Ventilation And Heat Exchange Under The Coupling Effects Of Buoyancy And Earth To Air Heat Exchanger(EAHE)

Natural ventilation as a simple and effective way has been widely used all over the world thousands of years ago. With the development of HVAC technology, a large number of air conditioning facilities began to appear. People showed great initiative in regulating indoor thermal environment. However, the problem of energy consumption and environmental pollution is getting worse and worse. In the face of these problems,the development of green energy-saving technology and sustainable development has become an important issue in today's society and even the country. Thus, natural ventilation or associated systems as a kind of energy saving, environmental protection and economic methods has been paid more attention to by researchers in the world.The earth-to-air heat exchanger(EAHE) system is a typical combined natural ventilation system, which can introduce outdoor air to the tunnels by hot pressing or mechanical ventilation and extract thermal energy from soil into buildings to improve indoor thermal environment. The combination of natural ventilation and EAHE system makes full use of the advantages of both to achieve sustainable development of energy conservation and environmental protection. However, the theoretical mathematical model, which is coupled with building and tunnel system, is comparatively rare,especially in thermal natural ventilation. Firstly, this paper considers that the existence of non-harmonic multi-frequency fluctuation behavior induced by the coupling between thermal mass and buoyancy-driven natural ventilation on the assumption of wave theory.A mathematical approach of the nonlinear coupling between thermal mass and buoyancy-driven natural ventilation in a simple two-opening one zone building is analyzed based on Fourier transform theory when the indoor temperature is always greater than or equal to the outdoor temperature. A new theoretical model which is linearized is provided to discuss the relationship between indoor air temperature,ventilation flow rate and structure size, thermal mass and heat loading, which makes the calculation more simple and effective.Secondly, a small size building model was built to demonstrate that the theoretical model which is compared with experimental data is reasonable in the periodic fluctuation of outdoor thermal environment when the indoor temperature is always greater than or equal to the outdoor temperature. Meanwhile, the theory also confirmed the possibility exists of non-harmonic in the nonlinear coupling betweenthermal mass and natural ventilation.Thirdly, in order to solve the problem about the penetration radius of heat diffusion inside the soil. An analytical model for the evaluation of the thermal performance of an EAHE in a periodically fluctuating thermal environment is proposed to account for the interaction between the thermal waver originated from the pipe and that possessed in the soil by introducing the “excess fluctuating temperature”. And the explicit expressions for the outlet air temperature in an EAHE pipe in a daily or annual periodicity are provided, which makes the analysis procedure more straightforward and efficient.Finally, the EAHE systems which coupled with natural ventilation and EAHE system are also analyzed on the basic of wave theory when the indoor temperature is always greater than or equal to the outdoor temperature. And the explicit expressions of the indoor air temperature and ventilation flow rate are obtained by the theoretical mathematical model. Furthermore, the CFD software is used to simulate the unsteady natural ventilation of EAHE system in the periodic outdoor environment. And the theoretical model which is compared with the simulation result proved to be reliable.Meanwhile, this model also provides a more accurate and simple processing method for the design of EAHE system.