Ev tipi buzdolabında buz oluşumunun sayısal ve deneysel olarak incelenmesi
Ev tipi buzdolabında buz oluşumunun sayısal ve deneysel olarak incelenmesi
Dosyalar
Tarih
2025-06-17
Yazarlar
Uçar, Ahmet
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
İTÜ Lisansüstü Eğitim Enstitüsü
Özet
Günümüzde ev tipi buzdolaplarının fonksiyonları artmaktadır ve bu fonksiyonların önemli olanlarından bir tanesi de tam otomatik buz makineleridir. Tam otomatik buz makinelerinin sağladıkları konforun yanında en önemli iki özelliği günlük buz üretim hızları ve saklama haznelerinin kapasiteleridir. Günlük buz üretim hızının arttırılması için hem üreticiler tarafından hem de literatürde çalışmalar gerçekleştirilmektedir. Günlük buz hızına etki eden faktörlerin belirlenmesi ve bu faktörlerle en optimum çözümün sağlanması çalışmaların konusunu oluşturmaktadır. Ayrıca, buz oluşum süresinin modellenmesine dair de literatürde çalışmalar bulunmaktadır. Buz oluşum süresininin modellendiği durumda, üreticiler günlük buz üretim hedeflerine erişmek için hangi koşulları sağlamaları gerektiğine dair güçlü tahminlere sahip olacaklardır. Bu tez çalışmasında, buz oluşum süresinin hesaplanmasını içeren bir model oluşturulmuş ve buz üretim hızına etki eden su kütlesi, kartuş üstündeki ve altındaki hava debisi oranı ve hava sıcaklığı etkilerinin günlük buz performansına etkisi parametrik olarak çalışılmıştır. Tez çalışmasında kullanılan tam otomatik buz makinesinde, buharlaştırıcı üzerinden geçerek soğuyan hava, fan aracılığı ile suyu soğutarak buz oluşumunu gerçekleştirmektedir. Buz oluşum süresinin modellenmesi için ısı yayılım denkleminden yararlanılmıştır. Isı yayılım denklemi termal problemlerde sıcaklık dağılımının elde edilmek istendiği durumlarda kullanılır. Buz oluşumunda faz değişimi esnasında sıcaklık 0°C sabit kalır. Isı yayılım denklemi buz oluşumuna uyguladığında sonsuz küçük ya da sonlu küçük elemanın sıcaklığı, komşu hücreleri ile olan sıcaklık farkından dolayı değişecektir. Faz değişiminde ise sıcaklığın 0°C'de sabit kalması gerekmektedir. Dolayısıyla, ısı yayılım denklemi doğrudan faz değişim problemine uygulanamaz. Bu sebeple, ısı yayılım denklemi, sıcaklık, entalpi ve özgül ısı ilişkisi ile entalpi formunda yazılmıştır. Sayısal yöntemlerle çözülecek olan denklem için sonlu farklar metodu kullanılmış, zamanda ileri konumda merkezi farklar ayrıklaştırılmasından yararlanılmıştır. Elma dilim olarak adlandırılan buz geometrisi model ortamına aktarılmıştır. Hava taşınım katsayısının elde edilmesi adına, buz kartuşunun altındaki ve üstündeki akış şartları belirlenmiştir. Literatürdeki korelasyonlarla, kartuşun alt bölgesinde türbülanslı iç akış, kartuşun üst bölgesinde hava hızlarına bağlı olarak plaka üzerinde laminer akış ve doğal taşınım denklemleri kullanılmıştır. Elde edilen taşınım katsayısı ile yüzeylerde taşınım katsayısı sınır koşulu uygulanmış, ısı yayılım denklemi su ve buzun iletim katsayıları ile tüm geometri için çözülmüştür. Model ortamında buzun ortasındaki bir noktanın sıcaklık değişimi, deneyde aynı noktaya konumlandırılan termokupl ile karşılaştırılmıştır. Model ile deney sonuçları arasındaki fark, faz değişim süresi için en fazla %13 olurken, sıvı bölgeden katı bölgeye toplam buz oluşum süresi için en fazla %11 olmuştur. Buz oluşum hızına etki faktörler olarak su kütlesi, buz kartuşunun üstündeki ve altındaki hava debisinin oranı ve hava sıcaklığı etkileri incelenmiştir. Su kütlesinin artışının artan ısı transfer yüzey alanı pozitif şekilde, artan su-buz kalınlığı sebebi ile iletim dirençlerinin artışıyla negatif şekilde günlük buz oluşum miktarını etkileyeceği düşünülmüştür. Bir buz döngüsü için 100 g, 125 g ve 150 g su kütleleri parametrik olarak incelenmiştir. Buz kartuşunun üst bölgesindeki havanın su ile temasının zayıf kaldığı gerçekleştirilen HAD analizlerinde görülmüş, kartuşun alt bölgesine daha yüksek oranda hava debisi gönderilmesine çalışılmıştır. Gerçekleştirilen HAD analizleri doğrultusunda tasarlanan parçalar ile, toplam hava debisi sabit tutularak 1:1, 1:1,25 ve 1:1,5 üst/alt hava debi oranları parametrik olarak incelenmiştir. Hava sıcaklığının düşük olması her durumda buz oluşum hızını iyileştirecektir. Hava sıcaklığının düşürülmesi için gerçekleştirilecek tasarımsal değişiklikler ve günlük buz üretim hedefleri bir arada düşünülerek uygun hava sıcaklığı seçimi yapılabilecektir. Bu doğrultuda, -13°C, -15°C ve -17°C hava sıcaklıkları parametrik olarak incelenmiştir. Toplamda 27 deney tam faktöriyel olarak gerçekleştirilmiştir. Sonuçlar, her 2°C hava sıcaklığı düşüşünde günlük buz oluşum miktarının ortalama %20 arttığını göstermiştir. Su kütlesinin 100 gramdan 125 grama çıkışında günlük buz oluşum miktarı %6, 125 gramdan 150 grama çıktığında %2 arttığı görülmüştür. 1:1 debi oranından 1:1,25 debi oranına geçişte günlük buz oluşum miktarı %5, 1:1,25 debi oranından 1:5 debi oranına geçişte ise %2 artmıştır.
