This study employed a two-fluid three-field model and numerical methods to investigate the dryout heat transfer characteristics of vapor-liquid two-phase flow in rifled tubes with varying geometric structures. The intensity of the swirling flow was quantitatively evaluated using the absolute vortex flux. The analysis focused on how rifled tube geometry influences the uneven distribution of wall temperature and dryout heat transfer characteristics. The results indicate that, compared to smooth tubes, all rifled tubes exhibit reduced wall temperature rise and delayed dryout initiation, demonstrating a significant inhibitory effect on dryout. The circumferential wall temperature shows a periodic distribution, with elevated temperatures occurring at the junction of the rib roots and sides due to fluid retention, leading to the disruption of the liquid film. As rib height increases, dryout initiation is delayed, the dryout region expands, and the wall temperature difference within the same cross-section becomes more pronounced. In contrast, an increase in rib width and rib pitch advances dryout initiation but has a negligible effect on the dryout region. Secondary flow intensity and heat transfer enhancement increase with axial position, while the degree of fluid disturbance induced by internal threads is reflected in the absolute vortex flux.

This study proposes a solar thermal-phase change heat storage packed bed system to address heating demand and centralized heating limitations in extremely cold and arid regions. A spherical-bed experimental setup and a validated 3D numerical model were used to test heat storage and simulate phase change material (PCM) melting. Parametric analyses of airflow direction, inlet velocity, PCM sphere diameter, central occlusion area ratio, tank height-to-diameter ratio (H/D), and conical taper angle. Results indicate that upward airflow reduces complete PCM melting time by 17.0% and increases the average heat storage rate by 4.7% relative to downward airflow. Increasing inlet velocity from 0.05 m/s to 0.15 m/s shortens melting time by 60.0% and enhances the average heat storage rate by 108.5%. Reducing PCM sphere diameter from 60 mm to 40 mm decreases full-melting duration by 50.4% and improves the average heat storage rate by 86.1%. Raising the central occlusion area ratio from 13.6% to 28.8% extends melting time by 10.0% while increasing the average heat storage rate by 40.1%. Elevating the H/D ratio from 0.75 to 1.25 reduces melting time by 13.7% and raises the average heat storage rate by 8.2%, albeit with a 37.9% increase in pressure drop. Finally, a conically tapered top-compared to a divergent top-mitigates thermal stratification, reducing melting time by 62.2% and increasing the average heat storage rate by 111.1%. Collectively, these results provide a quantitative basis for the performance-oriented design and optimization of solar-integrated packed-bed thermal energy storage systems.

In this study, the enhancement mechanism of single-layer hexagonal boron nitride nanosheets (BNNS) doping on the thermal characteristics of Polyethylene glycol (PEG)-based composite PCM was investigated using MD simulations. Initially, the interfacial characteristics between BN Nanosheets with varying numbers of layers and PEG were examined. It was found that BNNS exhibits the lowest interfacial thermal resistance. The influence of BNNS doping at different mass fractions on the thermal conductivity of composites was additionally examined. The results indicated that as the mass fraction of BNNS increased, the thermal conductivity of the composites was substantially improved, increasing by 40.20%, 108.85%, and 156.86% compared to pure PEG, respectively. However, excessive doping of BNNS can result in poor dispersion and a reduced rate of thermal conductivity enhancement. By analyzing the phonon density of states, it was found that the phonon matching coefficient peaks at 12 wt%, thereby effectively enhancing phonon coupling and heat transfer. Interaction energy calculations reveal that the binding strength between BNNS and PEG increases with the doping amount. The microscopic mechanism by which BNNS enhances thermal conductivity is systematically elucidated through the reduction of interfacial thermal resistance, optimization of the thermal conductivity network, and improvement of low-frequency phonon contributions.

