Axial Temperature Profiles Research Articles

Catalytic partial oxidation of ethanol over Rh-coated monoliths investigated by the axially resolved sampling technique: Effect of H2O co-feed

The catalytic partial oxidation (CPO) of ethanol with H2O co-feed was studied in a laboratory scale adiabatic reactor on Rh/α-Al2O3 catalysts deposited on honeycomb monoliths. In order to understand the behavior of the homogeneous and the heterogeneous reactions taking place in the CPO process, axial temperature and concentration profiles were collected using a spatially resolved sampling technique. The effect of steam co-feed was studied at C/O ratio of 0.65, varying the H2O molar concentration in the feed from 0% to 10 %. H2O was added to the system by replacing part of the diluting N2, keeping a constant ethanol concentration. At increasing H2O feed content, ethanol conversion grew close to 100 %. The water gas shift reaction was promoted, which resulted in increased H2 yield and H2/CO ratio. Globally, thermal effects of the water co-feed were negligible. Noteworthy, a significant drop of the concentration of cracking products such as CH4, and C2H4 in the initial part of the reactor was observed in the presence of water co-feed. Beneficial effects on the catalyst stability were also observed by performing tests of CH4-CPO (a reference reacting system that, due to the sensitivity of the temperature profile to the catalyst activity, is used to detect catalyst deactivation processes in between ethanol CPO tests) and by TPO measurements on the used catalysts. Less pronounced drift of the temperature-profile in CH4-CPO and lower carbon deposition were in fact associated with the water co-feed operation of the reactor.

Experimental feasibility of tailored porous media burners enabled via additive manufacturing

The advantages in performance and emissions from combustion within the voids of a porous material, as compared to those of conventional open-flame combustion, have motivated numerous studies to understand and harness the connection between the underlying porous structure topology and the desired system-level properties. The current study examines the feasibility and performance of additively manufactured complex ceramic structures with geometric functionalization to actualize novel porous media burner design concepts for enhanced performance. Smoothly graded porous structures are synthesized using triply periodic minimal surfaces and manufactured via lithography-based ceramic manufacturing. Experiments are performed on these structures to characterize flame stability, axial temperature profiles, and pressure drop for three different pore-scale topologies. These measurements are complimented by computational predictions from one-dimensional volume-averaged models. X-ray computed tomography was used to verify relevant geometric properties of the structures, as well as to examine the material after combustion. This work demonstrates the first smoothly graded burner, leveraging recent advancements in additive manufacturing to design, fabricate, and test the performance of burner concepts unattainable by traditional ceramic foam manufacturing techniques.

Investigating azeotropic separation of hydrochloric acid for optimizing the copper-chlorine thermochemical cycle

In this paper, an atmospheric-pressure distillation system is designed and constructed for partial to separation of hydrochloric acid and water. The system concentrates HCl(aq) between the electrolyzer and hydrolysis processes of the Copper–Chlorine (Cu–Cl) cycle for hydrogen production. The motivation behind this study is to investigate azeotropic separation of HCl(aq), as needed for integration of unit operations in the Cu–Cl cycle. The separation is only partial, as the mixture is unable to cross the azeotrope with only a single pressure. The distillation system consists primarily of one packed distillation column, which employs heating tapes and thermocouples to achieve a desired axial temperature profile. The column can be operated in batch or continuous mode. The distillate is H 2O(l) and the bottoms is HCl(aq) near the azeotropic concentration; feed concentrations are less than azeotrope. Thus, the degree of separation is determined to be independent of the feed concentration. The bottoms concentration varies from experiment to experiment, but does so independently of feed concentration, likely the result of corrosion impurities affecting the calculation of its concentration. It is found that HCl(aq) can be concentrated up to approximately 0.1068 mol/mol from an initial concentration of 0.0191 mol/mol. A simulation of pressure-swing distillation (PSD) is also performed, but due to safety constraints (a column operating at 10 atm must be certified to CSA B51), a single-pressure (single-column) distillation is physically performed. A single-pressure column is beneficial to the Cu–Cl cycle because it partially recycles HCl, which reduces the cost of the cycle, and still provides valuable results for analysis. The maximum HCl concentration achieved experimentally is 0.1068 mol/mol and the maximum HCl concentration determined from simulation is 0.11 mol/mol (the azeotropic concentration). The novelty of this research is that the experimental column built to study HCl partial separation is designed to be simple yet safe to integrate within the Cu–Cl cycle for hydrogen production.

