External Heat Flux Research Articles

Coupled thermo-electric-elastic piezoelectric vibration energy harvester with axial movement: Modeling, verification, and analysis

Abstract In the field of rail transport and aerospace field, vibration energy harvesting is inevitably subjected to coupled excitations, including train wheel-track interaction induced friction heat and forced vibration, periodic thermal radiation, and vibration excitation. This paper investigates a coupled thermo-electric-elastic piezoelectric vibration energy harvester with axial movement under external heat flux and mechanical force load. The coupled forced vibration equation, coupled electric equation, and coupled thermoelastic heat conduction equation are derived and solved by Green's function theory. To analyze the effect of excitations on the response characteristics, the decoupled method is utilized to solve the coupled multi-field equations and obtain the displacement, electric, and temperature distribution closed-form solutions. The displacement coupling effect induced temperature distribution and the thermo-electric coupling effect triggered displacement are respectively decoupled and analyzed. The obtained closed-form temperature distribution and displacement solutions are verified by the finite element method. To further verify the obtained solutions, a numerical method is conducted by decoupling the coupled multi-field equations and comparing them with prior solutions. Additionally, the different height-to-length ratios, axially moving speeds, and external force load are analyzed in detail. The results indicate that the displacement, temperature distribution, and output voltage vary with external conditions due to the coupled multi-field effect. Overall, this work investigates the thermo-electric-elastic coupling effect on the axially moving piezoelectric energy harvesting, which is beneficial to promote theoretical investigations of the coupled multi-field energy harvesting system and accelerate the practical applications in the aerospace field.

Design and Verification of Thermal Control System of Communication Satellite

The multiple working modes, complex working conditions, frequent changes in external heat flux, and high power consumption of communication satellites all pose great difficulties to their thermal design. This paper mainly describes the design of a thermal control system for high-power communication satellites. Firstly, new efficient heat transfer technologies and thermal control materials for spacecraft are introduced. Secondly, the structure and internal heat source of the satellite are introduced. Thirdly, the external heat fluxes are analyzed, and the position of the heat dissipation surface and extreme conditions are confirmed. Then, a thermal control system is designed around the difficulties of thermal control. With heat pipes, the temperature uniformity of +Y deck, −Y deck, and +Z deck increased by 8 °C, 9.9 °C, and 34.2 °C, respectively. Furthermore, the maximum temperature of the power controller, secondary power supply, bidirectional frequency converter, and solid discharge decreased by 32.5 °C, 22.0 °C, 14.0 °C, and 164 °C, respectively. Finally, a thermal balance test is performed. The test results show that the temperatures of the solid-state power amplifier, on-board computer, power controller, secondary power supply, and bidirectional frequency converter meet the requirements of the thermal control indices. In addition, the temperature of thermal-sensitive components such as batteries and the storage tank also meets the requirements. The thermal design scheme is reasonable and feasible, and the thermal balance test verifies the correctness of the thermal design.

Molecular dynamics method to investigate the interaction energy and mechanical properties of the reinforced graphene aerogel with paraffin as the phase change material in the presence of different external heat fluxes

BackgroundGraphene aerogels (GA), known for their exceptional lightweight and sturdy characteristics, present a promising avenue for improving thermal energy (TE) storage and transfer efficiency. It might be possible to make better thermal management systems in fields like electronics, aerospace, and energy storage by studying how heat flux (HF) affects the strength and stability of graphene aerogels. MethodsThe study used molecular dynamics (MD) simulation to investigate how the mechanical properties of graphene aerogels strengthened with paraffin as phase change material (PCM) change in response to external heat flux (EHF). These simulation methods provided a detailed view of molecular interactions and dynamics at the atomic level, allowing researchers to understand the behavior of materials under various conditions. The change in toughness, interaction energy (IE), Young's modules (YM), and ultimate strength (US) was examined for this reason. Significant findingsThe results indicate that when the HF increased from 0.1 to 0.3 W/m2, the ultimate strength and Young's modules increased from 8.91 and 5.37 GPa to 14.546 and 8.59 GPa, respectively. These values declined when HF increased by more than 0.3 W/m2. When EHF went up to 0.3 W/m², these graphene aerogel properties went up. This was because the atoms moved around more and there were more bonding contacts among the graphene sheets, which made the structure of material stronger. However, at heat flux levels exceeding 0.3 W/m², excessive thermal energy may lead to thermal degradation, causing bond breakage and loss of structural integrity, ultimately resulting in a decrease in these mechanical properties. Also, the results reveal that interaction energy increased from -1522.098 to -1546.325 eV as external HF increased to 0.3 W/m2. The thermal motion of atoms enhanced as the HF increased, enabling closer clustering and better alignment of graphene sheets, thereby strengthening their interactions. This study gave us useful information about how to improve the mechanical properties of graphene aerogels in different HF conditions. This made it more likely that these materials can be used in energy storage systems and thermal management.

