Analysis On Effective Length Of Earth-Air Heat Exchanger

As a kind of energy-saving energy saving measure,Earth-Air Heat Exchanger?EAHE?has good application prospect,and is popularized and applied in some passive buildings or low-energy buildings.There are many factors to consider when designing the EAHE system,including operation mode and buried depth.Through calculation and analysis of thermal performance,determine the appropriate length and depth.Or under the condition of specific depth and length,it is important to evaluate its thermal performance.Firstly,the principle of EAHE,the process of heat exchange between air and soil and the change of original temperature of soil are introduced in detail.Among them,the heat transfer between air and soil in summer includes two processes of"equal-wetness cooling"and"desiccating cooling",while in winter,only equal-wetness heating process is available because it is only heated.Based on the analysis,the calculation formula of soil original temperature and the delay time of soil temperature wave are obtained.Secondly,the EAHE physical model is equivalent to a square pipe,and the air-soil heat transfer model is established,that is,the air one-dimensional unsteady convection-diffusion model and the soil two-dimensional unsteady heat conduction model.The implicit discrete equation of air and explicit discrete equation of soil are established by finite volume method.An algorithm to solve the sequence of air temperature,humidity and soil temperature is presented.The independence test of the grid is carried out,and the calculation grid is suggested to be?z=3m,?x=?y=0.3m.Taking the one-dimensional circular pipe model as the benchmark,the annual clearance operation example shows that the relative error frequency distribution of the equivalent square pipe is concentrated within 5%.The proportion of the annual total time is respectively:the square pipe with equal cross-sectional area accounts for93.1%,the circumference square pipe with equal cross-sectional area accounts for 94.8%,and the square pipe with equal hydraulic diameter accounts for 91.4%.The error is within the acceptable range.The model is verified by two verification methods of shallow and deep burial respectively,and the reliability and universality of the model are proved.Finally,based on the intermittent operation mode of an office building in Chengdu area,the numerical model of two-dimension heat exchange is used to simulate the whole year.The annual relative temperature difference,relative enthalpy difference and frequency of relative temperature difference and enthalpy difference greater than 0.9 are defined.A method is proposed to determine the effective length with the frequency of relative temperature difference and enthalpy difference greater than 0.9 and the annual accumulative cooling capacity/heat.The depth,the diameter,the flow rate and the accumulated cold heat of the buried pipe were analyzed in detail.The frequency of annual relative enthalpy difference or temperature difference greater than 0.9 and the annual accumulated cold heat under the working conditions of buried depth H₁=2,4,6,10m,pipe diameter d=0.1,0.2,0.3,0.5 m and flow rate u=2,4,6 m·s^-11 were calculated and analyzed,and the effective length of the buried pipe can be used as a reference for EAHE engineering.The flow velocity and the pipe diameter have a great influence on the effective length of the buried pipe.The buried depth of more than 4m has no obvious effect on the effective buried pipe length,but it has obvious effect on the annual cold heat.It is recommended to bury the pipe within 4 m below the surface.