INTRODUCTION This Data Item presents design recommendations for improving the performance of axial diffusers for incompressible flow. Sections 4 to 11 give@ where possible@ design recommendations and guidance on optimum performance improvement by various techniques in addition to general consideration of the methods. The information given is based on data from a limited range of tests for each method and any extrapolation must be applied with caution. A tentative basis for interpolation or extrapolation is given in Section 2.3. Table 2.1 provides information that will assist in the selection of suitable control methods for various circumstances. For optimum performance a modest wall angle (??lt; 5??s required. Therefore the first consideration when designing a diffuser is to utilise all the available length in order to minimise the wall angle. The effect of wall angle on diffuser performance is discussed in Section 3. Restrictions on length may result in a wide-angle diffuser@ some examples of which are: (i) gas turbine pre-combustor diffusers@ (ii) wind tunnel diffuser sections@ (iii) inlet channels of glycol injection heat exchangers@ (iv) electro-static precipitator pre-entry diffusers. Inlet flow conditions also affect diffuser performance. For optimum performance a uniform (one-dimensional) inlet velocity profile is required. In practice conditions at inlet will not be uniform@ resulting in less than optimum performance. The effects of non-uniform inlet conditions at entry are discussed in Item No. 76027. A comprehensive study of diffusers in incompressible flow is given in the following Data Items. Item No.76027@ Introduction to design and performance data for diffusers 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.