Characterization of expiration air jets and droplet size distributions immediately at the mouth opening

Size distributions of expiratory droplets expelled during coughing and speaking and the velocities of the expiration air jets of healthy volunteers were measured. Droplet size was measured using the interferometric Mie imaging (IMI) technique while the particle image velocimetry (PIV) technique was used for measuring air velocity. These techniques allowed measurements in close proximity to the mouth and avoided air sampling losses. The average expiration air velocity was 11.7 m/s for coughing and 3.9 m/s for speaking. Under the experimental setting, evaporation and condensation effects had negligible impact on the measured droplet size. The geometric mean diameter of droplets from coughing was 13.5 μm and it was 16.0 μm for speaking (counting 1–100). The estimated total number of droplets expelled ranged from 947 to 2085 per cough and 112–6720 for speaking. The estimated droplet concentrations for coughing ranged from 2.4 to 5.2 cm^?3 per cough and 0.004–0.223 cm^?3 for speaking.     

Measurements of Kinetic Energy Loss for Particles Impacting Surfaces

Incoming and rebounding particle velocities were measured to within several particle diameters of the impaction surface using laser Doppler velocimetry. Impacts occurred normal to the surface and ranged from 1 m/s, near the threshold for particle bounce, to 100 m/s, well into the plastic damage regime. Monodisperse ammonium fluorescein spheres, 2.6–6.9 μm in diameter, impacted target surfaces including polished molybdenum and silicon, cleaved mica, and a fluorocarbon polymer. The incident kinetic energy recovered on rebound depended on particle size and target composition at low velocity (< 20 m/s), where the adhesion surface energy is important. No dependence on target composition was found at higher velocities where up to half of the impact energy was lost to plastic deformation. Plastic deformation was a significant component of energy loss even at impact velocities near critical velocity. Critical velocities for the onset of bounce decreased with a stronger power-law dependence on particle diameter than expected from classical adhesion theory or the elastic flattening model proposed by Dahneke. This is consistent with kinetic energy loss contributions from both plastic deformation and surface forces. Auxiliary experiments conducted with and without continuous discharge of the impact surface indicated the absence of a significant electrostatic contribution to particle adhesion.     

多喷嘴对置式撞击流反应器流场的数值模拟

为探讨多股撞击流反应器内流场的特点,利用Fluent软件对两喷嘴对置式和四喷嘴对置式撞击流反应器内流场进行模拟,研究了在不同工况下这两种撞击流反应器内的流场结构,设定进口流速分别为5 m/s、10 m/s、15 m/s、20 m/s、25 m/s。模拟结果表明,进口流速由5 m/s增加到25 m/s时,反应器内流体流动的平均速度及压力波动增大为原来的4.1倍和16.2倍,其速度梯度和压力平均波幅也增大为原来的5.0倍和25.4倍。通过对两种不同结构的撞击流反应器的比较可以得到四喷嘴对置式撞击流反应器内流体流动产生的速度梯度和压力平均波幅更大,其值都约为二喷嘴对置式撞击流反应器的1.2倍,因而四喷嘴对置式撞击流反应器内剪切力场更强,脉动更强烈,更有利于反应器内的湍动混合。     

Quantitative Assessment of the Effect of Surface Deposition and Coagulation on the Dynamics of Submicrometer Particles Indoors

Exposure to airborne particles indoors depends on particle concentration, which is affected by air filtration, ventilation, and particle dynamics. The aim of this work was quantitative assessment of the effects of coagulation, surface deposition, and ventilation on the submicrometer particle concentration indoors. The assessment was obtained from measured particle loss rate and deposition velocity parameters. The experiments were conducted in an experimental chamber for three different types of aerosols: environmental tobacco smoke, petrol smoke, and ambient air aerosols. Particle number concentration and size distribution were measured in the size range between 0.017 and 0.898 w m by SMPS. The average values for the overall deposition loss rates varied from 4.3 2 10 m 5 s m 1 (0.16 h m 1 ) to 1.1 2 10 m 4 s m 1 (0.39 h m 1 ). The overall deposition velocities associated with surface deposition and coagulation ranged from 9.6 2 10 m 4 cm s m 1 to 2.4 2 10 m 3 cm s m 1 , and for surface deposition only from 2.8 2 10 m 4 cm s m 1 to 6.3 2 10 m 4 cm s m 1 . For indoor conditions with an air exchange rate above 1.3 h m 1 , (natural ventilation, no filters) only a reduction in particle number of about 20% is attributed to the surface deposition and coagulation.     

