Reduced Pressure Environment Research Articles

Fluorescence imaging of plume-surface interaction in large-scale reduced pressure environments

Planar laser-induced fluorescence (PLIF) was applied for the first time in the 20-ft vacuum chamber at the Marshall Space Flight Center to visualize the plume-surface interaction (PSI) of a nitrogen jet seeded with nitric oxide (NO). A Mach 5.3 nozzle was used to simulate the exhaust of a landing spacecraft for two different jet stagnation pressures and one jet stagnation temperature. A flat plate was used to simulate the landing surface, and two different dimensionless altitudes were investigated. The chamber pressure was reduced such that both lunar-relevant environments at 0.01–28 Pa and Martian-relevant environments at ∼600 Pa were investigated. PLIF flow visualization was performed using a pulsed, tunable, ultraviolet laser, which entered the vacuum chamber through a window, and was directed to the test article using remote-controlled mirrors. Fluorescence at ultraviolet wavelengths was imaged using an intensified camera, which was placed inside a pressurized enclosure located inside the vacuum chamber. For the Martian-relevant condition, a Mach disk and stagnation bubble were observed at h/De = 10, whereas a pair of oblique stagnation shocks were observed at h/De = 3. Significantly complex flows, such as different stagnation shock behaviors, were observed for the lunar-relevant conditions based on the h/De and Reynolds number. The results presented here are the first NO-PLIF measurements of the PSI flowfield within rarefied environments. The unique information on jet expansion and plume structure will be useful to aid researchers in validating complex computational simulations and to inform engineering designs of extraterrestrial landing systems.

Vortex Ring Formation Following Shock Wave Diffraction in Low-Pressure Environments

Compressible vortex rings have been widely investigated for decades under ambient atmospheric conditions, and understanding this transient phenomenon is important for improving the thrust vector and avoiding surface impingement and contamination. However, how the vortex ring behaves in a reduced pressure environment remains unknown. This work provides schlieren imaging and pressure measurement results of the vortex ring when the environmental pressure is lower than 1 atm. The basic structure of the compressible vortex ring in low-pressure environments has been captured. The reduced environmental pressure will degenerate the internal flow structure, including the shock wave, the CRVRs, and the vortices due to the Kelvin–Helmholtz instability, which is consistent with the conclusion of previous numerical work. The vortex ring is confirmed to exist when the environmental pressure is approximately 1.0 kPa.

Linking digital image intensity to carrier density in low-pressure corona discharges

The electrification of transportation, and aircraft electrification in particular, is experiencing rapid development due to the more efficient use of energy. Since the dielectric strength of air decreases at the cruising altitudes of commercial aircraft due to the reduced pressure environment, there is a need to control and minimize the risks associated with electrical discharges. This paper shows from experimental and computational data that there is a relationship between the electrical and optical phenomena involved in the discharge process. To this end, this paper analyzes corona discharges generated using different electrode geometries under a wide range of pressures from 100 kPa to 10 kPa. It is shown that the densities of charged particles or charge carriers generated during the discharge process are positively correlated with the intensity of corona images acquired with a digital imaging sensor sensitive to the near UV and visible wavelength ranges. Therefore, the intensity of the images can be used as a reliable and accurate indicator of the corona activity, the values of which are related to the ionization processes involved in the discharges. The results presented in this paper can be applied in various physical and engineering fields, such as high voltage engineering, power line monitoring, or ozone generation, among others.

Zircon U–Pb ages and Hf–O isotopes of Xiaomasa Miocene granite in the Ximeng area, western Yunnan, southwest China: Constraints on petrogenesis and tectonic setting

Characteristic granites of the East Tethyan Tectonic Domain (ETTD) are primarily exposed along the Himalayan orogenic belt and Gangdese batholith in the north, extend from western Yunnan in the centre, and through the Burma batholith to Peninsular Malaysia in the south. Western Yunnan is an important part of the ETTD magmatic arc, featuring characteristic granites that mainly developed between the Jurassic and Paleogene. Nonetheless, compared to other regions of the ETTD, Miocene magmatism has been poorly documented in western Yunnan. The Xiaomasa granite occurs in the Ximeng area of western Yunnan. In this study, we present U–Pb ages, and Hf–O isotope data for zircon crystals from the Xiaomasa granite pluton. Laser Ablation–Multicollector–Inductively Coupled Plasma–Mass Spectrometry (LA–MC–ICP–MS) zircon U–Pb dating yields precise emplacement ages of 15.8–16.1 Ma for the Xiaomasa granite, indicating that Miocene granitic magmatism occurred in western Yunnan. Zircons from the Xiaomasa granite have negative εHf(t) values ranging from −6.6 to −11.6, relatively high δ18O values (6.8%–10.2‰), and two‐stage Hf isotopic model ages (1.51–1.83 Ga). Zircon Hf–O isotopic characteristics indicate that the Xiaomasa granite was mainly derived from partial melting of ancient continental crust. The Ximeng Precambrian metamorphic basement is a probable material source for the granite. The Xiaomasa granite was probably produced by partial melting of the thickened Ximeng ancient metamorphic basement under a reduced pressure environment, reflecting a cumulative effect of the escape of Sundaland caused by the dynamics of the continuous India–Asia collision.

