| Energy conservation measures of building envelope are one of the effective ways to reduce building energy consumption,and it is of great significance in the energy shortage of today's society.Taking the use of the air flow through itself,hollow block ventilated wall enhances the performance of thermal insulation,and recycles low-grade energy,which makes it equipped with considerable energy saving potential.In this paper,the single-hole hollow block ventilated wall is taken as the object of study.The dynamic heat transfer characteristics of this structure are studied.Firstly,without considering the temperature change of the airflow and wall along the cavity direction,the frequency domain finite difference method is used to establish the two-dimensional frequency domain finite difference model(FDFD model)to analyze the thermal characteristics under the multiple external disturbances.This model considers the convective heat transfer of the inner and outer surfaces of the hollow block ventilation wall,the convective heat transfer between the inner wall surface of the cavity and the airflow,and the radiation heat transfer between the inner wall of the cavity.Using the Fourier series expansion,the dynamic disturbances in frequency domain is obtained from the temperature boundary conditions in time domain,and input to the FDFD model.Then the inverse Fourier transform is used to obtain the temperature response in time domain.Considering the temperature change of the airflow along the cavity direction,the heat exchange NTU model of the airflow in the cavity and the wall structure is established.Combining FDFD model and NTU model,the three-dimensional quasi-dynamic heat transfer model of the hollow block ventilated wall is obtained.The summer heat transfer characteristics of hollow block ventilated walls with different working conditions were studied by means of experimental method.The experimental results show that the average temperature of the inner surface of the hollow block wall is reduced by 3.1?when the wall is ventilated and the airflow velocity is 1.93 m/s,which indicates that the wall effectively blocks the heat from outdoor.The quasi-dynamic heat transfer model and the experimental results are validated.The maximum relative error of the temperature of the inner surface of the wall is 3.24%,and the maximum relative error of the temperature of the cavity outlet airflow is 3.6%,which indicate the reliability of the heat transfer model.Utilizing the established heat transfer model,the influencing factors of the thermal performance of the wall are compared and analyzed,including the airflow velocity in the cavity,the airflow temperature in the cavity,the wall orientation and the cavity size.The thermal properties of the wall in different climate zones were evaluated by the wall equivalent thermal resistance,the decay times,the delay time and the heat transfer of one day.The results of the simulation calculation show that the faster the airflow is,the higher the airflow temperature is,or the bigger the cavity size is,the larger the thermal resistance of the ventilated wall is,the closer the inner surface temperature is to the indoor air temperature,the better the thermal comfort is.When the cavity size is 130mm×130mm,the cavity wind speed is 1.8 m/s and the air temperature is 26?,the wall equivalent thermal resistance is 2.13 m~2??/W,the daily heat transfer is 0.44 MJ,which can reduce 75.3%heat transfer.When the indoor air conditioning exhaust is used as the gas source of the cavity,the best orientation of the hollow block ventilation wall is south in cold regions and hot summer and cold winter areas,and north in severe cold regions and hot summer and warm winter area.The ventilated wall of different orientations in this four climate zones can averagely decrease 55.73%heat transfer.When taking the use of outdoor cool breeze as a gas source,the average indoor heat taken away by the hollow block ventilation wall of different orientations is 0.23 MJ/day in the mild area. |
PDF Full Text Request