International Journal of Thermal Sciences最新文献

As one of the most commonly used energy-saving methods in buildings, thermal insulation layer is widely used in building façade and it also endangers the human safety in buildings due to its combustibility. U-shaped facade is a commonly used structure in high-rise buildings since it could improve both the light and ventilation conditions indoors. This paper investigated the flame acceleration over the thermal insulation in u-shaped building facade fires. It was found that the flame spread rate over u-shaped facade had an exponential growth with time as the direction of preheating zone location is the same direction of the heat transfer from the pyrolysis zone. The air entrainment and the upward induced airflow played an important role in the acceleration. As the side wall length increased or the back wall length decreased, the flame spread rate accelerated more rapidly. Furthermore, a mathematical prediction model of the unsteady flame spread rate over u-shaped structure façade was established and validated through experiments. This study provides technical guidance for the fire safety design of building facade.

The gas turbine blade tip might face substantial heat loads because of leakage flow between the blades and the casing. For blade tip cooling, a composite cooling structure with film holes and broken ribs is first used on GE-E³ blade in this work. The flow and cooling characteristics of the innovative structure are studied by numerical simulation under various blowing ratio (BR) conditions. Meanwhile, the impact of modifying both the rib angle and the rib height on the adiabatic film cooling effectiveness (AFCE) at the tip of the squealer is analyzed. According to the results, adding rib structures to the squealer tip can effectively regulate the paths of cavity vortices and kidney-shaped vortex pairs (KVP) at the tip. As a result, the averaged AFCE at the blade tip is improved. The notch pressure-side broken rib structure has good aerothermal performance, and the highest AFCE at BRs of 0.50, 1.00, and 1.50 basically occur under the “R60-100 %” condition (R60 refers to the rib structure of 60°, and 100 % is the ratio of rib height to notch depth), and the corresponding AFCE are 27.71 %, 26.00 %, and 32.47 % higher than those of the no-rib case, respectively. The corresponding AFCE increased by 27.71 %, 26.52 %, and 32.47 %, respectively, compared to the no-rib condition. The highest AFCE at a BR of 1.50 occurs at “R75-70 %“, which is a 38.20 % increase in AFCE compared to the no rib case. The improvement in AFCE is due to the difference in the flow of the cooling jets, which are subject to cavity vortices at different BRs. The analysis shows that the addition of ribs disrupts the formation of KVPs and weakens the influence of the cavity vortex, thus reducing the low AFCE region at the lower end of the tip groove and increasing the AFCE. However, due to the blocking effect of the ribs, the pressure loss at the blade tip is elevated. The proposed blade tip cooling structure is expected to provide new ideas for the next generation of advanced gas turbine cooling designs.

The interfaces between SiC and the corresponding substrate largely affect the performance of SiC-based electronics. Understanding and designing the interfacial thermal transport across the SiC/substrate interfaces is critical for the thermal management design of these SiC-based power electronics. In this work, we systematically investigate the heat transfer across the 3C-SiC/Si, 4H-SiC/Si, and 6H-SiC/Si interfaces using non-equilibrium molecular dynamics simulations and diffuse mismatch model. We find that the room temperature ITC for 3C-SiC/Si, 4H-SiC/Si, and 6H-SiC/Si interfaces is 932 MW/m²K, 759 MW/m²K, and 697 MW/m²K, respectively, which is among the highest values for all interfaces made up of semiconductors (Yue et al., 2011; Cheng et al., 2020; Wilson et al., 2015; Ziade et al., 2015) [[1], [2], [3], [4]]. The ultrahigh ITC of SiC/Si heterointerfaces at room temperature and high temperatures results from the dictating elastic scatterings at interfaces. We further find the ITC contributed by the elastic scattering decreases with the temperature but remains at a high ratio of 67%-78% even at an ultrahigh temperature of 1000 K. The reason for such a high elastic ITC is the large overlap between the vibrational density of states of Si and SiC at low and middle frequencies (<∼18 THz), which is also demonstrated by the diffuse mismatch model.

