Hot Wall Research Articles

CFD Simulation of Particle-Laden Flow in a 3D Differentially Heated Cavity Using Coarse Large Eddy Simulation

Particulate flow in closed space is involved in many engineering applications. In this paper, the prediction of particle removal is investigated in a thermally driven 3D cavity at turbulent Rayleigh number Ra = 109 using Coarse Large Eddy Simulation (CLES). The depletion dynamics of SiO2 aerosol with aerodynamic diameters between 1.4 and 14 µm is reported in an Euler/Lagrange framework. The main focus of this work is therefore to assess the effect of the subgrid-scale motions on the prediction of the particulate flow in a buoyancy driven 3D cavity flow when the mesh resolution is coarse and below optimal LES standards. The research is motivated by the feasibility of modeling more complex particulate flows with reduced CPU cost.The cubical cavity of 0.7 m side-length is set to have a temperature difference of 39 K between the two facing cold and hot vertical walls. As a first step, the carrier fluid flow was validated by comparing the first and second-moment statistics against both previous well-resolved LES and experimental databases [Kalilainen (J. Aero Sci. 100:73–87, 2016); Dehbi (J. Aero. Sci. 103:67–82, 2017)]. First moment Eulerian statistics show a very good match with the reference data both qualitatively and quantitatively, whereas higher moments show underprediction due to the lesser spatial resolution. In a second step, six particle swarms spanning a wide range of particle Stokes numbers were computed to predict particle depletion. In particular, predictions of 1.4 and 3.5 µm particles were compared to LES and available experimental data. Particles of low inertia i.e. dp < 3.5 µm are more affected by the SGS effects, while bigger ones i.e. dp = 3.5–14 µm exhibit much less grid-dependency. Lagrangian statistics reported in both qualitative and quantitative fashions show globally a very good agreement with reference LES and experimental databases at a fraction of the CPU power needed for optimal LES.

Effect of porous wall on Mach 6 boundary-layer transition over a flat plate

Stabilization of the Mack second mode is of great importance to control hypersonic boundary-layer transition, and the porous wall is one of the promising passive techniques. In this paper, the transition onset of a two-dimensional Mach 6 boundary layer over a flat plate with porous coating is predicted by the eN method based on the linear stability theory. The results show that the second mode instability over the smooth wall can be significantly stabilized by the porous coating. As a price, the other instabilities, mostly the lower-frequency first mode instability, are destabilized, which is more prominent over a hotter wall. Consequently, the transition onset over the porous wall is not always delayed obviously compared to the smooth wall, and even a premature transition is obtained for some porous walls, especially for a hot wall. To achieve an effective delay of transition onset over a hot wall, a sectionalized strategy is proposed in this paper. By partially using the porous wall at downstream location, the destabilization of first mode instability is relieved while the significant stabilization of second mode instability is retained for the most unstable wave, and thus the transition onset is delayed more effectively over a hot wall.

Impact of a hot constructal tree-shaped fin on the convection flow of single wall carbon nanotube water nanofluid inside a sinusoidal enclosure

Heat transfer due to natural convection in different types of fins embedded closed enclosures are widely used in various fields including petrochemicals, solar collectors, heat exchangers, gas turbines, semiconductor devices, automobile radiators, etc. Herein, the natural convection flow of a water-based single-wall carbon nanotube (SWCNT) nanofluid inside a novel sinusoidal closed geometry is investigated. The horizontal bottom and top boundaries of the enclosure are presumed to be hot and thermally adiabatic, respectively. The left and right vertical sinusoidal boundaries are assumed to be cold. A hot constructal tree-shaped fin is placed at the center of the hot bottom wall. The impact of the simple constructal heated tree-shaped fin on steady, two-dimensional, laminar, and incompressible flow of the water-based SWCNT nanofluid inside the sinusoidal enclosure is analyzed. The mathematical formulation of water-based SWCNT nanofluid flow inside the enclosure is formulated using the Navier–Stokes equations under the Boussinesq approximation. A modified effective thermal conductivity model of carbon nanotubes including the radius of water molecules and carbon nanotubes is employed. Galerkin finite element simulations are conducted to study the flow and heat transfer characteristics via stream function, isotherms, local Nusselt number, and averaged Nusselt number graphs. The combined effect of the amplitude of the sinusoidal wall (A = 0.1, 0.15, and 0.2) with the Rayleigh number (Ra = 104–106), nanotube volume fraction (φ = 0.01–0.05), and the thickness of the tree-shaped fin (B = 0.02–0.06) are comprehensively studied. The temperature gradient decreases with increasing nanotube volume fraction and the sinusoidal wall amplitude, while it increases above the fin with the thickness of the tree-shaped fin. Moreover, the averaged Nusselt number increases with the thickness of the tree-shaped fin by 14.72%, 16.74%, and 19.6% for A = 0.1, 0.15, and 0.2 along the fin, respectively. The averaged Nusselt number along the fin is an increasing function of the Rayleigh number, nanotube volume fraction, thickness of the tree-shaped fin, and amplitude of waviness. Additionally, a novel correlation for the averaged Nusselt number along the fin is derived.

