The system of turbulent thermal convection is introduced. Progresses in recent decades in the four major areas of research in turbulent convection are briefly reviewed. Some of the recent trends of the field are then ...The system of turbulent thermal convection is introduced. Progresses in recent decades in the four major areas of research in turbulent convection are briefly reviewed. Some of the recent trends of the field are then discussed, which also serve to point out that the future directions in this important field of fluid mechanics lie in the extension to the non-standard or non-classical Rayleigh-Bénard configuration.展开更多
The distribution of the thermal effects of the ion thruster plume are essential for estimating the influence of the thruster plume, improving the layout of the spacecraft, and for the thermal shielding of critical sen...The distribution of the thermal effects of the ion thruster plume are essential for estimating the influence of the thruster plume, improving the layout of the spacecraft, and for the thermal shielding of critical sensitive components. In order to obtain the heat flow distribution in the plume of the LIPS-200 xenon ion thruster, an experimental study of the thermal effects of the plume has been conducted in this work,with a total heat flow sensor and a radiant heat flow sensor over an axial distance of 0.5–0.9 m and a thruster angle of 0°–60°. Combined with a Faraday probe and a retarding potential analyzer, the thermal accommodation coefficient of the sensor surface in the plume is available. The results of the experiment show that the xenon ion thruster plume heat flow is mainly concentrated within a range of15°. The total and radial heat flow of the plume downstream of the thruster gradually decreases along the axial and radial directions, with the corresponding values of 11.78 k W m^(-2) and 0.3 k W m^(-2) for the axial 0.5 m position, respectively. At the same position, the radiation heat flow accounts for a very small part of the total heat flow, approximately 3%–5%. The thermal accommodation factor is0.72–0.99 over the measured region. Furthermore, the PIC and DSMC methods based on the Maxwell thermal accommodation coefficient model(EX-PWS) show a maximum error of 28.6% between simulation and experiment for LIPS-200 ion thruster plume heat flow, which, on the one hand, provides an experimental basis for studying the interaction between the ion thruster and the spacecraft, and on the other hand provides optimization of the ion thruster plume simulation model.展开更多
Underground subway platforms are among the world’s busiest public transportation systems,but the airborne transmission mechanism of respiratory infections on these platforms has been rarely studied.Here,computational...Underground subway platforms are among the world’s busiest public transportation systems,but the airborne transmission mechanism of respiratory infections on these platforms has been rarely studied.Here,computational fluid dynamics(CFD)modeling is used to investigate the airflow patterns and infection risks in an island platform under two common ventilation modes:Mode 1-both sides have air inlets and outlets;Mode 2-air inlets are present at the two sides and outlets are present in the middle.Under the investigated scenario,airflow structure is characterized by the ventilation jet and human thermal plumes.Their interaction with the infector’s breathing jet imposes the front passenger under the highest exposure risk by short-range airborne route,with intake fractions up to 2.57%(oral breathing)or 0.63%(nasal breathing)under Mode 1;oral breathing of the infector may impose higher risks for the front passenger compared with nasal breathing.Pathogen are efficiently diluted as they travel further,in particular to adjacent crowds.The maximum and median value of intake fractions of passengers in adjacent crowds are respectively 0.093%and 0.016%(oral breathing),and 0.073%and 0.014%(nasal breathing)under Mode 1.Compared with Mode 1,the 2nd mode minimizes the interaction of ventilation jet and breathing jet,where the maximum intake fraction is only 0.34%,and the median value in the same crowd and other crowds are reduced by 23–63%.Combining published quanta generation rate data of COVID-19 and influenza infectors,the predicted maximum and median infection risks for passengers in the same crowds are respectively 1.46%–40.23%and 0.038%–1.67%during the 3–10 min waiting period,which are more sensitive to ventilation rate and exposure time compared with return air.This study can provide practical guidance for the prevention of respiratory infections in subway platforms.展开更多
For an enclosed space like an industrial plant where devices and equipment generate a large amount of heat,the fluctuation characteristics of thermal plume become more crucial.This paper aims to study the unique air d...For an enclosed space like an industrial plant where devices and equipment generate a large amount of heat,the fluctuation characteristics of thermal plume become more crucial.This paper aims to study the unique air distribution of the point-source plume by shutting down all valves in a thermostatic chamber.250 consecutive instantaneous flow fields(in each zone)were measured by a two-dimensional particle image velocimetry(PIV)system at a sampling frequency of 3 Hz to quantify the air distribution of the plume.Based on the experimental data,the indoor instantaneous velocity and vorticity fields were analyzed,and the transient velocity fluctuation characteristics of typical points were discussed.The fluctuation periods of the plume at different heights were ini-tially estimated.Through the proper orthogonal decomposition(POD)analysis,the dominant coherent structures in complex instantaneous flow fields were excavated,and the turbulent kinetic energy of coherent structures was calculated.In addition,POD analysis provided a new method for validating and improving transient numerical simulation models.展开更多
