Numerical analysis of a building with a combination of windcatcher and exhaust chimney

There is an increasing interest in utilizing from natural ventilation systems in modern buildings. Many designers have been conducting researches on improvement of natural ventilation systems with the aim of reducing operation cost. Compared to mechanical systems, natural ventilation systems provide healthier and more comfortable working and living environments for occupants. Considering the sustainability and thermal comfort, natural ventilation systems are offering better indoor air quality by reducing the effects of sick building syndrome caused by mechanical ventilation systems. Natural ventilation is a method for the buildings to reduce the energy consumption by eliminating the use of mechanical ventilation systems. Researchers have been working on adaptation of ancient applications such as windcathers used in East Asia to the modern buildings for many years. Windcatchers have been used in many modern buildings. Researchers have improved windcatchers called "Monodraught" which can be controlled automatically according to the weather conditions. In this study, one storey building with combination of windcatcher and exhaust chimney which is located behind the windcatcher is simulated and numerically investigated. Dimensions of the building is 18 m x 14.5 m x 4 m. Building area is 261 m2. Natural ventilation efficiency is investigated by changing exhaust chimney heights behind the windcatcher. 10 models according to exhaust chimney heights have been created, analysed numerically and compared by Ansys FLUENT. A 2 m height of guide plate has been placed between two chimneys inside the building in order to prevent by-pass of incoming air flow between chimneys. Maximum height of the exhaust chimney has been selected equal to windcatcher chimney's height in model 10. The initial height of exhaust chimney has been modelled as 1 m with 0.5 m increment on models from 1 to 10. Exhaust chimney and windcatcher chimney openings have been modelled as 1.5 m x 1 m and 3 m x 1.5 m, respectively. Windcatcher height is 5.5 m for all models. These analysis results for this building are planned to be used in Solar Decathlon Competition held in China. Therefore meteorological climatic datas of China/Dezhou area have been used in Ansys simulation models. The constant mean wind velocity 3 m/s and dominant wind direction have been chosen from these data. The weather data consists of 12 months of last 5 years. After creating geometry of whole building with chimneys, control volume has been added around geometry. Control volume has 54 m width, 223 m length and 36 m height. 5 times of the building width of control volume is created in front of the building while 10 times of the building width of control colume is created behind the building. In Fluent Mesh program, the geometry has been divided into tetrahedron & Hex mesh elements. About 2.700.000 elements have been generated for the geometry volume of all models in order to examine the analysis in Ansys FLUENT. Maximum and minimum element numbers are 2.805.046 and 2.682.110 respectively. Control volume has been divided into 9 volume in order to obtain good mesh quality. 25 m x 25 m x 16 m control volume has been created around the building in order to obtain boundary layer thickness by using 5 layer inflation with 1.15 growth rate. Grid density has been larger in the regions which includes more flow over the building. The next step was to assign the boundary conditions to the surfaces of the geometry. Boundary conditions of the exterior parts of the building have been described as "Wall" while the inlet and outlet parts of control volume geometry have been described as "inlet" and "outlet", respectively. Ansys Fluent has defined the interior parts of the building as "interior". The numerical analysis have been carried out by CFD commercial code Fluent which is commonly used in most researches nowadays by the scientists. CFD Fluent is mostly accepted to deliver the reliable and accurate results for large and complex geometry models by the scientists. CFD model solves the models analytically by using Navier Stokes equations in order to give exact data for properties of indoor environment. k-ε turbulence model has been used for the analysis of the models due to giving a better approximation results for the models including near-wall flow. The analysis has been conducted for steady state conditions. k-ε standard turbulence model has been chosen because there are some similar successfull researches in literature which use same turbulence models. Pressure based solver is chosen for this study. Air flow around and inside the building and the control volume is simulated by CFD Fluent package which solves the governing equations on individual control volumes to construct an algebraic equation for pressure coefficient and velocity values. In this study, wind driven ventilation strategy takes an important role in natural ventilation of this building without any temperature differences between outside and inside. Only one ventilation mechanism which uses pressure coefficient differences takes part in this study. Buoyancy ventilation is ignored because the temperature differences between outside and inside is not considered. Aim of this study is to examine the mass flow rates with velocity and pressure coefficient distributions and correlate between mass flow rate and pressure coefficients. Precious relation between mass flow rate and pressure coefficient can't be obtained. Mass flow rate efficiency is not just related to the pressure coefficient. After obtaining the analysis results, the most efficient model has been chosen according to mass flow rates. The model extracting the maximum mass flow rate into the building has been determined as the most efficient one. Most efficient model has 2 m height exhaust chimney. This most efficient model has been modified by curving the edged corners of the wind catcher. This modified model has been taken into analysis again in order to achieve better efficiency and this modified model has been determined as most efficient final model. Comparing the final model to the previous most efficient model, it seems that there has been a considerable mass flow difference. In this study, mass flow rate, velocity and pressure coefficient distributions has been obtained by numerical methods and tried to find most efficient model according to extracted mass flow rates. In future, experimental study can be conducted for this study in order to compare the results of CFD Fluent package.