Evaluation of local energy dissipation rate using time resolved PIV

Numerous industrial approaches input large quantities of power to achieve a desired outcome. The question is whether that power is most efficiently transformed into process to achieve the specific purpose of heat, mass transfer, mixing etc. To optimize heat/mass transfer, small scale eddies should have maximum energy and they should be close to the interface. In contrast, optimum mixing performance is achieved by large eddies distributed uniformly over entire volume of contactor. In summary, the 'Energy budget'; i.e. how the supplied energy is distributed between eddies of different sizes is very important. Such a distribution is represented by the turbulent energy spectrum. In the current work, we utilize high speed particle image velocimetry (PIV) to record the variation of liquid velocity over a plane in time-resolved fashion. The local energy spectrum is obtained by taking FFT of velocity time series at each point in planar PIV data. A model energy spectrum by Pope ('Turbulent Flows' Cambridge University Press, 2000) was fitted to experimental spectrum giving the energy flux (equivalent to the turbulent energy dissipation rate) at each point. The salient feature of this method is the ability to calculate local energy dissipation rate without using the estimates of velocity gradients. The results obtained in current work are less susceptible to the noise in PIV velocity data. The local energy spectrum and energy dissipation rate measurements will enable us to put a close check on the energy budget in the process equipment. It will allow tailoring the turbulence to the specific application, significantly enhancing the operating efficiency.