Search Results (1,170)

In the current work, pulsating airflow and an external heat source are proposed to improve the evaporation performance of falling films. A volume of fluid (VOF) multiphase flow model is established based on the commercial software ANSYS Fluent 2023 R1 (ANSYS, Inc., Canonsburg, PA, USA) to simulate the falling film evaporation process. Numerical simulations are conducted under combined working conditions with variable average inlet velocity (u₀), relative pulsating amplitude (A), and wall heat flux (q_w). The spatial and temporal distributions of physical fields are visualized via numerical contours and characteristic curves, and the heat and mass transfer enhancement mechanism is revealed from three aspects, namely, the promotion of driving potential difference by wall heat flux, the increase in heat and mass transfer gradients induced by pulsating airflow, and the synergistic effect of the above two factors. The results indicate that the evaporation ratio (Ψ) increases monotonically with the rise of u₀, with a maximum growth rate of 83.8%. By contrast, under the condition of fixed A₀ = 1 m/s while varying u₀, the evaporation ratio exhibits a convex variation with the relative amplitude A = A₀/u₀, and the global optimal value is achieved at A = 1/6, corresponding to u₀ = 6 m/s and A₀ = 1 m/s. Comparative analysis demonstrates that wall heat flux exerts a more significant influence on evaporation performance than pulsating airflow. Specifically, the evaporation ratio at q_w = 10,000 W/m² is 3~4 times higher than that under the adiabatic wall condition. The reliability of the numerical model is first confirmed by comparing the predictions with published experimental data for vertical falling film evaporation. Based on this validated model, the quantified parametric effects and optimal operating conditions provide practical design references for falling film evaporators in seawater desalination and related thermal separation applications. Full article

Drying is a key operation for extending the shelf life of agricultural products and maintaining food quality, and its efficiency and product outcomes are governed by coupled heat and mass transfer. This review critically summarizes the mechanisms, technological characteristics, research methods and application prospects of agricultural-product drying from a heat- and mass-transfer perspective. The moisture-migration pathways, including surface evaporation, internal diffusion, capillary flow, vapor diffusion and bound-water desorption, are first discussed within a porous-medium framework. Governing equations based on Fourier’s law, Fick’s law, energy conservation and convective transfer are then introduced to clarify the theoretical basis of drying models. Typical convective, radiative, conductive and combined drying technologies are compared in terms of transfer mechanisms, drying efficiency, energy consumption, product-quality retention, carbon-footprint potential and industrial feasibility. Particular attention is given to the effects of drying-induced heat and mass transfer on color, texture, rehydration, bioactive compounds, antioxidant activity and microstructure. Current theoretical, experimental, numerical and data-driven research methods are further reviewed, and the limitations of existing studies are identified, including simplified homogeneous assumptions, insufficient model validation, limited quantitative comparison and weak scale-up applicability. Finally, future directions are proposed, including refined multi-scale and multi-field coupled models, advanced in situ characterization, multi-energy-field synergistic drying, digital twins, predictive modeling and multi-objective intelligent optimization. This review aims to provide a more mechanism-based and application-oriented reference for developing efficient, low-carbon and quality-preserving drying systems for agricultural products. Full article

To satisfy the electricity demands and further improve the cooling performance of scramjets, the supercritical CO₂ Brayton cycle is adopted as a promising cooling and power supplying scheme. However, with the extremely high heat flux level of MW/m², a cooling channel featuring a rectangular cross-section with non-uniform heat flow and different positions of the heated wall relative to the direction of gravity make the flow and heat transfer characteristics of supercritical CO₂ within scramjet cooling channel extremely complex. In this study, the effects of different angles of the heated wall normal direction relative to the direction of gravity (θ) on the flow and heat transfer behavior of supercritical CO₂ within a scramjet cooling channel are comprehensively investigated. The results show that the spanwise heat transfer coefficient on the heated wall decreases while on the opposite wall it increases as θ increases. Moreover, the overall heat transfer performance is insensitive to variations in θ, but the heat transfer characteristics on different walls are sensitive to variations in θ. Moreover, the cases with lower θ provide better heat transfer performance for the heated wall, which is more significant for scramjet cooling due to having the highest heat load. In detail, the averaged HTC on the HW of θ = 180 deg for G = 900 kg/(m²s) is reduced by 11.79% in comparison to that of θ = 0 deg, while it is enhanced by 18.70%, 6.39% and 6.39% for the OW, LW and RW, respectively. Full article

