High-Frequency Pulsatile Pipe Flows Encompassing All Flow Regimes

Time-varying pipe flows driven by a harmonically pulsating inlet velocity and spanning all flow regimes have been investigated by means of numerical simulations. The Reynolds number varied from 1000 to 5000 in response to the inlet velocity oscillations. The frequency of the pulsations was varied from 1 to 10 Hz. These frequencies are markedly higher than those previously studied (maximum value of 0.025 Hz). The motivation for the use of the elevated frequency range was engendered by practical applications such as cardiovascular and respiratory systems of mammals in addition to numerous industrial applications. The simulations made use of the modified Menter transitional model. The key conclusion found here is that the use of a quasi-steady model for the prediction of fully developed friction factors is not applicable for the higher frequencies considered here. The deviations between the actual and quasi-steady friction factor values increase markedly with increasing frequency. Backflow occurs near the wall as the flow transists from deceleration to acceleration. This transition gives rise to a change in the sign of the axial pressure gradient. The amplitudes of the pressure oscillations generated by the imposed velocity variations increase markedly with increasing frequency and diminish with increasing downstream distance from the pipe inlet. The effect of modifications of the Menter model was assessed by carrying out separate numerical solutions for the unmodified and modified models. The pressure oscillations corresponding to the respective models were compared, and it was found that the deviations are insignificant.     

Design and optimization of biomass power plant

Among the renewable energy sources, biomass offers some benefits due to its low cost and presumed zero-carbon emission when compared with fossil fuels. However, the moisture content of biomass is often high that lowers its heating value, reduces the combustion temperature and causes operational problems. Because of these, when burning biomass for power generation, biomass is often dried prior to the combustion. To lower the drying cost or to maximize the power output of a biomass power plant, proper heat integration in between the steam power plant and the drying process has to be considered. In this work, heat integration studies are performed to a biomass power plant that burns empty fruit bunches (EFB) as fuel. Composite curves of all studied cases are plotted to visualize the intensity and to identify opportunities of heat integration among the drying and power generation systems. A multi-stage drying process is proposed that employs steam and waste-heat from the power plant and the drying process, respectively. Results of this study show that with proper drying and heat integration, the overall efficiency of a biomass power plant can be significantly improved.