Joonkon Kim

Instability-induced extinction of diffusion flames established in the stagnant mixing layer

Nonlinear dynamics of diffusional-thermal instability in diffusion flames is numerically investigated by employing a diffusion flame established in the stagnant mixing layer as a model. Particular attention is focused on the pulsating-instability regime, which arises for Lewis numbers sufficiently greater than unity. Once the steady flame structure is obtained for a prescribed value of the initial Damkohler number, transient evolution of the flame is calculated after a finite amount of the Damkohler-number perturbation is imposed on the steady flame. Depending on whether the initial Damkohler number is greater than the bifurcation Damkohler number or not, evolution of the transient flame structures can be differently characterized. If the initial Damkohler number is smaller than the bifurcation Damkohler number, pulsating instability can be triggered without any external perturbations, while if the initial Damkohler number is greater than the bifurcation Damkohler number, flame oscillations can be amplified only for the perturbed Damkohler number smaller than the threshold Damkohler number. Therefore, character of the nonlinear instability is subcritical. Once the oscillation amplitudes grow too large, flames are eventually led to extinction. Locus of the threshold Damkohler number is presented, which could be used as a revised extinction criterion for diffusion-flamelet library in the laminar flamelet regime of turbulent combustion.

Structure and acoustic-pressure response of hydrogen-oxygen diffusion flames at high pressure

The flame structures and extinction characteristics of undiluted hydrogen–oxygen strained diffusion flames at high pressures with detailed and reduced chemistry are numerically studied with an intention of application to acoustic instabilities of rocket engines. The numerical results of extinction strain rate for undiluted hydrogen–oxygen flames are found to be qualitatively different from those for hydrogen–air flames in that extinction strain rate for undiluted hydrogen–oxygen flames increases linearly with pressure up to 100 atm whereas extinction strain rate for hydrogen–air flames saturates around 50 atm. Comparison of the characteristic flow time with the characteristic chemical time shows that extinction strain rate varies linearly with pressure for flames controlled by two-body chain-branching reactions that are found to be dominant up to 100 atm. The four-, three-, and two-step reduced mechanisms are also tested to exhibit that the linearity of the extinction strain rate with pressure is preserved. Based on these results, the asymptotic methods, previously used in low-pressure hydrogen–air flames, can be extended to predict the asymptotic structure of hydrogen–oxygen flames at high pressures. On the other hand, the fall-off effect and real-gas effect are found to be minimal on extinction characteristics. Since the characteristic flow time is estimated to be several orders of magnitude shorter than the characteristic acoustic time in rocket engines, acoustic responses are satisfactorily reproduced from the quasisteady flame structures. Finally, the sensitivity analysis identifies the reaction step, H + O2 + M → HO2 + M as a favorable reaction path to stabilize acoustic oscillations by depressing the reaction rate.

Silicon nanodisk array design for effective light trapping in ultrathin c-Si

The use of ultrathin c-Si (crystalline silicon) wafers thinner than 20 μm for solar cells is a very promising approach to realize dramatic reduction in cell cost. However, the ultrathin c-Si requires highly effective light trapping to compensate optical absorption reduction. Conventional texturing in micron scale is hardly applicable to the ultrathin c-Si wafers; thus, nano scale texturing is demanded. In general, nanotexturing is inevitably accompanied by surface area enlargements, which must be minimized in order to suppress surface recombination of minority carriers. In this study, we demonstrate using optical simulations that periodic c-Si nanodisk arrays of short heights less than 200 nm and optimal periods are very useful in terms of light trapping in the ultrathin c-Si wafers while low surface area enlargements are maintained. Double side texturing with the nanodisk arrays leads to over 90% of the Lambertian absorption limit while the surface area enlargement is kept below 1.5.

Structure of the edge flame in a methane–oxygen mixing layer

The structure of an edge flame in a mixing layer of two uniformly flowing pure CH4 and pure O2 streams has been investigated numerically by employing a detailed methane–oxidation mechanism in order to investigate the influence of using pure oxygen, instead of air, as the oxidizing agent. The results exhibited similar behaviour to the CH4-air counterpart in the premixed-flame front, through which the carbon-containing compound leaked mainly in the form of CO and H2 instead of HC compounds. However, while passing through the rich premixed-flame region, the most pronounced distinction of using pure oxygen was that the major route for CO production, in addition to the direct CH4 decomposition, is C2H m compound formation followed by their decomposition into CO, thereby giving continuous CO production, contrary to the rich CH4-air premixed-flame region in which CO consumption existed. In the downstream region from the rich premixed flame front, CO is further oxidized into CO2 in a broad diffusion-flame-like reaction zone located around the moderately fuel-rich side of the stoichiometric mixture by the OH radical produced from the oxygen leakage from the fuel-lean premixed-flame front. Since the secondary CO production through C2H m decomposition has a relatively strong reaction intensity, an additional heat-release branch appears and the resulting heat-release profile can no longer be treated as a tribrachial structure.

Vertical Bridgman techniques to homogenize zinc composition of CdZn Te substrates

In order to improve the Zn homogeneity along the axial direction of CdZnTe boule, we have employed a modified Bridgman technique using a (Cd, Zn) alloy source in communication with the melt, whose temperature has been gradually changed from 800 to 840°C during growth. Electron probe microanalysis (EPMA) measurements of Zn composition in the boule shows an excellent homogeneity of Zn along the axis of the CdZnTe boule compared with results in a boule grown by using a fixed source temperature. We have performed a numerical simulation to obtain the approximate temperatures of additional heating and cooling needed to improve the radial Zn homogeneity. CdZnTe boule has been grown by seeded vertical Bridgman furnace with two zones of heater and cooler. Ultraviolet/visible spectroscopic measurements of Zn composition over the length of the boule indicate that the radial distribution of Zn composition is very homogeneous in the body region of the boule, where the radial variation of Zn composition is ±0.0005.

