Shrouded Probe Performance: Variable Flow Operation and Effect of Free Stream Turbulence

Performance of a shrouded aerosol sampling probe, operated in accordance with the variable flow protocol of EPA Methods 5 and 17, was compared with its performance in a fixed flow rate mode. The effect of free stream turbulence intensity on the transmission ratio of the shrouded probe was also evaluated. Shrouded probes were developed for continuous emission monitoring of nuclear aerosols, which generally involves sampling at a constant flow rate even though the stack velocity may vary. However, this study shows that the sampling flow rate through a shrouded probe can be varied in proportion to the stack velocity, and representative samples can be obtained. This variable sampling flow rate protocol is appropriate to nonnuclear stacks and to nuclear stacks that have the potential to emit significant quantities of radioactive aerosol particles. A shrouded probe, designed for nominal conditions of 57 L/min at a stack velocity of 12 m/s was tested in a wind tunnel over a range of free stream velocities of 6-24 m/s. When the flow rate through the shrouded probe was varied proportionally with the free stream velocity, the transmission ratio (ratio of aerosol concentration at the probe exit plane to the aerosol concentration in the free stream of the stack or duct) of 10-μm aerodynamic diameter (AD) particles varied between 0.97 and 1.04. In comparison, sampling at a fixed flow rate over this same range of free stream velocities resulted in transmission ratios of 0.88-1.32. The previously noted tests were run with liquid aerosol particles, and tests comparing transmission ratios with solid and liquid 10-μm AD aerosol particles at velocities of 8 and 16 m/s showed less than a 6% difference. The effect of free stream turbulence intensity on shrouded probe sampling was experimentally evaluated at intensities of 4%, 8%, 12%, 16%, and 28% in the free stream at the test section. For all these conditions, the change in transmission ratio of the shrouded probe was less than 5% for 10-μm AD liquid aerosol particles.