Unheated Gas Research Articles

Hydrodynamic Response of the Intergalactic Medium to Reionization

Abstract The intergalactic medium is expected to clump on scales down to M ⊙ before the onset of reionization. The impact of these small-scale structures on reionization is poorly understood despite the modern understanding that gas clumpiness limits the growth of regions. We use a suite of radiation-hydrodynamics simulations that capture the Jeans mass of unheated gas to study density fluctuations during reionization. Our simulations track the complex ionization and hydrodynamical response of gas in the wake of ionization fronts. The clumping factor of ionized gas (proportional to the recombination rate) rises to a peak value of 5–20 approximately Δt = 10 Myr after ionization front passage, depending on the incident intensity, redshift, and degree to which the gas had been preheated by the first X-ray sources. The clumping factor reaches its relaxed value of ≈3 by Δt = 300 Myr. The mean free path of Lyman-limit photons evolves in unison, being up to several times shorter in unrelaxed, recently reionized regions compared to those that were reionized much earlier. Assessing the impact of this response on the global reionization process, we find that unrelaxed gaseous structures boost the total number of recombinations by ≈50% and lead to spatial fluctuations in the mean free path that persist appreciably for several hundred million years after the completion of reionization.

Non-diffusive ignition of a gaseous reactive mixture following time-resolved, spatially distributed energy deposition

Systematic asymptotic methods are applied to the compressible conservation and state equations for a reactive gas, including transport terms, to develop a rational thermomechanical formulation for the ignition of a chemical reaction following time-resolved, spatially distributed thermal energy addition from an external source into a finite volume of gas. A multi-parameter asymptotic analysis is developed for a wide range of energy deposition levels relative to the initial internal energy in the volume when the heating timescale is short compared to the characteristic acoustic timescale of the volume. Below a quantitatively defined threshold for energy addition, a nearly constant volume heating process occurs, with a small but finite internal gas expansion Mach number. Very little added thermal energy is converted to kinetic energy. The gas expelled from the boundary of the hot, high-pressure spot is the source of mechanical disturbances (acoustic and shock waves) that propagate away into the neighbouring unheated gas. When the energy addition reaches the threshold value, the heating process is fully compressible with a substantial internal gas expansion Mach number, the source of blast waves propagating into the unheated environmental gas. This case corresponds to an extremely large non-dimensional hot-spot temperature and pressure. If the former is sufficiently large, a high activation energy chemical reaction is initiated on the short heating timescale. This phenomenon is in contrast to that for more modest levels of energy addition, where a thermal explosion occurs only after the familiar extended ignition delay period for a classical high activation reaction. Transport effects, modulated by an asymptotically small Knudsen number, are shown to be negligible unless a local gradient in temperature, concentration or velocity is exceptionally large.

ICONE15-10808 Scaled Experimental Modeling of VHTR Plenum Flows

The Very High Temperature Reactor (VHTR) is the leading candidate for the Next Generation Nuclear Power (NGNP) Project in the U.S. Various scaled heated and unheated gas and water flow facilities were investigated for modeling VHTR upper and lower plenum flows during the decay heat portion of a pressurized conduction-cooldown scenario and for modeling thermal mixing and stratification ("thermal striping") in the lower plenum during normal operation. It was concluded, based on phenomena scaling and instrumentation and other practical considerations, that a heated water flow scale model facility is preferable to a heated gas flow facility and to unheated facilities which use fluids with ranges of density to simulate the buoyancy effect of heating. For a heated water flow lower plenum model, both the Richardson numbers and Reynolds numbers may be approximately matched for conduction-cooldown natural circulation conditions. Thermal mixing during normal operation may be simulated but at lower, but still fully turbulent, Reynolds numbers than in the prototype. Natural circulation flows in the upper plenum may also be simulated in a separate heated water flow facility that uses the same plumbing as the lower plenum model. However, scaling distortions will occur due primarily to the necessity of using a reduced number of channels connected to the upper plenum than in the prototype in an otherwise geometrically scaled model. Experiments conducted in either or both facilities will meet the objectives of providing benchmark data for the validation of codes proposed for NGNP designs and safety studies, as well as providing a better understanding of the complex flow phenomena in the plenums.

Laser-Driven Aerosol Synthesis of Nickel Nanoparticles

Nanoparticles of nickel have been prepared by laser-driven decomposition of nickel carbonyl. In this method, an infrared laser rapidly heats a dilute mixture of nickel carbonyl and a photosensitizer in a carrier gas to decompose the precursor and initiate particle nucleation. To produce nickel nanoparticles, nickel carbonyl was generated in situ from activated nickel powder and CO at room temperature, so that we never maintained any inventory of this highly toxic compound. During the synthesis process, laser heating allows for rapid cooling of the freshly nucleated particles by mixing with unheated gas. By varying the precursor flow rate, laser energy, and unheated gas flow rate to change the residence time, precursor concentration, and reaction temperature, the average particle size can be controlled over a range of primary particle diameters from 5 to 50 nm. The particle size and crystalline structure have been characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen...

