Properties Of Gas Mixtures Research Articles

Multi-period operation of integrated electricity and gas systems with hydrogen blending considering gas composition dynamics

Green hydrogen from renewable sources can be blended with natural gas and serves as a potentially feasible measure for contributing to the net zero energy sector. The time-varying nature of hydrogen injection, influenced by stochastic renewable generations, can result in fluctuations in gas concentrations in the entire network. It poses a potential threat to the secure regulation of integrated electricity and gas systems (IEGS). For managing the operating condition and guaranteeing the security of IEGS during operation, a multi-period operation framework with alternative gas (e.g., hydrogen) blending is developed. First, a convex gas security range is derived using the Dutton method. Then, the multi-period operation framework is devised to mitigate the impacts of alternative gas injection on gas security over the entire operational period. Both the dynamics from gas composition and gas flow are modelled, accurately describing the real-time travel of alternative gas concentrations. The dynamics in the gas mixture properties (e.g., relative density) are fully revealed with time-varying gas concentrations. To tackle the high non-convexities in the optimization problem, second-order-cone relaxation is well-tailored and firstly used in the case of varying gas compositions, making the motion equations and advective transport equations more tractable. An advanced second-order-cone sequential programming is devised to drive the relaxation tight more efficiently. Finally, our operation strategy is illustrated in IEEE and Belgium Electricity and Gas Systems. Results indicate that hydrogen concentrations take about 12.5 h to travel from the injection point to the end of the pipeline route in the Belgium gas system. By incorporating this unique characteristic into the model, operational scheduling and dispatch in IEGS can be more practical when integrating hydrogen in the future.

Innovative approaches to optimizing Li-Ion battery cooling performance using gas mixtures

This study investigates the critical issue of thermal management in lithium-ion batteries, a vital concern for enhancing battery performance, safety, and longevity. While more complex cooling systems like liquid cooling, phase change materials, and two-phase systems offer superior thermal performance due to their higher heat transfer capabilities, they come with significant drawbacks. These include increased system complexity, higher costs, added weight, and the potential for leaks or other failures that can compromise safety and reliability. Consequently, simpler air-based cooling systems are often preferred in many applications, despite their inherently lower h coefficient. To bridge the performance gap between air and liquid cooling without resorting to complex engineering solutions, this study focuses on enhancing the h coefficient of air-based systems using innovative refrigerant gas mixtures. The use of refrigerant gas mixtures, specifically a blend of R1234ze(E), nitrogen, and neon, as an alternative to traditional cooling methods was explored. The study employed a comprehensive FEM model and experimental validation to evaluate free and forced convection in a slender, vertical cylindrical configuration. To analyze the impact on convective heat transfer, the key parameters CRate, T0 and Tamb were systematically varied. A dimensionless analysis then evaluated how thermodynamic properties influence h coefficient. This analysis led to the derivation of a dimensionless heat transfer function h(ρ, Cp, k, μ). The study aimed to significantly improve the efficiency of h by optimizing the gas mixture properties. The “Mixture Study” identifies the optimal refrigerant composition, where a specific mixture α of 86 % R1234ze(E), and 14 % neon demonstrated the best balance between heat transfer efficiency and enhanced thermal capacity. The heat transfer coefficient increases by around 65 % during natural convection and by around 80 % during forced convection. The percentage reduction of the thermal difference in temperature reaches approximately 10 % with a maximum of 4 K in natural convection conditions and a maximum of 36.5 % and a reduction of approximately 10 K compared to air cooling alone, effectively maintaining the battery within the optimal operating range of 293.15 K to 333.15 K. Notably, this composition increased the thermal power absorbed by the mixture by over 100 % under the same thermal difference. This research therefore provides valuable insights for developing more effective gas cooling strategies that ultimately enhance the performance, safety, and durability of batteries in various applications. By integrating the benefits of both cooling methods, the study paves the way for innovative thermal management solutions in battery technology.

