A Study On Polymorphic Stack Effect Pressure Distribution Of Natural Ventilation In Underground Buildings

Thermal ventilation driven by temperature difference widely exists in underground buildings.Even with the same boundary conditions and geometry,there are many possible distributions of thermal pressure.The different distributions of thermal pressure will correspond to different indoor environment and natural ventilation of underground buildings.Therefore,it is necessary to study the formation mechanism,to establish a theoretical model,and to summarize the criterion,so as to better guide the design of thermal ventilation in underground buildings,and to ensure the safe and comfortable indoor environment.This research was conducted under the grant by the National Natural Science Foundation of the People's Republic of China(Grant No.51678088)The polymorphism of thermal pressure distribution is actually a problem of solution multiplicity of natural ventilation,which belongs to a branch of natural ventilation research.In the literature,the research of natural ventilation is increasing popular in recent years.However,most of the existing research is aimed at the single zone aboveground buildings driven by wind and thermal pressure.There are still some gaps:(1).the limitations of knowledgement,it is considered that the combat between buoyancy and wind pressure is compulsory for the existence of solution multiplicity,in fact,the phenomenon of solution multiplicity can exist with thermal pressure alone;(2).the lack of research on solution multiplicity of natural ventilation in underground buildings,there are obvious differences between underground buildings and aboveground buildings,such as complex geometry,single thermal pressure as driven force,etc.;(3).lack of research on the formation process of multiple solutions of thermal buoyancy ventilation;(4).lack of general criteria to determine the stability and existence of multiple solutions of buoyancy ventilation.CFD or theoretical analysis method should be applied to analyze each specific case every time.Therefore,it is necessary to carry out a detailed analysis and research on the multiple solutions of natural ventilation of underground buildings to better guide the design,construction and operation of underground thermal ventilation.This paper summarizes and introduces the research methods involved in the research of thermal ventilation and polymorphism.It includes multi-zone network model,CFD simulation,small scaled model experiment and non-linear dynamical analysis.Firstly,this paper summarizes and improves the one-dimensional multi-zone network model of underground building ventilation based on looped method.The model is coupled with the ventilation and heat transfer model,and can simulate the ventilation problem of underground buildings with complex structures.A case study was conducted to demonstrate how to reproduce the multiple steady states in a hydro power station by using LOOPVENT.Then,the scaled model experiment is used to study the multi-solution of buoyancy ventilation,and a 1:20 scale model of a hydropower station is established.Subsequently,the two-stage CFD simulation method is introduced to obtain different steady-states.Finally,the theoretical analysis method based on non-linear dynamics is introduced.It includes the definition of vector fields and phase portrait of ordinary differential equations,the analysis of fluid bifurcations of nonlinear dynamical systems and the numerical method of solving the nonlinear dynamical systems.In order to analyze the formation process of multi-solution of thermal ventilation in underground buildings under the action of local heat source,this paper adopts the method of combining experiment with CFD.Firstly,two kinds of steady-state buoyancy ventilation are visualized by smoke generator.Then,the air temperature and airflow velocity of the model was tested.The simulation results were compared with different turbulence models.The RNG k-e model is selected.Subsequently,a two-stage CFD simulation method is used to simulate the multi-solution of buoyancy ventilation.By changing the intensity of heat source,the initial wind speed and the location of local heat source,we studied the strength of buoyancy ventilation,the relationship between local convection and overall ventilation,the formation process and the transition of multiple steady states.To obtain the criteria for predicting the stability and existence of the thermal ventilation of a typical underground buildings,the nonlinear dynamical analysis is conducted.The existence and stability of solution multiplicity is analyzed for three typical cases,i.e.single heat source,constant heat strength ratio with different height ratios,different heat strength ratios with the same height ratio.Flow bifurcation graphs,phase portraits and direction fields are drawn.Furthermore,the multi-solution criterion of double-opening underground building based on heat source ratios and height ratios is derived and summarized.This paper takes a hydropower station in Xinjiang as an example and carries out case application analysis.Through the field measurement in summer,the heat transfer of the envelope,the heat dissipation of internal space and the impedance of the whole system is obtained.Based on this,the ventilation model described by the non-linear ordinary differential equations is established.Then,the stability and existence of the solution multiplicity is analyzed.Compared with the criterion of solution multiplicity for double-opening underground buildings,the validity and universal adaptability of the criterion is verified.It shows how to use this criterion to guide design better and to induce favorable buoyancy ventilation.