The effect of amplitude of heat flux on the adsorption of doxorubicin by MOF11 bio-carrier using molecular dynamics simulation

A common chemotherapy drug, doxorubicin's effectiveness is restricted by its quick excretion from the body and poor solubility. Because of their large surface area and adjustable pore size, bio MOF11 carriers demonstrated promise as drug delivery systems. Examining how external heat flux amplitude (EHFA) affects bio MOF11's ability to adsorb doxorubicin can reveal ways to improve drug loading and release, which will improve drug delivery. Moreover, by shortening the time needed for adsorption (Ads) and desorption, using EHFA in drug Ads processes can increase energy efficiency. Through comprehending the effect of EHFA on the Ads procedure, researchers can ascertain the ideal circumstances for optimizing drug loading while reducing energy usage. The current work examined the effect of EHFA amplitude on doxorubicin Ads via a bio MOF11 carrier using molecular dynamics (MD) modeling. According to MD data, EHFA was expected to have a significant effect on the atomistic evolution of the proposed drug-MOF11 system. The system's interaction energy (IE) and diffusion coefficient rose from −937.27 kcal/mol and 61.40 nm2/ns to −984.08 kcal/mol and 75.16 nm2/ns when EHFA changed from 0.01 to 0.05 W/m2. Increasing EHFA to 0.05 W/m2 resulted in a mean square displacement (MSD) parameter of 69.16 Å2. Therefore, based on the numerical results from this study, it can be said that the doxorubicin drug-MOF11 system changed and atomically evolved when the applied EHFA changes in magnitude.

Small scale pool fires: The case of toluene

Industrial applications adopting toluene as a solvent have been largely extended in recent years, including solutions within the framework of the energy transition and energy storage technologies. The potential use of this flammable compound in a different set of operative conditions and compositions requires a comprehensive and complete knowledge of its fire behaviour and combustion kinetic. To this scope, an innovative experimental procedure applicable to liquid reactive systems was developed for this scope and implemented at different boundary conditions. More specifically, the specimen was exposed to air and heat fluxes between 7 and 50 kW/m2, at a constant sample surface of 0.01 m2, an initial sample thickness of 0.01 m, and a distance between the sample and the horizontally oriented conical-shaped heater of 0.025 m. Measurements were compared with data from the current literature, when available, demonstrating the robustness and validity of the adopted procedure. Although an increase in the external flux leads to growing mass burning rates (i.e., from 0.47 g/s to 0.85 g/s), negligible effects on the ignitability were observed. Conversely, a peak in the heat release rate at 35 kW/m2 was measured. The observed reduction at higher external heat fluxes was attributed to less effective combustion, demonstrating that the maximum expected heat flux cannot be considered as an aprioristic worst-case scenario for the evaluation of pool fires. The collected data were, then, further utilized to obtain insights on the formation of the main products, including soot tendency. Based on the collected data a simplified kinetic model suitable for the computational fluid dynamics was proposed to reproduce the chemistry of the system.