Bubble rising near a heated vertical wall is a common scenario in industrial applications. However, the specific coupling between near-wall bubble migration patterns-governed by bubble-wall distances (S), Galilei number (Ga), and E & uml; otv & uml;os number (Eo)-and the resultant convective heat transfer characteristics remains less explored. To fill this gap, this study investigates the rise of an air bubble near a heated wall using the Volume of Fluid (VOF) method. By analyzing these factors, the study examines the migration patterns of rising bubbles and their effect on the wall-to-fluid heat transfer characteristics. The core of the heat transfer enhancement mechanism lies in the systematic disturbance of the fluid caused by the dynamic behavior of bubbles, including the rising trajectory of bubbles, bubble-induced agitation, and transition to turbulence. The results indicate that the Eo, Ga, and S are all critical factors influencing thermal performance. The heat transfer enhancement introduced by bubbles was analyzed by comparing the variation of the Nusselt number (Nu). A reduction in S has a clear enhancement effect on the heat transfer. Compared to the reference case of S = 4D (D represents the bubble diameter), configurations with S = 2D and S = D increased the instantaneous maximum local Nu on the heated wall by approximately 2.91% and 29.49%, respectively. Concurrently, the time-averaged, wall-averaged Nu increased by approximately 0.46% and 2.1%, respectively. The maximum relative enhancement in local Nu occurred in the oscillatory regime, approximately 376.22%, whereas the axisymmetric regime showed the minimum enhancement of only 27.14%.

To address the challenge of predicting the thermal-hydraulic behavior of a steam generator (SG) under seismic excitation, this study developed an integrated numerical simulation methodology. Based on the SG design of the Daya Bay Nuclear Power Plant (DBNPP), a refined unit tube bundle model was constructed. The computational framework incorporates the coupling of multiphase flow equations with the Multiple Size Group model using Computational Fluid Dynamics (CFD), and the boiling heat transfer model under oscillatory conditions was validated via a User-Defined Function (UDF). Seismic loading was simulated by applying tri-directional accelerations based on the EI Centro seismic record, and the variation in thermal-hydraulic characteristics under multiple working conditions was systematically investigated by controlling the peak ground acceleration. The results demonstrate that: the frequency-domain response of the vapor mass fraction at the outlet under both transverse and longitudinal waves is predominantly concentrated in the low-frequency region; an increase in vapor fraction along the flow path significantly intensifies velocity fluctuations in both vapor and liquid phases; the overall system temperature stability and thermal inertia of the tube bundle effectively suppress drastic temperature fluctuations at the outlet; the coalescence and fragmentation of bubbles of varying sizes are identified as the mechanism causing fluctuations in vapor-related parameters at the secondary-side outlet. This study provides a theoretical foundation for the seismic design and safety assessment of SGs in nuclear power plants.

The investigation of the thermal-hydraulic characteristics of single-phase fluids under mechanical vibration is essential for ensuring equipment safety and stability in polar shipping and marine engineering. An integrated pipeline vibration-shock experimental system was designed and constructed, which innovatively employed a complex triaxial vibration waveform as the input boundary via a dedicated vibration-shock input system to quantify the temperature and pressure responses of single-phase flow under vibrational excitation. To complement the experiments, a corresponding numerical simulation model was also established to enable in-depth analysis of the transient flow processes, thereby authentically replicating the multi-directional nonlinear vibration environment relevant to practical engineering through a combined approach. The results indicate that under short-term vibration, the temperature field maintains stability, while pressure fluctuations show a positive correlation with vibrational acceleration and flow velocity; flow velocity fluctuations depended on vibration intensity, pipeline orientation, and initial flow velocity.; good agreement was observed between the simulation and experimental data, with relative errors in pressure fluctuations of 14.60%. This work elucidates the vibration-thermal-flow coupling mechanisms, offering insights for optimizing anti-vibration design and piping system safety. Given the limitations to specific configurations and single-phase flow, future work should address complex geometries, multiphase flows, and multi-physics couplings.

