Thermal Accommodation Research Articles

Investigation into the thermal effect of the LIPS-200 ion thruster plume

The distribution of the thermal effects of the ion thruster plume are essential for estimating the influence of the thruster plume, improving the layout of the spacecraft, and for the thermal shielding of critical sensitive components. In order to obtain the heat flow distribution in the plume of the LIPS-200 xenon ion thruster, an experimental study of the thermal effects of the plume has been conducted in this work, with a total heat flow sensor and a radiant heat flow sensor over an axial distance of 0.5–0.9 m and a thruster angle of 0°–60°. Combined with a Faraday probe and a retarding potential analyzer, the thermal accommodation coefficient of the sensor surface in the plume is available. The results of the experiment show that the xenon ion thruster plume heat flow is mainly concentrated within a range of 15°. The total and radial heat flow of the plume downstream of the thruster gradually decreases along the axial and radial directions, with the corresponding values of 11.78 kW m−2 and 0.3 kW m−2 for the axial 0.5 m position, respectively. At the same position, the radiation heat flow accounts for a very small part of the total heat flow, approximately 3%–5%. The thermal accommodation factor is 0.72–0.99 over the measured region. Furthermore, the PIC and DSMC methods based on the Maxwell thermal accommodation coefficient model (EX-PWS) show a maximum error of 28.6% between simulation and experiment for LIPS-200 ion thruster plume heat flow, which, on the one hand, provides an experimental basis for studying the interaction between the ion thruster and the spacecraft, and on the other hand provides optimization of the ion thruster plume simulation model.

A robust trivariate approach for characterizing the primary particle size distribution of soot aggregates using time-resolved laser-induced incandescence

This study proposes a new trivariate retrieval method based on a moderate-fluence laser-induced incandescence (LII) model. By changing the undesirable mathematical properties present in our previous work, this upgraded new method is able to obtain robust estimates of the two primary particle size distribution parameters and one assembled parameter, and still maintains the advantage of not requiring prior knowledge of thermal accommodation coefficient and aggregate structure. The robustness of the upgraded method is verified on a large number of numerical results, as evidenced by significantly improved retrieval accuracy and substantially reduced estimation uncertainty. The generality of the upgrade method is demonstrated over a large range of three variables. Furthermore, the influence of several key factors (the number of shots used for averaging noisy LII signal, the intensity ratio of the measurement termination, and the sublimation sub-model error) on the retrieval results of the upgraded method is investigated numerically.

Methodical selection of thermal conductivity models for porous silica-based media with variation of gas type and pressure

If the effective thermal conductivity of a silica powder in any gas atmosphere is to be calculated analytically, one is faced with a whole series of decisions. There are a lot of different models for the gas thermal conductivity in the pores, the thermal accommodation coefficient or the effective thermal conductivity itself in the literature. Furthermore, it has to be decided which input parameters should be used. This paper gives an overview and recommendations as to which calculation methods are best suited for the material classes of precipitated silica, fumed silica, silica gel and glass spheres. All combinations of the described methods result in a total of 2800 calculation models which are compared with pressure-dependent thermal conductivity measurements of 15 powdery materials with 7 different gases using Matlab computations. The results show that with a model based on a spherical unit cell, which considers local Knudsen numbers, the measuring points of all powder-gas combinations can be determined best with an average variance of about 18.5%. If the material class is known beforehand, the result can be predicted with an average accuracy of about 10% with the correspondingly determined methods.

Thermal Conductivity Decomposition and Accommodation Coefficient Extraction for Thermal Insulation Materials

Thermal Conductivity Decomposition and Accommodation Coefficient Extraction for Thermal Insulation Materials

Measurement of soot primary particle size in co-flow laminar diffusion flames of n-heptane at elevated pressures

