International Journal of Heat and Fluid Flow最新文献

We conducted a direct numerical simulation (DNS) study to investigate the impact of surface undulation steepness on rough wall turbulent heat transfer. The flow geometry was turbulent open-channel flow over three-dimensional sinusoidal rough surfaces. To examine the effects of steepness, we systematically varied the streamwise and spanwise wavelengths of the sinusoidal roughness while keeping the roughness height constant. The friction Reynolds number ranged from 180 to 600, and we considered a passive scalar with the fluid Prandtl number was 0.7, assuming air flow conditions. In the fully rough regime, the velocity roughness function is expressed as a function of the inner-scaled equivalent sand grain roughness $k_s^{+}$ independent of steepness, whereas the steeper surfaces with shorter wavelengths result in larger temperature roughness functions at the same $k_s^{+}$ value. Analysis of the physical mechanisms that increases the roughness function shows that the pressure drag primarily contributes to the increase in the velocity roughness function, while the temperature roughness function is mainly augmented by the roughness-induced wall heat transfer term, correlating with the steepness of the surface undulations. It is also suggested that the effective slope, which quantifies the steepness of rough surfaces, could improve the predictive accuracy of existing correlations for the temperature roughness function.

This paper introduces an original plate-fin precooler designed for potential applications in extreme low-pressure environments. Utilizing additive manufacturing technology, the heat transfer plate thickness is constrained to 2.0 mm, with channel widths inside the plate achieving 0.8 mm. Through experimental methods, this study aims to assess the precooler’s performance and identify critical influencing factors. Experimental results indicate that the hot fluid inlet temperature to the precooler exceeds 1100 K, with static pressures dropping below 30 kPa. Despite these conditions, the precooler demonstrates an impressive pressure recovery coefficient exceeding 97 % and achieves a maximum temperature drop of 731.4 K for the hot fluid. Furthermore, it is observed that the overall performance of the precooler diminishes with increasing mass flow rates of the hot fluid, showing fluctuations of up to 25 % when assessed by the j/f^1/3 factor. Additionally, while the hot fluid inlet velocity exceeds 90 m/s, laminar flow predominates during the heat transfer process. Moreover, regardless of whether the cooling fluid experiences a phase change within the precooler, its heat transfer performance show priority than that of the hot fluid. Thus, changes in the mass flow rate of the cooling fluid have minimal impact on the overall precooler performance. Finally, the first-stage heat exchanger plays a critical role in the heat transfer process, accounting for over 2/3 of the total temperature and pressure drop for the hot fluid. This research is expected to contribute to the design of high-efficiency, low-resistance precoolers, particularly those applied for operation under negative pressure conditions.

The application of an electric field in flow boiling has been proven to effectively enhance heat transfer and reduce pressure drop instability. This study aims to elucidate the mechanism of the impact of electric fields on flow boiling bubble dynamics through the pseudo-potential lattice Boltzmann method (LBM). The interplay between the externally imposed electric field incoming velocity in different gravity conditions examined. These factors can regulate flow boiling heat transfer in horizontal channel. The results demonstrate a competitive relationship between electric field and gravity and between incoming velocity and gravity. Therefore, under higher gravity condition, an electric field is less effective to enhance flow boiling heat transfer than in low gravity condition and vice versa. Additionally, there exists a synergistic relationship between incoming velocity and the electric field that mitigates their competition. Moreover, when considering multipoint nucleation processes, applying an electric field can attenuates bubble–bubble interactions and inhibit large bubble formation so as to accelerates bubble condensation in supercooled flows and enhance boiling heat transfer. This work provides comprehensive physical insights into the mechanism of electric field to enhance the heat transfer in flow boiling, which is instructive for the development of electrohydrodynamic technique in flow boiling enhancement.