This study investigates heat transfer enhancement in the cooling channels of gas turbine blade turbulators using modified transverse baffles with isosceles triangular sawtooth perforations. The proposed baffle design aims to improve convective heat transfer by promoting flow mixing and disrupting the thermal boundary layer. Experiments were conducted in a rectangular channel with an aspect ratio of 3.75 under constant heat flux conditions using air (Pr = 0.7) as the working fluid. The effects of Reynolds number (Re = 6000–24,000), sawtooth width ratio (a/W = 0.0, 0.0625, 0.125, 0.25, and 0.5), and sawtooth height ratio (b/e = 0.0, 0.25, 0.5, 0.75, and 1.0) were systematically investigated. The blockage ratio (e/H) and pitch ratio (P/H) were maintained at 0.3 and 1.5, respectively. Heat transfer characteristics were evaluated using the thermochromic liquid crystal (TLC) technique, while thermal–hydraulic performance was assessed in terms of the Nusselt number (Nu), friction factor (f), and thermal performance factor (TPF). The results demonstrate that introducing sawtooth perforations significantly enhances heat transfer compared with a smooth channel, yielding Nusselt number ratios (Nu/Nu_s) between 1.6 and 2.6. The highest heat transfer enhancement was achieved at a/W = 0.0625 and b/e = 0.25, where the relatively small sawtooth openings generated stronger jet impingement, enhanced flow mixing, and more effective disruption of the thermal boundary layer. However, these geometric modifications also increased the pressure loss due to intensified flow blockage and recirculation, resulting in friction factor ratios (f/f_s) ranging from 8.9 to 14.9. The maximum pressure-drop penalty occurred at b/e = 0.25 because the smaller openings produced stronger turbulence and increased flow resistance. Despite the increased friction loss, the optimum configuration (a/W = 0.0625 and b/e = 0.25) achieved the highest thermal performance factor of 1.2 at Re = 6000. Full article

Fractures exert a significant influence on rock mass deformation and seepage pathways, thereby posing a serious challenge to the safe and efficient extraction of deep mines. This problem is particularly evident in deep mines located near the sea, where fractures are extensively developed. For such mines, the overlying seawater represents a considerable potential risk to mining safety. Therefore, investigating the distribution characteristics of deep fractures and clarifying the coupling relationships among the fracture, stress, seepage, and temperature fields are important for ensuring safe and efficient production in deep mines near the sea. Taking the auxiliary shaft of the Sanshandao Gold Mine as the engineering case, this study uses extensive measured fracture data, determines fracture locations by their centroids, and adopts kernel density estimation to non-parametrically characterize the fracture spatial distribution. Fourier convolution is then employed to rapidly reconstruct fracture positions in the discrete fracture network (DFN) model. The results demonstrate that the proposed kernel density estimation method can effectively identify the spatial distribution characteristics of fractures. Subsequently, the fracture field of the underground rock mass is reconstructed by the Monte Carlo method, and a thermal–hydro–mechanical multi-field coupling model incorporating the fracture field is established. The numerical results indicate that fluid flow is primarily concentrated along fractures, and that heat transfer within fractures is markedly faster than that in the rock matrix. The presence of fractures significantly affects the stress field of the underground rock mass, and their influence on the stress distribution increases as fracture length becomes greater. Accordingly, the effects of fractures should not be neglected in numerical analyses. The findings provide reliable support for mine stability calculations and safety evaluations. Full article