Thermal response function method: A method for predicting the transient surface temperature of black-box objects

In engineering practice, thermal analysis of objects with unknown internal structure and/or thermophysical properties, and uncertainties in contact thermal resistances, is very challenging and even impossible using the traditional approach of direct solving the heat transfer equation. In this work, a thermal response function method (TRFM) is proposed for predicting the transient surface temperature of ‘black-box’ objects (i.e., unknown internal structure and thermophysical properties). The method relies on an introduced measurable quantity called thermal response function, which characterizes the thermal response characteristics of an object. Using the measured thermal response functions as input parameters, the transient temperature distribution on the surface of a black-box object under arbitrary external heat flux boundary condition can be predicted through linear superposition. Proof-of-concept simulations and experiments are conducted to demonstrate the feasibility and effectiveness of the TRFM method. The predicted surface temperature distribution under various external heat fluxes using TRFM agree well with the reference results. The results show that the TRFM is very promising as a solution of the challenging problem of predicting the transient surface temperature of black-box objects, with potential application for thermal imaging modeling of complex objects.

Assessing thermal hazards and toxicity of raw biomass particles from prevalent agricultural crops in China

Fire and explosion hazards pose significant safety concerns in the processing and storage of biomass particles, warranting the safe utilization of these particles. This study employed scanning electron microscopy, thermogravimetric analysis, and cone calorimetry to investigate the thermal hazards and toxicity of raw biomass particles from four prevalent agricultural crops in China: rice, sorghum, corn, and reed. Among the samples, corn exhibited the highest heat output of 8006.82 J/g throughout the thermal decomposition process. The quantitative evaluation of critical heat flux, heat release rate intensity, fire growth rate index (FIGRA), post-ignition fire acceleration (PIFA) and flashover potential (X) revealed a substantial fire risk inherent to all the examined straw samples. Notably, corn displayed the lowest FIGRA value of 8.30 kW/m2 s, while rice demonstrated the minimum PIFA value of 16.11 kW/m2 s. Moreover, the X values for all four biomass particle types exceeded 10 under varying external heat flux levels, indicating their high propensity for fire hazards. Analysis of CO and CO2 emissions during combustion showed all four biomass samples exhibited high concentrations throughout, from the initial stages to the end. The present study offers crucial insights for formulating comprehensive fire safety guidelines tailored to the storage and processing of biomass particles.

Experimental Study on the Influence of External Heat Flux and Air Pressure on the Combustion Characteristics of Solar Panels

Experimental Study on the Influence of External Heat Flux and Air Pressure on the Combustion Characteristics of Solar Panels

Effect of the wood species on the fire behavior in vertical orientation

The objective of this study was to investigate the parameters controlling auto-ignition, degradation and auto-extinction of wood. For this purpose an extensive set of experiments was conducted varying extrinsic parameters such as external heat-flux but also the type of wood. Varying the wood species allowed to explore the role of thermal properties and wood composition. Seven wood samples were tested, some light and some heavy, both hardwood and softwood. The experimental setup was based on a double-cone calorimeter, which allowed to accurately change the imposed heat flux at a predefined moment. More than 600 tests were carried out in a vertical orientation, allowing a statistical analysis. For each test, mass loss, surface temperature and in-depth temperature of the samples were measured using a precision scale, an infrared camera and thin wire thermocouples embedded using a special machining, respectively. The auto-ignition study showed that the time to auto-ignition increases linearly with density. Despite a wide range of these times to ignition, the surface temperatures at ignition were in the same order of magnitude for all species considered: between 450 and 700 °C for auto-ignition before 2 min and between 700 and 800 °C for auto-ignition after 2 min. The onset of char oxidation was observed at low heat fluxes. It occurs at different times depending on the wood species, but at similar surface temperatures, between 380 and 400 °C. The sliding double heating cone made it possible to identify the criteria for auto-extinction: the heat flux for auto-extinction can vary from 40 to 55 kW.m−2 depending on the wood species, and a linear correlation was found between the mass loss rate at extinction and the initial density of each sample studied. The study highlights the dominant role of density for auto-ignition and auto-extinction.

Experimental observation of crack formation on surface of charring timber

Crack formation on the charring surface of burning wood is an important factor increasing the burning rate by offering a passage for heat and oxygen, but it remains a poorly understood process. This work considers crack formation on pyrolyzing Norway spruce, Scots pine and birch timbers. Timber specimens of different sizes were tested under various radiative heat fluxes in nitrogen atmosphere. The cracking process was followed with an infrared camera mounted above the specimen. The obtained recordings were used to determine the formation times and lengths of cracks and to estimate the validity of an existing thermomechanical model for crack formation. The results show that the crack formation time has no significant dependence on the specimen geometry. Further, the inverse of the square root of crack formation time follows grows linearly with external heat flux, which is a similar dependence as with time for ignition, according to the thermal model of ignition. The analytical model predictions were of correct order of magnitude, but not consistently accurate at all experimental conditions. This could be accounted for the simplifying assumptions within the analytical model, and therefore creating a more detailed three-dimensional numerical model for crack formation is suggested as future research.

