查看更多>>摘要：Applying partially miscible lubricating oil-refrigerant mixtures instead of completely miscible ones in refrigeration systems is a promising way to avoid the refrigerant insufficiency, and the application of partially miscible oil requires the clarity of nucleate pool boiling characteristics of partially miscible mixtures. As partially miscible mixtures at boiling conditions show an entirely different stratification process compared to completely miscible mixtures or partially miscible mixtures at non-boiling conditions studied by former researchers, the existing researches cannot be directly applied to evaluate pool boiling characteristics of the partially miscible mixture. The purpose of this study is to experimentally investigate and model both stratification and heat transfer performance of nucleate pool boiling of the partially miscible oil-refrigerant mixture. The stratification phenomenon at boiling of partially miscible mixtures is observed, and up to four layers (i.e., an oil-rich micro-layer, a middle oil-poor layer, a top oil-rich layer, and a gas phase layer) are found. The boiling process of partially miscible mixtures stratifying from a single layer into four layers is modelled, and calculation methods of mass transfer rates among layers are proposed. The model validation shows that predicted thicknesses of layers accord with experimental images, which covers the conditions of heat flux ranging from 5 to 15 kW/m~2, initial oil mass fraction ranging from 5 to 15%, and initial height ranging from 80 to 120 mm. Predicted heat transfer coefficients agree with 80% of the experimental data within the deviation of ± 25%, and the mean deviation is 15.3%.
查看更多>>摘要：In this paper, we experimentally investigate the pool boiling heat transfer on pillar-structured surfaces with different wettability patterns and different values of the channel-to-pillar width ratio. Five types of wettability patterns are compared, i.e., homogeneous hydrophilicity (HPi), homogeneous hydrophobicity (HPo), homogeneous near-superhydrophilicity (SHPi), a mixed pattern with hydrophilic bottom and hydrophobic pillar top (MPiPo), and another mixed pattern with hydrophobic bottom and hydrophilic pillar top (MPoPi). The effects of homogenous wettability patterns on the boiling performance of pillar-structured surfaces are basically consistent with those on plain surfaces, but the differences between the HPi and HPo patterns are found to be gradually enlarged with the decrease of the channel-to-pillar width ratio. Moreover, compared with the base HPi pattern, the MPoPi mixed pattern leads to a leftward shift of the boiling curve and an upward shift of the heat transfer coefficient (HTC) curve but gives a lower value of the critical heat flux. Such a tendency of the MPoPi mixed pattern is not affected when the channel-to-pillar width ratio is decreased from 2.0 to 0.5. In contrast, the boiling performance of the MPiPo mixed pattern is found to be significantly affected by the channel-to-pillar width ratio as it performs best among the five types of wettability patterns when the channel-to-pillar width ratio γ≥1, but it deteriorates the boiling heat transfer at high heat fluxes when the ratio γ is reduced to 0.5.
查看更多>>摘要：In the direct energy deposition (DED) process of IN718, the microstructure significantly affects the properties of the workpieces, but the evolution mechanism of the microstructure has not been systematically studied, so it is valuable to understand the solidification process in the molten pool. In this study, a simplified temperature field model is established and the cellular automata (CA) model is used to explore the evolution of the microstructure during DED, which can effectively simulate the microstructures of IN718 with different process parameters. Then a columnar to equiaxed crystal transition (CET) model is established to quantitatively reveal the relationship between solidification parameters and microstructures throughout the solidification process, and the mechanism of grain growth during the solidification of molten pool is further explored, finding that increasing the laser power and scanning speed will promote the CET process with the equiaxed crystal content increasing from 6.18% to 33.8%. This study can be used to simulate the solidification process of IN718 during DED and obtain the required microstructures by optimizing the processing parameters.