To comprehensively understand soot formation in flames of liquid fuel, the soot primary particle size (PPS) in n-heptane\\air diffusion flame at pressures up to 5 bar was investigated in this work. A series of experiments were conducted in a high-pressure laminar flame burner system, and the spatial distributions of soot primary particle size in the flames at atmospheric and elevated pressures were evaluated by two-dimensional Time-Resolved Laser-Induced Incandescence (TiRe-LII). The results indicated a tendency that the conunt mean diameter (Dp,mean) of soot primary particle increased with enhancing laser fluence, which might be attributed to inaccuracy of the thermal accommodation coefficient at the increasing laser fluences. The spatial distribution of Dp,mean varied with pressures. Further, the evolution of Dp,mean along the centerline and the radial distribution of Dp,mean at the pressures of 1.0–3.0 bar were also presented. The zone of the maximum Dp,mean moved from the flame centerline to the flame edge as the pressure increases. The spatial distribution of Dp,mean at 3.5–5.0 bar showed the area of maximum Dp,mean located at the annular wing region in flames and being floated as the pressure increased. This study complemented the existing theoretical knowledge of spatial primary particle size distribution (PPSD) in n-heptane laminar diffusion flames.

Use of geothermal resources in Konya Plain Project (KOP) Region for tourism, health purposes, agricultural, industrial and distict heating

This study has been prepared in accordance with the objectives of evaluating and developing geothermal resources in order to bring the geothermal resources in the Konya Plain Project (KOP) Region, which includes Aksaray, Kırşehir, Konya, Nevşehir, Niğde and Yozgat provinces, to the country?s economy with maximum capacity, and to develop proposals for investments to be made. The number of overnight stays and the number of tourists outside the region should be increased by utilizing alternative tourism opportunities in the region. Their use in terms of geothermal energy, tourism and health has an important investment and income potential for the region. It has been determined by the laboratory results that the contents of thermal waters have inductions against many diseases. New investment projects have been recommended with the strategy of highlighting different practices in different provinces within the KOP Region. In this respect, each province and region within each province were evaluated separately, and suggestions suitable for their strategic positioning were presented. In addition, rather than offering new thermal accommodation facilities, modernization of existing facilities or businesses and infrastructures that offer services that all facilities can benefit from are proposed. However, it was seen that there was a need for accommodation capacity and new business models in some regions and suggestions were made accordingly.

Determination of thermal accommodation coefficients on [formula omitted] and [formula omitted] using molecular dynamics and experiments

The thermal accommodation coefficient α has been assumed, although lacking any experimental proof, to be near unity for most gases so far, which denotes no influence. However, it plays a contributing role in the field of the effective thermal conductivity of highly porous insulation materials based on SiO2 or CaSiO3 as it is shown in this work. Besides, this work investigates a possible influence on α for Ar,N2,He within parameters like temperature, roughness and contamination as this has not been examined on such materials so far. More importantly, it answers the question whether the assumption of α = 1 is valid. By using a parallel plates device, very similar to the guarded-hot-plate, following EN 12667 it was possible to determine α on a dense CaSiO3. It occured that the assumptions α = 1 (for Ar,N2) and α = 0.3 (for He) are valid for measurements near room temperature. Further, physical adsorption was found to increase α. The determination of the influence of roughness has been started showing an interesting effect, but it still remains an open topic. In a collaborative study molecular dynamics (MD) simulations were performed showing a strong equivalence of α between SiO2 and CaSiO3. These results can be considered a lower limit of α as neither roughness nor adsorption processes have been included in the simulation. Therefore, any deviations between experiments and MD could be considered as an apperance of physical adsorption.

Experimental measurements of thermal and tangential momentum accommodation coefficients on solid surfaces: Water vapor in comparison with noble gases

In the high Knudsen number flows, like micro gaseous flows and flows in vacuum, the gas-surface interaction plays an important role. Among many gas species, water is a common and useful fluid in wide area of engineering applications, and it is one of the main residual gas components in the high vacuum environment. However, experimental measurements on the gas-surface interaction of water molecules are still limited. In this study, we focus on the accommodation coefficient, which represents the mean behavior of gas molecules on the scattering process from a surface. We measure the thermal accommodation coefficient (TAC) and the tangential momentum accommodation coefficient (TMAC) of water molecules. TAC is measured from the heat flux between two surfaces with different temperatures in vacuum. TMAC is derived through the viscous slip coefficient from the measurement of the mass flow rate through a microtube. To investigate the characteristics of water molecules clearly, the measured accommodation coefficients are compared with those of noble gases, helium, neon, and argon, measured with the same apparatus and procedures. For the accommodation coefficients of noble gases, TAC increases with increasing the molecular weight, while TMAC decreases, which are in good agreement with the previous studies. The TAC and TMAC of water are slightly larger than those estimated from the corresponding molecular weight of noble gases.

