| Summary: | 博士 === 國立成功大學 === 航空太空工程學系碩博士班 === 98 === Turbulence modulation is an interaction that occurs in the multiphase flow. Essentially, turbulence modulation modulates an inconsistency between loading particle motion and the eddy fluctuation of turbulence flow which will alter the continuous phase structure. It can used to describe turbulence enhancement or suppression between the particle and gas phases, and is distinguished from turbulence intensity which is traditionally an indication of estimated turbulence variation. Here, an innovative experimental method generates a narrow-distribution mono-dispersed droplet stream, with the particle loaded into a homogeneous turbulence. Moreover, the electro-hydrodynamic (EHD) principle was applied to suppress droplet collision and create a dipole spray in the downstream. The particle size and mass loading ratio (LR) can be precisely controlled by the dipole mono-sized spray. This method can improve the drawbacks of previous experiments, specifically that influence on LR of increasing particle sizes. A phase Doppler anemometer (PDA) was used to measure the dynamic information of particle-laden flow (droplets and tracers). Following the acquisition of the particle dynamic data, several post-processes analysed the effects of particle size and LR on turbulence intensity and turbulence modulation through different programming tools.
The experimental results show that loading mono-sized droplets of 45μm and 60μm at LR=1.1x10-3 enhanced single-phase turbulence intensity by 20% and 9%, respectively. On the other hand, loading 80μm and 95μm at LR=2.3x10-3 enhanced turbulence intensity by 15% and 18%, respectively. For the LR effect, three different LRs (LR=1.1x10-3, 2.3x10-3, 6.4x10-3), with a fixed particle size of 60μm, were observed and the result shows that the fluid turbulence intensity varied from 9% to 3% to -17% as the LRs increased in turn. The wakes produced by large particles cause extreme variation of fluid turbulence intensity in higher particle concentrations. It can be supposed that the higher particle concentration increases the reactive area for energy exchange between the particle and gas phase, and has a modulation effect on fluid turbulence. The combined particle concentration cp and particle surface area can be defined as a parameter (cp x pi x dp2) which represents the effective reactive area. The turbulence intensity follows this parameter linearly in the all experiments. In a larger effective reactive area, fluid turbulence intensity is suppressed, indicating that particle density is important to the study of two-phase flows.
Turbulence modulation quantity , was reviewed from the concept of property transport. However, it’s difficult to obtain turbulence modulation quantity from experiment because of the high post-process threshold. To obtain the velocity cross-correlation, researchers need to know the arrival time, size, and velocity of each particle. One of this study’s contributions is that it reduces this limitation by an optimum strategy of a particle-fluid cross-correlation function, allowing the particle-fluid correlation ＜u’pu’g＞ to be determined and the further analysis of the variation of turbulence modulation quantity in the two-phase flow. Due to instrument limitations, the turbulence modulation quantity was replaced by [＜u’pu’g＞-＜u’gu’g＞]. The experiment results show the larger particles did not easily achieve a dynamic balance in the gas phase, and therefore caused a reduction in ＜u’pu’g＞. Since the larger particles had deterministic characteristics, the [＜u’pu’g＞-＜u’gu’g＞] would stay constant after achieving the balance. Since the characteristic scale of smaller particles is closer to one of gas phase, they interact easily with the gas phase eddy and caused a higher ＜u’pu’g＞. The [＜u’pu’g＞-＜u’gu’g＞] gradually approaches zero downstream, indicating that the energy exchange is complete. For the LR effect, a higher LR implies a larger reactive area, which would benefit the energy exchange between the particle and gas phases, therefore resulting in turbulence modulation.
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