Çift Cidarlı Cephelerde Farklı Havalandırma Koşullarında Duman Hareketinin Sayısal Yöntemle İncelenmesi

Abstract

Çift cidarlı cephe sistemlerinin kullanımı sağladığı çeşitli avantajlar nedeniyle mimari cephe tasarımlarında giderek yaygınlaşmaktadır. Çift cidar arasında bırakılan boşluk sayesinde doğal havalandırmaya imkan vermesi en önemli avantajları arasında sayılabilir. Ancak aynı boşluk ortamı, baca etkisi yardımıyla cepheyle bağlantılı bir mekanda çıkan bir yangının oluşturduğu dumanın diğer katları etkilemesini kolaylaştırmaktadır. Dumanın katlar arasında yayılmasıyla ilgili en önemli kriterler çift cidarın genişliği, havalandırma koşulları ve bina cephesine etki eden rüzgarın yönü ve miktarıdır. Bu çalışmada çift cidar cephe sistemlerinde yangının oluşturduğu dumanın cephe içindeki boşlukta yayılma koşulları, farklı genişlik ve havalandırma koşullarına ve cepheye etki eden farklı yönlerdeki rüzgara bağlı olarak incelenmiştir. Yangın modeli için zemin+6 kaltı bir ofis binasi planlanmış olup yangının ikinci katta odanın ortasında çıktığı varsayılmıştır. Cidar genişlikleri, havalandırma boşlukları ve cepheye etki eden rüzgar yöunü değiştirilerek toplam 24 senaryo oluşturulup ve bu senaryoları 8 gurup halinde her gurup içinde iç cidar ve dış cıdar yüzeyinde sıcaklık ve hız değerleri incelenmiş ve dumanın üst katlara geçişi ve çift cidar arasında hareketi incelenmiştır. İnceleme, sayısal ortamda CFD tabanlı PYROSIM programı kullanılarak gerçekleştirilmiştir. Programla ilgili görsel veriler SMOKEVIEW programından elde edilmiştir. Yapılan çalışmanın sonuçları, Farklı derinlik ve havalandırma açıklıklarına sahip çift cidarlı cephelerde yangın nedeniyle oluşan dumanın hareketi miktar ve hız açısından incelendiğinde, çift cidarın derinliğinin dumanın tahliyesinde çok etkili bir unsur olduğu ancak bunun da cidarın havalandırma koşullarına ve rüzgar yönüne bağlı olarak değiştiği gözlenmiştir. Yine havalandırma koşulları cidar boşluğunu dolduran dumanın soğutulup sıcaklığının düşürülmesi ya da tersine yükselmesinde etkili olmaktadır. Bu nedenle derinliği yüksek olan cephelerde istenilen sonuçların alınmasının daha güç olduğu; buna karşın derinliği görece daha dar olan cephelerde tahliye hızı yüksek olmasına karşın duman ve sıcak gazların yangının bulunduğu katın üstündeki katları, seçilen cam türüne de bağlı olarak, daha hızlı etkileyebildiği söylenebilir. Üretilmesi olası duman miktarına ve havalandırma koşullarına ve cepheye etki eden rüzgar yönüne bağlı olarak çift cidar cephe derinlikleri belirlenmelidir.

