International Journal of Thermal Sciences最新文献

The present study analyze the thermal behaviour of multiple jets impinging on a convex heated surface for two different nozzle geometries (circular and elliptical) and its orientations. Tests are performed with 5 different nozzles at different values of non-dimensional nozzle to surface distance (z/d = 2–10) and Reynolds number (5000–28000). At smaller value of non-dimensional nozzle to plate distance, a distinct pattern of temperature variation is observed that depend on the nozzle shape and orientation, and this pattern diminishes as surface to nozzle distance increases. In the farthest region, elliptical nozzle is found to improve the uniformity (up to 60 %) in Nu variation compared to the circular jets even at the largest surface to nozzle distance. The overall heat transfer is found to increase up to 18 % and the uniformity is found to enhance up to 60 %) for N-2 and N-3 nozzles. An improvement in the thermal performance is observed in the elliptical nozzle in the fountain and impingement zones. The non-uniformity in the Nu behaviour is found to increase with the increase in Reynolds number.

Applications requiring high heat transfer rates, such as cooling of high-density electrical equipment, cooling of gas turbine components, cooling of rocket launcher components, cryosurgery, etc., are frequently use impinging jets. Non-uniformity in the heat transmission from the impingement surface is the main drawback of jet impingement heat transfer. In order to achieve uniform heat transfer, the current study examines the presence of porous carbon foam on a targeted surface. Using a thin metal foil and infrared thermography, the local heat transfer distribution of a porous carbon foamed surface is determined. The findings of the porous carbon foamed surface are compared to the bare surface (smooth surface without foam) for local Nusselt number and uniformity in the heat transfer (coefficient of variance). The effects of Reynolds number, foam height, and the distance between the nozzle exit to the targeted plate are examined. The results of the carbon foamed surfaces are also compared with the aluminium metal foamed surface results available in the literature. The current work also describes the separation of the modes of heat transfer that exist with porous carbon foamed surfaces while under jet impingement. The findings imply that, depending on the height of the carbon foam, the porous carbon foam on a targeted surface gives a lower or equivalent heat transfer rate compared to a bare surface. In comparison to a bare surface, carbon foam on a targeted surface provides uniform heat transfer that is independent of foam height. The study of the separation of modes of heat transfer suggests that heat from the porous carbon foamed surface is conveyed by conduction induced by carbon foam and convection induced by jet fluid. The convection provided by the jet fluid is compromised by the carbon foam on a targeted surface. The conduction induced by carbon foam makes the heat transfer from the targeted surface more uniform. The conduction and convection factors can be used to present the conduction and convection heat transfer from porous carbon foamed surfaces, respectively. Regression analysis is used to develop a region-wise correlation for the conduction and convection components. The local Nusselt number of a carbon foamed flat plate can be predicted using the local Nusselt of a bare surface utilizing the provided correlations for conduction and convection factor.

Airflow in wall-to-wall junctions is known to have a major hygrothermal impact on building performance. However, current and validated modeling options to simulate such phenomena are limited. This paper develops and compares two numerical models to study the heat and moisture transfer due to air infiltrations through a prefabricated wall-to-wall junction. The first model explicitly accounts for the airflow with a pipe flow approach. The second model is a modification to a typical approach to simulate ventilated cavities in building envelope simulation tools and mimics the effect of the airflow through source terms. Both approaches were introduced in a heat and moisture transfer 2D finite element model. Additionally, laboratory measurements were conducted in a climatic chamber to validate the simulation results. Six scenarios were tested experimentally under steady-state conditions. These datasets were used to calibrate different parameters of the models, such as material properties, the junction air gap thickness, and the magnitude of the heat and moisture source terms. Both sets of numerical results provided reasonable agreement with the measurements. The first approach outputs more accurate temperature and relative humidity values than the second one. However, considering uncertainties, no method predicted a perfect fit with the relative humidity profiles. Close to the junction, the first method estimates better the relative humidity than the second one. This work provides guidelines to better model and account for wall junctions in building envelope simulators.

In the two-dimensional heat transfer experiments of aero-engine rotating disk cavities, the inverse heat transfer problem method can be used to obtain the wall heat flux numerically, which uses the two-dimensional measured wall temperature to solve the rotating disk heat conduction equation. A back propagation (BP) neural network data approximation method is proposed to reduce the ill-posedness of the two-dimensional inverse heat transfer problems in rotating disk cavities in this paper. The priori knowledge of wall temperature characteristics expressed by two-dimensional wall temperature first-order radial partial derivative distribution is used for BP neural networks’ regularization. The distribution characteristics of the wall temperature first-order radial partial derivative in a typical preswirl rotating disk cavity were investigated by the flow-thermal coupling numerical simulation. Based on these characteristics, the BP neural network construction and training method with uncertain regularization coefficient is adopted. The numerical experiment results show that compared with the traditional polynomial fitting methods, the BP neural network approximation methods in this paper show significant advantages in data processing performance and stability; The fluctuation amplitude of the wall heat flux relative error on the disk surface can be reduced by 1–3 orders of magnitude, reducing the relative error of wall heat flux in most areas of the disk to within 20 % of the original value; The maximum wall heat flux relative error suppression area where |δq_r,cal/δq_r,mea × 100 %| < 100 % of BP neural network approximation method can reach 1.93 times that of the traditional fitting method, and 3.18 times for the area where |δq_r,cal/δq_r,mea × 100 %| < 30 % in the current study.

