Yu-Kang Chen

The capture envelope of a flanged circular hood in cross drafts.

The flow patterns of an exterior circular hood subject to the influence of various uniform cross drafts were experimentally studied in an apparatus consisting of hood-model/wind-tunnel assembly. A two-component laser Doppler anemometer was employed to measure the velocity field on the symmetry plane. The streamline patterns were obtained from the measured velocity data. The cross draft caused a characteristic envelope similar to a half Rankine body-of-revolution to form in the flow field. The boundary of the envelope is described by a dividing streamline. All streamlines within the envelope lead to the opening; those outside the envelope evolve to the downstream area. The normalized geometry of the capture envelope is theoretically justified and correlated by modifying the potential theory of point-sink-plus-rectilinear-flow. The domain and shape of the envelope enclosing the hood opening are determined primarily by the velocity ratio between the cross draft and hood suction. The correlated formula is applicable to design the hood parameters, including the sizes of opening and flange as well as the location of contaminant sources.

Capture envelopes of rectangular hoods in cross drafts.

The suction fields of the rectangular hoods of various aspect ratios varying from 0.1 to 10 that are subject to the influence of cross drafts were experimentally studied in an apparatus consisting of a hood model/wind tunnel assembly. The velocity field on the symmetry plane was measured with a two-component laser Doppler anemometer. Being under the influence of cross draft, the suction field presents a characteristic capture envelope, which is described by a dividing streamline. The characteristics of the capture envelope were found to be determined by the cross-draft to hood-suction velocity ratio R and the hood-opening aspect ratio AR. The flow characteristics of the hoods with aspect ratios less than unity were dramatically different from those with aspect ratios greater than one. If areas of the hood openings had the same values, the hydraulic-diameter normalized characteristic length scales of the capture zone of the square hood were as same as those of the circular hood. When the diameter of a circular hood was equal to the width of a square hood, the physical dimensions of the capture zones created by these two hoods coincided with each other.

Experimental Study on the Effect of Fiber Orientation on Filter Quality

Most filtration models assume that the air stream runs perpendicularly to the orientation of the filter fibers. However, cigarette filters remove aerosol particles apparently by a different filter configuration in that the fiber orientation almost parallels the air streamlines. To focus on the effect of fiber orientation, cellulose acetate filters were used in this work to facilitate the filter performance comparison. A piece of original round cigarette filter was molded to form a cube. The same piece of filter was used for both perpendicular and parallel orientations, to avoid the variability caused by the non-uniform filter media distribution. DOP aerosol particles used in the tests were generated by either a constant output aerosol nebulizer or an ultrasonic atomizing nozzle. A Po-210 radiation source was used to neutralize the challenge aerosols to the Boltzmann charge equilibrium. A scanning mobility particle sizer (for < 0.8 μm) and an aerodynamic particle sizer (for > 0.8 μm) were used to measure aerosol number concentrations and size distributions upstream and downstream of the cigarette filters. The results showed that parallel and orthogonal filters behave similarly. However, the pressure drop across parallel filter was lower than for the perpendicular filter, indicating that the airflow is more laminar passing through the parallel filters. Possibly for the same reason, aerosol penetration through parallel filter was higher than the orthogonal filter, although the difference may not be statistically significant. When a comparison of the fiber orientation is based on filter quality, orthogonal filter performs better than parallel filter, if face velocity is lower than 60 cm/s. Parallel filter performs better only when the particles are smaller than the most penetrating size and under high face velocity.

Development and Characterization of a Wake-Controlled Exterior Hood

A wake-controlled exterior hood was developed to overcome the negative influence of cross draft on an exterior hood and avoid the operation inconvenience caused by the enclosure of an airflow capture booth. This new type of local exterior hood used the hood suction flow to stabilize the dynamic vortex shedding that was induced when a crossflow passed over a blockage plate, and therefore formed a hydrodynamics-stabilized local isolation area for efficient removing of the contaminant. The development process was performed in a test section of an open-circuit wind tunnel. The blockage plate and the exterior hood model were placed in a wind-tunnel test section so that the crossflow could be freely supplied by the airstream of the wind tunnel. The laser light sheet flow visualization method and the laser Doppler velocimeter were employed to reveal the characteristics of the flow field. Primary influential parameters were factored out of the measured velocity results so that a design procedure was proposed. Experiments using hot-wire type alcohol sensors to measure the toluene vapor concentration distributions showed that the capture efficiency of this type of actively controlled hood was remarkably higher than that of an uncontrolled hood.

Effects of Flange Size on Dividing Streamlines of Exterior Hoods in Cross Drafts

Effects of flange size on the flow patterns of an exterior circular hood subject to the influence of various uniform cross drafts were studied in an apparatus consisting of hoodmodel/wind-tunnel assembly. The cross draft to suction velocity ratio covered the range from 0.056 to 0.792. The flange width to hood diameter ratio spanned from 1.2 to 3.0. The hood equipped without flange was also probed. A two-component laser Doppler anemometer was used to measure the velocity field on the symmetry plane. The streamline patterns were obtained from the measured velocity data. The cross draft caused a capture envelope. The boundary of the envelope was described by a dividing streamline. It was found that the flange width presented complex effects on the capture envelope and the dividing streamline. Corresponding to a cross draft to suction velocity ratio, a critical flange width existed. If the flange width was smaller than the critical value, the dividing streamline would terminate at the downstream tip of the flange. The behaviors of the capture envelope and the dividing streamline of the hood in cross draft under different flange widths are presented and discussed.