Helium Mole Fraction Research Articles

Performance assessment of closed Brayton cycle-organic Rankine cycle lunar base energy system: Thermodynamic analysis, multi-objective optimization

The long lunar night, which cannot be powered by solar energy, brings a huge challenge to the lunar base energy system. Closed Brayton cycle (CBC) system is considered as an effective solution, but cannot be driven by low temperature heat sources. Organic Rankine cycle (ORC) system is used to couple into the CBC system to recover waste heat and produce more electricity. In this paper, a mathematical model of CBC-ORC system driven by collector or heat storage unit is developed, the variation of thermal efficiency, exergy destruction, and Brayton-Rankine rotating unit (BRRU) mass is evaluated during the whole lunar day. Results are as follows: when the helium mole fraction is 0.9, CBC stops on the day of 7.7 at night, which is earlier than the stop time for other helium mole fractions. The maximum power generation can reach 169.21 kW. Thermal and exergy efficiency can reach 34.49% and 31%, respectively. After three-objective optimization, the results of thermal efficiency (30.07%), exergy destruction (169.62 kW) are similar to the basic working condition, and the BRRU mass (720.3 kg) can be extremely reduced by 78.76% compared to the basic working condition, which is essential to the practical applications.

Effect of flow rate of shielding gas on distribution of particles in coaxial double-tube helium atmospheric pressure plasma jet

In the application of atmospheric pressure plasma jet, the influence of ambient gas cannot be ignored, especially in some specific scenarios which are highly sensitive to ambient particles. Coaxial double-tube plasma jet device is a promising method of controlling the chemical properties of jet effluent by restraining the mutual diffusion between jet effluent and ambient gas. In this work, the discharge characteristics and chemical properties of coaxial double-tube helium atmospheric pressure plasma jet at different flow rates of shielding gas are studied numerically, and the model is validated by experimental optical images. The results illustrate the enhanced discharge at the high flow rate, the weaker discharge at the low flow rate, and discharge behaviors without shielding gas as well. With the increase of shielded gas flow rate, the particle density increases in the discharge space, which can be attributed to the wider main discharge channel caused by the increase of shielding gas flow rate. In addition, the analysis shows the great difference in ion fluxes affected by the flow rate of the SG between the contour lines of different helium mole fractions. This study further reveals that different discharge positions have a great influence on the generation of nitrogen and oxygen particles, thus deepening the understanding of influence of shielding gas flow rate on discharge behavior, and may open up new opportunities for the further application of plasma jet.

Combustion enhancement in rearward step based scramjet combustor by air injection at step base

Numerical simulations were performed to model the non-reacting and reacting flow behind a rearward step flameholder in Mach 1.6 supersonic flow with fuel injection at the step base. The combustor geometry was based on the University of Florida scramjet experimental facility. Turbulence was modeled using k-ω shear stress transport (SST), laminar flamelet was used for combustion modeling. Wall static pressure showed good agreement with experimental data for non-reacting and reacting flow. For non-reacting flow, dummy fuel helium mole fraction distribution in the recirculation region behind the step was validated with planar laser induced fluorescence (PLIF) images in experiments. To improve the combustion characteristics, air was injected in tandem with hydrogen at step base using various configurations. With all fuel injection as baseline, the case with 2 air jets around each fuel jet and air injected at 2 times the stagnation pressure of fuel showed the most improvement compared to other cases. It was most effective in reducing the local fuel richness, shortening the flame length and increasing combustion efficiency.

Numerical studies on downstream uniformity of atmospheric pressure plasma jet array modulated by flow and electric multi-field coupling control

Atmospheric pressure plasma jets (APPJs) have potential applications in many aspects ranging from traditional surface treatment to growing biomedicine. An array structure of such APPJs is the most efficient way to enlarge the treatment area. Nevertheless, the majority of APPJ arrays have shown mottled patterns downstream, a disadvantage for applications. Particularly, in biomedicine and certain other areas, improving the plasma homogeneity downstream of APPJ arrays is crucially needed. In this work, we numerically study synergistic effects of APPJ arrays on plasma propagation and homogeneity downstream based on a model coupling electric, flow, and temperature fields. Taking a two-dimensional three-tube APPJ array as an example, we study the influence of gas velocity and component, as well as applied voltages on plasma distributions. In addition, essential strategies for merging plasma bullets are acquired. Results show that the ionization rate between adjacent jets is important to provide electrons for jet merging. The helium mole fraction controls the plasma distribution and thus directly decides whether adjacent jets merge. After merging, the plasma bullets affect each other through the electric field to control the homogeneity downstream. Therefore, the plasma distribution is a result of the synergy of flow and electric fields. Then, a homogeneous plasma distribution downstream can be realized by the fine control of both fields, which provides an effective way to uniform the plasma downstream in plasma processing.

