International Journal of Heat and Mass Transfer最新文献

In a lead-lithium (PbLi) based breeding blanket, the bred tritium remains dissolved in the PbLi, which flows towards the Tritium Extraction Unit (TEU). In a TEU, tritium must be extracted at a fast enough rate that guarantees the plant’s self-sufficiency and safety. Different TEU concepts exist, one of the most promising being Permeation Against Vacuum (PAV), based on the extraction of tritium from the PbLi through a highly permeable membrane.

Even in the so-called low velocity blanket concepts, PbLi is expected to flow at relatively high total mass flow rates. This means that the high tritium extraction efficiencies that safe operation requires must be obtained partitioning the flow into several extraction channels, but this solution increases both cost and complexity. However, less partition channels may be required should turbulence be induced in the flow. Since turbulence increases the flow mixing, it should favor tritium extraction. Hence, in the range of velocities where turbulent phenomena start —i.e., the transition region—, extraction efficiency is expected to grow rapidly with velocity due to turbulence acting as a new transport mechanism. Thus, a turbulent flow in the TEU may achieve the high extraction efficiencies required with a more moderate partitioning scheme.

In this work, a PbLi flow in a prototypical PAV channel was modeled using Direct Numerical Simulations (DNS). To trigger turbulence, two different methods were implemented, namely an instability-inducing oscillating boundary condition, and a physical turbulator consisting of a geometrical obstacle. Both methods proved successful as they resulted in greater extraction efficiencies than those seen in the analogous laminar regimes. In fact, up to a 15% increase in extraction efficiency or up to a 5-fold increase in total extraction rate were obtained.

The under-rib convection, driven by the pressure differences between neighboring channels, plays a crucial role in determining the performance of flow-field structured vanadium redox flow batteries. However, the correlation between key geometric characteristics and under-rib convection is unclear, limiting the exploration of flow-related transport mechanisms and the development of high-performance flow fields. In this work, a three-dimensional model integrating fluid flow, mass transport, and electrochemical reactions is developed, and the under-rib convection is enhanced by tailoring the critical geometric characteristics adopting a rotary serpentine flow field as an example. Results show that variations in channel fraction and channel depth can mitigate concentration polarization by influencing the active material transport velocity within a localized region (i.e., under-rib convection intensity) without deteriorating the distribution of active material transport. More remarkably, the battery with the optimal geometric characteristics is able to deliver a preferable total pump-based efficiency (91.4 %) at a current density of 150 mA cm⁻² and a flow rate of 40 ml min⁻¹. This study offers a comprehensive analysis of the correlation between flow field geometric characteristics and under-rib convection to serve as guidance for the design of high-performance vanadium redox flow batteries.

In this research, the quantitative identification of the characterization limits of lock-in infrared thermography applied to buried flat flaws in materials is aimed. With this goal, first, a numerical dimensionless model is developed in order to obtain an experimentally unconstrained understanding of the technique. In this frame, the complete thermographic problem is noticeably reduced to a set of dimensionless parameters without lack of generality. Second, after successfully validating the model against experimental data, a global sensitivity analysis is fed with the developed numerical model. As a result, the maximum sensitivity and predominancy ranges are quantitatively identified for each dimensionless parameter, leading to an experimental guideline for optimum thermographic inspection. Coming out from this analysis, the particular suitability of infrared thermography for the quantification of very thin delaminations is demonstrated.

