High Wall Temperature Research Articles

An alternative solution for insulating a burning chamber with high temperature walls

Usually, the industrial halls, who are in most cases poorly insulated and have high permeability, are heated with hot air generators or convecto-radiators. Using convecto-radiators is improper and hot air generators, although it is more appropriate, it consumes a very high amount of energy. This paper continues the topic discussed in a previous article [1] in which we presented an alternative heating solution to the existing ones. The heating system mentioned uses an environmentally friendly and cheap fuel (biomass). This paper presents an alternative solution to insulate a burning chamber (BC) with high temperature walls that uses pellets as fuel. This technical solution has the benefit of supplying heating agents to various types of heat consumers and to optimize the construction costs through the materials used. The BC is design with external ribs on here longitudinal direction. For insulating and cooling the BC, a metallic case was design through which atmospheric air is circulated. Three constructive solutions of BCs were analysed. The paper presents the modelling results of the constructive solutions and the calculation method. The calculation was made applying the physical laws that govern this type of heat transfer and was made with discretization segments. Determining the parameters of each segment will allow the user to optimize the equipment design. The main purpose of the study is to conceive an alternative technical solution for insulating the BC, that should be cheaper, easy to implement and able to prepare a second thermal agent for heating.

Effect of coal particle size on gasification performance of two‐stage entrained‐flow coal gasifier

AbstractTo clarify the effect of coal particle sizes on gasification performance of an advanced two‐stage entrained‐flow coal gasifier for IGCC (integrated gasification combined cycle) application, a comprehensive three‐dimensional numerical model is established by incorporating the shrinking core model combined with the Langmuir‐Hinshelwood kinetic rate expression, which considers the inhibitory effect of CO on char‐CO2 reaction. The flow, temperature, and species distributions were obtained, and the results are consistent with the operating data. Results show that the helical flow and eight recirculation zones in the gasifier improve carbon conversion efficiency through extending the residence time of coal particles. Slow devolatilization of large particles caused by slow heating retards volatiles combustion, and thus char combustion and gasification. As a result, less char gasification and higher gas temperature appear in the injection region of the first stage. Higher inertia of larger particles produces higher slip velocity, which enhances heat transfer and mass diffusion to char particles and increases char consumption rate in diffusion‐limited regions. The regions of high inner wall temperature spread from locations around burners to the whole inner wall of the injection and bottom regions in the first stage with increase of coal particle sizes.

Effects of thermal boundary conditions on spontaneous combustion of coal under temperature-programmed conditions

Coal spontaneous combustion (CSC) is one of major disasters that threaten the safety and production of coal mines. A method to calculate the key parameters of coal oxidation reaction based on temperature-programmed tests was proposed. Moreover, a one-step global numerical model for exploring coal oxidation considering the effects of multi-component material was established. The evolution of the oxygen consumption rates (OCRs) in multiple stages of coal oxidation was investigated and the changes in temperature and reaction rate were assessed by changing thermal boundary conditions including gas temperature supplied and wall temperature. The results show that the coal oxidation can be simulated based on the obtained parameters; the process of coal oxidation can be partitioned into three stages (i.e. temperature-dominated, co-dominated by temperature and oxygen, and oxygen-dominated) as the reaction continues. Under thermal conditions involving high-temperature gas, the coal always exhibits a high temperature at the gas inlet and the dimensionless number Φ of the coal self-heating temperature presents three trends including slight variation, a gradual increase, and gradual reduction. Under conditions involving the high-temperature wall of coal-storage tanks in contact with coal (briefly called the contact wall hereinafter), the dimensionless number Φ of the coal self-heating temperature increases exponentially.