Effect of light gas on condensation heat transfer of steam–air mixture and gas stratification

Condensation has been considered for various industrial applications. However, the characteristics of condensation heat transfer have not been fully elucidated. In this study, we identify the characteristics of condensation heat transfer deterioration due to light gas. We measure the condensation heat transfer coefficient by varying the mass fraction of noncondensable gas and the mole fraction of helium to noncondensable gas. We discuss parameters that can affect the condensation heat transfer coefficient, i.e., the gas density, diffusion coefficient, and mole fraction of helium. Gradual deterioration of condensation heat transfer coefficient by increasing mole fraction of helium to noncondensable gas is observed. Furthermore, conditions for helium stratification under condensation are identified. Models for predicting condensation heat transfer are evaluated by varying an effective diffusion coefficient model based on experimental data.

A method for resolving changes in atmospheric He ∕ N2 as an indicator of fossil fuel extraction and stratospheric circulation

Abstract. The atmospheric He/N2 ratio is expected to increase due to the emission of He associated with fossil fuels and is expected to also vary in both space and time due to gravitational separation in the stratosphere. These signals may be useful indicators of fossil fuel exploitation and variability in stratospheric circulation, but direct measurements of He/N2 ratio are lacking on all timescales. Here we present a high-precision custom inlet system for mass spectrometers that continuously stabilizes the flow of gas during sample–standard comparison and removes all non-noble gases from the gas stream. This enables unprecedented accuracy in measurement of relative changes in the helium mole fraction, which can be directly related to the 4He/N2 ratio using supplementary measurements of O2/N2, Ar/N2 and CO2. Repeat measurements of the same combination of high-pressure tanks using our inlet system achieves a He/N2 reproducibility of ∼ 10 per meg (i.e., 0.001 %) in 6–8 h analyses. This compares to interannual changes of gravitational enrichment at ∼ 35 km in the midlatitude stratosphere of order 300–400 per meg and an annual tropospheric increase from human fossil fuel activity of less than ∼ 30 per meg yr−1 (bounded by previous work on helium isotopes). The gettering and flow-stabilizing inlet may also be used for the analysis of other noble-gas isotopes and could resolve previously unobserved seasonal cycles in Kr/N2 and Xe/N2.

Effects of nitrogen impurity on the atmospheric pressure helium plasma jets exposed to a nitrogen environment

In this study, the effects of small nitrogen impurity on the behaviors of helium APPJs propagating into nitrogen surrounding gas are investigated numerically based on a two-dimensional fluid model. The results show that the addition of N2 impurity can change the APPJ length and there exists an optimal N2–helium ratio which enables the plasma jet to reach the maximum length. This is attributed to the variation of helium mole fraction and the ionization rate in the helium flow channel resulting from N2 impurity. Like the APPJ propagating into ambient air, with the addition of nitrogen, the profile of APPJ exposed to a nitrogen environment varies from an annular shape to a nearly uniform structure, though their reaction chemistries inside the jet effluent differ remarkably. Also, under different impurity concentrations, the density distribution and evolution of active species throughout the effluent of APPJ, as well as their dominant reaction pathways, behave differently. The physics mechanisms underlying these behaviors are discussed in detail.

