查看更多>>摘要：This work uses an asymptotic expansion technique to derive analytical solutions for the acoustic properties of one-dimensional combustors in general, and Rijke tubes in particular, assuming different endpoint conditions and thermal profiles. By leveraging Green's functions and integral formulations, the analysis provides closed-form expressions for acoustic frequencies and mode shapes, including those of the oscillatory pressure, velocity, temperature, heat release, density, entropy, energy density, intensity, boundary layer, and heat flux. In this process, attempts are made to clarify the dependency of various oscillatory properties on fundamental parameters such as the temperature gradient, thermal gain, heat source length and location, and stability regions. Special emphasis is placed on a logistic-exponential thermal profile, which offers a smooth and physically representative alternative to traditional piecewise models. At the outset, deeper insights are gained into the trends affecting frequency modulation and mode shape variations for diverse properties due to the attendant temperature distributions, thermal gradients, and endpoint boundary conditions. Moreover, a general interpretation of thermoacoustic phenomena in one-dimensional combustors with heat sources is confirmed, with tangible implications for the design and control of thermoacoustic instability in various combustors and solid rocket motors.
Parham DehghaniDushyant M. ChaudhariMatthew J. DiDomizioJason E. Floyd...
127190.1-127190.17页
查看更多>>摘要：This study investigates methods to enhance the accuracy and grid independence of convective heat transfer predictions in engineering-scale fire simulations using Fire Dynamics Simulator (FDS). Direct numerical simulations (DNS), validated against experiments involving a vertical, heated plate, served as benchmarks for large eddy simulations (LES) performed with grid sizes ranging from 1 mm to 38.4 mm. Three approaches were evaluated to improve LES predictions. The first approach modified how FDS applies natural convection correlations by considering the boundary layer thickness and selecting the free stream temperature from beyond the boundary layer, as opposed to the current method, which uses the gas temperature in the first cell adjacent to the wall. The second approach introduced a new Nusselt number correlation based on dimensionless parameters (Grashof and Reynolds numbers), normalized energy flux, and surface-free stream temperature difference. The third approach applied a logarithmic law of the wall using normalized temperature profiles. Both the first and second approaches improved accuracy and reduced grid dependency compared to the existing FDS model, with the first approach demonstrating greater effectiveness. The third approach still exhibited grid-dependent behavior, indicating the need for further refinement. The findings suggest that integrating one of the first two approaches into FDS would enhance its prediction accuracy, particularly for convective heat transfer. This study also establishes a foundation for applying these improvements to more complex fire scenarios, providing a pathway for advancing the modeling of real-world fire scenarios.
查看更多>>摘要：Supercritical natural circulation loops (NCLs) promise passive cooling for critical systems like nuclear reactors and solar collectors, eliminating the need for mechanical pumps. However, instabilities similar to those seen in two-phase systems can emerge in supercritical NCLs, leading to undesirable oscillatory behaviour, marked by system-wide fluctuations in density, temperature, pressure, and flow rate. This study investigates the stability of NCLs at supercritical pressures (73.7 ≤ p ≤ 110.0 bar) using CO_2 in an experimental setup with vertical cooling and vertically adjustable heaters to control convective flow rates and to oppose flow reversal. Oscillations were found to originate in the heater of the NCL, and demonstrated a high sensitivity to the thermodynamic state and proximity to the pseudo-critical line of the system. Increased mass flow rates and added resistance upstream of the heater suppressed the oscillations, while increased pressures and reduced heating rates dampened them. A static model which takes into account the non-ideality of the heat exchangers is introduced to assess the presence of multiple steady states. The system is concluded to be statically stable, and the oscillations are considered to be dynamically induced. In particular, the modulation of the NCL velocity by the traversal of the current oscillations in density is assumed to periodically re-incite non-ideality in the heater. These findings intend to refine our understanding of the stability boundaries in NCLs, to ensure a safer operation of prospective passive cooling and circulation systems employing fluids at supercritical pressure.
