Temperature Of Surrounding Gas Research Articles

Role of volatile secondary char on the combustion behavior of cellulose-based hydrochars

Hydrothermal carbonization (HTC) can transform a wide range of biomass into renewable biofuels. Hydrochar, the solid fuel resulting from HTC, has a similar energy density to low-rank coals. Despite an extensive body of literature detailing the thermogravimetric analysis (TGA) of hydrochar under slow pyrolysis and oxidation, there remains a limited understanding of hydrochar's behavior in realistic combustion settings. In this work, we integrate combustion experiments and TGA to understand the combustion characteristics of a model hydrochar fuel. Cellulose-based hydrochars produced at different temperatures along with solvent-extracted chars are tested in a Hencken burner across varying temperatures and oxygen concentrations in the oxidizer gas. Simultaneous CH* chemiluminescence, particle image velocimetry, and two-color pyrometry are used to measure ignition delay time and identify homogeneous/heterogeneous ignition mechanisms and combustion phases. Incorporating TGA with combustion results shows that the ignition mode and combustion processes are strong functions of surrounding gas temperature and oxygen mole fraction. The level of carbonization of the hydrochar dictates the ignition delay time and combustion modes. Further, the presence of a tar-like secondary char on the as-carbonized hydrochars leads to more rapid ignition due to high volatility.

Experimental and simulation study on evaporation characteristics of E85 impinging spray under DISI cold-start condition

The direct injection of ethanol-gasoline blended fuel into the engine cylinder near the top dead center (TDC) is evitable to suffer from the wall impingement due to more injection mass required to maintain the same air/fuel ratio with traditional fossil fuels. In this study, the evaporation characteristics of E85 cold-start impinging spray were investigated by experiments and numerical simulation. A constant volume vessel was used to produce the direct-injection spark-ignition (DISI) engine-like conditions. The ultraviolet (UV) – visible (Vis) dual-wavelength laser absorption scattering (LAS) technique was employed to quantitatively measure the individual components evaporation in E85, with the emphasis on the effects of impinging distance, wall temperature and injection pressure on the mixture formation. The results show that although the larger impinging distance rarely influences the evaporation before impingement, it facilitates the E85 evaporation process after impingement due to the better atomization by low boiling point (LBP) components (e.g. ethanol). The effects of cold-start wall temperature on the E85 impinging spray are greatly dependent on the impinging distance. Under the small impinging distance, the ethanol and low boiling point (LBP) components of gasoline evaporate faster after impingement at higher wall temperature, but such effect becomes absent under long impinging distance. Therefore, the smaller impinging distance shows a higher sensitivity to wall temperature. Lower wall temperature leads to a high wall heat transfer, thus potentially extend the evaporation time at the near wall region along the spray axis. At either impingement timing or initial impingement, the LBP component with high latent heat evaporates fast, leading to a decrease of surrounding gas temperature. Increasing injection pressure helps to greatly improve the evaporation of the high boiling point (HBP) component in gasoline under cold-start impingement. The evaporation ratio of E85 increases up to about 18% ∼ 30% than that of E100 at the initial impinging period under cold-start condition.

Lipid-microparticles for pulmonary delivery of active pharmaceutical ingredients: Impact of lipid crystallization on spray-drying processability

Spray-drying is an extensively used technology for engineering inhalable particles. Important technical hurdles are however experienced when lipid-based excipients (LBEs) are spray-dried. Stickiness, extensive wall deposition, or simply inability to yield a solid product have been associated to the low melting points of LBEs. In this work, solutions containing polyglycerol esters of behenic acid (PGFA-behenates), or other high melting point LBEs, were spray-dried to produce ibuprofen (IBU)-loaded inhalable lipid-microparticles. Prior to spray-drying, rational boundaries for the outlet temperature of the process were defined using LBE-IBU phase diagrams. Despite spray-drying the solutions at outlet temperatures below the boundaries, process performance and yield among LBEs were entirely different. Lipid crystallization into polymorphs or multi-phases negatively impacted the yield (10–47%), associated to liquid fractions unable to recrystallize at the surrounding gas temperature in the spray-dryer. The highest yields (76–82%), ascribed to PGFA-behenates, resulted from monophasic crystallization and absence of polymorphism. Lipid-microparticles, composed of a PGFA-behenate, were characterized by a volume mean diameter of 6.586 µm, tap density of 0.389 g/cm3 and corrugated surface. Application as carrier-free dry powder for inhalation resulted in high emitted fraction (90.9%), median mass aerodynamic diameter of 3.568 µm, fine particle fraction of 45.6% and modified release in simulated lung fluid.

