Pure Conduction Research Articles

Simulation of turbulent flow subjected to conjugate heat transfer via a dual immersed boundary method

In the context of turbulent flows, this study presents a new methodology for the high-fidelity simulation of conjugate heat transfer between solid and fluid. The numerical strategy is mainly based on the immersed boundary method to ensure the expected thermal boundary condition at the fluid/solid interface. The approach is dual since the solid domain is the immersed region for the fluid and, conversely, the fluid for the solid. The resulting method is an extension of an already existing immersed boundary method based on the reconstruction of the solution inside the immersed region. The duality of the technique requires to define one temperature field for the fluid and another one for the solid. The interaction between the two temperature fields is ensured, at the fluid/solid interface, through an efficient weak coupling preserving numerical stability and accuracy. Thanks to the reconstruction in its own immersed zone, each temperature field solution is perceived as smooth by the numerical differentiation method, for any fluid-to-solid ratio of thermal conductivities and diffusivities, while representing a desirable sharp interface. This feature is crucial for the high-order finite difference schemes used in this work. The method is validated for the plane channel and pipe flow configurations and a demanding pure conduction case with variable conductivity in a composite wall. Its ability to preserve second-order accuracy in space is checked in the laminar and pure conduction cases by comparison with analytical solutions. In the turbulent case, the assessment is based on comparisons of basic turbulent statistics with reference data obtained by direct numerical simulation. The step-by-step analysis of thermal statistics shows that the present immersed boundary method can ensure accurately imposed temperature, imposed heat flux, and conjugate heat transfer conditions at the fluid/solid interface.

Calculated versus measured pure tone bone conduction 3 kHz thresholds in sudden sensorineural hearing loss

To compare the measured bone conduction threshold at 3 kHz with the calculated threshold in newly diagnosed sudden sensorineural hearing loss. A retrospective chart review was conducted of pure tone audiograms in confirmed sudden sensorineural hearing loss cases. Of 157 patients with sudden sensorineural hearing loss, 144 had idiopathic hearing loss, 8 had vestibular schwannoma and 5 had Ménière's disease. The r value for the correlation between the two methods of 3 kHz assessment for all patients was 0.887 (p < 0.001). The mean difference between the measured and calculated 3 kHz thresholds was 0.76 ± 7.96 dB, 0.4 ± 8.08 dB and 1.5 ± 1.8 dB in the sudden sensorineural hearing loss, idiopathic and Ménière's disease groups, respectively. The mean difference between the measured and calculated 3 kHz thresholds was significantly greater in the vestibular schwannoma group (6.86 ± 4.38 dB) than in the idiopathic group (p = 0.013). The 3 kHz frequency may encompass important audiometric information. A discrepancy between the measured and calculated bone conduction 3 kHz thresholds raises suspicion of an underlying vestibular schwannoma as an aetiology for sudden sensorineural hearing loss, and these thresholds should therefore be measured independently and routinely.

Examination of Gadolinium Doped Ceria Mixed with Yttrium Stabilized Zirconia Mixed Ionic Electronic Conducting Solid Electrolytes for Use As Reversible High Temperature Cells

