Fan Model Test at Varying Ambient Pressure - Efficient Product Validation at Full Scale Reynolds and Mach Number

1.3 Fan Efficiency Metric & Market Surveillance

Common and standardized test rigs for fans operate at standard ambient conditions. An increase of Mach and Reynolds number is realized with increasing rotating speeds, but both dimensionless parameters cannot be changed individually. This is only possible, if the impeller diameter is changed, which leads to higher requirements to the manufacturing process as well as increased measurement uncertainties. The relative roughness of all surfaces and the geometry, including the relative gap, have to be scaled. When changing ambient conditions, this problem disappears. An increase in static pressure causes an increase in density and Reynolds number. The speed of sound and the Mach number are independent of the ambient pressure for ideal gases. With pressure and rotational speed as variable parameters all combinations of Mach and Reynolds numbers are possible within a specific range. The effect of the Reynolds number on performance and efficiency is examinable at constant Mach number. Compressible effects are measureable for increased Mach numbers at constant Reynolds number, because they are small for fans in comparison to Reynolds number effects.
In this paper, the physical background for a variation of ambient conditions are presented, as well as the design of the investigated pressurized test rig. This method is applicable for all types of fan models, which fit into the pressurized chamber. The limits for Mach and Reynolds number are derived, based on power and torque limits and the measurement range of all measurement techniques. The experimental investigations and the results are presented and discussed. An additional advantage of this introduced measurement procedure is the enlarged Reynolds number range without changing the fan model and test rig. Thus, the systematic measurement errors can be reduced. A variation of the gas and its properties allows a further increase in Mach number, which is necessary for a deeper investigation of compressible effects in fans or other turbomachines at subsonic operating conditions.
Summarized, this paper shows the possibilities for fan test rig operations at different ambient pressures. The advantages are the investigation of compressible effects in fans and the enlarged Reynolds number range without changing fan model or test rig.