Numerical study of supersonic boundary-layer modal stability for a slightly rarefied gas using Navier—Stokes approach

In this paper, a systematic study on the supersonic boundary-layer modal stability for a slightly rarefied gas is conducted by considering velocity slip and temperature jump effects in the Navier–Stokes (NS) equations. The effects of slip boundary on the first- and second-mode instability at different conditions are presented in detail. The laminar flow is obtained by solving the NS equations along with no-slip and slip boundary conditions, which shows that the slip boundary causes the boundary layer becoming thinner and the supersonic region near the wall becoming narrower. The perturbation slip boundary conditions at the wall and their influence on the stability are carefully discussed. The tangential momentum accommodation coefficient and the thermal accommodation coefficient are set equal or unequal for a broad range to study the combined or leading effects of velocity slip and temperature jump, respectively. It is found that velocity slip significantly stabilizes the second-mode disturbances while largely destabilizes the first-mode perturbations. On the contrary, the temperature jump apparently enhances the second-mode instability, while it has little influence on the first mode. When velocity slip and temperature jump are both present, the first mode is more destabilized, while a competitive effect acts on the second mode. Additional results show that the neutral stability curves for the second and third modes as well as the synchronization between fast and slow modes are delayed further downstream due to velocity slip. These findings are shown consistently regardless of the wall cooling for both supersonic and hypersonic flows.

Thermal Conductivity Analysis of High Porosity Structures with Open and Closed Pores

Developing a robust, cost-effective, super-thermal insulation material with an ultralow thermal conductivity, i.e., below that of stationary air (0.026 W/(mK), 300 K, 1.0 atm), is challenging. A significant number of building blocks (e.g., polymers, oxides, ceramics, and carbon materials) have been explored to fabricate super-thermal insulators with various pore structures (i.e., interconnected “open” pores/channels or sealed “closed” pores, separated by matrix material) over the past decades. Understanding heat transfer in highly porous structures is essential for both geometric design and practical thermal insulation applications. Herein, we adopted two analytical models to analyze the influence of both geometric (e.g., pore size, pore type and porosity) and thermophysical parameters (e.g., solid thermal conductivity, thermal accommodation coefficient, and pressure) on the effective thermal conductivity of porous structures with open and closed pores. Our results indicate that, as porosity increases, the effective thermal conductivity of porous structures, regardless of pore type (open or closed), approaches the thermal conductivity of the gas inside the pores. Effective techniques to diminish the effective thermal conductivity of a porous structure to less than that of stationary air include decreasing gaseous thermal conductivity by implementing nm-scale pores in the materials or by reducing the pressure inside existing pores. Additionally, the thermal conductivity of porous structures with µm-scale pores may also be lower than that of stationary air, if the thermal conductivity of the solid matrix is reduced to much lower than that of the air inside the pores. Decreasing the thermal accommodation coefficient helps to reduce the effective thermal conductivity of porous structure if the nm-scale pores are included. This work is expected to prove helpful in understanding heat transfer in high porosity structures, analyzing experimental measurements, and guiding the design and fabrication of super-thermal insulation materials.

At what ambient temperature can thermal runaway of a burning metal particle occur?

A simplified heat and mass transfer model is considered to predict at what ambient temperature can thermal runaway occur for a burning metal particle exposed to an oxidizing gas. The model accounts for transition transport phenomena important for particles with sizes comparable to the mean free path of the gas molecules. The quasi-steady model can be reduced to a set of algebraic equations that can be readily solved numerically if both the reaction kinetics law and the value of thermal accommodation coefficient (TAC) are assumed. Such assumptions are made and the solutions are produced and discussed for particles of different sizes for aluminum, magnesium, and boron burning in air. Different reaction kinetics expressions reported in the literature and different values for the TAC are considered for all three fuels. For aluminum, different reaction kinetics lead to a widely different range of air temperatures, at which the particle thermal runaway is predicted. However, the runaway is expected for particles of all sizes, including nano-particles if the air temperature is sufficiently high. Similarly, thermal runaway is also predicted to occur for boron particles of different sizes, although respective air temperatures must be substantially higher than for aluminum. The effect of TAC on the predicted result is strong; smaller values of TAC lead to a reduced effect of particle size on the gas temperature required for the thermal runaway to occur. It is found that neglecting transition heat and mass transport regimes can be always justified for very large, 200 µm particles. For smaller particles, both higher reaction rates and smaller values of TAC may require considering transition transport when modeling behavior of burning particles.

