Modeling of cold storage tank for dıfferent cooler arrangements

Abstract

Birçok Avrupa ülkesinde ve Amerika 'da elektrik tarifeleri günün saatlerine göre değişkenlik göstermektedir. Soğutma yükünün maksimum olduğu gündüz saatlerinde elektrik pahalı, soğutma ihtiyacının duyulmadığı gece saatlerinde ise daha ucuzdur. Bu durum .soğıı enerjisinin, gece saatlerinde depo edilip gündüz saatlerinde ise soğutma ihtiyacını karşılamak için kullanılması fikrini ortaya çıkarmıştır. Bu çalışmada, içerisinde su bulunan dikdörtgensel bir soğıı depolama tankına yerleştirilen farklı geometrilerin, buz oluşumuna etkilerinin karşılaştırılması amaçlanmıştır. Ayrıca tank içeresindeki faz değişim malzemesi olan suyun, başlangıçta farklı sıcaklıklarda olmasının da katılaşmaya olan etkileri incelenmiştir. Bu amaçla FLUENT paket programı kullanılarak; akış alanının zamana bağlı sayısal çözümü yapılmıştır. Dikdörtgensel tank iki boyutlu olarak modellenmiş ve akış alanı simetrik olduğundan, analizler tankın yarısı için yapılmıştır. ank içerisindeki su sıcaklığı () ÜC ve 0C olarak alınıp, silindir yüzey sıcaklığı -10 ÜC kabul edilerek, farklı geometrilerdeki silindir modelleri için, Al,/A, oranı (Buz alanı kesit alanı), sıcaklık dağılımı ve sıvı oranları hesaplanmıştır. Elde edilen sonuçlarda, tüm modellemeler için 900 s, 1800 s, 3600 ve 7200 deki sıcaklık ve sıvı oranlarının dağılımları incelenmiştir. Ayrıca farklı silindir düzenlemelerinin katılaşma oranlarının (Ab/A,) zamanla değişimi de değerlendirilmiştir. Dairesel olan silindir, elips ve altıgen modellerinde katılaşma oranlarının birbirine oldukça yakın olduğu görülmüş, üçgensel ve kare gibi keskin köşeli modellerde ise daha az buz oluşumu gözlenmiştir. Bununla beraber düzenlemelerin her birinde; su sıcaklığının ÜC olması durumunda, katılaşma oranlarının daha büyük olduğu tespit edilmiştir.

For the past few decades, the world ’s energy supply has not been keeping up with increasing demand. Burgeoning countries undergoing industrial reform are consuming an increasing amount of crude oil, coal and electricity, which has increased overall energy prices to an unprecedented level (Sönmez et al., 2009). Ice cool thermal energy storage systems are often classified as static or dynamic, according to the way ice is delivered to the storage tank. In dynamic systems, ice is produced outside the storage tank and removed from the ice-making surface continuously (Dinçer, 2002). In static systems, an ice-making pipe is installed in the storage tank where ice is formed and later melted. Ice-on-coil systems produce ice outside coil. The ice may be melted using either an external melting system, which melts ice from the outer side, or an internal melting system (Dinçer, 2002). Habeebullah (2007) investigated growth rate of ice on the outside of' cooled copper tubes. Ice formation around an isothermally cooled horizontal cylinder and eflect of' natural convection were studied by Cheng et al (1988). Fertelli et al. (2009) studied analytically and experimentally the solidification around the horizontal tube by considering fully developed velocity profile in the tube. Experiments are performed to investigate the effects of' diflerent heat transfer fluid inlet temperatures on ice formation around the tube. Finite volume method (FLUENT program) was used to solve solidification around cylinders placed in fixed volume as cylindrical, elliptical, hexagon, triangle, square geometry of cooled cylinder. Quadrilateral grid was usedfor simulation and/iner grid distribution was used near cylinder surfaces. Grid spacing increases awayfrom cylinder surfaces, with total of20000 to 25000 elements The‘following assumptions were made to simulate solidification around cylinders in fixed space: i) Flow is two dimensional, laminar and incompressible; Water as Newtonian fluid for phase changing material; ii) Unique thermal conductivities and specific heats (ks, k1, Cs, Cı) are considered/or solid and liquid phases; and iii) Efects of viscous dissipation and radiation are ignored. Validation studies of the present model shows that variations of area ratio (A), /A_,) match with experimental data in entire time range for solidification around single cylinder. To examine eflect of' different cooled cylinder geometries (cylindrical, elliptical, hexagon, triangle, square) in rectangular cavity on transient natural convection in water with density inversion, calculations were carried out for different initial water temperatures. All calculations were carried out under condition of transient natural convection in water with cooled tubes. Diameter of'tubes was taken (d, 0.0254 m), and tank heights was taken as 11d. Water temperature in tank an cylinder surface temperature were assumed as °C, 0C and -10 °C respectively. Natural convection is not observed at ,: 0C for all models and thermal stratification is not established in the storage tank. The water which was at 0C temperatures in the beginning was observed to have reached different temperature values at different points of the tank at the 900 s. At the end of this period, phase change was observed in the tank and the ice layer on the cylinders was reached to specific thickness. Thermal stratified region is established by warm water placing at the bottom of the tank and cold water placing at the top ofthe tank This situation can also be seenfrom the temperature contours at the 900 s. At 1800 and other durations, the cold water at the top was observed to be colder when the ice thickness was increased. More temperature decrements of accumulated water at the bottom were observed to have decreased more; and diflerent thermal layer was seen to have appeared in the following period. When the temperature distributions at the end of the 3600 and 7200 are analyzed, it can be seen that the water temperature has decreased up to ÜC at the whole part of the tank except the bottom part. For the solidification rate obtained for all models, in case the cylinders are cylindrical, elliptical and hexagonal, the model for the highest solidification rate is obtained in the case of cylindrical. It was found that when the cylinders are triangular and square models, the solidification is affected negatively and more formation of' ice is seen for circular models.