Shell-and-tube heat exchangers are widely employed as heat exchange devices in industrial applications, with the optimization of their heat transfer performance representing a persistent focus of research. However, traditional heat transfer correlations often exhibit significant prediction errors and limited adaptability in complex operational conditions. In recent years, machine learning and physics-informed neural networks (PINNs) have been introduced to heat transfer modeling, yet their application in complex scenarios remains challenging. Currently, there is a notable absence of systematic research on efficient hybrid models for correcting heat transfer correlations in tube-shell heat exchangers, particularly in the context of multi-case validation and performance enhancement, which remains an underexplored area. This study develops a hybrid framework that integrates ensemble learning with the physics-informed neural network (PINN). Based on 1900 data sets, a hybrid model combining segmented correlation, PINN, and data-driven ensemble models is constructed to refine the heat transfer correlation for shell-and-tube heat exchangers. The ablation experiments demonstrate the hybrid model's superiority in all four typical working scenarios, with an average R2 of 0.9930 and MAPE of 1.66%. This hybrid model can effectively predict the heat-transfer performance of shell-and-tube heat exchangers across various engineering applications and is expected to provide valuable references for performance optimization.

Triply Periodic Minimal Surfaces (TPMS) are widely used in heat exchange due to their high heat transfer area. However, most studies ignore that sheet-TPMS has a larger heat-exchange area. In this study, we increased the thickness of sheet-TPMS and used it as a flow channel to construct Diamond and Gyroid three-channel heat exchangers. In each three-channel heat exchanger, we investigated three distinct fluid flow configurations. Longitudinal vortices are generated in the sheet-Diamond channel, significantly enhancing heat transfer performance. The results showed that sheet-TPMS channels not only have lower flow resistance but also pressure drop losses that are only 31 %-35 % of those of the solid-TPMS channel. By considering both heat transfer performance and flow resistance through j/f and j/f 1/3, the higher the value, the better the overall performance. Compared to solid-TPMS, all sheet-TPMS j/f values increased by more than 197 %, and j/f 1/3 values increased by more than 30 %. Different flow arrangements have a limited effect on three-channel heat exchangers. In the same TPMS heat exchanger, the overall performance is best when both the hot and cold fluids are arranged in countercurrent flow.

Steam generators are pivotal heat exchange devices between the primary and secondary sides of nuclear systems, making their safe and stable operation paramount. Casing once-through steam generators (OTSGs) have garnered significant attention due to their compact size, high heat exchange efficiency, ability to produce superheated steam, and elimination of the need for steam separators. This study focuses on the Russian ABV series of casing OTSG as its research prototype. A three-dimensional unit-tube model, incorporating the support plate structure, was developed using a similarity modeling method. This model was then used to simulate the coupled flow and heat transfer characteristics under primary and secondary side conditions. Subsequently, fluid temperature and pressure loads were transferred to the solid structure domain. Through thermal-fluid-solid interaction simulations, the stress and deformation distributions of the heat exchange tubes and support plates were determined. The results indicate that under coupled conditions, the secondary side wall temperature increased by approximately 70 K. In the support plate region, the primary side fluid velocity and pressure exhibited periodic variations, while the outer heat exchange tube wall temperature initially increased and then decreased. The coupled stress in the heat exchange tubes was primarily influenced by thermal stress. Mechanical stress, thermal stress, and coupled stress all showed periodic fluctuations circumferentially, with increased amplitudes at the support plate locations. When the support plate was blocked, the stress value in the contact area doubled, reaching twice that of a structure with gaps. The heat exchange tubes exhibited minimal deformation at both ends due to constraint from the end plates, with the maximum deformation, approximately 0.23 mm, occurring at the dry-out point.

