arrangement, fin thickness, fin spacing, inlet flow conditions of the refrigerant and the air and calculates the performance characteristics, which are overall heat transfer coefficient, heat transfer capacity, pressure losses on the refrigerant and the air side, outlet conditions of the refrigerant and air, of the evaporator under steady state condition. xxAir side heat and momentum transfer correlations implemented to the model were obtained from the experimental study. Refrigerant side heat and momentum transfer correlations were taken from the literature and implemented to the model. A computer program based on the computational procedure was developed. The program is used as a design tool for the sizing of the evaporator. The program also to be used as a subroutine program of a software which was developed in Arçelik A.Ş. R&D Center. This software is used to simulate the refrigeration system cycle of the refrigerator under steady state conditions. XXI
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