Mechanical and Aerospace Engineering

A novel flow reactor is proposed for the study of heat-loss effects on the propagation of turbulent premixed flames. This Taylor-Couette flow reactor allows the stabilization of a premixed flame in a stationary, high-intensity turbulent flow and the examination of the role of heat transfer from the flame (to the walls of the apparatus) on the experimentally observed quenching of the premixed flame at high turbulence intensities. The key advantages and design parameters of the flow reactor are identified and discussed.

The purpose of this research was to develop tools for numerical simulations of flame propagation with mesh-free radial basis functions (RBFs). Mesh-free methods offer many distinct advantages over traditional finite difference, finite element, and finite volume methods. Traditional Lagrangian methods with significant swirl require mesh stiffeners and periodic remeshing to avoid excessive mesh distortion; such codes often require user interaction to repair the meshes before the simulation can proceed again.

The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately. This article was published in an Elsevier journal. The attached copy is furnished to the author for non-commercial research and education use, including for instruction at the author's institution, sharing with colleagues and providing to institution administration.

The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately. This article was published in an Elsevier journal. The attached copy is furnished to the author for non-commercial research and education use, including for instruction at the author's institution, sharing with colleagues and providing to institution administration.

The influence of buoyancy on turbulent flame propagation in a Taylor-Couette flow reactor is investigated using dimensional analysis. Based on this analysis, it is concluded that a microgravity environment is needed to fully eliminate buoyancy effects on turbulent-flame structure at high turbulence intensities.

A fully nonlinear evolution equation governing the propagation of a premixed flame through a large-scale spatially periodic shear flow is derived, and steady-state solutions are obtained numerically. The gas density is assumed to be constant across the flame, but the local normal burning speed is allowed to vary with the local strain and curvature along the flame front in order to investigate the influence of the length scale of the external flow on the average propagation speed of the wrinkled flame. At fixed values of the amplitude of the flow-field variations an increase in the length scale (relative to the flame thickness) is found to result in an increase in the average flame propagation speed, in accordance with the predictions of earlier theoretical investigations and with experimental observations for the regime of large-scale turbulence. The propagation speed of the wrinkled flame is calculated to exhibit the experimentally observed bending effect, the tendency of the rate of change of the burning velocity to decrease with increasing turbulence intensity at low fixed turbulence Reynolds numbers. It is shown also how the average flame speed depends on the ratio of the transverse to longitudinal length scale associated with the periodic flow.

A novel flow reactor is proposed for the study of heat-loss effects on the propagation of turbulent premixed flames. This Taylor-Couette flow reactor allows the stabilization of a premixed flame in a stationary, high-intensity turbulent flow and the examination of the role of heat transfer from the flame (to the walls of the apparatus) on the experimentally observed quenching of the premixed flame at high turbulence intensities. The key advantages and design parameters of the flow reactor are identified and discussed.

Laminar, premixed methane-air flames propagating through the annulus of a Taylor-Couette burner were studied experimentally. Such flames ignited at the top, open end of the burner and propagating downward toward the closed end are susceptible to the Darrieus-Landau hydrodynamic instability as well as to thermoacoustic instabilities that develop. Flame speeds measured during a stage of quasiplanar propagation near the middle of the burner attained through action of the primary thermoacoustic instability were found to agree well with laminar-flame speeds reported in the literature. Axial and circumferential velocity fluctuations of the flow field were measured during flame propagation using laser Doppler velocimetry (LDV), and critical acoustic-velocity amplitudes for saturation of the primary thermoacoustic instability and for the onset of the secondary thermoacoustic instability were determined. The influence of the equivalence ratio on the critical acoustic-velocity amplitude for the onset of the secondary thermoacoustic instability determined from the LDV measurements is shown to agree well with theoretical predictions. This influence is consistent with an increase in the stabilizing effect of flame stretch with increasing equivalence ratio for methane-air flames. The Markstein number increases with increasing equivalence ratio for these flames, resulting in growth rates of the secondary thermoacoustic instability for rich methane-air mixtures that are lower than those for lean mixtures with similar laminar-burning speeds.

