ABSTRACT
In this chapter, the technique of dimensional analysis is applied to turbomachines. To this end, the variables of interest are identified, and using the exponent method the various dimensionless groups, also called pi groups, are derived. It is shown that the most relevant groups are the flow coefficient, head coefficient, and power coefficient. By combining the head and flow coefficients, two very useful dimensionless parameters called the specific speed and specific diameter are defined. For all geometrically similar machines, the dimensionless numbers are the same. The value of the specific speed determines the shape of the machine. As the specific speed increases, the shape of the impeller changes from purely radial, to mixed, to purely axial turbomachine types. By plotting the specific speed against the specific diameter, the Cordier diagram is obtained, which is a useful tool for preliminary sizing. Using the concept of model testing, the efficiencies of the model and prototype are related using Moody’s formula.
