Performance characterization of a positive pressure virtual cyclone working at high aerosol loading
Date Issued
2014
Date
2014
Author(s)
Hsu, Chia-Wei
Abstract
In the wake of the March 1995 sarin attack in the Tokyo subway, as well as other recent terrorist incidents, governments and publics are viewing with the growing concern about the potential threat posed by chemical, biological, radiological, or nuclear (CBRN) weapons. The risk for terrorist events has led to the development of new approaches to sampling, testing, and controlling for both indoors and outdoors ambient air. A standard military armored vehicle is composed of a blower, a virtual cyclone, a pleated HEPA filter unit, and a charcoal pack. A positive pressure type virtual cyclone has been used for high aerosol loading and low loading influence. This study used three steps to improve the efficiency of the cycole.
The first step is to expert on Performance Improvement of the Stairmand Cyclone Design. This work adopts the cyclone quality factor as the performance index, and the cyclone designed by Stairmand is adopted as the basic configuration in the present study. With the cyclone body diameter (D) fixed, we vary the cyclone length (H), inlet height (a) and width (b), cyclone length without the cone (hb), cone height (hc), cone bottom diameter (B), and vortex finder length (S). The results show that each of these design parameter has a different impact on the cyclone performance. Based on the cyclone quality factor, it is found that the H and S of the Stairmand cyclone are already the optimal conformations. The cyclone performance could be further improved, if further changes could be made to the other conformations, for example, narrowing down the inlet (adjusting a/b from 13/5 to 20/3.2), reducing the cone bottom diameter (adjusting B from 9 mm to 4 mm) and decreasing the vortex finder diameter (adjusting De from 13 mm to 8 mm), The cyclone quality factor appears to be a function of the volumetric flow rate. Results demonstrate that while the air flow is at 15 L/min, the newly improved cyclone could perform 12 times better than the Stairmand type cyclone, according to the cyclone quality factor.
The second step is to improve the Performance characterization of a positive pressure virtual cyclone working at high aerosol loading. This step work adopts the clean air delivery rate (CADR), which is commonly used in air-cleaning devices as an indicator to understand the cyclone loading effect of different parameters such as the inflow rate, the position of bottom hole, the cone shape, and the back pressure.The results demonstrate that while increasing the air flow, the CADR would be well improved. As the bottom hole becomes smaller, the CADR of big particles will increase. There is no obvious differences between hole positions. However, when the bottom hole locates in the center, the air flow will turn to draw air instead of discharged. Therefore, the best position of the bottom hole is the margin of the circle and a small cone will produce a better CADR. When the back pressure caused by pleated HEPA filter unit and charcoal pack is bigger, the CADR is lower. If we replace the high pressure inflow air with general centrifugal fan, the Qminor will decrease as well as the total flow, thereby lowering the collection efficiency.
The third step is to expert study of the positive pressure virtual cyclones as particle concentrators. There are lots of studies evaluating the fisibility of positive pressure virtual cyclones used as a particle concentrator. However, only a few studies diccussed the characteristics of parameters of the positive pressure virtual cyclone. An ideal particle concentrator is capable of controlling the particle concentration rate, particle size and its distribution. In this study, the particle concentration rate was proved to be effected by the above factors and the minor flow was effected by inlet flow, hole diameter at the cyclone bottom and the bottom diameter. When inlet flow rate was higher or the hole diameter was bigger, the particle concentration rate was lower. On the contrary, as the bottom diameter (B) was smaller, the concentration rate was higher. The position of the hole had no significant effect on particle concentration rate except particle diameter at 3-6 m.
The particle number fraction in the minor flow was increased with increasing hole diameter and the distance between the hole to the center of bottom. When the inlet flow rate was higher, the hole diameter and the cone diameter were smaller, the GSD (Grain Size Distribution Curve) was narrower.
Subjects
虛擬旋風分徑器
品質
CADR
微粒濃縮器
SDGs
Type
thesis
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