In recent years, the adoptions of an earth-to-air heat exchanger (EAHE) system to many buildings have been increasing. This system is classified into two types of tube system and underground air tunnel system. There is a problem that an underground air tunnel which is the object of this study is difficult to establish the method of performance prediction since it has a complicated structure. Moreover, this system has an anxiety of air pollution derived from occurrence of dew condensation and continuation of high-humidity environment in a system at operational phases. The occurrences of dew condensation have been predicted by the three-dimensional CFD analysis in several studies. However, these studies are limited to steady state calculation or representative points. The verification of countermeasures of dew condensation has not been carried out by long period and detailed predictions. Originally, EAHE systems should be designed after the identification of the high probability parts of occurrence of dew condensation and the estimation of the countermeasure effect of dew condensation by numerical predictions. Moreover, the prediction about pre-cooling and pre-heating effect of introduced outside air should be also carried out.
The purpose of this study is to establish the design and operation method that can prevent the occurrence of dew condensation and obtain the high pre-cooling and pre-heating effect of the outside air. In this paper, we propose the evaluation method of dew condensation by the unsteady CFD analysis for the existent underground air tunnel. Moreover, we verify the technique of introducing outside air into the system considering the above two purposes of this research.
We carry out the proposed CFD simulation (uncoupled simulation) coupled with humidity transport equations. After some flow fields depending on introduced outside air volume are obtained in advance for the uncoupled simulation, only temperature and humidity transport equations using these obtained flow fields are solved. Therefore, it can reduce the calculation load for the CFD analysis. The target of underground air tunnel is introduced in the existing office building located in Kitakyushu, Fukuoka, Japan. Case studies are carried out such as introducing outside air according with the schedule, implementing of the dew point temperature control depending on surface temperature of concrete and dew point temperature, or changing the introduction path of outside air.
Following results were obtained; if the number of outside air intake (introduction path) was one, the pre-cooling and pre-heating effect was higher than the case of two intakes under the same air volume conditions. It was suggested that there was a possibility of dew condensation in a winter rainy day. Therefore, the restriction of the use of this system is effective as countermeasures in the rainy weather. Even if the introduction path of outside air is complicated, it can prevent to some extent occurrence of dew condensation by the dew point temperature control. Using the dew point temperature control, the pre-cooling effect in the summer was decreased about 10 to 30 percent, but the pre-heating effect in the winter was slightly decreased. Given those results, it was suggested that EAHE systems has the relationship of trade-offs between air quality of introduced outside air and the effect of energy conservation in summer. In the case of uncoupled simulation coupled with humidity transport equations, the computing time was greatly reduced. Moreover, it was possible to evaluate the property of dew condensation in the system and the annual pre-cooling and pre-heating effect in short calculation time by this simulation method.
