Experimental investigation of premixed ammonia combustion at high Karlovitz number conditions

This thesis aims to acquire deep knowledge on turbulent premixed combustion at high Karlovitz (Ka) number conditions with the help of laser-based diagnostic measurements.Considering that ammonia (NH3), as a carbon-free energy carrier, is a promising candidate for replacing conventional fossil fuels in the future, all the investigations in this work were carried out on ammonia/air premixed flames. Different optical diagnostic techniques, including planar laser-induced fluorescence (PLIF), laser Doppler anemometry (LDA) and Rayleigh scattering thermometry, have been employed for the measurement of key species, flow velocity and temperature, respectively.Firstly, experimental research on ammonia/air premixed flames was conducted on the Lund University Pilot Jet burner (LUPJ). The flame structure was visualised through the simultaneous measurement of the temperature field together with NH radical distribution or with NO distribution. Five stoichiometric flames withKarlovitz (Ka) numbers ranging from 274 to 4720 were studied. The NH layer, used as a marker of fuel consumption layer, remains thin at the burner exit, but becomes progressively thickened along the flame height with increasing turbulent intensity (u'/SL) when the Ka is higher than 1900. This thickness increase is attributed to the penetration of the small eddies and the merging of flame branches. The NO pollutant, mainly generated in the reaction zone, was observed to exist in a wide region, across the whole flame, because of the turbulent diffusivity and the flow convection.Limited by the geometric scale, the turbulent Reynolds number (Ret) in the LUPJ flame is much smaller than the operational ranges in industrial applications. For a better understanding of highly turbulent premixed combustion, a DRZ (distributed reaction zone) burner was introduced. This burner has integral scales (l0) between 30 – 40 mm and wider working conditions with a maximum turbulent intensity (u'/SL) and Karlovitz (Ka) number up to 240 and 1008. OH-/NH-PLIF and LDA measurements have been carried out to expand fundamental understanding. The results show that NH layer thickness remains almost constant and independent of turbulent intensity (u'/SL), although all the cases are located in the distributed reaction zone regime (Ka > 100). The turbulent eddies can only wrinkle the flame surface instead of penetrating into the inner structures. The ratio of turbulent to laminar burning velocity (ST/SL) increases monotonously with the Karlovitz (Ka) number. All the signs indicate that the flamelet theory is still applicable for ammonia premixed combustion at high Karlovitz (Ka) number conditions, which is inconsistent with Peters’ assumption in the Borghi-Peters diagram

MTR4 drives liver tumorigenesis by promoting cancer metabolic switch through alternative splicing.

The metabolic switch from oxidative phosphorylation to glycolysis is required for tumorigenesis in order to provide cancer cells with energy and substrates of biosynthesis. Therefore, it is important to elucidate mechanisms controlling the cancer metabolic switch. MTR4 is a RNA helicase associated with a nuclear exosome that plays key roles in RNA processing and surveillance. We demonstrate that MTR4 is frequently overexpressed in hepatocellular carcinoma (HCC) and is an independent diagnostic marker predicting the poor prognosis of HCC patients. MTR4 drives cancer metabolism by ensuring correct alternative splicing of pre-mRNAs of critical glycolytic genes such as GLUT1 and PKM2. c-Myc binds to the promoter of the MTR4 gene and is important for MTR4 expression in HCC cells, indicating that MTR4 is a mediator of the functions of c-Myc in cancer metabolism. These findings reveal important roles of MTR4 in the cancer metabolic switch and present MTR4 as a promising therapeutic target for treating HCC

The effect of the interaction of sleep onset latency and age on ischemic stroke severity via inflammatory chemokines

ObjectiveProlonged sleep onset latency (PSOL) and age have been linked to ischemic stroke (IS) severity and the production of chemokines and inflammation, both of which contribute to IS development. This study aimed to explore the relationship between chemokines, inflammation, and the interplay between sleep onset latency (SOL) and age in influencing stroke severity.MethodsA cohort of 281 participants with mild to moderate IS was enrolled. Stroke severity was assessed using the National Institutes of Health Stroke Scale (NIHSS), and SOL was recorded. Serum levels of macrophage inflammatory protein-1alpha (MIP-1α), macrophage inflammatory protein-1beta (MIP-1β), monocyte chemoattractant protein-1 (MCP-1), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-α) were measured.ResultsNIHSS scores of middle-aged participants with PSOL were significantly higher than those with normal sleep onset latency (NSOL) (p = 0.046). This difference was also observed when compared to both the elderly with NSOL (p = 0.022), and PSOL (p < 0.001). Among middle-aged adults with PSOL, MIP-1β exhibited a protective effect on NIHSS scores (β = −0.01, t = −2.11, p = 0.039, R2 = 0.13). MIP-1α demonstrated a protective effect on NIHSS scores in the elderly with NSOL (β = −0.03, t = −2.27, p = 0.027, R2 = 0.12).ConclusionThis study reveals a hitherto undocumented association between PSOL and IS severity, along with the potential protective effects of MIP-1β in mitigating stroke severity, especially among middle-aged patients

