International Journal of Heat and Fluid Flow最新文献

The paper introduces a novel composite heat transfer structure integrating sensible heat and latent heat utilizing water and phase change material, facilitated by a high thermal conductivity contact melting process. A numerical model is developed and validated. Optimization of the trapezoidal structure for the refractory zone during PCM melting ensures the volume proportion of water and PCM remains unchanged. The study compares and analyzes the melting properties of different structures (melting time, heat charging rate, energy storage rate of different media, dimensionless temperature response, etc.), and explores the impact of heat source temperature and initial temperature conditions. The findings indicate that adding water enhances the thermal conductivity and convective effect of upper PCM, however, at the end of melting, the square structure exhibits a refractory zone. It is worth noting that compared with the square initial structure Case 0, the positive trapezoid structure (Case 3) reduces the heat storage time of PCM by 6.14 % and increases the average heat storage rate of PCM and water by 5.81 % and 4.62 %, respectively, while the inverted trapezoid structure weakens the heat transfer process. Additionally, an increase in heat source temperature from 340.15 K to 354.15 K leads to a 67.10 % rise in the mean heat transfer rate of PCM and 101.75 % for water. Conversely, augmenting the initial temperature negatively affects the heat transfer rate and total heat storage of water, while reducing the melting time. This study holds significance for the development of new contact melting methods and enhancing heat transfer mechanisms.

The dual optimization of heat transfer and fluid flow constitutes subjects of paramount interest in heat exchange equipment. Non-uniform thermal boundaries are common in supercritical heat exchangers, interacting with drastic changes in properties to make the heat transfer process very complex. This paper first reviews the forms of boundary conditions in supercritical heat exchangers, including plate heat exchangers, shell-and-tube heat exchangers, printed circuit heat exchangers and some emerging heat exchangers. It attempts to translate structural impacts into non-uniform thermal boundary conditions. Then, based on single-tube analysis, the effects of axially and circumferentially non-uniform heat flux on the flow and heat transfer characteristics of supercritical fluids are examined. To reduce wall temperature and improve heat transfer performance, some methods for optimizing structures are introduced. Finally, referencing constructal theory, some optimization strategies and evaluation methods from laminar heat exchangers, which are similarly sensitive to thermal boundary conditions, are introduced into the supercritical domain. This establishes the spatiotemporal matching theory of supercritical substance flow and energy flow, providing reference and guidance for the intelligent design and safe, efficient operation of supercritical heat exchangers. Additionally, some reasonable suggestions for future research are given, including study and utilization of the supercritical thermal entrance effect and the exploration of optimal heating strategies.

This study investigated effects of flow pulsation on heat transfer performance of the surface with teardrop-shaped dimples. The flow structures and heat transfer characteristics were simulated by large eddy simulation with a Lagrangian dynamic sub-grid scale model. The cases of steady flow and pulsating flow (the Strouhal number of 0.3 and rms velocity amplitude normalized by bulk velocity of 0.14) were examined for dimple inclination angle of 30 deg, 45 deg, and 60 deg with in-line arrangements and for the bulk Reynolds number of 25,000. Surface-averaged results indicated that the flow pulsation increased the Nusselt number ratio by 9–12 %, the friction factor by 18–21 %, and the heat transfer efficiency index by 3–6 %. Using the phase-averaged results, it was clarified that the increased Nusselt number was due to the appearance and disappearance of flow-separation bubbles induced by the flow pulsation at the leading edge of inclined dimples and the time-averaged swirling flow intensity was well correlated with the surface-averaged Nusselt number and the friction factor.

