10 research outputs found
Investigation of Spatter Trajectories in an SLM Build Chamber under Argon Gas Flow
Spatter particles ejected from the melt pool during selective laser melting processes can get redeposited on the build plate region and impact final part quality. Although an inert gas flow is used to purge the spattered particles away from the build plate region, some of the spatter particles get redeposited on the plate region leading to increased porosity and surface roughness. In this regard, the current study focuses on the numerical modeling of the interactions between the inert gas flow and spatter particles by using the discrete phase model. A Renishaw AM250 build chamber is used as the base geometry and the flow field within the build chamber is evaluated for various inert gas flow rates and nozzle diameters of 6 mm and 12 mm. For the first time, spatter trajectories are tracked at specific spatter diameters and ejection angles to pinpoint the influence of drag and gravitational forces on the evolution of spatter trajectories. The findings reveal that the spatter particles between 120 and 180 μm diameter travel beyond the build plate only at specific gas ejection angles and gas flow rates (≥750 L/min). Reducing the nozzle diameter to 6 mm increases the inert gas flow velocity in the build region and enhances the range of spatter particles. New correlations are proposed to relate the range of particles and inert gas flow rates, which can be used to identify the spatter diameters, ejection angles, and inert gas flow rates required to transport the particles beyond the sensitive build plate region
Impact of H<sub>2</sub> Blending of Methane on Micro-Diffusion Combustion in a Planar Micro-Combustor with Splitter
An investigation into the non-premixed combustion characteristics of methane in a planar micro-combustor with a splitter was performed. The impact of blending methane with hydrogen on these characteristics was also analyzed. Additionally, the effects of inlet velocity and global equivalence ratio on flame location, flame temperature, combustion efficiency and outer wall temperature were studied for three different fuel compositions: pure methane (MH0), 60% methane with 40% hydrogen (MH40), and 40% methane with 60% hydrogen (MH60)). A heat recirculation analysis of the combustor wall was conducted to determine the amount of heat recirculated into the unburnt gas at various inlet velocities for all three fuel compositions. The results demonstrated that the stability limit of methane in terms of inlet velocity (1–2 m/s) and global equivalence ratio (1.0–1.2) was significantly enhanced to 1–3 m/s and 0.8–1.2, respectively, with the addition of hydrogen. At an inlet velocity of 2 m/s, the flame location of 3.6 mm for MH0 was significantly improved to 2.2 mm for MH60. Additionally, outer wall temperature exhibited a rise of 100 K for MH60 compared to MH0. Furthermore, from heat recirculation analysis, when the ratio of heat recirculated to heat loss exceeded unity, the flame started exhibiting the lift-off phenomenon for all the fuel compositions
Heat Transfer Enhancement in Microchannel Flow: Presence of Microparticles in a Fluid
In the present study, a numerical model was developed for laminar flow in a microchannel with a suspension of microsized phase change material (PCM) particles. In the model, the carrier fluid and the particles are simultaneously present, and the mass, momentum, and energy equations are solved for both the fluid and particles. The particles are injected into the fluid at the inlet at a temperature equal to the temperature of the carrier fluid. A constant heat flux is applied at the bottom wall. The temperature distribution and pressure drop in the microchannel flow were predicted for lauric acid microparticles in water with volume fractions ranging from 0 to 8%. The particles show heat transfer enhancements by decreasing the temperature distribution in the working fluid by 39% in a 1 mm long channel. Meanwhile, particle blockage in the flow passage was found to have a negligible effect on pressure drop in the range of volume fractions studied. This work is a first step towards providing insight into increasing heat transfer rates with phase change-based microparticles for applications in microchannel cooling and solar thermal systems
High-Temperature Experimental and Theoretical Study of the Unimolecular Dissociation of 1,3,5-Trioxane
Unimolecular dissociation of 1,3,5-trioxane
was investigated experimentally
and theoretically over a wide range of conditions. Experiments were
performed behind reflected shock waves over the temperature range
of 775–1082 K and pressures near 900 Torr using a high-repetition
rate time of flight mass spectrometer (TOF-MS) coupled to a shock
tube (ST). Reaction products were identified directly, and it was
found that formaldehyde is the sole product of 1,3,5-trioxane dissociation.