Effect of Surfactants on the Synthesis and Dissociation of Gas Hydrates

The synthesis and dissociation of methane hydrate and carbon dioxide hydrate were studied. Nonflammable gas hydrates can be used to extinguish flames in confined spaces. To increase the extinguishing efficiency, it is necessary to increase the dissociation rate (gas release rate) by using surfactant. The work investigates gas hydrates synthesized using sodium dodecyl sulfate (SDS). Experimental studies were carried out in wide ranges of surfactant concentration, the number of the stirrer revolutions and the initial water volume. To achieve the maximum rate of synthesis and dissociation, optimization of the specified parameters was performed. The influence of the key parameters on the dissociation rate was investigated experimentally and theoretically. The novelty of the work lies in solving a complex of interrelated tasks on the synthesis and dissociation of gas hydrate. It is shown that in order to achieve the maximum dissociation rate of carbon dioxide hydrate, it is necessary to optimize the following parameters: the diameter of the particles and their porosity, the porosity of the layer and the external heat flux.

Three-dimensional transient multilayer heat conduction sphere comprising metallic particles

A numerical approach employing finite element methodology is utilized to analyze the thermal behavior of a three-dimensional multilayer sphere composed of copper, single-wall, and multi-wall carbon nanotubes. The study investigates the external heat flux, temperatures within the three spherical layers, and their associated contours. Notably, this research diverges from prior studies by opting for copper, single-wall, and multiwall carbon nanotubes instead of generic metallic layers. Graphs are generated to illustrate the relationships between various effects and profiles. Analysis of temperature contours and distributions under non-uniform heat flux reveals that temperatures are more concentrated towards the center when the initial temperature is non-zero compared to scenarios where the inner surface starts at zero temperature. Temperature contours at t = 2, 5, 7, 9, 11, 13, 15, 27, 35, and 45 s with non-zero initial temperature are calculated. For non-zero initial temperature contours are drawn for t = 2, 45 s temperature contours are drawn. Temperature in radial azimuthal and tangential directions for t = 1, 1.5, 2, 2.5, 3 s and steady state are drawn. The initial temperature of the ith surface is kept at fi(r,θ,ϕ), where ro≤r≤r3,0≤θ≤π,0≤ϕ≤2π. It has been concluded that for zero initial temperatures after 45 s, the temperature is at 0.005K and Temperature contours for non-zero inner layer temperature t = 5 s shows 10.7K.

Geometries, electronic structures, mechanical properties and stabilities of Fe128Hen and Fe127VacHen: An uniaxial compression effect

Owning to the factors of external mechanical loads and high heat flux deposition, various degrees of strain or stress are inevitably introduced into the structural materials of nuclear fusion reactors. The uniaxial compression effects on the geometries, electronic structures, mechanical properties, and stabilities of Fe128Hen and Fe127VacHen were studied by first-principles calculations in this work. The results show that the lattice suffers from a noticeable inclination or disorder as the strain reaches 15%. The maximum stress of the stress–strain curves implied that the mechanical characteristics decay considerably as the number of He atoms increases, but the presence of a vacancy mitigated this effect. Compressive strain decreases the stability of Fe atom in the perfect pure Fe system, but it slightly elevates the monovacancy formation energy from Fe128He3 onward. According to the binding energy of He-He, compressive strain enhanced the aggregation of He. The migration barriers of an interstitial He atom exhibit anisotropy under uniaxial compressive strain, which was explained by the variations of the surrounding architectures.

