Thermal Gas Flow Research Articles

Tracing Thermal Creep and Thermophoresis in Porous Structures at Low Ambient Pressure and Low Gravity

We carried out two types of drop tower experiments to quantify gas and particle motion induced by temperature gradients inside a porous structure in a low pressure environment. In one setup 400 μm sized particles were traced inside heated channels at pressures of a few Pascal. Their motion is consistent with pure thermophoresis. In the second setup tracer particles were used to track the thermal creep gas flow through a porous dust bed. Here, the flow was traced outside of the dust bed and without thermophoretic motion. The results are consistent with a simple capillary model of the dust bed.

MEMS-Based Smart Gas Metering for Internet of Things

Utilities have traditionally employed or contracted meter readers to collect natural gas usage data, which is expensive and time consuming, and thus necessitates the need of smart natural gas metering. Existing gas metering systems mainly focus on measuring the amount of gas flowing through an microelectro mechanical system (MEMS) thermal gas flow sensor and simply ignore the detailed gas composition. From computational fluid mechanics simulations, however, we discover that gases with different compositions will cause different effects on the reading of an MEMS sensor. Based on a thorough analysis of the working principle of MEMS thermal gas flow sensor, we propose an innovative mechanism to compensate the errors caused by different types of natural gases on the sensor’s reading. The proposed solution first measures the physical property of metered gas to derive the composition correction coefficient that will then be used to correct the meter’s reading errors, considering the relation between the calorific value and physical property of natural gases. In this way, the proposed solution realizes a real-time multicomposition gas metering via thermal gas flow sensors. We implement and evaluate the proposed gas metering technique in various Internet of Things systems, including industrial flow metering, gas metering in smart home, and gas metering in low-power wide-area networks. Experiment results verify the innovative design and confirm that the proposed solution features high sensitivity, high precision, and high range ratio.

Analog Experiments on Tensile Strength of Dusty and Cometary Matter

The tensile strength of small dusty bodies in the solar system is determined by the interaction between the composing grains. In the transition regime between small and sticky dust (µm) and non cohesive large grains (mm), particles still stick to each other but are easily separated. In laboratory experiments we find that thermal creep gas flow at low ambient pressure generates an overpressure sufficient to overcome the tensile strength. For the first time it allows a direct measurement of the tensile strength of individual, very small (sub)-mm aggregates which consist of only tens of grains in the (sub)-mm size range. We traced the disintegration of aggregates by optical imaging in ground based as well as microgravity experiments and present first results for basalt, palagonite and vitreous carbon samples with up to a few hundred Pa. These measurements show that low tensile strength can be the result of building loose aggregates with compact (sub)-mm units. This is in favour of a combined cometary formation scenario by aggregation to compact aggreates and gravitational instability of these units.

Asymptotic modeling of thermal binary monatomic gas flows in plane microchannels—Comparison with DSMC simulations

Asymptotic models are constructed to investigate the basic physical phenomena of thermal flows of a mixture of two monatomic gases inside a two-dimensional microchannel. The steady flows are described by the Navier-Stokes-Fourier balance equations, with additional coupling terms in momentum and energy equations, and with first-order slip boundary conditions for the velocities and jump boundary conditions for the temperatures on the two walls. The small parameter equal to the ratio of the two longitudinal and transverse lengths is introduced, and then an asymptotic model is proposed. It corresponds to small Mach numbers and small or moderate Knudsen numbers. Attention is paid to the first-order asymptotic solutions. Results are given and discussed for different cases: the mass flow rates, the molecular weights of the gases, and the temperature gradients along the walls. Comparisons between the first-order asymptotic solutions and Direct Simulation Monte Carlo (DSMC) simulations corresponding to the same physical data show rather good agreement. It should be noted that obtaining an asymptotic solution is very fast compared to obtaining a DSMC result.

Optical Sensor of Thermal Gas Flow Based on Fiber Bragg Grating.

This paper aims at solving the problem of explosion proof in measurement of thermal gas flow using electronic sensor by presenting a new type of flow sensor by optical fiber heating. A measuring unit based on fiber Bragg grating (FBG) for fluid temperature and a unit for heat dissipation are designed to replace the traditional electronic sensors. The light in C band from the amplified spontaneous emission (ASE) light source is split, with one part used to heat the absorbing coating and the other part used in the signal processing unit. In the heating unit, an absorbing coating is introduced to replace the traditional resistance heating module to minimize the risk of explosion. The measurement results demonstrate a fine consistency between the flow and temperature difference in simulation. The method to enhance the measurement resolution of flow is also discussed.

