With the increasingly stringent CO2 emission regulations, the degree of strengthening of the engines is increasing. Under high-pressure conditions, the airway throat parts of the intake and exhaust ...systems have a great influence on the flow loss of the diesel engine. The reasonable distribution of the throat area of the intake and exhaust ports in the limited cylinder headspace is key to improving the performance of supercharged engines. This study took a large-bore, high-pressure ratio diesel engine as the research object. Firstly, the three-dimensional (3D) flow simulation method was used to reveal the influence law of different throat areas on the engine intake and exhaust flow under steady-state conditions, and a steady-flow test bench was built to verify the accuracy of the simulation model and law. Secondly, based on the 3D steady-state calculation and test results, a more accurate one-dimensional simulation model was constructed, and a joint optimization simulation platform was established based on the dynamic data link library. On this basis, the mathematical description of the multi-objective optimization of airway throat size was established using machine learning methods, such as a genetic algorithm, the design domain and boundary conditions of variable parameters were clarified, and the collaborative optimization objective of integrated flow coefficient and flow loss is proposed to achieve the fast and accurate optimization of intake and exhaust throat diameters.
The high entropy alloy coating is considered as one of the most promising methods to improve the high-temperature oxidation resistance of titanium aluminum alloys due to its fine mechanical property ...and thermal stability. However, the high entropy alloy coating prepared so far has poor coating quality, and low coating forming efficiency and there are certain requirements for the substrate alloys. To this end, a NiCoCrAl high entropy alloy coating was developed using the double glow plasma alloying technique on the TiAl alloy. The morphology, phase structure, and high temperature oxidation resistance of the coating were comprehensively studied. The results indicate that a uniform and dense coating with a single solid solution of face-centered cube phase formed on the alloy surface. Furthermore, isothermal oxidation test was performed, and the oxidation failure process of the TiAl substrate and the NiCoCrAl-coated sample with different oxidation times was analyzed. It is found that the coating efficiently prevented the TiAl alloy from degeneration for up to 100 h at 900 °C.
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The experimental study on the boiling heat transfer performance and visualization of Al2O3-H2O, SiO2-H2O and Al2O3-SiO2-H2O nanofluids with 0.01% mass concentration were carried out. Alumina and ...silica nanoparticles with average particle diameters of 30 nm and 50 nm were selected. Al2O3-SiO2-H2O was the mixed aqueous nanofluids with a mass ratio of Al2O3 to SiO2 of 1: 1. The results show that the effect of particle size on the boiling heat transfer performance is small. For nanofluids with a particle size of 30 nm, the boiling heat transfer performance of Al2O3-H2O nanofluids is better than that of Al2O3-SiO2-H2O and SiO2-H2O nanofluids under the working pressure of 101 kPa. The critical heat fluxes increased by 7.9% and 22.1% respectively, and the maximum heat transfer coefficients increased by 18.3% and 32.6% respectively. The critical heat flux and the maximum heat transfer coefficient are 162.1 W/cm2 and 7.01W/(m2·K). The working pressure has an important effect on the boiling heat transfer performance of nanofluids. Compared with the boiling heat transfer performance under high pressure conditions, the boiling heat transfer performance of nanofluids is better under low pressure.
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•The novel hybrid coating solves the thermal barrier problem of graphene.•Modification makes the coating have rich chemical bonds and complex microstructure.•The oxygen-containing ...functional groups promote the capillary effect of coating.•Concentration has little effect on structure and element content of hybrid coating.
During the boiling process, the heating surface has a substantial influence on the growth of the bubbles, the wetting of dry spots, and the deposition of the particles. Therefore, improving the surface properties is one of the main ways to enhance heat transfer. In this study the heating coatings were prepared by boiling deposition using GNP nanofluids, GNP/Ag mixed nanofluids, and GNP-Ag hybrid nanofluids with different mass fractions (wt% = 0.001,0.002,0.003) as raw materials. The boiling heat transfer characteristics of DI water on the bare copper surface and heating coatings were studied, and the heat transfer mechanism was analyzed. The results show that GNP-Ag hybrid nanoparticle coating has the most outstanding enhanced heat transfer effect. DI water achieves the highest critical heat flux (CHF) of 163.65 W/cm2 and heat transfer coefficient (HTC) of 9.69 W/cm2K on the coating, which are respectively 45.84% and 82.14% higher than the smooth copper surface. The wettability was analyzed by measuring the contact angle. Scanning electron microscope image was used to analyze the microstructure of the coating. The microstructure of Ag modified functionalized GNP nanoparticle has a great change compared with mixed nanoparticles. It was observed that the hybrid nanoparticle deposit into a large number of bulges and cavies, and the bulges accumulate in clusters and form self-assembled flower-shaped bulges. Combined with the energy spectrum test results of the heating coating, the coverage of the nanoparticles in the coating on the copper substrate and the variation trend of each deposited element with the concentration are analyzed. The novel heating coating enhances heat transfer under the combined action of nanoparticle functional groups, coating microstructure, and wettability. GNP-Ag hybrid nanoparticle coating was applied towards the heat dissipation of electronic devices. When the input power is 300 W, the maximum temperature of electronic devices is 108.52 °C, which is much lower than that of other coatings.