Energy consumption have reached to top in recent years in regard to technological and structuring improvements. This situation has created worries about depletion of energy resources and has led to the search for renewable energy. A huge portion of energy consumption consisted of lighting, heating and cooling energy use. Thus, architectural designs which decreases those energy usage have become the most perefered ones. One of the prominent of energy examples of energy efficient building are also double-skinned façade systems. It is seen that double-skinned façade systems are efficient for reducing operational costs whereas have disadvantages in case of fire compared to single skinn façade. A gap between two skinns are designed for providing a natural ventilation by chimney effect in these sytems. However, it also provides a suitable environment for the spread of smoke and flames in the case of fire. Different strategies in double-skinned façade system designs are developed and lots of studies on fire safety are conducted to prevent this situation. Smoke, a more dangerous life-critical factor than flames in case of fire, behaves like a natural air movement. Thus, ventilation conditions are determinant for smoke movements during fire. As a result, analysis of smoke movements in different ventilation conditions is important for creation of the safest designs. Experimental methods for simulation of fire scenarios provide vauable results. On the other hand they are lack of efficiency in time and costs and of generalizability. Since they can be tested in short time and give generalizable and highly predictable results, numerical methods are more preferred for these systems. In the current study, smoke movements are analyzed with numerical methods in double-skinned facades. Studies on double-skinned facades focus on energy efficiency, lighting, shadowing, glass choice, ventilation and skinn width. On the other hand, studies on fire safety concentrate on effects of chimney effect on smoke movements in the gab between two skinns. Pressure difference in the ventilation gap may cause smoke to move upwards and to transport to other floors. Fire can rapidly spread from bottom elevation to upper elevation independently of smoke movements due to high pressure. This situation should be analysed apart from smoke movements. Floors in high buildings are usually designed as wide and open plan. This type of plan poses a risk for spreading of flames and smoke during fire. In low-rise buildings, spaces are typically divided into more sections. Heat, flame and smoke permeability of walls, installations and rebates located between sections should be checked and preventions against passing of smoke should be taken. Fans that are made for ventilation support and other systems that serve various purposes (sprinkling, wlaking areas for maintenance, panels, etc.) on the facade should be fire-resistant. Possible scenarios after the case of fire in double-skinned facades: After starting in closed spaces, smoke and fire tend to spread toward outside of the building, also depending on ventilation conditions. Glass on the internal skinn brakes earlier because of its proximity to fire and the effect of thermal radiation. Thereby, smoke and flames pass through glass to buffer zone between two skinns. Then, smoke move upwards and toward outer skinn due to chimney effect. After that, outer skin may be broken under the effect of flames and smoke. By this way, flames and smoke are thrown out with carrying lower life-threatening risk by preventing them to spread other floors. In the other scenario, however, flames and smoke lick the outer facade and move upwards with the chimney effect. In this scenario, flames and smoke brake glasses on the inner skin and spread to spaces in upper floors during its upward movement. There are several factors which effect these scenarios. These are can be listed as width of gap between two skins, ventilation conditions (wind), heat emisson, type of glass, and protrusions on the inner skin. Usage of double skin façades sytems in artchitectural façade designs is gradulally becoming more common because of its numerous advantages. Natural ventilation that is created by the gap between two façades is the most important advantage of this systems. However, this gap also causes spreading of smoke by chimney effect to other floors that is connected with the skin.The most important criterieas for spreading of smoke are width of double skin, ventilation conditions and direction and the amount of wind that affects building façade. In this study, the spreading conditions of smoke in the gap between two façades is analyzed according to different width and ventilation conditions and to wind that affects the façade from different directions. A six-storey ofice building with +5 floor is diesgined as numerical model. Every floor of building is 4 meters, and the place where the fire is and every place in vertical axis of it have 2,5 m x 3m windows in their outer façades (interior of double skin). Windows of this floor and of all other floors except the top floor is thought to be closed. In the top floor, however, only one window module is thought to be opened. Wall thicknes is designed as 20 cm, parapet is designed as 80 cm, and joist is designed as 70 cm in the model. The size of the every places in the same axis that are consecutively located is 36 m2. The fire is thought to be occurred in the middle of the place at the first floor that is analyzed. The size of the fire is chosen as 5 MW. Depth of the double skin façade is estimated as 60 cm, 120 cm and 180 cm. Tempered glass is used on the transparent surface of interior façade of double skin façade. The results of studies on these kind of glasses show that they cracked and broken in 600 ºC heat. Based on those results, breaking of glasses when this temperature is reached indoor and between double skins is used as design data in numerical analysis. On the other hand, the glass on the outside façade of the double skin façade has lower risk of breaking, and further breaking of it creates advantage for smoke removal. Thus, breaking of that glass is not included to the model. Different ventilation options are created by openings at the top and bottom of outer skin of double skin façade. Openings are planned as 20 cm x40 cm though the façade. Wind that comes from three different directions to outer façade of double skin façade with 2.19 m/Sec (mean air speed for Istanbul according to datas of DMI Göztepe Station ffor years) is entered to program as data to analyze effects of wind movement. Incidence angle of wind estimated as 0º, 45º and 90º. Total 24 scenarios are formed by using width of façade, ventilation gaps and three different incident angles of wind. Then, these scenarios are divided into 8 groups. After that, these groups are compared according to temperature of both inner and outer façades and speed values by usng grpahics. Also, how smoke affects upper floors and its behavior are analyzed. Analyzes are made by using CFD based PYROSIM program. Visual datas are obtained from SMOKEVIEW program. Speed and temperature grpahics for each gorup is analyzed. Results show that scenarios with narrower façade width have higher values of speed and temperature on double skin façades and in gap between double skin. Therefore, it is concluded that double skin depth has an important effect on smoke evacuation. However, this effect also changes according to ventilation conditions and direction of wind. They are also affects cooling or increasing the temperature of smoke which fills the gap between double skin. Thus, it can be said that expected results is hard to be obtained in deeper façades. Also, it can be concluded that even tough smoke evacuation is faster in less deep façades respectively, smoke and hot gassess affect upper floors faster based on glass. Double skin façade depth should be determined according to possible quantity of smoke, ventilation conditions and direction of wind that affects the façade.