Slider liquid phase epitaxial growth of Hg0.8Cd0.2Te, Hg0.7Cd0.3Te and Hg0.3Cd0.7 Te with precise control of alloy compositions

A slider liquid phase epitaxial (LPE) technique, by which the solution composition can be kept constant during growth, has been developed to grow Hg1− xCd ϰ Te epi-layers with ϰ = 0.2, 0.3 and 0.7. A graphite button between the solution and HgTe wells in the slider boat is shown to be very effective in controlling Hg loss from the solution. Data are presented on compositional uniformity and reproducibility, surface morphology, and electrical properties.

Simulation of a diffusion flame in turbulent mixing layer by the flame hole dynamics model with level-set method

The partial quenching structure of diffusion flames, arising from the phenomenon of turbulent flame lift off, is investigated in a turbulent mixing layer by the method of flame hole dynamics. Modification of the flame hole dynamics by including the level-set method is specifically aimed to properly take into account the effect of slow flame-edge response near the crossover scalar dissipation rate at which the edge propagation speed vanishes. Simulating the flame hole dynamics with the level-set method results in two major improvements. The first improvement is observed in stabilizing lifted turbulent diffusion flames. The three necessary conditions for lifted stabilization are proposed and numerically tested to show that rapid variation of the edge propagation speed near the crossover scalar dissipation rate helps lifted stabilization. Secondly, an improvement in the statistical properties of the stationary turbulence reacting state is observed in that (1) the lift-off height is found to be higher because of the streamwise flow pushing the turbulent edge front to the downstream direction and (2) the partial burning probability, conditioned with the scalar dissipation rate, exhibits a realistic smooth transition across the crossover scalar dissipation rate.

Modelling of lifted turbulent diffusion flames in a channel mixing layer by the flame hole dynamics

The partial quenching structure of turbulent diffusion flames in a turbulent mixing layer is investigated by the method of flame hole dynamics as an effort to develop a prediction model for the turbulent flame lift off. The essence of the flame hole dynamics is derivation of the random walk mapping, from the flame-edge theory, which governs expansion or contraction of the quenching holes initially created by the local quenching events. The numerical simulation for the flame hole dynamics is carried out in two stages. First, a direct numerical simulation is performed for a constant-density fuel–air channel mixing layer to obtain the background turbulent flow and mixing fields, from which a time series of two-dimensional scalar-dissipation-rate array is extracted. Subsequently, a Lagrangian simulation of the flame hole random walk mapping, projected to the scalar dissipation rate array, yields a temporally evolving turbulent extinction process and its statistics on partial quenching characteristics. In particular, the probability of encountering the reacting state, while conditioned with the instantaneous scalar dissipation rate, is examined to reveal that the conditional probability has a sharp transition across the crossover scalar dissipation rate, at which the flame edge changes its direction of propagation. This statistical characteristic implies that the flame edge propagation instead of the local quenching event is the main mechanism controlling the partial quenching events in turbulent flames. In addition, the conditional probability can be approximated by a heavyside function across the crossover scalar dissipation rate.

Enhanced efficiency of crystalline Si solar cells based on kerfless-thin wafers with nanohole arrays

Several techniques have been proposed for kerfless wafering of thin Si wafers, which is one of the most essential techniques for reducing Si material loss in conventional wafering methods to lower cell cost. Proton induced exfoliation is one of promising kerfless techniques due to the simplicity of the process of implantation and cleaving. However, for application to high efficiency solar cells, it is necessary to cope with some problems such as implantation damage removal and texturing of (111) oriented wafers. This study analyzes the end-of-range defects at both kerfless and donor wafers and ion cutting sites. Thermal treatment and isotropic etching processes allow nearly complete removal of implantation damages in the cleaved-thin wafers. Combining laser interference lithography and a reactive ion etch process, a facile nanoscale texturing process for the kerfless thin wafers of a (111) crystal orientation has been developed. We demonstrate that the introduction of nanohole array textures with an optimal design and complete damage removal lead to an improved efficiency of 15.2% based on the kerfless wafer of a 48 μm thickness using the standard architecture of the Al back surface field.

Fabrication of thin diamond membranes by using hot implantation and ion-cut methods

A thin (2 μm) and relatively large area (3 × 3 mm2) diamond membrane was fabricated by cleaving a surface from a single crystal chemical vapor deposition (CVD) diamond wafer (3 × 3 mm2× 300 μm) using a hot implantation and ion-cut method. First, while maintaining the CVD diamond at 400 °C, a damage zone was created at a depth of 2.3 μm underneath the surface by implanting 4 MeV carbon ions into the diamond in order to promote membrane cleavage (hot implantation). According to TEM data, hot implantation reduces the thickness of the implantation damage zone by about a factor of 10 when compared to implanting carbon ions with the CVD diamond at room temperature (RT). In order to recover crystallinity, the implanted sample was then annealed at 850 °C. Next, 380 keV hydrogen ions were implanted into the sample to a depth of 2.3 μm below the surface with the CVD diamond at RT. After annealing at 850 °C, the CVD diamond surface layer was cleaved at the damage-zone due to internal pressure from H2 gas arising fro...