Influence of kinetic energy sources and internal obstructions on room air conditioning strategy, efficiency of ventilation and room velocity conditions
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Doctoral thesis (article-based)
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Report / Helsinki University of Technology, Laboratory of Heating, Ventilating and Air Conditioning. A, 4
AbstractThere is a variety of different methods consulting engineers use to design room system, room air diffusion, such as assumption of perfect mixing, design methods employing the empirical relations determined through research, air jet theory and computational fluid dynamics (CFD) codes. The most common design methods based on air jet theory allows only for the prediction of extreme values of air velocities and air temperatures in the occupied zone. However, the results of most analytical and experimental studies has been received from tests in empty rooms and do not reflect the influence of the obstructions or other kinetic energy sources on the room conditions, air distribution and ventilation efficiency. The objectives of the study have been to investigate the influence of different factors on the room air conditions, airflow pattern and efficiency of ventilation and to utilize the collected information to improve current design practices. Scale models and full scale experiments and computational fluid dynamics simulations were conducted in order to study the influence of an occupied zone obstruction level, air distribution method, air change rate, heat and contaminant source plus non-uniformity on the room system performance and the efficiency of ventilation. A new room air conditioning strategy classification was developed. In classification the zonal strategy is introduced to separate room flow situations that cannot be explained by mixing strategy. It is suggested the room air conditioning strategy should be used as a target for design of the room air conditioning system. A simple method for the calculation of the room average velocity conditions was developed. The method is based on the kinetic energy balance of the room space, thus taking into account both air jets and heat sources. Following the presented design algorithm, a designer can estimate the average velocity level within a ventilated room and furthermore utilize it for evaluation of comfort conditions. The calculation method developed is reasonably accurate in mixed conditions, but additional development is needed to take into account zone effects. The room obstructions do not influence on room contaminant distribution within the studied ranges and air distribution methods. The room heat sources are important factors for contaminant removal effectiveness and contaminant uniformity inside the occupied zone with zonal air distribution methods. There exists non-uniformity of the contaminant concentrations within the occupied zone that should not be neglected when designing room air distribution. The non-uniform distribution of heat and contaminant sources within the ventilated space can have a remarkable influence on the contaminant removal effectiveness and especially on the contaminant distribution within the occupied zone. A straightforward comparisons between the measurement data and CFD simulation results is difficult because the comfort oriented, omni-directional air speed measurements are not directly comparable with the air velocity computed with the turbulence models. The use of an artificial, modified velocity method gives especially in low speed areas better correspondence between the measured and calculated speeds.
indoor air, contaminants, air velocity, air distribution, computational fluid dynamics, CFD, simulation, heat sources, calculation methods
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