Synthesis of a novel Si-N-S flame retardant and its application on cotton cellulose biomacromolecule

A novel flame retardant containing Si, N, and S elements, ((2-(triethoxysilyl)ethyl)thio)ethan-1-amine hydrochloride (TETEA), was synthesized via a click reaction and characterized using nuclear magnetic resonance spectroscopy (NMR) and fourier transform infrared spectroscopy (FTIR). Subsequently, the flame-retardant cotton fabric was fabricated by sol-gel method. The results indicated that TETEA was successfully loaded on cotton fabric and formed a uniform protective layer on the surface of cotton fabric, exhibiting excellent flame retardancy. The flame-retardant cotton fabric achieved limiting oxygen index (LOI) of 28.3 % and passed vertical combustion test without after-flame or afterglow time at TETEA concentration of 500 g/L. Thermogravimetric analysis revealed that the residual carbon content of the flame-retardant cotton fabric was much higher than that of the control under air and N2 conditions. Besides, the flame-retardant cotton fabric was not ignited in cone calorimeter test with an external heat flux of 35 kW/m2. The peak heat release rate and the total heat release decreased from 133.4 kW/m2 to 25.8 kW/m2 and from 26.46 MJ/m2 to 17.96 MJ/m2, respectively. This phosphorus-free flame retardant offers a simplified synthesis process without adverse environmental impacts, opening up a new avenue for the development environmentally friendly flame retardants compared to traditional alternatives.

Implementation of Numerical Model for Prediction of Temperature Distribution for Metallic-Coated Firefighter Protective Clothing

The aim of this study is to predict the distribution of temperature at various positions on silver-coated firefighter protective clothing when subjected to external radiant heat flux. This will be helpful in the determination of thermal protective performance. Firefighter clothing consists of three layers, i.e., the outer shell, moisture barrier and thermal liner. The outer shell is the exposed surface, which was coated with silver particles through a physical vapor deposition process called magnetron sputtering. Afterwards, these uncoated and silver-coated samples were exposed to radiant heat transmission equipment at 10 kW/m2 as per the ISO 6942 standard. Silver-coated samples displayed better thermal protective performance as the rate of temperature rise in silver-coated samples slowed. Later, a numerical approach was employed, contemplating the impact of metallic coating on the exterior shell. The finite difference method was utilized for solving partial differential equations and the implicit method was employed to discretize the partial differential equations. The numerical model displayed a good prediction of the distribution of temperature at different nodes with respect to time. The comparison of time vs. temperature graphs at different nodes for uncoated and silver-coated samples acquired from numerical solutions showed similar patterns, as witnessed in the experimental results.

EHFE refinement and extension for variable physical Earth radiation in spacecraft uncertainty thermal analysis

Spacecraft uncertainty thermal analysis is an important part of ensuring robust and reliable spacecraft thermal control system design. The calculation of the Earth external heat flux is an important part of it, however, the current model for calculating the Earth external heat flux is the constant physical Earth radiation model. In this paper, a model of the Earth’s variable physical radiation is developed based on the long-term averaging method. We refine and extend the application of the external heat flux expansion (EHFE) formula to accommodate our proposed model. This study further includes an uncertainty analysis of Earth radiative external heat flux, utilizing the enhanced EHFE formula in the context of a representative spacecraft. We provide statistical data, such as mean values, standard deviation, and probability density functions, derived from our analysis. The augmented EHFE formula, when properly generalized, can be applied to compute the celestial body’s variable physical radiative external heat flux affecting the spacecraft. This methodological advancement offers theoretical underpinning for the thermal design of spacecraft intended for deep space exploration.

Calibration and verification of CFD-VOF models for the analysis of pressurization scenarios in LH2 tanks

Due to its high power density, the aviation sector is pushing towards its electrification by designing new propulsion systems based on cryogenic liquid hydrogen (LH2). From an engineering point of view, the efficient design of tanks constitutes one of the main challenges. They must guarantee efficient isolation of the hydrogen to avoid its evaporation and, at the same time, ensure reasonable weight and dimensions. Using numerical simulations is fundamental for studying heat transfer between the external environment and the liquid fuel. This work focuses on the simulation of multiphase liquid-gas flow to analyze the tank pressurization as a function of different external heat fluxes. A commercial CFD software is used to perform simulations. VOF simulations with the Lee mass transfer model are employed, and some critical numerical parameters are analyzed to provide information on the optimal values.

