International Journal of Heat and Mass Transfer最新文献

A simple analytic model for the thermal resistance of the oscillating (pulsating) heat pipe (OHP or PHP) is proposed and verified. It is based on the Taylor liquid film formed by moving liquid slugs, film heat conduction, and interfacial evaporation/condensation. It uses a unidirectional flow (characteristic of use of check valves) in a horizontal (no gravity effect) single-loop OHP to derive a relationship between the slug velocity and heat flux. The model predictions are verified with experiments. In addition, the CFD results for the slug-deposited liquid film thickness δ_l support that the transition from laminar flow (at Reynolds number, Re_D = 2000, D is the diameter) tends toward a constant film thickness regime, previously observed (experimentally) and correlated. Prior direct 1-D simulations also show that at yet higher Re_D (3700), a constant slug velocity regime is reached. The evaporator specific thermal resistance, A_eR_e = A_e/G_e = δ_l /k_l, (k_l is liquid thermal conductivity) and the total OHP thermal resistance R, is predicted and compared with the current and available related experimental results (for R134a and butane) with good agreement. The suggested ideal high heat flux (constant film thickness ${\delta }_{l,c}$ limit for Re_D > 2000) dimensional total ideal resistance is R =$\frac{1}{G}=$ $\frac{{\delta }_{l,c}}{k_l}\left(\frac{1}{A_e}+\frac{1}{A_c}\right)$, with ${\delta }_{l,c}$ = $120D{\mathrm{La}}^{-2/3}$, providing a lower limit on R (upper limit on G). The filling ratio, flooding, and local dryout effects causing deviation from the ideal conductance are addressed. This analytical model can be extended to non-circular channels by using the hydraulic diameter.

The axle box bearing is a critical component of high-speed trains. Investigating the thermal characteristics of axle box bearings under in-service conditions is essential for developing an effective temperature monitoring strategy. In this study, a non-linear thermal field model for axle box bearings, considering the vehicle-environment coupling effects, is established based on the bearing-vehicle coupled dynamics and finite element method. The validity of the proposed thermal model is demonstrated by comparing the temperature results with those obtained from a field test and calculated via the traditional method. Furthermore, the influence mechanism of vehicles and environment on bearing temperature was revealed, and the thermal characteristics of the axle box bearing under in-service conditions were analyzed. The results show that the vehicle affects the bearing temperature through the boundary load and wheelset speed, and these two parameters can have a significant impact on the power loss of the bearing and the convective heat transfer coefficient of the grease. The environment influences the bearing temperature by affecting the convective heat transfer coefficient on the surface of the axle box and wheelset. In addition, the track line parameters, such as curve radius and superelevation, may lead to the changes in bearing loads, consequently affecting the thermal behavior of the bearings. The maximum temperature of each bearing component increases with higher vehicle speed and higher ambient temperature. It is thus essential to consider the vehicle-environment coupling effects when analyzing the temperature characteristics of axle box bearings under operating conditions.

Proper design of the flow field in bipolar plates is important for improving proton exchange membrane fuel cells in water transport, gas distribution and net power density enhancement. Inspired by the fact that the streamlined design of an airplane airfoil makes the flow resistance reduced, a new airfoil cross flow field is proposed. Airfoil cross flow field is composed of a regular pattern of airfoil pins which are categorized in pin-type flow fields. The effects of different airfoil-pin shapes and arrangements on cell performance were explored. The results showed that airfoil cross flow field improved water management by finely modulating the airflow significantly increasing the gas flow rate in the channel. In addition, the airfoil cross flow field design achieves higher and more uniform oxygen distribution at the interface between the gas diffusion layer and the catalyst layer. The proper shape and arrangement of the airfoil-pins can effectively increase the net power density of the cell by 10.65 % and 2.49 % compared to the conventional parallel flow field and the square-pin flow field. Due to the streamlined design of the airfoil cross flow field, the voltage drop is approximately 60 % of that of the conventional parallel flow field and even slightly lower than that of the square-pin flow field, which demonstrates its high practicality. In summary, the airfoil cross flow field well coordinates the high current density and low voltage drop requirements of proton exchange membrane fuel cells, and some new points of view on flow field design are presented.