Warming of insufflation gas during laparoscopic hysterectomy: effect on body temperature and the autonomic nervous system

Hypothermia is a known side effect of laparoscopic operations. It may increase the sympathetic activity of the autonomic nervous system (ANS), which can be evaluated noninvasively by heart rate variability (HRV). We tested the hypothesis that warming of the delivered CO2 insufflation gas helps to maintain the normal body temperature. Thirty-seven healthy women undergoing laparoscopic hysterectomy were randomized into heated (37 degrees C, n=18) or unheated (24 degrees C, n = 19) gas insufflation groups. Anesthesia was induced with propofol and maintained with sevoflurane in O2-air. Tympanic (ttymp) temperature was recorded before, during and after the operation. Nasopharyngeal (tnaso) temperature was recorded only during operation. Electrocardiograms were recorded and stored to evaluate changes in HRV. The individual changes in HRV were compared after decibel (dB) transformation. A median decrease in tympanic temperatures during the operation was 0.7 degrees C in the heated and 0.3 degrees C in the unheated group (P = 0.01 between groups), and in nasopharyngeal 0.3 degrees C and 0.1 degrees C (P = 0.03), respectively. Preanesthetic tympanic values were reached within 90 min after anesthesia. After dB transformation, HRV high frequency power differed between the groups. It was better preserved in the patients receiving unheated gas. The heating of insufflation gas does not prevent a decrease in body temperature and is thus unnecessary during laparoscopic hysterectomy.

A two-dimensional analysis of laser heat addition in a constant absorptivity gas

The two-dimensional equations of motion describing the interaction between a laser beam and a flowing gas are considered. An implicit numerical scheme is used to solve these equations for unchoked flow through a converging-diverging nozzle. Separate grids are used for the fluid dynamics and the radiation equations. The ef- fects of beam focusing and cross-beam intensity profiles are included. The calculations are based upon real gas properties for all quantities except the gas absorptivity, which is taken as a constant. The solutions contain the expected hot central core region with cool gas near the walls. This results in steep temperature gradients in both the streamwise and cross-stream directions. The absorption zone acts as a blockage in the nozzle causing a nonuniform velocity profile at the inlet and an overall decrease in mass flow. The absorption region also forces the streamlines to move away from the axis of symmetry, although this effect is not strong. ASER thermal propulsion promises to deliver higher specific impulse from a rocket nozzle than can be at- tained with conventional chemical propellants. The laser thruster derives its energy by absorbing an incoming beam of electromagnetic radiation, rather than from the combustion process which is utilized in a conventional system. Because its energy supply is external to the working fluid, the heat added per unit mass in the laser engine is not rigidly limited. Instead, the temperature rise and hence, the specific impluse, depends upon the details of the radiation/gasdynamic in- teraction. This interaction is highly complex and strongly nonlinear. Improved understanding is required if this poten- tially attractive concept is to be developed. Experimental studies must certainly be undertaken (Refs. 1 and 2 describe experiments currently under way), but analytical results are also necessary to guide and interpret the experimental studies and to scale laboratory results to full-scale applications. The general picture of a laser thruster is similar to that depicted in Fig. 1. Both the laser beam and the cool unheated gas enter the nozzle from the upstream end. As the gas flows toward the throat, it is heated by absorption of ra- diant energy, causing it to accelerate. The converging nozzle walls also aid in accelerating the gas. For the space propul- sion application, the nozzle remains choked at all times and the flow becomes supersonic downstream of the throat. Although the mixing and recombination phenomena which take place in the supersonic portion of the nozzle are of im- portance to propulsive performance, they do not affect the absorption process and are considered to be secondary in the present discussion. In an attempt to focus on the radia- tion/gasdynamic interaction, the present analysis considers the unchoked nozzle where the Mach number remains sub- sonic throughout. The unchoked nozzle flowfield contains the essential physics of the absorption process and provides considerable insight into this complex interaction. The exten- sion to choked throat conditions is straightforwa rd and re- quires no additional computational advances. In a practical laser thruster, the absorption heating will probably be restricted to the region near the centerline of the nozzle, leaving a cool, unheated layer of gas near the wall3'4

Analysis of Arc-Heating Phenomena in a Tube

This paper presents an analysis of the flow and energy transport processes in an arc-heating device in which an electric arc and a gas flow pass concurrently through a cylindrical tube. The particular region of interest in the tube is the inlet length over which the hot gas is always separated from the tube wall by a layer of unheated gas. The analysis demonstrates the manner in which the flow parameters in the tube may be computed by the matching of the unheated flow and the heated flow at the thermal boundary for given end boundary conditions. Experimental data obtained with an arc heater are presented in support of the analytical model.