Blast freezer (şok dondurucu) içindeki sıcaklık dağılımı ve hava akışının incelenmesi
Investigation of temperature distribution and air flow in a blast freezer
Yazar: NIMA MOLANI
Danışman: DOÇ. DR. MELİKE NİKBAY
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Uçak ve Uzay Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Havacılık Mühendisliği = Aeronautical Engineering ; Makine Mühendisliği = Mechanical Engineering

Özet:
Gıdaların uzun süreli muhafaza ihtiyacı, insanlık tarihi boyunca, bir amaç ve zorunluluk olmuştur. Bu doğrultuda uygulanan en yaygın yöntemler, kurutma, tuzla salamura yapma, konserveleme ve soğutmadır. Bütün muhafaza yöntemleri, gıdaların besin değerlerini ve kalite değerlerini en üst düzeyde tutmayı amaçlamaktadır. Ancak gıdaların gerek kurutma, gerekse salamura veya konserveleme işlemlerinde, besin değerlerinde (özellikle vitamin) meydana gelen kayıplar oldukça fazladır. Bu nedenlere bağlı olarak soğutma yöntemi bir çok gıdaya uygulanabilirlik, besin / kalite değerlerinin korunması ve uygulama kolaylığı bakımından en etkili yöntemdir. Günümüz teknolojik imkanları dahilinde en yüksek verimli soğutma sistemi dondurma / şoklama / derin dondurma teknolojisidir. Blast Freezer sistemlerine yönelik olarak günümüzde oda tipi ve buzdolabı tipi olmak üzere iki genel kullanım sınıfı bulunmaktadır. Mevcut durumda oda tipi sistemler ülkemiz genelinde de üretilebilir iken, buzdolabı tipi sistemlerin boyut ve ağırlık kriterleri nedeniyle teknik altyapısının daha zor olduğu görülmüştür.
Dondurma teknolojisi ile muhafaza etmenin temel prensibi, -18〬C derece ve daha düşük sıcaklık değerlerinde gıdayı hızlı bir şekilde dondurarak, gıda bozulmasına sebep olabilecek mikroorganizmaların çalışma ve çoğalma faaliyetlerinin tam olarak durdurulmasıdır.
Bu çalışmanın amacı Hesaplamalı Akışkanlar Dinamiği (HAD) tekniğini kullanarak Blast Freezer buzdolabında akışın kabin içerisinde hedeflenen şekilde homojen şekilde dolaşmasını ve gıda kütle merkezinin ısısını 270 dakikada 100〬C dereceden -18〬C dereceye düşürülmesini sağlamaktır.
Bu çalışmayı literatürdeki çalışmalardan farklı kılan özellik, şok dondurucu üzerinde deneysel ve HAD yöntemleri yardımı ile tasarımın doğrulanması ve daha verimli bir prototipin imalatıdır.
Çalışmada ileriki aşamada farklı fan motorları kullanılarak alternatif incelemeler yapılmıştır. Bu analizlerin sonucunda toplam debi ve her bir fandan çıkan debi değerleri bulunmuştur. Ayrıca kabin içindeki sıcaklık dağılımları ve deneysel çalışmalarla uyumluluğu gösteren ölçüm paketlerinin sıcaklık dağılımları da sunulmuştur. 
Yapılan deneyler ve bunları destekleyen analizler doğrultusunda görülmüştür ki kabin içinde sıcaklık duvarlara yaklaştıkça artmaktadır. En soğuk paketler orta şok dondurucuda bulunmuştur. Sıcaklığın hızlı şekilde çok düşük değerlere düşmesi nedeni ile üst ve alt taraflardaki sıcaklık farkı çalışmamızı etkileyen önemli bir parametredir. Ayrıca kontrol panellerini kullanmadan, uyguladığımız sınır şartları ile çekirdek sıcaklığını istediğimiz sürede -18 C dereceye düşürülmesi mümkün kılınmıştır.

Abstract:
Preserving food was historically one of man's main challenges. The general methods of protection include drying, bedding in salt, conserving and freezing. All protection methods aim to maintain the content and quality of food at the highest possible level. However, drying, salting and conserving operations all result in the loss of some contents, especially the vitamins. Therefore, considering the quality and content of the food, freezing could be applied to many foods and this method is easier. 
Regarding the modern technological possibilities, the highest level of quality could be achieved using freezing, blasting and deep-freezing where food is reserved safe for storage and later consumption. Blast freezing  is commonly used in food catering and recently, in preparation of 'instant' foods, as it ensures the safety and the quality of the food product. A blast freezer is an appliance which reduces the core temperature of a cooked food from 100° C to -18° C in 270 minutes using 3 powerful fans and a cooling unit. 