New analytical thermodynamic models developed for pure H2, CH4, CO2 and H2 containing mixtures based on molecular simulations

Pressure-volume-temperature-composition (PVTX) properties of gas mixtures are important to model and predict flow and compression performance in a range of industrial processes. It is time consuming and sometimes impractical to obtain these properties from experiments. Here, molecular dynamics (MD) simulations were conducted to compute the thermodynamic volumetric properties of pure H2, CH4, CO2, and binary mixtures of H2/CH4 and H2/CO2 at temperatures of 310.9–470 K, pressures up to 172 MPa. MD simulation results were used to develop five analytical equations explicit in pressure as a function of density or volume, temperature, and composition, for the pure gases and mixtures. Our simulated results matched very well with the predictions from the established GERG-2008 equation and data published by NIST. The simplified equations of state (EOS) developed in this work will be more computationally efficient than other EOS for mixed systems. This can be significant in reservoir simulations.

Computing Mixture Adsorption in Porous Materials through Flat Histogram Monte Carlo Methods.

Mixture adsorption properties of porous materials are critical to determine their potential as adsorbents in separation applications. Toward the discovery of optimal adsorbents, in silico screening studies typically employ the grand canonical Monte Carlo (GCMC) technique to compute adsorption properties of gas mixtures in materials of interest at a given condition (i.e., composition, total pressure, and temperature) or to compute their adsorption properties for each component, followed by utilizing methods to predict mixture adsorption isotherms. However, the former approach results in the need for repeated calculations when different conditions such as compositions are considered. For the latter, the predictions may involve uncertainties, sometimes originating from the fitting quality to the pure component isotherms, and repeated simulations may also be needed for different temperatures. To this end, this study demonstrates the potential of flat histogram Monte Carlo methods in addressing the abovementioned shortfalls. Specifically, the so-called NVT + W method, first reported by Smit and co-workers, is extended herein to determine the macrostate probability distribution (MPD) of binary mixtures in porous materials. The obtained MPD can be reweighted to any conditions, yielding accurate adsorption isotherms of any desired compositions and temperatures. This approach, denoted as 2D NVT + W, is also compared with the widely adopted ideal adsorbed solution theory (IAST) method, and the former is found to offer more reliable predictions. Overall, the 2D NVT + W approach represents an efficient and effective alternative to compute mixture adsorption isotherms for porous materials, and the obtained MPD can be conveniently reused by peer researchers. A user-friendly Python code is also provided along with this article to employ this method.

The equivalence ratio concept in fire safety science

The equivalence ratio is a well-established metric used to evaluate a fuel–air mixture in combustion. Since its inception, many have used this metric to evaluate the reactive properties of gas mixtures and their impact on combustion. Some have expanded beyond this concept using the compartment (i.e., global), plume, upper layer, and local equivalence ratios as criteria for predicting species yields in fire experiments, particularly carbon monoxide generation. In this work, an overview of the equivalence ratio concept in fire safety science is discussed, focusing on research conducted in multi-scale fire experiments. Gas analysis techniques used to measure the equivalence ratio, such as the phi meter, gas chromatography, or multi-gas sensor systems, are examined. The notable takeaways from works examining the equivalence ratio concept across various fire experiments suggest opportunities to address challenges in fire safety sciences.

Molecular simulation study on the separation of CO2 and N2 in poly(4 methyl-1-pentene) membrane

• Molecular simulation is used to research the transport properties of gas mixtures. • Mechanisms of the gas diffusion and adsorption processes are studied. • The separation ratio of carbon dioxide and nitrogen in the poly(4 methyl-1-pentene) membrane was 4.83. • The presence of impurity gases affects the gas separation ratio. In this paper, the separation characteristics of C O 2 and N 2 in the poly(4 methyl-1-pentene) [PMP] membrane was studied via molecular dynamics simulation. The model for the separation of the mixed gas containing C O 2 and N 2 was constructed. The diffusion and adsorption processes of gas molecules in the PMP membrane were simulated. Separation performances of gas molecules in PMP membranes is obtained, including the solubility coefficients, the penetration coefficients and the separation factors. The effect of impurity gases on the separation of C O 2 and N 2 was analyzed by using oxygen or hydrogen as impurity gases. The simulation results showed that the separation factor of C O 2 and N 2 through the PMP membrane was 4.83 under the temperature condition of 298 K. The separation factor would increase to 5.73 with O 2 doping, and it decreased to 3.81 with H 2 doping under the conditions of this paper. Then, the dissolution and diffusion mechanisms of PMP membrane were analyzed, the separation performance of C O 2 and N 2 in PMP membrane and the influence of doping different gases on the separation of C O 2 and N 2 were studied. This study make up for the deficiency of the current experimental research, and provides a corresponding reference for the separation factor test of C O 2 and N 2 in PMP membrane.