高Ka 数下分子扩散效应对氨气/氢气/空气预混火焰结构的影响

Simultaneous planar laser induced fluorescence (PLIF) and Rayleigh scattering thermometry (RST) were applied to measure the key species and temperature fields of premixed ammonia/ hydrogen/air jet flames to investigate the effects of differential diffusion on flame structure. NH-PLIF technique was developed to properly characterize the reaction zone of ammonia flames. Three flames with similar laminar combustion characteristics but different Lewis numbers (Le) were investigated. Results show that the reaction zone are locally thickened for all flames at high Karlovitz number (Ka). Furthermore, the reaction layer thickness increases with the Le, indicating that the differential diffusion still plays a role in the turbulent combustion even in the distributed reaction zone regime

Planar laser-induced imaging of CH3 for high resolution single-shot reaction-zone visualization in premixed methane/air flames over broad stoichiometric ratios

We report a novel approach for single-shot planar imaging of CH3 radicals in premixed methane and air flames. A 213 nm beam from the 5th harmonic of an Nd:YAG laser was resonantly absorbed by the CH3 radicals, which were excited to the highly pre-dissociative upper level and dissociated to H2 and CH (X), as the main dissociation channel. The CH radicals were consequently excited by a 388 nm beam from an alexandrite laser, and the fluorescence from the excited CH radicals was collected off-resonant at 431 nm. Using this Photo-Fragmentation Planar Laser-Induced Fluorescence (PF-PLIF) technique, instantaneous flame front structures, represented by CH3 radicals, can be visualized with high spatial resolution over a broad range of stoichiometric ratios. Signal-to-noise ratios up to 50 were observed for premixed methane/air flame with stoichiometric ratio as low as 0.26. The CH radicals naturally presented in flame front are more than 400 times lower in concentration than the CH3 radicals in premixed methane/air flames even at the conditions close to stoichiometric or slightly fuel rich cases where the highest CH concentrations exist, and the CH3/CH concentration ratios increase dramatically moving towards fuel lean conditions. By adopting a structured illumination of the 213 nm pump beam, the naturally presented CH radicals were visualized simultaneously with CH3 at slightly fuel rich laminar flames, where the CH signal intensity was 5 times lower than that from CH3. The results indicate that the CH3 PF-PLIF technique can provide much stronger signal than the CH PLIF and presented a much promising potential for applications in fuel-lean flames. Finally, the CH3 PF-PLIF was performed in premixed turbulent flames to demonstrate the feasibility of this technique for flame front visualization in turbulent premixed flames

Structure and scalar correlation of ammonia/air turbulent premixed flames in the distributed reaction zone regime

Instantaneous structures of turbulent premixed ammonia/air flames on a piloted jet burner were investigated using simultaneous planar laser-induced fluorescence (PLIF) and Rayleigh thermometry measurements. Two-dimensional spatial distributions of temperature were simultaneously measured with those of the NH radical and the NO pollutant, respectively. Experiments were conducted for stoichiometric flames under five jet velocities. All flames are located in the distributed reaction zone regime of the Borghi-Peters diagram with the Karlovitz (Ka) number ranging from 274 to 4720. The NH PLIF images are used to characterize the fuel consumption layer of the reaction zones since the formation of NH is associated with the consumption of ammonia. The NH PLIF results show that under all flame conditions investigated, the NH layer remains thin in the proximity of the burner while it becomes progressively thickened and distorted by turbulence with increasing turbulent intensity and axial distance. For flames with Ka of 274, the NH layer essentially remains thin, while at Ka of 590 or higher, significant broadening of the NH layer is observed. Probability density functions (PDFs) of the NH layer thickness show that the NH layer can be broadened by 3 – 4 times as the flames are developed downstream. The broadening of the NH layer is considered to indicate that the flames are in the distributed reaction zone regime. The boundary between the thin-reaction zone regime and the distributed reaction zone regime occurs at a much larger Ka than that in methane/air flames. The broadening of the NH layer is due to the penetration of the turbulent eddies and the merging of flame branches. The latter occurs mainly near the flame tips. NO is shown to exist in a wide region in space, across the preheat zone, reaction zone, and postflame zone. NO formation occurs mainly in the reaction zone, however, it is transported by turbulence eddies to the preheat zone and by convection to the postflame zone. The temperature measurements indicate that the preheat zone is broadened in all flames investigated. The broadening of the preheat zone is moderately sensitive to the Ka number while it is more sensitive to the integral length scale of the flames