To ensure that the necessary gas-phase safety space is retained after the filling of a liquid hydrogen cylinder and to reduce the risk of rapid pressure rise caused by overfilling, a 34 L vehicle-mounted liquid hydrogen cylinder was taken as the research object. A non-isothermal two-dimensional numerical model considering gas–liquid two-phase flow, heat transfer, and phase change was established. The dynamic and thermal characteristics of cylinders with and without a gas phase space were compared under different filling rates. The results show that, during liquid hydrogen filling, the liquid phase first accumulates at the bottom of the main chamber. Then, the liquid level rises and compresses the upper gas phase, and part of the liquid hydrogen enters the gas-phase space in the later stage. The gas-phase space can delay the occupation of the safety gas cushion by liquid hydrogen, allowing a certain volume of compressible gas to remain during overfilling. The pressure variation presents three stages: a rapid increase in the initial stage, a slower increase in the middle stage, and another rapid increase in the final stage. These stages are related to liquid hydrogen flash evaporation, gas-phase cooling and condensation, and compression of the remaining gas, respectively. The tank temperature generally shows a rapid decrease followed by a slower decrease. As the filling rate increases, the liquid level rises faster, the gas–liquid interface disturbance becomes stronger, the liquid hydrogen enters the gas-phase space earlier, the pressure rise rate increases, and the buffering effect weakens. The results indicate that the gas-phase space structure can improve the safety margin in the final stage of liquid hydrogen cylinder filling, but the filling rate should still be reasonably controlled in actual filling processes. Full article

Plate–fin heat exchangers (PFHEs) are widely used in petrochemical, energy and electric power, aerospace, and other industries with large heat transfer requirements. The development of performance prediction and optimization methods for PFHEs has become increasingly important in the design and operation of such heat exchangers (HEs). This paper establishes a database of flow and heat transfer characteristics for four types of PFHEs with different structural parameters. Based on this database, the back-propagation artificial neural network (BP-ANN) model was optimized using the particle swarm optimization (PSO) algorithm to form the PSO-BP-ANN model for the performance prediction of these four types of PFHEs. This combination has been found to improve the prediction accuracy and generalization ability of the BP-ANN model. Additionally, the non-dominated sorting genetic algorithm II (NSGA-II) method was used to characterize the relationship between four structural parameters to be optimized (the length, height, spacing, and thickness of the HE fin) and the two objective functions (j and f) of the serrated PFHE in laminar flow. This enables the Pareto optimal solution to be obtained. The results show that, under laminar flow conditions (Re = 800), the serrated fin HE achieves the best heat transfer performance when the fin height, spacing, thickness, and length are 9.29, 1.22, 0.16, and 3.06, respectively. Full article

The reliance on a single evaluation parameter represents a major limitation in traditional geothermal reservoir assessment models, hindering accurate and effective evaluation of geothermal extraction performance. Moreover, mechanical deformation induced by cold fluid injection exerts a significant influence on both fluid flow behavior and geothermal energy recovery. In this study, a thermo-hydraulic–mechanical (THM)-coupled single-fracture model is developed based on the physical properties of the solid matrix and the seepage characteristics of the fluid, using a finite-element framework for heat and mass transfer. This model enables a multi-parameter evaluation of geothermal extraction efficiency as well as reservoir rock deformation. The simulation results indicate that reservoir temperature decreases progressively from the injection well to the production well, resulting in a gradual decline in the outlet temperature after an initial stable production period of approximately 200 days. The presence of a preferential “fastest flow path” between the injection and production wells plays a critical role in sustaining the stable production phase, whereas the development of a tongue-shaped isotherm pattern is a primary factor responsible for the reduction in outlet temperature during the later stages of extraction. In addition, thermally induced rock deformation further modifies geothermal extraction efficiency, mainly through its effects on reservoir permeability and top vertical displacement. Overall, this study provides reliable and effective fundamental data for geothermal exploitation in specific geological reservoirs, thereby supporting the role of geothermal energy as a viable supplement to fossil fuel resources. Full article

