The need to know the rate of natural convection heat transfer in the annular region between concentric cylinders is an important engineering problems because of several practical applications which are associated with this basic geometry. There are numerous applications to be found in energy-conversion, energy-storage, and energy-transmission such as nuclear reactors, thermal storage systems, electric transmission cables,solar collectors, and electronic component cooling. This book is designed for use in a standard engineering Natural heat transfer analysis procedure in the annulus passages. Natural convection heat transfer was investigated experimentally and numerically in an inclined open cylindrical annulus to study the effect of the heat flux and the angle of inclination on the heat transfer process.