Non-thermal gliding arc discharge assisted turbulent combustion (up to 80 kW) at extended conditions : phenomenological analysis

Plasma assisted combustion has been proposed as an efficient technique to enhance combustion, especially under the extreme conditions. For shedding light on the interactions between plasma and turbulent flame at extended conditions, a burner design with integrated electrodes was used to couple a non-thermal gliding arc (GA) discharge to a turbulent flame. The morphology and dynamic behaviors of the GA assisted flame under extended flow rates and gas temperatures were investigated by high-speed video imaging. It is found that two distinct types of flame (named as Flame A and Flame B) can be sustained by the GA discharge depending on the local flow conditions. Flame A was sustained by the GA on stable anchor points, while Flame B moved together with the thin plasma volume of the gliding arc, behaving as an unstable flame. When the fed air gas temperature was increased, Flame A became more stable while Flame B became fragile and extinguished easily. Furthermore, the phenomenological findings under different flow conditions imply typical four flame types for the GA discharge assisted combustion system, including the self-sustained flame at relatively low Reynolds number (Re), the GA sustained stable flame at moderate Re number, the GA sustained unstable flame and the GA assisted auto-ignited and propagating flame at relatively large Re number. In all, the GA discharge seems to provide various effects on combustion depending on the overall turbulence as well as the local equivalence ratio, the gas temperature, etc

Flame structure and burning velocity of ammonia/air turbulent premixed flames at high Karlovitz number conditions

This paper presents experimental studies of the structures and burning velocities of premixed ammonia/air jet flames at high Karlovitz (Ka) number conditions. Simultaneous planar laser-induced fluorescence (PLIF) imaging of imidogen (NH) and hydroxyl (OH) radicals was performed to investigate the local flame structure and Laser Doppler Anemometry (LDA) measurements were employed for extracting complement relevant turbulent quantities from the flow field. All the selected cases are located in the regime of distributed reaction zones (DRZ) in the Borghi-Peters diagram, with a maximum Karlovitz (Ka) number and turbulent intensity (u′/SL) up to 1008 and 240, respectively. The OH- and NH-PLIF data were used to determine the flame surface density, flame-surface area ratio, and turbulent burning velocity (ST). The main findings include: (a) The NH layer remains thin and continuous over the investigated range of turbulent intensity and Karlovitz number, and the thickness keeps constant statistically without any significant broadening by turbulent eddies; (b) Spatial correlations of the NH and OH radicals show that overlap of NH and OH layers always exists in a thin region where OH has a weaker signal intensity; (c) The ratio of turbulent to laminar burning velocity (ST/SL) shows a nearly linear increase with turbulent intensity, while the ratio of wrinkled flame surface area to that of ensemble-averaged flame surface area increases only slightly with turbulent intensities. The slower increase of wrinkled flame surface area with turbulent intensity can be attributed to under-resolution in the current state-of-the-art PLIF experiments, the neglection of 3D flame wrinkles in 2D experiments, and the increase in flame stretch factor at high turbulent intensities

Turbulent burning velocity and its related statistics of ammoniahydrogenair jet flames at high Karlovitz number : Effect of differential diffusion

To clarify the role of differential diffusion in highly turbulent premixed flames, a series of turbulent premixed ammonia/hydrogen/air flames were investigated using the NH-PLIF diagnostics. The investigated flames have almost the same laminar burning velocity, SL, but are characterized by different Lewis number, Le, from 0.56 to 1.77. The Karlovitz number, Ka, of these flames ranges from 11 to 1052, and the turbulence intensity, u'/SL, covers from 10 to 156. It is observed that the global consumption speed, ST,GC/SL, of sub-unity Le flames is much larger than that of super-unity Le flames at high Ka, indicating that the differential diffusion plays a significant role in highly turbulent flames. The flame surface density and the area ratio of turbulent flames with different Le are, however, similar under wide turbulent conditions. The stretch factor of sub-unity Le flames is estimated to be significantly larger than that of super-unity Le cases. The enhanced ST,GC of sub-unity Le flames is suggested to be attributed to the promotion of local burning rates by the couple effect of differential diffusion and turbulent flame stretch within the flame brush, rather than the enlargement of flame surface area at high Ka. Furthermore, three correlations for the ST,GC were developed based on Damkohler's second hypothesis with consideration of the Le effect. The correlation of ST,GC/SL - (ReTLe-2)0.5 is further validated by using small-scale methane/air and large-scale ammonia/air flames at high Ka, where ReT is turbulent Reynolds number. It suggests that the ST,GC is roughly inversely proportional to the Le, and the differential diffusion effect should be included in the theoretical analysis and numerical simulation of highly turbulent flames