Reversible cells can be used as solid oxide fuel cells (SOFC) and solid oxide electrolyzer cells (SOEC). The solid electrolytes used for such cells frequently have pure ionic conduction with insignificant electronic conduction. A previous study (Virkar and Tao, 2015) has shown that cells made of ceria doped yttrium stabilized zirconia (YSZ), a mixed ionic electronic conducting (MIEC) material, can be made to operate at the same efficiency level and with superior stability compared to a pure ionic conductor, like pure YSZ. This work is a continuation of that study, examining the use of gadolinium doped ceria (GDC) mixed with YSZ. The MIEC cell, 8mol%GDC+YSZ, will be further tested and designed to consume fuel or water vapor, generate power or hydrogen, and release heat at the same rate as the pure ionic conducting cell, YSZ, when operating in SOFC or SOEC mode. Future testing will also determine if the GDC MIEC cell can match or exceed the durability of the ceria doped YSZ.The electrolyte layer, made of either pure 8YSZ or 8YSZ doped with 8mol%GDC, was die pressed and sintered at 1400oC (2oC/min) for five hours. On one side of the electrolyte, the anode interlayer is mixed with isopropyl alcohol and painted on the electrolyte's surface. The anode interlayer is composed of 50wt% NiO and 50wt% 8YSZ. The cell was heated to 950oC (2oC/min) for one hour. The anode functional layer, 70wt% NiO and 30wt% 8YSZ, was painted on top of the anode interlayer. The sample was then again heated to 950oC (2oC/min) for one hour to burn off any organic compounds. A cathode interlayer was then painted on the other side of the electrolyte layer. The cathode interlayer was also painted on and consists of 50wt% lanthanum strontium manganite (LSM) and 50wt% YSZ. The cell is heated to 1200oC (2oC/min) for two hours. Finally, a pure LSM layer is painted on top as the cathode current collector material and heated to 1100oC (2oC/min) for one hour to ensure good porosity. The cells were then sealed by compression-loading at the edges using glass-mica seals.Results of tests will be reported for both SOEC and SOFC modes at 800oC. Data to be reported include open-circuit voltage, area-specific resistance, corresponding ionic currents based on an applied voltage, and long-term durability of the two cells in both SOFC and SOEC modes.

Convective heat transfer in suspensions of prolate ellipsoids

Particle Resolved Simulations (PRS) using the Immersed Boundary Method (IBM) are performed for flow through suspensions of ellipsoids with aspect ratios of 2.5, 5, and 10 for solid volume fractions from 0.1 to 0.3 in the Reynolds number range from 10 to 200. The mean Nusselt number increases as Re1/2but shows only a weak dependence on the aspect ratio while increasing between 10-15% with an increase in solid fraction from 0.1 to 0.3. Two common practices of calculating Nusselt number in past literature are reconciled. It is shown that the suspension mean Nusselt number based on individual particles, by definition is always greater than or at the least equal to the Nusselt number based on the internal developing flow analogy. It is established that for Re≤50, the suspension heat transfer coefficient is very sensitive to the spatial distribution of particles or local-to-particle solid fractions. For the same mean solid fraction, suspensions dominated by particle clusters or high local solid fractions can exhibit Nusselt numbers which are lower than the minimum Nusselt number imposed by pure conduction on a single particle in isolation. This results from the dominant effect of thermal wakes at low Reynolds numbers. As the Reynolds number increases to 100 and beyond, the effect of particle clusters on heat transfer becomes less consequential. Unlike heat transfer, particle clustering has a less significant effect on mean fluid forces such as drag.

Anisotropic heat transfer of ferro-nanofluid in partially heated rectangular enclosures under magnetic field

The natural convection and anisotropic heat transfer of ferro-nanofluids in partially heated enclosures is studied for revealing the influence of the anisotropy of thermal conductivity, which is generated by the existence of external magnetic field. The viscosity of the ferro-nanofluid is assumed to vary with the nanoparticle concentration. The constitutive model of the anisotropic thermal conductivity is derived based on the principle of material frame indifference of Continuum Mechanics, and the numerical solver is built based on the library of OpenFOAM. Both problems of pure heat conduction and natural convection are investigated, and a series of numerical simulations are conducted for different relevant parameters such as the types of the magnetic field, nanoparticle concentration, Hartmann number and Rayleigh number. The numerical results show that the heat transfer along the magnetic field direction is apparently enhanced with Hartmann number of 0.1 and nanoparticle concentration of 0.05, which implies the feasibility of controlling the heat transfer of the ferro-nanofluids by adjusting the external magnetic field. Furthermore, for pure heat conduction, the increasing of the Hartmann number and nanoparticle concentration raises the anisotropic thermal conductivity; and in the natural convection case, increasing the intensity of magnetic field could also raises the Lorentz force resistance. In addition, the Nusselt number is larger with higher Rayleigh number and lower Hartmann number, as the Rayleigh number increases from 1×103 to 1×105, the average Nusselt number on the heater increases from 0.888 to 3.139, and it decreases from 3.789 to 2.866 when Hartmann number is 0 and 10.