Experimental study on a high efficient and ultra-lean burn meso-scale thermoelectric system based on porous media combustion

Current combustion-based meso-scale thermoelectric systems generally operate at stoichiometric ratio, which leads to high emissions. Lean burn is beneficial to the burning of low-concentration gas and emissions reduction. To exploit the thermal energy of low-concentration gas, this paper proposed a meso-scale thermoelectric system based on porous media combustion to reduce the lean burn limit while utilizing thermal energy by thermoelectric generators. Experimental results showed that the lean burn limit is reduced to 0.46 which is the lowest obtained so far. It was found that the relationship between system efficiency and equivalence ratio or inlet velocity is not monotonous, but is related to wall temperature distribution and flame situations. The system efficiency reaches its peak when the equivalence ratio is around 0.70. To improve the system efficiency, four layouts of porous media were proposed. All four layouts are found to be useful to stabilize the flame in a confined position. However, for the same thermal power input, the combustor with the downstream porous media gives higher wall temperature and higher system efficiency because the downstream porous media functions as a heat sink to absorb more heat from the flame and redirects it to thermoelectric generators. Experimental results provided a useful reference for the development of the combustion-based meso-scale thermoelectric system.

Hot surface ignition of a wall-impinging fuel spray: Modeling and analysis using large-eddy simulation

The accidental ignition of liquid fuels is an industrial safety concern due to the storage and transport of pressurized flammable liquids near components at elevated temperatures. In this work, a liquid n-dodecane fuel spray is considered that impinges on a hot surface, undergoing thermal ignition. Surface temperatures above the minimum hot surface ignition temperature (MHSIT) for n-dodecane ignition at atmospheric pressure are considered. At these temperatures, the interaction of the spray with the hot surface is governed by the Leidenfrost effect, resulting in inelastic reflection of impinging droplets. Large-eddy simulations are employed with finite-rate chemistry using a realistic 54-species chemical mechanism with low-temperature ignition chemistry. An Eulerian-Lagrangian approach is taken to the describe the spray dynamics. Ignition kernel formation and propagation are discussed in physical and composition spaces, and the flow field structure is compared to theory. At temperatures near the MHSIT, the secondary flow resulting from the spray impingement in the form of a toroidal vortex is shown to enhance scalar mixing. Low-temperature ignition within the vortex is seen to significantly precede high-temperature ignition near the wall and subsequent rapid flame propagation. At higher wall temperatures, the ignition delay is greatly reduced such that high temperature ignition occurs prior to the establishment of mixing structures, and hence the extent of flame propagation is diminished, with transition occurring rapidly to a steady-burning regime. This study provides fundamental understanding of the physical phenomena involved in the thermal ignition of impinging sprays in different temperature regimes toward the goal of improved industrial safety.

Single-phase and Two-phase Flow and Heat Transfer in Microchannel Heat Sink with Various Manifold Arrangements

The manifold microchannel (MMC) heat sink, which possesses better heat transfer performance and less pressure drop than the traditional microchannel heat sink, has attracted much attention from researchers in recent years. But not much of them, either single-phase or two-phase ones, focused on the problems about flow maldistribution and wall temperature nonuniformity that might ultimately lead to system breakdown. To investigate the flow distribution in the MMC heat sink, six types of manifold, namely Z-type, C-type, H-type, U-type, ZU-type, and HU-type are considered in the current study. All six manifolds are investigated in single-phase cases with water, while the latter three are adopted in two-phase cases with HFE7100. The cases are solved with the self-developed codes based on the OpenFOAM package. The manifold arrangement has a strong impact on the overall heat transfer performance and pumping power required. The HU-type and ZU-type MMC heat sinks are recommended to be fabricated in the single-phase heat transfer application due to their small thermal resistance lower than 2×10−5 m2·K/W and pressure drop less than 500 Pa. Besides, if the pumping power is not a restriction factor in the system, Z-type MMC is also recommended. In the two-phase cases, the flow distribution is naturally not uniform within a parallel microchannel system, and the microchannels with large flow velocity appear to have higher wall temperatures. The authors recommend using HU-type MMC heat sink in the flow boiling application due to its bottom wall temperature distribution is relatively uniform with a difference of less than 2 K.

Effects of porous media on partially premixed combustion and heat transfer in meso-scale burners fuelled with ethanol

Porous media (PM) have the potential to enhance the flame stability and improve thermal performance in a micro/meso-scale burner. In this work, zirconia foam and high-porosity nickel foam are used in the meso-scale burners and partially premixed submerged combustion of liquid ethanol is achieved. An experimental investigation of the effects of foamed PM on flame structure, detailed temperature distributions, emissions, and thermal performance is undertaken. A blue flame can be observed in the nickel foam and the transient features of flame front is obtained. The flame front propagates intermittently when the combustion zone moves upstream. The maximum instantaneous flame speed in the PM is 6.519 mm/s and the average flame speed is 0.225 mm/s. The foamed PM in the meso-scale burners are shown to significantly improve the heat transfer efficiency. The burner in submerged combustion has a lower reaction zone temperature (806 °C vs. 1721 °C), lower exhaust gas temperature (419 °C vs. 1227 °C) and higher wall temperature than that with free flame. The heat transfer efficiency of the submerged flame is about twice that of the free flame.

