Study of Twisted Tapes and Vortex Generators on the Enhancement of Ultrafine Particle Deposition and Flow Resistance in Air Channel

風道中螺旋扭帶和渦流發生器的超細顆粒沉積强化及流動阻力研究

Student thesis: Doctoral Thesis

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Award date6 Aug 2021

Abstract

Particle-laden two-phase flow is very common and of great significant importance in a wide of engineering industries such as stand-alone air cleaners, heating, ventilation, and air conditioning (HVAC) systems, heat transfer systems, steam generators, chemical process plant, and distillation processes. Especially, aerosols (solid-gas mixtures, such as dust, smoke, smog, fog, or pathogens in the air) not only harm the visibility outdoors but also directly affect the indoor air quality and our health. There are many indoor and outdoor sources for airborne ultrafine particles (UFPs). These airborne UFPs can be biological or non-biological particles, which are smaller than 100 nm. Outdoor UFPs from ambient air also can transport indoors through HVAC systems. It is always beneficial to remove the particles from the airstream.

Fibrous filters have a long history of uses and they are the most used filters. Removing ultrafine particles (UFPs) from airstream is even more challenging. The use of medium-grade fibrous filters is common in most commercial buildings, but studies have shown that these filters are less effective for removing UFPs. High-efficiency fibrous filters have a very high-pressure drop penalty, while they require frequent replacement and are only used for selected built environments with extremely high operating and running costs. Thus, an alternative approach for air cleaning without too much pressure drop penalty must be sought.

This work proposes novel methods to develop a low-energy “filter-less” system for the filtration of UFPs. In the literature, turbulators have been applied to enhance heat transfer very successfully. In theory, turbulence can also promote UFP diffusional deposition. The thesis aims to examine and assess the flow resistance performance and enhancement of UFP deposition in channel airflow with two different kinds of turbulators: twisted tapes (TTs) and vortex generators (VGs). The present project provides a comprehensive analysis of the deposition of nano-sized particles in air channels. Overall, the investigations mainly consist of the following four parts.

First, airborne particle deposition in empty channels was described in three different aspects: (i) Fast-predicted empirical models; (ii) Experiments; (iii) Numerical simulations. Theory and empirical models on airborne particle transport and deposition in channel flows were systematically introduced. Particle penetration tests and pressure drop measurements were conducted in the lab-scaled empty channel (or duct) under both laminar and turbulent conditions. Different particle diameters and Reynolds numbers were considered. Numerical simulations were performed by modeling the airflow and the airborne particles. The airflow was modeled by using the Reynold stress model (RSM) with the correction of turbulent wall-normal velocity fluctuation. A discrete particle model using the Lagrangian approach was employed to predict particle motion. Experimental results for the empty duct were compared with modeled results by empirical equations in the literature, as well as the results of numerical simulations. The results showed the measurements agreed well with the results predicted by empirical equations and numerical results. The results also demonstrated that the deposition velocity of UFP increases with the Reynolds number and scales inversely with the particle size due to the impact of Brownian diffusion and turbulent diffusion.

Second, by considering the particle deposition and flow resistance simultaneously, under the same fanning power for empty ducts and ducts fitted with inserts, the overall particle deposition-hydraulic performance index was established for laminar and turbulent flows, respectively. The friction factor, size-resolved particle penetration efficiencies, and size-resolved particle deposition velocities were determined for the empty duct and ducts with inserts under identified-pumping power conditions. For a single TT configuration, different TT diameters were tested. An active approach was proposed for enhancing UFP deposition. Active (rotating) and passive (stationary) TT were compared in terms of flow resistance and particle deposition performance. The results showed that all inserts could enhance the UFP deposition in duct air flows. A rotating TTs yielded a higher particle deposition velocity than stationary TTs, while a marginal difference in friction factor between static and rotating TTs was seen.

The third part is for testing in a bigger duct fitted with multiple TTs. For filtration of practical larger duct tens of centimeters in size, most particles flow in the core of the pipe and would not be affected. In this regard, multiple TTs and duct partitions were proposed for applications in different channel sizes. A bigger scaled duct was built. Different numbers of TTs, twist ratios, and duct partitioning arrangements were investigated. The results showed the ratios of nanoparticle deposition velocities for ducts equipped with TTs to the empty duct ranged from 1.8 to 13.7, while the corresponding ratios of friction factor varied from 1.4 to 3.2. Besides, the thesis also proposed empirical correlations of friction factor and UFP deposition parameters for ducts fitted with inserts, which are very useful for further engineering applications and comparison.

The fourth part is for studying the VGs’ effect on the hydraulic-particle deposition enhancement performance. This work studied two kinds of VGs: V-shaped rectangular winglet VGs and LVGs (longitudinal vortex generators or called discrete V-winglets). Different pitches and heights of VGs were investigated and compared. Also, the difference between LVGs at common-flow-down and common-flow-up configurations was studied and discussed. Simulations were also conducted to reveal more details of flow patterns and the structure of vortices induced by VGs with different geometries and arrangements. The “method of images” was employed to further explain the vortex movement.

The innovation of this work is to systematically design experiments and computer simulations to study turbulence enhanced by TTs and VGs for UFP deposition for duct air flows. The proposed TTs and VGs are novel technologies for particle removal. This research area has not been explored by others. The reported data is the first set of evidence on the UFP deposition by using TTs and VGs. Furthermore, both a small-scale mock-up and a bigger ducting system with partitions were tested. This further enhances the research value with an applied component. The investigation of flow resistance and UFP deposition would be valuable for developing advanced air cleaning devices or systems, with the features of high filtration performance and less pressure drop.