Mediating heat transport by microbubble dispersions: The role of dissolved gases and phase change dynamics

• Developed a new model for buoyant convection with a dispersed microbubble phase. • Predicts microbubble phase fraction controls additional HT mechanism. • Scale of the additional heat transfer ranges from 5–45% with phase fractions 2–20%. • Demonstrates that dominant feature for HT in larger domains is the boundary layer. • Conclusions hold qualitatively with simple model for gas exchange between phases. Recently a theory for additional heat convection by microbubbles was posited. The mechanism proposed is that the latent heat of vapor of the liquid is carried by microbubbles from hot zones that vaporize more liquid to cold zones where condensation releases the latent heat. In this paper, the proposition that the additional heat flux is controlled by the phase fraction of microbubbles, is tested by steady state solutions of the canonical hot wall / cold wall buoyant convection problem. The simulations show that the range of additional heat transfer varies monotonically with length scale of the cavity, between 5 and 45% with phase fractions varying from 0.02 and 0.2. The larger characteristic lengths introduce insufficient heat flux from the hot wall to maintain a “driven cavity” flow structure, so that the steady state structure that emerges is a stable stratification with thin boundary layers near the hot and cold walls, with weak shear flow convection. The stable stratification resultant at higher characteristic lengths suppresses the additional heat flux due to microbubble mediation, yet only moderately deviating from proportionality. These conclusions hold qualitatively for a variant of the model which simply treats the gas exchange between the microbubble phase and dissolved gases in the liquid, resulting in variation of the microbubble phase fraction with temperature, hence with position in the domain. Quantitatively, the percentage increases are 3–30% in the parameter regime above, with the effects of dissolved gases captured dynamically.

Temperature dependence of Raman and photoluminescence spectra of pure and high-quality MoO3 synthesized by hot wall horizontal thermal evaporation

In this work, a novel system for the hot wall evaporation of MoO3 in a horizontal furnace and at low vacuum atmosphere was settled. Pure MoO3 material was obtained without the presence of other phases and with a strong preferential orientation, as revealed by electron microscopy, micro-Raman spectroscopy and x-ray diffraction. Photoluminescence and Raman spectra were measured as a function of temperature. As a result, the first-order temperature coefficient of the fifteen measured vibrational modes is reported and compared with the scarce literature data available. The relation of these coefficients with the thermal expansion coefficient is discussed in the framework of the Grüneisen model. Moreover, for the first time, the temperature dependence of the different contributions of photoluminescence spectra for MoO3 is reported, and an anomalous increase in PL intensity of all transitions is observed in the range of 40–140 K, followed by a normal quenching at high temperatures. The Shibata model was used to interpret the origin of this anomalous effect.

Effect of the position of the hot source on mixed convection in a rectangular cavity

This work presents a study by numerical simulation of mixed convection in a rectangular cavity with a sinusoidal temperature imposed on the right vertical wall while the other wall on the left is kept at a cold temperature. The upper and lower walls are thermally insulated, the inlet and outlet ports are respectively located on the hot wall to the bottom and on the top to the left. The enclosure represents a practical system such as an air-cooled electronic device, the heat source represents a radiator or an electronic component located in three different positions towards the left side at the bottom, the center and towards the right side at the top in such an enclosure. All calculations are made for a range of Richardson number from 0 to 10 and Reynolds numbers from 50 to 200. The influence of Richardson number, the position of the heat source and the influence of amplitude and phase deviation of the temperature imposed on the Nusselt number on the hot surface is studied. The results are presented in the form of streamlines and isotherms graphs as well as the variation in maximum temperature and mean Nusselt number under different conditions. The numerical resolution of the governing equations is obtained using the software of Multiphysics Comsol based on the finite element method.