With the rapid development of the economy and society, the contradiction between energy supply and the deterioration of the ecological environment has become increasingly prominent. Efficient thermal management, capable of reducing water and energy consumption while maintaining environmental sustainability, has emerged as a pivotal factor in addressing the prevailing energy crisis, garnering significant attention. Due to latent heat induced super thermal conductive capability, robustness and localized hotspot mitigation ability, vapor chambers (VCs) have emerged as one of the most efficient thermal management solutions for high heat flux cooling. Numerous researchers have diligently worked on enhancing VC heat transfer performances from the aspects of wick design, evaporation/condensation, and liquid-vapor transport. Based on the specific functions triggered by structural changes, this review comprehensively summarizes the latest research progress of novel VC designs, encompassing aspects such as heterogeneous/hybrid wettability design, biomimetic wick design, vapor-liquid space design, nanoengineered and new materials, extreme scale design, and integrated design. Subsequently, the latest advancements in the optimization of wick-level thin film evaporation/boiling and the enhanced capillary performance are discussed, while offering comparisons of their respective advantages and disadvantages. Furthermore, the diverse applications of VCs in electronic cooling, proton exchange membrane fuel cells, battery thermal management systems, solar cells, waste heat recovery, and space exploration are introduced. Finally, this review is concluded by providing the existing challenges and suggested directions for future research directions.

Nanomaterials with low thermal conductivity (TC) are ideal candidates as thermoelectric materials. In this direction, we design innovative multilayer graphene/h-BN (GBN) van der Waals (vdW) heterostructures with gradient composition distribution (C, B and N atoms), inspired by the newly synthesized boron carbonitride nanosheets. Three types of 26-layer GBN models are constructed and investigated, i.e. U-shape, X-shape, A-shape, representing uniform, symmetrical, and asymmetrical distributions of carbon atoms along the cross-plane direction, respectively. Based on non-equilibrium molecular dynamics (NEMD) simulation, we confirm that X-shape GBN model possess the lowest cross-plane TC, approximately 7 times smaller than that of the uniform U-shape graphene. The cross-plane TC can be further reduced by applying tensile strains or decreasing interlayer coupling strength. These findings elucidate the significant role the composition distribution plays in the thermal transport of GBN vdW heterostructures. However, the mechanical properties (Young's nodulus and tensile strength) of the new vdW heterostructures are insensitive to the composition distribution. Our work provides a new perspective for manipulating the interfacial thermal transport of multilayer GBN vdW heterostructures by means of material design and offers a useful guide for rationally designing thermoelectric materials with tailored thermal properties based on vdW heterostructures.

In this study, the heat transfer and frictional pressure drop characteristics of R-32 and R-410A in a plate heat exchanger are experimentally evaluated. The study comprises two-phase condensation, single-phase vapor, and liquid cooling experiments of R-32 and R-410A, as well as an independent single-phase water heat transfer experiment. The effects of heat flux, mass flux, condensing pressure, mean vapor quality, and refrigerant Reynolds number on heat transfer and pressure drop are assessed for comparisons of R32 and R410A. The results demonstrate that the heat transfer performance and frictional pressure drop increase with rising mass flux and mean vapor quality and decrease with declining condensing pressure in the two-phase condensation experiments. Furthermore, increasing the heat flux enhances the heat transfer performance while having negligible impact on the frictional pressure drop. In the single-phase cooling experiments, both heat transfer performance and frictional pressure drop rise with increasing refrigerant flow rate. Based on the experimental results, Nusselt number and friction factor correlations for two- and single-phase flows of R-32 and R-410A are developed and compared with the correlations from other studies. Finally, energy-saving performance is evaluated and compared between the refrigerants.

Energy management efficiency is crucial for the increasingly severe global energy challenges. Low global warming potential (GWP) refrigerant R452B used in air conditioning and refrigeration systems could reduce the global warming effect for applications. The objective of this paper is to conduct the numerical and experimental studies for examining the thermal fluid behaviors of flow boiling evolution of refrigerant R452B in the vertical microchannels. In the experimental approach, a gear pump and a programmable DC power supply are employed to regulate the mass flux and heat flux for determining the heat transfer and pressure drop outcomes across the microchannel at varied vapor qualities. A high-speed camera with a LED fiber optical light source is used to observe the close-up optical images over the complex flow boiling process. Theoretically, the volume-of- fluid (VOF) method built in the computational fluid dynamics (CFD) software ANSYS/Fluent® is employed to simulate the progression of bubble nucleation processes for resolving the distributions of velocity, pressure, temperature and liquid volume fraction in the microchannel. The predictions are compared against the measured heat transfer coefficients and pressure drops for the CFD model validation. The time sequences of vapor volume fraction contours are also simulated to characterize the evolving vapor-liquid interfaces for better grasping the detailed behaviors of nucleation, growth, departure, coalescence of bubbles and the transformations of dominant flow patterns. The measured results estimate the average heat transfer coefficients and pressure drops up to 14.1 kW/m²K and 76.5 kPa at a mass flux of 600 kg/m²s in the microchannel. This research further conducts the performance assessments by testing the popular correlations, and thereby reveals the effectiveness of Bertsch as well as Sun and Mishima correlations to reasonably calculate the heat transfer coefficients and pressure drops of R452B refrigerant, involving the mean absolute errors of 12.5 % and 22.6 %, respectively.