查看更多>>摘要：Due to the extreme increase of thermal load and the constraint of ram-air utility by stealth requirements, fuel becomes an important heat sink in an aircraft and plays a role in the heat collection, transport, and dissipation. Additionally, the fuel temperature is limited by the engine inlet temperature and fuel stability. Therefore, dynamically predicting the fuel temperature in aircraft flight is essential for the thermal management and propulsion systems. This paper proposes a rapid dynamic prediction method to calculate the fuel temperature under a complete flight profile, which is based on surrogate modeling, heat transfer theory of a fuel tank, and CFD technique. The CFD results show that the distributions of fuel temperature and wall convective heat transfer coefficient are uniform, and thus employing the average values of these two quantities in the prediction method is reasonable. Based on a sample database from CFD simulations, a response surface method surrogate model performs well in predicting the wall convective heat transfer coefficient. Ultimately, combined with the theory of heat transfer, an efficient algorithm is designed to predict the fuel temperature under a complete flight profile. The fuel temperature is predicted to increase from 298 K to 317.3 K by the operation of thermal management system under the typical profile. The computation results are explicable and in qualitative agreement with engineering data.
查看更多>>摘要：The objective of this study was to explain and quantify the effect of blinds on the heat transfer paths causing thermal decay and the resulting effects on diffuser discharge temperatures, temperature profiles, supply airflow rate, and energy consumption. To analyze the direct effects of convective heat transfer on thermal decay as blind angle changes, two processes must be examined. The first is the effect of the return temperature on convective heat transfer during operating hours, and the resulting change in conduction rate at the bottom surface of the concrete slab. The second is the effect of change in solar radiation at the top surface of the raised floor on convective heat transfer from its bottom surface, which affects thermal decay. This paper performed an analysis of thermal decay and cooling energy consumption according to the blind angle through a comparison of experimental data and simulation results. Phi means stratification in the room; lower values mean more stratification, which should be considered as a UFAD systems design. The most notable result of this study is the return temperature. It can be seen that during better temperature stratifications, the return temperature was higher due to the plume phenomenon. Although blinds decrease the total indoor cooling load with their shielding effect on the solar radiation load, the rise in return temperature increased the cooling coil load of the air conditioner and the heat storage in the concrete slab. This study will help to understand the relationship between the blind angle, the thermal stratification, and cooling energy usage in the UFAD systems.
查看更多>>摘要：For the challenge of burgeoning anti-icing applications in the space vehicles and airplanes territory, droplet self-dislodging phenomena provide a promising prospect owing to a distinctive quick removal and refresh ability. In recent decades, the high-end micro/nano-fabrication of the superhydrophobic textures has motivated self-dislodging icephobic application. In this work, recent advances of synergic morphology optimizing strategies in micro/nano-scale functional surface according to the droplet self-jumping, self-migration and condensate self-ejecting mechanisms are systematically introduced. Further, it is revealed that existing droplet self-dislodging regulation can be categorized as: a) combining multi-scale effects with physical heterogeneities, b) considering surface tension manipulation and Laplace pressure gradient, and c) controlling droplet frequent departure and droplet programmable self-migration, for slower ice propagation velocity and longer freezing retarding time. Strategies of micro/nano-morphologies and chemical characterization are evaluated. The contradiction or merits, remaining challenges, and future endeavor of nanoscale mono or hierarchical veins for highly effective anti-icing procedures, are proposed. In the end, this review is concluded by proposing outlooks with photo-thermal assist and well adaptability to extreme cold and wet environment.
查看更多>>摘要：Superhydrophobic surface-modified zeolite 13X has been utilized to regulate nonadsorbed liquid water in an open-loop adsorption heat pump system for steam generation. The zeolite is modified by vinyltrimethoxysilane to achieve a superhydrophobic surface effect. The measured static water contact angles of the modified zeolite reached 152°. Thermogravimetric analysis results show good thermal stability at temperatures less than 400℃. The measured adsorption capacity and adsorption heat slightly decrease for the modified zeolite. During the regeneration process dry air at 140℃ drives wet zeolite to remove adsorbed and free water. Then generation process proceeds for steam generation at a maximum of 220℃ from water at 72℃. The mass and temperature of the steam generated from the modified zeolite bed increase compared with those of unmodified zeolite 13X. This is because of the apparent decrease in the total amount of free water, which allows the released adsorption heat to be transferred to passing water quickly for steam generation. Energy consumption during regeneration simultaneously reduces. System indicators such as the coefficient of performance for heating (COP_h), specific heating power (SHP), gross temperature lift (GTL) and exergy efficiency (η_e) increase with increasing hydrophobicity of zeolite particles. The surface hydrophobicity decreases the affinity of zeolite for useless free water which cannot contribute to increasing the overall adsorption heat.
