Study On Characteristics Of Air Distribution And Thermal Comfort Of Deflection Ventilation For Heating Mode

In modern society,with the rapid development of economy,people’s demand for the thermal environment of air-conditioned rooms continues to increase.A good indoor environment can make body and mind cheerful,thereby improving work efficiency.For winter,warm-air heating is a common heating method,and air distribution is a key factor affecting its heating performance.However,for traditional warm-air heating(such as mixing ventilation),due to the influence of buoyancy,airflow short-circuit and serious indoor temperature stratification are likely to occur,which will cause thermal discomfort of the human body and cause energy waste.Therefore,a new type of air distribution-deflection ventilation is innovatively applied to heating in winter,and it is expected to create a comfortable indoor environment efficiently.Deflection ventilation is the combination of the air supply outlet of vertical wall jets and the deflector of a certain height.Due to "Coanda effect",the jet attach to the wall and spreads downwards and its momentum loss is smaller than that of the mixing ventilation.It deflects after hitting the deflector,directly send heat to the workspace.This paper takes a typical office as the research object,and studies the characteristics of air distribution and thermal comfort of deflection ventilation for heating through experimental research and CFD numerical simulation.Firstly,11 cases of deflection ventilation in heating mode are designed,which consist of different combinations of supply air velocities(0.81m/s,1.17m/s and1.81m/s),supply air temperatures(24℃,26℃ and 28℃)and angles of deflector(0°and 35°),and design a set of corresponding mixing ventilation conditions for comparison.Arrange measuring points indoors to measure speed and temperature,combined with the experimentally verified CFD numerical simulation results and experimental results,the indoor air distribution is analyzed.It is found that the velocity distribution of the deflection ventilation in the vertical direction of the workspace presents a "V" shape.And the temperature distribution was uniform,the temperature difference between 0.1m and 1.7m was less than 3℃,and no obvious temperature stratification.Under the same supply air parameters,the maximum air velocity in workspace of deflection ventilation is 0.24m/s,while the mixing ventilation is lower than 0.1m/s.The average temperature of the workspace of deflection ventilation is about0.7℃ higher than that of the mixing ventilation,indicating that the hot jet is effectively sent to the workspace.Secondly,comprehensive experimental and CFD numerical simulation results,using predicted mean vote(PMV),vertical temperature difference,draft rate(DR)and energy utilization coefficient to evaluate the performance of deflection ventilation for heating.The results show that increasing the air supply velocity and angle of deflector,and reducing the air supply temperature can effectively reduce the vertical temperature difference and increase the energy utilization coefficient,but high air velocity and low temperature will cause draft risk and reduce PMV.Under most cases,the PMV of the area near the human body is between-0.5～+0.5,the vertical temperature difference is less than 3℃,the draft rate is less than 20%,which meets thermal comfort requirements of ISO7730 Class B;the energy utilization coefficient ranges from 1 to 1.13,indicating that it has good energy saving potential.Compared with mixed ventilation,the PMV value is higher than that of mixing ventilation by 0.05,the vertical temperature difference is lower than that of mixing ventilation by 0.16℃,and the energy utilization coefficient is higher than that of mixing ventilation by 0.47,indicating that deflection ventilation for heating can more efficiently create a comfortable thermal environment.Finally,the Box-Behnken design is used to determine the simulation conditions,the index data is obtained through CFD numerical simulation,and the response surface model of each evaluation index is constructed.Therefore,the entropy weight method and the satisfaction function method are used for multi-objective optimization.The non-linear relationship between the indexs and the parameters leads to the contradiction in the change of the indexs,and the change trend is complicated,and judging the best combination of parameters is difficult.By constructing an optimized objective function and using the simulated annealing algorithm to perform optimization calculations in the feasible region to determine the best parameter combination: air supply velocity,the air supply temperature and the angle of deflector are 1.5m/s,23.7°C,and 70°,respectively.In summary,Under suitable air supply parameters and angle of deflector,the air distribution of deflection ventilation for heating is well,which can efficiently create a comfortable indoor environment,and has the potential to be applied to warm-air heating in winter.The results of this research can provide guidance for the engineering design of deflection ventilation for heating.