The current study proposed a novel crescent-dimpled film cooling hole and investigated its aerothermal performance on the turbine endwall when a row of the film holes are arranged in front of the stator. The mainstream Reynolds number based on the inlet velocity and the axial chord length of the vane was 150,000, and the blowing ratio ranged from 0.5 to 1.5. Besides, a hybrid configuration combining the advantages of the cylindrical and crescent-dimpled holes was also investigated. RANS simulations using Shear Stress Transport (SST) k–ω turbulence model were conducted. The numerical simulations show that the pure crescent-dimpled hole design enhances the adiabatic film cooling effectiveness by 32.3 %, 52.3 %, and 43.6 % at blowing ratios of 0.5, 1.0, and 1.5, respectively. Correspondingly, the net heat flux reduction (NHFR) values are 22.9 %, 57.9 %, and 63 % higher than the cylindrical holes. The high film cooling effectiveness behind the film cooling hole prevents the additional thermal load caused by the dimple-induced heat transfer enhancement. Using cylindrical holes near the leading edge and crescent-dimpled holes elsewhere, the hybrid arrangement suppresses the passage vortex and further enhances the film cooling effectiveness and NHFR by 51.9 % and 93.8 % at BR = 1.5, respectively. The streamlines and vortex structures show that the crescent dimple at the hole’s exit diffuses the coolant, thereby enhancing the film cooling in the lateral direction. Flow separation occurs behind the dimple, which reduces the jet momentum and attracts the jet towards the wall. The curved surface of the dimple directs the horseshoe vortex in front of the jet to the side, and anti-CRVP is formed. These are responsible for the film cooling enhancement by the crescent dimple.

Two-dimensional Reynolds-Averaged Navier–Stokes simulations are performed to study a single, round impinging jet heat transfer problem, utilizing the generalized k-omega (GEKO) turbulence model as a benchmark. The simulations are performed at a jet Reynolds number of 23,300 and a nozzle-to-plate distance of 2.0 where a second peak in surface Nusselt number is observed. The effects of the three primary ($C_{sep}$, $C_{mix}$ and $C_{nw}$) and three auxiliary ($C_{bf,l}$, $C_{bf,t}$ and $C_{nw,sub}$) GEKO calibration parameters are investigated. The results indicate that $C_{mix}$ has the most significant impact on the laminar-turbulent transition zone. A deep learning based regression model is developed and trained using the simulation outputs for fast predictions of the heat transfer curve. Using $C_{sep}=1.1$, $C_{mix}=-0.7$, $C_{nw}=2.0$, $C_{nw,sub}=2.25$ and $C_{bf,t}=3.0$ along with laminar-to-turbulent transitional modeling values, provides the best agreement with experimental results from previous studies.

We discuss wall-modeling of turbulent heat transfer of water channel flows with temperature-dependent viscosity via direct numerical simulations. We considered a top-cooled wall (293[K]) and a bottom-heated wall (353[K]) and varied the friction Reynolds numbers from 300 to 1000. The fluid viscosity varied depending on the local fluid temperature, whereas the other physical properties were assumed to be constant. The results show that semi-local scaling based on the local viscosity and wall friction velocity reasonably accounts for the effects of variable viscosity on turbulent flows, except in the vicinity of the wall, where wall cooling intensifies the turbulent vortical motion, leading to increased semi-locally scaled eddy diffusivities compared with those near the heated wall. In the vicinity of the cooled wall, turbulent transport is enhanced by increased viscous transport, which transfers more turbulent kinetic energy toward the cooled wall. The effectiveness of semi-local scaling for wall-modeling was validated by performing a wall-modeled large-eddy simulation at $R{e}_{\tau }=1000$, where we incorporated the semi-local viscous length scale into the classical mixing-length model. The modified mixing-length model reasonably reproduced the effects of variable viscosity on turbulent flows.

In practice, film cooling on gas turbine blade inevitably works in an unsteady environment, which is introduced by periodical rotor/stator interaction and unsteady combustion. Previous studies have introduced two methods to simulate the realistic unsteady condition: 1) the pulsation of mainstream velocity; and 2) the pulsation of coolant injection. However, up to this point, the differences in instantaneous film cooling behaviors between these two methods remain unclear. This work presents a series of large eddy simulations to exhibit the unsteady flow and film cooling behaviors under steady and the two unsteady flow conditions. The numerical strategy is validated against our time-resolved experimental data. Time-averaged results show that the difference between the two pulsations is not significant if the averaged blowing ratio remains the same. However, the pulsation type plays a dominant role on the transient mode of film coverage. Under the steady condition, film coverage instability is induced by the unsteady trajectory of near-wall vortex structure; but with pulsed environments, the unsteadiness magnitude increases, and the area with high unsteadiness level enlarges. The pulsation of the mainstream velocity induces a more severe film coverage instability compared to the pulsation of the cooling air injection, because of the higher fluctuation energy of the mainstream bulk. Under mainstream pulsation, the probability distribution of instantaneous cooling effectiveness is the most scattered, and the corresponding fluctuation range is the largest.

