ESDU 93018 presents Nusselt number data in laminar flow for conditions of either uniform wall temperature or uniform heat flux in fully-developed flow, flow that is thermally-developing (where the heated section is preceded by an isothermal length exceeding the hydrodynamic entry length), and flow that is developing simultaneously both thermally and hydrodynamically. The effect of variable fluid properties is considered and a correction for radial viscosity variation in liquids is recommended, but no correction is felt necessary for gases, following an assessment of the available data. For mixed convection conditions only a uniform wall temperature condition is considered. Nusselt number data are given for buoyancy-assisted flow in vertical tubes (heated upflow and cooled downflow) and for buoyancy-opposed flow. Data are also provided for flow in horizontal tubes. The ranges of conditions for which the recommended correlations have been tested are tabulated for the mixed convection conditions. For transitional flow an interpolation function in Nusselt number is suggested to be used between the limiting values in laminar and turbulent flow at Reynolds numbers (based on tube diameter) of 2000 and 8000 respectively. Worked examples illustrate the use of the equations and methods. ESDU 92003 provides guidance on the boundaries between the flow regimes.