The present study employs an entropy analysis to investigate the boundary layer flow and heat transfer characteristics of an electrically conducting fluid subjected to the influence of a constant transverse magnetic field over a flat plate with viscous dissipation. The Keller box method is utilized to solve non-similar boundary layer equations. The effects of magnetic interaction, viscous dissipation (Eckert number), and group parameters on heat transfer, entropy generation, Bejan number, velocity, and temperature profiles are calculated. The findings demonstrate that a reduction in the magnetic parameter and Eckert number is associated with an enhancement in the heat transfer parameter. Furthermore, the surface behaves as a significant source of irreversibility due to higher entropy generation parameters near the surface. A new correlation is also obtained for the local skin friction and heat transfer parameters. An Artificial Neural Network is constructed to forecast the desired output values. Finally, the problem is mathematically defined with symbolic regression trees using genetic programming and artificial bee colony programming. The performances of the algorithms are evaluated according to coefficient of determination and root mean squared error values. The best model is obtained by ABCP algorithm with three arithmetic operators (+, -, *). The model’s coefficient of determination is found to be 0.9762, while its root mean squared error value is calculated as 0.0129. Full article

Physical tempering is widely used to enhance the mechanical strength and thermal stability of glassware. Traditional numerical studies commonly adopt the uniform heat transfer coefficient assumption, which significantly deviates from the actual non-uniform jet cooling conditions, especially for glassware with complex three-dimensional curved surfaces. In this work, a fluid–thermal–structure sequential coupling numerical model for low-borosilicate glassware was developed using STAR-CCM+. The Realizable k-ε turbulence model, temperature-dependent thermophysical properties of glass and air, and transient non-uniform convective heat transfer boundaries were employed. Flow characteristics, heat transfer behavior, and residual stress distribution during air cooling were systematically investigated. The simulation results were verified using a polarizing stress instrument. Results indicate that obvious flow separation and vortices occur at the curved regions, resulting in highly non-uniform heat transfer. Temperature uniformity first decreases and then rebounds, while stress uniformity finally stabilizes above 90%. The through-thickness stress exhibits a parabolic profile with surface compression and internal tension. The maximum relative error between simulation and experiment is below 6%, demonstrating the reasonable engineering accuracy of the sequential coupling framework. Ultimately, these numerical observations quantify the fluid–thermal–structural interactions and underscore the critical importance of integrating realistic non-uniform aerodynamic boundaries. Full article

This study presents a novel one-dimensional solution method to demonstrate the effects of fuel composition and channel roughness on phase-change heat transfer in spiral regenerative cooling systems. The calculated models are grounded in an experimental correlation of liquefied natural gas (LNG) flow boiling, and their accuracy is validated through ignition experiments conducted on a 1 kg/s-class thrust chamber. The experimental data shows that the physical characteristics of LNG contribute to an extended reach within the two-phase region, resulting in a calculated pressure drop that exceeds that of pure liquid methane. Variations in surface roughness influence the pressure drop by altering the frictional coefficient. Specifically, an increase in surface roughness from 2 µm to 8 µm results in a 47.8% rise in pressure drop. The proposed model demonstrates high accuracy, with deviations in the coolant temperature rise and the pressure drop being less than 9.0% and 7.6%, respectively, when compared to experimental data. The findings serve as an engineering guide for designing and optimizing heat transfer in LOX/LNG rocket engine cooling systems. Full article

High-power electric air heaters are key charging components in air thermal energy storage systems, but the dominant heat-loss regions and retrofit basis of existing devices remain unclear. In this study, a three-dimensional conjugate heat-transfer model was developed for an existing 1200 kW vertical electric air heater and validated using three steady-state experimental cases, with a maximum outlet-temperature deviation of 2.17%. Based on the validated model, temperature-field characteristics and segmental heat-loss distributions were analyzed under different mass flow rates. The results show that heat loss was highly non-uniform: Segments 2 and 3 accounted for 37.26% and 54.51% of the total heat loss, respectively, contributing 91.77% in total. A targeted local retrofit scheme was, therefore, proposed by filling the non-flowing inner-cylinder region in Segments 2 and 3 with glass wool and enhancing insulation near local cooling boundaries. After optimization, the average total heat loss decreased from 31.94 kW to 17.69 kW, corresponding to a 44.6% reduction. Under the rated condition, the outlet temperature increased from 1421.6 K to 1482.0 K, providing 584.8 kWh of additional effective thermal storage per cycle and an estimated payback period of 399 d. This study provides a diagnosis-guided retrofit approach for existing high-power electric air heaters. Full article