A critical assessment of the implementation of phase change materials in the VCC of refrigerator

The ultimate heat storage system is an alternative way of improving the heat exchange in the home refrigerator. This research is a systematic analysis of the studies that are being performed with its advantages and limitations. Comments of the researcher on potential improvements in the current work to enhance efficiency are also checked. Key parameters influencing system performance and key issues related to effective system integration are discussed here. PCM integration with the standard setup is restricted by the use of low-temperature PCMs, which is why PCM analysis with Nano-particle additives can be performed. Most experiments are limited to a hot wall form only, the investigation of a PCM application for another form of the condenser is subject to further work. Besides, a combination of two thermal enhancement methods, such as LSHX and PCM, can be used to test the performance.

Characterization of thin films of Ga 2 Se 3 on Si fabricated using electron beam evaporation technique

Many studies have been conducted in the past on compound semiconductor and wide-gap semiconductor for their possible usage in place of oxide in metal-oxide-semiconductor structures for high-speed metal-oxide-semiconductor field-effect transistors application. The best candidates for this purpose, however, are compounds with stoichiometric vacant A 2 III B 3 VI type sites. A 2 III B 3 VI type compounds are wide-gap semiconductors with stoichiometric vacancies and low concentration of free and localized charged carriers. It has that gate insulators can be constructed from wide-gap semiconductor layers with thicknesses smaller than the Debye Screening distance. These new MS'S (metal-Wide bandgap semiconductor-semiconductor) with a good lattice match have less of interface states thus giving better stability to the structure. The idea was to use these MS'S structures for photovoltaic applications with insignificant number of interface states. During literature survey, it was also found that Ga2Se3 is the most prospective candidate for forming a heterostructure with Silicon because of its various properties. Structurally, perfect and homogeneous in thickness Ga2Se3 layers have already been formed in quasi-closed volume by the Hot Wall technique by B. I. Sysoev in the Voronezh Technological Institute, Russia. However, this technology is not cost-efficient for photovoltaic applications, although it has the advantage of better control of Ga and Se components. Over the last few years, many attempts have been made using many other techniques such as Sol-gel technique, vapour phase epitaxy, molecular beam epitaxy, etc. for preparing the thin film of GaSe. The current work is an attempt to prepare the Ga2Se3 film using the electron beam evaporation technique as it appeared to be much simpler. Film of thicknesses varying from 70 to 1000 Å were grown. The films have been studied for their morphology and properties for future use in photovoltaics and other device fabrication like IR detectors, etc. Various techniques were used to study the behavior of the films like XRD, SEM, and TEM.

Ratchetting and Creep Failure in Twin-Wall Turbine Blades Experiencing Severe Thermal and Centrifugal Loading

Abstract Twin-wall structures can be cooled both externally and internally, raising great potential for use in high-temperature applications. However, their increased geometric complexity imposes a range of potential failure mechanisms for consideration in design. The primary aim of this study is to identify the nature of such mechanisms by constructing Bree type interaction diagrams for idealized double-wall systems under cyclic thermomechanical loading that shows the combination of loading conditions for which cyclic plasticity (leading to fatigue failure)-creep ratchetting occur. Through an extension of the classical Bree analysis, we determine analytical boundaries between different regimes of behavior. We also quantify the effects of wall thickness ratio, temperature field, and yield and creep material properties. Local cyclic plasticity is shown to dominate over structural/global ratchetting when the yield strength reduces with temperature and/or when the temperature gradient through the hot wall thickness dominates over the temperature difference between the walls. Thus, we conclude that global ratchetting is unlikely to occur in the practical loading range of Nickel-based twin-wall turbine blades, but instead these systems suffer from local fatigue at cooling holes and excessive creep deformation. This is verified by 3D cyclic finite element (FE) simulations, demonstrating that the analytical approach provides a powerful, cost-effective strategy for providing physical insight into possible deformation mechanisms in a range of thin-walled components; highlighting the key trade-offs to be considered in design; and directing the use of computer methods toward more detailed calculations.

