Coupled Heat And Moisture Transfer Model And Experiments Of Earth-to-air Heat Exchanger Exposed To Harmonically Fluctuating Thermal Environments

The Earth-to-air heat exchangers(EAHEs)represent a promising passive energyefficient technology that maintains acceptable indoor thermal comfort.As China’s urbanization continues to achieve new achievements,people’s living standards have improved significantly,and building energy consumption has grown rapidly with the urbanization process,and its proportion in China’s energy consumption structure continues to remain high.Now most of the energy comes from fossil energy,and its nonrenewability will exacerbate the energy crisis and severely damage the environment.It is urgent to alleviate the energy crisis and protect the natural ecological environment.At present,China vigorously promotes green buildings and building energy-saving technologies.The EAHEs have the characteristics of simple system,energy saving and economy,which attract the attention of scholars at home and abroad,and are respond to China’s energy development strategy.Therefore,it is of great significance to study the operation mechanism of the underground wind system for its energy saving evaluation.Quantification of heat and moisture transfer process in EAHEs is crucial for the evaluation of EAHE cooling/heating performance.The previous studies have noticed the coupled heat and moisture transfer inside EAHEs and its effects on heating or cooling capacity provisions.However,the coupled heat and moisture transfer in a harmonically fluctuating thermal environment has rarely been investigated.This paper aims at the humid climate,and takes the EAHE with wet pipe wall as the research object.A model for evaluating coupled heat and moisture transfer of EAHEs exposed to harmonic external thermal environment is developed in this study.The temporal profiles of air temperature and moisture content are decomposed into timeaveraged and fluctuating ones,and the solutions of amplitude dampening and phase shifting for both air temperature and moisture content along the EAHEs are obtained.Based on the time-dependent solutions for the air temperature and moisture content,the air enthalpy at the outlet is noted to follow an approximately harmonic profile,and its temporal solution can thus be defined.This paper makes full use of the advantages of small-scale model experimental research,which not only shortens the experimental period and improves the experimental efficiency,but also simulates the harmonic profile of atmospheric temperature and moisture content,and conducts an experimental study on the coupling of heat and moisture in EAHEs.Both the proposed coupled heat and moisture transfer model and the model that considers only the sensible heat transfer are validated with well-regulated experiments.It was concluded that the mathematical model proposed in this paper to evaluate the internal coupled heat and moisture transfer performance of EAHEs has high reliability and accuracy.Subsequently,the enthalpy fluctuations at the EAHE outlet are assessed using the proposed model,and the advantages of the proposed model over the model considering only the sensible heat transfer are demonstrated.It is noted that the moisture transfer between air and the EAHE walls intensifies the phase shift but considerably attenuates the fluctuation amplitude of the air enthalpy at the EAHE outlet.The findings indicate that it is necessary to consider the coupled effects of the heat and moisture transfer in the EAHEs when evaluating the dynamic behavior of the cooling or heating capacity of the EAHEs.Finally,based on the coupled heat and moisture transfer model,a single variable analysis was performed on the key factors,including buried pipe depth,length,radius and ventilation,which affecting the EAHEs.The qualitative impacts of these key factors on the outlet air temperature,moisture content and enthalpy of EAHEs under annual fluctuation cycle conditions were described.In a word,the coupled heat and moisture transfer model proposed in this paper makes a more comprehensive analysis of the coupled heat and moisture transfer process of Earth-to-air heat exchangers,which provides technical support for the comprehensive design and actual operation of the system.