Radiation and convection treatment of nanomaterial within a Linear Fresnel Reflector unit

To absorb more solar flux, the tube of LFR system was equipped with Y-shaped fins in current investigation. The fluid in tube is H2O with inclusion of alumina nanoparticles. Installing mirrors with special angles and installing parabolic reflector above the tube lead to greatest optical performance. Turbulent regime was assumed and K-ɛ approach was used for modeling. Reasonable accommodation with data of empirical correlations indicates nice accuracy of simulation. Influences of operation factors on carrier fluid thermal behavior were presented. Also, exergy efficiency, irreversibility and Be were scrutinized. DO model with employment of conditions of pure radiation was utilized to compute the solar heat flux. Better flow mixing takes places if values of Vin, L1 and L2 augment because residence time grows. Increase in Tin offers lower cooling rate and higher wall temperature appears. Increasing Vin, L1 and L2 makes tube temperature to decline about 0.457%, 0.109% and 0.194% with assuming lowest level of other factors. Also, increasing Tin offers augmentation of tube temperature about 9.93%. Darcy factor drops with grow of L1 and L2 about 47.27% and 37.42% when Vin = 0.16, Tin = 293.15 K. As Vin augments, Nu intensifies about 82.71% when L1 = L2 = 2 mm. Nu increases with augment of L1 about 15.67%. Sgen,h declines about 9.82% with augment of L1 while Sgen,f augments about 51.68% with rise of this variable. Be decreases about 1.13% and 0.108% with intensify of Vin and L2, respectively. The value of $$\eta_{{{\text{th}}}}$$ augments with rise of Vin about 1.08% while it reduces about 23.43% with growth of Tin. With rise of L1, $$\eta_{{{\text{ex}}}}$$ and $$\eta_{{{\text{th}}}}$$ enhance about 0.67% and 0.09% when L2 = 2 mm, Vin = 0.16, Tin = 293.15 K.

Experimental study on characteristics of hydrogen production from exhaust gas-fuel reforming in a catalytic fixed-bed reactor

Since the hydrogen-rich gas can be generated on-board by the catalytic reforming of exhaust gas from the engine, reformed exhaust gas-fuel recirculation is an attractive method for waste-heat recuperation and performance enhancement for the engines fueled by natural gas (NG). In the present study, the industrial Ni/Al2O3 catalyst was selected to investigate the effects of wall temperature, feed ratio and gas hourly space velocity (GHSV) on hydrogen production from exhaust gas-fuel reforming in a catalytic fixed-bed reactor. Meanwhile, Ni/Al2O3 fresh catalyst was characterized by the X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) techniques. The simulated exhaust gas was prepared based on the exhaust gas from a marine NG engine at 75% propeller load. The experimental results show that the higher wall temperature is favored for steam reforming to deliver higher H2 concentration in the reformate gas, which result in the increase of the H2 volume fraction at reformer outlet, H2 yield and CH4 conversion with the wall temperature. The maximum values of H2 yield and H2 volume fraction from the reformer outlet can reach up to approximately 0.96 and 22%, respectively. From the view of reforming process, that CH4/O2 = 2 and H2O/CH4 = 2 is considered as the optimum matching feed ratio. When a GHSV is approximately 7500 h−1, the H2 volume fraction at reformer outlet can be maximized. Under the given conditions, a more preferable reforming energy efficiency can be achieved when H2O/CH4 = 2, CH4/O2 = 3, GHSV = 6000 h−1 and high wall temperature.