Intermittent flame lengths of buoyant jet flames in reduced and normal pressures: Experiments and theoretical analysis

Quantifying the intermittent flame length scale of buoyant jets is valuable for the understanding of the physical mechanisms of flame stabilization, risk assessment of fuel leakage fire in storage facilities, as well as the practical design of effective industrial burners. In this work, experimental studies are conducted to study the intermittent flame lengths in both reduced (0.64 atm) and normal (1.0 atm) pressures. The intermittent flame lengths are quantified by flame probability contours of propane flame using five nozzles with different diameters ranging from 4 to 10 mm with different heat release rates ranging from 4.31 kW to 49.52 kW. The intermittent flame lengths are found to be positively related to the heat release rate Q˙ and nozzle diameter d. Besides, results show that lower ambient pressure can bring about significantly higher intermittent flame lengths as compared with the results obtained in normal pressure. Moreover, based on physical analysis of the flow character of unburned fuel pressure dependence of flame length scale, a general model coupling with Froude number and nozzle diameter (d) and pressure effect (P∞/1.0atm)2/3 is established as Δlf/d(P∞/1.0atm)2/3=C1Frm−0.4(2−C2Frm−0.6) for the quantification of the intermittent flame lengths in both pressures. And the correlating results show that all the data in both pressures and those from previous literature can be well normalized with a satisfactory agreement. The current work can provide some supplemental knowledge on flame instability in the reduced pressure environment, a scientific evaluation model of fire length scale in fire safety strategies in fuel storage facility leakage cases, and good data reference for CFD validation.

Virtual impaction in compressible flows with pressure recovery

Virtual impactors have been utilized towards classifying and increasing aerosol concentrations in sampling and measurement systems for half a century; in an ideal virtual impactor, sampled particles larger than a critical diameter (known as the cutsize) are directed inertially into a small fraction of the sample flow, known as the minor flow. Concentrating submicrometer particles with virtual impactors while maintaining near-atmospheric pressure downstream in the minor flow remains a challenge, due the low mass-to-drag ratios of these particles. Increased inertial behavior for small mass-to-drag ratio particles can be overcome by increasing flow velocities; however, in most prior work with virtual impaction of submicrometer particles, pressure recovery (keeping the aerosol outlet pressure similar to the inlet pressure) in the receiving tube at high sample flow rates was not investigated. Here, we describe the development and testing of virtual impactors functioning with flow velocities near the sonic limit, capable of submicrometer particle concentration, and with near-complete pressure recovery in the minor flow. We specifically designed, built, and tested two round-nozzle virtual impactors (denoted as VI-1 and VI-2) and evaluated their performances using fluorescent polystyrene latex particles and uranine-doped oleic acid particles with varying particle diameter. The results show that both virtual impactors, operating with 12 L per minute sample flow and a 10:1 sample flow-to-minor flow ratio, have submicrometer cutsizes (∼700 nm and 200 nm for VI-1 and VI-2, respectively) and near-complete pressure recovery. However, significant particle losses were observed in VI-2 with larger particle diameters. To understand such losses and particle trajectories in compressible flow virtual impactors, we also carried out computational fluid dynamics (CFD) simulations coupled with particle trajectory simulations. Importantly, trajectory calculations incorporated a recently developed drag model capable of predicting particle drag coefficients in a wide Mach number and Knudsen number (and hence Reynolds number) range. Trajectory calculations yielded cutsizes and large particle penetration curves in good agreement with experiments, indicating that large particle losses are due to overfocusing (radial inertial motion leading to deposition) within the nozzle and major flow chamber. Overfocusing always leads to an upper size limit in any virtual impactor system, but it is of greater concern in higher velocity systems. In addition to comparing simulations to measurements sampling at atmospheric pressure, we used trajectory calculations to examine compressible flow virtual impactor performance with variable sampling flow pressure and varying the driving pressure in the major flow. Such simulations reveal that sub-atmospheric pressure sampling leads to a reduced penetration “window” for virtual impaction, such that although it is possible for virtual impactor operation in reduced pressure environments, virtual impactors are applicable to a decreasing range of sizes at low pressure.

