Characteristics Of Mixed Convection Flow Field Under Double-buoyancy Effect

With the rapid development of the economy,the problem of indoor pollution in industrial buildings has become increasingly prominent.There are often strong heat sources and harmful substances in industrial buildings.Local ventilation is an effective energy-saving ventilation control mode.Thermal buoyancy caused by heat source,solutal buoyancy caused by pollution source and momentum of local air supply interact to form complex mixed convection flow.The contaminants are usually regarded as passive in the civil building due to the small concentration difference and it is considered that follows the air flow completely without the influence on the air flow field.In industrial buildings,however,the concentration of pollutants varies greatly,neglecting the solutal buoyancy will cause greater errors.It is of important theoretical significance and engineering application value to study the characteristics of mixed convection flow field under Double-buoyancy effect for precise design of local ventilation technology with strong heat and pollution sources in industrial buildings.Firstly,the applicability of density model for mixed convection with high temperature heat source is studied.An opposing mixed convection experiment platform with the bottom high-temperature heat source and the upper air supply was built,and the law of convective flow under different combinations of air supply velocity and heat source temperature was obtained.The applicability of three density models,Boussinesq approximation,incompressible-ideal-gas and piecewise-linear,are explored by numerical simulation.The experimental results verify the effectiveness of incompressible-ideal-gas and piecewise-linear,while the Boussinesq approximation has restractions.Furthermore,it is explored that the limitation of the Boussinesq approximation to the mixed convection problems is related to Archimedes number and Reynolds number,not just the temperature difference.Secondly,the characteristics of mixed convection flow field under isothermal solutal buoyancy are studied.The solutal buoyancy dynamics model was established by theoretical analysis.The effects of molar fraction and relative molecular mass difference on solutal buoyancy were revealed,and the dynamic mechanism of contaminants migration was obtained.A relative solutal buoyancy dynamics model was proposed.The dynamic mechanism of the interaction between airflow and gaseous pollutants was clarified.How to increase or weaken the influence of solutal buoyancy on contaminants transport was clarified.Based on the theoretical analysis,a physical model of mixed convection under isothermal conditions is established.Three factors,contaminant concentration,contaminant type and flow field velocity,are studied by numerical simulation.The correctness of the theoretical model is verified and the flow field characteristics of mixed convection under isothermal conditions are obtained.Finally,the characteristics of mixed convection flow field under the combined action of thermal buoyancy and solutal buoyancy are studied.A Double-buoyancy dynamic model was established through theoretical analysis,and the effects of molar fraction,relative molecular mass difference,and temperature difference were revealed.The dynamic mechanism of contaminants migration was obtained.Four basic models of synergistic and antagonistic of thermal buoyancy and solutal buoyancy were obtained.Based on theoretical analysis,a physical model of mixed convection with Doublebuoyancy effect is established.The effects of buoyancy ratio on the flow field characteristics of mixed convection under four modes are studied by numerical simulation.Furthermore,the control strategy of mixed flow ventilation under the action of strong heat source and strong pollution source is given.