Entrainment performance enhancement of hvac systems with active chilled beams
Date of Issue2017-05-29
School of Electrical and Electronic Engineering
Heating, ventilation and air conditioning (HVAC) systems with active chilled beams (ACB) terminal units are attracting increasing attention in both theoretical research and practical application, due to their superior benefits compared with variable air volume (VAV) systems. In conventional VAV systems, a large volume of recirculation air is distributed through ductwork by big exhaust and supply fans. The energy consumed by these fans is enormous. As water has a much higher heat capacity compared with air, the ACB system utilizes water instead as the media of heat exchange, saving energy from delivering less volume of air. The volume of water transferring the same cooling is much smaller than air. Therefore, ACB systems are more energy efficient, by taking advantage of using less energy in distributing the chilled water directly to the air-conditioned space. Besides the energy saving benefits, ACB system can save the space above suspended ceilings. As most of the space is occupied by air supplying duct, the bottleneck for buildings with VAV system diminishing the space is to decrease the size of duct. As the ACB system requires less air supplying, it is an inherent character of ACB system to occupy less space above the ceiling, so that the floor-to floor height of a building can be decreased. Our research is dedicated how to enhance the energy efficiency performance of HVAC systems with space-friendly ACB terminal units. To achieve this, we make use of different tools to investigate the energy efficiency of ACB systems in many aspects, especially by confining the geometric factors of terminal unit. The energy efficiency of the whole system can thus be increased. In the meanwhile, we also suggest reduction of the height of terminal unit, so that the ACB system can show a greater advantage in term of space saving. By these means, the economic value of the ACB system, as well as the user acceptance can be increased. Accurate measurement of experimental data is the foundation of computations and the subsequent studies. An innovative method of acquiring entrainment ratio is proposed and validated through our experimental studies. With this method, the entrainment performance of a commercial terminal unit is tested by a series of experiments. The performance of the commercial terminal unit is also considered as a benchmark to compare and contrast. We build a numerical model of the terminal unit in computational fluid dynamics (CFD) software. The software can simulate the air flow pattern inside the terminal unit. By iterative computations, the velocity of every single point inside the terminal unit can be numerically solved. Compared with the experimental data, the model is verified to accord with the experimental data and is thus proved valid. After the model is established, we make numerous simulations and modification, and finally achieve a model that can largely increase the performance of the terminal units. It is found that by changing the geometry of the mixing chamber and lengthening the nozzle, the modified structure can increase entrainment ratio by 32% under the same working condition and primary air volume flow rate. The height of terminal unit is also diminished to fit more tight space. The results obtained prove that the entrainment ratio of a terminal unit, as well as the efficiency of the whole air conditioning system, can be effectively increased by proper modification on its geometry. The findings about the nozzle can also guide the adjustment of other geometric characters of the terminal unit.