Seo Youngjin, McFarland Andrew R, Ortiz Carlos A, O'Neal Dennis L
Aerosol Technology Laboratory, Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843, USA.
Health Phys. 2006 Jul;91(1):47-57. doi: 10.1097/01.HP.0000203325.62672.39.
Tests were conducted to characterize the uniformity of velocity and tracer gas profiles in a square and a rectangular duct with respect to defining the suitability of locations for single point sampling of gaseous contaminants. Several configurations, such as a straight duct with unidirectional flow at the entrance section and straight ducts preceded by mixing elements (a 90 degrees mitered bend and double 90 degrees bends in S- and U-type configurations) were tested. Results are compared with those from circular ducts. For a straight duct of square cross section, which is not preceded by a mixing element, the coefficients of variation (COV) of tracer gas concentration at 19 duct diameters downstream of the gas release location is 143% (center release of tracer gas). COVs of velocity and tracer gas concentration in a straight square duct 9.5 duct diameters downstream of a 90 degrees mitered bend are 6% and 24.3% (top inside release), respectively, which does not meet the ANSI N13.1 limit of 20% for the tracer gas COV. In case of the rectangular duct with a 3:1 (width to height) aspect ratio, COVs of velocity and tracer gas concentration at 9 duct diameters downstream of a 90 degrees mitered bend are 29% and 62% (bottom inside release), respectively. A mixing element in a square duct comprised of two 90 degrees mitered bends in a U-configuration produces results similar to those obtained with a single 90 degrees bend. However, COVs of velocity and tracer gas concentration in a square duct 6 duct diameters downstream of an S-type double bend are 10.6% and 8.3% (top inside release), respectively, which comply with the ANSI tracer gas and velocity criteria for single point representative sampling. When mixing elements were employed in square ducts, the COV results were comparable with those of other researchers for circular ducts.
为了确定气态污染物单点采样位置的适用性，对方形和矩形管道中速度和示踪气体分布的均匀性进行了测试。测试了几种配置，如入口段单向流动的直管以及在直管前设置混合元件（90度斜接弯头以及S型和U型配置的双90度弯头）的直管。将结果与圆形管道的结果进行了比较。对于没有混合元件的方形横截面直管，在示踪气体释放位置下游19个管道直径处，示踪气体浓度的变异系数（COV）为143%（示踪气体从中心释放）。在90度斜接弯头下游9.5个管道直径处的方形直管中，速度和示踪气体浓度的COV分别为6%和24.3%（从顶部内侧释放），这不符合美国国家标准学会（ANSI）N13.1中示踪气体COV 20%的限值。对于宽高比为3:1的矩形管道，在90度斜接弯头下游9个管道直径处，速度和示踪气体浓度的COV分别为29%和62%（从底部内侧释放）。方形管道中由两个呈U型配置的90度斜接弯头组成的混合元件产生的结果与单个90度弯头的结果相似。然而，在S型双弯头下游6个管道直径处的方形管道中，速度和示踪气体浓度的COV分别为10.6%和8.3%（从顶部内侧释放），这符合ANSI示踪气体和单点代表性采样的速度标准。当在方形管道中使用混合元件时，COV结果与其他研究人员对圆形管道的研究结果相当。