查看更多>>摘要：Recent advances in near-field radiative heat transfer (NFRHT), taking advantage of evanescent modes, promise a wide variety of interesting applications in material science and thermal energy management at nanoscale. However, the lack of knowledge on suitable materials poses a bottleneck to the deployment of NFRHT concepts in practical applications. In this paper, the NFRHT is studied in a well-known category of two-dimensional (2D) materials, MX (M = Ge, Sn; X = S, Se, Te) phase of monolayers of group-IV monochalcogenides. Such material systems can significantly improve the ability to confine and control heat radiation thanks to its highly anisotropic plasmonic properties. Super-Planckian radiation enhancement of more than three orders of magnitudes over the blackbody limit is reported when the vacuum gap scales down to ≈ 100 nm. The effect of changing the chalcogen species on the performance of near-field radiative heat transfer has also been discovered, that originates from the modulation of electronegativity. This enables the deep near-field (DNF) regime to extend even at significantly high vacuum gap sizes (≈ 300 nm) when the appropriate doping concentration is chosen. Additionally, it has been shown that electrochemical doping, injecting electrons, can strongly modulate NFRHT responses of MX monolayers. So that, the peak frequency of spectral heat flux is being shifted about 0.02 eV at any n = 1×10~(12) cm~(-2) of the charge density step in the GeTe monolayer. Moreover, the amplitude of spectral heat flux relevant to the GeTe monolayer increased by approximately 1 nJm~(-2)rad~(-1) for the aforementioned charge density step. This work lays the foundation for a novel cooling strategy for next-generation integrated circuits (ICs), harnessing the remarkable potential of the MX family of materials.
查看更多>>摘要：In nuclear reactors, the corrosion-related unidentified deposit (CRUD) is a widely observed thin surface layer that grows due to the long-term layered deposition of corrosion products nanoparticles on the fuel cladding. As a typical porous-hydrophilic surface medium featured by the randomly distributed boiling chimneys and high wickability, the presence of CRUDs not only forms the boron-enrichment layer of high neutron absorptivity on the fuel cladding, causing the axial offset anomaly (AOA), but also drastically changes the micro-morphology features of boiling surfaces, leading to the significant variations in multiphase flow dynamics and boiling heat transfer mechanisms. These CRUD-related problems put great challenges to the safe, long-term and economical operation of advanced nuclear reactors. Regarding these issues, this paper reviews and summarizes the recent progresses about the CRUDs' effects on boiling heat and mass transfer mechanisms in nuclear engineering. Wherein, some advanced experimental and theoretical methods are identified, compared and highlighted, including the nano-deposition techniques for artificial CRUD synthesis, the framework of fractal-based bubble dynamic theory, and enhancement mechanisms of CRUD-affected transitional boiling sub-regimes. By applying the insights from this review, the practitioners can improve their understanding on the CRUD-affected heat transfer phenomena in nuclear engineering, which would help to promote the design and analysis methods for safe, long-term and economical operations of the advanced nuclear reactors.
查看更多>>摘要：Although vibrational excitation holds significant research value in influencing the flow and heat transfer characteristics of supercritical pressure fluids, the underlying mechanisms remain poorly understood. In this study, large eddy simulation (LES) is employed to investigate the effects of overall vibrational excitation on the flow and heat transfer characteristics of supercritical pressure fluids within pipes. The research primarily focuses on analyzing its suppressive effect on typical heat transfer deterioration events, aiming to elucidate the fundamental mechanisms by which the coupling of physical property variations and vibrational excitation induces heat transfer enhancement. The results show that near-wall streamwise vortex sheet structures (SVS), generated by vibrational inertial forces, and Prandtl's third-kind secondary flows, resulting from vortex tilting, are key mechanisms contributing to heat transfer enhancement. Transport equations for streamwise vorticity and turbulent kinetic energy, outline the conditions needed for the occurrence of these two types of flows: the former is periodically generated within the high radial density gradient layer near the pseudocritical temperature through the vibrational inertial generation. The latter arises from the enhancement of the lift-up mechanism, which facilitates the self-sustaining recovery of turbulence. The streamwise vortices experience increased tilting and deformation due to the entrainment action of the SVS, leading to the formation of turbulent secondary flows. The results demonstrate that turbulent secondary flows can be induced by applying transverse vibrational excitation in pipe flows with strong density stratification. This constitutes a novel mechanism for maintaining turbulent secondary flows and provides a new strategy for enhancing turbulent heat transfer.
查看更多>>摘要：The condensation of water in wet hydrogen occurs in various applications such as fuel cells and nuclear power plants. However, the microscopic process of water condensation in wet hydrogen is not well understood. In the present study, the molecular dynamics (MD) was used to investigate the impact of various conditions on the condensation of saturated water vapor from a microscope perspective. It was found that the liquefaction ratio of H_2O molecules increased from 72.33% to 83.10% as the initial pressure increased from 1 MPa to 1.5 MPa when the cooling temperature was fixed at 380 K, while it increased from 72.33% to 87.05% as the cooling temperature decreased from 380 K to 350 K when the initial pressure was fixed at 1 MPa. Furthermore, hydrogen gas was introduced into the system to study the impacts of different initial pressures and temperatures on the condensation of saturated water vapor in the mixed gas. It was observed that the number of H_2O molecules contained in the final cluster increased with increasing initial temperature. As the initial pressure increased, plenty of H_2 molecules were adding to the system, hindering the nucleation of H_2O molecules. Through the comparison of nucleation rates, it was found that the computation of the nucleation rate of water in wet hydrogen flow concurs well with the rate determined by classical nucleation theory (CNT) under this simulation condition. However, the nucleation model proposed by Kantrowitz is closer to the actual condensation process of H_2O in pure steam at high temperatures and pressures and the nucleation rate of CNT is 1-2 orders of magnitude higher than that of MD in this situation.