Altering the Supply of Shielding Gases to Fabricate Distinct Geometry in GMA Additive Manufacturing

Wire arc additive manufacturing (WAAM) is the process by which large, metallic structures are built, layer-by-layer, using a welding arc to melt wire feedstock. In this process, the proper selection of the shielding gas plays a vital role in the achievement of structurally acceptable part geometries and quality surface finishes. In this study, the authors used either a ternary mix (He, Ar and CO2) or a binary mix (Ar and CO2) of shielding gases to deposit wall geometries using an open loop-controlled WAAM system developed at Oak Ridge National Laboratory’s Manufacturing Demonstration Facility. The binary blend produced a wider and shorter geometry, while the ternary blend resulted in a narrower build that was more equivalent to the CAD geometry. The data indicated that the binary blend provided a higher oxygen concentration in the weld as compared to that of the ternary blend. The results imply that the arc characteristics and heat input had a significantly higher impact on the weld penetration than the surface tension effect of surface active elements. This was further verified by developing and applying a high-fidelity computational fluid dynamics (CFD) model of the thermophysical properties of gas mixtures. The results from the model showed that, while the influence of increased oxygen concentration on the surface tension for the binary blend led to a deeper penetration, the ternary blend gave rise to heat flux to the workpiece.

Optimal design of CH4 pyrolysis in a commercial CVD reactor using support vector machines and Nelder-Mead algorithm

Chemical vapour deposition (CVD) of pyrocarbon (PyC) effectively fabricates advanced carbon materials. Controlling the nanotexture of PyC is critical for the desired application. Reactor operating conditions, including temperature, pressure, inlet flow rate, reactant concentration, govern thickness, and film uniformity. The optimal film performance can be obtained by selecting appropriate process conditions. However, the optimisation of the CVD reactor is challenging due to the highly nonlinear and multi-variable nature of the process. In this study, the support vector machine, a robust supervised learning algorithm and Nelder-Mead algorithm are coupled to optimise the CH4 pyrolysis in a commercial CVD reactor. To this end, a comprehensive CFD model for CH4 pyrolysis in a commercial CVD reactor is first constructed. The model accuracy is improved by considering temperature-dependent transport properties of gas mixture and incorporating the detailed gas and surface chemistry (14 species and 32 reactions). The deposition rate and film uniformity are then obtained using a parametric study. Subsequently, the support vector machine (SVM) is employed to deduce the correlation between the PyC growth rate/film uniformity and operating variables. It has been found that the accuracy of SVM is better than the linear regression model. Finally, SVM coupled with the Nelder-Mead algorithm is proposed to optimise the CVD process parameters to maximise the PyC film quality.

Measuring the permeabilities of binary gas mixtures with a novel time‐lag technique

AbstractMeasuring permeation properties of gas mixtures is an important but challenging issue. This work reports on a novel time‐lag technique by selectively depositing (ie, desublimation) one of the gas components in a binary mixture at the downstream side using a liquid nitrogen cold trap, thereby allowing for determination of the permeation properties of both permeating components from the pressure response in the permeate chamber. The measurement technique was demonstrated for the permeation of several binary gas mixtures (H2/CO2, and He/CO2 with various compositions) through a carbon molecular sieve membrane. It was found that the true permeation properties of a binary mixture can be significantly different from the ideal values based on the permeation of pure gas components. This method is time and cost efficient and works well for binary gas mixtures where the difference in the boiling points of the gas components is sufficiently large so that one of the components in the permeate can be selectively desublimated at the permeate side.

Predominant reaction products and dielectric breakdown properties of gas mixtures consisting of SF6 and ablation products of C2F4/BN in the temperature range of 300–3000 K

