تجزیه و تحلیل عملکرد از یک طراحی جدید از سیستم تهویه سقفی پراکنده اداری

This paper aims to document and analyse performance of a new design of diffuse ceiling ventilation system in a typical office room. A full scale measurement is carried out in a climate chamber with an office setup at the Technical University of Denmark. Indoor air temperatures, air speeds, wall surface temperatures, pressure loss of the ceiling and ventilation effectiveness are measured for an air change rate of 3.5 h−1 and 5.1 h−1 respectively. A computational fluid dynamics model of the office with the diffuse ceiling ventilation system is built and validated by the full scale measurement. The measurements of pressure loss across the ceiling show a low pressure drop between the plenum and the occupied zone. Ventilation effectiveness is measured to be close to 1 on average under the tested conditions. It is shown that the diffuse ceiling ventilation system is able to remove indoor pollutant in an efficient way. The draught risk is found to be insignificant by both experimental and theoretical investigations. A design chart based on “flow element” method is created for the diffuse ceiling ventilation system by calculations with the validated CFD model. The design chart serves as a guideline for design and dimension of the investigated diffuse ceiling terminals as an air distribution system.

A good design of an air distribution system requires not only supplying clean air with appropriate temperature and flow rate to the occupants, but also that the system is designed in an energy efficient and cost effective way so that the occupants are able to experience high air quality and thermal comfort in the occupied zone with minimum energy consumptions. Major air distribution principles used in mechanical ventilation are mixing ventilation (MV) [1] and displacement ventilation (DV) [2]. Indoor pollution concentration is diluted in a room with mixing ventilation, while with a displacement ventilation, fresh air is supplied from the lower level of the occupied zone to separate stale air from fresh air in the room. An alternative ventilation principle for room air distribution system is downward ventilation or diffuse ceiling ventilation (DIFCV). The design principle of DIFCV is that air is supplied through ducts or direct openings from façade into a pressurized plenum. As there is a small pressure difference between the plenum and the occupied zone, the air penetrates through the entire ceiling into the occupied zone. Numerous studies have been carried out to investigate ceiling radiant cooling system without/with ventilation. Chakroun [3] investigated the air quality in rooms conditioned by chilled ceiling and mixed displacement ventilation. The energy saving of such a system is evaluated. Taki [4] and Keblawi [5] investigated the performance of a combination of chilled ceiling and displacement ventilation systems. Tian et al. [6] and [7] investigated the performances of a chilled ceiling panel system with/without mechanical ventilation for a typical office room in both cooling model and heating model. The thermal environment and thermal comfort in the room were fully measured and evaluated. It is found that ceiling ventilation improves the general thermal comfort and reduces the risk of local discomfort in the combined mode. The research on diffuse ceiling ventilation system for office environment is limited. In Denmark, DIFCV is extensively used in livestock buildings [8]. However in terms of indoor space occupied by human being, the results are preliminary. The design of the diffuse ceiling ventilation system such as the shape and distribution of the air passages, the porosity of the ceiling, etc. has a significant influence on performance of the ventilation system. Diffuse ceiling ventilation systems with different designs were investigated in the literature [9], [10], [11], [12], [13] and [14]. Nielsen [9], [10] and [11] carried out experimental work with an aim to evaluate the ventilation performance of a ceiling mounted low-impulse textile terminal and a diffuse ceiling in comparison with DV and MV systems. The diffuse ceiling was made of 60 cm × 120 cm painted mineral wool plates suspended from the ceiling, impenetrable to air, i.e., air was supplied through cracks in the suspension system hypothetically forming small microjets. Consequently, the ceiling installation determines the cracks, their locations and the microjets, which potentially affects the performance negatively. To reduce the uncontrolled leakages, another design with perforated suspended ceiling plates penetrable to air was experimentally investigated by Hviid [12]. The plates had perforation percentages of 16–17% and 17–64% of the supplied air entered through cracks in the suspension system. Both Nielsen and Hviid claim that DIFCV is superior to creating draught free environment in the traditional design range. Application of full scale experiments of diffuse ceiling ventilation was investigated by Jacobs [13] and [14] in a Dutch classroom. The measurement showed that even at extreme condition with high ventilation flow rate and large temperature difference (11 dm3/s per child, ΔT = 18 K), there was no draught problem and air quality in the classroom is significantly improved. Furthermore the results showed modest investment costs and very low fan power use due to low pressure loss compared to conventional diffusers. The objective of this paper is to experimentally and theoretically investigate performance of a new design of office diffuse ceiling product with controlled microjets. It is generally interesting to investigate whether the new design of diffuse air distribution system is able to provide occupants a high air quality and a comfort environment in the occupied zone. Based on calculations by validated CFD models, a design chart will be developed for the investigated DIFCV system as room air distribution device.

Performance of a new design of diffuse ceiling ventilation system for office uses is investigated experimentally and theoretically. Experimental investigations are carried out to examine ventilation effectiveness and thermal comfort of an office room with DIFCV system under two cooling scenarios with an air change of 3.5 h−1 and 5.1 h−1 respectively. The experiments show a good mixing in the occupied zone under both scenarios. The ventilation flow rate has insignificant influence on ventilation effectiveness in the investigated setup. There is no significant draught risk present in both scenarios, showing that DIFCV is superior to create a draught free environment even at a high flow rate. DR is evaluated to be Class A for an air change rate of 3.5 h−1 while it is rated as Class B for an air change rate of 5.1 h−1. The PMV and PPD are calculated at the different part of the room, indicating that 5% and 8% of people feeling discomfort for an ACH of 3.5 h−1 and 5.1 h−1 respectively. The correlation between the pressure drop across the ceiling and the ventilation flow rate is measured. It is shown that this new design of DIFCV has a significantly lower pressure drop than a conventional DIFCV with for instance perforated ceiling diffuser, which is considered valuable in a sustainable building design aiming at optimal use of energy for buildings. A CFD model of the test chamber with the investigated DIFCV is created and validated against the measurement. Both the experiment and the CFD calculations show no evidence of draft risk in the occupied zone under the tested conditions. A parametric analysis is carried out using the validated CFD model in order to investigate the influences of ventilation flow rate and heat sources on thermal comfort. A design chart of the DIFCV system is created based on parametric analysis. The design chart serves as a guideline for design and dimension of the DIFCV air distribution system.