Çok birimli bir binanın doğal havalandırma davranışının sayısal analizi
Numerical analysis of natural ventilation in multi-unit building
Yazar: OSMAN AVCI
Danışman: DOÇ. DR. YAKUP ERHAN BÖKE
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Isı-Akışkan Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Doğal havalandırma, basit bir şekilde yapı, bina veya belli bir hacim içinde uzun süredir bulunan yaşlanmış/kirli havanın taze hava ile yer değiştirmesi olarak tanımlanabilir. Rüzgar ve sıcaklık farklılıkları gibi doğal kuvvetlerin kullanılarak ortama taze havanın sağlanması doğal havalandırmanın temel prensiplerindendir. Doğal havalandırma, sürdürülebilirlik ve yeşil enerji kavramları konseptlerini içerdiğinden günümüzde yaygın olarak bilinmektedir. Diğer bir yandan, doğal havalandırma, diğer havalandırma sistemlerine göre daha az enerji ve yapısal donanıma ihtiyaç duyarak daha ucuz bir şekilde havalandırma sağlamaktadır.  Kentsel alanlarda kurulan yüksek katlı binaların sayılarının günden güne artması bölgeye bulunan hakim rüzgarların doğal havalandırma davranışlarının önemli ölçüde değişmesine sebebiyet vermektedir.
Yapılan tez çalışması kapsamında etrafı açıklık olan ve yaklaşık 345 m2 alan üzerine kurulu 4 katlı ve birçok birimden oluşan binanın hesaplamalı akışkanlar dinamiği yöntemi ile doğal havalandırma analizi yapılmıştır. 
Oluşturulan çok birimli bina toplamda 4 kattan oluşmaktadır ve her katta karşılıklı olarak 4'er oda bulunmaktadır. Toplamda bina tasarımı için 32 ofis oluşturulmuştur. Kaldırma kuvveti ve rüzgar kuvvetlerinin etkisi yapılan analizlerde incelenmiştir. Tasarımı yapılan bina üzerinde belirli alanlarda açıklıklar ve baca oluşturulmuştur. Açıklıklar ofis olarak adlandırdığımız birimlerde pencere ve kapı üzerine karşılıklı gelecek şekilde konumlandırılmıştır. Rüzgar hızı, yönü ve sıcaklığının belirlenmesi için Meteoroloji Genel Müdürlüğü'nden 2011 yılına ait saatlik veriler alınmıştır ve bu veriler irdelenrek sınır koşulları belirlenmiştir. Hakim yön olan kuzey, ortalama sıcaklık 15 oC ve ortalama hız 1,5 m/s verilerin incelenmesi sonucu analizlerde kullanılmıştır. Bacanın ve açıklıkların açıklık durumuna göre toplamda 4 adet sayısal  analiz gerçekleştirilmiştir.   
Sayısal anliz çalışmalarından önce binanın üç boyutlu geometrisi GAMBİT 2.4.6 programında oluşturulmuştur. Geometrisi oluşturulan modelin sonlu hacimler metodu kullanılarak ayrıklaştırma işlemi ve sınır koşul ve süreklilik türlerinin atanması  yine GAMBİT programında yapılmıştır. Sayısal çözümler genel olarak yaygın kullanılan ve hesaplamalı akışkanlar dinamiği yazılımı olan FLUENT programında yapılmıştır. Bina içerisindeki sayısal çözümlerde hava akışı özelliğinden dolayı viskoz çözüm yöntemi olarak türbülans çözüm yöntemi ve    türbülanslı akışın modellenmesinde RNG k-ε modeli kullanılmıştır.
Elde edilen sanuçlar ile baca ve açıklıklardardaki sıcaklık, basınç değişimleri ve kütlesel debi değerleri hesaplanmıştır.

Abstract:
There is an increasing demand for higher quality office buildings. Occupants and developers of office buildings ask for a healthy and stimulating working environment. They also demand buildings that create less environmental damage. In the 1990's, concern about global warming has resulted in a resurgence of interest in naturally ventilated offices.
Natural ventilation is a promising strategy for improving the indoor air quality while reducing environmental damages. It is also a method to eliminate or reduce the energy consumption of air conditioning systems, especially for buildings that are located in temperate weather areas.
Some research results have shown good agreement between prediction and measurement of natural ventilation in small simple buildings, however in large scale complex buildings, such as commercial buildings, natural ventilation is much more difficult to predict. These large naturally ventilated buildings can have a much more significant impact on energy usage than small buildings.
In this study, the natural ventilation efficiency of a multi-unit building is investigated by computatioal fluid dynamics. Building area is 345 m2 and it has four floors. Each floor has 8 office rooms which are located mutually. The office building comprises 32 office modules in total, distributed over four floors and two orientations: 4 office modules/floor at each of the two orientations. 
The dimensions of room are 4.1 m wide, 3.5 m high, 4.65 m long. There is an internal wall between office modules and corridor has an openable window above the door to facilitate the air flow between northern and southern spaces. Each office has two windows (one is located to top of the bigger window and other one is located to top of  the door) to allow natural day or night ventilation. 