Effect of hydrogen enrichment of laminar ethylene diffusion flames on thermal structure and soot yields at pressures up to 10 bar

A series of high-pressure experiments were conducted to assess the influence of hydrogen enrichment of laminar diffusion flames of nitrogen-diluted ethylene on the thermal flame structure and soot yields at pressures above atmospheric. In parallel experiments, added hydrogen is replaced by helium, either in equal mole fractions or in mass fractions, to evaluate the thermal, dilution, and direct chemical interaction effects of hydrogen in soot formation. Experiments covered pressures from atmospheric to 10 bar. In the first set of experiments, conducted at 3, 6, and 10 bar pressure, base fuel was an ethylene-nitrogen mixture with 33.3% ethylene and 66.7% nitrogen (by mole as well as by mass). This base fuel was doped with either hydrogen or helium such that hydrogen and helium mass fractions and mole fractions in the fuel stream are matched in two cases. In the second set of experiments, which were conducted at 1.2 bar pressure with ethylene as the base fuel, hydrogen or helium is added such that additive mole fraction in the fuel stream was 44%. Temperature measurements in the first set of experiments indicate that, when hydrogen is added to nitrogen-diluted ethylene, the changes in the temperature field of the co-flow diffusion flames are negligible, except at lower in the flame where hydrogen added flames display slightly higher temperatures. When helium is added instead of hydrogen, however, the temperatures were measurably lower than those of the base fuel. Results show that, once the dilution effects are accounted for, the hydrogen addition to ethylene does not suppress soot formation by direct chemical interaction at elevated pressures. These findings, which are not in agreement with the previous experimental results obtained at atmospheric pressure, are discussed in terms of the higher molecular diffusivity of hydrogen and shorter residence times of high-pressure flames.

Systematic errors in mixing measurements using filtered Rayleigh scattering in supersonic flows

When performing non-reacting mixing enhancement experimental studies in supersonic flow, helium is often used as an injectant which serves as a simulant gas for hydrogen fuel. The resulting distribution of helium mole fraction in the binary mixture of air and helium can be quantified using filtered Rayleigh scattering (FRS). The FRS technique requires two independent experiments in supersonic flow, one with helium injection and the other with air injection. The helium mole fraction is retrieved under the key assumptions that the number density profiles and the extent of the Doppler shift that is associated with each of the two independent experiments at the measuring plane are identically the same. This work is centered on the analysis of the impact of a departure from the aforementioned assumptions with the aid of a reduced-order model developed for a canonical rectangular jet in supersonic flow. The results, derived from the implementation of the model, are used to identify key driving phenomena that concurrently contribute to violate the key assumptions and are compared and discussed in the light of available experimental data. Additionally, the analysis suggests that the newly developed model can be used in the design of FRS experiments by minimizing the extent of mismatch in the number density profile and thus reducing the systematic bias error associated with the mixture’s composition measurements.

Comparison of Intrusive and Non-Intrusive Mixture Composition Measurements in Supersonic Flow

Accurately quantifying mixture composition in supersonic flows is vital to the experimental development of fuel–air mixing enhancement strategies for scramjet propulsion applications. Mixture composition can be experimentally quantified by using either intrusive probe-based techniques or nonintrusive laser-based optical diagnostic techniques, with the different methods each offering unique advantages and disadvantages. To this end, the work presented herein provides a comparison of mixture composition measurements made with an intrusive gas-sampling probe to measurements of the same quantity made with the nonintrusive filtered Rayleigh scattering technique. The measurements are made downstream of a strut injector, which sonically injects helium parallel to a Mach 2.5 airflow, creating a two-dimensional planar shear layer composed of a binary mixture of helium and air. The experimental results show that the helium mole fraction measurements obtained using these two different techniques compare quite favorably. Specifically, at the primary downstream station of interest, all of the points surveyed with the gas-sampling probe lie within the 95% uncertainty bands of the filtered Rayleigh scattering measurements, with an average absolute difference in helium mole fraction of between the two techniques.