Markstein numbers characterizing the influence of stretch on the speed of thin laminar flames are determined computationally for lean and rich methane-air mixtures using results from a recent experimental study of thermoacoustic instability in a Taylor-Couette burner. A model describing the response of a thin laminar flame to acoustic flow-field excitation, which is found to exhibit dependence of thermoacoustic-instability characteristics on Markstein number, is used to determine Markstein numbers from measurements of the critical acoustic-velocity amplitudes associated with the onset of the secondary thermoacoustic instability. The model accounts for the Saffman-Taylor influence of viscosity on thermoacoustic instability for adiabatic flame propagation between closely separated walls.

We demonstrate a simple synthetic strategy for the fabrication of single-phase rare earth (RE) doped gadolinium oxide (Gd 2 O 3 :RE where RE = terbium (Tb), ytterbium (Yb), and erbium (Er)) nanorods (NRs) as multimodal imaging probes. The NRs are ultranarrow and exhibit both emission and magnetic characteristics. The Tb-doped and Yb/Ercodoped Gd 2 O 3 NRs exhibit down-and up-conversion fluorescence respectively, and also exhibit paramagnetism. Importantly, these codoped NRs possess excellent magnetic characteristics, as shown in their longitudinal relaxation time (T1) -weighted image contrast, which is closer to that of commercial Gadovist for magnetic resonance imaging (MRI) applications. This property opens up new avenues in the development of contrast agents.

Fluorescent optical probes have been intensively used in the area of bio-imaging. In this review article, we describe the recent advancements in the synthesis and application of bimodal magnetic–fluorescent probes for bioimaging. The bimodal probes consist of fluorescent [semiconducting quantum dots (e.g., CdSe/ZnS) or rare-earth doped (e.g., NaYF4:Yb,Er)] nanoparticles (NPs) and magnetic (iron oxide or gadolinium based) NPs for optical and magnetic resonance (MR) imaging. Microsc. Res. Tech., 2011. © 2010 Wiley-Liss, Inc.

The synthesis and characterization of various rare-earth (RE)-doped and codoped yttrium oxide (Y 2 O 3 ) nanomaterials were undertaken in the current work. RE dopants such as terbium (Tb), europium (Eu), and erbium (Er) were successfully doped, and ytterbium (Yb)-Er was codoped into Y 2 O 3 nanomaterials. The as-synthesized nanomaterials are highly dispersed in the organic phase. X-ray diffraction peaks can be assigned to cubic Y 2 O 3 , which confirms the crystallinity of the as-prepared Y 2 O 3 samples. Room temperature photoluminescence (PL) spectra showed characteristic emission peaks of Tb-, Eu-, and Er-doped Y 2 O 3 samples. Up-conversion spectra of green and red emissions were also shown for Er-Yb-codoped Y 2 O 3 samples. It was found that the intensities of the green and red emissions could be altered by varying the dopant concentration. The nanocrystals were surface-functionalized with an amine (NH 2 ) group via a reverse microemulsion method. The amine groups render the nanocrystals water-soluble and also afford them with the possibility of further functionalization by other biomolecules. in Vitro cytotoxicity studies showed that the synthesized nanocrystals have no appreciable toxicity on human hepatocellular carcinoma (Hep-G2) cells at concentrations of 0.007-0.063 mg/mL. Because of the Y 2 O 3 :RE nanomaterials' well-dispersity in water, low toxicity, and good PL, they can potentially be used as fluorophores in bioimaging.

A new class of fluorescent and superparamagnetic bifunctional nanocrystal has been successfully prepared by a facile, non-hydrolytic method. The synthesized Tb-doped g-Fe 2 O 3 nanocrystals are highly monodispersed with a diameter of 13 nm, show superparamagnetic behaviour with saturation magnetism strength of 30 emu g À1 , and exhibit photoluminescence at room temperature. The nanocrystals are amine-functionalized to render them water-dispersible and ease of further functionalization by other biomolecules. In vitro cytotoxicity tests indicate that these nanocrystals are non-toxic. Nanocrystals of such bifunctionality are envisioned to have potential as probes in integrated imaging technique, of which at least two imaging modalities (for example magnetic resonance imaging (MRI) and fluorescence microscopy) are combined to provide enhanced visualization at tissues and cellular levels.