The “seesaw relationship” between thermal conductivity and thermal expansion coefficient (CTE) in most high temperature ceramics has become an obstacle to the design of long-life multilayer ...thermal/environmental barrier coatings (T/EBC). Due to low thermal conductivity and CTE, defect fluorite type high-entropy rare earth (RE) hafnates have drawn a lot of interest for potential application in T/EBC systems. This work designs and synthesizes the (Dy0.2Ho0.2Er0.2Tm0.2Lu0.2)2Hf2O7 with comprehensive thermal performance and investigates the thermophysical mechanism from the phonon scale. In addition to the lattice distortion effect caused by the point defects of multicomponent substitutional atoms in (Dy0.2Ho0.2Er0.2Tm0.2Lu0.2)2Hf2O7, the oxygen vacancies in the defect fluorite lattice also play a critical role in reducing the thermal conductivity. From microscopic thermal expansion behavior, the low-frequency optic phonons originated from the vibration of RE atoms are the key factors in altering CTE for hafnates. And doping smaller RE ions is beneficial for enhancing the RE–O bond strength and further reducing CTE. The results contribute to the understanding of high-entropy strategic design and suggest that (Dy0.2Ho0.2Er0.2Tm0.2Lu0.2)2Hf2O7 is a promising top layer material in the implementation of T/EBC.
Graphene nanoplatelets (GNP) have natural hydrophobicity and are not easy to disperse in deionized (DI) water. In this study, Ag modified functionalized GNP (f-GNP) was used to prepare novel hybrid ...nanoparticles. Through phase and structural analysis, it is found that the material has high purity, and is dispersed in DI water to produce the nanofluids that are stable. The boiling heat transfer properties of GNP nanofluids, GNP/Ag mixed nanofluids and GNP-Ag hybrid nanofluids of three weight concentrations (wt% = 0.001, 0.002, 0.003) were investigated. Among the nine samples, the GNP-Ag hybrid nanofluids with weight concentration of 0.001% has the best heat transfer property, with a critical heat flux (CHF) of 170.74 W/cm2, which is 52.31% more than the one of DI water. After the boiling experiment, the heating surface was analyzed by scanning electron microscope (SEM) to observe the deposition structure of nanoparticles. It is found that the self-assembled structure of the hybrid nanoparticle deposition is more complex. Under the combined action of the oxygen-containing functional groups of the nanoparticles, the lateral heat transfer of the working medium is strengthened, and the separation of the vapor bubbles is promoted. In the wettability analysis, the contact angle of the nanofluids were measured, and it was found that the hybrid nanofluids was hydrophilic, which improved the liquid supply mechanism on the heating surface and was beneficial to increase the CHF. The bubble formation characteristics of nanofluids were observed by high-speed camera, and it was found that the bubble formation period of hybrid nanofluids was short and the bubble separation diameter was small. Results from tests and experiments indicate that covalent modified hybrid nanofluids have good heat transfer performances.
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•Novel hybrid nanoparticle is prepared to solve the problem of graphene agglomeration.•Compared heat performance of physical mixed nanofluid and chemical hybrid nanofluid.•Functional groups in the hybrid nanoparticle affect the heat transfer intensity.•Analyzed heat transfer mechanism by testing the properties of nanofluid and the surface.
•The preparation methods for amphiphilic surfaces are divided into template coating, conventional photolithography, and laser texturing.•‘Hydrophobic island’ mode effectively enhances both CHF and ...HTC.•Patterns close to blunt circles are more conducive to CHF, and pronounced contrast of wettability between adjacent patterns is advantageous for enhancing HTC.•Capillary criterion is equally applicable to biphilic surfaces.•The biphilic surface has the potential to suppress intermittent boiling.