Differential heat and mass transfer rate influences on the activation efficiency of laminar flow condensation particle counters

Laminar flow condensation particle counters (CPCs) are uniquely sensitive detectors for aerosol particles in the nanometer size range (i.e. below 10 nm in size they can have single particle sensitivity). Their operation hinges upon the creation of supersaturation of a working fluid; particles exposed to supersaturated vapor grow by condensation to optically detectable sizes. The degree of supersaturation is fully controlled via differential rates of heat transfer and working fluid vapor mass transfer. Because of the Kelvin relationship governed vapor pressure of small particles, in all CPCs there is a critical size/cut-size (diameter), and particles smaller than this size do not grow and are not detected efficiently. While efforts have been made to control the CPC activation efficiency (i.e. the fraction of particles detected as a function of size), prior studies have not examined how differential heat and mass transfer in CPCs are governed by changes in gas composition. Here, we measure and model CPC activation efficiencies (with 1-butanol as the working fluid) in mixtures of gases of disparate thermophysical properties, namely helium and molecular nitrogen. Our experiments show that the activation efficiency of smaller particles (i.e. below 8 nm in the tested CPC) can be increased by adding a modest amount of helium to the aerosol (mole fractions near 0.20). This is expected based upon the increased Lewis number brought about by Helium addition, and supported by predictions of CPC activation efficiency based upon thermophysical property variable models of coupled heat, mass, and momentum transfer within the CPC condenser region. Interestingly, we find that when operating with a constant precision orifice diameter (choked flow), the activation efficiency for a given sub-10 nm particle diameter first increases with increasing Helium mole fraction and then decreases as the Helium mole fraction increases beyond 0.67. In comparison, experiments with constant mass transfer Peclet number (Pem = 77) show an increase in CPC activation efficiency up to a helium mole fraction of 0.67, but then the activation efficiency decreases more modestly beyond this helium mole fraction. We attribute these contrasting results to the increased flowrate through the instrument under constant orifice diameter conditions, which affects the performance of the CPC saturator. Finally, through modeling we show that the ability to enhance the activation efficiency of a CPC via a modest amount of helium addition is general, and can be applied with other heavy working fluids. The results presented in this study elucidate the importance of gas composition and Lewis number controlled differential heat and mass transfer rates on the performance of condensation based nanoparticle detectors.

Reactive Species Production In Argon And Helium Plasma Jets: Reactivity Transfer From Plasma To Liquid Layers

Atmospheric pressure plasma jets are often used in plasma medicine as sources of reactive species in biomedical applications including the treatment of wounds and cancerous tumors. A rare gas or a mixture of a rare gas with a small percentage of oxygen are often used as a plasma-forming gases [1]. The biological responses are attributed to the production of reactive oxygen and nitrogen species. The reactive species generated by the plasma diffuse through a thin layer of a biological liquid which usually covers the treated sample. In this work, we discuss results from computational investigations of properties of a single jet and jets array operated in He and Ar. We quantify densities and fluxes of reactive species produced by jets in the bulk plasma (Figure 1). We then investigate the reactivity transfer from the plasma into the liquid. As this process typically occurs on a large dynamic ranges of timescales, the plasma–liquid interactions and liquid phase chemistry are considered on a short (ns) and long (ms) time scales. This investigation is conducted using the 2D modeling platform nonPDPSIM, which solves transport equations for charged and neutral species, Poisson’s equation for the electric potential, the electron energy conservation equation for the electron temperature and Navier–Stokes equations for the neutral gas flow. The model is essentially the same as used in [2]. The reaction mechanism includes plasma and neutral chemistry for He/humid air and Ar/humid air as well as liquid phase chemistry. Download : Download high-res image (388KB) Download : Download full-size image Figure 1. (a) and (c): Flow patterns for helium and argon jet into ambient air. Numbers at the lines show helium and argon mole fraction. (b) and (d): OH radicals production during a short plasma time scale for helium and argon jet, respectively.

Development of a three-stage looped thermoacoustic electric generator capable of utilizing heat source below 120 °C

Thermoacoustic engine, with the outstanding characteristics of high reliability and environmental benignity, is one of the most attractive and promising energy conversions to recover low-grade thermal energy. In this paper, a three-stage looped thermoacoustic electric generator, consisting of a three-stage looped thermoacoustic engine and a linear alternator, was proposed and built. To obtain a better coupling between the thermoacoustic engine and the linear alternator, the acoustic impedance of the linear alternator has been adjusted by modulating the external electric resistance/compliance and the frequency. The frequency was adjusted by adopting the He-Ar mixture. Additionally, the location and the length of compliance tube have been optimized to improve the acoustic field in the system. Upon optimization, a maximal thermal-to-electric efficiency of 1.51% can be achieved at the hot temperature of 120 °C with He-Ar mixture as the working fluid, in which the mole fraction of helium was 0.63. Furthermore, the system has also been tested in the hot temperature range of 90 °C–200 °C. Results show that the thermal-to-electric efficiency stabilizes at around 1.5% when hot temperature is in the range of 120 °C–170 °C.