The loop thermosyphon is an efficient two-phase heat transfer device for heat recovery and energy saving. Generally, the Lee model is commonly applied in the Computational Fluid Dynamics (CFD) methods to predict the simultaneous evaporation and condensation in the loop thermosyphon. However, the Lee model has the non-conservation of mass problem due to the mismatch between the condensation mass transfer time relaxation parameter (r_c) and the evaporation mass transfer time relaxation parameter (r_e) in the two-phase closed system. This paper improves the Lee model with dynamic-r_c by considering the mass balance in the loop thermosyphon. The value of the r_c is related to the total mass of the liquid working fluid, the evaporation intensity, and the condensation intensity. The CFD simulated temperature distributions were in good agreement with the experimental temperature data. This study investigates the effect of different r_c and saturation temperatures (T_sat) on the simulation results. The results show that the refined model ensures the total working fluid mass remains stable during the calculation process, and the simulation of the gas-liquid two-phase distribution in the loop thermosyphon is more accurate. The value of the T_sat will have a more significant effect on the wall temperature distribution of the loop thermosyphon but less on the working fluid distribution. Additionally, the impact of filling ratios is investigated while setting the T_sat as constant in the CFD models. The results experimentally and numerically indicate that when the filling ratio is 50 %, the overall thermal resistance is the smallest, and the liquid working fluid distribution is optimal. This work provides a method and approach for analyzing the heat transfer procedure inside the loop thermosyphon and improving the accuracy of the numerical model.

Synchronous ultra-high frequency (UHF) induction-assisted laser deposition is an effective approach to tackle the extreme heat input and common defects that originate from laser direct deposition. The added auxiliary induction heat source can reduce the energy input of the laser heat source which guarantees the geometric integrity of the embedded tube and decreases temperature gradient. Meanwhile, the effective preheating of the induction heat source assures the metallurgical bonding among the substrate, deposited track and embedded tube. Experiments showed that the effective bonding can be formed among substrate-deposited track and deposited track-embedded tube under suitable combination parameters of the laser heat and induction heat. To gain insight into the hybrid deposition, a 3D numerical model coupled with multi-physical fields was established to explore the thermal process and flow behavior with assistance of induction heat. Results indicated that the electromagnetic force produced by induction coil promotes the flow velocity, which increases heat convection. Investigation on the induction heat parameter demonstrated that raising current intensity can accelerate the flow velocity from 0.07 m s^-1 to 0.09 m s^-1 while the maximum temperature of the molten pool declined from 2610 K to 2440 K owing to the reinforced heat convection. The experimental cross-section of the deposited tracks and the detected element distribution in transition areas are well tested against the numerical simulation results. The grain size in the top region of the deposited track is refined with increasing current intensity, which is experimentally and numerically verified by observed microstructure and G*R value. Meanwhile, the fabricated deposited track with average friction coefficient of 0.445 can be obtained when the laser and induction parameters are 1000 W and 400A, respectively.

In the billet reheating process during steel rolling, the real-time and accurate prediction of the temperature field is a prerequisite for the dynamic regulation of the heating process, which is crucial for ensuring the quality of billet reheating and reducing the energy consumption of the reheating furnace. The most commonly used finite element thermal field simulation and analysis methods are unable to meet the demand for real-time prediction under dynamic working conditions. Furthermore, traditional machine learning methods struggle to handle irregular data that includes spatial location information, resulting in inaccurate temperature field predictions, which in turn affect the furnace control efficiency and the quality of the product. In light of these limitations, this study proposes a Graph Neural Network (GNN)-based temperature field prediction method for steel rolling reheating furnaces. This method considers the interactions between the nodes within the reheating furnace and constructs a temperature field topology graph to effectively capture these interactions, including heat conduction and convection. Additionally, in the feature fusion and state updating mechanism of the model, we introduce process parameters, such as the air-fuel ratio, as additional inputs to enhance the ability of the GNN to handle complex variables. This enables real-time accurate simulation of the internal temperature distribution of the reheating furnace. The experimental results demonstrate that the model prediction error is controlled within 2.9 % and the response time is maintained within 20 ms, thereby validating the reliability and efficacy of the method in practical applications.

