Dynamic Thermal Analysis On Vertical Wall Attached Night Ventilation System For Commercial Buildings

The energy consumption in China is increasing year by year with the continuous process of urbanization.At the meantime,the energy consumptions are increasingly resulted from buildings that are newly constructed and also due to the operation and maintenance of existing buildings.Take the year 2015 as an example,the yearly energy consumptions due to the operation of buildings take up approximately 20%of the total energy consumption in China,and the energy consumptions from public buildings become especially remarkable.Ministry of Housing and Urban-Rural Development put forward the key work of the year 2018 as follows:Strictly follow the Thought on Socialism with Chinese Characteristics for a New Era raided by President Xi Jinping.Fully implement the 19th CPC spirit.Put into practice the new development concept.Give full play to the strategic support from the scientific and technological innovation.Orienting towards the goal of green city construction,we should further advance the building efficiency improvement as well as the green building development.Therefore,reducing the use of non-renewable energy and making full use of sustainable energy sources are still the urgent issue.Furthermore,the design of low-energy buildings should follow the principles of low-energy consumption,high efficiency,environmental friendliness and low cost,which will then lead to a comfortable,health and hygeian indoor environment,as well as improved building energy efficiency.Night ventilation is an important low-energy approach to adjust the indoor thermal environment,to improve indoor air quality and to reduce the energy consumption of air conditioning,which is both economic and environmental-friendly.In Hot summer and Cold Wind Climate Zone（along the Yangtze River）in China,however,the application of night ventilation should be applied with caution.In this type of Climate Zone,temperature difference between the outdoor air and indoor air during the night is insignificant in summer time,making it challenging to implement conventional night ventilation schemes（natural ventilation or mechanical ventilation）for effectively releasing the stored heat within the wall and the room.In view of these problems,this thesis proposes a novel mechanical ventilation system,the wall-amounted attached ventilation（WAV）,for night cooling of an office-type room,based on the vertical wall-attached jet principle.The main idea of WAV system is that by introducing the outdoor air over the internal wall surface（of the west wall）vertically with a high jet air velocity,enhanced heat transfer is to be expected,which will result in higher efficiency of heat removal from walls during the night cooling period.Therefore,by using simple ventilation installations and minimum energy consumption of the fan,the indoor thermal environment can be improved with reduced energy consumption of air conditioning during summer.Taking Chongqing as an example,following work are carried out in this thesis.The wall-amounted attached night ventilation is proposed.A series of experiments have been carried out to evaluate the indoor temperature field and the velocity field of the proposed WAV system for night cooling under hot summer conditions.The experiments have been carried out during the summers of 2015,2017 and 2018.On the base of the experimental measured data of the brick wall applying with WAV,transient heat transfer models are established.A 1-D transient conduction model is established in terms of finite-difference formulation and numerically solved in Matlab.The radiation model of wall surfaces is also included and solved in Matlab.According to the heat balance on the wall surface,the internal surface convection and the heat extraction from the walls are further obtained.Therefore,with reasonable simplification and assumption on the conduction model and radiation model,the transient heat transfer model can be applied to solve the nonlinear problem of night ventilation coupled with structural mass.A series of evaluation indices of performance of wall-amounted attached night ventilation system are proposed,including the surface cooling effectivenessη_surf,the night ventilation releasing effectivenessη_ventto evaluate the heat removal efficiency of the chamber and the coefficient of performance（COP）to estimate the energe saving potential.A transient CFD model of the WAV system is established in which the effect of and then validated by the experimental data.This numerical model is afterwards compared with mixing night ventilation system and further used to optimize the ventilation schedule.The results show that with WAV,the attached air can be observed and effective heat release of the wall can be achieved.With the increasing of supply air velocity,enhanced convection is achieved which benefit the heat removal from the total chamber.The operation of WAV has an influence on the supply air temperature,and the continuous ventilation after sunrise will lead to re-warming of the chamber with the outdoor environment.The CFD model is helpful for the optimized design of WAV system.The combination of WAV with PCM wall is also introduced and tests are carried out with the main focus on attenuation and delay of the room air.The results show that the PCM material added contributes to a stable indoor climate in a closed chmaber.In the WAV system,the PCM material added is helpful to extend the comfortable duration of the room.The combination system based on WAV and latent thermal mass is benefit in broaden the application of night ventilation system.