In recent years, studies on patterned surfaces with different local wettabilities have emerged and developed. A number of peculiar structures are the most promising topographic ones in simultaneously enhancing CHF and HTC due to their high efficiency, and compatibility with other strengthening methods. Many studies have attempted to explore the optimal expression of wettability patterned surfaces for preparation of a boiling surface, pattern design, wettability, and diverse combination. There are hundreds of relevant literature works, and a comprehensive analysis and summary is required.
This article first provides a detailed introduction to the manufacturing processes of these engineering surfaces, which are technically divided into wettability acquisition and pattern combination methods. It continues with different factors affecting surface boiling and the unique bubbling attributes which led to these differences. A review of visualization studies aims at providing an analytical basis for these effects. Finally, this article summarizes the prediction models and proposes challenges which confront future research works. This work provides a comprehensive introduction to the most recent research on wettability patterned surfaces.
This work is based on a new research direction to ameliorate the hydrophobicity of graphene nanoplatelets (GNP) by functionalizing it without affecting its good thermal properties and to prepare ...novel hybrid nanofluids for enhancing heat transfer. In this work, a novel type of GNP-Fe3O4 hybrid nanoparticles is prepared, and the crystal structure test proves that it has high purity, contains a variety of hydrophilic functional groups, and has a strong binding force between elements. The blending of metal or non-metal nanoparticles in the GNP-Fe3O4 hybrid nanofluids is beneficial to improve the deposition effect of the nanofluids and reduces the heat transfer resistance of the heating surface. Therefore, the boiling heat transfer characteristics of GNP nanofluids, GNP-Fe3O4 hybrid nanofluids, GNP-Fe3O4 + Al2O3 hybrid nanofluids, and GNP-Fe3O4 + SiO2 hybrid nanofluids with different mass concentrations (0.0005 wt%, 0.001 wt%, 0.002 wt%, and 0.003 wt%) were studied experimentally. The heat transfer effects of different nanofluids at the same concentration were also compared. The experimental results show that the heat transfer characteristics of nanofluids are significantly enhanced compared with the deionized water (DI water). Among them, GNP-Fe3O4 + Al2O3 hybrid nanofluids and GNP-Fe3O4 + SiO2 hybrid nanofluids have the best heat transfer effect, when the mass concentration is 0.003 wt% critical heat flux (CHF) is 180.23 W/cm2 and 185.29 W/cm2, which are 60.61% and 65.12% higher than DI water. Also, the maximum heat transfer coefficient (HTC) is 9.73 W/cm2K and 10.23 W/cm2K, which are 82.74% and 92.13% higher than DI water. Both of them have similar heat transfer intensity at the same mass concentration. Through the observation of the macroscopic morphology and microstructure of the deposition surface, the wettability test and the study of the bubble generation characteristics, the boiling heat transfer mechanism of the new hybrid nanofluid is qualitatively analyzed. The results show that the wettability and deposition characteristics of GNP-Fe3O4 hybrid nanofluids are significantly improved after blended with other nanoparticles, the microscopic deposition structure is more complex and porous, the macroscopic deposition amount is significantly reduced, and the heat transfer resistance is reduced; the characteristics of bubble formation show that the fluid is violently disturbed and the heat transfer intensity is greatly improved. At the same time, the functional groups introduced by acid mixing treatment and metal modification in the preparation of the new hybrid nanoparticles also plays a great role in promoting the heat transfer of the fluid.
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A closed-loop type experimental system of pool boiling heat transfer with condensed reflux in the bottom was developed to study the boiling heat transfer of copper foams in different working fluids, ...including deionized water, water-based Al2O3 nanofluids, ethylene glycol/deionized water mixed fluid and ethylene glycol/deionized water-based Al2O3 nanofluids. The pore densities of copper foams are 5, 30 and 60 pores per inch (PPI) and the foam thickness are 2, 4, 6 and 8 mm, while the porosity remains a fixed value of 0.9. Al2O3 nanoparticles with an average diameter of 50 nm are used in this study and their mass concentrations are 0.01%, 0.05% and 0.1%. The results show that the concentrations and types of nanofluids and the structural parameters of copper foams have significant effects on the boiling heat transfer performance. Introducing the condensed reflux back to the bottom of boiling chamber can enhance the boiling heat transfer performance compared with the conventional pool boiling experimental condition. Under the current experimental settings, the gravity-capillary driven condensed reflux can offer extra liquid replenishment to the copper foam surface, improving the rewetting ability and therefore enhancing the boiling heat transfer. In addition, the scouring effect of condensed reflux can promote the bubble departure and decrease the bubble release resistance, and inhibit the deposition of nanoparticles on the heating surface.
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