The effects of outlet geometry on automotive demister performance

The established method of clearing a misted car windshield or of maintaining a clear view under misting conditions is through the application of an air supply via jet outlets in the instrument panel. The ability of such arrangements to perform adequately is a function of the prevailing environmental conditions, the vehicle speed, the condition of the demist air source and the geometry and arrangement of the jet outlets. This paper presents experimental data obtained in a purpose built environmental chamber designed to accommodate simple rectangular jets impinging on a misted glass surface. The facility consists of three conditioned air sources applied to a test chamber designed to represent the external, internal and demist air flows. Mist conditions on the glass surface are determined using a novel technique employing a CCD camera acquiring grey scale images which are digitally analysed to generate mist detection, grading and clearing contour data. The test facility and mist analysis techniques are described. Transient jet performance is investigated in a parametric study that addresses the effects of jet aspect ratio, offset, impingement angle and mean volume flow rate on clearing performance. Results are presented in terms of parameters derived from the images recorded during the clearing process. These parameters include rate of clearing terms, breakthrough time and location of clearing centre relative to geometric impingement point. Steady state performance is presented for a number of representative geometries by examining the size of the clear area for a range of interior humidity levels.