This study focuses on the thermoelastic fracture behavior of quasicrystals, a class of solids that exhibit unique properties between traditional crystals and amorphous materials. The complex atomic structure of quasicrystals poses challenges in understanding their fracture mechanisms under thermoelastic loading conditions. Central to our investigation is an analytical examination of a penny-shaped crack in an infinite three-dimensional body composed of two-dimensional hexagonal quasicrystals, where the upper and lower crack surfaces are applied to a pair of uniformly antisymmetric heat fluxes. This crack problem requires simultaneous consideration of thermal-phason-phonon multiple field coupling. According to the symmetry of field variables with respect to the crack plane, the thermal-phason-phonon coupled crack problem is transformed into a mixed boundary value problem in the upper half space. The extended displacement discontinuities, encompassing both phonon and phason displacement discontinuities, as well as the temperature discontinuity, are chosen as the basic unknown variables to construct the boundary integral-differential equations governing the mixed boundary value problem. Based on these boundary governing equations and Fabrikant's potential theory method, the problem with the crack surface subjected to uniform antisymmetric heat fluxes is solved. The solutions of thermal-phason-phonon fields on the crack plane, and in the full space are given in closed-form. Numerical results are employed to validate the obtained analytical solutions and visually illustrate the spatial distribution of thermal-phonon-phason coupling fields in the vicinity of the crack. The study provides fundamental insights into the behavior of cracks in quasicrystals under thermal loading, with potential implications for the design of new materials and structures.

In this study, the bubble-induced thermal convection in nucleate boiling and microbubble emission boiling (MEB) under highly subcooled conditions is investigated through using high-speed schlieren and chronophotography techniques. The spatio-temporal evolution of thermal plumes in nucleate boiling and MEB is revealed based on schlieren technique and power spectral density (PSD) analysis. The results show that bubble condensation in nucleate boiling generates the most intense thermal plumes, particularly at high heat fluxes. Using chronophotography technique, it is found that the thermal convection in MEB, driven by bubble oscillations, exhibits a wave-like pattern over time, while bubble collapses generate densely striated schlieren patterns. PSD analysis highlights significant differences in the decay profiles, cutoff frequencies, and spectral indices of fluctuations of schlieren grayscale between nucleate boiling and MEB. Comparative analysis with previous particle image velocimetry studies suggests that MEB may facilitate efficient heat transport from the heating surfaces to the cold bulk, thereby preserving the subcooling of the liquid near the heating surface even at very high heat flux. These insights deepen our understanding of the fundamental differences in heat transfer mechanisms between nucleate boiling and MEB.

The thermal concentrating efficiency of a thermal concentrator is determined by the ratio of its interior to exterior temperature gradients, serving as a crucial indicator influenced by the interaction of geometrical and thermal conductivity parameters. Finding simpler and more effective ways to improve thermal concentrating efficiency has been a key concern in this field. In our study, we present a method to enhance the concentrating efficiency of an isotropic multilayer circular thermal concentrator by introducing gradient-distributed thermal conductivities or layer thicknesses within the multilayer circular structure. Our goal is to identify the optimal structural setup parameters for achieving enhanced thermal concentrating efficiency using an optimization approach that combines stepwise refinement search with machine-learning predictions. Initial investigations explore the impacts of different gradient schemes on thermal concentration performance. The gradient distribution function with high thermal concentrating efficiency is established through the stepwise refinement search strategy and the machine-learning model. Subsequently, a detailed search process is carried out in small increments, followed by finite element simulations to validate the thermal concentrating efficiency and ascertain the optimal design parameters of the thermal concentrator. Our findings reveal that the optimally designed gradient thermal concentrator showcases an 8.56 % increase in thermal concentrating efficiency compared to a single-layer structure without gradients. Moreover, applying the gradient function to the outer and inner rings elucidates the inherent influence of the inner and outer layered ring structures on thermal concentrating efficiency. The optimization methodology, combining stepwise refinement search and machine-learning predictions, succeeds in improving the efficiency with easy, fast and efficient operation. This approach can be extended to advance the development of various other thermal metastructured devices.

Pure thermal plumes have been extensively studied in recent decades. It has been established that for the flow that formed over horizontal surfaces with local heating areas of various shapes, a change in flow regimes is observed: the stationary regime of an axisymmetric plume is replaced by a regime with periodic destruction of the near-wall layer, and the emergence of toroidal vortex structures. The main interest for the authors of the published studies is apparently in the fully developed flow regimes (both stationary and periodic). In contrast, the causes of the destruction of the near-wall layer, leading to the emergence of the puffing regime, remain explored poorly. This paper will discuss the changes in the flow patterns of the plume formed above a locally heated horizontal surface. The main study method was axially symmetric numerical simulation in a wide range of Rayleigh numbers. Numerical data are supplemented by physical experiment results obtained in similar conditions. The flow structure close to the disk's surface is given special consideration, along with the circumstances leading to a change in flow regimes. The nature of the evolution of thermal disturbance propagating in the air above the disk surface at different values of the Rayleigh number is considered separately.