Modelling the minimally fluidized state under reduced pressure

Several aspects of numerically modelling a minimally fluidized gas–solid system have been investigated in this work. The numerical results show that voidage and the resulting pressure drop are not a function of the fluidizing cycle. More interestingly, the pressure drop was not impacted by introducing the lateral axis of gas and solid flow in the 3D models. Under a reduced pressure environment, none of the well-known drag models could capture the effect of the slip flow. A relatively new but not well-known slip flow drag model showed the ability to capture the impact of the slip flow regime. However, improvements in its overall accuracy are desirable. To this extent, the Ergun pressure drop equation was modified to introduce the effect of the slip flow regime. The losses in the slip flow regime were captured by deriving a new correlation using experimental work that predicted a linear relationship between the laminar coefficient and the Knudsen number. The modified Ergun equation showed notable improvement in its pressure drop accuracy. Furthermore, the modified Ergun equation was implemented as a modified Gidaspow drag model. It showed better accuracy in predicting pressure drop and minimum fluidization velocity at reduced pressure for various alumina particle sizes.

High-temperature evolution behavior of MK silicon resin derived SiOC ceramics under different environments

Silicon oxycarbide (SiOC) ceramics has been prepared by polymethylsilsesquioxane (MK silicon resin) under inert environment at 1000 °C in the present work. The evolution behavior of compositions and structures of MK silicon resin derived SiOC ceramics at high temperatures in different environments were studied systematically. It was found that in inert environment, the rearrangement of Si–C and Si–O bonds took place at 1200 °C–1500 °C and the carobothermal reductions of SiOC ceramics were activated at 1500 °C. While in reduced pressure environment, lower pressure reduced the activation temperature of carbothermal reductions to 1400 °C. In air environment, partial Si[(O,C)4] network oxidation and free carbon oxidation occurred at high temperatures, which would generate a large amount of SiO2 and played a protective role in the interior of SiOC ceramics.

Observation of high-speed droplet splashing under reduced pressure environment

Observation of high-speed droplet splashing under reduced pressure environment

Active flow control using plasma actuators in a reduced pressure environment

This paper proposes a lightweight, energy-efficient airflow control device that uses plasma discharge to realize stratospheric flight. Wind-tunnel experiments were performed in a reduced-pressure environment, and it was noted that plasma can suppress the flow separation around an airfoil, thereby dynamically changing the performance of the airfoil. The results demonstrate that even if the electron mean free path, reduced electric field for the plasma, and the aerodynamic Reynolds number for the stratospheric flight are very different from those at ground level, the plasma is effective in an airflow control device. Moreover, the proposed device operates by simply adhering thin tapes on a ready-made airfoil or hull of the airship and applying voltage to them, which contrasts with the conventionally developed plasma propulsion system. The airflow control technique using plasma will be a key technology in extending human activity to the stratosphere or regions at higher altitudes.

Ion Manipulation in Open Air Using 3D-Printed Electrodes.

Ambient ionization techniques provide a way to sample materials via creation of ions in the air. However, transferring and focusing of these ions is typically done in the reduced pressure environment of the mass spectrometer. Spray-based ambient ionization sources require relatively large distances between the source and mass spectrometer inlet for effective desolvation, resulting in a small fraction of the ions being collected. To increase the efficiency of ion transfer from atmosphere to vacuum, 3D-printed focusing devices made of conductive carbon nanotube doped polymers have been designed and evaluated for ion focusing in air. Three main classes of electrodes are considered: (i) conic section electrodes (conical, ellipsoidal, and cylindrical), (ii) simple conductive and non-conductive apertures, and (iii) electrodes with complex geometries (straight, chicane, and curved). Simulations of ion trajectories performed using the statistical diffusion simulation (SDS) model in SIMION showed a measure of agreement with experiment. Cross-sectional images of ion beams were captured using an ion detecting charge-coupled device (IonCCD). After optimization, the best arrangements of electrodes were coupled to an Agilent Ultivo triple quadrupole to record mass spectra. Observations suggest that electrode geometry strongly influences ion trajectories in air. Non-conductive electrodes also assisted in focusing, due to charge buildup from ion deposition. We also observed minimal spreading of the ion packet after exiting the focusing electrodes indicating that atmospheric collisions do not reduce collimation of the beam. The study suggests that high pressures need not be viewed as a hindrance to ion transport, but as a potentially useful force.