Window which is located to top of bigger window has height and length of 2,75 m, 0,5  m, respectively. In addition to this, other window is located to top of the door has heigth and length of 2 m, 0,5 m, respectively. In two cases, windows and openings of the chimney are let to be full opened and in other two cases windows and openings of the chimney are half opened.
During the analysis, a radiator is placed onto each room for heating purpose. Dimensions of the radiator are 1 meter height and 0,6 meter lenght. In total, 32 radiators were used for analysis. Capacity of the each radiator is 1400 W/m2. 
Furthermore, there is a chimney which is located to top of the whole building is used for analysis and examined its effects on natural ventilatin of the building.  Chimney has an openable windows on itself to facilitate the air flow between northern and southern spaces. The window on chimney has lenght and height of 19 meter and 0,5 meter respectively. 
The simulations were performed with climatic data of Goztepe (region of Istanbul, Turkey). The weather data were recorded by The Turkish State Meteorological Service and consist of 12 actual months, representative of the average climate at Goztepe. The meteorological data was presented hourly from January to December. On the other hand, the meteorological data gives information about wind direction, magnitute of velocity and temperature of air hourly. Furthermore, the data was researched extensively.  
Dominant wind direction is understood from the meteorological data that the direction of north for the year 2011 in Goztepe, Istanbul. In 2011, according to data from Turkish State Meteorological Service, wind had blown 3484 hours towards the north. For this study, the results which are obtained from data are showed that average temperature is 15,79 oC and average magnitude of velocity is 1,52 m/s for the north. So, for this study, we chose constant temperature value (15 oC) and constant wind speed (1,5 m/s) . We did not also change the wind direction during the study. 
There is one building model is modelled within the scope of this study. According to opening condition of the chimney and windows openings 4 numerical analysis were determined and obtained totally. Firt case is, whole windows and openings of the chimney are open. Second case, whole windows are open but southern opening of the chimney is closed. Third case is, all of the southern and northern windows are half open and openings of the chimney are also half open. Last case is, southern and northern windows are half open and southern opening of the chimney is closed but northern opening of the chimney is half open.   
The most CFD is based on the finite-volume method and the following comments relate to that. . Subsequently the process of generating unstructured grids, was done for this design. The choice of calculation domain and the associated grid (or mesh) has long been recognised as being very important to the results of the calculations. It is not just a matter of the size of the individual cells that comprise the grid, but also their shape and orientation. Generally speaking, the density of the grid needs to be larger in regions of high spatial gradients and in regions of particular interest.
A geometrical representation of the whole building including rooms and control volume was produced in Gambit program. The method which is held in this study is first to create a room with a dimension as the same as other room in Gambit program. And openings are located onto rooms. After create the room, corridor, chimney and control volumes are generated. The next step to gain a successful conclusion was the use of the "wall" and "interior" boundary conditions. 
In Gambit program, Submap and TGrid were used as element type. In addition to this, Hex and Tet/Hybrid element construction were used to generate to nodes. 0,1 – 0,3 – 1 – 2 interval sizes were used for constitute the whole volume. In total 5.657.754 elements were generated by volume for analysis of navural ventilation in multi-unit building. 
Boundary conditions have to be specified at the surfaces of the model and at solid surfaces within the control volume. In this way the openings models could be easily produced when two or more "walls" were substituted for two or more "interiors". To make this process simple, again the "interior" and "wall" boundary conditions play important roles. For the external part of the control volume, boundary conditions for velocity and temperature have to be specified at the inlet, outlet and remaining surfaces.
The numerical methodology is based on the finite volume numerical solution of the Navier–Stokes equations, using the CFD commercial code FLUENT. Currently, computational fluid dynamics (CFD) model is used extensively in the analysis of airflow, temperature and contaminant distributions. CFD simulation can provide detailed thermal  environment and contaminant information. In recent years, CFD has become a more reliable tool for the evaluation of indoor thermal comfort and air quality. 
In contrast, CFD uses turbulence models to solve the Navier– Stokes equations to predict detailed information on the indoor environment. Turbulence is simulated by the standard k- ε model which is reported to be a good approximation especially for near-wall flows. 
The k-ε RNG model was used in the CFD simulation because this model has steady and easily convergent advantages. In this study a commercial CFD package FLUENT, was used to simulate the air flow in and around building and rooms. The analysis was conducted at steady state and with 3-dimensional models. Natural convection has been modeled by employing Boussinesq method which yields good results provided with small difference in extremum values of temperature within the space in question.
In this work, buoyancy ventilation was initially considered without any wind over the building exterior. Buoyancy ventilation is a widely used strategy for natural ventilation design. Under real conditions, the outside wind is very difficult to describe and simulate because of its fluctuation. To simplify this problem, in this model experiment, the outside (ambient) air was controlled at a constant temperature of  15 oC and constant wind speed of 1,5 m/s. Also, heat flux of the each radiator is 1400 W/m2 designated for each numerical analysis. 
The mass flow rate is calculated from results of the numerical solutions at the inside, all of northern and southern windows of the building and chimney openings. Also, CFD simulation provides detailed spatial distributions of air velocity, air pressure, temperature, contaminant concentration and turbulence by numerically solving the governing conservation equations of fluid flows. It is a reliable tool for the evaluation of thermal environment and contaminant distributions. So, velocity field, pressure distribution and temperature distribution throughout the building are examined in detail.