Comparative study of the effects of increasing heat transfer area within compression and expansion chambers in combination with modified pistons in Stirling engines. A simulation approach based on CFD and a numerical thermodynamic model

The aim of this research is to ascertain the potential benefits of increasing heat transfer area in the expansion and compression spaces of Stirling engines. This is achieved, as proposed in an accepted patent, by simultaneously attaching pins or fins to the chamber walls and modifying the geometry of the displacer. Different configurations involving pinned and finned surfaces were proposed for comparison with a hollow cylindrical chamber. In order to compute the heat transfer rate as well as the convective heat transfer coefficient for each suggested geometry, a CFD (Computational Fluid Dynamics) model was run under different temperatures, displacer speeds and pressures. For a selected geometry based on longitudinal cylindrical pins, the computed heat transfer coefficients were correlated with input parameters in order to obtain a simple equation. The equation was subsequently introduced into a three-camera second-order thermodynamic model of a gamma Stirling engine with crankshaft mechanism. Five different cases were evaluated. The models were run for different operational conditions (speed and charge pressure), for a hot wall temperature of 493 K and a cold wall temperature of 293 K. Pressure was varied from 1 to 2.5 atmospheres and the engine rotational speed ranged between 50 and 1000 rpm. Results showed an increase in engine power between 1.3 and 2.5 times when compared to a conventional engine without heat exchangers.

Analysis of mixed convection of a power-law non-Newtonian nanofluid through a vented enclosure with rotating cylinder under magnetic field

The impact of an inner adiabatic rotating cylinder within a vented cavity on the mixed convection of a hybrid nanofluid is investigated in this paper using a numerical solution. The governing equations of mixed convection motion are assumed to be two-dimensional, steady, and laminar for an incompressible power-law non-Newtonian hybrid nanofluid. Using the finite element technique, these equations are numerically solved. (Al2O3–Cu/CMC) is presented as a nanofluid in this study. The effects of significant parameters such as the Hartman number (0 < Ha < 100), cylinder radii (0.1 < R < 0.25), cylinder positions (0.25 < AR < 0.75), angular rotational speed (−10 ≤ Ω ≤ 10), Grashof number (103 ≤ Gr ≤ 105) and Reynolds number (50 ≤ Re ≤ 500) are studied. The gathered information is displayed using a variety of qualitative and quantitative numbers. The findings report that rotating the stationary cylinder counterclockwise enhances convective heat transfer, whereas rotating it clockwise has the opposite effect. Furthermore, when the cylinder is rotated counterclockwise, the heat transfer improves as the cylinder approaches the hot wall.

Multiple Optical Characterization Of A Solid/Liquid Material

Various industrial configurations use Phase Change Materials (PCM) for thermal management. Their use generates complex flows that must be studied and characterized to better understand the phase-change mechanisms and to help model development. In particular, the works considering large containers where turbulent effects are encountered are poorly addressed in the literature. The present study proposes the experimental analysis of PCM flows with a simultaneous flow and energetic characterization. Heat and mass transfer are analyzed in a large (0.4 meter) cubic volume undergoing various lateral temperature conditions. 2-dimensional fields of velocities by a Particle Image Velocimetry (PIV) technique and temperature by thermocouples (TC) scanning and by Background-Oriented Schlieren (BOS) are measured. The measurement methodology highlights the local transfer as reflected by the Nusselt number for the two latter techniques. The local Nusselt numbers are also integrated and compared to a global Nusselt assessed by the heat transfer through the hot wall regulating system. In this study, the different techniques with their theoretical approaches and the experimental setup are described. The conditions and the conducted procedure of the tests are detailed. The results of local measurements for two particular temperature configurations are given. The global heat transfer and the global Nusselt number are also provided as a function of the Rayleigh number and the solid fraction. The results and the techniques are discussed. The results corresponding to various Rayleigh numbers form a database that is used to understand phenomena in large PCM containers and validate numerical models involving melting phenomena.

Entropy Optimization and Thermal Behavior of a Porous System With Considering Hybrid Nanofluid

The aim of this research work is to study the performance of hybrid eclectically conducting nanofluid (MWCNT-Fe3O4) with entropy optimization impacts using the CVFEM approach. 3D and contour plots are two types of outcomes that extend the discrepancy of analyzed variables. The Newtonian liquid is presumed as a testing fluid, and the non-Darcy model is employed to simulate the permeable domain. The geometry has two adiabatic walls and one wavy hot wall with unvarying flux, and the other is kept at a constant temperature. The impact of Brownian motion in the hybrid nanofluid (MWCNT-Fe3O4) is considered. The irreversibility effect is also taken into account. This effect acts against the performance of the system and reduces the efficiency of the system. To solve the final equations with higher accuracy, we utilized the method of CVFEM. The validation of the results obtained through CVFEM is clearly and graphically presented with the available literature. The outputs of the simulation showed that there is an inverse relation between permeability and the Bejan Number, and the boundary layer thickness augments with intensification in Lorentz force. These variations in the form of streamline, isotherm, and various contour plots have been reported at Ra=103 and Ra=105 for different values of Da, Ha, and ϕ.