Reaction rate coefficients were extracted by the best fit to the experimentally
measured concentration–time histories. Additionally, high-level
quantum chemical and RRKM calculations were employed to study the
falloff behavior of 1,3,5-trioxane dissociation. Molecular geometries
and frequencies of all species were obtained at the B3LYP/cc-pVTZ,
MP2/cc-pVTZ, and MP2/aug-cc-pVDZ levels of theory, whereas the single-point
energies of the stationary points were calculated using coupled cluster
with single and double excitations including the perturbative treatment
of triple excitation (CCSDÂ(T)) level of theory. It was found that
the dissociation occurs via a concerted mechanism requiring an energy
barrier of 48.3 kcal/mol to be overcome. The new experimental data
and theoretical calculations serve as a validation and extension of
kinetic data published earlier by other groups. Calculated values
for the pressure limiting rate coefficient can be expressed as log<sub>10</sub> <i>k</i><sub>∞</sub> (s<sup>–1</sup>) = [15.84 – (49.54 (kcal/mol)/2.3<i>RT</i>)] (500–1400
K)
Investigation of the Thermal Performance of Salt Hydrate Phase Change of Nanoparticle Slurry Flow in a Microchannel
Computational study was conducted to investigate the thermal performance of water-based salt hydrate S44 nanoparticles as the phase change material (PCM) in a microchannel heat sink. Constant heat dissipation was applied on the top wall of the heat sink. Forced internal convection of the PCM slurry flow was performed through a homogeneous approach. Three thermal performance parameters, including effectiveness ratio, performance index, and Merit number, were used to quantify the cooling performance of S44 for various concentrations of the PCM nanoparticles. The thermal performance of the salt hydrate S44 slurry was also compared with a similar study conducted for lauric acid nanoparticle slurry found in the literature. Specific operating conditions were identified. The salt hydrate S44 would provide better thermal performance than lauric acid, and vice versa. Finally, Nusselt number correlations have been developed for the microchannel PCM heat sink for Reynolds numbers in the range 12.23 to 47.14 and Prandtl numbers in the range 3.74 to 5.30. A design guideline for manufacturing PCM particles and microchannel heat sinks is provided. With this guideline, the heat absorption ability of the heat sink is maximized, and the pumping power and the losses related to the addition of the particles are minimized
Theoretical Study of the Reaction Kinetics of Atomic Bromine with Tetrahydropyran
A detailed theoretical analysis of
the reaction of atomic bromine
with tetrahydropyran (THP, C<sub>5</sub>H<sub>10</sub>O) was performed
using several ab initio methods and statistical rate theory calculations.
Initial geometries of all species involved in the potential energy
surface of the title reaction were obtained at the B3LYP/cc-pVTZ level
of theory. These molecular geometries were reoptimized using three
different meta-generalized gradient approximation (meta-GGA) functionals.
Single-point energies of the stationary points were obtained by employing
the coupled-cluster with single and double excitations (CCSD) and
fourth-order Møller–Plesset (MP4 SDQ) levels of theory.
The computed CCSD and MP4Â(SDQ) energies for optimized structures at
various DFT functionals were found to be consistent within 2 kJ mol<sup>–1</sup>. For a more accurate energetic description, single-point
calculations at the CCSDÂ(T)/CBS level of theory were performed for
the minimum structures and transition states optimized at the B3LYP/cc-pVTZ
level of theory. Similar to other ether + Br reactions, it was found
that the tetrahydropyran + Br reaction proceeds in an overall endothermic
addition–elimination mechanism via a number of intermediates.
However, the reactivity of various ethers with atomic bromine was
found to vary substantially. In contrast with the 1,4-dioxane + Br
reaction, the chair form of the addition complex (<i>c</i>-C<sub>5</sub>H<sub>10</sub>O–Br) for THP + Br does not need
to undergo ring inversion to form a boat conformer (<i>b</i>-C<sub>4</sub>H<sub>8</sub>O<sub>2</sub>–Br) before the intramolecular
H-shift can occur to eventually release HBr. Instead, a direct, yet
more favorable route was mapped out on the potential energy surface
of the THP + Br reaction. The rate coefficients for all relevant steps
involved in the reaction mechanism were computed using the energetics
of coupled cluster calculations. On the basis of the results of the
CCSDÂ(T)/CBS//B3LYP/cc-pVTZ level of theory, the calculated overall
rate coefficients can be expressed as <i>k</i><sub>ov.,calc.</sub>(<i>T</i>) = 4.60 × 10<sup>–10</sup> expÂ[−20.4
kJ mol<sup>–1</sup>/(<i>RT</i>)] cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup> for the temperature range of
273–393 K. The calculated values are found to be in excellent
agreement with the experimental data published previously
Measurements of Positively Charged Ions in Premixed Methane-Oxygen Atmospheric Flames
<p>Cations and anions are formed as a result of chemi-ionization processes in combustion systems. Electric fields can be applied to reduce emissions and improve combustion efficiency by active control of the combustion process. Detailed flame ion chemistry models are needed to understand and predict the effect of external electric fields on combustion plasmas. In this work, a molecular beam mass spectrometer (MBMS) is utilized to measure ion concentration profiles in premixed methane–oxygen argon burner-stabilized atmospheric flames. Lean and stoichiometric flames are considered to assess the dependence of ion chemistry on flame stoichiometry. Relative ion concentration profiles are compared with numerical simulations using various temperature profiles, and good qualitative agreement was observed for the stoichiometric flame. However, for the lean flame, numerical simulations misrepresent the spatial distribution of selected ions greatly. Three modifications are suggested to enhance the ion mechanism and improve the agreement between experiments and simulations. The first two modifications comprise the addition of anion detachment reactions to increase anion recombination at low temperatures. The third modification involves restoring a detachment reaction to its original irreversible form. To our knowledge, this work presents the first detailed measurements of cations and flame temperature in canonical methane–oxygen-argon atmospheric flat flames. The positive ion profiles reported here may be useful to validate and improve ion chemistry models for methane-oxygen flames.</p