"INTRODUCTION A diffuser is a section of closed duct along which the mean static pressure of a flowing fluid increases as a result of decreasing kinetic energy of the flow without energy input from an external* source. For subsonic flow this generally requires a duct with a cross-sectional area increasing in the direction of flow@ although an increase in mean static pressure can occur in ducts of other geometries@ such as parallel pipes@ when a distorted velocity profile settles to a more uniform condition. Diffusers are used to match components in internal flow systems by producing an increase in static pressure without further energy input@ by providing an efficient reduction in flow velocity or simply by joining a small area duct to a larger one. For example@ diffusers are used to improve the performance of compressors by converting some kinetic energy to pressure energy (often also called ""flow work"") and to achieve the necessary velocity reduction between compressor and combustion chamber in gas turbines. In air-conditioning systems and elsewhere@ diffusers may be used to reduce the flow velocity and hence losses and noise. For convenience in installation and to reduce cost@ small pipes may be used for transmission of fluids and are then connected to larger working components by a diverging section. In all these cases the adoption of a ""nearer-to-optimum"" design@ as opposed to one that simply satisfies the primary requirements@ can lead to improvements in overall system performance. Inadequate design leads not only to poor pressure recovery@ non-uniformity of exit flow and high energy losses but also to noise and vibration due to fluctuating separation. Furthermore@ adverse effects on the performance of duct components downstream of the diffuser may be caused by separated@ asymmetric or fluctuating flows. This Data Item provides a comprehensive survey of diffuser performance which gives background information for non-specialists and enables specialists and non-specialists to assess the effects of geometrical and flow variables on diffuser performance. More detailed performance data for a range of particular diffuser geometries are given in the following Data Items. Item No. 73024@ Performance of conical diffusers in incompressible flow. Item No. 74015@ Performance in incompressible flow of plane-walled diffusers with single-plane expansion. Item No. 75026@ Performance of circular annular diffusers in incompressible flow. In the present Item@ Section 2 introduces some parameters used to define diffuser performance and Section 3 briefly discusses some analytical performance-prediction methods. Section 4 describes the principal features of performance trends with overall diffuser geometry and includes comments on the effects of cross-sectional shape and surface finish while Section 5 covers the effect of inlet flow conditions on performance. Section 6 summarises briefly some methods that have been suggested for improving diffuser performance@ especially of wide-angle diffusers@ and guides the user to sources of more detailed information. Some of these methods have particular application as remedial measures for diffusers whose performance has been found to be unsatisfactory in service. * External energy input or output may be used to improve diffuser performance@ for example by removal of potentially separating boundary layers."