Impacts of variable thermal conductivity and mixed convective stagnation‐point flow in a couple stress nanofluid with viscous heating and heat source

AbstractThe current study examines mixed (combined) convection stagnation‐point couple stress nanofluid over a stretched cylinder of variable thermal conductivity in the presence of viscous dissipation and internal heat source. The basic governing partial differential equations have been converted to coupled nonlinear differential equations by using adequate similarity transformations. By applying semi‐analytic technique (BVPh2.0), the equivalent ordinary differential equations are successfully solved and validated with a bvp4c solver. Graphs are presented to study the impact of various parameters on axial velocity, temperature, and volumetric nanofluid concentration profiles. The coefficient of skin friction (quantifying resistance) and the rate of heat and mass transfer on the surface due to flow variables are computed and explained. The axial velocity and momentum thickness are decreased with increasing couple stress parameter, whereas the reverse trend is noted with mixed convection and buoyancy ratio parameters. The temperature distribution increases for increasing Brownian motion and thermal conductivity parameter, whereas it decreases for increasing stagnation parameter.

Thermal analysis of electroosmotic flow in a vertical ciliated tube with viscous dissipation and heat source effects

Thermal analysis of electroosmotic flow of Newtonian fluid in the presence of viscous dissipation and heat source in a vertical ciliated tube is discussed. Three different cases of electroosmotic flow with heat source, viscous dissipation and buoyancy effects are presented, and their solutions are calculated analytically. Exact solutions for velocity and temperature field are calculated for the first case (presence of heat source and buoyancy force) and second case (presence of viscous dissipation), but the approximate solution is calculated by the Adomian decomposition method for the third case (presence of buoyancy force and viscous dissipation). The modeled equations of electroosmotic flow are reduced into simple equations by the long wavelength and low Reynolds number approximation. Influences of physical parameters are graphically computed for axial velocity, temperature profile, pressure gradient and stream function. It is observed that electroosmosis shows the significant effect on velocity and temperature profile in the presence of viscous dissipation and buoyancy force. This mathematical model gives a deep understanding to describe the thermal analysis in biomimetic ciliary flow and for the efficient flow of highly viscous fluid due to artificial cilia on glass tube used in laboratory.

Partial oxidation of o-xylene to phthalic anhydride in a fixed bed reactor with axial thermowells

The present study analyses the thermowell system for monitoring temperatures in a fixed bed catalytic reactor for the highly exothermic partial oxidation of o-xylene to phthalic anhydride. Particle-resolved computational fluid dynamics (CFD) simulations are presented for the cases of randomly-packed spheres inside cylindrical tubes with tube-to-particle diameter ratios (N) of 3.96, 5.96 and 7.99. Results are obtained for each N value for cases with inner thermowell radius to outer tube radius ratios of 0.0 (no thermowell), 0.05, 0.1 and 0.2. Comparisons of radial void fraction and axial velocity profiles, axial pressure drop and axial temperature profiles under reaction conditions are presented. The presence of the thermowell did change the mass and heat transfer and reaction inside and around the particles, especially at the bed center. The configurations of catalyst particles were rearranged, leading to changes in voidage, flow and temperature profile. The axial temperature profiles from the thermowells differed from the profiles in the undisturbed beds.

Experimental Validation of a Multidimensional Model for an Indirect Temperature Swing Adsorption Unit

AbstractConventional temperature swing adsorption (TSA) is mainly applied for the removal of trace contaminants. Indirectly heated and cooled adsorbers were developed to make bulk separation economically feasible. A quasi‐continuous TSA process to remove CO2 from an N2/CO2 mixture with a pilot plant is established. The experimentally determined data are taken to validate and to adjust a 2D simulation model. For the validation, the CO2 desorption, the N2 recovery rate as well as the axial temperature profile are compared. The successful model validation can be seen in the good agreement between simulation and experimental results. Moreover, this process is able to separate high amounts of CO2 and to produce a nearly CO2‐free product stream.

