Thermal Gas Flow Research Articles

Spatial assessment of pollutants concentration in air and soils impacted by industrial wastes in lagos state, Nigeria

The mega city of Lagos, Nigeria is plagued by various environmental issues, chief amongst them being environmental pollution induced by poor and disjointed urban industrial waste management practices. This study aimed to identify and quantify the types of urban industrial waste generated in Lagos State. The State plays host to the largest population base in Nigeria with over twenty million people. The study adopted a quasi-experimental design. Soil, industrial waste generation and pollutant levels were determined with the aid of a thermal mass gas flow meter. Industrial wastes were measured in situ using a weighting scale, while surface soil samples were collected at depths of 0–30 cm. Results show that 20% of the industrial waste generated in the metropolis was ignitable (possibility of spontaneous combustion). The results were thereafter compared with international standards on the permissible limits for various pollutants in air and soil of Lagos State. Ikorodu had the greatest contribution of industrial waste generated in Lagos, followed by Ikeja, Oshodi, Ojo/Alaba and Surulere. The implications of these results and findings is that a lot needs to be done in terms of appropriate legislation, enforcement and tracking of industrial waste generation in Lagos State for effective monitoring and implementation of management strategies.

State-of-the-Art on Advancements in Carbon-Phenolic and Carbon-Elastomeric Ablatives.

Ablative composites serve as sacrificial materials, protecting underlying materials from high-temperature environments by endothermic reactions. These materials undergo various phenomena, including thermal degradation, pyrolysis, gas generation, char formation, erosion, gas flow, and different modes of heat transfer (such as conduction, convection, and radiation), all stemming from these endothermic reactions. These phenomena synergize to form a protective layer over the underlying materials. Carbon, with its superb mechanical properties and various available forms, is highlighted, alongside phenolics known for good adhesion and fabric ability and elastomers valued for flexibility and resilience. This study focuses on recent advancements in carbon-and-phenolic and carbon-and-elastomeric composites, considering factors such as erosion speed; high-temperature resistance; tensile, bending, and compressive strength; fiber-matrix interaction; and char formation. Various authors' calculations regarding the percentage reduction in linear ablation rate (LAR) and mass ablation rate (MAR) are discussed. These analyses inform potential advancements in the field of carbon/phenolic and carbon/elastomeric ablative composites.

Dry Downhill Particle Motion on Mars

We recently flew a new setup on parabolic flights for the first time to study particle motion on Martian slopes under Martian gravity. Here, we describe the initial experiments. We used dust/sand beds at varying ambient pressure of a few hundred pascals. The inclination of the particle bed was varied from 0° to 45° and parts of the surface were illuminated under varying conditions. We could observe downhill motion of material related to the insolation at the lowest light flux used of 591 ± 11 W m−2 for JSC Martian simulant. Motion occurred at significantly lower inclinations under illumination than without illumination, i.e., down to about 10° compared to about 20°–30°, respectively. We attribute this reduction in slope to thermal creep gas flow in the subsoil. This induces a Knudsen compressor, which supports grains against gravity and leads to smaller angles of repose. This is applicable to recurring slope lineae and slopes on Mars in general.

Reliability of Domestic Gas Flow Sensors with Hydrogen Admixtures.

Static flow sensors (e.g., thermal gas micro electro-mechanical sensors-MEMS-and ultrasonic time of flight) are becoming the prevailing technology for domestic gas metering and billing since they show advantages in respect to the traditional volumetric ones. However, they are expected to be influenced in-service by changes in gas composition, which in the future could be more frequent due to the spread of hydrogen admixtures in gas networks. In this paper, the authors present the results of an experimental campaign aimed at analyzing the in-service reliability of both static and volumetric gas meters with different hydrogen admixtures. The results show that the accuracy of volumetric and ultrasonic meters is always within the admitted limits for subsequent verification and even within those narrower of the initial verification. On the other hand, the accuracy of the first generation of thermal mass gas flow sensors is within the limits of the verification only when the hydrogen admixture is below 2%vol. At higher hydrogen content, in fact, the absolute weighted mean error ranges between 3.5% (with 5%vol of hydrogen) and 15.8% (with 10%vol of hydrogen).