Influence of gaseous mixture parameters on the maximum burning temperature and combustion rate of carbon particle

The method is developed to determine the maximum temperature of burning carbon particle and corresponding value of particle diameter by using the maximum temperature as a parameter depending on surrounding gas temperature and oxygen content. This method allows finding a limiting value of oxygen content below which ignition of a carbon particle is not possible in cold oxygen-nitrogen mixture. It is shown that the particle temperature attains a maximum value in the transitional regime of chemical reactions. The rise of the burning temperature with the particle size decrease is caused by relative decrease of radiation heat loss.

Thermophoresis of nanoparticles hotter/colder than the surrounding dilute gases

Aerosol particles suspended in a diluted gas with non-uniform temperature distribution are expected to experience a thermophoretic force. In theoretical treatment of thermophoresis, it is usually assumed that the particle temperature is equal to the surrounding gas temperature. However, this might not always be the case. In some particular applications, the particle temperature can significantly differ from the gas temperature. In the present paper, we theoretically investigate the effect of the particle temperature on the thermophoresis of nanoparticles in the free molecule regime. Theoretical formulas for the thermophoretic force and thermophoretic velocity are obtained based on the gas kinetic theory. As examples, a spherical Ag nanoparticle suspended in a dilute He gas is considered, and the Rudyak–Krasnolutski potential is employed to model the gas–particle interaction. It is found that the influence of the particle temperature on the thermophoresis of nanoparticles can be significant. With increasing particle size, the error due to the equal gas–particle temperature assumption can be neglected.

Experimental investigation of wall heat transfer due to spray combustion in a high-pressure/high-temperature vessel

Combustion chamber wall heat transfer is a major contributor to efficiency losses in diesel engines. In this context, thermal swing materials (adapting to the surrounding gas temperature) have been pinpointed as a promising mitigative solution. In this study, experiments are carried out in a high-pressure/high-temperature vessel to (a) characterise the wall heat transfer process ensuing from wall impingement of a combusting fuel spray, and (b) evaluate insulative improvements provided by a coating that promotes thermal swing. The baseline experimental condition resembles that of Spray A from the Engine Combustion Network, while additional variations are generated by modifying the ambient temperature as well as the injection pressure and duration. Wall heat transfer and wall temperature measurements are time-resolved and accompanied by concurrent high-speed imaging of natural luminosity. An investigation with an uncoated wall is carried out with several sensor locations around the stagnation point, elucidating sensor-to-sensor variability and setup symmetry. Surface heat flux follows three phases: (i) an initial peak, (ii) a slightly lower plateau dependent on the injection duration, and (iii) a slow decline. In addition to the uncoated reference case, the investigation involves a coating made of porous zirconia, an established thermal swing material. With a coated setup, the projection of surface quantities (heat flux and temperature) from the immersed measurement location requires additional numerical analysis of conjugate heat transfer. Starting from the traces measured beneath the coating, the surface quantities are obtained by solving a one-dimensional inverse heat transfer problem. The present measurements are complemented by CFD simulations supplemented with recent rough-wall models. The surface roughness of the coated specimen is indicated to have a significant impact on the wall heat flux, offsetting the expected benefit from the thermal swing material.

Analysis of surrounding gas temperature influence on thermocouple measurement temperature

When thermocouple is used to measure high temperature gas flow, the measured temperature, i.e., the thermocouple bead temperature is not exactly the gas temperature. The bead temperature results from the bead energy balance including the convection with the gas flow, the radiation with the environment and the gas, the conduction with the thermocouple wire near the bead. The wire temperature is determined by its own energy balance relating to the gas temperature contacting the wire. Through the wire conduction, the bead temperature is related to the gas temperature around the wire near the bead. The one-dimensional thermocouple heat transfer simulation code was developed and validated by the elaborate experiment. In the current study, this code is used to study the influence of the surrounding gas temperature on the bead temperature. Of course, the gas temperature far away from the bead will not influence the bead temperature. The size of the surrounding region which could influence the bead temperature is identified and its relation with the flow condition and the thermocouple size is analyzed. The quantitative bead temperature variation with the temperature gradient inside this region is studied.