Giovanni A. LongoSimone MancinGiulia RighettiClaudio Zilio...
13页
查看更多>>摘要：This paper presents the local heat transfer coefficients of R32 and R410A boiling inside a Brazed Plate Heat Exchanger. A new and innovative test section, with one channel on the refrigerant-side and two channels on the water-side, was specifically designed and manufactured for measuring the local heat transfer coefficient in refrigerant two-phase heat transfer in BPHE. The test section includes two thick corrugated plates instrumented with 36 copper-constantan thermocouples for measuring the plate surface temperature, the heat flux and the heat transfer coefficient in 9 different positions along the refrigerant channel. The experimental tests were carried out at a saturation temperature around 10℃, in the refrigerant mass velocity range 9.7-39.3 kg m~(-2)s~(-1) with an inlet vapour quality ranging from 0.28 to 0.38 and an outlet vapour quality around 1.00. R32 and R410A exhibit boiling heat transfer coefficients very similar both in trend and magnitude. Nucleate boiling seems to be the governing heat transfer regime both in R410A and R32 tests. Consolidated correlations for boiling inside BPHE show a fair agreement with all the experimental data.
查看更多>>摘要：The melting and dripping of burning thermoplastics can cause a new ignition and form a plastic pool fire, resulting in a significant fire risk. This work investigates the burning dynamics of polyethylene (PE) vs polypropylene (PP) pools fully melted at 380-410℃ on a hot plate with a controlled area and initial temperature. For PE, three burning patterns are observed and defined under different bottom boundary temperatures. When the boundary temperature is lower than the melting point of thermoplastic, burning Pattern I (near-limit flame) appears shortly and extinguishes quickly. Above the melting point of PE, the flame becomes stronger and can last for the longest period before quenching (Pattern II: transitional flame). PP does not have this transitional-flame stage due to a higher melting point and lower pyrolysis point. When the plastic pool temperature exceeds its flashpoint of about 300℃ (～60℃ below its pyrolysis point), the flame becomes intense and quickly burns out the molten pool (Pattern III: intensive flame). The burning processes of molten thermoplastics show a clear difference from the burning of ethanol and paraffin wax. This study promotes the understanding of the melting and burning of plastics in real fire scenarios and helps quantify the hazards of dripping and flooring fires.
查看更多>>摘要：The heat sink with vapor chamber heat spreader has been the emerging design as it offers tremendous reduction in spreading resistance. However, this type of heat sinks suffer from the fin side temperature non-homogeneity, which hinders the heat transfer enhancement. The present study aimed to develop parallelogram fins integrated with heat pipes to increase the heat transfer performance of the heat sink with the vapor chamber heat spreader. The experimental and numerical investigations were performed on different heat sinks with different heat spreaders, including flat plate, vapor chamber, and vapor chamber combined with heat pipe. The air-cooled heat sink features a parallelogram fin configuration is proposed to tailor the temperature non-uniformity alongside the fins. The results showed that the overall thermal resistance of the heat sink with the vapor chamber and the vapor chamber combined with heat pipe spreader (no-cut fins) is about 50% and 55% lower than in the flat plate case. The proposed heat sink with vapor chamber combined with heat pipe spreader having a parallelogram fin structure offered about 61% reduction in thermal resistance and 45-50℃ reduction in chip temperature over the heat sink with flat plate heat spreader.
NSTL
Elsevier