Modeling of three-dimensional exciplex pumped fluid Cs vapor laser with transverse and longitudinal gas flow

Abstract Considering the thermodynamical fluid mechanics in the gain medium and laser kinetic processes, a three-dimensional theoretical model of an exciplex-pumped Cs vapor laser with longitudinal and transverse gas flow is established. The slope efficiency of laser calculated by the model shows good agreement with the experimental data. The comprehensive three-dimensional distribution of temperature and particle density of Cs is depicted. The influence of pump intensity, wall temperature, and fluid velocity on the laser output performance is also simulated and analyzed in detail, suggesting that a higher wall temperature can guarantee a higher output laser power while causing a more significant heat accumulation in the cell. Compared with longitudinal gas flow, the transverse flow can improve the output laser power by effectively removing the generated heat accumulation and alleviating the temperature gradient in the cell.

Comparison of H2S high-temperature corrosion characteristics of three kinds of low-alloy and heat-resistant-steel tubes

ABSTRACT This study investigates the H2S high-temperature corrosion characteristics of three common heat – exchanging tubes for supercritical boilers, namely SA-213T12, 15CrMoG and 12Cr1MoVG. The high-temperature corrosion rates and wall thickness reduction rates of these three materials were investigated. The results indicate that the spalling of corrosion fragments produced by high-temperature H2S corrosion is the main cause for the reduction in the tube wall thickness. Moreover, as a pressure component, the thinning of the water wall tube thickness is an important index to judge its service life. The bulk aggregation of corrosion products can easily cause the corrosion layer to fall off and make the corrosion reaction go inside. By comparison, the abilities of the three kinds of tubes for anti-H2S high-temperature corrosion are ranked from weak to strong as follows: SA-213T12、15CrMoG and 12Cr1MoVG. Besides, the high-temperature H2S corrosion process is the most sensitive to temperature.

Effects of variable heat flux at supercritical pressure in vertical pipe flow

It is a well-established fact that the heat transfer at supercritical pressure is complex and several numerical investigations have been conducted to address this issue. However, the focus of such studies remains uniform heat flux. Therefore, here we report the outcomes of direct numerical simulations performed with different profiles of non-uniform heat flux for supercritical carbon dioxide. Flow in a small pipe with a diameter of 2 mm is considered and vertical orientation with upward direction of fluid flow was chosen. The heat flux was varied in the axial and circumferential direction of the pipe. The circumferential heat flux induces the thermal stratification which resulted in a higher wall temperature at one side where heat flux was maximum. While axially varying wall heat flux results in the peak and valley in the wall temperature. The study also suggests that axially varying heat flux can be a possible strategy to control the heat transfer deterioration.

Thermal characteristics and thermal stress analysis of a superheated water/steam solar cavity receiver under non-uniform concentrated solar irradiation

Solar cavity receiver is a vital component in a solar power tower (SPT) system, where concentrated solar irradiation is absorbed, converted into heat and carried away by a kind of heat transfer fluid. The cavity receiver usually suffers from the high-flux and non-uniform concentrated solar irradiation, which would cause local overheating, large temperature gradient and even thermal stress fracture of the receiver and bring challenges for the safety of its daily operation. Therefore, it is necessary to investigate the thermal and mechanical performance of the solar cavity receiver. In the present study, a superheated water/steam solar cavity receiver with three boiling panels and a superheater was chosen as the physical model. An integrated simulation method, which couples the Monte-Carlo ray tracing (MCRT) method, the appropriate heat transfer correlations, the finite volume method (FVM) and the finite element method (FEM), was employed to simulate the complicated heat transfer process inside the receiver and conduct the thermal stress analysis for the absorber panels. With this method, the three-dimensional wall temperature and thermal stress distributions were gained both on the boiling panels and the superheater, and the causes of thermal stress induced by different temperature gradients were analyzed in detail. The following results were obtained: 1) The heat flux, wall temperature and equivalent thermal stress distributions all appear highly non-uniform on the absorber panels; 2) The hot spots can be found on the membrane wall between the elbows of the boiling panels, which can easily generate the high thermal stress due to the large temperature gradient with their surroundings; 3) The bi-axial stress at the outer and inner radius of the boiling tubes can be created by the large radial temperature gradient, and the thermal stress varies as a cosine distribution across the circumference of the superheated tubes, which is induced by the large temperature gradient in the circumferential direction. Besides, the spot-like areas with high thermal stress can also be found on the superheated tubes due to the considerable nonlinear axial temperature gradient. Moreover, the thermal stress on the membrane wall connected with both the boiling and the superheated tubes has the definite directionality, which shows parallel to the tubes or perpendicular to the tubes; 4) The thermal stress concentration always occurs at the high-temperature and structural discontinuity locations. Here, the thermal stress of the superheater is 360 MPa higher than that of the boiling panels. In general, the superheater has both the higher wall temperature and thermal stress, which is more crucial for the safe operation of the superheated water/steam solar cavity receiver.