A molecular dynamics study of collisional heat transfer to nanoclusters in the gas phase

Nanoclusters in the gas phase grow by condensation and coagulation, which heat up nanoclusters, creating thermal non-equilibrium with the surrounding gas. The extent of non-equilibrium for nanoclusters is dependent upon the rate of heating relative to the rate of cooling brought about by nanocluster-gas molecule collisions, hence the latter process plays an important role in nanocluster energy evolution during growth. We apply molecular dynamics simulations to investigate heat transfer between metal nanoclusters and gas molecules relevant to high temperature aerosol systems. In analysis, we first define a thermal reemission coefficient (TRC), which is a correction factor for the portion of energy possessed by reemitted gas molecules after collision. The TRC differs from the thermal accommodation coefficient (TAC), which is a correction factor for the maximum possible energy transfer rate from nanoclusters to gas molecules based on the diffusive collision model. We find the TRC is insensitive to nanocluster size but is affected by the nanocluster to gas temperature ratio and the nanocluster to gas atomic mass ratio. Comparison to the traditionally-defined TAC shows that the TAC is insensitive to temperature, a phenomenon that has been widely indicated in literature but not quantitatively explained. We show how the temperature insensitivity of the TAC arises because of the manner in which TRCs vary with both nanocluster and gas temperature, i.e. the manner in which the reemitted gas molecule energy from a surface varies with the temperature of that surface relative to that of the gas. In addition, while calculations ultimately reveal temperature insensitive TACs and justify their continued use in modeling gas-phase heat transfer, we argue that the TRC is more appropriate to calculate a priori in analyzing simulations; it is physically more realistic to first introduce a correction factor solely for the energy of reemitted gas molecules after a collision (as TRCs) instead of a correction factor applying to both the reemission energy and the initial energy (as TACs), as the latter is exactly calculable and independent of nanocluster properties. In line with prior work, for monoatomic gases we find the TAC decreases as the nanocluster to gas atomic mass increases, approaching 0.08 for gold nanoclusters in helium.

Modeling laser-induced incandescence of Diesel soot\u2014Implementation of an advanced parameterization procedure applied to a refined LII model accounting for shielding effect and multiple scattering within aggregates for {varvec{alpha}}_{{varvec{T}}} and {varvec{E}}left( {varvec{m}} right) assessment

An extensive set of LII signals measured in a Diesel spray flame has been simulated using a refined LII model built upon a comprehensive version of soot heat- and mass-balance equations. This latter includes terms standing for saturation of linear, single- and multi-photon absorption processes, cooling by sublimation, conduction, radiation and thermionic emission in addition to mechanisms depicting soot oxidation and annealing, non-thermal photodesorption of carbon clusters as well as corrective factors allowing considering shielding effect and multiple scattering (MS) within aggregates. A complete parameterization of the so-proposed model has been achieved by means of an advanced optimization procedure coupling design of experiments with a genetic algorithm-based solver. Doing so, the values of different factors involved in absorption and sublimation terms have been assessed for a 1064-nm laser excitation wavelength including the multi-photon absorption cross section for C2 photodesorption and the saturation coefficients for linear- and multi-photon absorption, among others. This parameterized model has then been demonstrated to effectively reproduce signals measured in different combustion media including a CH4/O2/N2 premixed flat flame and a diffusion ethylene flame. As a result of the data derived from the analysis of the Diesel flame, a thermal accommodation coefficient value of 0.49 has been assessed against 0.34 when neglecting the shielding effect. In addition, values of the soot absorption function (Eleft( m right)) comprised between 0.18 and 0.31 have been inferred depending on the particle maturation stage. On the other hand, Eleft( m right) 24% higher on average have been estimated when neglecting MS thus illustrating the importance of aggregate characteristics on soot properties derived through LII modeling. Eventually, the Eleft( m right) evolution observed herein has been compared with results issued from studies conducted with varied hydrocarbons which led to highlight the crucial role played by the soot maturity level over the nature of the burnt fuel as far as optical properties are concerned.