Experimental and numerical study on a SI engine fueled with gasohol and dissociated methanol gas blends at lean conditions

Hydrogen-rich dissociated methanol gas (DMG) and ethanol are both promising alternative fuels for engines. The study investigates the performance of a spark ignition engine using various gasohol-DMG blends under lean conditions, aiming to evaluate the influence of fuel composition, excess air ratio, and operational parameters including engine load and speed. The results demonstrate that the introduction of DMG substantially enhances the engine's fuel economy, combustion efficiency, combustion stability, and the HC and CO emissions. The addition of ethanol has a distinct impact on engine performance, including increased peak cylinder pressure at lower DMG ratios and a subsequent decrease at higher DMG ratios. It also leads to substantial reductions in HC and NOx emissions but shows a non-linear effect on CO emissions with an increasing ethanol blending ratio. An increase in excess air ratio results in simultaneous reductions in peak cylinder pressure, HC, CO, and NOx emissions. Moreover, variations in engine load and speed respectively impact the cylinder's thermal environment and gas flow, consequently influencing combustion and emission characteristics.

A high sensitivity and low noise thermal gas flow sensor based on silicon-doped thermocouples

A high sensitivity and low noise thermal gas flow sensor based on silicon-doped thermocouples

Surface brightness discontinuities in radio halos

Context. Dynamical motions in the intra-cluster medium (ICM) can imprint distinctive features on X-ray images that map the thermal bremsstrahlung emission from galaxy clusters, such as sharp surface brightness discontinuities due to shocks and cold fronts. The gas dynamics during cluster mergers may also drive large-scale turbulence in the ICM, which in turn generates extended (megaparsec-scale) synchrontron sources known as radio halos. Aims. Surface brightness edges have been found numerous times in the thermal gas of clusters based on X-ray observations. In contrast, edges in radio halos have only been observed in a handful of cases. Our goal is to search for new radio surface brightness discontinuities in the ICM. Methods. We inspected the images of the Bullet Cluster and the other 25 radio halos reported in the MeerKAT Galaxy Cluster Legacy Survey. To aid the identification of surface brightness discontinuities, we applied a gradient-filtering edge-detection method to the radio images. Results. We find that the adopted filtering technique is helpful in identifying surface brightness edges in radio images, allowing us to identify at least one gradient in half of the radio halos studied. For the Bullet Cluster, we find excellent agreement between the locations of the four radio discontinuities detected and the X-ray edges. This similarity informs us that there is substantial interplay between thermal and nonthermal components in galaxy clusters. This interplay is likely due to the frozen-in ICM magnetic field, which mediates the advection of cosmic rays while being dragged by thermal gas flows. Conclusions. We conclude that radio halos are shaped by dynamical motions in the ICM and that they often display surface brightness discontinuities, which appear to be co-located with edges in the thermal gas emission. Our results demonstrate that new and future generations of radio telescopes will provide an approach to efficiently detecting shocks and cold fronts in the ICM that is complementary to X-rays.

Peak heat flux prediction of hypersonic flow over compression ramp under vibrationally excited free-stream condition

In hypersonic shock tunnel experiments, the high-temperature reservoir gas expands and accelerates so rapidly that there is not enough time for vibrational energy relaxation. As a result, thermal nonequilibrium gas flow is frequently encountered in the test section, and this significantly affects the measured heat flux. In this paper, hypersonic compression-ramp flows are studied numerically to investigate the effect of incomplete vibrational energy accommodation on the separation flow structure and peak heat flux in the reattachment region under low-to-medium Reynolds number and high Mach number conditions. Numerical results and theoretical analysis suggest that the vibrational energy accommodation has no noticeable impact on the length scale of the separation zone, but strongly influences the peak heat flux of the separated ramp flows. Decomposing the peak heat flux into translational–rotational energy and vibrational energy components, qtr and qv, respectively, we find that qv/qtr characterizes the nonequilibrium degree of the vibrational energy accommodation. A formula for predicting the peak heat flux is then proposed, taking the effect of incomplete vibrational energy accommodation into consideration. Finally, surface heat flux measurements in a hypersonic shock tunnel indicate that a deviation of up to 13% in total peak heat flux could arise if vibrational energy accommodation is not considered under the vibrationally excited free-stream condition.