For a high voltage SF6 circuit breaker equipped with a boron nitride (BN)-filled polytetrafluoroethylene ((C2F4)) nozzle, the predominant reaction products in a high-temperature SF6 mixture with vapor decomposition of C2F4 and BN at temperatures of 300–3000 K are determined based on calculations of equilibrium composition. The calculation is performed by considering the molecular products obtained by chemical reactions with boron atoms (e.g. BN, BS, BC, BF, BF2 and BF3), nitrogen atoms (e.g. N2, N3, CN, NS, NF, NF2 and NF3) and carbon atoms (e.g. CS, CS2, C2F4, CF2, CF3 and CF4). The results obtained for the C2F4–BN molar fraction of 20%–80% reveal that BF3, N2 and CF4, the molecules originating from the nozzle material, are produced at high molar fractions over a range of 300–3000 K. Such an evaluation of the gas composition allows the derivation of a reduced collision ionization coefficient α/N and a reduced electron attachment coefficient η/N of the gas, considering various electron impact processes such as BF3 + e BF2 + F− and CF4 + e CF3 + F−. The evaluation subsequently enables us to determine a critical reduced electric field strength causing . The results suggest that the decomposed C2F4–BN mixture at of 80% causes the reduction of to 110 Td in the range of 1000–2000 K at a gas pressure of 1.0 MPa. This strength is 20 Td lower than that determined separately for the SF6 gas mixture with only 80% C2F4 vapor decomposition, because of the CF4 molar fraction reduction in the SF6/C2F4–BN mixture.

Impact of hydrogen injection on thermophysical properties and measurement reliability in natural gas networks

In the context of the European decarbonization strategy, hydrogen is a key energy carrier in the medium to long term. The main advantages deriving from a greater penetration of hydrogen into the energy mix consist in its intrinsic characteristics of flexibility and integrability with alternative technologies for the production and consumption of energy. In particular, hydrogen allows to: i) decarbonise end uses, since it is a zero-emission energy carrier and can be produced with processes characterized by the absence of greenhouse gases emissions (e.g. water electrolysis); ii) help to balancing electricity grid supporting the integration of non-programmable renewable energy sources; iii) exploit the natural gas transmission and distribution networks as storage systems in overproduction periods. However, the hydrogen injection into the natural gas infrastructures directly influences thermophysical properties of the gas mixture itself, such as density, calorific value, Wobbe index, speed of sound, etc [1]. The change of the thermophysical properties of gaseous mixture, in turn, directly affects the end use service in terms of efficiency and safety as well as the metrological performance and reliability of the volume and gas quality measurement systems. In this paper, the authors present the results of a study about the impact of hydrogen injection on the properties of the natural gas mixture. In detail, the changes of the thermodynamic properties of the gaseous mixtures with different hydrogen content have been analysed. Moreover, the theoretical effects of the aforementioned variations on the accuracy of the compressibility factor measurement have been also assessed.

Propagation of weakly stretched premixed spherical spray flames in localized homogeneous and heterogeneous reactants

Propagation of weakly stretched spherical flames in partially pre-vaporized fuel sprays is theoretically investigated in this work. A general theory is developed to describe flame propagation speed, flame temperature, droplet evaporation onset, and completion locations. The influences of liquid fuel and gas mixture properties on spherical spray flame propagation are studied. The results indicate that the spray flame propagation speed is enhanced with increased droplet mass loading and/or evaporation heat exchange coefficient (or evaporation rate). Opposite trends are found when the latent heat is high due to strong evaporation heat absorption. Fuel vapor and temperature gradients are observed in the post-flame evaporation zone of heterogeneous flames. The evaporation completion front location considerably changes with flame radius. For larger droplet loading and a smaller evaporation rate, the fuel droplet tends to complete evaporation behind the flame front. Flame bifurcation occurs with high droplet mass loading under large latent heat, leading to multiplicity of flame propagation speed, droplet evaporation onset, and completion fronts. The flame enhancement or weakening effects by the fuel droplet sprays are revealed by the enhanced or suppressed heat and mass diffusion process in the pre-flame zone. Besides, for heterogeneous flames, heat and mass diffusion in the post-flame zone also exists. The mass diffusion for both homogeneous and heterogeneous flames is enhanced with a decreased Lewis number. The magnitude of the Markstein length is considerably reduced with increased droplet loading. Moreover, post-flame droplet burning behind the heterogeneous flame influences the flame propagation speed and Markstein length when the liquid fuel loading is relatively low.