Hydrothermal fluids, as significant carriers of marine renewable energy, hold importance for ocean resource utilization, and the efficient exploitation of the high temperature difference energy between them and the surrounding cold seawater is crucial. To address the technical limitations of traditional Organic Rankine Cycles and the low energy capture efficiency of thermoelectric effects, this study innovatively applies the supercritical CO2 Brayton cycle for power generation from deep-sea hydrothermal temperature differences. The design of key components, such as heat exchangers and expanders, is conducted. Consequently, a power generation system tailored for deep-sea high temperature difference energy is proposed. A thermodynamic model of the system was developed and solved using self-written Python code. Furthermore, a three-objective optimization framework for net electrical efficiency, power density and levelized cost of energy was established. To address the tendency of the Harris Hawks Optimization algorithm to be trapped in local optima, an improved algorithm, named MOHHO-SEIR, was developed by integrating multiple enhancement strategies. The Pareto optimal front was obtained within the multi-objective optimization framework. The results demonstrate that the optimization scheme derived from MOHHO-SEIR achieves superior overall performance. This study confirms the advantages of applying the supercritical CO2 Brayton cycle for power generation appled the high temperature differential of deep-sea hydrothermal vents.

The Drum level is an important parameter that affects the safe and stable operation of the marine supercharged boiler. Predicting the drum level quickly and accurately is beneficial for optimizing control and improving security of the entire system. In this paper, Pearson correlation analysis, maximum mutual information coefficient (MIC) and RF model are used to screen the characteristic parameters of marine supercharged boiler drum level. LSTM and GRU regression models were established, and drum level prediction was conducted based on three feature parameter schemes. The effects of different feature selection methods and neural network models on the accuracy and time consumption of drum water level prediction tasks were explored. The conclusion is that the RF model has the advantage of eliminating redundant parameters when selecting characteristic parameters, so the prediction error can be reduced by more than 50

Gravity heat pipes have attracted much attention in the field of geothermal energy development due to their excellent heat transfer performance and high adaptability. This present work first proposes a fusion control strategy based on the proportion of vapor and liquid phases and the balance of evaporation and condensation, and modifies the condensation mass transfer time relaxation parameter beta c in the Lee model. The accuracy of the modified model is verified by comparing it with the experimental results. Secondly, in view of the difficulties in extracting the thermal energy of submarine hydrothermal fluids, the influence of liquid filling rate, whether to install an internal flow guidance tube and the length of the flow guidance tube on the heat transfer performance of the gravity heat pipe is deeply explored based on the modified model. The research results indicate that the dynamic correction of beta c effectively controls its rate of change, thereby maintaining the heat and mass transfer balance within the heat pipe and improving the accuracy of the computational results. The modified model can control the pipe wall temperature error within +2.5 %. During the stable calculation phase, the deviation of the liquid phase fraction within the heat pipe remains stable within +10 %. Additionally, installing an appropriate length of flow guidance tube in the heat pipe can timely transport the liquid phase back to the evaporation section of the heat pipe when the liquid accumulation phenomenon occurs, thereby improving the heat transfer performance of the heat pipe. Especially when a long flow guidance tube is installed at a liquid filling rate of 0.4, the performance improvement effect is particularly prominent, and the equivalent convective heat transfer coefficient of the heat pipe increases by 27.23 %. This study can provide a reference for optimizing phase change heat transfer models and understanding the heat transfer mechanism of gravity heat pipes.

Magnesium-based metal hydride alloys have garnered significant attention in recent years due to their high hydrogen storage density and excellent safety profile, emerging as a focus in hydrogen storage. This paper presents the design of hydrogen storage heat exchange structure that integrates spiral and straight tubes, incorporating double-layer fins. The objective is to enhance the heat transfer performance of the hydrogen storage reactor, thereby improving its hydrogen storage capacity. Through numerical studies, the effects of various factors, including the flow direction of the heat transfer fluid, fins material, quantity, and dimensions, on the hydrogen absorption performance are analyzed. The results indicate that the outlet temperature of the heat transfer fluid varies with the direction of flow. During the entire hydrogen absorption, the inflow through the outer spiral tube provides greater heat exchange capacity, resulting in slightly superior hydrogen storage performance. However, the structural parameters of the fins significantly impact hydrogen storage efficiency. Specifically, under the same volume fraction, comprehensive adjustments to the height, width, thickness, and quantity of the fins lead to a reduction in the hydrogen storage reaction time from 506 s to 460 s, representing a nearly 9.1% decrease in the time required to complete the hydrogen absorption.