Influence of surrounding gas temperature on thermocouple measurement

When thermocouples are used to measure gas temperature, the measured temperature, i.e., the thermocouple bead temperature, is not equal to the gas temperature. The bead temperature results from the bead energy balance including the convection with the gas flow, the radiation with the environment and the gas, the conduction with the thermocouple wires. The wire temperature is determined by its own energy balance relating to the gas temperature contacting the wire. Through the wire conduction, the bead temperature is in fact related to the gas temperature surrounding the bead. The influence of the surrounding gas temperature on the thermocouple bead temperature is studied numerically with the one-dimensional code here. The effect length, which is the size of the influencing region, is identified and its relation with the flow condition and the thermocouple property are analyzed. Analytical expressions are also deducted to calculate the effect length. The analytical and numerical results show very good agreement. The analytical effect length follows the same trend as the numerical value when the size and thermal conductivity of the thermocouple and the velocity and temperature of the incoming gas are varied. The analytical effect length deviates from the numerical value by less than 14.3%.

High temperature gasification of high heating-rate chars using a flat-flame reactor

The increasing interest in gasification and oxy-fuel combustion of biomass has heightened the need for a detailed understanding of char gasification in industrially relevant environments (i.e., high temperature and high-heating rate). Despite innumerable studies previously conducted on gasification of biomass, very few have focused on such conditions. Consequently, in this study the high-temperature gasification behaviors of biomass-derived chars were investigated using non-intrusive techniques. Two biomass chars produced at a heating rate of approximately 104K/s were subjected to two gasification environments and one oxidation environment in an entrained flow reactor equipped with an optical particle-sizing pyrometer. A coal char produced from a common U.S. low sulfur subbituminous coal was also studied for comparison. Both char and surrounding gas temperatures were precisely measured along the centerline of the furnace. Despite differences in the physical and chemical properties of the biomass chars, they exhibited rather similar reaction temperatures under all investigated conditions. On the other hand, a slightly lower particle temperature was observed in the case of coal char gasification, suggesting a higher gasification reactivity for the coal char. A comprehensive numerical model was applied to aid the understanding of the conversion of the investigated chars under gasification atmospheres. In addition, a sensitivity analysis was performed on the influence of four parameters (gas temperature, char diameter, char density, and steam concentration) on the carbon conversion rate. The results demonstrate that the gas temperature is the most important single variable influencing the gasification rate.

Gasification of High Heating-rate Biomass-derived Chars at Elevated Temperatures

Abstract In this study, gasification behavior of high heating-rate chars were investigated using an entrained flow reactor with an optical particle-sizing pyrometer. Two biomass chars produced from forest residue and torrefied forest residue as well as one coal char produced from low sulfur Black Thunder subbituminous coal have been gasified at temperature over 1700 K. Both the char surface temperatures and surrounding gas temperatures were precisely measured. Despite large discrepancies in constituents, the two biomass chars showed rather similar surface temperatures at the tested conditions. However, a slight lower surface temperature was obtained by the coal char gasification. A comprehensive model was also applied to aid understanding of the conversion process of char gasification. One set of intrinsic kinetic parameters for both the heterogonous water-gas reaction and the Boudouard reaction were found to effectively capture the experimental data at the two investigated conditions.

Critical condition limits for the high-temperature oxidation of gases on a catalyst particle

The critical conditions have been analytically determined for the high-temperature oxidation of gases over a catalyst. At certain catalyst particle sizes and in a certain range of surrounding gas temperatures, there are gas autoignition and induced ignition regions provided that the reactor walls remain cold. Inside the autoignition region, there can be a catalytic gas autoignition on the particle surface at any initial temperature of the latter. In the induced ignition region, the catalytic combustion of gases occurs at a catalyst temperature above some critical value. The limiting critical concentrations of the combustible component for gas autoignition and induced ignition have been analytically estimated. A gas temperature effect on the limiting critical conditions and on the parameters at which these conditions undergo degeneration has been revealed for catalyst particles with different emissivities. Depending on the catalyst particle size, the upper boundaries of the gas autoignition and induced ignition regions are determined by the effect of radiative heat loss.