Boiling of Argon flow in a microchannel by considering the spherical geometry for roughness barriers using molecular dynamics simulation

In this research, the method of molecular dynamic (MD) is used to consider the influences of the spherical roughness barrier on the boiling flow regime of Argon particles, flowing into microchannel with a square cross-section. To prepare boiling conditions, different constant temperatures from 84 K to 133 K are applied on the walls of microchannels. Results show that roughness elements help boiling thermal forces to the distribution of fluid atoms in central layers of a microchannel, especially at high time steps between 750,000 and 1,000,000. This phenomenon is noticeable under applied high wall temperatures of 114 K and 133 K. Also, a summation of velocity values indicated that increasing boundary wall temperatures of rough microchannels results in a small reduction of the velocity of fluid flow, whereas; it is almost unchanged in the smooth microchannel. The statistical approach shows that the presence of spherical roughness does not have a destructive influence on the boiling flow properties of Argon into the microchannel. Also, they are useful to enhance effective surfaces of heat transfer, which empower the boiling process. Therefore, investing in polishing microchannel surfaces and removing the spherical shape of roughness components can even be a serious mistake for some of the practical applications.

Diminishing vortex intensity and improving heat transfer by applying magnetic field on an injectable slip microchannel containing FMWNT/water nanofluid

In this study, heat transfer and entropy generation were investigated in a microchannel containing FMWNT/water nanofluids given the slip condition. The main focus was on utilizing injection technique in the presence of the magnetic field. The injection from the upper high-temperature wall was incorporated into the flow field. Injection at high Reynolds number causes vortex formation, which ultimately reduces local heat transfer in the adjacent injection zone. By applying the magnetic field, the vortex intensity as well as boundary layer thickness was diminished which in turn improved the heat transfer. Based on numerical results, at higher nanoparticle volume fraction, the effect of the magnetic field on heat transfer enhancement was amplified. Moreover, at higher Reynolds numbers, the magnetic field efficacy is more obvious. The highest heat transfer occurred at the highest values of the Hartmann and Reynolds numbers and eventually the nanoparticle volume fraction. Owing to applying the magnetic field on the injectable microchannel containing nanofluid, heat transfer improvement can reach up to 79%. From the second law prospective, the entropy generation intensified by 82.8%.

Wall temperature effects on shock wave/turbulent boundary layer interaction via direct numerical simulation

Shockwave/turbulent boundary layer interaction on flat plate are investigated by using direct numerical simulation at different wall temperature of Tw/Tr=0.5,1.0,2.0 respectively. Responses of skin friction, velocity profile and root mean square velocity profile are examined. Analysis on velocity profiles indicate that streamwise velocity in the outer part of boundary layer experiences a reduction with wall temperature going up. Skin friction decreases while wall is heating mainly caused by a lower velocity gradient near the wall at a high temperature. A lower skin friction on high wall temperature results in a stronger separation. The comparison of root mean square velocity profile shows turbulent kinetic energy is augmented on a heating wall. The maximum value moves closer to the wall with wall temperature increasing. The scale of separation bubble is amplified with wall temperature increasing. Near-wall streaks become unstable and get an increment in spanwise distance. The process of near-wall strip vortices evolving to complicated 3-dimonsional structures is accelerated and amplified in a high wall temperature. Counter-rotating vortex in reattachment process is evaluated in the three cases. The scale of such vortex gets larger when wall temperature gets higher. The spanwise distance between counter-rotating vortices rises obviously and break down to smaller coherent structures quicker.

