Theoretical And Experimental Study On Hybrid Displacement Ventilation Model

Energy is the driving force for social development and the focus of international politics,national finance and domestic security game.The construction of ecological civilization is the cornerstone of realizing socialist modernization.The improvement of living environment is an important goal of ecological civilization construction.Hybrid displacement ventilation combines the advantages of natural displacement ventilation and mechanical ventilation,which not only reduces air conditioning energy consumption,but also improves indoor air quality and thermal comfort.The research on hybrid displacement ventilation is of great significance for creating a comfortable living environment and establishing a green,low-carbon,safe and efficient building energy system.Since the relevant design specifications are still not perfect,how to determine the important design parameters such as the composite ventilation rate and its main influencing factors is an urgent problem to be solved in practical engineering applications.Natural displacement ventilation plays an important role in the research of hybrid displacement ventilation.Firstly,aiming at the stack-driven natural displacement ventilation,based on the classical thermal plume theory and the existing calculation model,an improved calculation model of natural displacement ventilation is proposed by considering the non-adiabatic condition of the building wall and the air temperature in the lower part of the room is higher than that of the outdoor air.The calculation formulas of natural displacement ventilation rate and thermal stratification height are obtained.The main factors affecting natural displacement ventilation rate and thermal stratification height are analyzed by using the calculation formula.It is found that the main factors affecting the two characteristic parameters are the effective opening area and the indoor heat source intensity.Under the same effective opening area,the natural displacement ventilation rate increases with the increase of the indoor heat source intensity.Under the same indoor heat source intensity,the thermal stratification height increases with the increase of the effective opening area.Secondly,according to the type of fan(exhaust fan,supply fan)and the relative strength of the the stack force and the mechanical force,the hybrid displacement ventilation driven by the stack force and the fan force together is divided into four flow modes.On the basis of natural displacement ventilation calculation model,considering thermal stratification and non-adiabatic wall condition,the calculation models of hybrid displacement ventilation under four flow modes are derived respectively by using the basic equations of fluid mechanics and heat transfer,such as mass conservation equation,orifice flow equation and energy balance equation.The calculation formulas of composite ventilation rate,ventilation mode switching point and thermal stratification height are obtained.The main factors affecting composite ventilation rate,ventilation mode switching point and thermal stratification height are analyzed by using the calculation formula.It is found that the main factors affecting the three characteristic parameters are mechanical ventilation rate,ratio of top opening to bottom opening area,effective opening area and indoor heat source intensity.In order to validate the improved model of natural displacement ventilation and the calculation model of hybrid displacement ventilation,a displacement ventilation experimental platform was designed and built.It is found that compared with the experimental values,the natural displacement ventilation rate calculated by the "fully-mixed" model is obviously larger.The natural displacement ventilation rate calculated by the “Emptying filling box” model is smaller,and that calculated by the improved model proposed in this paper is less than 5% of the relative error,with higher accuracy.The relative error between the composite ventilation rate calculated by the derived model proposed in this paper and the experimental value is less than 15%.In the hybrid displacement ventilation driven by stack force and fan force together,when the stack force dominates the ventilation mode,the indoor thermal stratification phenomenon is obvious.With the gradual increase of mechanical ventilation intensity,it becomes the mechanical force dominated ventilation mode.The top ventilation outlet changes from exhaust outlet to intake.The indoor thermal stratification phenomenon gradually disappears,and the indoor temperature tends to uniform distribution.Finally,the calculation model of hybrid displacement ventilation is validated by numerical simulation,which provides a reference for the study of hybrid displacement ventilation using CFD simulation technology.In this paper,theoretical analysis,experiment and numerical simulation are used to study the important design parameters and their main influencing factors,such as hybrid ventilation rate,thermal stratification height.All of this can provide some reference for related research and engineering design.