Vapor compression refrigeration systems account for a substantial share of global electricity consumption in residential and commercial applications, with environmental impacts arising from both energy use and refrigerant leakage. Refrigerant charge optimization offers an economical means of improving system performance without hardware modifications. Oil-free linear compressors mitigate lubricant-induced degradation of heat transfer, yet the combined influence of charge amount on the coefficient of performance (COP) and total equivalent warming impact (TEWI) has not been thoroughly quantified. An experimental investigation was conducted on a vapor compression refrigeration system equipped with an oil-free linear compressor using R134a. The experiments covered refrigerant charges of 220–330 g, piston strokes of 9–12 mm, and pressure ratios of 2.0–3.5. Component-level refrigerant distribution and system performance characteristics were analyzed systematically. The condenser holds 74.7% of the total refrigerant charge at the optimal charge of 280 g. Rising charge reduces superheat and increases subcooling, both of which serve as practical indicators of the charge level. The mass flow rate, cooling capacity, and COP all exhibit characteristic non-monotonic trends. The maximum COP of 4.67 and the maximum cooling capacity of 472.7 W are both achieved at 280 g, which is identified as the optimal operating condition. The oil-free design eliminates lubricant interference and yields a clearly condenser-dominated refrigerant distribution. The TEWI increases by only 3.6% when the charge is raised to 330 g, and this slight environmental drawback is offset by the gain in energy efficiency. A distinct COP reduction is observed at a charge of 220 g. The charge of 280 g achieves the best balance between energy efficiency and lifecycle CO₂ emissions. This work provides quantitative guidance for charge selection in oil-free linear compressor refrigeration systems. Full article

Compared with straight microchannels, wavy microchannels have been shown to significantly improve the heat transfer capability of microchannel heat sinks. The present study introduces a bidirectional curved wavy microchannel design aimed at enhancing performance. The thermo-hydraulic performance of bidirectional curved and ordinary wavy microchannels within the Reynolds number range of 300–800 is analyzed numerically under a constant heat flux. The results indicate that the bidirectional curved microchannel achieves optimal performance at an inlet velocity of 0.6 m/s. Compared with the ordinary wavy microchannel, the comprehensive performance factor of the bidirectional curved wavy microchannel with A₂ = 2 mm and λ₂ = 8 mm increases by 48% under the same inlet Reynolds number. For the preferred bidirectional curved wavy microchannel with A₂ = 2 mm and λ₂ = 12 mm, the average secondary flow intensity is enhanced by 153%, the comprehensive performance factor reaches 1.35, and the minimum entropy generation rate decreases by 6.87%. The enhanced heat transfer is attributed to the increased main flow velocity and the secondary flow intensity due to the bidirectional curve, which promotes coolant mixing. Full article

The present study investigates heat transfer characteristics in the thermally developing region of a porous tube under the local thermal nonequilibrium (LTNE) model. The influence of magnetohydrodynamic (MHD) flow on an electrically conducting fluid flowing through a porous medium under a transverse magnetic field is examined. Under the LTNE framework, two separate energy equations are employed to describe the temperature fields of the fluid and solid phases. The coupled governing equations are solved numerically using the Keller-box method. The results indicate that increasing the interphase heat transfer parameter strengthens thermal coupling between the fluid and solid phases, thereby reducing temperature differences and promoting local thermal equilibrium. In contrast, an increase in the Prandtl number reduces thermal diffusion, leading to larger temperature gradients and greater disparity between the two phases. Furthermore, the magnetic field suppresses both the velocity and temperature distributions through the Lorentz force. An increase in permeability reduces the velocity profiles due to the combined effects of the MHD and Prandtl numbers while increasing the temperature profiles. Increasing the interphase heat transfer rate drives the system from the LTNE to the LTE phase. The study confirms that LTNE effects play a significant role in thermal transfer processes in porous media and are relevant for various industrial heat transfer applications. Full article