Amplification of dust loading in Martian dust devils by self-shadowing

Insolation of the Martian soil leads to a sub-surface overpressure due to thermal creep gas flow. This could support particle entrainment into the atmosphere. Short time shadowing e.g. by the traverse of a larger dust devil would enhance this effect. We find in microgravity experiments that mass ejection rates are increased by a factor of 10 for several seconds if a light source of 12.6 kW/m2 is turned off. Scaled to Mars this implies that self-shadowing of a partially opaque dust devil might lead to a strongly amplified flux of lifted material. We therefore suggest that self-shadowing might be a mechanism on Mars to increase the total dust loading of a dust devil and keep it self-sustained.

Application of the Streamlined Body for Properties Amplification of Thermoresistive MEMS Gas Flow Sensor

The main problems of the gas flow meters are the improvement of the sensitivity and velocity range. In this paper thermoresistive membrane gas flow sensor is considered. The sensor is based on the calorimetric principle of operation and developed for the thermal gas flow meters. Special streamlined body was used to improve sensor’s characteristics. It is shown that streamlined body raises sensitivity by 2 times and improves velocity range of the sensor.

Thermal creep-assisted dust lifting on Mars: Wind tunnel experiments for the entrainment threshold velocity

In this work we present laboratory measurements on the reduction of the threshold friction velocity necessary for lifting dust if the dust bed is illuminated. Insolation of a porous soil establishes a temperature gradient. At low ambient pressure this gradient leads to thermal creep gas flow within the soil. This flow leads to a sub-surface overpressure which supports lift imposed by wind. The wind tunnel was run with Mojave Mars Simulant and air at 3, 6 and 9 mbar, to cover most of the pressure range at martian surface levels. Our first measurements imply that the insolation of the martian surface can reduce the entrainment threshold velocity between 4 % and 19 % for the conditions sampled with our experiments. An insolation activated soil might therefore provide additional support for aeolian particle transport at low wind speeds.

Thermal multicomponent lattice Boltzmann model for catalytic reactive flows.

Catalytic reactions are of great interest in many applications related to power generation, fuel reforming and pollutant abatement, as well as in various biochemical processes. A recently proposed lattice Boltzmann model for thermal binary-mixture gas flows [J. Kang, N. I. Prasianakis, and J. Mantzaras, Phys. Rev. E. 87, 053304 (2013)] is revisited and extended for the simulation of multispecies flows with catalytic reactions. The resulting model can handle flows with large temperature and concentration gradients. The developed model is presented in detail and validated against a finite volume Navier-Stokes solver in the case of channel-flow methane catalytic combustion. The surface chemistry is treated with a one-step global reaction for the catalytic total oxidation of methane on platinum. In order to take into account thermal effects, the catalytic boundary condition of S. Arcidiacono, J. Mantzaras, and I. V. Karlin [Phys. Rev. E 78, 046711 (2008)] is adapted to account for temperature variations. Speed of sound simulations further demonstrate the physical integrity and unique features of the model.

A boundary integral equation formulation for the thermal creep gas flow at finite Peclet numbers

The integral equation formulation developed previously by the authors to study isothermal micro flows under shear slip boundary condition is extended in this work to consider the case of non-isothermal micro gas flows with thermal creep effects at finite Peclet numbers. The effect of thermal creep over the flow patterns with and without considering the effect of shear slip is investigated in detail using a boundary element method. In this work the boundary integral approaches for both fluid velocity and temperatures fields are used to solve the problem of shear-driven cavity flow at finite Peclet number. This is obtained by considering the diffusive-convective heat equation using the Dual Reciprocity Method to transform the corresponding volume integral of the convective terms into equivalent surface integrals.

Gas sensor using a multi-walled carbon nanotube sheet to detect hydrogen molecules

In this paper, a gas sensor with a fabricated multi-walled carbon nanotube (MWCNT) sheet, which can sense hydrogen (H2) gas, is proposed and its output properties were evaluated. The key contribution of this study is a novel fabrication technique that introduces a simple transfer method. Easy transfer from initial substrate to another substrate, the simple process and integration ability, and low cost make CNT-based sensors more suitable for future nano-material based sensor. Moreover, the MWCNT sheet does not need a complex purification process to be used as a sensor, making it suitable for practical applications. The MWCNT-sheet gas sensor had a large surface area and many contact points; these facts were further confirmed through morphological analyses. In addition, the effects of thermal annealing and carrier gas flow on gas desorption were investigated in order to improve the recovery response of the gas sensor. It was found that a thermal annealing temperature of 100°C and a larger flow (2000sccm) of carrier gas recovered original resistance values, resulting in high performance, faster response time, and improved reproducibility for the MWCNT-sheet gas sensor.

