Research On Improving Indoor Thermal Environment In Cooling Season By Impinging Jet Ventilation Combined With Ductless Personalized Ventilation

Ventilation and air conditioning system undertakes the task of creating a suitable environment with acceptable air quality.As one type of stratified ventilation,impinging jet ventilation,which has a high supply momentum,has been proposed and developed to achieve the goal of high ventilation efficiency.To satisfy different occupant’s air movement preferences,based on traditional personalized ventilation,ductless personalized ventilation has been proposed without influencing the indoor layout and design.Facing the risk of large vertical air temperature differences in impinging jet ventilation,and the status that a background ventilation system with high ventilation efficiency is the prerequisite to benefit from ductless personalized ventilation.In this study,the combined system with impinging jet ventilation and ductless personalized ventilation was proposed,which is designed for cooling mode.The performance of the integrated system has been evaluated by some aspects including thermal comfort,air quality,air movement preference,etc.Firstly,the CFD method was adopted to investigate the interaction between the impinging jet ventilation and ductless personalized ventilation,as well as the performance when they were integrated.24 Cases,considering various supply parameters,were designed and applied after the verification of the turbulent models.The evaluation included the indoor thermal distribution,vertical air temperature air difference,local cooling effect,draft rate,etc.Based on the numerical simulation results,human response trials were carried out to evaluate the actual performance of the integrated system in practice.20 participants were invited to take part in 9 experimental conditions(the three ambient temperature is25 ℃,27℃,and 29℃,respectively),and their feedback on the thermal environment was collected by filling out questionnaires.The questionnaires included thermal comfort votes,thermal sensation votes,air movement preference votes,perceived air quality votes,sick building symptoms,etc.The CFD results indicated that ductless personalized ventilation could provide an efficient cooling effect on occupant’s head region when combined with the impinging jet ventilation,and the air temperature at the head zone could be decreased up to 3.7℃.The local cooling air provided by the ductless personalized ventilation would not influence the thermal stratification characteristic of the impinging jet ventilation.When the supply velocity of the impinging jet ventilation was 1.5 m/s,the vertical air temperature difference could be decreased from higher than 3℃ to lower than 0.5℃.The cooling effect of ductless personalized ventilation increases with its supply flow rate.And when the supply flow rate of the ductless personalized ventilation is higher than 8 L/s,the cooling effect mainly depends on the supply velocity but rather the temperature difference between the occupant’s skin and the surrounding air.The draft rate under each case was lower than 20% in this study.The obtained results of the human responses demonstrated that the skin temperature at the forehead part of the occupant could be significantly decreased by the local airflow provided by the integrated system,therefore,the local thermal sensations at the head and neck were decreased.Compared to being exposed to the environment created by impinging jet ventilation,occupants’ overall thermal sensation was significantly decreased under the integrated system,and the overall thermal sensation was close to neutral even the ambient temperature was 29℃.Under the user-control mode,occupants’ thermal comfort was improved,and the thermal acceptability was above 90%.Ductless personalized ventilation provided local cool air to occupant’s breathing zone,resulting in decreasing occupants’ inhaled air temperature and increasing the perceived air quality.In addition,a linear relationship between the perceived air temperature and perceived air quality is revealed.The occupant’s air movement preference was satisfied under the integrated system,and the local airflow did not irritate the occupant’s local body parts.Meanwhile,no obvious sick-building symptoms were reported.From the obtained results,it could be concluded the risk of large vertical air temperature differences under impinging jet ventilation could be significantly decreased when combing with ductless personalized ventilation.Ductless personalized ventilation is beneficial for improving the thermal environment based on the high ventilation efficiency of impinging jet ventilation.Under the proposed integrated system,thermal comfort,perceived air quality and air movement preference will be achieved.The integrated system also shows the potential for application under higher ambient temperature indoors,and furthermore shows the potentials of energy savings,without increasing the complexity of system design.