Heat transfer measurements in a hydrogen-oxyfuel combustor

ABSTRACT In the context of emission-free electricity generation, the authors developed a novel hydrogen-oxygen combustor, which is based on swirl-stabilized combustion technology and steam dilution. Following previous burner characterization, the current study investigates the heat transfer conditions in the combustion chamber wall. To this end, a carefully controlled co-flow of air is used to cool the combustion chamber in an annular duct, which surrounds it. Temperature measurements enable the evaluation of the heat flux from the combustor flow to the walls, local wall temperatures, and the Nusselt numbers on the hot and the cold side. The extremely high wall temperatures, caused by the H2/O2 flame, can be reduced by steam dilution down to approximately 900 K. Even better cooling could be reached by using the dilution steam as the coolant before it flows to the plenum. The Nusselt numbers at the inner combustion chamber wall are in the order of 10–50 and increase with the thermal power and the steam dilution ratio.

Molecular dynamics study on the effect of hydrophilicity on thin film evaporation over biphilic surface

The objective of this study is to understand the effect of surface wettability and how nano-scale heat transfer is affected by different surface wetting conditions over biphilic surface by molecular dynamics simulations. The system of 3 nm liquid Argon film over Platinum surface is equilibrated at 90 K and then wall temperature is raised to higher value to study the effect of temperature. According to this study at low wall temperature and low surface wetting conditions, liquid temperature increases proportionally with the increment of hydrophilic portion but at higher wall temperature and better surface wetting conditions, maximum value of liquid Argon temperature increases with increasing hydrophobic portion. Evaporation number, evaporative mass flux and heat flux increase significantly with slight increment of hydrophilic portion. Inception time decreases with increasing hydrophilic portion and surface wettability. Also, for better surface wetting conditions, average evaporative mass flux is more sensitive to wall temperature and increases significantly at higher wall temperature.

Effects of Heat Flux, Mass Flux, Inlet Fluid Subcooling, and Channel Orientation on Subcooled Flow Boiling in a High-Aspect-Ratio Rectangular Microchannel

Abstract An experimental investigation of subcooled flow boiling in a high-aspect-ratio, one-sided heating, silicon-based rectangular microchannel was conducted utilizing de-ionized water as the working fluid. The microchannel was 5.01 mm wide and 0.52 mm high, having a hydraulic diameter of 0.94 mm and an aspect ratio (AR) of 10. The heat flux, mass flux, and inlet fluid subcooling were in the ranges of 0–30 W/cm2, 200–500 kg/m2 s, and 5–20 °C, respectively, while the orientations were vertical and horizontal. Parametric study on heat transfer characteristics including the onset of nucleate boiling (ONB), heat transfer coefficient (HTC), and critical heat flux (CHF) was carried out combined with flow visualization. Significant appearance of ONB without boiling hysteresis was observed in the boiling curve, accompanied with bubble nucleation. Nucleate boiling occurred first near the exit, where the HTC increased more sharply, while easier bubble nucleation was found near the sides. Unique time-dependent flow pattern consisting of isolated bubbly flow, elongated bubbly flow, partial dry-out, and rewetting process was observed. More nucleation sites were activated at higher heat flux, while higher initial heat flux and wall superheat for ONB as well as higher CHF value were obtained at higher mass flux and inlet subcooling. Compared to the vertical channel, higher wall temperature and pressure drop with larger oscillation amplitudes were found for the horizontal counterpart, where the merged bubbles agglomerated in the heating section, resulting in earlier dry-out which deteriorated heat transfer.

Quality Performance Evaluation of Thin Walled PLA 3D Printed Parts Using the Taguchi Method and Grey Relational Analysis

This paper investigates the quality performance of FDM 3D printed models with thin walls. The design of experiments method (DOE) was used and nine models of the same size were fabricated in a low-cost 3D printer using polylactic acid (PLA) material. Two limited studied parameters were considered (extraction temperature and wall thickness), each one having three levels. External X and Y dimensions were measured using a micrometer, as well as four surface roughness parameters (Ra, Rz, Rt, Rsm) with a surface tester. Two optimization techniques (the Taguchi approach and Grey relational analysis) were utilized along with statistical analysis to examine how the temperature and wall thickness affect the dimensional accuracy and the surface quality of the parts. The results showed that high extraction temperature and median wall thickness values optimize both dimensional accuracy and surface roughness, while temperature is the most important factor.