Abstract
Passive cooling systems, which control or utilize natural energy sources, can contribute to the cooling/heating of a building without the need for mechanical equipment. Underground air tunnel systems, which are a type of EAHE (Earth-to-Air Heat Exchanger), have been often adopted as a method of passive cooling in relatively large office buildings. However, to evaluate these effects precisely, reproducing the air velocity and temperature distributions in the system by applying a three-dimensional unsteady CFD (Computational Fluid Dynamics) analysis is desirable. Estimating the annual cooling/heating effects by CFD analysis is nontrivial, however, due to the large number of calculations required.
The purpose of this study is to propose and validate the prediction method of the short/long period thermal performance for underground air tunnel systems by three-dimensional unsteady CFD analysis. In this paper, we proposed an annual prediction method for the pre-cooling and pre-heating performance of an EAHE by unsteady CFD analysis, which does not analyze flow fields to reduce calculation loads. Our method (uncoupled simulation) reduces the calculation load for CFD analysis by using flow fields simulated in advance, and linearizing the heat equation by loading these flow fields. The following effects are expected by the proposed uncoupled simulation: (1) The conventional method (full coupled simulation) calculates equations for continuity, momentum, heat, turbulence energy , and turbulence energy dissipation rate for every time step. However, uncoupled simulations need to solve the only heat equation by loading flow fields. (2) Velocity, turbulence energy, and the turbulence energy dissipation rate become constant in the uncoupled simulation. Thus, the flow fields and temperature fields are coupled indirectly, meaning that heat equation is linearized, and this linearization improves the convergence and expansion of time intervals comparison with the full coupled simulation.
Moreover, we carried out case studies using the conventional method and proposed method with two types of outside air flow rate, and verified the validity of this proposed method comparing the conventional method. The target office building of this investigation was located in Kitakyushu, Fukuoka, Japan. In that case study, the initial temperature distribution of underground and air tunnel system elements were set temperature distributions that based on the results of the one-dimensional heat conduction analysis at 24:00 on December 31. Time intervals in uncoupled simulation were an hour. Flow fields in the uncoupled simulations were obtained at a representative date and time in the summer or winter. The following results were obtained from the case study: 1) It was suggested that using an EAHE which outside air flow rate were 10,350 m3/h and 2,070 m3/h, annual sensible heat loads of outdoor air-conditioning units were reduced by 20.6 % and 38.8 %, respectively. 2) In all cases by the proposed method, the relative errors of an annual integrated pre-cooling and pre-heating quantity based on the results for the conventional method were under 3 %. 3) The required time for the annual prediction by full coupled simulation were about 50 days, in contrast, uncoupled simulation were able to carry out in 6 hour in all cases. Comparing the conventional method with the proposed method, computing times for the annual prediction were reduced by about 0.5 %.
Accordingly, it was assumed that the proposed method has a practical use in terms of calculation times and precisions for estimating the annual thermal performance of EAHE in design phase.