A numerical model is developed using the interIsoFoam solver within the OpenFOAM-v2106 framework to investigate gas-liquid two-phase annular flow in inclined tubes, with a specific focus on upward flow at five different inclination angles. The tube, which is 700 mm long with an inner diameter of 11.7 mm. The working fluid consists of air and water at atmospheric pressure, with the gas phase exhibiting a superficial velocity of 18 m/s, and the liquid film characterized by a liquid film Reynolds number (Ref) of 350. Near the inlet, highfrequency, low-amplitude initial waves are present. As the flow develops, these waves evolve into slower ripples at the top of the tube, particularly in the horizontal tube. Meanwhile, at the bottom, they transition into high-amplitude disturbance waves. This progression reflects the distinct evolution of wave types at the top and bottom as the flow progresses. The base liquid film thickness and interfacial wave amplitude in the inclined tube exhibit pronounced circumferential non-uniformity, with both decreasing gradually from the top to the bottom. This non-uniformity becomes more pronounced as the inclination angle decreases. An edge detection algorithm is used to identify the characteristic lines of ripple and disturbance waves, aiding in the investigation of circumferential wave velocity variations within the tube.

Dryout is a key limitation to heat transfer efficiency in high heat flux systems, and its triggering mechanism must be elucidated through the analysis of liquid film interfacial behavior in diabatic annular flow. This study examines vertical upward vapor-liquid diabatic annular flows of water at 7 MPa, with a mass flux of 750 kg/(m2 & sdot;s), and an inlet vapor quality of 0.5, under three distinct heat flux conditions representative of operating conditions in once-through steam generators (OTSGs). The spatial-temporal distributions of liquid film thickness are examined, along with the axial variations in base film thickness and wave amplitude. The relationship between liquid film thickness distribution and wall temperature was explored, with particular attention to the correspondence between the timing of temperature rise and the liquid film thickness reaching a local minimum. The results indicate that the mass source is higher in the base film region, and the rupture of the base film leads to direct vapor-wall contact, causing localized vapor superheating. Higher heat flux results in an increased intermittent dryout fraction.

This study proposes a fault diagnosis method, SNN-MSU(H), to address the issue that the diagnosis model and support set cannot be updated and improved during the supercharged boiler fault diagnosis process. The proposed method sends samples that are not correctly diagnosed by the model to experts for re-diagnosis. These expert-re-diagnosed samples are then collected for model re-training. The results show that as the number of test samples increases or noise decreases, the model's accuracy increases and the need for expert intervention decreases. The SNN-MSU's accuracy is greater than 95%, and the number of expert interventions in every 10 test samples does not exceed 1 when there are no samples in the initial support set. When the model is pre-trained with a few supercharged boiler fault samples, the SNN-MSU's accuracy exceeds 97%, and that of SNN-MSUH exceeds 99%. Finally, actual system tests demonstrate that SNN-MSU(H) can accurately detect new fault categories, such as valve disturbances, and request expert intervention in time. After expert intervention, the accuracy of the model diagnosis can be improved based on the knowledge provided by the experts.

The substantial thermal energy of two-phase NaCl-H2O hydrothermal fluids makes them a significant target for utilizing deep-sea energy. A fundamental understanding of the thermal performance of two-phase NaCl-H2O hydrothermal fluids during condensation is crucial for heat energy harnessing. It aims to obtain the flow and condensation heat transfer characteristics of two-phase NaCl-H2O fluids during the heat transfer process outside a spiral coil structure in this paper. The properties of the NaCl-H2O binary systems were adopted instead of pure water, and the salt transport equation was considered to reflect the impact of salinity change on heat transfer, making the simulation more representative of natural venting fluids. The results indicated a significant tangential velocity when the fluids externally swept the spiral coil. Secondly, the heat transfer area can be divided into the "normal region" and the "island region," of which convection and condensation contribute to the heat transfer mechanisms. The "island region" formation was attributed to both vapor condensation and the spoiler effect caused by the spiral structure. Moreover, the heat flux increased with decreasing salinity in the "island region," and a weak interaction was observed between heat flux and salinity in the "normal region." Finally, the spiral structure's heat transfer performance was superior to the straight structure's. This investigation may provide a basis for designing and optimizing the heat transfer equipment for deep-sea hydrothermal extraction.