Regulated multi-stage turbocharging is an indispensable technology to achieve high boosting pressure and hence power recovery for High-Altitude Long-Endurance Unmanned Aerial Vehicle (HALE UAV). Inevitably, unsteady interstage coupling has become a key factor restricting the pursuit of higher aerodynamic performance in multi-stage turbochargers. This study is to investigate the unsteady gasdynamic behavior of two-stage radial turbines and aimed to obtained the knowledge of unsteady aerodynamic interaction at pulsating conditions. Results reveal an obvious difference in the performance discrepancy mechanisms of high-pressure turbine (HPT) and low-pressure turbine (LPT). HPT shows more unsteady than that of LPT according to the discussion on mass accumulation and local temporal gradient. Furthermore, this unsteadiness is drastically enhanced when the valve is open. For HPT with open valve, a sharp reduction of up to 10.7% in the cycle-averaged efficiency is observed. Both the volute and the rotor are the main components causing the deterioration of efficiency. Swirling flow precession leads to a dramatically loss generation in the volute. Leading-edge separation and tip leakage flow are the dominated factors in rotor. The former is primarily attributed to inlet swirling flow and the latter is predominantly attributed to the pulsating effect. For LPT, the performance is not sensitive to the valve state, with no more than 3% discrepancy on efficiency. At specific velocity ratio, the negative swirling flow tends to exert a positive effect to the rotor efficiency, whereas the positive swirling flow leads to a higher entropy generation in rotor passage.

A flexible wing with a large aspect ratio emerges in many modern engineering applications (e.g. solar planes and super large wind turbine blades) and its interaction with incident flow differs markedly from conventional fluid–structure interactions (FSI), exhibiting frequently a large three-dimensional (3D) deformation. Yet, there is little information on how such deformation may change wing aerodynamics. This work investigates the aerodynamic performance of deformed and cantilever-supported NACA0012 rigid wings with an aspect ratio of 9, using ANSYS Fluent with the SST-κ-ω turbulence model at a chord-length-based Reynolds number Re_c of 1.5 × 10⁵. Numerical simulation is validated experimentally. The wing tip bending displacement is up to 4.14c, and the maximum twisted angle is up to 7°. The angle α of attack varies from 0° to 20° at mid span of the wing. It has been found that the torsional deformation can significantly advance the local flow separation, reattachment, bubble length, and transition from laminar to turbulence, resulting in a drop in the critical angle α_cr of attack, at which the separation bubble size reaches the maximum. Accordingly, the lift and drag coefficients increase, as well as the bending and pitching-up moments, though the stall is advanced due to a change in local α. The tip vortex is also enhanced, inducing strong downwash postponing the separation bubble on the wing and resulting in redistributed force near the wing tip that increases markedly the local bending moment but decrease the local pitching-up moment. On the other hand, the bending deformation tends to produce an effect opposite to the torsion on the flow structure and causing little change in the lift coefficient, though reducing the induced drag and moments appreciably. With both deformations in place, the torsion overwhelms the bending in general in terms of its impact upon aerodynamics and flow structures, the latter acting to cancel at least partially the effect of the former.

In this paper, a new kind of bionic spoiler rib based on the shape of a shark shield scale is proposed, and the structural parameters of the bionic rib with optimal jf factor are determined. The thermal properties of bionic fins with different lengths, widths, and heights and plate-fin heatsinks with different positions were studied utilizing computational fluid dynamics. Then the optimal structural parameters of the bionic fins were obtained by the Taguchi method and multi-objective optimization. The study finds that the jf factor of the bionic rib decreases by up to 15 % with increasing rib height at Reynolds number 6000. Conversely, it increases by up to 10.38 % with increasing rib length, decreasing by up to 2.07 % with increasing rib width at the same Reynolds number. In terms of location, the bionic rib can exert its effect of enhancing the jf factor when it is close to the channel outlet. Finally, using the Taguchi method and multi-objective optimization method, the optimal structural parameters of the bionic rib are H_r = 6.0 mm, W_r = 3.47 mm, and L_r = 3.0 mm when Re = 6000. The findings of this study can provide insights for research on flow spoiler structures.

This study investigates the manipulation of convective heat transfer through spanwise wall oscillations in a turbulent channel flow. Direct numerical simulations are performed at $R{e}_{\tau }=180$ and $Pr=1$.