Time-Resolved Laser-Induced Incandescence Measurements on Aerosolized Nickel Nanoparticles.

This work presents the first quantitative analysis of time-resolved laser-induced incandescence (TiRe-LII) measurements on aerosolized nickel nanoparticles in several gases and over a range of laser fluences. A measurement model composed of spectroscopic and heat transfer submodels is used to recover the particle size distribution parameters and the thermal accommodation coefficient (TAC). A qualitative analysis of the results reveals evidence of nonincandescent laser-induced emission temporally aligned with the laser pulse, and more laser energy is absorbed than can be accounted for from the modeled spectral absorption cross section of the nanoparticles. The TiRe-LII inferred particle size parameters were generally consistent with values found from ex situ transmission electron microscopy (TEM) analysis. The TACs for nickel nanoparticles in polyatomic gases were larger than those in monoatomic gases, which may indicate chemisorption.

Transport behavior of pressure-driven gas flow in a nanochannel and the variation of thermal accommodation coefficient

A three-dimensional molecular dynamics simulation of a rarefied gas flow confined between two parallel solid walls has been carried out to study the gas transport behavior in nanoscale pressure-driven flows. To simulate the effects of pressure-driven flows in nanoscale systems; a reservoir is created at the beginning of the channel. A force, in the form of gravity, is applied to every gas molecule occupying a specific region in the reservoir thereby facilitating to establish a streamwise inhomogeneous flow in a finite length nanochannel. Further, the model is tested for its structural stability and steady-state characteristics. The flow characteristics are evaluated at various bins (A, B, C, and D) along the length of the channel. The presence of a pressure drop established along the length of the channel results in a significant variation of flow properties in the channel. The characteristics of the system are further studied using the spatial distribution of flow properties. To investigate the streamwise variation, thermal accommodation coefficients (TACs) at various sections of the channel are calculated. The results indicate that the TACs vary along the length. Further, the effect of pressure drop on TACs is also analysed. The effect of confinement is also studied with velocity autocorrelation function (VACF) and mean squared displacement (MSD). The effect of confinement is evident in the fluid region away from the wall too.

Application of Additively Manufactured Pentamode Metamaterials in Sodium/Inconel 718 Heat Pipes.

In this study, pentamode metamaterials were proposed for thermal stress accommodation of alkali metal heat pipes. Sodium/Inconel 718 heat pipes with and without pentamode metamaterial reinforcement were designed and fabricated. Then, these heat pipes were characterized by startup tests and thermal response simulations. It was found that pentamode metamaterial reinforcement did not affect the startup properties of sodium/Inconel 718 heat pipes. At 650–950 °C heating, there was a successful startup of heat pipes with and without pentamode metamaterial reinforcement, displaying uniform temperature distributions. A further simulation indicated that pentamode metamaterials could accommodate thermal stresses in sodium/Inconel 718 heat pipes. With pentamode metamaterial reinforcement, stresses in the heat pipes decreased from 12.9–62.1 to 10.2–52.4 MPa. As a result, sodium/Inconel 718 heat pipes could be used more confidently. This work was instructive for the engineering application of alkali metal heat pipes.

Analysis of Temperature-Jump Boundary Conditions on Heat Transfer for Heterogeneous Microfluidic Immunosensors.