The flow of “settling” slurries in tubes with internal spiral ribs

The secondary flows created by an internal spiral rib in an otherwise conventional smooth tube are illustrated by reference to the axial, and combined tangential and radial velocity components for a fluid flowing in a 2-inch diameter tube containing a rib with a pitch-to-diameter ratio of 3 The effect of such secondary flows on the transportability of “settling”(suspensions)was investigated by measuring pressure gradients for the flow of two sand/water slurries in a smooth 2-inch tube and in 2-inch diameter tubes with ribs of pitch-to-diameter ratio 5.15, 3.32 and 1.80. Average flow velocities ranged from 0.5 to 8 feet/sec. and delivered concentrations ranged from 5 to 18 per cent by volume While the ribs were found to be a disadvantage at relatively high average mixture velocities because of increased pressure gradients, they were found to be advantageous at relatively low velocities, i.e. velocities less than the critical deposit velocity for flow in the smooth tube, in that the pressure gradients, for a given sand/water slurry with a given delivered concentration and average velocity, were lower in the ribbed tubes than in the smooth tube. Because of this, the power consumption per unit mass of solid transported was reduced by the presence of a rib. The optimum pitch-to-diameter ratio was estimated to be about 5 Pressure gradients measured for one of the slurries with air added as a third phase showed the presence of air to be a definite disadvantage     

循环流化床提升管中颗粒速度的径向分布及其沿轴向的发展

采用5光纤速度探头对f100mm?5.1m循环床提升管8个高度截面上11个径向位置的局部颗粒速度进行了实验测试,并采用径向不均匀指数(RNI)对颗粒速度径向分布的不均匀性及其沿轴向的变化进行了定量描述。研究结果表明:在高气速、高颗粒循环量操作时,操作条件对颗粒上升速度和下降速度的径向分布的影响在加速段和充分发展段呈现出不同的规律;颗粒上升速度和下降速度沿轴向的变化在核心区和边壁区也表现出不同的趋势。当颗粒循环速率大于200 kgm-2s-1时,颗粒的加速段长度大大延长,以至于大于提升管的高度(15.1m)。颗粒速度径向分布的不均匀性沿轴向是逐渐增大的,并且与截面平均颗粒速度存在很强的相关性。     

Velocity Exponent in Solid-Particle Erosion of Silicon

The velocity dependence of the solid-particle steady-state erosion rate of (111) silicon single crystals was measured for a range of particle types (Al₂O₃, SiC, and SiO₂) and two sizes (40 and 270 μm) from velocities of 30 to 150 m/s. Power-law velocity dependence was found in all cases, but the velocity exponent n decreases with increasing size, contrary to current theories which predict a constant n. One possible interpretation of this effect is considered .     

Dwell and Interface Defeat on Borosilicate Glass

We have conducted impact experiments using gold long rods into borosilicate glass and the measured the penetration velocity as a function of impact velocity. At sufficiently low-impact velocities, the glass target resists penetration and there is dwell; dwell is observed to approximately 450 m/s for bare glass. If a copper buffer is placed over the glass to eliminate the impact shock, significant dwell can be seen at impact velocities as high as 890 m/s. These impact velocities correspond to Bernoulli stresses of approximately 2.0 and 7.6 GPa, respectively. The paper describes the experimental data, and summarizes the results and our findings.     