In recent years, domestic refrigerators have gained more advanced functions to meet users' changing needs, and one of the most notable of these is the integration of fully automatic ice makers. These systems offer significant convenience, allowing users to obtain ice without manual effort, but they are also expected to perform efficiently in terms of production rate and storage capacity. The most critical performance metrics for such systems are the total amount of ice that can be produced per day and the ice that can be stored in the system's container. To increase daily ice production rates, both manufacturers and researchers have investigated the effects of various system parameters. These parameters typically include the total airflow supplied to the ice-forming region, the temperature of the circulating air, and the physical shape and volume of the ice being produced. Determining the impact of these factors and finding the optimal operating conditions are key steps toward improving the overall performance. Moreover, modelling the time required for water to completely freeze is also essential for predicting system behaviour and optimizing performance. Several modelling strategies have been proposed in the literature to address this challenge. These include modifications to the classical Stefan solution to better account for dynamic interfaces between ice and water, empirical modelling approaches based on experimental measurements, simulations using commercial software, and numerical modelling based on enthalpy formulations that capture the physics of phase change. When the freezing process is modelled with sufficient accuracy, manufacturers can better estimate the system requirements to achieve specific production targets. In this thesis, a simulation model was developed that solves the heat diffusion equation in an enthalpy-based format. The convective heat transfer coefficients were calculated separately for the upper and lower surfaces of the ice cartridge, depending on airflow conditions. The ice geometry, which resembles a slice of an apple, was incorporated into the model to preserve its actual physical shape. The model was used to calculate how temperature and phase change evolve over time of water. To confirm the accuracy of the model, experiments were conducted where thermocouples were placed in the geometric center of different ice samples. These temperature readings were then compared with the simulation results. In addition to model development and validation, the study also included a detailed parametric analysis to evaluate how water mass, airflow ratio, and air temperature influence ice formation. A total of 27 experiments were conducted using combinations of three different air temperatures, three different water masses, and three airflow ratios above and below the ice cartridge. The only measured result in these experiments was the total amount of ice produced per day, expressed in kilograms. The fully automatic ice maker used in the experiments operates by blowing air that has been cooled by an evaporator across the water-filled ice cartridge. This cold air removes heat from the water, causing it to freeze over time. The phase change where the freezing process of water occurs, the temperature remains constant at 0°C. Because of this, the classical heat diffusion equation cannot be applied in its standard form, as it does not account for the thermal energy transferred during phase change without temperature variation. For this reason, the equation was reformulated using enthalpy, which considers both sensible heat and latent heat. The model used the finite difference method to numerically solve the enthalpy-based heat diffusion equation, with discretized using a forward in time and central in space. The ice geometry was defined precisely within the computational grid, allowing for accurate spatial resolution of temperature fields. In order to simulate realistic boundary conditions, the airflow around the ice cartridge was characterized in detail. Turbulent internal flow was assumed for the bottom of the cartridge using established correlations, while laminar external flow or natural convection equations were applied to the upper surface depending on local air velocities. These heat transfer coefficients were applied to the model as boundary conditions. After solving the equations across the simulation domain, the temperature distribution and phase change evolution were obtained. The model's results were validated experimentally in four different test conditions, which included two different air temperatures and two different ice samples. In each case, the temperature profile measured by a thermocouple placed in the center of the ice was compared to the simulation prediction at the same location. The maximum observed error between the model and the experimental result was 13% for the duration of the phase change and 11% for the total freezing time, confirming the reliability of the numerical model under the tested conditions. The parametric study in this thesis examined the effects of water mass, cartridge top/bottom flow ratio, and air temperature on daily ice production. It is intuitive to think that increasing the water mass would increase the time needed to freeze it, but daily performance is not only a matter of how long a single cycle takes. If the additional mass causes a proportional increase in cycle time, daily production remains unchanged. However, because larger water volumes also increase the surface area available for heat transfer, the freezing process becomes more efficient. As a result, the freezing time does not increase at the same rate