Laser probe beam deflection technique is used for the analysis of laser-induced plasma shock waves in air and distilled water. The temporal and spatial variations of the parameters on shock fronts are studied as funct...Laser probe beam deflection technique is used for the analysis of laser-induced plasma shock waves in air and distilled water. The temporal and spatial variations of the parameters on shock fronts are studied as functions of focal lens position and laser energy. The influences of the characteristics of media are investigated on the well-designed experimental setup. It is found that the shock wave in distilled water attenuates to an acoustic wave faster than in air under the same laser energy. Good agreement is obtained between our experimental results and those attained with other techniques. This technique is versatile, economic, and simple to implement, being a promising diagnostic tool for pulsed laser processing.展开更多
The thermal buoyancy generated by the difference in air density in a building can drive hot-pressed natural ventilation,which is an energy-efficient means of ventilation used to obtain higher air quality.Therefore,the...The thermal buoyancy generated by the difference in air density in a building can drive hot-pressed natural ventilation,which is an energy-efficient means of ventilation used to obtain higher air quality.Therefore,the effect of a single-point heat source with limited sides at different heights on stratified flow was studied in a naturally ventilated room in this paper.Based on the classical plume diffusion law of an independent point heat source and the mirroring principle,a calculation model of the thermal stratification height with a restricted source elevated to different levels was derived and validated.The quantitative effects of the heat source height from the floor,the effective opening area and other factors on the natural ventilation of hot pressure were analyzed.A threshold X_(T)for the separation between a point source and a sidewall was defined to estimate whether the thermal plume was independent or restricted by a sidewall.And a method for calculating the threshold xt was obtained.This research can provide a reference basis for designing natural ventilation for buildings with a restricted heat source at different levels to achieve a desired indoor environment.展开更多
基金supported by the Research Grants Council of Hong Kongin particular through the General Research Funds (CUHK403811 and CUHK403712)through the NSFC/RGC Joint Research Scheme (N CUHK462/11)
文摘The system of turbulent thermal convection is introduced. Progresses in recent decades in the four major areas of research in turbulent convection are briefly reviewed. Some of the recent trends of the field are then discussed, which also serve to point out that the future directions in this important field of fluid mechanics lie in the extension to the non-standard or non-classical Rayleigh-Bénard configuration.
基金National Natural Science Foundation of China (No. 12005087)the Science and Technology Program of Gansu Province (Nos. 2006ZCTF0054, HTKJ2019KL510003, and 20JR10RA478)。
文摘The distribution of the thermal effects of the ion thruster plume are essential for estimating the influence of the thruster plume, improving the layout of the spacecraft, and for the thermal shielding of critical sensitive components. In order to obtain the heat flow distribution in the plume of the LIPS-200 xenon ion thruster, an experimental study of the thermal effects of the plume has been conducted in this work,with a total heat flow sensor and a radiant heat flow sensor over an axial distance of 0.5–0.9 m and a thruster angle of 0°–60°. Combined with a Faraday probe and a retarding potential analyzer, the thermal accommodation coefficient of the sensor surface in the plume is available. The results of the experiment show that the xenon ion thruster plume heat flow is mainly concentrated within a range of15°. The total and radial heat flow of the plume downstream of the thruster gradually decreases along the axial and radial directions, with the corresponding values of 11.78 k W m^(-2) and 0.3 k W m^(-2) for the axial 0.5 m position, respectively. At the same position, the radiation heat flow accounts for a very small part of the total heat flow, approximately 3%–5%. The thermal accommodation factor is0.72–0.99 over the measured region. Furthermore, the PIC and DSMC methods based on the Maxwell thermal accommodation coefficient model(EX-PWS) show a maximum error of 28.6% between simulation and experiment for LIPS-200 ion thruster plume heat flow, which, on the one hand, provides an experimental basis for studying the interaction between the ion thruster and the spacecraft, and on the other hand provides optimization of the ion thruster plume simulation model.