Quantitative analysis of the influence of thermal barrier coating spallation on the thermo-mechanical behaviors and creep lifetime of turbine vanes is crucial for their maintenance and reliability improvement. The temperature and stress distribution of vanes with preset coating spallation damage are investigated in this study, using the thermal-fluid-solid coupling method. Additionally, a further computational analysis is conducted to predict the hazardous regions and creep lifetime of the vanes. The results indicate that coating spallation leads to significant changes of the temperature and stress distribution at the spalled regions of vanes. The remaining coating on the unspalled regions continues to provide effective protection. Stress concentration primarily occurs at the upstream and downstream of the leading edge film holes, while high-stress regions are observed between adjacent rows of film holes, forming a serrated shape. The creep lifetime of the vanes decreases significantly at the region with coating spallation. When the same spallation area is considered, the coating spallation at the leading edge has a more serious influence on creep lifetime, which is more likely to cause the vanes failure. This study reveals the influence of coating spallation characteristics on the thermo-mechanical behaviors and creep lifetime of vanes, providing valuable insights for durability assessment of coated high-temperature components.

Accurate analysis of slab heat transfer characteristics is crucial for optimal control of reheating furnaces. This paper established a half furnace model to describe the heat and mass transfer and slab step heating processes in a walking beam reheating furnace. The contact heat conduction and beams misalignment are fully considered. The distribution of total heat transfer coefficient and the effect of stepping strategies on skid mark severity are investigated. The results show that the contact thermal resistance accounts for 1/8 of the conductivity thermal resistance of skid buttons and welding pads and has a lighter effect on the skid marks. The misalignment of the water-cooled beams effectively eliminated the original skid marks and minimized the range of the new ones, but the new skid mark severity before discharge from the furnace remains at 121 °C. The total heat transfer coefficient showed a wave distribution on the slab bottom surface and varied in the range of about −0.72 to 1.71 due to the beam shielding. On the slab top surface, it followed a stepped distribution. Stepping while advancing in soaking zone effectively improve the new skid mark temperature and reduce the skid mark severity by 24 °C. Increasing the stepping number can also reduce the influence of skid marks on the temperature of slab end, thus improving the temperature uniformity of the slab head and tail.

In this study, a stable hydrophilic thin layer resembling SiO_x is formed on the copper surface by combining plasma polymerization using hexamethyldisiloxane (HMDSO) and Ar plasma activation. The effect of coating on the Heat Transfer Coefficient (HTC) and Critical Heat Flux (CHF) at two different subcooling temperatures is investigated through pool boiling experiments. It is found that the HTC and CHF of the modified surface improved by 42 % and 97 %, respectively. The chemical composition of the coating, as well as changes in surface roughness, wettability, and porosity, are studied using the Scanning Electron Microscope (SEM), Energy Dispersive X-ray spectrometer (EDX), Fourier Transform Infrared (FT-IR) spectroscopy, and contact angle measurement. The boiling/cooling experiments for the plasma-coated surface show good stability, demonstrating that the surface characteristics remain stable even after three boiling/cooling cycles.

The Oberbeck–Boussinesq equations and boundary conditions resulting from the conservation laws and thermodynamics principles provide the basis for mathematical modeling of evaporative convection in a bilayer liquid–gas–vapor system. The processes of fluid dynamics and heat and mass transfer in the volume phases and through the interface are successfully described with the help of a partially invariant solution of the constitutive equations. The solution is the efficient tool for studying regularities of physical phenomena as well as for describing heat-mass exchange processes with respect to the Ludwig–Soret mass transport and the diffusion thermoeffect appeared in the gas phase due to the presence of a volatile component. An exact solution of convection equations is derived under the assumption that evaporation/condensation is a process of the diffusive type and has an inhomogeneous character along the interface. Based on the comparison of the calculated and experimental values of the evaporation mass flow rate, the correct problem statement is specified that provides acceptable qualitative and quantitative agreement. The influence of the kinematic characteristics of the gas on the parameters of convective regimes arising in a horizontal mini-channel is investigated within the frame of the selected problem statement for the ethanol–air fluid system under the terrestrial gravity field. The topological structure of the bilayer flows, pattern of the temperature and vapor concentration fields, evaporation rate variations as well as the stability of the convective flows are analyzed with respect to different values of the gas flow rate. The destabilizing influence of the pumping gas on the threshold characteristics of the stability for the two-layer flow is ascertained. Three different wave modes of instability are predicted.