基于TDLAS技术的双基系推进剂装药排气羽流流速特性研究

基于可调谐半导体激光吸收光谱(TDLAS)技术,利用吸收光谱中1392nm附近的H2O吸收谱线,测量了双基推进剂和改性双基推进剂装药在Ф50mm标准缩比发动机中燃烧后产生的排气羽流的流速。结果表明,双基推进剂装药的流速在达到第1个峰值后有稍许下降,随后缓慢上升,进入位于0.90~1.39s的平台区;而改性双基推进剂装药的流速在达到1100m/s后直接进入位于0.33~0.88s的平台区,而流速在该区域内呈现下降趋势;上述两种装药的排气羽流流速曲线的变化趋势与燃烧室内压强曲线一致,但由于流速测量点距喷口有一定距离,导致其与压强曲线相比有一定程度的滞后;双基和改性双基两种推进剂装药在距发动机喷口30cm处的平均羽流流速分别为831.8和1057.5m/s。     

Hydrodynamics of three-phase fluidized bed systems examined by statistical, fractal, chaos and wavelet analysis methods

Hydrodynamic studies were conducted in gas-liquid-solid systems (0.1 m ID, 2 m high) of 3.0 mm glass beads and of 2.1 mm polypropylene low-density particles, with particles densities of 2471 and 1290 kg/m³, respectively. Simultaneous measurement of differential pressure and bubble conductivity probe signals sampled at 500 Hz for 60 s enabled the investigation of the change in flow structure in relation to the flow regime transitions. Superficial gas velocities ranged between 0.010 and 0.052 m/s for polypropylene particles, and extended to 0.12 m/s for glass beads, while the superficial liquid velocities covered the ranges of 0.0007-0.045 m/s for polypropylene particles, and ranged up to 0.056 m/s for glass beads.Spectral analysis of the pressure fluctuations revealed a transition from dispersed to coalesced bubbling flow with decreasing liquid velocity for a given superficial gas velocity. The use of a conductivity probe facilitated characterization of the local flow structure in terms of bubble movement. The measurements were extensively analyzed using fractals and chaos, power spectra frequency analysis and wavelet decomposition in addition to the standard statistical analyses. The coefficient of variation of the bubble probe signals was found to be the most effective in deducing the transition velocity between coalesced and dispersed bubbling flow regimes, while wavelet energy confirmed the similarity in the distribution between two axial positions once operated in the dispersed flow regime. Comparison of the flow structure between glass beads and polypropylene particles showed that both the minimum liquid fluidization velocity and the transition velocity between the bubble flow regimes were much higher for the glass beads than for the lighter polypropylene particles. Furthermore, the standard deviations of the decomposed bubble probe signals through wavelet transformation successfully highlighted the difference between the two systems of particles.     

超声脉冲反射法和激光吸收光谱法同步测量流动液膜厚度

液膜流动现象在工业过程中广泛存在,对流动液膜厚度测量方法的测量研究至关重要。首先利用已知液膜厚度的标准具（100～1000 μm）对超声脉冲反射法和激光吸收光谱法精度进行验证;结果表明,超声脉冲反射法测量液膜厚度的平均测量误差为1.07%,激光吸收光谱法测量液膜厚度的平均测量误差为1.29%。同时结合两种方法对流动液膜进行研究,结果表明,当液膜在低速/中速/高速流动时,两种方法测量流动液膜的平均厚度吻合良好,平均厚度差值分别为16.59、16.26、13.36 μm,流动液膜厚度的标准方差的相对偏差分别为0.29%、7.71%、25.37%,且在三种不同速度下两种方法测得在1s的周期内液膜波动次数一致。     

Experimental and numerical investigations of scale-up effects on the hydrodynamics of slurry bubble columns

