| A reasonable ventilation strategy is one of the preconditions and guarantees for air conditioning system to provide good indoor thermal comfort,indoor air quality(IAQ)and reduce energy consumption.The widely used supply mode is divided into two types according to the air flow pattern,i.e.,displacement ventilation(DV)and mixing ventilation(MV).Although the DV system can achieve the goal of energy saving and provide good IAQ,it cannot be used during winter heating seasons because of the low supply velocity.The MV system can be used for both summer cooling and winter heating,but it has the drawbacks of low ventilation efficiency and high energy consumption.The impinging jet ventilation(IJV)system as an alternative ventilation strategy was proposed,in which the supply air with a high momentum is discharged from a nozzle at the lower part of the room,impinges onto the floor and then the airflow direction changes and spreads along the floor in the form of a thin air layer.In theory,IJV has the advantages of DV when both are used for space cooling.When IJV is used for winter heating,its warm supply air is directly distributed into the occupied zone and mixes with the room air from bottom-up due to the high supply velocity.Therefore,IJV not only overcomes the drawbacks of DV which cannot be used in winter heating season and but also avoids the flaws in MV which cannot distribute the warm air into the occupied zone effectively.That is to say,the IJV system has the potentials to achieve the goals of energy saving,good indoor thermal environment and IAQ simultaneously.However,most of the existing studies about IJV are carried out under cooling and isothermal scenarios,only few studies have been conducted to explore the ventilation performance of IJV operated in winter heating season and there is a lack of studies about theory and performance optimization,which limits the application of IJV in practice.In the present study,the ventilation performance of IJV used for space heating(such as energy consumption and IAQ)are investigated and analyzed in detail to provide theoretical support and design basis for the application of IJV.Laboratory experiments and CFD simulations were used to evaluate the ventilation performance of IJV.A series of experiments were carried out in a chamber equipped with IJV in heating mode,and the CFD techniques used in the present study were validated by the measured data of typical experiment conditions.Based on this,the airflow and temperature fields in rooms ventilated by IJV under heating and cooling scenarios were simulated by using the validated numerical method.The differences of the flow behavior between the warm and cool jets were compared and analyzed,and the influences of different factors on the warm jet flow behavior were investigated.The numerical results show that the warm supply jet of IJV would rise upward to the ceiling after spreading along the floor for a certain distance under the effect of thermal buoyancy,leading to a limited warm air spreading distance(L).This is quite different from the flow behavior of the cool jet.Except the supply nozzle shape and room height,both the supply air temperature difference,supply velocity,jet discharge height and nozzle area of IJV have obvious effects on L.When IJV operates in its heating mode,the warm jet spreading distance will be shorten greatly if the effect of thermal buoyancy is too strong,so a heating blank area may appear in the region far away from the supply inlet,thus resulting in a low heating energy efficiency and poor indoor thermal comfort.Hence,to accurately predict the warm jet spreading distance has great practical significance to the design of IJV in heating mode.In view of this,the airflow and temperature fields in an IJV heating room with different combinations of design variables were simulated.And then the prediction model for L was established by applying multiple regression analysis coupled with the CFD results.By analyzing the developed model for L,it can be known that L has a maximal value,and the requirement of L greater than 12 m cannot be met in IJV for any supply parameters(supply air temperature difference and supply velocity)and nozzle configuration(jet discharge height and nozzle area).It is also found that the height and area of the nozzle should be controlled in particular ranges and the supply air volume should also be greater than a critical value and for any specific requirement of L.Otherwise,the requirement of L will not be met no matter how to adjust the design variables.The most widely used warm air supply modes is the MV system up to now.However,the supply air of MV is usually delivered to the room from the upper part of the room,the heating energy is difficult to distribute into the lower part of the room effectively due to the thermal buoyancy,so the temperature in the MV heating is characterized by lower in the occupied zone and higher in the upper zone.In order to meet the thermal comfort requirement for occupants,the temperature in the upper zone is much higher than the heating design temperature,which results in the averaged indoor temperature being too high and causes much heating energy to be consumed.This adverse effect caused by the thermal buoyancy is particularly significant in large-height spaces,but it is obviously that the problem of high energy consumption in MV cannot be solved effectively.On the contrary,the warm supply air of IJV is directly delivered to the occupied zone.In order to evaluate the possibility of IJV replacing MV,numerical simulations were carried out to investigate the temperature distributions in a large-height space ventilated by IJV and MV during winter heating season,respectively,and the differences of indoor thermal environment and energy consumption between IJV and MV were then discussed.The numerical results indicate that the temperature difference between the ceiling and floor is up to 8 ?C in MV,while this difference is less than 1 ?C in IJV when the temperature in the occupied zone of the two rooms is maintained at the same level.For a large-height space with a height of 10 m,the IJV can save at least 20 % heating energy when compared with MV and the energy saving rate increases with increased room height.The IJV system is a low-level supply mode,and the fresh air is distributed into the lower part of the room directly,so IJV also has an obvious advantage in terms of improving the air quality in the breathing zone when compared with MV.To investigate the distribution of the IAQ in the IJV heating room,the spatial distributions of the active and passive contaminants(represented by the CO2 released from occupants and the 2.5?m particles caused by occupants’ activities,respectively)in spaces equipped with IJV in heating mode were simulated and compared with MV.The numerical results show that both of the CO2 and 2.5?m particles are distributed uniformly in the IJV room,while the CO2 is mainly distributed in the middle region of the room height and the 2.5?m particles concentrate at the region that far away from the exterior wall in MV,this results in the contaminant removal efficiency is much higher in IJV than in MV and the contaminant concentrations in the breathing zone in the former are significantly lower than those in the latter.The numerical results of the local mean age of air show that the mean age of air in the breathing zone in IJV is 37 to 45 % less than that in MV.The main reason for high energy consumption in large-height spaces is the serious thermal stratification of indoor air during winter heating season,and one of the incentives is that the cold outside air invades into the heating room and accumulates at the floor level.For spaces with occupants coming in and out frequently,e.g.,airport terminals and commercial buildings,the intermittent opening of the entrance door causes masses of cold outside air to invade the heating space in winter,and this leads to the temperature in the occupied zone is much lower than that in the upper parts of the room,and thus resulting in an increase in the heating energy demands.It seems that IJV has a promising application for the above mentioned public spaces with cold air invading according to the existing research results.To prove this,a series of experiments were conducted to investigate the temperature distribution and energy consumption of IJV used in a space with cold air invading intermittently and compared with those in MV.The measured results indicate that the door opening frequency only affects the amplitude of temperature fluctuation with time for a fixed door opening duration.Although the thermal stratifications in both IJV and MV become serious as the duration of door opening increases,the floor-to-ceiling temperature difference in the former is always less than 60 % of that in the latter,resulting in the heating energy consumption is much less in IJV than in MV.Moreover,the invading cold air would cause a bad thermal environment near the entrance door,as for MV,it is almost impossible to improve this state by adjusting the supply parameters,but the adverse effects caused by the invading cold air on the indoor thermal environment can be weakened effectively in IJV by changing the supply parameters. |
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