Preparation and characterization of a heat storage material: Shape-stabilized KNO3 using a modified diatomite-based porous ceramic as the skeleton

The applications of salts-based phase change materials (PCMs) are greatly restricted by their corrosion to containers and low thermal conductivity. To address these issues, a modified diatomite-based porous ceramic (MDPC) was prepared to shape-stabilize KNO3 salt in this paper. Particularly, effects of graphite content on the pore structure of diatomite-based porous ceramic (DPC) and MDPC on the thermophysical properties of shape-stabilized KNO3 were investigated. The results showed that 30 wt% graphite increased apparent porosity of DPC up to 67.21% and enlarged average pore diameter from 0.71 to 3.58 μm, while weakened its compressive strength from 19.12 to 5.51 MPa. The MDPC effectively prevented leakage of molten KNO3 to avoid its corrosion problem, and possessed an excellent chemical compatibility with KNO3 salt. Additionally, the MDPC hardly changed phase transition temperature of shape-stabilized KNO3, while increased latent heat of composites from 51.70 to 60.21 J/g due to its higher apparent porosity. The MDPC was also shown to enhance thermal conductivity of shape-stabilized KNO3 from 1.16 to 1.52 W/(m·K) at 25 °C, which was attributed to that the MDPC with a larger pore size was more easily infiltrated by molten salt, contributing to a lower apparent porosity of composites. Compared with pure KNO3, thermal conductivity and thermal stability of shape-stabilized KNO3 were seen to be significantly improved by 238% and 60 °C, respectively. This work therefore solved two key challenges for the application of KNO3 salt, and proposed an effective way to improve thermophysical properties of such heat storage material.

Sensitivity analysis of transient heat and moisture transfer in a bio-based date palm concrete wall

Hygrothermal mathematical models are commonly used to describe heat and moisture transfer in porous and bio-based building construction materials. This allows to evaluate their thermal insulation capacity, as well as their ability to regulate external climatic conditions and ensure indoor comfort for inhabitants. In this paper, a sensitivity analysis is performed on Kunzel's model, while it is applied to simulate the hygrothermal behavior of a wall structure made of a new bio-based building material (cement composite including date palm fibers). In a first part, the effects of finite variations of material properties/boundary conditions on the model's outcome are investigated. In a second step, specific transfer modes are neglected in the model, in order to study their influence on the numerical predictions. The results of this parametric study show that special attention should be paid to few parameters (heat capacity and density for heat transfer, sorption isotherm and water vapor resistance factors for moisture transfer) at the expense of others. Uncertainties on these influent parameters may result in large error accumulation, especially when modeling the moisture transfer process. Furthermore, initial boundary conditions and sensors position appear to be possible sources of discrepancies in the calculated RH profiles. Finally, the pure conduction model is found to provide good estimation of the temperature profiles compared to the full model, whereas liquid transfer must always be taken into account in the model to ensure accurate RH predictions through a bio-based date palm concrete wall.

Transcutaneous and percutaneous bone conduction sound propagation in single-sided deaf patients and cadaveric heads

Objective To investigate transcranial transmission (TT) and the dampening effect of the skin in patients and cadaver heads. Design In patients a pure tone bone conduction audiogram for ipsilateral and contralateral stimulation was performed. The TT was defined as the difference between ipsilateral and contralateral hearing thresholds. In cadaver heads ipsilateral and contralateral promontory motion was measured using a three-dimensional Laser Doppler Vibrometer system. Study sample Seven single-sided deaf patients fitted with a Baha® Connect, fifteen single-sided deaf patients without a bone conduction hearing aid and five Thiel-embalmed cadaver heads were included. Results The TT decreased with increasing frequency in patients and cadaver heads. No significant difference was seen between patients and cadaver heads. Measurements on patients and cadaver heads showed increasing skin attenuation with increasing frequency. However, the dampening effect was 3–12 dB higher in patients than in cadavers at all frequencies. Conclusion The TT was not significantly different for patients compared to cadaver heads. The value of promontory motion to estimate TT in patients need to be further evaluated. The skin attenuates a BC stimulus by 10–20 dB in patients and by a smaller amount in cadaver heads, probably due to changes in the properties of the Thiel-conserved skin.