Production of thin film of multicomponent inorganic semiconductors under quasi-equilibrium conditions

Issues of improving the properties of semiconductor thin film and their reproducibility, as well as improving and reducing the cost of manufacturing technology stimulate research and development of new, advanced methods. Therefore, it is important to optimize the technology of getting reproducible, competitive, high-tech thin films of multicomponent semiconductor compounds with predetermined properties. In the given article it is shown that constructive and technological improvements of a method of thermal spraying in vacuum allow to minimize nonequilibrium conditions of film growth, keeping the advantages of thermovacuum spraying, such as high reproducibility, processability and productivity, a wide range of variations in the synthesis conditions, and, accordingly, the properties of condensates, maximum purity of growth processes, as well as ease of performing and management and cost-effectiveness of the process of getting perfect condensates. For this purpose, we have developed a special construction of a quasi-fusion evaporator and a device for getting semiconductor film in vacuum, as well as a version of a transparent “hot wall”. The resistivity, cross section and geometric dimensions of the cover and the heater of the developed structures were selected so that in the mode of resistive heating of the evaporator temperature gradient due to the difference in their electrical resistance, and, accordingly, the Joule heat of current, in the temperature range 673… 1473 K provided the temperature of the cover 1.1 ... 1.3 times higher than the temperature of the heater. Due to the elevated temperature of the cover, the solid fraction is either repelled on the sublimating (evaporating) surface and the walls of the crucible, or undergoes sublimation (evaporation) from the surface of the cover. Depending on the values of the sputtering rate, the grain size of semiconductor polycrystalline film varied from units of nanometers to several micrometers. Crystallinely ordered films were got at relatively low values of the sputtering rate (0.5...5 nm·s-1). It was set up the technological conditions for getting thin films of multicomponent semiconductors, which ensure the independence of the chemical composition of condensates from the evaporation rate in the wide range from 0.05 to 20 nm·s-1, uniform composition of the gas phase during sublimation, absence of inhomogeneous solids in films, wide range properties of condensates and their high reproducibility.

Growth of vanadium doped semi-insulating 4H-SiC epilayer with ultrahigh-resistivity

4H-SiC epitaxial layers with ultrahigh resistivity of over 1010 Ω cm were successfully grown by using a hot wall chemical vapor deposition system with vanadium doping. The resistivity of the vanadium doped epilayer was found to be strongly dependent on the types of dopant impurities. The resistivity of n-type-based vanadium doped semi-insulating 4H-SiC epilayers showed stronger dependence on vanadium incorporation than that of p-type-based epilayers. This means that the carrier trap characteristics of vanadium atoms may differ with respect to electrons and holes. As the result, an ultrahigh resistivity of over 1010 Ω cm was realized on an n-type-based 4H-SiC epilayer with vanadium doping.

Unsteady natural convection of nano-encapsulated phase change materials (NEPCMs) inside a random porous medium considering local thermal non-equilibrium condition

A local thermal non-equilibrium analysis is undertaken to simulate unsteady free convective heat transfer inside a square enclosure filled with a random porous medium. There is a time-varying temperature profile at the hot wall. The fluid is a suspension comprising nano-encapsulated phase change materials (NEPCM) particles dispersed in water while aluminum foam, copper foam, or glass balls are considered for the solid matrix. It is found that when the hot wall temperature reaches its minimum value during one oscillation period, most of the cavity is occupied by the NEPCM particles in the solidious phase. For the metallic foams, the heat transfer of the medium is substantially higher than the NEPCM particles. Meanwhile, the Nusselt number of the glass balls is much lower than the suspension. A rise in the dimensionless frequency or amplitude of the temperature profile as well as approaching the dimensionless fusion temperature to 0.5 elevates Nusselt number of the suspension. However, no general conclusion can be drawn for their consequences on the heat transfer in the solid matrix. Among the current alterable parameters, only the contributions of the non-dimensional interface heat transfer coefficient and the non-dimensional amplitude of the temperature profile are found significant.

