Measurement of the Boundary Layer Growth Due to Transpiration Flow Through Nacelle Acoustic Liners

Abstract

To determine the effect of transpiration flow through the nacelle acoustic inlet liner on the boundary growth along the liner, measurements were made in the Boeing Wichita 6”x6” flow duct. A two pronged boundary layer probe was used to measure the boundary layer growth along a 48 inch test panel with transpiration mass flow rates of 0.0%, 4.2% and 5.7% of the tunnel mass flow rate. The boundary layer growth was quantified with respect to displacement thickness, momentum thickness and shape factor. The boundary layers profiles were not highly affected by Mach number even with transpiration flow. Introduction As airplane community noise rules and regulations become more stringent, strategies for reducing engine noise will result in moving the acoustic lining forward of the inlet throat to the anti-ice bulkhead to obtain more acoustically lined area. Since the acoustic lining on the lower barrel has axial drain channels which collect water from circumferential core drain slots, the pressure difference along the inlet causes air to flow through the drain channels and into the inlet through the acoustic liner face sheets. A generic inlet contour and surface pressure plot are shown in Fig. 1. This transpiration of air through the face sheets will affect the boundary layer growth in the inlet and the engine performance. The acoustic perforates in the inlet have hole diameters, percent open areas and transpiration flow rates that are too large to have a positive effect on engine performance as might be the case for microperforates with lesser flow rates. The goal was not to measure the increase or decrease in drag due to the transpiration flow, but to ∗Senior Specialist Engineer, Boeing Commercial Airplane Group, Wichita Division †Associate Technical Fellow, Boeing Commercial Airplane Group, Wichita Division ‡Technical Fellow, Boeing Commercial Airplane Group, Wichita Division assess how much the boundary layer would grow along the acoustic liner. The effect of transpiration flow on boundary layer growth was measured in the Boeing Wichita 6”x 6” flow duct for Mach numbers up to 0.5. The pressure gradient along the Boeing Wichita 6”x 6” duct was minimal compared to the work of Andersen et. al., and the boundary layer measurements were made with a traversing total pressure probe as opposed to hot wire measurements.