Reliability evaluation of thermal barrier coatings for engine combustion chambers based on Monte-Carlo simulation

Due to their outstanding high temperature resistance, thermal barrier coatings are commonly applied in hot section such as aero-engine combustion chambers to protect components from high-temperature airflow. However, its material properties are insecure, the service environment is harsh, and the service life is uncertain. This paper uses a Monte-Carlo simulation-based dependability evaluation method to resolve this uncertainty. To begin with, the failure criteria for thermal barrier coatings were defined, as was the limit state equation. Second, using the Monte-Carlo simulation approach, the failure probability was calculated. Finally, the TBCs' dependability in a thermal gas flow environment was assessed and compared to actual measurement results. The thermal barrier coating in the area where the surface temperature of the combustion chamber is above 1100 °C begins to fail after 4000 h of service under the action of the hot air flow, with a failure rate of more than 60 %. The service life of the thermal barrier coating at surface temperatures of 1000, 1100, and 1197 °C was 6327, 3125, and 1642 h, respectively, based on the time it took to attain 60 % failure probability under the impact of hot air flow. The numerical relative error of the actual failure probability PT of the thermal barrier coating of the combustion chamber obtained by hole detection technology is less than 10 % (except for 3000 h), demonstrating the dependability and applicability of the reliability assessment technique. The reliability of thermal barrier coatings is influenced by temperature and TGO development.

Efficient DSBGK simulations of the low speed thermal transpiration gas flows through micro-channels

Low speed thermal transpiration gas flows through micro-channels are simulated by the direct simulation BGK (DSBGK) method that is based on the Bhatnagar-Gross-Krook model equation. Different from the ordinary application of BGK equation having the viscosity matched, we selected its relaxation time according to the thermal conductivity coefficient and thus significantly improved the accuracy. In comparing the detailed flow field solutions of a 2D Knudsen pump, the validation by the standard direct simulation Monte Carlo (DSMC) method shows that the improved BGK equation has about the same accuracy as the Shakhov equation that is solved by the well-developed gas kinetic unified algorithm (GKUA). We also present the ordinary application of BGK equation with noticeable error to clearly show the difference made in our study. Then, the efficient DSBGK method is applied to simulate the low-speed rarefied gas flows in 3D real-size thermal transpiration problem to show its applicability and its validity is verified by comparing with the experimental measurement of mass flow rates. The DSBGK simulation results have excellent agreement with experimental data for three types of gases over a wide range of Knudsen number. Additionally, the channel end effect is also studied and discussed as a prerequisite for high-fidelity modeling, otherwise it will be mistakenly interpreted as the variation of accommodation coefficients of the molecular reflection process on the solid boundary.

MEMS thermal gas flow sensor with self-test function

In this paper, the design, fabrication and characterization of a MEMS thermal gas flow sensor with self-test function are presented. The flow sensor is composed of a platinum heater and thermopiles, where the heater is served as a heat source and the thermopiles are used for voltage output. In order to improve the performance of the flow sensor, the heavily doped N/P-polysilicon is utilized to form the thermocouple and XeF2 front-side isotropic etching is adopted to realize thermal isolation of the device. At the same time, the effects of the chip position in the gas channel and heater voltage on device performance are also studied from both experiment and simulation. Moreover, a self-test method and corresponding test system based on the thermal flow sensor are proposed, which use an equivalent heater voltage to simulate the gas flow rate for monitoring the dynamic response of the flow sensor to the gas. This method is simple and effective compared with traditional methods for detecting the performance flow sensors. In addition, the basic output performance of the flow sensor using nitrogen as the test gas is characterized. The experimental results show that the flow sensor owns a relatively high sensitivity of 123.006 mV (m/s)−1 (no amplification), a low response time of about 250 ms and good accuracy of ±1.97%.

An assessment of gas power leakage and frictional losses from the top compression ring of internal combustion engines

A multi-physics integrated analysis of piston top compression ring of a high-performance internal combustion engines is presented. The effects of transient ring elastodynamics, thermal gas flow through piston crevices upon chamber leakage pressure and parasitic frictional losses are investigated. The multi-physics analysis comprises integrated flexible ring dynamics, ring-liner thermo-mixed hydrodynamics and gas blow-by, an approach not hitherto reported in literature. The predictions show close conformance to frictional measurements under engine motored dynamometric conditions. It is shown that power losses due to gas leakage can be as much as 6 times larger than frictional losses, which are usually considered as the main sources of inefficiency.

