Fouling experiments were carried out with and without pulsation using an aqueous solution of calcium sulphate. The test runs without pulsation showed the expected decrease of the fouling resistance with increasing flow velocity. Test runs with pulsation led to a little improvement of heat transfer which displays a negative value of the fouling resistance. This is caused by small crystals growing on the heat transfer surface which increase turbulence. Optical observations with a CCD-camera showed that single crystals or small crystal clusters that grew at the surface were removed continuously and started growing at another part of the heat transfer surface. The adhesion force between crystals and surface is lower than the hydrodynamic forces acting on the crystals which means that the crystals can be removed. A quasi-stationary state ensues.
Based on these results further investigations were carried out varying the oscillation frequency. Single strokes of higher velocity superimposed on the stationary flow and the time interval were varied. The fouling behaviour of the test runs with short intervals was simular to the experiments with continuous pulsation. Going to longer intervals leads to a limiting value where no difference could be observed between experiments with and without pulsation.