Examination of radiative fraction of small-scale pool fires at reduced pressure environments

Determining flame radiative fraction is necessary in the estimation of radiation heat transfer of a fire, while the radiative fraction for a given fuel is usually regarded as constant, even at sub-atmospheric pressures. To ascertain the factors affecting this parameter, a series of small-scale n-heptane pool fires with diameters of 4–10 cm are performed at low pressures by means of an altitude chamber in this work. Based on a point source model, the radiative fractions of tested pool fires are determined, and coupled effects of pressure and pool diameter on this parameter are observed. A dimensional scaling theory is further proposed to interpret these effects, i.e. Xr~P0.3−0.175αD0.15, and the experimental results can be well correlated with this model. The findings will be helpful in evaluating the radiative heat fluxes of pool fires at reduced pressures more accurately, further providing guidance on the detection of incipient fires at sub-atmospheric pressure environments.

Laser-produced uranium plasma characterization and Stark broadening measurements

This work reports the spatiotemporal diagnostics of uranium species in plasma plumes produced by nanosecond near-infrared laser pulses in a low-pressure environment. Spatially and temporally resolved emission spectroscopy experiments are combined with the modeling of uranium emission for investigating the dynamics of the plume. The Saha-Eggert equation and Boltzmann plots generated from numerous U I transitions are used to infer temperature. This work also reports the measurements of uranium Stark broadening parameters for U I 499.01 nm and U II 500.82 nm transitions. The Stark widths of select U transitions were measured by comparing their linewidths with the broadening of the O I 777.19 nm line. The electron density was found to be of the order of 1016 cm−3, while the temperature was found to be in the range of 3000–9000 K. In addition to enhancing the fundamental understanding of high-Z plasmas in reduced-pressure environments, the knowledge of Stark broadening parameters could improve the modeling capabilities and analytical performance of techniques that rely on emission plasma spectroscopy.

Automatic pressure-assisted dual-headspace gas-liquid microextraction. Lab-in-syringe platform for membraneless gas separation of ammonia coupled with fluorimetric sequential injection analysis

A novel pressure-assisted dual-headspace lab-in-syringe microextraction technique is presented as an alternative approach for automatic on-line membraneless gas separation of volatile compounds. The developed gas-liquid microextraction procedure is based on the lab-in-syringe (LIS) concept by using two independent micro-syringe pumps which are connected to each other for the application of negative and positive pressure inside the common headspace area of the syringe barrels. The adoption of reduced and increased pressure conditions is facilitated by the programmable LIS strategy resulting in increased extraction rates. The analytical process includes the in-situ ammonia vapor generation in the headspace of the first microsyringe, under reduced pressure environment, and its subsequent transportation into the headspace of the second microsyringe. Then, positive pressure is applied inside the second microsyringe enabling the ammonia vapor dissolution into the extraction solution to produce a fluorescent product (isoindol-1-sulfonat). The reaction is time and temperature affected, thus after an optimized time of delay inside the thermostated syringe barrel at 60 °C, it is delivered into the flow-cell of the miniSIA system where it is quantified at 425 nm (excitation wavelength, 365 nm). The proposed preconcentration system has been fully tested and optimized regarding the relevant parameters affecting the generation of gaseous ammonia, its effective transportation into the headspace of the second syringe barrel and its quantitative dissolution and reaction with the extraction solution. For a sample volume of 3000 μL, the sample frequency is 8 h−1, the precision expressed as relative standard deviation (RSD) is 3.6 (at 5.0 μg L−1) and a detection limit (3s) of 0.05 μg L−1 for ammonium is obtained. The detection is linear in the concentration range of 0.15 and 10.0 μg L−1 with a correlation coefficient of 0.9987. The accuracy of the proposed method has been evaluated by analyzing a standard reference material (relative error: 3.8%) as well as using the Certified Method (relative error < 5.5%) for ammonium determination. The potential of this novel schema has been demonstrated for ammonia determination in natural water samples.

Experimental Study on Pyrolysis of Black Non-Charring Polymers in the Reduced-Pressure Environment

A series of experiments on a black non-charring polymer in the low-pressure chamber is conducted under different external heat fluxes. The surface and bottom temperatures and the mass loss of the sample are measured. A parameter T p is introduced to describe the impact of pressure on the surface temperature. There is a loose layer of the char residue left with significant pyrolysis bubbles under the low heat flux, and the bubble size decreases with pressure. The parameter T p is found to exhibit a significant decline trend with increasing pressure, and the mass loss rate of the sample decreases apparently as the pressure increases. However, under a high heat flux, the char residue is denser, and the pyrolysis bubbles are not observed. The value of T p and the mass loss rate of the sample have no obvious relationship with pressure. The average pyrolysis rate is linearly proportional to p a .