The present study presents a new approach for evaluating in vitro cytotoxicity of nanoparticles. The approach is based on American National Standard ISO 10993-5. Hepatoma HepG2 and fibroblast NIH3T3 cell lines were incubated with nanoparticles, and their associated extracts were derived at 70 and 121°C. Nanoparticles proposed as potential biomedical imaging probes were evaluated on the basis of the detection of metabolic activities and cell-morphology changes. In general, nanoparticles incubated directly with cells showed higher cytotoxicity than their associated extracts. CdSe and core-shell CdSe@ZnS quantum dots resulted in low cell viability for both cell lines. The cytotoxicity of the quantum dots was attributed to the Cd ion and the presence of the nanoparticle itself. A statistically significant (p < 0.05) decrease in cell viability was found in higher dosage concentrations. Rare earth nanoparticles and their extracts appear to affect NIH3T3 cells only, with cell viability as low as 71.4% ± 4.8%. Magnetic nanoparticles have no observable effects on the cell viabilities for both cell lines. In summary, we found the following: (1) both direct incubation and extracts of nanoparticles are required for complete assessment of nanoparticle cytotoxicity, (2) the rare earth oxide nanoparticles are less cytotoxic than the Cd-based quantum dots, and (3) the extent of cytotoxicity is dependent upon the cell line. © 2009 Wiley Periodicals, Inc. J Biomed Mater Res, 2010

Engineered nanomaterials have become prevalent in our everyday life. While the popularity of using nanomaterials in consumer products continues to rise, increasing awareness of nanotoxicology has also fuelled efforts to accelerate our understanding of the ill effects that different nanomaterials can bring to biological systems. In this study, we investigated the potential cytotoxicity and genotoxicity of three nanoparticles: titanium dioxide (TiO 2), terbium-doped gadolinium oxide (Tb-Gd 2 O 3), and poly(lactic-co-glycolic acid) (PLGA). To evaluate nanoparticle-induced genotoxicity more realistically, a human skin fibroblast cell line (BJ) with less mutated genotype compared with cancer cell line was used. The nanoparticles were first characterized by size, morphology, and surface charge. Cytotoxicity effects of the nanoparticles were then evaluated by monitoring the proliferation of treated BJ cells. Genotoxic influence was ascertained by profiling DNA damage via detection of cH2AX expression. Our results suggested that both TiO 2 and Tb-Gd 2 O 3 nanoparticles induced cytotoxicity in a dose dependent way on BJ cells. These two nanomaterials also promoted genotoxicity via DNA damage. On the contrary, PLGA nanoparticles did not induce significant cytotoxic or genotoxic effects on BJ cells. V

X-ray luminescent nanoparticles (NPs), including lanthanide fluorides, have been evaluated for application to deep tissue in vivo molecular imaging using optical tomography. A combination of high material density, higher atomic number and efficient NIR luminescence from compatible lanthanide dopant ions indicates that particles that consist of ALnF 4 (A = alkaline, Ln = lanthanide element) may offer a very attractive class of materials for high resolution, deep tissue imaging with X-ray excitation. NaGdF 4 :Eu 3+ NPs produced an X-ray excited luminescence that was among the most efficient of nanomaterials that have been studied thus far. We have systematically studied factors such as (a) the crystal structure that changes the lattice environment of the doped Eu 3+ ions within the unit cell; and extrinsic factors such as (b) a gold coating (with attendant biocompatibility) that couples to a plasmonic excitation, and (c) changes in the NPs surface properties via changes in the pH of the suspending mediumall with a significant impact on the X-ray excited luminescence of NaGdF 4 :Eu 3+ NPs. The luminescence from an optimally doped hexagonal phase NaGdF 4 :Eu 3+ nanoparticle was 25% more intense compared to that of a cubic structure. We observed evidence of plasmonic reabsorption of midwavelength emission by a gold coating on hexagonal NaGdF 4 :Eu 3+ NPs; fortunately, the NaGdF 4 :Eu 3+ @Au core−shell NPs retained the efficient 5 D 0 → 7 F 4 NIR (692 nm) luminescence. The NaGdF 4 :Eu 3+ NPs exhibited sensitivity to the ambient pH when excited by X-rays, an effect not seen with UV excitation. The sensitivity to the local environment can be understood in terms of the sensitivity of the excitons that are generated by the high energy X-rays (and not by UV photons) to crystal structure and to the surface state of the particles.