查看更多>>摘要：In the growing lithium-ion battery market, an efficient battery simulation plays a crucial role in assessing performance and lifetime of Li-ion battery products. Computationally thermal models are in high demand for the battery simulation. In this work, a 1-D simplified thermal model considering cell heat generation was developed to correlate the steady-state thermal resistance under dynamic duty cycles for a 48 V lithium iron phosphate (LFP) battery pack with fourteen cells in series. The thermal resistance was correlated based on the proposed thermal model and thermal data collected by thirty-three thermal sensors placed in the thermal experiments under a representative dynamic drive cycle profile used in practical applications. Also, the influence of the coolant flow rate on the steady-state thermal resistance between the cell and the coolant was comprehensively studied. It was found that the cell-averaged steady-state thermal resistance decreases from 1.31 ～ 1.97 K/W to 0.88 ～ 1.46 K/W as the coolant flow rate increases from 0.5 L/min to 15 L/min. Furthermore, the 'Tab' and 'Bottom' region was found to have the largest and smallest averaged steady-state thermal resistance, respectively. This thermal resistance correlation work is expected to benefit a computationally efficient battery thermal and electrical performance, and lifetime prediction.
查看更多>>摘要：The two-fluid model is crucial in many industrial applications for optimizing system performance and ensuring safety. Interfacial area concentration, when multiplied by the corresponding driving potentials, represents the typical equation that expresses the transfers between mass, momentum, and energy. As a result, interfacial area concentration modeling is necessary to complete the two-fluid model. The two-group interfacial area transport equation is suitable for interfacial area concentration modeling; the equation classifies bubbles into two groups based on their drag coefficients. The two-group drift-flux model simplifies the procedure without adding more transport equations. This study introduces a new two-group drift-flux model developed for dispersed two-phase flow in medium-diameter pipes in upward flow. The asymptotic distribution parameter was determined to be 1.00 for group-one bubbles and 1.25 for group-two bubbles based on the collected data. Additionally, previously developed drift velocity correlations were applied, and reasonable agreement was demonstrated with the experimental data. The group-one and group-two void fractions were predicted by the developed model with mean relative absolute errors of 37.2 % and 30.6 %, respectively. The two-group drift-flux model is applicable to a wide range of flow conditions, including varying hydraulic diameters and gas-liquid systems, such as air-water and steam-water systems. Due to limited data availability, the asymptotic distribution parameters for group-two bubbles were determined on a preliminary basis for medium-to-large pipes using a linear interpolation method; the parameters ranged from 1.25 to 1.40, for the non-dimensional hydraulic diameters between 18.6 and 40.0. This study demonstrates the effectiveness of the model in predicting two-phase flow parameters in medium-diameter pipes and contributes to expanding the applicability of two-group drift-flux model for engineering applications.
查看更多>>摘要：The topology optimization is becoming a highly promising technique in engineering with the development of the additive layer manufacturing technology. Compared to the structural design, however, there are necessities to further improve its methodology in the field of fluid flow and heat transfer, such as the suppression of the gray region. In this study, a topology optimization technique for a heat transfer problem is developed based on the finite volume method, adopting the continuous adjoint method. For the objective of minimizing the difference between the temperature fields and desired temperature, adjoint equations and sensitivity field are derived from the primal equations, which are the continuity, momentum, and energy equations considering Boussinesq approximation. Filtering and projection techniques are implemented to obtain a distinctly optimal structure by eliminating gray elements. A gradual variation of steepness parameter value, consisting of exponential and linear functions, is proposed in the projection process to ensure numerical stability. The suggested algorithm consists of two-step: i) to consider the consistency of the initial condition, which estimate sensitivity fields only, and ii) to obtain a distinctly optimal solution, which update the design variables using an optimizer. Topology optimizations are conducted for a benchmark case of natural convection problem. Optimal performance and the level of constraints satisfaction are evaluated corresponding to the parameter values of filtering and projection. As a result, a guidance of handling parameter values is suggested for natural convection problem. Finally, the physical aspects of the generated optimal structure for the objective function are discussed.
NETL
NSTL
Elsevier