Polycarbonate panels (PC panels) are state-of-the-art transparent insulating materials widely used in the construction industry due to their cavity structure, which provides exceptional thermal insulation and optimal optical performance. However, the inherent anisotropy of the three-dimensional cavity structure complicates radiative transfer and requires consideration of both azimuth and zenith angles in optical performance evaluation. This aspect has received limited attention in existing research. This study aims to accurately characterize the optical performance of PC panels through numerical simulations. A three-dimensional radiative transfer model based on the discrete ordinate radiation model is developed to solve the radiation transfer equation. The model's independency regarding mesh division, angular discretization, and accuracy is validated. The effects of incidence angle, geometric parameters, and optical properties of PC panels on optical performance are analyzed. The findings reveal a strong correlation between transmittance and absorption with variations in incident zenith and azimuth angles. The transmittance exhibits a consistent monotonic variation expressible as a rational bifunction. Notably, absorption peaks occur within specific solid angle ranges, with increased structural complexity resulting in heightened absorption and greater uncertainty. For conventional PC materials, maximum transmittance ranges from 46.9 % to 73 %, while maximum absorption ranges from 2.3 % to 13.5 %. Increasing absorption coefficients, refractive index, and surface scattering coefficients nonlinearly decrease transmittance while increasing absorption. Additionally, deviations in transmittance and absorption with azimuth angle amplify with an increase in non-horizontal structures. Sensitivity analysis indicates a significant influence of zenith angle on transmittance, and absorption coefficient predominantly affects absorption.

Experiments were carried out to study hydrodynamics and heat transfer in a channel with discrete roughness elements on walls. Semicircle ribs with the height of 1.4 % of the hydraulic diameter of the channel were considered. Various rib geometries and positions were examined: solid ribs and ribs with slits, various streamwise and spanwise pitches, inline and staggered rib arrangements. Heat transfer enhancement and thermal-hydraulic performance were estimated. Integral and local values of the heat transfer coefficient were measured. Optical measurements yielded the fields of velocities and turbulent characteristics of the flow. Simultaneous analysis of hydrodynamic and thermal parameters of flow was carried out. Formation mechanisms of the kinematic structure of flow and heat transfer on roughened walls were considered. The distribution of heat transfer coefficient was shown to depend on the turbulent structure of flow, namely the vertical fluctuation of velocity in the near-wall region.

The study examines the coupling effect between air temperature and time-varying ventilation wind speeds within the tunnel, abstracting their influence on tunnel coldness. This investigation introduces a novel indicator—the equivalent mean air temperature within the tunnel—derived through fluid dynamics and heat transfer theories based on the principle of equivalent convective heat transfer. Case studies using the Xinjiaodong Tunnel and BSLL Tunnel illustrate the indicator's applications, including optimal anti-freezing axis identification, insulation layer thickness design, and active-controlled ventilation implementation. The optimal anti-freezing axis orientation angle for the Xinjiaodong Tunnel entrance section is 133.3°, deviating significantly by 160.8° from the actual axis, indicating a lower level of equivalent mean annual air temperature (5.5 °C) at the entrance section. This underscores the necessity to reinforce anti-freezing measures specifically at the entrance section of the Xinjiaodong Tunnel. Determining a 10 cm-thick insulation layer requirement at the Xinjiaodong Tunnel entrance section based on the equivalent mean air temperature. Through on-site investigation and published findings, it was observed that a 5 cm-thick insulation layer failed to prevent freezing, resulting in water leakage and ice formation on the lining, thus validating the calculation results. The BSLL Tunnel requires an insulation layer thickness exceeding 10 cm based on the equivalent mean air temperature, necessitating the implementation of active-controlled ventilation. Calculation results reveal that, with active-controlled ventilation wind speeds increasing from 1 m/s to 4 m/s at a temperature threshold of 4 °C, the equivalent mean air temperature during cumulative negative temperature periods within the BSLL Tunnel rises sharply from 1.0 °C to 2.7 °C. These findings demonstrate that the equivalent mean air temperature not only guides the identification of optimal anti-freezing axis, the design of insulation layer thickness considering time-varying ventilation wind speeds, and the implementation of active-controlled ventilation but also provides new methods and technologies for anti-freezing design in cold-region tunnels.