Developing buoyant convection in vertical porous annuli with unheated entry and exit

AbstractThe open‐ended vertical double‐passage annular space between three vertical concentric coaxial cylinders is an important physical configuration portraying many practical applications. Hence, in the present analysis, the developing buoyant convection in vertical double‐passage annuli filled with fluid‐saturated porous media is studied numerically by imposing unheated entry and unheated exit thermal boundary conditions. The numerical solutions of the mathematical model equations are found through finite difference technique. The velocity profiles in radial as well as axial directions and temperature profiles have been depicted for vast range of nondimensional numbers, baffle position, and heating and un‐heating ratio. The velocity and thermal gradients decreases as heating section length decreases. Maximum velocity and heat transport occurs in a narrow annular passage rather than equal or wider passages. The presence of porosity causes a reduction in flow velocities and thermal gradients.

Entropy generation and temperature-dependent viscosity in the study of SWCNT–MWCNT hybrid nanofluid

Nanofluids are of excellent significance to scientists, because, due to their elevated heat transfer rates, they have important industrial uses. A new class of nanofluid, “hybrid nanofluid,” has recently been used to further improve the rate of heat transfer. The current phenomenon particularly concerns the analysis of the flow and heat transfer of SWCNT–MWCNT/water hybrid nanofluid with activation energy through a moving wedge. The Darcy–Forchheimer relationship specifies the nature of the flow in the porous medium. Further the impact of variable viscosity, velocity and thermal slip, thermal radiation and heat generation are also discussed in detail. The second law of thermodynamics is utilized to measure the irreversibility factor. The numerical technique bvp4c is integrated to solve the highly nonlinear differential equation. For axial velocity, temperature profile, and entropy generation, a comparison was made between nanofluid and hybrid nanofluid. The variable viscosity parameter enhances the axial velocity and diminishes the temperature distribution for both nanofluid and hybrid nanofluid. Furthermore, the solid volume fraction diminishes the velocity and concentration profile while enhancing the temperature distribution.

Collisionless electron cooling in a plasma thruster plume: experimental validation of a kinetic model

A central challenge in the modeling of the near-collisionless expansion of a plasma thruster plume into vacuum is the inadequacy of traditional fluid closure relations for the electron species, such as isothermal or adiabatic laws, because the electron response in the plume is essentially kinetic and global. This work presents the validation of the kinetic plasma plume model presented in (Merino et al 2018 Plasma Sources Sci. Technol. 27 035013) against the experimental plume measurements of a SPT-100-ML Hall thruster running on xenon presented in (Giono et al 2018 Plasma Sources Sci. Technol. 27 015006). The model predictions are compared against the experimentally-determined axial profiles of electric potential, electron density, and electron temperature, and the radial electric potential profile, for 6 different test cases, in the far expansion region between 0.5 and 1.5 m away from the thruster exit. The model shows good agreement with the experimental data and the error is within the experimental uncertainty. The extrapolation of the model to the thruster exit plane and far downstream is consistent with the expected trends with varying discharge voltage and mass flow rate. The lumped-model value of the polytropic cooling exponent γ is similar for all test cases and varies in the range 1.26–1.31.

Modeling and Design of a Multi-Tubular Packed-Bed Reactor for Methanol Steam Reforming over a Cu/ZnO/Al2O3 Catalyst

Methanol as a hydrogen carrier can be reformed with steam over Cu/ZnO/Al2O3 catalysts. In this paper a comprehensive pseudo-homogenous model of a multi-tubular packed-bed reformer has been developed to investigate the impact of operating conditions and geometric parameters on its performance. A kinetic Langmuir-Hinshelwood model of the methanol steam reforming process was proposed. In addition to the kinetic model, the pressure drop and the mass and heat transfer phenomena along the reactor were taken into account. This model was verified by a dynamic model in the platform of ASPEN. The diffusion effect inside catalyst particles was also estimated and accounted for by the effectiveness factor. The simulation results showed axial temperature profiles in both tube and shell side with different operating conditions. Moreover, the lower flow rate of liquid fuel and higher inlet temperature of thermal air led to a lower concentration of residual methanol, but also a higher concentration of generated CO from the reformer exit. The choices of operating conditions were limited to ensure a tolerable concentration of methanol and CO in H2-rich gas for feeding into a high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) stack. With fixed catalyst load, the increase of tube number and decrease of tube diameter improved the methanol conversion, but also increased the CO concentration in reformed gas. In addition, increasing the number of baffle plates in the shell side increased the methanol conversion and the CO concentration.