A complete lattice Boltzmann model combined with immersed boundary method (LB-IBM) is developed to address radiative heat transfer problem in irregularly shaped media. This method investigates radiative heat transfer in two-dimensional uniform/gradient refractive index media with various geometric shapes. The thermal effects generated by irregular boundaries are represented in the form of thermal density and interpolated onto adjacent lattices in the lattice Boltzmann model (LBM). Then, the four-point discrete delta function is used as the interface scheme of the immersed boundary method. Therefore, the standard LBM can effectively solve radiation problems in irregular geometries. The accuracy of the LB-IBM is validated through a comparative analysis with the results predicted by the finite volume method, embedded boundary method, and other numerical methods. Moreover, this paper promotes the application of LBM in radiative heat transfer in irregularly shaped media by providing a straightforward and efficient mesoscopic tool. This lays the foundation for establishing a framework of LBM for unified treatment of convection, conduction and thermal radiation.

A comprehensive understanding of the development characteristics of multiple fires in tunnels holds significant importance in estimating the thermal safe distance required for both people and facilities. In this paper, a series of numerical and experimental works are performed to examine the ceiling gas temperature, fire merging, and flame length of twin fires in a tunnel. Varied thermal hazard scenarios were simulated by altering the ambient pressure, heat release rate, and pool spacing. The findings indicate that as the ambient pressure reduces, the air entrainment coefficient decreases, resulting in a higher ceiling gas temperature. Large pool spacings demonstrate two peak impact points in ceiling gas temperature. However, as the pool spacings decrease further, only one peak impact point appears above the center of two fire sources. As pressure mounts, the low-oxygen zone at the tunnel ceiling contracts progressively, and it primarily appears in the additional region between two fire sources. The temperature processing method is adopted to determine the fire merging and flame length. The fire merging probability is predicted by introducing a piecewise model. Furthermore, a physical model is proposed based on the air entrainment theory to establish the relationship between flame length and the effects of pool spacing, ambient pressure, and heat release rate, which can be applied to both open spaces and tunnels.

In this work, we explore the possibility that a hexagonal ring structure can be used as a solar absorber and a thermal emitter for multiple applications. By using FDTD (finite-difference time-domain) Solutions for numerical simulation, the light source is set to 280 nm–2500 nm, and the following properties of the structure are studied. Firstly, the structure achieves an average absorption efficiency of 92.57 % and 97.88 % at AM (Air Mass) 1.5, and the bandwidth is 283 nm–2006 nm (absorption efficiency greater than 90 %), which achieves ultra-broadband perfect absorption. Secondly, the structure can theoretically work at 1500 K, at which the thermal radiation efficiency is 89.13 %. When considering the oxidation and decomposition of materials in practical applications, the structure can work up to 700 K, and the thermal radiation efficiency decreases to 77.07 %. Therefore, the structure has excellent absorption and radiation performance, and has a wide range of applications as a solar absorber or thermal emitter.

Extensive research has been conducted on the heat transfer characteristics related to the boundary conditions present in phase-change applications. However, there remains a fundamental gap in understanding the local heat transfer characteristics of mixed convective laminar flow exposed to a uniform wall temperature boundary condition. Furthermore, there is a disparity between numerical and experimental studies investigating this boundary condition. This study addresses these gaps by being the first to experimentally investigate the local heat transfer characteristics of developing laminar flow through a horizontal tube exposed to a uniform wall temperature boundary condition. A novel experimental setup was developed to measure the mean fluid temperatures along a 5 m-long copper tube with an inner diameter of 4.9 mm. While the local results indicated an increase in wall temperature along the test section, the average Nusselt numbers correlated well with literature, indicating that similar temperature trends existed in prior experimental studies. The local heat transfer characteristics for developing laminar uniform wall temperature flow were divided into four regions: (1) Free Convection Developing, (2) Free Convection Governing, (3) Sustained Free Convection, and (4) Diminishing Heat Transfer. Free convection effects were found to increase near the inlet of the tube and the associated secondary flow assisted the flow in becoming fully developed. However, due to the decreasing wall-fluid temperature differences, free convection effects could not be sustained, and heat transfer eventually diminished as the fluid temperatures approached the wall temperatures.

Effusion cooling characteristics of the cylindrical and fan-shaped hole configurations are studied under realistic swirl flows at blowing ratios ranging from 1.2 to 6.0. RANS computations with the k-ω SST model are used to evaluate the interaction between swirl mainstream and cooling air. The results show that the cooling effectiveness distribution for the cylindrical and fan-shaped hole configurations are similarly controlled by swirl impact. Two high-temperature regions emerge near the impact location of the swirl main flow on the liner wall. The fan-shaped hole configuration has higher cooling effectiveness, and the difference is relative to location. Quantitatively analyzing, the fan-shaped holes are 19.7 %–53.2 % higher than the cylindrical holes in impact zones. In the corner recirculation zone, the difference ranges from 39.1 % to 84.2 %. The computations reflect the interaction between swirl flows and cooling jets is stronger for fan-shaped holes due to lower outlet velocity. Therefore the cooling air is easier to be suppressed by swirl impact under low BR, while the increasing blowing ratio can enhance the resistance of cooling air against swirl flows.