Air return ratio measurements at the solar tower Jülich using a tracer gas method

The air return ratio is a key factor for the overall efficiency of the open volumetric receiver concept. Although first measurements of the air return ratio exist for smaller setups of the open volumetric receiver concept, so far no measurement of the air return ratio has been presented for the Solar Thermal Test and Demonstration Power Plant Jülich (≈1.5MW, ≈10(kg air)/s; ≈700°C). This paper describes the application of a tracer gas method at the solar tower Jülich to determine this substantial ARR.As tracer gas the environmentally friendly helium has been chosen. The helium is injected dynamically into the circular air flow of the system and the helium mole fraction is measured using a mass spectrometer. The dynamic concentration response of the system is used to determine the air return ratio. This dynamic method only requires one location of measurement. First measurements with this dynamic method were conducted at the solar tower Jülich. The ARR of STJ was measured with and without irradiation of the main receiver with high accuracy.Under low-wind conditions and without irradiation of the main receiver the air return ratio was measured to be (67.7±0.5)% for an air mass flow of (9.96±0.04)kg/s. A slightly higher air return ratio of (68.6±0.7)% was measured under irradiation with an air mass flow of (9.94±0.04)kg/s. The air return ratio was sensitive to the air mass flow, showing significantly lower rates when moving further away from the 10kg/s design air mass flow to 5kg/s.

Experimental investigation of the auxiliary gas circuit of a diffusion absorption chiller with natural and forced circulation

The auxiliary gas circuit has a significant influence on the cooling capacity and the efficiency of a diffusion absorption chiller. Nevertheless, there are only a few theoretical studies and experimental investigations concerning this topic. The reason is the difficulty to gain measurement data of the volume flow rate and the partial pressures without affecting the natural circulation of the auxiliary gas circuit negatively.This paper presents a detailed experimental investigation of the auxiliary gas circuit. The gas mixture in the circuit mainly consists of the vapour of the refrigerant (ammonia) and the inert gas (helium). A clamp-on ultrasonic flowmeter measures the volume flow rate of the gas mixture and the mole fraction of helium and ammonia continuously. The influence of an increasing volume flow on the mass transfer in the auxiliary gas circuit is shown by using a propeller to force the circulation of the gas mixture.

Argon metastable production in argon-helium microplasmas

Microwave resonator-driven microplasmas are a promising technology for generating the high density of rare-gas metastable states required for optically pumped rare gas laser systems. We measure the density of argon 1s5 states (Paschen notation) in argon-helium plasmas between 100 Torr and atmospheric pressure using diode laser absorption. The metastable state density is observed to rise with helium mole fraction at lower pressures but to instead fall slightly when tested near atmospheric pressure. A 0-D model of the discharge suggests that these distinct behaviors result from the discharge being diffusion-controlled at lower pressures, but with losses occurring primarily through dissociative recombination at high pressures. In all cases, the argon metastable density falls sharply when the neutral argon gas fraction is reduced below approximately 2%.

Investigation of gas flow dependence of plasma jet produced by pulsed power

Nonthermal plasmas were generated using repetitively operated pulsed power generators, demonstrating the technology’s potential in creating discharges relatively easily at atmospheric pressure. Control of plasma jets at atmospheric pressure facilitates a wide range of applications in material modification, medicine, and energy. This paper investigates the effects of gas flow velocity and helium mole fraction on plasma length under dielectric-barrier discharge. Helium gas was used for easy Please check if “easy observance” can be rephrased as “easy observation.” observance of light emission.