Numerical modelling of thermal jet assisted rock cutting with double PDC cutters

AbstractPreconditioning of rock for drilling operations is a potential method to facilitate the mechanical breakage and mitigate the tool wear. This paper numerically investigates one such preconditioning technique, namely, the thermal jet assisted rock cutting. For this end, a numerical method for solving the governing thermo‐mechanical problem is developed and validated. The continuum approach is chosen to describe the rock failure, being based on a damage‐viscoplasticity model with the Drucker–Prager yield surface and the modified Rankine surface as the tensile cut‐off. In the damage part, separate scalar damage variables are employed for tension and compression. The cutters are modelled as rigid bodies and their interaction with the rock is modelled by imposing contact constraints by the forward increment Lagrange multiplier method, which is compatible with the chosen explicit time integration scheme. A damage‐based erosion criterion is applied to remove the contact nodes surrounded by heavily damaged elements. The thermal jets are modelled with a moving external heat flux boundary condition. The global thermo‐mechanical problem is solved with a staggered approach explicitly in time while applying mass scaling to increase the critical time step. The novel 3D numerical simulations involving two cutters demonstrate the capabilities of the method with a special emphasis on cutter‐thermal jet configurations. Therefore, the present method provides a potential tool for the bit design in thermal jet assisted drilling.

The pool boiling heat transfer of ammonia/Fe3O4 nano-refrigerant in the presence of external magnetic field and heat flux: A molecular dynamics approach

Pool boiling is distinguished by its capacity to eliminate excessive heat fluxes (HFs) at low temperatures. In recent decades, the optimal design of flooded evaporators elevated the significance of pool boiling HT with refrigerant to conserve natural resources and energy. The industry highly regards this process on account of its superior heat transfer (HT) coefficient in comparison to other HT mechanisms. Among the types of boiling, pool boiling has a special place due to its ability to remove HFs at low temperatures. This study was the first to investigate the boiling characteristics of the ammonia/Fe3O4 nano-refrigerant in a copper (Cu) nanochannel (NC) through molecular dynamics (MD) simulations. The primary goal was to investigate the effect of external HF (EHF) and external magnetic field amplitude (EMFA) on nanostructures' atomic behavior (AB) and thermal behavior (TB). The research findings indicate that increasing the applied EHF led to increased particle movement and the HT rate. By changing the EHF, boiling behavior in the nano-refrigerant may also be seen. Maximum (Max) velocity (Vel.) increased to 8.970 Å/ps when the EHF increases to 0.5 W/m2. Atomic collisions and particle mobility both increase when the EHF increases. Therefore, the maximum temperature value increases to 359.46 K. When the EMFA applied to the nano-refrigerant reaches to 0.5 T, the maximum values of the parameters, such as the Temp. and the velocity, reach to 410.07 K, and 11.802 Å/ps, respectively.

The Influence of the Heat Transfer Mode on the Stability of Foam Extinguishing Agents

The mass loss mechanisms of an aqueous film-forming foam (AF foam), an AR/AFFF water-soluble film-forming foam extinguishing agent (AR foam), and a Class A foam extinguishing agent (A foam) at different levels of thermal radiation, thermal convection, and heat conduction intensity were studied. At a relatively low thermal radiation intensity, the liquid separation rate of the AF, AR, and A foams is related to the properties of the foam itself, such as viscosity and surface/interface tension, which are relatively independent of the external radiation heat flux of the foam. At low radiation intensity (15 kW/m2 and 25 kW/m2), the liquid separation rate of the AF and A foams is relatively stable. When the heat flux intensity is 35 kW/m2, the liquid separation rate of the AF and A foams increases notably, which may be mainly due to the rapid decrease in foam viscosity. And the mass loss behavior is dominated by liquid separation in the AF, AR, and A foams under the influence of thermal radiation and thermal convection. Under the same experimental conditions, the liquid separation rate of AF is the fastest. There is no significant difference in the evaporation rates of the three kinds of foam in the same heat conduction condition. In addition, the AR and A foams usually have a 25% longer liquid separation time (t) under thermal radiation and thermal convection, and the thermal stability is better than AF foam. The temperature reached by the AF foam layer under thermal convection was lower than that of the AR and A foams, and the time for the foam layer to reach the highest temperature under heat conduction was longer than that of the AR and A foams.