as the mass, which leads to a net improvement in daily ice production. On the other hand, thicker layers of water and ice lead to higher conduction resistance, which could reduce freezing rate. Therefore, both positive and negative effects were considered. The tested water masses were 100 g, 125 g, and 150 g. Computational Fluid Dynamics (CFD) simulations showed that the airflow coming from the top part of the cartridge had limited contact with the water surface, reducing heat transfer efficiency. This finding led to a design change that increased the proportion of air sent below the cartridge. Airflow distribution ratios of 1:1, 1:1.25, and 1:1.5 referring to the proportion of air directed above and below the ice cartridge were tested while keeping the total airflow constant. Additionally, three air temperatures -13°C, -15°C, and -17°C were tested to evaluate the benefits of colder operating conditions. The results showed that reducing air temperature by 2°C increased daily ice production by approximately 20%. Increasing water from 100 g to 125 g led to a 6% improvement and increasing it further to 150 g resulted in an additional 2%. Changing the airflow distribution from 1:1 to 1:1.25 increased production by 5%, while moving from 1:1.25 to 1:1.5 brought another 2% gain. Even though 150 g water mass and 1:1.5 airflow provided the best performance in terms of ice yield, both were found to have drawbacks. The larger ice size made ejection more difficult and less reliable, so 125 g was selected as the most suitable water amount, offering similar performance with fewer problems. Likewise, the 1:1.5 airflow ratio created increased pressure losses, which could reduce fan performance in actual systems. This effect was not observed in the test environment since the total airflow was kept constant by adjusting the fan speed. Therefore, under real conditions, the 1:1.25 airflow ratio was seen as a more balanced and practical option. Finally, although colder air clearly improves ice formation, it could also lower the cooling performance of other refrigerator compartments or require a larger compressor, which would increase energy consumption. Therefore, the positive effect of cold air must be weighed against these system-level costs, and the optimum air temperature should be chosen carefully for both performance and efficiency.
In recent years, domestic refrigerators have gained more advanced functions to meet users' changing needs, and one of the most notable of these is the integration of fully automatic ice makers. These systems offer significant convenience, allowing users to obtain ice without manual effort, but they are also expected to perform efficiently in terms of production rate and storage capacity. The most critical performance metrics for such systems are the total amount of ice that can be produced per day and the ice that can be stored in the system's container. To increase daily ice production rates, both manufacturers and researchers have investigated the effects of various system parameters. These parameters typically include the total airflow supplied to the ice-forming region, the temperature of the circulating air, and the physical shape and volume of the ice being produced. Determining the impact of these factors and finding the optimal operating conditions are key steps toward improving the overall performance. Moreover, modelling the time required for water to completely freeze is also essential for predicting system behaviour and optimizing performance. Several modelling strategies have been proposed in the literature to address this challenge. These include modifications to the classical Stefan solution to better account for dynamic interfaces between ice and water, empirical modelling approaches based on experimental measurements, simulations using commercial software, and numerical modelling based on enthalpy formulations that capture the physics of phase change. When the freezing process is modelled with sufficient accuracy, manufacturers can better estimate the system requirements to achieve specific production targets. In this thesis, a simulation model was developed that solves the heat diffusion equation in an enthalpy-based format. The convective heat transfer coefficients were calculated separately for the upper and lower surfaces of the ice cartridge, depending on airflow conditions. The ice geometry, which resembles a slice of an apple, was incorporated into the model to preserve its actual physical shape. The model was used to calculate how temperature and phase change evolve over time of water. To confirm the accuracy of the model, experiments were conducted where thermocouples were placed in the geometric center of different ice samples. These temperature readings were then compared with the simulation results. In addition to model development and validation, the study also included a detailed parametric analysis to evaluate how water mass, airflow ratio, and air temperature influence ice formation. A total of 27 experiments were conducted using combinations of three different air temperatures, three different water masses, and three airflow ratios above and below the ice cartridge. The only measured result in these experiments was the total amount of ice produced