基金financially supported by the Natural Science Foundation of Guangdong Province,China(2019A1515012121)Natural Science Foundation of Zhejiang Province,China(Y20E080078)NIAID center of excellence for influenza research and surveillance(HHSN2722014000006C)。
文摘Underground subway platforms are among the world’s busiest public transportation systems,but the airborne transmission mechanism of respiratory infections on these platforms has been rarely studied.Here,computational fluid dynamics(CFD)modeling is used to investigate the airflow patterns and infection risks in an island platform under two common ventilation modes:Mode 1-both sides have air inlets and outlets;Mode 2-air inlets are present at the two sides and outlets are present in the middle.Under the investigated scenario,airflow structure is characterized by the ventilation jet and human thermal plumes.Their interaction with the infector’s breathing jet imposes the front passenger under the highest exposure risk by short-range airborne route,with intake fractions up to 2.57%(oral breathing)or 0.63%(nasal breathing)under Mode 1;oral breathing of the infector may impose higher risks for the front passenger compared with nasal breathing.Pathogen are efficiently diluted as they travel further,in particular to adjacent crowds.The maximum and median value of intake fractions of passengers in adjacent crowds are respectively 0.093%and 0.016%(oral breathing),and 0.073%and 0.014%(nasal breathing)under Mode 1.Compared with Mode 1,the 2nd mode minimizes the interaction of ventilation jet and breathing jet,where the maximum intake fraction is only 0.34%,and the median value in the same crowd and other crowds are reduced by 23–63%.Combining published quanta generation rate data of COVID-19 and influenza infectors,the predicted maximum and median infection risks for passengers in the same crowds are respectively 1.46%–40.23%and 0.038%–1.67%during the 3–10 min waiting period,which are more sensitive to ventilation rate and exposure time compared with return air.This study can provide practical guidance for the prevention of respiratory infections in subway platforms.
基金The work was supported by Natural Science Foundation of Shanghai under Grant no.16ZR1423200Machinery Industry Innovation Platform Construction Project under Grant no.2019SA-10-07Shandong Con-struction Department Research and Development Project under Grant no.K2019370.
文摘For an enclosed space like an industrial plant where devices and equipment generate a large amount of heat,the fluctuation characteristics of thermal plume become more crucial.This paper aims to study the unique air distribution of the point-source plume by shutting down all valves in a thermostatic chamber.250 consecutive instantaneous flow fields(in each zone)were measured by a two-dimensional particle image velocimetry(PIV)system at a sampling frequency of 3 Hz to quantify the air distribution of the plume.Based on the experimental data,the indoor instantaneous velocity and vorticity fields were analyzed,and the transient velocity fluctuation characteristics of typical points were discussed.The fluctuation periods of the plume at different heights were ini-tially estimated.Through the proper orthogonal decomposition(POD)analysis,the dominant coherent structures in complex instantaneous flow fields were excavated,and the turbulent kinetic energy of coherent structures was calculated.In addition,POD analysis provided a new method for validating and improving transient numerical simulation models.
基金supported by the National Natural Science Foundation of China (No.60578015)the Teaching and Research Award Program for Outstanding Young Professor in Higher Education Institute,Ministry of Education,P.R.Chinathe National Key Opening Experiment Foundation of Laser Technology (No.2005)
文摘Laser probe beam deflection technique is used for the analysis of laser-induced plasma shock waves in air and distilled water. The temporal and spatial variations of the parameters on shock fronts are studied as functions of focal lens position and laser energy. The influences of the characteristics of media are investigated on the well-designed experimental setup. It is found that the shock wave in distilled water attenuates to an acoustic wave faster than in air under the same laser energy. Good agreement is obtained between our experimental results and those attained with other techniques. This technique is versatile, economic, and simple to implement, being a promising diagnostic tool for pulsed laser processing.
基金supported by the Natural Science Foundation of Shaanxi Province(No.2020JQ-670)China Postdoctoral Science Foundation(No.2020M673353).
文摘The thermal buoyancy generated by the difference in air density in a building can drive hot-pressed natural ventilation,which is an energy-efficient means of ventilation used to obtain higher air quality.Therefore,the effect of a single-point heat source with limited sides at different heights on stratified flow was studied in a naturally ventilated room in this paper.Based on the classical plume diffusion law of an independent point heat source and the mirroring principle,a calculation model of the thermal stratification height with a restricted source elevated to different levels was derived and validated.The quantitative effects of the heat source height from the floor,the effective opening area and other factors on the natural ventilation of hot pressure were analyzed.A threshold X_(T)for the separation between a point source and a sidewall was defined to estimate whether the thermal plume was independent or restricted by a sidewall.And a method for calculating the threshold xt was obtained.This research can provide a reference basis for designing natural ventilation for buildings with a restricted heat source at different levels to achieve a desired indoor environment.