NH4Cl-induced low-temperature formation of nitrogen-rich g-C3N4 nanosheets with improved photocatalytic hydrogen evolution

The high-temperature secondary calcination (>500 °C) of bulk g-C3N4 usually suffers from a very low yield of g-C3N4 nanosheets owing to its serious and massive depolymerization. In this study, a NH4Cl-induced low-temperature second-calcination approach has been used to synthesize nitrogen-rich g-C3N4 nanosheets with a high yield (ca. 32 wt%), which includes the initial intercalation of NH4Cl into the interlayers of bulk g-C3N4 and the following direct low-temperature calcination at 400 °C. It is found that during the calcination process, the thermal gas flow (HCl and NH3) from NH4Cl decomposition not only can efficiently facilitate the delamination and depolymerization of the g-C3N4 structure, but also can introduce many amino groups on the g-C3N4 surface, resulting in the successful synthesis of nitrogen-rich g-C3N4 nanosheets at such a low temperature. Experimental data suggests that the resulting nitrogen-rich g-C3N4 nanosheets show a distinct enhancement for the H2-evolution performance mainly owing to the introduction of amino groups, which can efficiently enrich H+ from water to facilitate the rapid generation of H2. This study may open up a fire-new insight for the preparation of high-efficiency nanometer materials.

Asymptotic modelling of thermal binary gas flows in circular micro-channels

Many industrial and medical tools include micro-metric channels. These are found, for example, in micro-filters. The DSMC methods are commonly used to compute gas flows in these sub-millimeter-sized channels. Nevertheless, these statistical methods are expensive in computation time (Bird in Gas dynamics and the direct simulation of gas flows. Clarendon Press, Oxford, 1994; Aktas et al. in J Microelectromech Syst 10:538–549, 2001). Consequently, the development of alternative methods, with lower computation cost, is an approach of main interest. This paper focuses on binary gas mixture flows with low Mach numbers and low to moderate Knudsen numbers. In a phenomenological point of view, this case corresponds to the slip regime. An asymptotic model for two compressible gases is presented in the case of axisymmetric flows in circular micro-pipes. This model is derived from a BGK-type model for the mixture. Its results are compared to DSMC simulations, and the impact on the flow of the mass flow rates, the wall temperature and the molecular weights are discussed.

Modeling of Knudsen Layer Effects in the Micro-Scale Backward-Facing Step in the Slip Flow Regime

The effect of the Knudsen layer in the thermal micro-scale gas flows has been investigated. The effective mean free path model has been implemented in the open source computational fluid dynamics (CFD) code, to extend its applicability up to slip and early transition flow regime. The conventional Navier-Stokes constitutive relations and the first-order non-equilibrium boundary conditions are modified based on the effective mean free path, which depends on the distance from the solid surface. The predictive capability of the standard ‘Maxwell velocity slip—Smoluchwoski temperature jump’ and hybrid boundary conditions ‘Langmuir Maxwell velocity slip—Langmuir Smoluchwoski temperature jump’ in conjunction with the Knudsen layer formulation has been evaluated in the present work. Simulations are carried out over a nano-/micro-scale backward facing step geometry in which flow experiences adverse pressure gradient, separation and re-attachment. Results are validated against the direct simulation Monte Carlo (DSMC) data, and have shown significant improvement over the existing CFD solvers. Non-equilibrium effects on the velocity and temperature of gas on the surface of the backward facing step channel are studied by varying the flow Knudsen number, inlet flow temperature, and wall temperature. Results show that the modified solver with hybrid Langmuir based boundary conditions gives the best predictions when the Knudsen layer is incorporated, and the standard Maxwell-Smoluchowski can accurately capture momentum and the thermal Knudsen layer when the temperature of the wall is higher than the fluid flow.

Low-rank coal gasification using a bubbling fluidized bed reactor at low operating temperature