Experimental Investigation of Flame Characteristics of H2/CO/CH4/CO2 Synthetic Gas Mixtures

ABSTRACT Chemical composition of synthetic gas and concentration of its components depend on gasification process and feedstock. Synthetic gas mainly consists of hydrogen (H2) and carbon monoxide (CO), and may contain trace amounts of carbon dioxide (CO2) and methane (CH4). In this study, combustion and emission characteristics of H2/CO/CH4/CO2 blending syngas mixtures with high H2/CO ratio were experimentally investigated in a swirl-stabilized premixed combustor. Furthermore, stable operating ranges (flashback and blowout equivalence ratios) of such mixtures and flame response of 67.5%H2-22.5%CO-5%CO2-5%CH4 mixture to acoustic forcing were also detected. During the experiments, CH4 amount in tested gas mixtures varied between 5% and 20% by volume (at intervals of 5%), and H2/CO ratio and CO2 concentration were kept constant at the values of 3 and 5%, respectively. As a consequence, mixtures of 67.5%H2-22.5%CO-5%CO2-5%CH4, 63.75%H2-21.25%CO-5%CO2-10%CH4, 60%H2-20%CO-5%CO2-15%CH4 and 56.25%H2-18.75%CO-05%CO2-20%CH4 were derived and tested under the same boundary and physical conditions. All experiments were conducted at 0.4 equivalence ratio (Φ) and 0.2 geometric swirl number (SN). Preliminarily, stable operating ranges of respective gas mixtures were determined to find an equivalence ratio at which all gas mixtures could be tested without causing any flame instability and then, CH4 addition effects on combustion and emission characteristics of such mixtures were evaluated via utilizing axial and radial temperature, CO and O2 profiles. Lastly, flame behavior of 67.5%H2-22.5%CO-5%CO2-5%CH4 mixture was evaluated under externally modified acoustic conditions. Acoustic field in the combustor was mechanically altered by using side-mounted loudspeakers in the frequency range of 30–200 Hz. To identify flame instabilities and to evaluate the degree of coupling between heat release and pressure oscillations, pressure sensors and photodiodes installed on combustion chamber and burner were utilized. Results of this study showed that CH4 addition to synthetic gas mixtures improves rich flammability limits, and increases flame temperature, flame height and emissions. Under externally modified acoustic conditions, flame behavior slightly alters (flashback and blowout does not occur). Additionally, pollutant emissions improve in an environment-friendly manner under such conditions.

Investigation of heat transfer coefficients in a liquid–liquid direct contact latent heat storage system

The study presents a latent heat storage system consisting of a phase change material (PCM) and a heat transfer fluid (HTF), placed in direct contact, for storing thermal energy derived from renewable energy sources or industrial waste heat. It is important that the two media are immiscible for the function of such a storage system. In this study the PCM was an eutectic mixture of the two salt hydrates, magnesium nitrate hexahydrate and magnesium chloride hexahydrate and the HTF was a mineral oil. The direct contact leads to intensified heat transfer compared to that in the indirect contact heat storage systems. This intensified heat transfer results in short loading and unloading periods of the storage tank with a high heat exchange performance. Furthermore, a higher storage density is observed in comparison to the sensible storage systems, particularly when combined with low temperature differences between the charged and discharged storage systems. In our study, the heat transfer between liquid PCM and liquid HTF (rises drop-shaped in the PCM) is investigated. For this purpose, a storage tank has been built with only one HTF inlet opening in the bottom. For the time and space resolved data, the axial temperature profile, and the surface temperature of the rising droplet, as well as the PCM temperature, are measured with a specially developed, fast, near infrared sensitive, and fiber-coupled technique. These measurements made it possible to determine a local heat transfer coefficient between the PCM and HTF. With the chosen parameters and materials in this study a heat transfer coefficient of 1845 W/m² K was calculated.

