Improving cooling system performance, thus, reducing the fan aerodynamic requirement is a key to quieter fans. Relative to an existing gasoline-engined medium truck cooling system, increased top tank temperature permits an airflow reduction estimated to allow an 11 dB reduction in fan noise. Use of a multi-pass radiator in this configuration adds a further 1.5 dB reduction, as estimated in a non-optimized case. Fan aerodynamic and acoustical performance is most significantly improved by reducing blade tip-to-shroud clearance. Fan designs which include integral (rotating) shrouds provide the best aerodynamic performance and least noise. Alternatively, low tip clearance fans using fixed engine-mounted shrouds or radiator-mounted fans (driven by a flexible coupling), will also provide superior performance. Aerodynamic test data provided by manufacturers tends to use tip clearances much smaller than obtainable in practice. Standard test procedures are also unavailable for acoustical testing of engine cooling fans. Consequently, current production fan noise performance is almost completely undocumented. Aerodynamic and acoustical fan test procedures for vehicle applications which reasonably represent installed fans should be developed and performance data on commercially available fans be catalogued to permit rational fan selection and encourage fan development.

"EPA-550/9-81-319"--Technical report standard title page. Includes bibliographical references. Microfiche.