In engineering practice, thermal analysis of objects with unknown internal structure and/or thermophysical properties, and uncertainties in contact thermal resistances, is very challenging and even impossible using the traditional approach of direct solving the heat transfer equation. In this work, a thermal response function method (TRFM) is proposed for predicting the transient surface temperature of ‘black-box’ objects (i.e., unknown internal structure and thermophysical properties). The method relies on an introduced measurable quantity called thermal response function, which characterizes the thermal response characteristics of an object. Using the measured thermal response functions as input parameters, the transient temperature distribution on the surface of a black-box object under arbitrary external heat flux boundary condition can be predicted through linear superposition. Proof-of-concept simulations and experiments are conducted to demonstrate the feasibility and effectiveness of the TRFM method. The predicted surface temperature distribution under various external heat fluxes using TRFM agree well with the reference results. The results show that the TRFM is very promising as a solution of the challenging problem of predicting the transient surface temperature of black-box objects, with potential application for thermal imaging modeling of complex objects.

Electronic device advancements in power, size reduction, and integration have resulted in increased heat flux and operating temperatures, negatively impacting the reliability of electronics. Ionic wind cooling is a new, environmentally friendly, and energy-efficient thermal management approach that effectively cools high heat flux local heat sources. However, enhancing ionic wind strength continuously can be challenging. In this study, an ionic wind blower consisting of an emitting electrode, a collecting electrode, and auxiliary electrodes is constructed. The experimental verification confirms the supplemental acceleration capacity of the auxiliary electrodes. When determining the optimal operating voltage applied to the auxiliary electrodes, the power consumption of the system and the intensity of the output ionic wind are taken into consideration. The blower’s operational and structural factors, such as the emitter structure, discharge gap, and distance between the auxiliary electrodes and collectors, are optimized according to how they affect the device’s functional qualities. The improved blower’s heat dissipation ability is evaluated by cooling an LED chip. The results demonstrate that the system performs optimally with an emitter having seven needles, a discharge gap of 5 mm, and 9 mm between the auxiliary electrodes and the collector. The wind speed reaches 2.47 m/s, while the power consumption is only 1.6 W. Compared to the absence of auxiliary electrodes (47.7 W/(m²∙K)), the system’s mean convective heat transfer coefficient can reach 61.12 W/(m²∙K), resulting in a temperature reduction of the LED chip by up to 41.6 °C. With increasing voltage, the heat transfer enhancement ratio improves, enabling a blower with auxiliary electrodes to provide significant cooling while consuming less power.

As a non-toxic, non-combustible natural working fluid, CO₂ is widely used in kinds of new power generation systems and low-grade waste heat recovery due to its stable chemical properties and excellent thermophysical properties, which not only significantly reduces the volume of the thermal system, but also effectively improves the circulating thermal efficiency. The thermophysical properties of supercritical CO₂ change drastically with temperature near the pseudo-critical point (T_pc), generating a complex boundary layer structure that triggers heat transfer enhancement and deterioration. Heat transfer deterioration typically manifests as a sudden increase in wall temperature and a corresponding decline in the heat transfer coefficient. This leads to irreversible losses in the heat transfer process, resulting in heightened system circulation, reduced thermal efficiency, accelerated tube corrosion, and, in severe instances, poses a significant threat to system safety, potentially resulting in tube bursting and considerable harm. Therefore, understanding and mastering the flow and convective heat transfer characteristics of supercritical fluids in tubes is the basis for designing more efficient heat transfer structures. This paper provides a comprehensive overview of the mechanisms underlying heat transfer deterioration in supercritical CO₂ systems, along with various strategies to enhance heat transfer efficiency. Additionally, it discusses the current state of research on Helmholtz self-oscillating cavities, which can serve to inhibit heat transfer deterioration in supercritical fluid tubes. This research not only serves as a reference for improving system performance but also offers new insights into the exploration of more efficient heat transfer technologies.