per day, expressed in kilograms. The fully automatic ice maker used in the experiments operates by blowing air that has been cooled by an evaporator across the water-filled ice cartridge. This cold air removes heat from the water, causing it to freeze over time. The phase change where the freezing process of water occurs, the temperature remains constant at 0°C. Because of this, the classical heat diffusion equation cannot be applied in its standard form, as it does not account for the thermal energy transferred during phase change without temperature variation. For this reason, the equation was reformulated using enthalpy, which considers both sensible heat and latent heat. The model used the finite difference method to numerically solve the enthalpy-based heat diffusion equation, with discretized using a forward in time and central in space. The ice geometry was defined precisely within the computational grid, allowing for accurate spatial resolution of temperature fields. In order to simulate realistic boundary conditions, the airflow around the ice cartridge was characterized in detail. Turbulent internal flow was assumed for the bottom of the cartridge using established correlations, while laminar external flow or natural convection equations were applied to the upper surface depending on local air velocities. These heat transfer coefficients were applied to the model as boundary conditions. After solving the equations across the simulation domain, the temperature distribution and phase change evolution were obtained. The model's results were validated experimentally in four different test conditions, which included two different air temperatures and two different ice samples. In each case, the temperature profile measured by a thermocouple placed in the center of the ice was compared to the simulation prediction at the same location. The maximum observed error between the model and the experimental result was 13% for the duration of the phase change and 11% for the total freezing time, confirming the reliability of the numerical model under the tested conditions. The parametric study in this thesis examined the effects of water mass, cartridge top/bottom flow ratio, and air temperature on daily ice production. It is intuitive to think that increasing the water mass would increase the time needed to freeze it, but daily performance is not only a matter of how long a single cycle takes. If the additional mass causes a proportional increase in cycle time, daily production remains unchanged. However, because larger water volumes also increase the surface area available for heat transfer, the freezing process becomes more efficient. As a result, the freezing time does not increase at the same rate as the mass, which leads to a net improvement in daily ice production. On the other hand, thicker layers of water and ice lead to higher conduction resistance, which could reduce freezing rate. Therefore, both positive and negative effects were considered. The tested water masses were 100 g, 125 g, and 150 g. Computational Fluid Dynamics (CFD) simulations showed that the airflow coming from the top part of the cartridge had limited contact with the water surface, reducing heat transfer efficiency. This finding led to a design change that increased the proportion of air sent below the cartridge. Airflow distribution ratios of 1:1, 1:1.25, and 1:1.5 referring to the proportion of air directed above and below the ice cartridge were tested while keeping the total airflow constant. Additionally, three air temperatures -13°C, -15°C, and -17°C were tested to evaluate the benefits of colder operating conditions. The results showed that reducing air temperature by 2°C increased daily ice production by approximately 20%. Increasing water from 100 g to 125 g led to a 6% improvement and increasing it further to 150 g resulted in an additional 2%. Changing the airflow distribution from 1:1 to 1:1.25 increased production by 5%, while moving from 1:1.25 to 1:1.5 brought another 2% gain. Even though 150 g water mass and 1:1.5 airflow provided the best performance in terms of ice yield, both were found to have drawbacks. The larger ice size made ejection more difficult and less reliable, so 125 g was selected as the most suitable water amount, offering similar performance with fewer problems. Likewise, the 1:1.5 airflow ratio created increased pressure losses, which could reduce fan performance in actual systems. This effect was not observed in the test environment since the total airflow was kept constant by adjusting the fan speed. Therefore, under real conditions, the 1:1.25 airflow ratio was seen as a more balanced and practical option. Finally, although colder air clearly improves ice formation, it could also lower the cooling performance of other refrigerator compartments or require a larger compressor, which would increase energy consumption. Therefore, the positive effect of cold air must be weighed against these system-level costs, and the optimum air temperature should be chosen carefully for both performance and efficiency.
Açıklama
Tez (Yüksek Lisans)-- İstanbul Teknik Üniversitesi, Lisansüstü Eğitim Enstitüsü, 2025
Anahtar kelimeler
makine mühendisliği,
mechanical engineering,
refrigerators,
buzdolapları