Experiments and simulations were conducted for bubble columns with diameter of 0.2 m(180 mm i.d.), 0.5 m(476 mm i.d.) and 0.8 m(760 mm i.d.) at high superficial gas velocities(0.12–0.62 m·s-1) and high solid concentrations(0–30 vol%). Radial profiles of time-averaged gas holdup, axial liquid velocity, and turbulent kinetic energy were measured by using in-house developed conductivity probes and Pavlov tubes. Effects of column diameter, superficial gas velocity, and solid concentration were investigated in a wide range of operating conditions. Experimental results indicated that the average gas holdup remarkably increases with superficial gas velocity, and the radial profiles of investigated flow properties become steeper at high superficial gas velocities. The axial liquid velocities significantly increase with the growth of the column size, whereas the gas holdup was slightly affected. The presence of solid in bubble columns would inhibit the breakage of bubbles, which results in an increase in bubble rise velocity and a decrease in gas holdup, but time-averaged axial liquid velocities remain almost the same as that of the hollow column. Furthermore, a 2-D axisymmetric k–ε model was used to simulate heterogeneous bubbly flow using commercial code FLUENT 6.2. The lateral lift force and the turbulent diffusion force were introduced for the determination of gas holdup profiles and the effects of solid concentration were considered as the variation of average bubble diameter in the model. Results predicted by the CFD simulation showed good agreement with experimental data.     

Gas holdup for high gas velocities in a gas–liquid cocurrent upward‐flow system

Gas holdup has been measured in an 83‐mm diameter, 2.2‐m high column at high gas superficial velocities — 0.22 to 2.7 m/s — and at liquid (water) superficial velocities of 0 to 0.47 m/s, by means of a differential pressure transducer. The equation of Hills (1976) based on the slip velocity gives good predictions of the gas holdup for 0.1 ≤ E_g ≤ 0.4. However, the holdups predicted by this approach are considerably higher than the experimental values at gas velocities high enough that E_g > 0.4. Other equations from the literature are also shown to be inadequate. The new data and earlier data at high gas velocities are therefore correlated with a new dimensional equation for U_l ≤ 0.23 m/s.     

Low-velocity impact damage in graphite-fiber reinforced epoxy laminates

An experimental investigation was conducted to identify the failure mechanism and to understand damage propagation in compression-loaded composite structures. The tests were conducted on several laminates of different ply orientation with thicknesses that ranged from 0.56 to 0.79 cm. The panels were damaged by 1.27-cm-diameter aluminum spheres propelled normal to the specimen surface at velocities ranging from 30 m/s to 140 m/s. Results indicate that there is significant internal laminate damage due to low-velocity impact with no surface damage. The internal damage consists of delamination and intraply cracking. Three damage propagation modes were identified as causing specimen failure; delamination, axial load-lateral deformation coupling, and local shear failure.     

Position and velocity of a large particle in a gas/solid riser using the radioactive particle tracking technique

The flow behavior of the solids phase in the fully developed region of a laboratory-scale circulating fluidized bed riser was studied using an assembly of sixteen NaI detectors to determine the position of a 500 μm radioactive particle, 100 times/s. The particle location was inferred from the number of γ-rays recorded by the assembly. The knowledge of the instantaneous positions enabled the determination of the instantaneous and mean velocity fields. Tests were conducted in a 0.082 m diameter, 7 m tall riser using 150 μm silica sand particles. Data were obtained at a gas superficial velocity of 4 m/s and solids mass fluxes from 23 to 75 kg/m²·s. Radial profiles of axial particle velocity showed that the solids velocity decreased with increasing solids circulation rates. Correspondingly, turbulent particle velocities and solids dispersion coefficient in the longitudinal direction were found to decrease as the solids circulation rate increased. The cross-sectional area where, on average, solids downflow took place, increased with increasing solids circulation rate.     

Dynamic fracture instability in glassy polymers as studied by ultrasonic fractography

Ultrasonic fractography studies were performed on poly(methyl methacrylate) of high molecular weight. The transient fracture velocity change at the slow-to-fast transition during discontinuous propagation has been measured precisely. Fast fracture starts with a characteristic velocity which falls in a narrow range between 90 to 150 m/s, nearly independent of the loading speeds and the specimen temperature from ?50 to 40°C. Parallel double-cantilever-beam specimens exhibited stick-slip type propagation whose velocity change was also evaluated. In these specimens, the fast fracture abruptly slows down to speeds on the order of 10° m/s. These intermediate velocities have never been obtained in the slow-to-fast transition. Velocity measurements under hydrostatic pressure have shown that fracture velocities decrease significantly with increasing pressure, and that the slow-to-fast transition tends to disappear at a pressure between 5 and 10 MPa. Models have been presented concerning the mechanism of the slow-to-fast transition, crazing and cracking under superposed cyclic stress field, and the relationship between dynamic toughness and fracture velocity in this material.     

