Preheat Effects Research Articles

Turbulent Burning Velocity of High Hydrogen Flames

Abstract The turbulent burning velocity (ST) is one of the most important combustion properties controlling combustor operability limits, directly influencing blowoff, flashback, and combustion instabilities. Hydrogen has particularly significant influences on the turbulent flame speed. This paper presents new H2/CH4 data of high pressure, high hydrogen turbulent burning velocities. The datasets were designed to address fundamental questions as well as provide engineering/design relevant insights. This paper presents new scaling analysis of fuel composition, pressure, and preheat temperature effects on turbulent burning velocity. We also discuss the importance of considering what is being held constant (temperature, flame speed, Reynolds number, etc.) when one is analyzing these sensitivities. Data show that hydrogen fraction and pressure cause an increase in turbulent flame speed; whether quantified as raw ST,GC or normalized as ST,GC/SL,0 or ST,GC/SL,max (where SL,0 and SL,max are the unstretched and stretched laminar flame speed, respectively). We also propose that observed increases in ST,GC with pressure are due to increases in Reynolds number and not a kinetics/stretch sensitivity effect. With increasing preheat temperature, ST,GC increases while its normalized value (ST,GC/SL,0 and ST,GC/SL,max) can either increase or decrease, depending upon fuel composition. We also show how these sensitivities vary, depending on whether these comparisons are made at constant raw turbulence intensity urms or at constant normalized urms/SL,0 or urms/SL,max.

Investigation of preheat induced degradation on material compression for double shock drive under different picket power

Material compressibility and entropy is of great significance for inertial confinement fusion research. Preheat induced degradation on material compression under double shock compression is studied on ShenGuang series laser facility. The deviations on rear surface expansion and multiple shock timing are contrasted under different picket power. The measured preheat besides shocks could provide more physical boundaries and benchmarks from experiment. 1% silicon dopants is mandatory to keep ablator/fuel interface stable with and in a double-shock compression. While pure GDP ablator is applicable to . This study demonstrates a new testbed to provide in-situ preheat evaluation for implosion experiment. It aims to validate the preheat level and effects on material compression for a double shock, low convergence ratio design. It could also provide valuable information for target dopant design and upcoming shock tuning process.

Conceptual design of a Rayleigh–Taylor experiment to study bubble merger in two dimensions on NIF

A preliminary design of an experiment meant to investigate the evolution of multimode Rayleigh–Taylor instability (RT) is presented. This experiment is intended to provide a direct measurement of the two-dimensional bubble front evolution in the hydrodynamic regime. RT growth for the proposed design has been analyzed using one-dimensional direct numerical simulations in Hyades and models of self-similar behavior. The models predictions are compared to results obtained in two-dimensional Dafna numerical simulations, with good agreement. The proposed design assures a significant bubble-merging process (∼3–4 bubble merger generations). The design takes advantage of the National Ignition Facility (NIF) capabilities to provide a large enough laser spot area (∼0.5–1 cm2), along with a low-enough drive that preheat effects remain small.

Compressible Rayleigh–Taylor Instability with Preheat inInertial Confinement Fusion

The compressible Rayleigh–Taylor instability of accelerated ablationfront is analysed in consideration of the preheat effects, and thecorresponding eigen-problem is solved numerically using thefourth-order accurate two-point compact difference scheme. Both thegrowth rate and perturbation profiles are obtained, and the obtainedgrowth rate is close to the results of direct numerical simulation. Ourresults show that the growth rate is more reduced and the cutoffwave length becomes longer as preheat increases.

Hohlraum X-ray deposition in indirect-drive ICF ablator materials

Accurate measurements of shock timing and ablator x-ray burnthrough will be essential for the successful ignition of an indirect-drive inertial confinement fusion (ICF) capsule. In previous work [1], measurements of ablator shock velocities, shock temperatures, and preheat temperatures were made using a 280nm Streaked Optical Pyrometer (SOP) [2]. The x-ray fluxes were supplied by hohlraums driven by the University of Rochester Omega Laser [3]. More recent ablator experiments at Omega have extended the previous work by using an absolutely calibrated 600-800 nm SOP [4] together with a line-imaging velocity interferometer [5] similar to the diagnostic proposed for accurate National Ignition Facility (NIF) ignition shock timing measurements [6]. Important new information has been obtained relating to ablator surface movement prior to shock breakout, ablator preheat temperature, and preheat effects on the anvil and window components of the shock timing diagnostic system.

Preheat effects on shock propagation in indirect-drive inertial confinement fusion ablator materials.

The velocities and temperatures of shock waves generated by laser-driven hohlraum radiation fields have been measured for several indirect-drive inertial confinement fusion capsule ablator materials. For the first time, a time-resolved measurement of the preheat temperature ahead of the shock front has been performed and included in the analysis. It is found that preheat ahead of the shock front can cause significant shock propagation variations in the ignition capsule ablator materials being considered for the National Ignition Facility (NIF). If unaccounted for, these preheat effects could potentially preclude ignition at the NIF.