Numerical simulations of inert and reactive highly underexpanded jets

In this study, the high-resolution numerical simulations of the two-dimensional (2D) multi-component inert and reactive highly underexpanded jets are conducted to quantify the influences of the injected gas mixture properties on the flow structure. First, the gas mixture with the specified species mass fractions is imposed to exhaust into the quiescent air with a Mach number of 1.0, of which the specific heat ratios (γe) range from 1.3 to 1.6. Our results indicate that the larger γe yields a relatively shorter and thinner jet core under the same inlet pressure ratio due to the sound speed increasing. Next, we focus on the chemical reaction effects on the jets with a premixed hydrogen–air mixture injection. The results reveal that the shock-induced combustion develops into a detonation, inducing numerous vortices behind the combustion wave, while the combustion in the mixing layer cannot be preserved due to the instability of the supersonic shearing. During the detonation process, the increasing pressure accompanied by the heat release forces the Riemann wave to move upstream compared with the inert one. The violent detonation periodically propagates between the two jet triple points. The detonation collision leads to the intersection of their slip lines, which causes distinct vortex formation. In addition, the main frequencies, corresponding to the Riemann wave movement, the oscillation of the shock-induced ignition positions, the periodical propagation of the detonation, and the collision of the detonation triple points, are explored to explain the unsteady process of the reactive highly underexpanded jet.

Modeling of wall condensation in the presence of noncondensable light gas

During a loss of coolant accident in nuclear reactor, significant amounts of steam and hydrogen can be released in the containment. Condensation of steam in the presence of noncondensable gases on the containment walls and structures is a key issue because of its role in removing heat from the atmosphere. Extensive experimental and theoretical studies have been carried out for a better understanding of this complex phenomenon which involves several physical processes and parameters. When condensation takes place in the presence of noncondensable gases a liquid film is formed and noncondensable gases accumulate at the interface. The diffusion of steam through the gaseous layer depends on the gas composition, velocity, temperature and pressure. The formation of a gaseous layer leads to a significant reduction of heat transfer. Simulations based on CFD or lumped parameter (LP) approaches use correlations to estimate heat and mass transfer due to condensation. In this work, we are interested in theoretical correlations based on the diffusion layer theory using heat and mass transfer analogy. Among the variables and parameters affecting wall condensation, the effect of the gas mixture properties in particular the diffusion coefficient modeling is investigated. Test cases of steam injection into an enclosure filled with air or air-hydrogen mixture are simulated with a LP code using two different correlations for the evaluation of heat and mass transfer. Each correlation is based on different formulations of the effective diffusion coefficient. The ISP47 test performed in the MISTRA facility was used for validation purpose. The results showed that the addressed heat and mass transfer correlations underestimate the condensation rate in the steady state. Furthermore, the impact of the effective diffusion coefficient modeling on the heat and mass transfer turned out to be significant compared to the effect of the heat and mass transfer correlation.

A real-time pressure wave model for knock prediction and control

This article develops a control-oriented real-time zero-dimensional knock pressure wave model for spark-ignited engines based on the reaction-based two-zone combustion model developed earlier. This knock pressure wave model is capable of predicting the in-cylinder pressure oscillations under knocking combustion in real-time and can be used for the model-based knock prediction and control. A pressure wave equation including the knock deadening behavior is proposed, simplified, and used to calculate the pressure perturbations generated by knock combustion. The boundary and initial conditions at knock onset are analyzed and the analytic solution of the pressure wave equation is obtained. The model is calibrated and validated over two engine operating conditions at knock limit. The chemical kinetic-based Arrhenius integral and the maximum amplitude of pressure oscillations are used as the evaluation methods for knock onset and intensity prediction, and the knock frequency is studied using a fast Fourier transform of the filtered in-cylinder pressure oscillations. The simulation results show that the proposed knock pressure wave model is able to predict the knock onset timing, frequency, and intensity accurately under two engine operating conditions. Furthermore, the knock characteristics associated with gas mixture properties at intake valve closing is analyzed based on the experimental data, and their effect to knock cycle-to-cycle variation is also studied for the proposed model.

Two-dimensional analytical investigation of conjugate heat transfer in a finite-length planar micro-combustor for a hydrogen-air mixture

This study aims to investigate the conjugate heat transfer in a finite-length planar micro-combustor by using a two-dimensional analytical model. The primary objective is to thermally analyze the effects of ambient, solid wall, and gas mixture properties on wall and gas temperature profiles and heat recirculation via the micro-combustor wall. The micro-combustor is divided into three sections composed of pre-flame, reaction, and post-flame zones. The energy equation is solved in each zone for both wall and gas mixture regarding the matching boundary conditions. As a result, an appropriate correlation that is a combination of hyperbolic tangent and linear functions for estimating the wall temperature profile is developed. Moreover, since the absolute amounts of positive wall temperature gradient are larger than the negative ones, heat is conducted intensely toward the inlet side of wall which results in heat recirculation at pre-flame zone. These findings can be used to propose a simple approach for preliminary micro-combustor design.

