Study of Natural Ventilation in Buildings by Large Eddy Simulation

Purdue University, West Lafayette IN

Investigators

Abstract

9877118 Chen MIT Study of Natural Ventilation in Buildings by Large Eddy Simulation U.S. buildings use about onethird of the total energy used in the nation, and the Heating, Ventilating, and Air Conditioning (HVAQ systems in the buildings use a major part of it. The use of HVAC systems does not necessarily create a more comfortable and healthy indoor environment. On the other hand, natural ventilation generally improves indoor air quality, provides the occupants a high comfort level, and saves the energy used by the HVAC systems. The U.S. has a great potential to use natural ventilation. In many regions in the U.S., no air-conditioning system is needed during the summer with proper natural ventilation. This will save the first costs of the airconditioning systems. The overall objectives of the proposed research are to develop a reliable method to study natural ventilation in buildings, and to study the impact of the thermal and geometrical conditions in buildings on natural ventilation. The empirical data and equations available in literature are not suitable for accurate estimation of natural ventilation. The study of natural ventilation requires either experimental measurements or computer simulations. The experimental approach needs a wind tunnel to study the airflow around buildings and a full-scale environmental chamber to study the airflow in buildings. Although the approach can generate reliable data, it is expensive and time consuming. The computational approach includes Direct Numerical Simulation (DNS), Reynolds Averaged Navier-Stokes (RANS) equation modeling and Large Eddy Simulation (LES). DNS requires a computer not available at present. RANS is not accurate enough and cannot predict single-side natural ventilation. Only LES seems most appropriate for natural ventilation study. The investigation will develop a reliable Sub-Grid-Scale model for LES. The new SGS model will be validated by the experimental data from the literature and an environmental chamber. Then the validated LES model will be used to study the impact of thermal and geometrical conditions of a building on natural ventilation. The thermal conditions to be studied are: forced convection (no thermal buoyancy); natural convection (no wind); and mixed convection (the momentum force due to the wind is comparable to the buoyant force due to the heat gain in the building and the solar radiation on the outside of the building). The geometrical conditions to be studied are cross natural ventilation (an indoor space with openings in two or more walls) and single-side natural ventilation (an indoor space with only one opening) The LES program will calculate instantaneous and averaged distributions of air pressure, air velocity, air temperature, and contaminant concentrations, such as CO 2, in and around the building. The study is to understand how momentum and buoyancy forces can change the airflow pattern and pressure distribution around the building and how natural ventilation is interacted with the building opening location. The results are useful to determine the ventilation rate as a function of the flow Reynolds and Grashof numbers. Measurements in a wind tunnel at a Taiwanese university and in the environmental chamber will be conducted to further validate the computed results. A Chinese university will conduct on site measurements in an actual apartment building in Beijing. The LES model will be used to simulate the situations measured on site. The LES results are useful to analyze the impact of wind velocity and direction on natural ventilation, the impact of thermal buoyancy on natural ventilation, and the feasibility to design single-side ventilation LES should be a next-generation tool to study natural ventilation in buildings. The results of this project will provide detailed and accurate information to analyze indoor air quality and thermal comfort in buildings with natural ventilation. The results present a scientific basis to architects and HVAC engineers to design healthy and comfortable buildings and to save energy used by the HVAC systems. This will increase occupants' productivity and efficiency.