Heat flow through a liquid–vapor interface in a nano-channel: the effect of end-grafting polymers on a wall

Heat transfer through a liquid–vapor interface is a complex phenomenon and crucially relevant in heat-removal and cryogenic applications. The physical coupling among confining walls, liquid and vapor phases is very important for controlling or improving cooling rates or condensation efficiency. Surface modification is a promising route, which has been explored to taylor the heat transfer through confined two-phase systems. We use coarse-grained molecular-dynamics simulations to study the heat transfer through a nano-confined liquid–vapor interface as a function of fluid filling. We set up a stationary heat flow through a liquid–vapor interface, stabilized with the liquid in contact with a colder wall and a vapor in contact with a hotter wall. For these physical conditions, we perform extensive simulations by progressively increasing the number of fluid particles, i.e. the channel filling, and measure the fluid distribution in the channel, density, pressure and temperature profiles We also compare the heat flux behavior between a bare-surfaces nano-channel and others where the hot surface was coated with end-grafted polymers, with different wetting affinities and bending properties. We take extreme cases of polymer properties to obtain a general picture of the polymer effect on the heat transfer, as compared with the bare surfaces. We find that walls covered by end-grafted solvophylic polymers change the heat flux by a factor of 6, as compared with bare walls, if the liquid phase is in contact with the polymers. Once the liquid wets the coated wall, the improve on heat flux is smaller and dominated by the grafting density. We also find that for a wall coated with stiff polymers, the jump in heat flux takes place at a significantly lower channel filling, when the polymers’ free ends interact with the liquid surface. Interestingly, the morphology of the polymers induces a ‘liquid bridge’ between the liquid phase and the hot wall, through which heat is transported with high (liquid-like) thermal conductivity.

Radiative magneto‐thermogravitational flow in a porous square cavity with viscous heating and Hall current effects: A numerical study of ψ–v scheme

AbstractA numerical study presented for the natural convection two‐dimensional incompressible flow of a magnetized fluid in a square enclosure filled with porous medium. Viscous dissipation, thermal radiation, and Hall current effects are included. The formulated computational domain comprising the mass, momentum, and energy conservation equations with appropriate boundary conditions, is rendered dimensionless via appropriate scaling transformations. The resulting nondimensional partial governing equations are solved by the ψ–v scheme. A detailed parametric investigation of the influence of Hartman number (magnetic body force parameter), Rayleigh number (thermal buoyancy parameter), Darcy number (permeability parameter), Eckart number for porous medium, Hall parameter, and radiation parameter on the streamline, temperature contours, local Nusselt number along the hot wall and mid‐section velocity profiles are computed. Thermal Rayleigh number can be a control key parameter on heat and flow in enclosures. In addition, the heat transfer rate is intensified with increasing thermal Rayleigh numbers. With increasing Darcy number (i.e., greater permeability of the porous medium and progressively lower Darcian drag force) there is a strong elevation in the average Nusselt number at the left hot wall.

Hydro-thermal performance on the two phase nanofluid convection heat transfer using the LBM-FD method

This paper is dedicated to the investigation of nanofluid natural convection inside a 2D cavity with various kinds of cold walls. To consider the different nanoparticle distribution in the cavity, Buongiorno’s two-phase model is used. The equations are solved by the homemade hybrid Lattice Boltzmann-Finite Difference (LBM-FD) code. It found that although the nanoparticle concentration is almost uniform inside the cavity, the apparent difference can be observed around the walls and edges of the vortices. The nanoparticles have higher concentrations around the cold wall and the lower concentration adjacent to the hot wall. For all cases, the local and average Nusselt number increases by increasing the solid volume fraction of nanofluid. The configuration of cold walls affects the flow pattern and distribution of temperature significantly. By applying the combination of right-angle and inclined walls, the best heat transfer performance for the present system.

A Novel Method for On-Line Characterization of Alkali Release and Thermal Stability of Materials Used in Thermochemical Conversion Processes

Alkali metal compounds are released during the thermal conversion of biofuels and fossil fuels and have a major impact on the efficiency of conversion processes. Herein, we describe a novel method for the simultaneous characterization of alkali release and mass loss from materials used in combustion and gasification processes including solid fuels, fluidized bed materials, and catalysts for gas reforming. The method combines the thermogravimetric analysis of selected samples with the on-line measurement of alkali release using a surface ionization detector. The technique builds on the careful treatment of alkali processes during transport from a sample to the downstream alkali monitor including the losses of alkali in the molecular form to hot walls, the formation of nanometer-sized alkali-containing particles during the cooling of exhaust gases, aerosol particle growth, and diffusion losses in sampling tubes. The performance of the setup was demonstrated using biomass samples and fluidized bed material from an industrial process. The emissions of alkali compounds during sample heating and isothermal conditions were determined and related to the simultaneous thermogravimetric analysis. The methodology was concluded to provide new evidence regarding the behavior of alkali in key processes including biomass pyrolysis and gasification and ash interactions with fluidized beds. The implications and further improvements of the technique are discussed.