A Multi-Fluid Investigation of the Membrane Supporting Grid Effects on the Richtmyer-Meshkov Instability

We present results of numerical experiments performed to evaluate the effects of the material interface supporting wire grid on the Richtmyer-Meshkov instability (RMI). An air-SF6 interface initially perturbed sinusoidally supported on a number of solid circular cylinders. These cylinders are introduced along the interface to mimic the presence of the grid thin wires. The resulted mixing and growth rate of the perturbation in the presence and absence of the supporting grid were analyzed and validated with experimental measurements. The small scales perturbation imposed by the cylinders are around two orders of magnitude smaller than the interface sinusoidal perturbation wavelength requiring the adaptive mesh refinement (AMR) to adequately resolve small scale features. Furthermore, an embedded boundary technique is used to handle the complex geometry stemming from the presence of these multiple. A multi-fluid formulation is utilized to form a multi-gas species interface and compute the gas mixture properties.

Radiative extinction of large n-alkane droplets in oxygen-inert mixtures in microgravity

Experimental observations are presented concerning radiative extinction of large n-alkane droplets in diluent-substituted environments at moderately varied pressures in microgravity onboard the International Space Station. The fuels considered are n-heptane, n-octane, and n-decane with carbon dioxide, helium, and xenon used as inerts, replacing nitrogen as diluents at varying amounts. It is shown that a simple scaling analysis, based on the assumptions that radiative extinction occurs when the flame temperature drops to a critical value and that the radiative heat loss rate is a fraction of the heat-release rate at the flame, is able to correlate the measured droplet diameter at extinction as a function of its initial diameter and of the ambient gas-mixture properties.

Relationship between Raman spectral features and fugacity in mixtures of gases

AbstractRaman spectroscopy yields information about the internal vibrational modes of covalently bonded molecules, which are sensitive to the local molecular environment. As such, Raman spectra of a gas composed of covalently bonded molecules collected at different temperatures and/or pressures show variations in band positions and other spectral features, reflecting molecular‐scale interactions (attraction and repulsion) among gas particles associated with the local physical and chemical environment. Because the molecules interact, gases are nonideal, and fugacity is the fundamental thermodynamic quantity that describes partial pressures of gases, adjusted to account for the nonideality. The fugacity of a substance is generally obtained from thermodynamic calculations rather than by direct analysis or measurement and, as such, is conceptually nebulous. Here, we perform spectroscopic analyses of a gas mixture of known composition at variable and known pressures and couple these results with thermodynamic calculations of gas fugacities. We show that Raman peak shifts in gas mixtures are directly correlated to fugacities of the component gas species. Our results thus provide experimental evidence that fugacities of gases can be estimated from Raman spectra collected in situ. By this approach, the thermodynamic quantities fugacity and the related thermodynamic property activity are linked directly with underlying molecular‐scale phenomena, according to the vibrational properties of molecular species. This represents a tractable method to efficiently obtain large datasets on thermodynamic properties of gas mixtures, and with some adjustments, may lead to a viable method for developing nonideal mixing rules for other substances, such as crystalline solid solutions, aqueous ions, and electrolytes.

Langmuir probe plasma diagnostics to investigate the dielectric properties of cryogenic gas mixtures

Use of the Langmuir probe plasma diagnostics to investigate the dielectric properties of gas mixtures is discussed as a continuation our previous experimental and theoretical work on understanding the dielectric strength of helium gas mixtures for superconducting and other cryogenic applications. Here we report the results of Langmuir probe experiments conducted on the gas mixtures to obtain plasma parameters including plasma density and the electron temperature. The experimental procedure used in the Langmuir probe plasma measurements, the derivation of the plasma characteristic parameters, and the implications of the results to the dielectric characteristics are discussed. The plasma characteristics obtained under various discharge power, gas pressure, and gas composition are also discussed. The results support the findings of the our previous theoretical and experimental studies that showed substantial enhancement of dielectric strength of He gas obtained by the addition of small mol% of H2 and relate the enhancements to the plasma characteristics observed.