Coal gasification is one of coal utilizations that produces less CO2 emission than coal combustion. Coal gasification technology that has been used in Indonesia is generally a fixed bed gasification. Fixed bed is designed for high-rank coal and the majority of Indonesian coal is of a low-rank. Low ash and high moisture content of the Indonesian coal in a fixed bed can affect mechanical and thermal fragmentation, pressure drop, gas and particle flow distribution. The operation of gasifier may cause unstable condition. Another gasification technology is bubbling fluidized bed, which is operated above 1,200°C, so ash can melt. High operating temperature causes agglomeration and makes unstable gasification process. Therefore, in this study, low-rank coal is gasified in bubbling fluidized bed reactor at low operating temperature. The purpose of this study is to determine the optimal conditions of bubbling fluidized bed gasification. The research was conducted in bubbling fluidized bed coal gasification Process Development Unit (PDU) at Coal Utilization Technology Centre of R&D Centre of tekMIRA, Palimanan. Coal was fed continuously as many as 20 kg/hour into a gasifier then was gasified to produce gas using air as a gasifying agent and silica sand as a bed material at 850- 950°C. The produced gas from the gasification was analyzed using the Orsat Analyzer. A simulation using a ChemCAD 7.1 CC steady state was applied to validate the experiment result. From the analyzed result of yield gas composition, the produced CO and CO2 were about 10-15 wt%. Gas compositions that are close to criteria of producer gas, no agglomeration, and stable process condition during study indicate that bubbling fluidized bed gasification at low operating temperature is suitable to be applied as gasification technology for Indonesian low-rank coal.

Mathematical modeling of active thermo-emission thermal protection system of aircraft in high speed air flow

A mathematical model of the process of nonstationary heat transfer between active thermo-emission thermal protection system (ATETPS) and convective gas flow is given. The effect of electron’s evaporation (emission) from the surface of the emitter on lowering its temperature is studied.

A Numerical simulation of delamination caused by internal gas pressure for mid-density CFRP

Interlaminar debonding (delamination) is a major problem for carbon fiber-reinforced plastic (CFRP) when it is used in thermal protection systems subjected to rapid heating, such as in a re-entry situation. In this study, the possibility of avoiding delamination by controlling material porosity was investigated experimentally and numerically. CFRP specimens having various initial porosities (mid-density CFRP) were prepared and rapid heating tests (∼6 MW/m2) were performed, and the specimens were examined to determine whether delamination had occurred. To predict the delamination, heat transfer and pyrolysis gas-induced internal gas pressure growth were simulated numerically as a function of heating time. The parameters regarding temperature-dependent chemical reaction rate, orthotropic coefficients for gas permeability, and flatwise tensile strength were determined by individual experiments using the CFRP specimens. The coupled analysis of thermal transfer and pyrolysis gas flow following Darcy’s law was conducted by finite-difference form, employing the experimentally obtained parameters. When the gas pressure exceeds the flatwise tensile strength of CFRP, delamination is assumed to occur. The numerical results show good agreement with the experimental results, and it is concluded that delamination can be eliminated by using mid-density CFRP.

Analog experiments on thermal creep gas flow through Martian soil

At the low atmospheric pressure on Mars, thermal creep within its soil is unavoidable. Soil is a complex mixture of particles and pores and it is not clear a priori how to quantify such subsoil thermal creep gas flow. A capillary model of flow through granular beds was set up recently based on laboratory experiments. Here, we extend the earlier experiment to probe different parameter ranges of the model. For a different granular bed we directly traced the resulting gas flow in a pressure regime of 2–300 Pa. We specifically analyze the dependence of thermal creep on different gas species - we used He, CO2, N2 and air. These data agree well to the model calculations. This latest evidence strongly supports the simple but analytical model of granular thermal creep. Including CO2 in the measurements the model now allows quantitative calculations of gas circulation and pressure distribution within Martian soil as a network of Knudsen pumps.

Analysis of the exact solution for the evaporative convection problem and properties of the characteristic perturbations

The exact solution for the Boussinesq approximation of the Navier – Stokes equations is analytically constructed to describe the joint flow of an evaporating viscous heat-conducting liquid and gas-vapor mixture in an infinite horizontal channel. The effects of thermodiffusion and diffusive heat conductivity in the gas-vapor phase are additionally taken into account in the governing equations and interface conditions. Possible flow types are classified with respect to the types of the boundary conditions for the vapor concentration on the upper solid wall of the channel. The importance of a solution of special type is that it gives a possibility to specify on the qualitative level the physical factors defining the basic flow mechanisms. The constructed solution has the group nature and allows one to describe the real flow regimes and a formation of patterns observed in the experiments. It describes three classes of flows: pure thermocapillary, mixed and Poiseuille flows according to the dominant force or interaction of effects. The stability of the two-layer flows of the liquid and gas-vapor phase is investigated for the equal values of the longitudinal temperature gradients on the channel walls. In this case the perturbations of the basic flow can lead to the formation of the vortex and thermocapillary structures. The influence of the thermal load intensity and gas flow rate on the type of the arising instability is studied. The evolution of the perturbations is investigated numerically.