The primary focus of this work is to explore the heat transfer response to oscillation parameters that promote drag increase, a regime that has received limited attention. By adopting an extended oscillation period ($T^{+}=500$) and amplitude ($W^{+}=30$), which have been reported to enhance drag, a remarkable dissimilarity between momentum and heat transport emerges. Under these conditions, the convective heat transfer undergoes a substantial 15% intensification, while the drag increases by a comparatively moderate 7.7%, effectively breaking the Reynolds analogy. To elucidate the physical mechanisms responsible for this dissimilar behaviour, a comprehensive statistical analysis is conducted. The control effect on the near-wall streaks and the associated mixing of momentum and heat is investigated by examining the energy distribution across scales and wall-normal locations. This analysis provides valuable insights into the control’s impact on the turbulent structures. Furthermore, the correlation between wall-normal velocity fluctuations and both streamwise velocity and temperature fluctuations is scrutinized to understand the modification of sweep and ejection events, which drive the transport of momentum and heat. The Fukagata–Iwamoto–Kasagi (FIK) identity is employed to identify the contributing factors to the changes in drag and heat transfer. The analysis highlights the importance of the pressure term in the streamwise velocity equation and the linearity of the temperature equation. Further investigation is necessary to fully unravel the complex mechanisms governing the decoupling of heat and momentum transport. The results of this study underscore the potential of using unconventional spanwise wall oscillations parameters to preferentially enhance convective heat transfer while minimizing the associated drag penalty.

Pores are a common class of structures with enhanced heat transfer properties. The pores on the surface can usually be categorized into two types: dilated pores and contracted pores. In order to deeply investigate the effect of the surface on the phase change of the liquid layer during the heating and cooling process, this study examines the successive processes of gradual heating of the heat source, constant temperature of the heat source and cooling of the heat source. The boiling heat transfer characteristics at the solid–liquid interface and the evaporation characteristics at the liquid–gas interface were studied in depth by calculating these parameters such as the water layer heat flux, the wall heat flux, and the number of evaporating water molecules. The results show that wettability affects the time, position and kinetic energy of bubble generation. It is notable that a surface with a dilated pore structure and composite wettability exhibits superior heat transfer characteristics compared to a single hydrophilic surface. In addition, this nanoscale porous surface exhibits excellent heat transfer performance at lower wall temperature conditions, making it ideal for applications at higher heat source temperatures. It is worth noting that both the hydrophobic dilated pore structure and the contracted pore structure possessing the hydrophobic edge are not favorable for heat transfer.

The development of turbine loss models, especially the loss models under the influence of film cooling were reviewed. From early loss models with only aerodynamic analysis to film cooling loss models, a series of experimental and numerical simulation validation, application of film cooling loss models at present were summarized and limitations of the current film cooling loss models were exposed. Development of film cooling loss models was found to have a delay in experimental application, though the mixing process had three-dimensional assumption. The feasibility of loss models needed to be improved. Some recent models with targeted optimization algorithms were gathered and analyzed. It can be predicted that the newly proposed film cooling loss model will be more closely related to optimization algorithms with more realistic assumptions and more detailed analyses. and will have more practical application value such as being able to be validated in experiments under extreme conditions.

The higher the injection pressure, the more serious the cavitation phenomenon of the injector control ball valve, which seriously affects the emission and reliability of the diesel engine. The selection of turbulence and cavitation models is the key to study the above-mentioned cavitation problems using numerical methods. Based on the Winklhofer micro-channel fuel test, four turbulence models and two cavitation models with strong representation are used to construct a micro-channel model, and the simulation results are compared with the test results. The combination of the LES and ZGB model is more accurate for the calculation of mass flow at the outlet and the cavitation distribution of the micro-channel. The combination of the SST k-ω and the SS model is more accurate for the calculation of flow rate at the micro-channel cross-section and the pressure gradient inside the micro-channel. The combination of LES and ZGB model is more suitable for numerical simulation of control ball valve. The numerical simulation of the control ball valve is carried out by using combination of LES and ZGB model, and the visualization test of the actual size injector control ball valve is verified, with good consistency. The conclusions of the study provide guidance for the simulation analysis and design of injector control ball valve.