Three-dimensional heat transfer in a particulate bed in a rotary drum studied via the discrete element method

We simulated three-dimensional heat transfer inside a horizontal rotating drum using the discrete element method and a thermal conduction model. The aim was to determine the effect of end-wall heating on thermal behavior of a granular bed. The simulation showed that the end-wall heating significantly affects the axial temperature profile of the bed, particularly when the length-to-diameter ratio is low. Particles near the wall heated faster and became more thermally uniform than those in the center of the drum. The region affected by the end heating gradually increased over time. Increasing the rotation speed enhanced the heat conduction rate, and increasing the fill level reduced the mean temperature and thermal uniformity of the granular bed. Heat transfer was also simulated for drums with different length-to-diameter ratios.

Radiative SWCNT and MWCNT nanofluid flow of Falkner–Skan problem with double stratification

This article discusses the important of single wall carbon nanotube (SWCNTs) and multiple walls carbon nanotube (MWCNTs) past a static wedge under the effects of magnetohydrodynamics (MHD). The significance of activation energy, thermal radiation, heat generation and double stratification are also taken into account. Utilizing the BVP-4c function of MATLAB the system of differential equations is solved numerically. To validate our results, a correlation with a previously studied problem is also directed and a brilliant concurrence is found; thus, reliable results are being introduced. The influence of sundry parameter on axial velocity, temperature, and concentration profile are deliberated and studied graphically. The temperature and concentration distribution diminish respectively with thermal and concentration stratified parameter. Further the solid volume fraction enhances the axial velocity and temperature profile for both carbon nanotubes.

Diagnosis of helicon plasma by local OES

Optical emission spectroscopy (OES) is a common method for characterizing radio frequency (RF) discharge plasmas. Particulary, helicon plasma is featured by its high plasma density among all RF-excited plasmas. In order to obtain the spatial-resolved information of a helicon plasma, local optical emission spectroscopy (LOES) with a 3 mm spatial resolution was proposed and carried out to evaluate the local electron density and temperature. The plasma emission intensity via LOES was measured and compared with the electron density obtained by a RF-compensated Langmuir probe (LP) in Ar, N2 and Air helicon plasmas, respectively. The results revealed that there existed a functional relationship between some specific lines (LOES) and electron density (LP). Further, helicon plasma characteristics under capacitive (E) , inductive (H), and helicon (W) modes were systemetically investigated based on LOES. Besides, two-dimensional (2D) contour maps for plasma distributions were made via LOES as well. It was found that in E- and H-modes, axial profiles of plasma density and electron temperature were consistent under two opposite magnetic field directions. However, in W-mode, the plasma presented an asymmetric axial profile along the tube. As for radial profiles, plasma distribution varied under three discharge modes due to different heating mechanisms in Ar, N2 or Air helicon plasma. A deeper analysis indicated that the bulk absorption comes from the coupling of the helicon wave in Ar helicon plasma while the power depositions in N2 and Air helicon plasma are mainly dominated by the TG wave.

Neutral Plume Simulation of Ion Thruster in Vacuum Facility With Cooled Target

The background pressure of vacuum chamber determines operation and performance of ion thrusters. In order to present the neutral plume and temperature distribution of a 30-cm-diameter ion thruster working under rated condition and in a large vacuum facility, theoretical calculation and numerical simulation are used to investigate the characteristics of thruster neutral plume. The results show that the neutral plume can be treated as an oriented molecular flow. Both theoretical calculation and numerical simulation results show that axial temperature profiles of neutral plume on the center line of facility decreases very slowly; however, in the area near the ion cooled target, temperature rapidly decreases from 500 to 100 K. In terms of pressure, it is found in both results that when the thruster is working, the vacuum degree of the chamber is in the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> Pa. The simulation result, however, obviously shows the vacuum degree in the area near the thruster outlet and cooled target is higher than that in the middle area. The velocity of the neutral flow appears directional and decreases almost exclusively along the center line of the facility. The simulation results present a similar trend and agree well with the experimental results, and the comparison error is less than 10%. However, the calculations have some errors with the experimental results due to neglecting the absorption effect and energy deposition of the ion cooled target to neutral atoms. The study shows that numerical simulation can better describe the neutral plume expansion map of the facility, and the theoretical calculation can be used to roughly estimate neutral plume characteristics.