Experimental Study on Pyrolysis of Black Non-Charring Polymers in the Reduced Pressure Environment

Experimental Study on Pyrolysis of Black Non-Charring Polymers in the Reduced Pressure Environment

Modeling the pyrolysis study of non-charring polymers under reduced pressure environments

In order to study the pyrolysis of non-charring polymers under reduced pressure environments, a series of experiments based on black acrylonitrile butadiene styrene (ABS) was conducted in a reduced pressure chamber under different external heat fluxes. The temperatures of the top surface and the bottom of the sample and the mass loss during the whole process were measured in real time. A one-dimensional numerical model was developed to predict the top surface and the bottom surface temperatures of ABS during the pyrolysis at different reduced pressures and external heat fluxes, and the model was validated by the experimental data. The results of the study indicate that the profiles of the top surface and the bottom surface temperatures are different at different pressures and heat fluxes. The temperature and the mass loss rate of the sample under a lower heat flux decreased significantly as the pressure was increased. However, under a higher heat flux, the temperature and the mass loss rate showed little sensitivity to the pressure. The simulated results fitted the experimental results better at the higher heat flux than at the lower heat flux.

Effects of vacuum drying on structural changes of bananas, pineapples, and apples

This work aims at investigating structural changes occurring in vacuum drying (VD) compared with conventional convective drying of bananas, pineapples, and apples, and to find the appropriate thin-layer equation for predicting the drying kinetic of products. Drying experiments were carried out at the drying air temperatures of 100 °C, 110 °C, and 120 °C at 4 kPa. The experimental results showed that the VD method at high temperature for all three types of fruits showed the fastest drying time, lowest degree of lightness, lower shrinkage, more pores, more crispness, and more rehydration ability compared with conventional drying and low-temperature drying. Consequently, the VD at drying air temperature of 120 °C was suggested as the best drying condition. Moreover, six mathematical models describing thin-layer drying were investigated. The Logarithmic equation was considered adequate for describing the thin-layer drying behavior of apples, while the Diffusion approach equation was deemed adequate for bananas and pineapples. Practical applications Dried agricultural products serve as important healthful snack items around the world. Vacuum drying (VD) is another process in which products are dried in a reduced pressure environment. The products obtained by the process were of exceptionally high quality. Moreover, none have investigated the effect of VD on structural changes (in terms of shrinkage, rehydration behavior, microstructure, texture, and color) of bananas, pineapples, and apples at drying air temperatures of 100 °C, 110 °C, and 120 °C and absolute pressure of 4 kPa. The result from this research can be applied to the development of dried agricultural product in Thailand.

Experimental Investigation of Droplet Impact on Metal Surfaces in Reduced Ambient Pressure

Impacting water droplets are capable of eroding steam turbine blades, high speed aircraft and even serve as a machining operation. In this latter process, high velocity (∼100m/s) water droplets impact a solid surface under conditions in which the ambient air pressure is sub-atmospheric. The physics of this erosion mechanism is not completely understood. To begin to unravel these physics, we present results pertaining to the effects of ambient air pressure on the impact force of low velocity (1m/s) droplets. It is well known that droplet splash is suppressed when the ambient air pressure is reduced. The effects on the associated impact force are, however, unknown. In this article we examine the impact force of 3.5mm diameter, low velocity droplets in a reduced pressure environment. A 38x38x50cm vacuum chamber fitted with a unidirectional piezoelectric force sensor to measure the transient force of impacting droplets. Preliminary results in atmospheric air pressure reveal that the impulse of the droplets depends linearly on impact velocity while the average impact force depends quadratically on impact velocity. The results under reduced pressures are compared and discussed relative to the underlying physics.

Enhanced microwave catalytic-esterification of industrial grade glycerol over Brønsted-based methane sulfonic acid in production of biolubricant

The industrial production of renewable lubricant glycerol trioleate was simulated using a microwave reactor through esterification of glycerol with oleic acid in the presence of methane sulfonic acid catalyst under solvent-free conditions. The interaction effects of operating temperature, catalyst concentration, and reaction time were investigated. A conversion of 90% was achieved in optimal conditions (191°C, 0.3wt% and 104min reaction time) and reduced pressure environment. This result was impressive, compared to the conversion abounded with conventional reactor (39.5%). Furthermore, two different industrial grade crude glycerols were used as starting materials to evaluate the effect of impurities. The findings confirmed that the presence of impurities reduced both selectivity and total conversion significantly.