CFD of a hybrid cooling system composed of a displacement ventilation unit and a radiant cooling floor

Abstract

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.

The combined use of displacement ventilation (DV) [1-2] and a radiant cooling floor (RCF) [3-5] is increasingly common in the field of building climatization. DV systems are based on injecting air directly into the occupied zone of the building, at low speed and at a temperature slightly below the comfort one. On the other hand, RCF systems are based on circulating cold water through a circuit of pipes embedded in the pavement. With this hybrid cooling system (DV+RCF) that combines both technologies [6], only the occupied zone is climatizated and a vertical stratification of the room air temperature is achieved [7]. In addition, indoor air quality is greater than when conventional cooling systems are used [8], because the updraft convection currents caused by heat sources in the occupied zone [9], move the hot air and contaminants to the roof level going through the occupied zone only once. DV+RCF systems work better in building with high ceiling (of 3 meters high or more), where the air stratification will improve the thermal efficiency and pollution control. Therefore, its use is appropriate in large public buildings, for example theatres [10], museums [11], train stations [12] or airport terminals [13], which are characterized by their high rise. Despite the advantages of implementing a DV+RCF system, their behaviour is not well characterized for an efficient dimensioning of the system. Therefore, the aim of this paper is to present two correlations to quantify, on the one hand, the amount of energy (as heat) the floor is able to absorb by convection, that is, the heat flux along the floor, and, on the other hand, the maximum distance of influence of the primary air stream driven by the DV system. These correlations have been found for the hybrid cooling system DV+RCF in a large enclosure, using computational fluid dynamics (CFD). Moreover, they take into account the presence of the cooling floor and the vertical stratification of air temperature in the enclosure, becoming a powerful tool to help in the cooling system dimensioning. Multiple simulations with different Reynolds, based on the size of the diffuser and its supply air flow rate, and Grashof, based on the temperature difference between the floor and the air supplied by the diffuser, numbers were used to obtain the correlations.