Automated Audiometry in Quiet and Simulated Exam Room Noise for Listeners with Normal Hearing and Impaired Hearing.

Up to 80% of audiograms could be automated which would allow more time for provision of specialty services. Ideally, automated audiometers would provide accurate results for listeners with impaired hearing as well as normal hearing. Additionally, accurate results should be provided both in controlled environments like a sound-attenuating room but also in test environments that may support greater application when sound-attenuating rooms are unavailable. Otokiosk is an iOS-based system that has been available for clinical use, but there are not yet any published validation studies using this product. The purpose of this project was to complete a validation study on the OtoKiosk automated audiometry system in quiet and in low-level noise, for listeners with normal hearing and for listeners with impaired hearing. Pure tone air conduction thresholds were obtained for each participant for three randomized conditions: standard audiometry, automated testing in quiet, and automated testing in noise. Noise, when present, was 35 dBA overall and was designed to emulate an empty medical exam room. Participants consisted of 11 adults with hearing loss and 15 adults with normal hearing recruited from the local area. Thresholds were measured at 500, 1,000, 2,000, and 4,000 Hz using the Otokiosk system that incorporates a modified Hughson-Westlake method. Results were analyzed using descriptive statistics and also by a linear mixed-effects model to compare thresholds obtained in each condition. Across condition and participant group 73.6% of thresholds measured with OtoKiosk were within ± 5 dB of the conventionally measured thresholds; 92.8% were within ± 10 dB. On average, differences between tests were small. Pairwise comparisons revealed thresholds were ∼3.5-4 dB better with conventional audiometry than with the mobile application in quiet and in noise. Noise did not affect thresholds measured with OtoKiosk. The OtoKiosk automated hearing test measured pure tone air conduction thresholds from 500 to 4,000 Hz at slightly higher thresholds than conventional audiometry, but less than the smallest typical 5 dB clinical step-size. Our results suggest OtoKiosk is a reasonable solution for sound booths and exam rooms with low-level background noise.

Telehealth Audiology and Virtual Hearing Screenings: A Case Report.

Clinic closures across the United States in response to the coronavirus disease 2019 (COVID-19) pandemic meant that many audiology services were reduced to phone calls, video visits, and modified services (e.g., curbside, mail-ins, drop offs). Audiologists and other providers needed a way to manage hearing- and ear-related complaints via telehealth. A simple pure tone air conduction threshold test performed at home was used to determine which patients needed to come into the clinic for an in-person appointment. This case will review a 56-year-old male who was being treated for an idiopathic sudden sensorineural hearing loss prior to clinic closure. He had on file a comprehensive audiogram at first onset of symptoms and was treated with oral steroids. Virtual hearing testing was then utilized for a variety of other patients from teenagers to adults to triage hearing- and ear-related complaints while clinics were closed for in-person visits.

Understanding the thermal problem of variable gradient functionally graded plate based on hybrid numerical method under linear heat source

The thermal problem of functionally graded materials (FGM) under linear heat source is studied by a hybrid numerical method. The accuracy of the analytical method and the efficiency of the finite element method are taken into account. The volume fraction of FGM in the thickness direction can be changed by changing the gradient parameters. Based on the weighted residual method, the heat conduction equation under the third boundary condition is established. The temperature distribution of FGM under the action of linear heat source is obtained by Fourier transform. The results show that the closer to the heat source it is, the greater the influence of the heat source is and the influence of the heat source is local. The temperature change trend of the observation points is consistent with the heat source, showing a linear change. The results also show that the higher the value of gradient parameter is, the higher the temperature of location point is. The temperature distribution of observation points is positively correlated with gradient parameter. When the gradient parameter value exceeds a certain value, it has a little effect on the temperature change in the model and the heat conduction in the model tends to be pure metal heat conduction, the optimal gradient parameters combined the thermal insulation property of ceramics and the high strength toughness of metals are obtained.

