Investigation On Thermal Stratification Model And Performance Optimization In Cooling Rooms With Impinging Jet Ventilation

The primary requirements of building ventilation and air conditioning is to provide a comfortable indoor thermal environment,good indoor air quality（IAQ）,and also to reduce system energy consumption.These requirements are achieved on the premise of a reasonable airflow pattern in the ventilation room.According to the different airflow patterns,the existing ventilation strategies can be commonly classified as mixed ventilation（MV）and displacement ventilation（DV）.The MV system can be used in both cooling and heating modes,but it has the disadvantages of high energy consumption and low IAQ in the rooms.The DV system has advantages of improving good IAQ and energy saving potential in the cooling rooms.When DV is used for room cooling,appropriate thermal stratification can be formed in the room and a portion of the room cooling load is handled by the DV system,which can meet the requirements of thermal comfort,IAQ and energy saving simultaneously.However,it cannot be used for room heating due to the low air supply momentum.Impinging jet ventilation（IJV）is a high-performance ventilation strategy proposed in recentyears.In an IJV room,supply nozzles with a low discharge height are attached to the columns or sidewalls.The high-momentum air jets are delivered from the nozzles,impinge on the floor and then spread over it,and form a thin air layer with large diffusion area on the floor.The IJV system is considered to combine the advantages of MV and DV.For example,the energy saving potential is much better than MV in heating mode,and the vertical temperature distribution in the room is similar to that of DV in cooling mode.However,the study of IJV in cooling mode lacks complete theoretical investigations as DV.Although the existing studies have investigated the supply airflow patterns,thermal comfort and IAQ,few studies have been conducted to explore the formation conditions of thermal stratification,vertical temperature profiles and the calculation methods of cooling load correction.Therefore,there is a lack of basic data and theoretical support for IJV cooling design,and thus seriously limits the practical application of IJV.In the present study,the thermal stratification characteristics and vertical temperature profiles ofIJV system are investigated using experimentally validated numerical simulations,and also a simplified calculation method is given for correcting the cooling load in thermal stratification environment.In addition,scientific ideas are proposed to solve the limitations of the low cooling load capacity of IJV,which can provide theoretical support and design basis for the practical application of IJV.Appropriate thermal stratification is the key to improving ventilation efficiency and IAQ for IJV system in cooling mode.The supply air parameters are represented by the thermal length scale（L_m）in this study,which is applied to evaluate the combined effect of inertial force and thermal buoyancy on the supply airflow,and then the influence of the main influencing factors on thermal stratification is discussed and analyzed.The results indicate that the driving force on supply airflow changes from buoyancy dominated to inertial dominated with increasing thermal length scale,leading to the thermal stratification profiles changing from thermal stratification to uniform distribution.There is a critical supply air parameter（L_m）for the formation of thermal stratification in IJV cooling room,i.e.,the critical value of L_m is less than 1.4.Thermal stratification height（H）and the temperature difference between head and ankle level（ΔT）are important evaluation indexes for the indoor environment in low-level supply systems.The results show that the values of H in the IJV cooling room is only affected by the L_m,and the heat source intensity and its distribution have negligible effect on the thermal stratification height.The prediction models of H andΔT are obtained using the simulation results,which could provide the theoretical basis for optimizing the design scheme of IJV system in office buildings.The vertical temperature profile has an important influence on energy saving effect,thermal comfort and IAQ in IJV cooling room.An analysis method using dimensionless parameters is proposed for predicting the vertical temperature profile,and the Box-Behnken design method is employed to determine the simulation cases of influencing factors for vertical temperature profile.The thermal performance in IJV cooling room is investigated using numerical simulations,and the effects of supply air parameters,inlet area and height on vertical temperature profile are discussed,respectively.The results show that the inlet area has little effect on the vertical temperature profile in the room for small L_m conditions.However,the larger inlet area promotes a more uniform temperature distribution for large L_m.In addition,the inlet height has little effect on the temperature gradient for both small and large L_m conditions,indicating that the vertical temperature profile is a weak function of inlet height.Finally,multiple regression models related to L_m,inlet area and inlet height are developed for predicting the vertical temperature profile,which can provide a reference basis for the determination of temperature distribution in IJV cooling rooms during the design stage.One of the advantages of IJV used in cooling mode is that IJV only has to handle a part of room cooling load in the thermal stratification environment,i.e.,the cooling load in the occupied zone.Therefore,the cooling load ratio is a key parameter to determine the actual cooling load of the IJV system.A simplified calculation method using cooling load ratio（α）is proposed for correcting the cooling load of IJV,and numerical simulations are conducted to explore the effects of thermal length scale L_m,supply air rate and the room cooling load on the cooling load ratio.The results show that the cooling load ratio increases with increasing L_m and supply air rate and decreases significantly with the increase of room cooling load.A prediction model ofαrelated to L_m,supply air rate and room cooling load is developed,and indicating that the variation ofαranges from0.55 to 1.0 for all values of the above influencing factors.The minimum value ofαis 0.55 at low values of L_m,supply air rate and high value of room cooling load,and the IJV system achieves the maximum energy saving potential at this time.The prediction model can be employed by designers to calculate the cooling load in thermal stratification environment of IJV based on the supply air conditions and the room cooling load,and can also be used to evaluate the energy saving potential of an IJV system.IJV system has the limitation of low cooling capacity similar to DV system.To fully illustratethe energy saving advantages of low-level supply systems,many studies have focused on improving the cooling capacity of DV.However,few studies have explored the solutions to improve the cooling capacity of IJV.Passive chilled beam（PCB）systems are widely used in indoor environments with high cooling loads,and can be a potential solution to address the limitation of low cooling capacity for IJV.The possibility of coupling the IJV and PCB systems is investigated in this study,and the influences of two main factors on thermal and ventilation performance of an IJV-PCB system are studied,i.e.,percentage of cooling load removed by PCB（η）and the supply air parameters（L_m）of IJV.The results indicate that PCB enhances the mixing of indoor air and then reduces the vertical temperature gradient in the IJV room.There exists a suitable range ofηthat can achieve the goals of improving cooling capacity and maintaining the advantages of IJV（i.e.,high ventilation performance）.The suitable range ofηin an IJV-PCB system is from 0 to 0.45,with the comprehensive consideration of thermal comfort,IAQ and energy saving potential.When the supply air parameters are controlled within a certain range（from 0.4 to 1.2）,high ventilation efficiency and good IAQ are maintained in the room.