Deaerator is a key equipment in the secondary circuit system, its operating parameters and structural size have a significant impact on the thermal efficiency and arrangement rationality of the system. In this research, a mathematical model of the marine nuclear power deaerator is established, and the influence of thermal and structural parameters on the weight and volume of the deaerator is analyzed. An improved particle swarm optimization algorithm is proposed by introducing Tent chaotic mapping, evolutionary factor and Metropolis criterion, and its performance is verified. Taking the weight and volume minimization of the deaerator as the optimization objectives, the optimal design of the deaerator is carried out using the proposed IPSO algorithm, while satisfying the structural and performance constraints. The optimization results show that the volume and weight of the deaerator can be reduced by 12.979% and 10.213%, respectively, and the feasibility of the optimization design method is proved theoretically.

This numerical study investigated the wall-to-fluid heat transfer enhancement caused by an air bubble rising with an oscillatory trajectory. The geometric volume of fluid method, isoAdvector, is used to simulate a bubble rising near a heated wall in a static fluid, considering different bubble-wall distances (S), Galilei (Ga) number, and E & ouml;tv & ouml;s (Eo) number. The results indicate that when the distance between the bubble walls is relatively small, the repulsive force from the walls predominates, causing the bubble to move away from the wall and thereafter spiral upwards initially. As the distance increases, the bubble movement approaches a stable periodic spiral trajectory, and the influence of wall disturbances diminishes. The rise in the quantity of Eo exacerbates bubble deformation, resulting in increased resistance and a decrease in ascent velocity. Simultaneously, the shedding frequency of the vortex increases, further disrupting the thermal boundary layer. An increase in the Ga number accelerates the shedding frequency of the tail vortex and significantly amplifies the disturbance intensity of the flow field. Compared to the case of S = 4D, the mean maximum Nusselt number on the wall surface increases by 27.07% and 11.70% for S = 2D and S = D, respectively. Compared to Ga = 50, the mean maximum Nusselt number at the wall increases by 187.58% and 238.71% for Ga = 279.58 and 559.17, respectively. Compared to Eo = 0.54, the average maximum Nusselt number of the wall for Eo = 1 and Eo = 5 increases by 2.69% and 17.16%, respectively.

Supercharged boilers are susceptible to seismic impact loads, which can cause severe structural responses and exceed their ultimate bearing capacity. Compared with the traditional simplified structural model for earthquake resistance research in cold state, this paper establishes a real and complete three-dimensional physical model of the supercharged boiler base, and uses the reverse calculation method to obtain the boundary conditions of the boiler based on the temperature of the experimental measurement points. The temperature field and pressure field of the boiler are reconstructed, combined with the coupling method of thermal mechanical solid multiple physical fields. Using time-domain data of peak acceleration in various directions under typical earthquakes as external input loads, this study explores the seismic characteristics of boilers in the X, Y, and Z directions using time-domain analysis methods, and clarifies the vibration response mechanism in the stress concentration area of the tube sheet. The results show that the measurement and simulation error of the temperature of the inner and outer walls of the drum is 0.10 %; The individual effects of thermal stress and mechanical stress account for 107.56 % and 66.46 % of the total stress, respectively. Thermal stress plays a dominant role, while mechanical stress produces impedance effects; Under the action of an earthquake, the tube sheet undergoes reciprocating oscillation motion, and the impedance effect generated by the flexibility of the tube bundle gradually attenuates. The local stress in the X and Y directions increases by 7.70 similar to 8.27 MPa, while the local stress concentration phenomenon occurs in the Z direction due to low damping and stiffness.