Characterization of Carbon Nanofibers Treated with Thermal Nitrogen as a Catalyst Support Using Point‐of‐Zero Charge Analysis

The chemical and physical purification of carbon nanofiber exposes more anchoring sites between meal precursors and carbon surface but thermal N2 gas flow maintains the crystal’s structure as well as its defect and edge sites, referred to as active sites or anchoring sites. After calcination in nitrogen at 450°C, samples were characterized by Raman spectra X‐ray diffraction, as well as thermogravimetric and nitrogen physisorption analyses. Results showed a relatively lower fraction of amorphous carbon to graphite, indicating a greater removal of amorphous carbon. Moreover, the disorder intensity of carbon nanofibers that were treated in N2 flow rate of 1 L/min and 3 hours, called 1Gcom‐3h sample, achieved far more defect sites compared with unmodified carbon nanofiber. In addition, the surface areas of mesoporous carbon nanofibers decreased over prolonged residence time. The carbon nanofiber support‐metal cation interaction therefore improved the deposition of iron when the point‐of‐zero charge reading was greater than four.

Thermal Flow Measurements by a Flexible Sensor, Implemented on the External Surface of the Flow Channel

A thermal gas flow sensor was developed and evaluated. The presented implementation requires only low-cost manufacturing techniques and readily available components, while maintaining a high level of detection range and sensitivity. Heater and sensing elements were integrated on a flexible substrate and the device was formed by bending the substrate so that the active elements were placed on the external surface of the formed channel, therefore zero flow interference is achieved and a wide variety of fluids can be measured without compromising the sensor integrity. Evaluation was made using air flow rates in the range of 0-65SLPM utilizing electrical measurements and IR imaging techniques simultaneously.

A robust thermal microstructure for mass flow rate measurement in steady and unsteady flows

A silicon micro-machined thermal gas flow sensor operating in anemometric mode has been designed, fabricated and investigated for continuous and pulsatile flows. The sensor is specifically designed to achieve high sensitivity, fast response time and high robustness. It is composed of four metallic resistors interconnected to form a Wheatstone bridge. Two of them act simultaneously as the heating and sensing elements and the two others are used as a temperature reference. The heating element consists of a metallic wire of platinum Pt (2 µm width, 2 mm length) maintained on each lateral side by periodic silicon oxide SiO2 micro-bridges. Finite element simulations show that this structure achieves a fast thermal response time of 200 µs in constant current operating mode and a coefficient of temperature rise close to 25 °C/120 µW based on bulk electrical resistivity and when the Pt wire and SiO2 thicknesses are close to 100 nm and 500 nm, respectively. This design allows the fabrication of a robust thermal flow sensor with heating elements as long as possible, which enables accurate measurements with high signal to noise ratio. The sensor is then characterised experimentally; its electrical and thermal properties are obtained in the absence of fluid flow. These results confirm the effectiveness of the thermal insulation as predicted by the simulations. In a second step, the fluidic characterizations are reported and discussed for both continuous and pulsatile flows. In continuous mode, the sensor response was studied for gas flow rate ranging from 0 L min−1 to 10 L min−1. In pulsatile mode, the sensor is integrated inside a channel of a micro-valve actuated at 200 Hz. The measurements are compared with those obtained by a classical commercial hot wire.

Dynamic simulation of impurity transport and chemical reactions in a Bridgman furnace for directional solidification of multi-crystalline silicon

Numerical results from a dynamic simulation of impurity transport and reactions in a Bridgman furnace for directional solidification of multi-crystalline silicon are presented and compared to experimental results. The simulation includes the calculation of the thermal field, melt and gas flow velocity field, transport and chemical reactions of oxygen and carbon impurities for the entire process based on heating, melting and solidification phases. Carbon and oxygen distribution in the ingot is analyzed experimentally by means of FT-IR spectroscopy and LECO combustion method, the CO development by means of an μ-GC gas analyzer.The simulated impurity distribution in the ingot and the CO development above the free melt surface are in good agreement with the experimental results. Furthermore the results indicate that the carbon solubility limit is already reached at the stage of melting and SiC precipitates are likely to form at the early stage of growth.

