High-rise buildings form deep urban street canyons and restrict the dispersion of vehicle emissions,posing severe health risks to the public by aggravating roadside air quality.Field measurements are important for und...High-rise buildings form deep urban street canyons and restrict the dispersion of vehicle emissions,posing severe health risks to the public by aggravating roadside air quality.Field measurements are important for understanding the dispersion process of tailpipe emissions in street canyons,while a major challenge is the lack of a suitable tracer gas.Carbon dioxide(CO_(2)),which is safe to the public and inexpensive to obtain,can be reliably measured by existing gas analysers.This study investigated the suitability of using CO_(2)as a tracer gas for characterising vehicle emission dispersion in a real-world street canyon.The tracer gas was released via a line or point source,whose dispersion was characterised by a sensors network comprising low-cost air quality sensors.The results showed that the CO_(2)contained in the exhaust gas of a test vehicle itself had unmeasurable effect at roadsides.Both the line and point sources produced obvious CO_(2)level elevations at approximately 30 s after the test vehicle passed by.In addition,for both line and point sources,the CO_(2)elevations were much more distinct at the roadside next to tailpipe exit than the opposite side,and were higher at 0.8 m than 1.6 m above the ground.The present study demonstrated that using CO_(2)as a tracer gas is feasible for investigating vehicle emission dispersion in real-world street canyons.Future studies are needed to improve the gas release rate of the developed tracer gas systems for more reliable measurements and larger street canyons.展开更多
This study utilizes a visualization nozzle and spray experimental platform to experimentally investigate the flow focusing/blurring nozzle.It is found that the working mode of the nozzle transitions from flow focusing...This study utilizes a visualization nozzle and spray experimental platform to experimentally investigate the flow focusing/blurring nozzle.It is found that the working mode of the nozzle transitions from flow focusing to flow transition and eventually to flow blurring as the gas flow rate increases or the tube hole distance decreases.Conversely,an increase in liquid flow rate only facilitates the transition from flow focusing to flow transition.Changes in the gas/liquid flow rate or tube hole distance influence the gas shear effect and the gas inertial impact effect inside the nozzle,which in turn alters the working mode.An increase in gas flow rate results in a shift of the droplet size distribution towards smaller particle sizes in the flow blurring mode,whereas an increase in liquid flow rate produces the opposite effect.Notably,the impact of the gas flow rate on these changes is more pronounced than that of the liquid flow rate.展开更多
The flow focusing nozzle is a new type of nozzle that performs effective atomization of the discrete phase by means of high-speed motion of the continuous phase.The flow pattern and its morphological changes have a si...The flow focusing nozzle is a new type of nozzle that performs effective atomization of the discrete phase by means of high-speed motion of the continuous phase.The flow pattern and its morphological changes have a significant effect on the atomization, but the influence of different parameters on the morphological change of the flow pattern remains unclear.The flow focusing pattern and morphological changes in the two-phase flow inside the nozzle were simulated numerically, based on the volume of fluid method.The results demonstrate that the ratio of the nozzle-to-capillary distance and capillary diameter, the gas–liquid velocity ratio, and capillary diameter have significant effects on the flow pattern.When the ratio of the nozzle-to-capillary distance H and capillary diameter D increases, or the capillary diameter D increases, the flow pattern tends to transform into a laminar form; however, when the gas–liquid velocity ratio V increases, the flow pattern tends to transform into a turbulence form.Furthermore, we define the cone-shaped expansion rate, cone-shaped focusing rate,and cone angle in order to study the morphological changes in the cone shape inside the nozzle.The results indicate that the morphological change of the cone shape and flow pattern transformation is interrelated.When the cone shape tends to be unstable, the flow pattern changes towards flow blurring, whereas, a stable cone indicates that the flow tends to exhibit a droplet pattern.展开更多
The interactions between droplets have an important influence on the atomization of liquid fuel,the combustion efficiency,and the reduction of particulate matter emissions for an engine.For this reason,this paper pres...The interactions between droplets have an important influence on the atomization of liquid fuel,the combustion efficiency,and the reduction of particulate matter emissions for an engine.For this reason,this paper presents results from an experimental study on the coalescence and break-up of droplets after collision.According to the shape and parameters of the droplets at different times after the collision of the droplets was captured by a high speed camera,analysis was done for the following effects of droplet collisions:the collision-coalescence motion for the collision between the droplets,the change history of the dimensionless length-to-width ratio of the oscillation motion,the critical size ratio of the breakup motion,and the liquid physical properties of the particles.The results show that the droplets collide and exhibit two forms of coalescence oscillation and break-up:for oscillating motion,at higher droplet collision velocities and dimensionless size ratios,there will be a larger dimensionless length-to-width ratio for the droplet oscillation;for the break-up motion,at higher collision velocities,there will be lower dimensionless size ratios,and lower liquid surface tension,shorter times over which the droplet breaks,and facilitated droplet break-up.The research results presented here can be used for atomization in engine cylinder,increasing the gas/liquid contact area and enhancing the combustion efficiency of gas/liquid heat transfer to improve the combustion efficiency of the engine.展开更多
基金supported by the Environment and Conservation Fund(No.ECF 14/2018)of the Hong Kong SAR Government,China.