Using triply periodic minimal surfaces (TPMS)-based metal foams structures as skeleton for metal-foam-PCM composites for thermal energy storage and energy management applications

Organic phase change materials (PCMs), such as paraffin wax, have shown great potential for their utilization in latent heat thermal energy storage systems (LHTES). However, due to their low thermal conductivity, they are often hybridized with high thermal conductivity metal foam, resulting in metal foam-PCM composites (MFPCMs) with enhanced heat transfer features. Conventional metal foam is usually idealized using the Kelvin cell. Owing to the recent advances in 3D printing, any complicated topology can however be easily manufactured, which paves a pathway for other complex cell types to be utilized in such energy systems other than Kelvin cell. In this exploratory work, three Triply Periodic Minimal Surfaces (TPMS), i.e., Gyroid, I-graph and wrapped package-graph (IWP), and Primitive cells, are (to the best of the knowledge of the present authors), used for the first time, as skeleton for MFPCMs composites to enhance the effective thermal conductivity of conventional PCMs. Transient numerical simulations were performed to compare the thermal energy storage performance of the used TPMS-based PCM composites with their counterpart based on the conventional Kelvin cell. All structures were tested at the same porosity level and unit cell size of 90% and 7 mm, respectively. Simulations were done under two boundary conditions namely isothermal and isoheat flux conditions. Steady state simulations were also performed to assess the effective thermal conductivity of the used MFPCM composites at temperatures below the melting temperature of used PCM (pure conduction). The obtained results stress that the effective thermal conductivity of MFPCMs strongly depends on the cell type and its unique architecture and not only on the cell porosity where significant increase in the effective thermal conductivity of the MFPCMs composites was achieved when the three TPMS structures are used. Under isothermal condition while considering the Kelvin-based MFPCM as the baseline case, the PCM melting time was reduced by approximately 31% for the Gyroid, 40.3% for the IWP, and 35.3% for the Primitive-based MFPCMs. In isoflux case, the PCM melting time did not show dependence on the type of metal foam structure. However, by considering the temperature homogeneity as a performance indicator (quantified by the maximum and minimum temperature difference in PCM domain), Kelvin-based MFPCM showed the highest value for the difference (least homogenous) whereas, IWP-based MFPCM on average was almost 5 K lesser than the Kelvin-based MFPCM during the entire melting process. Therefore, TPMS structures showed superior performance than the Kelvin cell, let alone than the case of PCM alone, which makes them promising candidates for potential utilization in LHTES applications.

Heat transfer and energy storage performances of phase change materials encapsulated in honeycomb cells

Thermal energy storage devices are vital for reducing the inconsistency between energy supply and demand as well as for enhancing the performance of solar thermal systems. The present study investigates the melting process in metallic honeycombed heat exchangers filled with n-octadacane as phase change material (PCM). Further, a parametric study is conducted for four different honeycomb cell sizes, fin thicknesses, and angles of inclinations of the computational domain. The findings are described in the form of heated surface average temperature variation, melt fraction of the PCM, and thermal energy stored. Results show that the honeycomb fin structure considerably improves the heat transfer in the PCM. Further, it was observed that the honeycomb cell size and fin thickness greatly influence the variation of the average front surface temperature, melt fraction, and energy storage rate, whereas the inclination angle doesn't have a significant effect when cell size is less than 0.01 m. From the study, it was also observed that when the computational domain is horizontal without honeycomb cells, the pure conduction in the PCM results in much higher front surface temperature, which is greatly reduced in presence of the honeycomb cells.

Elucidating the dominant mechanisms in burn rate increase of thermite nanolaminates incorporating nanoparticle inclusions