An investigation of the action of porcine pancreatic α-amylase on native and gelatinised starches

The action of pancreatic α-amylase (EC 3.2.1.1) on various starches has been studied in order to achieve better understanding of how starch structural properties influence enzyme kinetic parameters. Such studies are important in seeking explanations for the wide differences reported in postprandial glycaemic and insulinaemic indices associated with different starchy foodstuffs. Using starches from a number of different sources, in both native and gelatinised forms, as substrates for porcine α-amylase, we showed by enzyme kinetic studies that adsorption of amylase to starch is of kinetic importance in the reaction mechanism, so that the relationship between reaction velocity and enzyme concentration [E 0] is logarithmic and described by the Freundlich equation. Estimations of catalytic efficiencies were derived from measurements of k cat/ K m performed with constant enzyme concentration so that comparisons between different starches were not complicated by the logarithmic relationship between E 0 and reaction velocity. Such studies reveal that native starches from normal and waxy rice are slightly better substrates than those from wheat and potato. After gelatinisation at 100°C, k cat/ K m values increased by 13-fold (waxy rice) to 239-fold (potato). Phosphate present in potato starch may aid the swelling process during heating of suspensions; this seems to produce a very favourable substrate for the enzyme. Investigation of pre-heat treatment effects on wheat starch shows that the relationship between treatment and k cat/ K m is not a simple one. The value of k cat/ K m rises to reach a maximum at a pre-treatment temperature of 75°C and then falls sharply if the treatment is conducted at higher temperatures. It is known that amylose is leached from starch granules during heating and dissolves. On cooling, the dissolved starch is likely to retrograde and become resistant to amylolysis. Thus the catalytic efficiency tends to fall. In addition, we find that the catalytic efficiency on the different starches varies inversely with their solubility and we interpret this finding on the assumption that the greater the solubility, the greater is the likelihood of retrogradation. We conclude that although α-amylase is present in high activity in digestive fluid, the enzymic hydrolysis of starch may be a limiting factor in carbohydrate digestion because of factors related to the physico-chemical properties of starchy foods.

Rear surface light emission measurements from laser-produced shock waves in clear and Al-coated polystyrene targets

The Nike KrF laser, with its very uniform focal distributions, has been used at intensities near 10 14 W/cm 2 to launch shock waves in polystyrene targets. The rear surface visible light emission differed between clear polystyrene (CH) targets and targets with a thin (125 nm) Al coating on the rear side. The uncoated CH targets showed a relatively slowly rising emission followed by a sudden fall when the shock emerges, while the Al-coated targets showed a rapid rise in emission when the shock emerges followed by a slower fall, allowing an unambiguous determination of the time the shock arrived at the rear surface. A half-aluminized target allowed us to observe this difference in a single shot. The brightness temperature of both the aluminized targets and the non-aluminized targets was slightly below but close to rear surface temperature predictions of a hydrodynamic code. A discussion of preheat effects is given.

Fuel preheat effects on soot-field structure in laminar gas jet diffusion flames burning in 0-g and 1-g

An experimental investigation conducted at the 2.2-s drop tower of the NASA Lewis Research Center is presented to quantify the influence of moderate fuel preheat on soot-field structure within 0-g laminar gas jet diffusion flames. Parallel work in 1-g is also presented to delineate the effect of elevated fuel temperatures on soot-field structure in buoyant flames. The experimental methodology implements jet diffusion flames of nitrogen-diluted acetylene fuel burning in quiescent air at atmospheric pressure. Fuel preheat of ∼100 K in the 0-g laminar jet diffusion flames is found to reduce soot loadings in the annular region, but causes an increase in soot volume fractions at the centerline. In addition, fuel preheat reduces the radial extent of the soot field in 0-g. In 1-g, the same fuel preheat levels have a more moderated influence on soot loadings in the annular region, but are also seen to enhance soot concentrations near the axis low in the flame. The increased soot loadings near the flame centerline, as caused by fuel preheat, are consistent with the hypothesis that preheat levels of ∼100 K enhance fuel pyrolysis rates. The results show that the growth stage of particles transported along the soot annulus is shortened both in 1-g and 0-g when elevated fuel temperatures are used.

NIF Capsule Design Update

We describe several ignition capsule designs, for use in the National Ignition Facility. We compare these designs for ablator efficiency, ignition margin, implosion and stability performance. This study includes capsule designs driven by x-ray drive profiles with both 300 eV and 250 eV peak temperatures. All of the 300 eV designs are tuned to implode the DT fuel in a nearly identical manner. Capsule designs consist of an ablator material (CH with Br dopant; polyimid; Be with Cu dopant; and B4C) encasing a layer of solid DT. The dopants alter material opacities sufficiently to 1) shield the DT fuel from preheat effects; and 2) develop an ablation front density profile favorable to implosion stability. B4C has sufficient opacity at 300 eV that a dopant is not necessary. Issues relating to material properties and fabrication are described.