The objective of the current study is to analyze numerically the effect of the temperature-jump boundary condition on heterogeneous microfluidic immunosensors under electrothermal force. A three-dimensional simulation using the finite element method on the binding reaction kinetics of C-reactive protein (CRP) was performed. The kinetic reaction rate was calculated with coupled Laplace, Navier−Stokes, energy, and mass diffusion equations. Two types of reaction surfaces were studied: one in the form of a disc surrounded by two electrodes and the other in the form of a circular ring, one electrode is located inside the ring and the other outside. The numerical results reveal that the performance of a microfluidic biosensor is enhanced by using the second design of the sensing area (circular ring) coupled with the electrothermal force. The improvement factor under the applied ac field 15 Vrms was about 1.2 for the first geometry and 3.6 for the second geometry. Furthermore, the effect of temperature jump on heat transfer rise and response time was studied. The effect of two crucial parameters, viz. Knudsen number (Kn) and thermal accommodation coefficient (σT) with and without electrothermal effect, were analyzed for the two configurations.

A proof-of-concept study on characterization of primary particle size distribution and thermal accommodation coefficient of soot aggregate by using conduction-dominated laser-induced incandescence

Laser-induced incandescence (LII) is a powerful tool for in-situ measurement of soot aggregate properties, including the primary particle size distribution (PPSD). When integrating the shielding factor in the LII model to quantify the aggregate shielding effect, thermal accommodation coefficient (TAC) and shielding factor appear as a product. It is impossible to determine them independently without their accurate prior values. To overcome this problem, this study defines a new parameter fractal-dependent TAC (αT, f) which combines the overall impact of both TAC and shielding factor on the all detected aggregates. Given the ill-posed inverse problem is sensitive to the uncertainty from model parameters, inferring αT, f simultaneously with μdp and σdp (two characteristics parameters of PPSD) from conduction-dominated LII has the remarkable advantage of being independent of most LII model parameters. To evaluate the feasibility of the inversion from the respective of model parameter uncertainty, three relatively simple two-dimensional (2D) invers problems are investigated: (1) inferring μdp and σdp when prior αT, f is biased; (2) inferring μdp and αT, f when prior σdp is biased; (3) inferring σdp and αT, f when prior μdp is biased. The results and analysis presented in this study provide a theoretical basis for inferring μdp, σdp, and αT, f from conduction-dominated LII.

LRO/LAMP observations of the lunar helium exosphere: constraints on thermal accommodation and outgassing rate

ABSTRACT We report a comprehensive study by the UV spectrograph LAMP (Lyman-Alpha Mapping Project) onboard the Lunar Reconnaissance Orbiter to map the spatial distribution and temporal evolution of helium atoms in the lunar exosphere, via spectroscopy of the He i emission line at 58.4 nm. Comparisons with several Monte Carlo models show that lunar exospheric helium is fully thermalized with the surface (accommodation coefficient of 1.0). LAMP-derived helium source rates are compared to the flux of solar wind alpha particles measured in situ by the ARTEMIS twin spacecraft. Our observations confirm that these alpha particles (He++) are the main source of lunar exospheric helium, representing 79 per cent of the total source rate, with the remaining 21 per cent presumed to be outgassing from the lunar interior. The endogenic source rate we derive, (1.49 ± 0.08) × 106 cm−2 s−1, is consistent with previous measurements but is now better constrained. LAMP-constrained exospheric surface densities present a dawn/dusk ratio of ∼1.8, within the value measured by the Apollo 17 surface mass spectrometer LACE (Lunar Atmosphere Composition Experiment). Finally, observations of lunar helium during three Earth’s magnetotail crossings, when the Moon is shielded from the solar wind, confirm previous observations of an exponential decay of helium with a time constant of 4.5 d

Simultaneous determination of primary particle size distribution and thermal accommodation coefficient of soot aggregates using low-fluence LII.

For the ill-posed inverse problem of LII-based nanoparticle size measurement, recovered primary particle size distribution (PPSD) is sensitive to the uncertainty of LII model parameters. In the absence of reliable prior knowledge, the thermal accommodation coefficient (TAC) and fractal-dependent shielding factor are often required to be inferred simultaneously with the PPSD. In the simplified LII model for low fluence regime, TAC and fractal-dependent shielding factor are combined to define a new fractal-dependent TAC. The present study theoretically verified the feasibility of inferring PPSD and fractal-dependent TAC from the normalized LII signals. Moreover, the inversion is independent of prior knowledge of most full LII model parameters, which is attributed to low laser fluence, normalized signal, and fractal-dependent TAC.