It was experimentally found that silica and gold particles can modify the combustion properties of nanothermites but the exact role of the thermal properties of these additives on the propagating combustion front relative to other potential contributions remains unknown. Gold and silica particles of different sizes and volume loadings were added into aluminum/copper oxide thermites. Their effects on the flame front dynamics were investigated experimentally using microscopic dynamic imaging techniques and theoretically via a reaction model coupling mass and heat diffusion processes. A detailed theoretical analysis of the local temperature and thermal gradients at the vicinity of these two additives shows that highly conductive inclusions do not accelerate the combustion front while poor conductive inclusions result in the distortion of the flame front (corrugation), and therefore produce high thermal gradients (up to 1010 K.m−1) at the inclusion/host material interface. This results in an overall slowing down of the combustion front. These theoretical findings contradict the experimental observations in which a net increase of the flame front velocity was found when Au and SiO2 particles are added into the thermite. This leads to the conclusion that the faster burn rate observed experimentally cannot be fully associated with thermal effects only, but rather on chemical (catalytic) and/or mechanical mechanisms: formation of highly-stressed zones around the inclusion promoting the reactant mixing. One additional experiment in which physical SiO2 particles were replaced by voids (filled with Ar during experiment) to cancel the potential mechanical effects while preserving the thermal inhomogeneity in the thermite structure confirms the hypothesis that instead of pure thermal conduction, it is the mechanical mechanisms that dominate the propagation velocity in our specific Al/CuO multilayered films.

Resistive-nanoindentation on gold: Experiments and modeling of the electrical contact resistance.

This paper reports the experimental, analytical, and numerical study of resistive-nanoindentation tests performed on gold samples (bulk and thin film). First, the relevant contributions to electrical contact resistance are discussed and analytically described. A brief comparison of tests performed on gold and on natively oxidized metals highlights the high reproducibility and the voltage-independence of experiments on gold (thanks to its oxide-free surface). Then, the evolution of contact resistance during nanoindentation is fully explained in terms of electronic transport regimes: starting from tunneling, electronic transport is then driven by ballistic conduction before ending with pure diffusive conduction. The corresponding analytical expressions, as well as their validity domains, are determined and compared with experimental data, showing excellent agreement. From there, focus is made on the diffusive regime. Resistive-nanoindentation outputs are fully described by analytical and finite-element modeling. The developed numerical framework allows a better understanding of the main parameters: it first assesses the technique capabilities (validity domains, sensitivity to tip defect, sensitivity to rheology, effect of an oxide layer, and so on), but it also validates the different assumptions made on current line distribution. Finally, it is shown that a simple calibration procedure allows a well-resolved monitoring of the contact area during resistive-nanoindentation performed on samples with complex rheologies (ductile thin film on an elastic substrate). Comparison to analytical and numerical approaches highlights the strength of resistive-nanoindentation for continuous area monitoring.

Numerical investigation of a dual-PCM heat sink using low melting point alloy and paraffin

For the purpose of reducing the weight and cost of low melting point alloy (LMPA) heat sink, a novel dual-phase change material (PCM) heat sink by using both LMPA and paraffin was proposed in this paper. A numerical model was developed by considering the conduction and natural convection in paraffin and the pure conduction in LMPA. The semi-implicit pressure-linked equation algorithm was adopted to obtain the flow field of liquid paraffin. The effects of volume fraction of LMPA, fin structure and heating power on the performance of dual-PCM heat sink were studied. The performance of dual-PCM heat sink was evaluated by the managed time and melting temperature interval. The results show that, the melting processes of the two PCMs are asynchronous due to the deference of thermophysical properties. The performance of dual-PCM heat sink is comparable with LMPA heat sink during the second melting stage. However, when the third melting stage is dominated by paraffin, the temperature management ability of heat sink deteriorates rapidly during the third melting stage. Moreover, by optimizing the fin structure and the LMPA volume fraction, we can effectively extend the second melting stage and improve the synchronization of the melting processes. With proper fin structure and LMPA volume fraction, the two PCMs can finish melting at the same time, then the third melting stage is eliminated and the melting temperature interval is minimized.