In-Plane Thermoelectric Generator of Carbon Nanotube Membrane Driven by Thermal Gas Flow

The in-plane thermoelectric generator (TEG) was ingeniously designed when the thermal gas flowed over the carbon nanotube (CNT) membrane at the modest speed of a few meters per second. It was composed of the glass substrate, aurum electrodes and CNT membrane synthesized by a floating catalyst chemical vapor deposition method. In the air under atmospheric pressure, the experimental results showed that the maximal output voltage could reach 1.7 mV. It related not only with the temperature difference between the hot-side and cold-side, but also the temperature gradient of the CNT membrane which was closely dependent on the velocity and temperature of the gas flow. The multi-physical power mechanism was applied to interpret the energy conversion, which included the coupling relation of the fluid dynamics, heat transmission and Seebeck effect. This novel method could effectively enhance the output voltage, extend the applied range of TEG and had a fine prospect.

Simulation of front motion in a reacting condensed two phase mixture

The computational technique is developed in order to provide the scale capturing for numerical simulation of the thermal processes. The thermal front motion and gas flow dynamics as well as the rate of particle growth during the Carbon Combustion Synthesis of Oxides (CCSO) were predicted using the numerical simulation. In CCSO the exothermic oxidation of carbon nanoparticles generates a self-sustained thermal reaction front that propagates through the solid reactant mixture converting it to the desired complex oxides. The combusted carbon is emitted from the sample as carbon dioxide and its high rate of release increases the product porosity and friability. It was shown that the complicated finger front instability can be developed during the carbon combustion synthesis. This phenomenon results from a vortex gas flow in the reaction zone fed by the carbon dioxide co-flow and oxygen counter-flow filtration.

Langmuir–Maxwell and Langmuir–Smoluchowski boundary conditions for thermal gas flow simulations in hypersonic aerodynamics

The simulation of nonequilibrium thermal gas flow is important for the aerothermodynamic design of re-entry and other high-altitude vehicles. In computational fluid dynamics, the accuracy of the solution to the Navier–Stokes–Fourier (N–S–F) equations depends on the accuracy of the surface boundary conditions. We propose new boundary conditions (called the Langmuir–Maxwell and the Langmuir–Smoluchowski conditions), for use with the N–S–F equations, which combine the Langmuir surface adsorption isotherm with the Maxwell/Smoluchowski slip/jump conditions in order to capture some of the physical processes involved in gas flow over a surface. These new conditions are validated for flat plate flow, circular cylinder in cross-flow, and the flow over a sharp wedge for Mach numbers ranging from 6 to 24, and for argon and nitrogen as the working gases. Our simulation results show that the new boundary conditions give better predictions for the surface pressures, compared with published experimental and DSMC data.

Thermo-Chemical Analysis of a Calcination Furnace to Produce Cathode Material for the Secondary Batteries

Lithium secondary batteries have been widely used in the portable electric devices as power source. Recently it is expected that the realm of its applications expands to the markets such as energy storage medium of hybrid electric vehicle (HEV), electric vehicle (EV). Cathode active material is crucial in terms of performance, durability, capacity of lithium secondary batteries. It is urgent to develope the technology for mass production of cathode material to cope with the markets' demands in the near future. In this study, a calcination furnace running in real production line is modelled in 3D, and the thermal flow and gas flow after chemical reaction in the furnace is analyzed through numerical computations. Based on the results, it is shown that large volume of CO2 gas is generated from chemical reaction. High concentration of CO2 gas and it's stagnation is clearly found from the reactant containers in which the reaction occur to the bottom area of the furnace. It is also studied that 15% or more CO2 mol fraction could affect to proper formation of LiCoO2 through TGA-DSC analysis. The solutions to evacuate carbon dioxide from the furnace are suggested through the change of furnace design and operating condition as well. Keywords : Secondary battery, cathode material, calcination furnace, LiCoO2

A silicon-glass-based microfabricated wide range thermal distribution gas flow meter

Recently, there has been high demand on miniaturizations of bio-instruments and wide range gas flux measurement in the field of chemistry and mechanics. This paper presents the design, fabrication, and characterization of a silicon-glass-based thermal distribution gas flow meter (20 mm × 10 mm × 1.6 mm) with a wide detection range. To facilitate the fabrication and maintain the stability of the sensor, a platinum (Pt) thin film was adopted as the heater and thermometers. Both the thermal property and temperature sensitivity of Pt thin film were characterized. SiO 2 passivation layers were deposited on top of the Pt film to prevent thermal and electrical shift of sensitive elements. Three pairs of thermometers were constructed beside the heater. Sensitivity and gas flux range of the gas flow meter can be increased by alternate use of these three sensor pairs. We also introduced a specific hardware control circuit system for real-time gas flux monitoring through the connection with a computer interface. The proposed gas flow sensor device was capable of measuring gas flux within the range of 0.8–2800 ml/min, thus demonstrating the potential for a wide range of applications.