The main principle of using the freezing method is to freeze food in  -18° C and to quickly  reduce the core temperature. This research aims to use the CFD (Computational Fluid Dynamics) methods to reduce the temperature from 100° C to  -18° C in 270 minutes so that activity and growth of microorganisms may be prevented which causes rinsing of the food.
In the present research, the model is designed in Solidworks CAD (Computer Aided Design)  program, then analyzed in Ansys Fluent CFD program. The present research differs from the previous research work and literature mainly by  finding a relation between the CFD and the experimental results which led to manufacturing of an improved prototype.
The air supply system is modelled as a closed-loop system while in former studies mass flow rates were given as boundary conditions. However, in this study the air supply system is completely modeled so that mass flow rates could be investigated numerically as well. Furthermore the CFD analysis was validated with experimental studies. 
The layout of the thesis is roughly as follows. First and second chapters include literature review in which existing refrigerator analysis, principles, food placement and CFD modeling of fan and cooling units are presented. Third chapter encompasses presenting the purchased refrigerator, testing environment, equipment and data logger. In the next step, a numerical model was generated and transferred from CAD to CFD environment and analyzed in Chapter 4. In Chapter 5, production line and prototype are presented in brief. Finally, the results are presented in the sixth chapter.   
In this thesis work, Unilab commercial program is utilized to select evaporator and condenser. Mesh generation using  Moving Reference Frame (MRF) method for modeling of the fan and the experimental approach of this study are based on the similar research and literature.
In experiments, the model was installed in a laboratory test room. The packages were placed in the Blast freezer in 14 1/1 GN trays. Then, the space between back of the model and rear wall of test room was adjusted to be 500 mm, as well as at least 1500 mm for space between sides of the refrigerator and side walls. The test was commenced at fixed 35〬C as ambient temperature. A package of 1kg was placed in each tray. In total there were 14 packages weighting 14 kg. During a 240-minute period the temperature of packages were recorded simultaneously utilizing 14 sensors.
During the experiments, core temperature of packages were recorded during a 4-hour process using data logger. The laboratory is conditioned to ensure the ambient temperature and relative humidity with respect to the tropical environment standards. 
For CFD, simplified CAD design of Blast freezer was exported to Ansys and was meshed using a number of meshing methods. In Fluent software, obtained meshed file was imported and boundary conditions were applied. The material specifications of air as a coolant and polyurethane as insulation material were applied in the CFD program. 
Consequently, pressure based k-εturbulence model was used to solve for the Navier-Stokes and energy equations. Then the airflow analysis and both airflow and temperature analysis were obtained for empty and full Blast freezer respectively. 
The research is carried out using different fan motors. Upon conclusion, the sum total debit (mass flow)  and the debit of each fan is given. In addition to debit, temperature distribution in the cabin and in measuring packs and its agreement with other experiments is presented.
Based on experiments and analyses, the temperature in the cabin increase as we approach the walls. The effect of weight of cooling air can be neglected due to the high speed of the exiting stream from fans. These fans also avoid downeard movement of cold stream. 
In terms of air circulation, the model refrigerator can be divided into three parts: The first part is affected by upper fan. Second part is under the influence of middle fan and the last part is affected by the bottom fan. The research shows that air stream is more dense in middle zone. The reason relates to the fact that one side of the upper and bottom fans are closed; while for the middle fan both sides are open and this leads to stream of more air and cooling middle pockets. Because of the decrease of temperature to very low levels, the difference of temperature in upper and lower parts will not influence our research. Moreover, without using the control system, -18° C could be achieved until we use the boundary conditions for obtaining the core temperature.
Validated from the computational analysis and the experimental studies, it is seen that the coolest packages are found to be in the middle regions while the warmest ones are in the upper and lower regions. The highest mass flow rates are at the fan outlet and the lowest rates are found at the fan inlet. 
The maximum velocity of 31.123 and 26.91 m/s were obtained for empty and full refrigerator respectively. Core temperature reduction in CFD and experimental model were agreeable. The average difference was 0.694° C which is a negligible value and it implies the CFD model is valid. Therefore the prototype production is carried out with the respect to the CFD solutions. 
After the prototype was manufactured, some tests according to the EN16825 have been performed and it is approved that the prototype complied with the performance requirements. Then, this Blast freezer is presented to the market.