Analysis of the turbulence in the suction chamber of an external gear pump using Time Resolved Particle Image Velocimetry

The use of air bubbles for the analysis of the turbulence in the suction chamber of a gear pump with Time Resolved Particle Image Velocimetry (TRPIV) is considered. It is the first time, as far as the authors know, that the flow inside a gear pump is observed with a non-intrusive technique. Although there are drawbacks of using air bubbles as flow tracers, it is the only option available in this case, since solid particles and water drops could drastically damage the steel-made gears. It is shown that if the bubbles are small enough (around 100 μm), the buoyancy is not a significant problem for the typical velocities involved in the experiments. On the other hand, if the number of particles per interrogation area is between 10 and 20, as is normally suggested in the literature, the modification of the speed of sound and, hence, the danger of having a compressible flow, is also negligible. Using air bubbles as tracers for the TRPIV, the turbulence for three experiments, at different diameter-based Reynolds numbers, is analysed. The time series of the velocity at different distances to the gearing zone of the gear pump are considered, as well as the wavenumber distribution of the density of energy, calculated from the vorticity field. The results show that for high enough Reynolds number the integral timescale of the turbulence, obtained from the autocorrelation function, is approximately the same as the gearing period. Also, the spectral distribution in frequency show an energy transport to large scale, as well as the spectral distribution in wavenumber. This latter shows also an stepper range related to the enstrophy cascade towards the small scales. That suggests the two-dimensionality of the turbulence created in the suction chamber.