Critical Temperature Differences of a Standing Wave Thermoacoustic Prime Mover with Various Helium-Based Binary Mixture Working Gases

Thermoacoustic prime movers are energy conversion devices which convert thermal energy into acoustic work. The devices are environmentally friendly because they do not produce any exhaust gases. In addition, they can utilize clean energy such as solar-thermal energy or waste heat from internal combustion engines as the heat sources. The output mechanical work of thermoacoustic prime movers are usually used to drive a thermoacoustic refrigerator or to generate electricity.A thermoacoustic prime mover with low critical temperature difference is desired when we intend to utilize low quality of heat sources such as waste heat and sun light. The critical temperature difference can be significantly influenced by the kinds of working gases inside the resonator and stack's channels of the device. Generally, helium gas is preferred as the working gas due to its high sound speed which together with high mean pressure will yield high acoustic power per unit volume of the device. Moreover, adding a small amount of a heavy gas to helium gas may improve the efficiency of thermoacoustic devices.This paper presents numerical study and estimation of the critical temperature differences of a standing wave thermoacoustic prime mover with various helium-based binary-mixture working gases. It is found that mixing helium (He) gas with other common gases, namely argon (Ar), nitrogen (N2), oxygen (O2), and carbon dioxide (CO2), at appropriate pressures and molar compositions, reduce the critical temperature differences to lower than those of the individual components of the gas mixtures. In addition, the optimum mole fractions of Hegas which give the minimum critical temperature differences are shifted to larger values as the pressure increases, and tends to be constant at around 0.7 when the pressure increases more than 2 MPa. However, the minimum critical temperature differences slightly increase as the pressure increases to higher than 1.5 MPa. Furthermore, we found that the lowest critical temperature difference for He-Armixture gas is around 66 °C which is achieved in pressure range of 1.5 MPa - 2.0 MPa and mole fractions of helium of 0.55 - 0.65. The He-N2 and He-O2 mixture gases demonstrate almost the same performances, both have the lowest critical temperature difference around 59 °C atpressures of 1.0 MPa - 1.5 MPa and helium's mole fractions of 0.35 - 0.55. For all tested gases, the lowest critical temperature difference of around 51 °C is provided by He-CO2 mixture gas at pressures of 0.5 MPa - 1.0 MPa with helium's mole fractions of 0.15 - 0.40.

Experimental study of the effect of non-condensable gases on steam condensation over a vertical tube external surface

An experimental investigation has been conducted to evaluate the steam heat removal capacity with non-condensable gases (e.g. air, helium) over a vertical tube external surface. Under steam/air condition, condensation heat transfer coefficients have been obtained under the wall subcooling ranging from 27 to 70°C, total pressure ranging from 2.0×105Pa to 6.0×105Pa and air mass fraction ranging from 0.10 to 0.80. The experiments for the influence of the wall subcooling on the steam condensation heat transfer with a fixed pressure and air mass fraction have been performed. An empirical correlation for the heat transfer coefficient is developed, covered all data points within 10%. Under steam/air/helium (simulating hydrogen) condition, the effect of helium mole fraction in non-condensable gases on the heat transfer coefficient was investigated under the wall subcooling ranging from 30 to 62°C, total pressure ranging from 2.0×105Pa to 5.0×105Pa, air mass fraction ranging from 0.09 to 0.76 and helium mole fraction in non-condensable gases from 0.03 to 0.35. The empirical correlation that got from the helium experiments covered data with 20%. There was not found the helium stratification under the experimental conditions.

Influence of helium mole fraction distribution on the properties of cold atmospheric pressure helium plasma jets

The influence of helium mole fraction distribution in air on the cold atmospheric plasma jets excited by 1.5 kHz rectangular high voltage pulse is studied in this work. Computational fluid dynamics (CFD) with incorporation of large eddy simulation (LES) model is used to simulate the helium mole fraction distribution in air under the helium flow from laminar to turbulent regime with increasing helium outlet velocity. Numerical simulation results are combined with experimental results in order to determine the influence of helium distribution on the cold plasma jets. It reveals that the structure of the helium distribution caused by diffusion or by turbulent mixing in turbulent regime determines the characteristics of the cold plasma jets. On the other hand, the curves of plasma jet length (L) versus helium outlet velocity (V) at different jet diameters (D) are unified in a map of jet Reynolds number (Re = ρHe·V·D/μHe, where μHe is the helium viscosity constant) versus dimensionless plasma jet length (l = L/D). The map is allowed to predict the flow pattern of helium jet in order to estimate and control the plasma jet length at different jet diameters.