Air distribution and thermal comfort for active chilled beam systems
Date of Issue2018-11-21
Interdisciplinary Graduate School (IGS)
Energy Research Institute @NTU
During the past few decades, the active chilled beam (ACB) system has become increasingly prevalent worldwide as a promising air conditioning and mechanical ventilation (ACMV) system. It is acknowledged that ACB systems have the advantages of providing high energy efficiency, good thermal comfort, and satisfied noise control with a lower space requirement. In modern society, the thermal discomfort and excessive energy consumption are the two major concerns for ACMV systems. People, especially in developed countries, spend substantial of their time in air-conditioned rooms. The poor air quality and inferior thermal condition incur plenty of uncomfortable sensations and even give rise to sicknesses such as sick building syndrome. One of the most common and serious problems is sensation of draught, which is mainly caused by the unpleasant cooling of air movement. Therefore, it is significantly important to evaluate the air distribution and thermal comfort for ACB systems. However, the previous studies mainly focused on cooling capacity, energy-saving effect and mechanical optimization of ACB systems. The studies on air distribution and thermal comfort are still inadequate to provide a comprehensive perspective on ACB systems. In order to fulfil the gaps, the airflow pattern is experimentally and numerically studied for ACB systems. Through experiments, an un-uniform air distribution is found near the nozzles in ACB terminals and this found distribution has a great chance to cause uncomfortable feeling such as draught in the occupied zone. A three-dimensional computational fluid dynamics (CFD) model for ACB terminal is built and validated by particular experiments. A proper strategy to balance the air distribution is proposed and validated by CFD simulation. Besides, the velocity contours near the ceilings are captured and the characteristics of Coandă effect, self-similarity, and turbulence intensity for air jet are discussed under various air supply temperatures and pressures. In addition, the effects of the heat sources’ configuration and strength on thermal comfort are experimentally investigated in a mock-up room with ACB systems. The full-scale distributions of air speed, temperature and humidity are mapped. Based on the data, the thermal comfort is comprehensively evaluated with indices of air diffusion performance index, predicted mean vote, draught rate and vertical air temperature difference. The uniformities of air speed, temperature and humidity are investigated in terms of standard deviation. Practical schemes are provided on how to set up sensors and controllers of ACB systems under various operation conditions. By making full use of the obtained characteristics of air distribution and thermal comfort, a model-based genetic algorithm is employed to achieve the minimal energy consumption of ACB systems while maintain the indoor thermal comfort for the occupied zone. The optimal energy consumption and thermal comfort are found by regulating the primary airflow rate and water flow rate. The optimization results show better performances compared with the original experimental results.
DRNTU::Engineering::Mechanical engineering::Energy conservation