In cases where technical or economic factors (for example size, rotational speed and material/fluid properties) make testing on an object or process infeasible under original conditions, measurements can also be made by means of model testing.
Particularly in the case of hydraulic turbomachinery (centrifugal pumps, water turbines, hydraulic torque converters) model testing can be conducted:
- on reduced scale models,
- and/or with reduced rotational speed,
- and using a substitute fluid as appropriate.
The advantages of model testing are:
- reduced dimensions of the test object and the test facility,
- lower power input/operating costs during testing,
- potentially easier handling of the fluid (e.g. cold water instead of hot water or toxic, flammable fluids),
- greater precision when transferring the results of highly precise model testing than when working with measurements taken under original conditions in the system.
When planning, performing and evaluating model tests and transferring the results to the original-size machine and/or the original operating conditions, the affinity laws must be observed and applied. Key considerations are:
- Geometric similarity must be maintained, including clearance gap widths, and, where possible, surface roughness. Changes in length due to elastic and thermal deformations must also be observed.
- Test results must be converted on the basis of model laws.
- Dynamic similarity cannot usually be fulfilled. If infringement is unavoidable, apply corrections to the converted results, e.g. efficiency scale-up in cases where the Reynolds number equivalence is comprised in model testing and original conditions.
- Cavitation model testing:
observe the fluid properties (germ count, gas content).