Using adiabatic surface temperature for thermal calculation of steel members exposed to localized fires

This paper provides a new way to calculate the temperature of steel components in localized fires. A newly developed concept named adiabatic surface temperature (AST) on fire-structural interface research has been used in the calculation to represent the fire exposed boundary conditions. Based on AST, a simple heat transfer model with the capability of considering heat conduction among different parts of the steel sections has been proposed. The popular CFD code FDS (version 5.3.0), which can output ASTs, has been adopted to simulate the heating of a steel beam and a steel column in a localized fire. Steel temperatures calculated by the proposed model agree very well with the results from CFD simulation. Studies also obviously show that using the surrounding gas temperatures for thermal calculation of steel members exposed to fire might yield unreasonable and unsafe results.

Effect of surrounding gas temperature on the morphological evolution of TiO2 nanoparticles generated by laser ablation in tubular furnace

Titanium oxide nanoparticles are synthesized by laser ablation of Ti target in oxygen atmosphere under well-controlled temperature profiles in a tubular furnace. The size and the shape of generated nanoparticles are varied by changing the temperature of furnace. The mobility-based size distributions of generated air-borne nanoparticles are measured using a scanning mobility particle sizer, and the size distributions of primary particles are analyzed by a scanning electron microscope. When the particles are generated by laser ablation at the room temperature, the particles are agglomerates in gas phase with the average mobility diameter of 117 nm and the mean diameter of primary particles of 11 nm. The primary particle diameter increases from 11 to 24 nm by raising the furnace temperature up to 800 °C. Since the mass of Ti vapor ablated from a target is found to be constant regardless of the furnace temperature, this particle growth may be attributed to the reduction in nuclei number as a result of mild quenching at higher temperatures. As the temperature reaches higher than 1,000 °C, the mobility diameter suddenly drops and the primary particle diameter increases due to sintering, and at 1,200 °C the mobility diameter coincides with the primary particle diameter. Since the laser oven method offers an independent control of vapor concentration and the temperature of surrounding atmosphere, it is an effective tool to study the formation process of nanoparticles from primary particles with a given size.

Multiplexed electrospray scaling for liquid fuel injection

Evaporation and space-charge requirements are evaluated to understand the effect of device scaling and fuel preheating for a liquid fuel injector using a multiplexed electrospray (MES) configuration in compact combustion applications. This work reveals the influence of the droplet diameter, droplet velocity and droplet surface temperature as well as the surrounding gas temperature on the size and performance of microfabricated MES. Measurements from MES devices are used in the model to accurately account for the droplet diameter versus flow rate relationship, the minimum droplet diameter and the relevant droplet velocities. A maximum extractor electrode to ground electrode distance of 3.1 mm required to overcome space-charge forces is found to be independent of voltage or droplet velocity for large levels of multiplexing. This maximum distance also becomes the required evaporation length scale which imposes minimum fuel pre-heating requirements for large flow densities. Required fuel preheating is therefore evaluated for both ethanol and 1-butanol with combustor parameters relevant to fuel reformation, thermoelectric conversion, thermophotovoltaic conversion and thermionic conversion.

Numerical analysis of the effect of the gas temperature on splat formation during thermal spray process

Thermal spray coatings are affected by various parameters. In this study, the finite element method with volume of fluid (VOF) procedure is used to investigate the deposition process which is very important for the quality of sprayed coatings. The specific heat method (SHM) is used for the solidification phenomenon. A comparison of the present model with experimental and numerical model available in the literature is done. A series of numerical calculations is carried out to investigate the effect of the surrounding gas temperature on the splat formation. The variation of the surrounding gas temperature has a significant effect on splat morphology and can affect the adhesion of the splat on the substrate.

Application of the CFAST zone model to the Fire PSA

The integrity of the cables located in the target room is very important in the Fire PSA, because the CDF and CCDP are changed according to the results of a cable integrity that depends on the surrounding gas temperature. The conservative assumptions used in the Fire PSA typically specify that all of the equipment and cables of a room would fail when a fire happens in the room. But the realistic assessment of a fire risk by using a fire simulation tool has become necessary in the Fire PSA as described in the ANS Fire PRA Standard. This paper evaluates the cable integrity of eight pump rooms in the nuclear power plant by using the CFAST zone fire model. The upper layer gas temperature of each room is estimated, and an analysis based on the results of model simulations is used to judge the cable integrity. According to the analysis results, the integrity of the cable located in the upper layer in the pump rooms is maintained without any thermal damage, and the CCDP is reduced to about the half of the original CCDP. A fire safety assessment for dominant pump rooms by using a fire modeling tool is seen capable of evaluating the consequences of a fire scenario, of reducing the uncertainty, and of developing a more realistic Fire PSA model. It is considered that the fire modeling can satisfy the requirements of the ANS Fire PRA standard related to a fire modeling, and improve the quality of the Fire PSA.