文摘High-rise buildings form deep urban street canyons and restrict the dispersion of vehicle emissions,posing severe health risks to the public by aggravating roadside air quality.Field measurements are important for understanding the dispersion process of tailpipe emissions in street canyons,while a major challenge is the lack of a suitable tracer gas.Carbon dioxide(CO_(2)),which is safe to the public and inexpensive to obtain,can be reliably measured by existing gas analysers.This study investigated the suitability of using CO_(2)as a tracer gas for characterising vehicle emission dispersion in a real-world street canyon.The tracer gas was released via a line or point source,whose dispersion was characterised by a sensors network comprising low-cost air quality sensors.The results showed that the CO_(2)contained in the exhaust gas of a test vehicle itself had unmeasurable effect at roadsides.Both the line and point sources produced obvious CO_(2)level elevations at approximately 30 s after the test vehicle passed by.In addition,for both line and point sources,the CO_(2)elevations were much more distinct at the roadside next to tailpipe exit than the opposite side,and were higher at 0.8 m than 1.6 m above the ground.The present study demonstrated that using CO_(2)as a tracer gas is feasible for investigating vehicle emission dispersion in real-world street canyons.Future studies are needed to improve the gas release rate of the developed tracer gas systems for more reliable measurements and larger street canyons.
基金the National Natural Science Foundation of China(52276026).
文摘This study utilizes a visualization nozzle and spray experimental platform to experimentally investigate the flow focusing/blurring nozzle.It is found that the working mode of the nozzle transitions from flow focusing to flow transition and eventually to flow blurring as the gas flow rate increases or the tube hole distance decreases.Conversely,an increase in liquid flow rate only facilitates the transition from flow focusing to flow transition.Changes in the gas/liquid flow rate or tube hole distance influence the gas shear effect and the gas inertial impact effect inside the nozzle,which in turn alters the working mode.An increase in gas flow rate results in a shift of the droplet size distribution towards smaller particle sizes in the flow blurring mode,whereas an increase in liquid flow rate produces the opposite effect.Notably,the impact of the gas flow rate on these changes is more pronounced than that of the liquid flow rate.
基金Supported by the National Natural Science Foundation of China(51776016,51606006)Beijing Natural Science Foundation(3172025,3182030)+2 种基金National Key Research and Development Program(2017YFB0103401)National Engineering Laboratory for Mobile Source Emission Control Technology(NELMS2017A10)the Talents Foundation of Beijing Jiaotong University(2018RC017)
文摘The flow focusing nozzle is a new type of nozzle that performs effective atomization of the discrete phase by means of high-speed motion of the continuous phase.The flow pattern and its morphological changes have a significant effect on the atomization, but the influence of different parameters on the morphological change of the flow pattern remains unclear.The flow focusing pattern and morphological changes in the two-phase flow inside the nozzle were simulated numerically, based on the volume of fluid method.The results demonstrate that the ratio of the nozzle-to-capillary distance and capillary diameter, the gas–liquid velocity ratio, and capillary diameter have significant effects on the flow pattern.When the ratio of the nozzle-to-capillary distance H and capillary diameter D increases, or the capillary diameter D increases, the flow pattern tends to transform into a laminar form; however, when the gas–liquid velocity ratio V increases, the flow pattern tends to transform into a turbulence form.Furthermore, we define the cone-shaped expansion rate, cone-shaped focusing rate,and cone angle in order to study the morphological changes in the cone shape inside the nozzle.The results indicate that the morphological change of the cone shape and flow pattern transformation is interrelated.When the cone shape tends to be unstable, the flow pattern changes towards flow blurring, whereas, a stable cone indicates that the flow tends to exhibit a droplet pattern.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.51776016,51606006)Beijing Natural Science Foundation(Grant Nos.3172025,3182030)+2 种基金National Key Research and Development Program(Grant No.2017YFB0103401)National Engineering Laboratory for Mobile Source Emission Control Technology(Grant No.NELMS2017A10)the Talents Foundation of Beijing Jiaotong University(Grant No.2018RC017)。
文摘The interactions between droplets have an important influence on the atomization of liquid fuel,the combustion efficiency,and the reduction of particulate matter emissions for an engine.For this reason,this paper presents results from an experimental study on the coalescence and break-up of droplets after collision.According to the shape and parameters of the droplets at different times after the collision of the droplets was captured by a high speed camera,analysis was done for the following effects of droplet collisions:the collision-coalescence motion for the collision between the droplets,the change history of the dimensionless length-to-width ratio of the oscillation motion,the critical size ratio of the breakup motion,and the liquid physical properties of the particles.The results show that the droplets collide and exhibit two forms of coalescence oscillation and break-up:for oscillating motion,at higher droplet collision velocities and dimensionless size ratios,there will be a larger dimensionless length-to-width ratio for the droplet oscillation;for the break-up motion,at higher collision velocities,there will be lower dimensionless size ratios,and lower liquid surface tension,shorter times over which the droplet breaks,and facilitated droplet break-up.The research results presented here can be used for atomization in engine cylinder,increasing the gas/liquid contact area and enhancing the combustion efficiency of gas/liquid heat transfer to improve the combustion efficiency of the engine.