Subcritical Crack Growth in Silica Optical Fibers in Wide Range of Crack Velocities

Crack growth in silica optical fibers was measured directly in a wide range of crack velocities d a/ d t (10⁻¹³ < d a /d t < 10⁻⁵ m/s) by improving our previous method. The slope in log-log plots of the crack velocity versus the stress intensity factor data was revealed to increase with the crack velocity, e.g., slope, 23.3 for d a /d t < 10⁻¹⁰ m/s, and 39.7 for d a /d t > 1(10⁻⁸ m/s. The data were also found to be fitted well by the exponential law in the whole range of d a /d t , rather than the power law. Additionally, the fiber data were shown to be close to those reported for bulk silica specimens having large cracks for high crack velocities where the bulk specimens were tested.     

Gas and particle flow patterns in cyclones at room and elevated temperatures

Experimental results are presented for a study of gas and particle flows in a 102 mm diameter conventional cyclone operated at temperatures between 300 and 2000 K. Inlet gas velocities ranged from 3 to 42 m/s. Particle deposition patterns and the measurements of local pressures were used to determine the flow patterns and velocity profiles within the cyclone. A “Reynolds Number” has been defined based on the mean inlet velocity and the hydraulic diameter of the annulus between the cyclone wall and the gas outlet duct. An empirical equation was derived to correlate the ratio of the wall tangential velocity to the mean inlet velocity with this Reynolds Number.     

Flow development in a gas-solids downer fluidized bed

The development of gas and solids flow structure was studied in a 9.5 m high and 0.10 m diameter, gas-solids cocurrent downflow circulating fluidized bed (downer). Local solids concentration and particle velocity were measured using two separate optical fibre probes at different radial positions on several axial levels along the downer. The results show that the flow development is significantly influenced by the operating conditions. For most of the conditions under which the experiments were conducted, the gas-solids flow reaches its fully developed zone within 3 to 8 m away from the entrance. On the other hand, the development zone can extend as long as the downer itself, under certain conditions. When the solids circulation rate is over 100 kg/m²s, an increasing solids circulation rate largely extends the length of radial flow development. It is found that the flow developments in the core and at the wall are not quite simultaneous. For solids concentration, the core develops more quickly at low gas velocities and the wall region develops faster at high gas velocities. For particle velocity, higher gas velocity speeds up the development of the wall region but does not significantly affect the development of the core region. The wall region is much more sensitive to the change of superficial gas velocity than the core region. At high superficial gas velocities (> 7 m/s), a “semi-dead” region is observed in the fully developed zone adjacent to the wall where the dilute solids are moving at a very low velocity.     

PREDICTION OF PRESSURE GRADIENT AND HOLDUP IN HORIZONTAL LIQUID–LIQUID SEGREGATED FLOW WITH SMALL EÖTVÖS NUMBER

This experiment utilized two-phase oil-water flows in a 26.1 mm diameter horizontal steel pipe to investigate segregated flow pattern. Data on pressure gradient and in situ phase distribution were obtained to come up with different combinations of phase superficial velocities, which ranged from 0.05 to 0.96 m/s. For the current liquid–liquid system having a small Eötvös number, we focused on identifying the effect of the interfacial tension and wall-wetting properties. Direct observation showed that oil and water started to mix when mixture velocity was increased. Also, at higher mixture velocities wherein the flows of two phases were initially separated, the droplets of one phase appeared in the mixed layer with negligible thickness. A closure relationship, which describes the interfacial mass transfer between liquid phases, was introduced into the framework of the two-fluid model. The model predictions agree well with the measured data. Similarly, an approach performance was evaluated based on the experimental data reported in previous literature.