Aiming to explore the boiling heat transfer characteristics in plate heat exchanger under ocean sloshing scenarios, a two-phase flow boiling experimental system integrated with visualized plate heat exchanger and three degree-of-freedom (DOF) motion platform is designed and built. With special focuses on two-phase flow pattern, temperature and pressure responses, the sloshing effect on flow behaviors and boiling characteristics of plate heat exchanger under different driven modes (pump-driven and self-driven), liquid filled charge ratio, mass flux, sloshing postures (inclined state and swing state) and sloshing orientation (frontal and lateral) are investigated. The results indicate that when plate heat exchanger is fully filled with working fluid under pump-driven mode, the heat transfer efficiency is insensitive to sloshing effect regardless of inclined state or swing state. When plate heat exchanger is not fully filled with working fluid under self-driven mode, the inclined angle poses a great influence on the heat transfer performance by changing the liquid-vapor distribution, with heat transfer coefficient variations of 44 % and 15 % at mass flux of 10 kg/(m²·s) during frontal inclined angle of ±20° and lateral inclined angle of ±20°, respectively. Moreover, there are heat flux variations of 24 % and 177 % at mass flux of 10 kg/(m²·s) during ±20° frontal incline and lateral incline, respectively. For swing state, heat transfer performances are apparently enhanced with the effect of inertial force, characterized by the increases of heat transfer coefficient, heat flux and pressure fluctuation with increasing swing amplitude. Under ocean sloshing scenarios, the operation of plate heat exchanger under high liquid filled charge ratio and mass flux is conducive to eliminate the sloshing effect on flow boiling.

We monitor the evaporation of a volatile liquid (ethanol) from an inkjet-printed liquid film, consisting of a mixture of ethanol and ethylene glycol. Interferometric video imaging technology is used for recording 2D vapor concentration profiles over the evaporating film. The vapor flow is reconstructed using numerical simulations. In this way, we reconstruct the complete flow velocity profile, and distinguish diffusive and convective gas transport, with quantitative tracking of the transport balances. The convective flows are driven by the buoyancy of the solvent vapor in the ambient air. In particular, we reconstruct the evaporation process from the interface of the two-component liquid. We monitor the evaporation flows, implement Raoult’s and Henry’s laws of vapor pressure reduction, as well as evaporation resistivity. We observe the edge-enhancement of evaporation flows at the wetting rims of the liquid film, and decompose the vapor flows in the diffusive and the convective contribution. We demonstrate how Langmuir’s evaporation resistivity can be identified using vapor pressure profiles in the gas phase data and mass transfer balances.

The thermal contact conductance (TCC) of rough surfaces is a fundamental issue in heat transfer. Thermal stress and asperity interactions have important impacts on the TCC. A new fractal model for predicting the TCC that considers thermal stress and asperity interactions is developed. First, an improved normal contact mechanics model is constructed that considers the asperity deformation, thermal stress, and interactions of a single asperity from a microscopic viewpoint. Then, a new TCC prediction model is proposed according to the improved contact mechanics model and classical heat conduction theory. Furthermore, the predicted values of the TCC are compared with published experimental results and reported models. Finally, the influences of surface topography, temperature differences, and material properties on the TCC are further revealed. This study can provide deep insight into the thermal design of sophisticated equipment.