雰囲気酸素濃度・温度分布の不均一性がディーゼル燃焼に及ぼす影響 : 第2報,温度の不均一性の影響(熱工学,内燃機関,動力など)

In order to clarify the effect of heterogeneity of distributions of temperature and oxygen concentration in a combustion chamber induced by EGR on the soot and NO emissions from the diesel engine, the combustion characteristics of diesel spray flame achieved in a rapid compression and expansion machine (RCEM) at various patterns of temperature distribution in the chamber are investigated. The distribution of surrounding gas temperature in a chamber is controlled by changing the location, phase, and duration of EGR gas injection with multi-port intake system. The multi-port intake system can make the circler distributions of gas temperature with different gradient. NO and soot emissions are measured at different temperature distributions. Results indicate that the NO_x and soot emissions from diesel combustion are governed mainly by the amount of enthalpy entrained into the spray upstream the ignition region, when the temperature distribution in a chamber is changed with keeping the oxygen fraction at constant.

Thermal equilibration of soot charge distributions by coagulation

Flame generated soot particles acquire a symmetric electrical charge distribution shortly after inception. Coagulation between soot particles causes the electrical charge to evolve to a Boltzmann distribution characterized by the flame temperature. When the soot is removed from the flame and allowed to coagulate further, the charge temperature (as defined via the Boltzmann distribution) is observed to fall to room temperature over a period of a few seconds. The resulting charge distribution is independent of the initial condition, e.g., the height in the flame at which the soot is sampled. In effect, coagulation acts to equilibrate the electrical charge distribution with the surrounding gas temperature even though the process is itself irreversible. The experimental observations are reproduced by a kinetic coagulation model that includes Coulomb forces between the colliding particles. In the case of diesel soot, a charge temperature of ∼ 1100 K is found at the vehicle tailpipe, presumably because cooling by coagulation remains incomplete over the transit time from engine to tailpipe.

Protection schemes for critical surface in vacuum environments

This article presents a study of particle formation during vacuum pump down and methods that can be incorporated to protect critical surfaces (like semiconductor wafers or masks) from those particles. Particle formation during pump down was reexamined with temperature measurements. Particles were intentionally produced with hard pump down for studying protection schemes. For the first step, a face-down approach for a critical surface was used to investigate the effect of protection from particle contamination. It was very effective to hold a critical surface face down for protection with the use of high gravitational settling velocity in vacuum environment. However, the face down approach did not sufficiently protect the critical surface. For the second step, a bottom protective plate was introduced below the critical surface to improve the protection efficiency. The bottom plate played a great role in protection of the critical surface with preventing particle formation near the critical surface by keeping the surrounding gas temperature high enough to avoid particle formation as well as with potential blocking of incoming particles toward the critical surface. Higher gas temperature intrinsically avoids formation of residue particles by the condensation process during pump down.

Simulation of mass transfer from an oscillating microdroplet

Mass transfer from micrometer and sub-micrometer airborne microdroplets arises in various chemical process, material science, and atmospheric phenomena, such as impinging flow reactor and unsteady pollutant diffusion, where microdroplet oscillation can substantially increase the mass transfer rate. Previous theories, however, do not adequately predict this enhancement of mass transfer, especially in the case of relatively large-amplitude oscillations. We have analyzed slow evaporation of an oscillating microdroplet having a sufficiently low vapor pressure such that it remains at the surrounding gas temperature and has a negligibly small rate of change of diameter. We solved the governing convective diffusion equation numerically to obtain the Sherwood number as a function of the system parameters. These include the oscillation frequency, the maximum velocity, and the initial microdroplet diameter. The theoretical results are compared with mass transfer data from the literature for a dodecanol microdroplet levitated in an electrodynamic balance (EDB) and oscillated by varying the dc levitation voltage and the ac amplitude and frequency. The predicted Sherwood numbers agree with the experimental results with a mean deviation of 9.2%. The analysis shows a distinct periodic change in the mass transfer rate or Sherwood number with a period that is one-half the period of oscillation of the microdroplet.