Heat transfer performance of a finned metal foam-phase change material (FMF-PCM) system incorporating triply periodic minimal surfaces (TPMS)

Metal foams have been used extensively to enhance the thermal conductivity of phase change materials (PCMs) in thermal energy storage (TESs) systems and thermal management systems (TMSs). The conventional metal foam structure, referred to commonly as the Kelvin cell, has been characterized well and the effects of its geometric and macroscopic parameters, such as porosity, pore size, and surface area density, on the performance of metal foam–PCM composites have been investigated extensively. With the advent of additive manufacturing technology, any intricate and complex architecture can be manufactured easily, thereby opening doors for the utilization of several other candidate foams and structures in TES systems and TMSs. In this work, three triply periodic minimal surface (TPMS)-based foams (Gyroid, IWP, and Primitive) were used in a finned metal foam–PCM (FMF–PCM) system, and their heat transfer performances were compared with that of the conventional metal foam. Pure conduction and natural convection-based transient phase change simulations were performed under isothermal conditions. The results indicated that all TPMS structures exhibited enhanced heat transfer performance by reducing the melting time of the PCM and increasing the average heat transfer coefficient. Hence, TPMS-based foams offer great promise for use in TES systems and TMSs.

Onset of thermal convection of a weakly rarefied Maxwellian gas: A continuum-slip approach

The onset of the Rayleigh–Bénard convection for a monatomic rarefied gas of Maxwellian type is investigated at small Knudsen numbers. Density variations due to compressibility have been considered without any restriction imposed on the magnitude of temperature difference. The steady-state conduction problem has been solved both analytically and numerically using a continuum approach with velocity-slip and temperature-jump conditions at the solid boundaries. The influence of the Froude number and the Knudsen number is comprehensively studied. To examine the onset of thermal convection, a linear temporal stability analysis is performed for the conduction state and the dispersion relation is calculated by a Chebyshev collocation method. The neutral stability curve obtained in the Froude–Knudsen number plane marks transition to convection from a pure conduction state. The critical wave number for the onset of convection is found to be close to 3.14, agreeing well with the existing findings. The convection only occurs for Knudsen numbers smaller than 0.027. A comparison between the Maxwellian model and the hard-sphere model is given and discussed. The encouraging results suggest the use of more realistic gas models in future studies.

Thermal performance analysis of a thermal enhanced form-stable composite phase change material with aluminum nitride

Phase change material is a competent candidate for thermal energy storage technology application of building envelope thermal regulation. But the low thermal conductivity of organic phase change material hinders its building thermal management utilization from fully functioning. In this study, a kind of thermal enhanced form-stable composite phase change material is prepared, aiming at enhancing the pure phase change material thermal conductivity by employing aluminum nitride as the matrix material. A series of tests are carried out attempting to figure out the thermal characteristics of the newly made compound. And an experiment is conducted, trying to find out its thermal performance in practice. The results turn out that the proposed composite possesses a thermal conductivity of 3.154 W∙(m·K)−1, 2903% improved than that of pure phase change material. Besides, the obtained experimental middle layer temperature of the newly made composite board sample is 27.93 °C. Compared with the phase change point of the pure material, the use ratio of phase change material in the board can be calculated as 100%, meaning that the newly made composite functioned well with little waste. Thus, these performances indicate a bright future for the aluminum nitride based thermal enhanced form-stable composite phase change material.

Determination of Oxide Ion Conductivity in Ba-Doped LaYbO3 Proton-Conducting Perovskites via an Oxygen Isotope Exchange Method

Proton-conducting oxides have garnered much attention as electrolyte materials of electrochemical devices operating at intermediate temperatures ranging 400–700 °C. For the implementation, quantifying the transport number of oxygen ions is crucial to predict the fuel utilization efficiency of solid oxide fuel cells (SOFCs). The acceptor-doped LaYbO3 proton conductors are one of the candidates for the electrolyte material of electrochemical devices such as SOFCs because of their high pure proton conduction. However, the oxide ion conductivity in this material is not quantified yet. Here, we evaluate the tracer diffusion coefficient (D*) of 5 mol % Ba-doped LaYbO3 by the oxygen isotope exchange and depth profile technique using secondary ion mass spectrometry. It is found that the oxygen ion conductivities in this material are lower than 10–4 S cm–1 below 800 °C, less than 2% to the total conductivity. We also visualize the domain of dominant conduction in this material. It reveals that proton conduction is dominant in fairly wide areas of the atmospheric condition, in particular below 600 °C, suggesting that the Ba-doped LaYbO3 is suitable for the SOFC electrolyte in terms of transport number.