AN INLET SAMPLING DUCT FOR AIRBORNE OR AND SULFURIC-ACID MEASUREMENTS/

An inlet assembly has been designed, tested, and used for the airborne measurements of OH and sulfuric acid. The inlet sampling duct, which incorporates a shroud connected to two nested, restricted flow ducts, slows air velocity by approximately a factor of 16 while maintaining a uniform and well-defined flow. Qualitative wind tunnel tests showed t hat an inlet shroud that incorporates a 3:1 inner surface and 4.5:I ou ter elliptical front surface can straighten the airflow at angles of a ttack of up to 18 degrees-20 degrees with no visible signs of turbulen ce. Tests using a Pitot tube to scan the flow velocity profile of the restricted flow ducts showed that the shroud, coupled to inlet ducts, could slow the flow and provide a relatively flat average velocity pro file across the central portion of the ducts at angles of attack up to 17 degrees. Tests performed using a chemical tracer showed that at an gles of attack where the Pitot tube measurements began to indicate sli ght flow instabilities (17 degrees-24 degrees), there was no mixing fr om the walls into the center of the inlet. The inlet assembly also pos sesses the ability to produce a fairly uniform concentration of OH in the relatively constant velocity portion of the inner duct for instrum ent calibration. Actual measurements of rapidly changing OH and H2SO4 provide both additional evidence of proper inlet operation and new ins ight into H2SO4 production and loss in and around clouds.