Dynamic modelling of material and process parameter effects on self-propagating high-temperature synthesis of titanium carbide ceramics

A dynamic, finite-difference model evaluation of titanium carbide (TiC) ceramic processing by self-propagation high-temperature synthesis (SHS) has revealed that material and process parameters have a significant influence on SHS reaction propagation kinetics. Examination of the effects of Ti∶C stoichiometry variations and the presence of pre-reacted TiC diluents in the initial (Ti+C) reactant powder mix indicates that off-stoichiometric ratios and dilution additions tend to lower SHS reaction velocities. Increasing compact preheat temperatures, lowering powder particle sizes and raising compact packing densities, on the other hand, cause a significant increase in this reaction variable. Model results are supported by experimental studies on dilution, stoichiometry and preheat temperature effects on SHS velocity, and other literature data on packing density and particle size effects on this parameter. Simulations also suggest that effects of these initial conditions are due to their influence on adiabatic reaction temperatures and heat transfer patterns produced during the process. Critical selection of initial material and process conditions thus appears to be of vital importance during SHS processing of TiC ceramics.

Thermal history of ht-9 weldment heat-affected zones during welding

The microstructure in weldments of HT-9, a commercial 12Cr-1Mo ferritic steel, has been observed to vary over short distances as a function of the varying thermal history during welding. The mechanical behavior response of “local” weldment microstructures can be characterized by simulation of the structure in larger usable volumes of material, if the “local” thermal history during welding is known. An empirical equation for describing the thermal history of HT-9 weldment heat-affected zones during gas tungsten-arc welding is presented in this paper. Peak temperatures and cool-down cycles are both predictable at any location in the base metal heat-affected zone for welding of plate under conditions of two-dimensional heat flow anticipated for first wall component fabrication. Power input and preheat effects are included in the equation. The equation is expected to apply to materials having a chemistry and thermal properties similar to HT-9. This includes low activation alloys based on the composition of HT-9.

Experimental scaling laws for ablation parameters in plane target–laser interaction with 1.06 μm and 0.35 μm laser wavelengths

Ablation parameters such as velocity, mass, momentum, pressure, and hydrodynamic efficiency have been investigated with plane targets irradiated in the range 3×1011-1015 W cm−2 with 1 nsec pulses and laser wavelengths of 1.06 μm and 0.35 μm. We show that ablation velocity, ablated mass, and momentum are in good agreement with ablation scaling laws deduced from analytical models taking into account inverse bremsstrahlung absorption below the critical density. Nevertheless, processes such as lateral conduction, hot spot, and preheat effects make inaccurate the comparison between ablation pressures, mass ablation rates, or hydrodynamic efficiencies measured for different laser wavelengths. Laser illumination nonuniformities are transmitted to the target in terms of pressure variations. The harmful consequence of a reduced lateral energy flow in 0.35 μm experiments can eclipse the increasing of ablation pressure and hydrodynamic efficiency.

Velocity measurements by X-ray backlighting of plane A1 targets irradiated with frequency-tripled Nd-laser pulses

The hydrodynamic behavior of thin A1 disk targets irradiated with a 1 ns frequency-tripled Nd-laser pulse is experimentally studied at irradiances ranging from 10 13 W cm -2 to 5 x 10 14 W cm -2 and compared to numerical simulations from a one-dimensional lagrangian hydrodynamic code. Velocity measurements as a function of target thickness — ranging from 0.75 μm to 25 μm — are performed by means of time and space resolved X-ray backlighting. Comparison is done with free surface velocity measurements deduced from visible interferometry and streak shadowgraphy, and with dense target velocities inferred from the double-foil technique. Results are discussed in terms of preheat effects, beam inhomogeneities and energy smoothing.

Study of preheat effects in ND-laser irradiated finite Al disk targets

Energy streaming around and through an Al disk target irradiated with a Nd-glass laser delivering a 0.8-ns pulse is discussed as a function of target thickness, disk diameter, and laser irradiance from 5×1012 to 5×1014 W cm−2. Front and rear surface behaviors of the disk are studied by means of frame and streak shadowgraphy, short-time exposure interferometry, ion calorimetry, and charge collectors. Experimental data clearly show that preheat of the rear side of a 500-μm-diam disk is due to plasma encompassing the target and occurs even at laser irradiance as low as 3×1013 W cm−2. Moreover, it is seen on time-resolved photography of double-disk targets, that the second foil expands before being struck by the first disk (irradiated disk) at laser irradiance as low as 1013 W cm−2. This effect is ascribed to preheating by particle bombardment. Numerical simulations do not match the experimentally observed temporal behaviors for the 500-μm-diam disks even with some preheat included in the hydrodynamic code, but they agree quite closely with the experimental results when the irradiated disk diameter is much larger than the characteristic density gradient length.