Adoption of 3D printed highly conductive periodic open cellular structures as an effective solution to enhance the heat transfer performances of compact Fischer-Tropsch fixed-bed reactors

Heat transfer is universally recognized as a key challenge for the intensification of the Fischer-Tropsch (FT) process in compact fixed-bed reactors. For the first time in the scientific literature we demonstrate experimentally that the adoption of a highly conductive periodic open cellular structure (POCS, 3D-printed in AlSi7Mg0.6 by Selective Laser Melting) packed with catalysts pellets is a promising solution to boost heat exchange in fixed-bed FT reactors. This reactor configuration enabled us to assess the performances of a highly active Co/Pt/Al2O3 catalyst packed into the POCS at process conditions relevant to industrial Fischer-Tropsch operation. Unprecedented performances (CO conversion ≈ 80%) could be thus achieved thanks to an outstanding heat management. In fact, almost flat axial and radial temperature profiles were measured along the catalytic bed even under the most severe process conditions (i.e. high CO conversions corresponding to high volumetric heat duties), demonstrating the effective potential of this reactor concept to manage the strong exothermicity of the FT reaction.The heat transfer of the packed-POCS reactor outperformed both packed-bed and packed-foam reactors, granting smaller radial temperature gradients in the catalytic bed, as well as smaller temperature differences at the reactor wall, with larger volumetric power releases. The strengths of the packed-POCS reactor configuration are its regular geometry, which enhances the effective radial thermal conductivity, and the improved contact between the structure and the reactor wall, which governs the limiting wall heat transfer coefficient.

Mode transition induced by gas pressure in dusty acetylene microdischarges: two-dimensional simulation

Radio-frequency microdischarge in acetylene is investigated by use of a fluid model and an aerosol dynamics model in a cylindrical discharge chamber. In this article, the results at a pressure of 100–500 Torr, a voltage of 80–150 V, and an electrode gap of 400–1000 μm are carefully analyzed and discussed. It is shown that two electron heating modes α and γ appear in the microdischarge, and the pressure-dependent transition from α to γ was accompanied by the abrupt decrease of electron density and electron temperature. The mode transition phenomenon is further confirmed by the variation of the electron temperature axial profiles, the profiles vary continuously from a center high at the pressure of 100 Torr to an edge high at the pressure of 500 Torr. Furthermore, in the α mode (100 Torr) the plasma density increases linearly with the increase of electrode gap, but decreases sharply with the increase of electrode gap in the γ mode (>100 Torr). The gas pressure and applied voltage effects on the nanoparticle density and degree of nonuniformity are also investigated. It has been shown that the gas pressure greatly influences the axial profiles of nanoparticle density and the values of the degree of nonuniformity, while the values of the plasma parameters (electron density and nanoparticle density) strongly depend on the applied voltage.

Experimental study and empirical modeling of CO and NO behaviors in a fluidized-bed combustor firing pelletized biomass fuels

The combustion and emission characteristics were studied in a fluidized-bed combustor burning pellets of five biomass feedstocks: cassava rhizome, eucalyptus bark, rubberwood sawdust, rice husk, and teak sawdust. During combustion tests with an individual pelletized fuel, the heat input to the combustor was ~ 200 kWth, with excess air varied from 20 to 80%. Temperature and gas concentrations were monitored along the combustor height and also at stack to quantify the major gaseous emissions and combustion efficiency of the reactor for the selected fuels and operating conditions. Experimental data revealed a good/fair similarity of the relative axial profiles of temperature, CO, and NO inside the combustor for different fuels fired at variable excess air. Empirical models for predicting the relative axial profiles of the temperature and major gaseous pollutants were derived via regression analysis of experimental data. Using the models, these variables can be predicted at any level in the combustor with an acceptable uncertainty for specified operating parameters. To meet the domestic emission standards for CO and NO, the selected biomass fuels should be fired at excess air of ~ 40%. Under these operating conditions, high (99.2–99.8%) combustion efficiency can be achieved during fluidized-bed combustion of the selected fuels.