With the aim to increase the overall heat transfer performance and reliability of tube receiver for parabolic trough solar collector system, asymmetric outward convex corrugated tube is introduced as ...the metal tube of parabolic trough receiver. An optical-thermal-structural sequential coupled method was developed to study the heat transfer performance and thermal strain of tube receiver for parabolic trough solar collector system. Heat transfer performance and thermal strain comparisons between conventional tube receiver and asymmetric outward convex corrugated tube receiver are conducted. The researches indicated that the usage of asymmetric outward convex corrugated tube as receiver can enhance the heat transfer performance and reduce the thermal strain effectively. By using asymmetric outward convex corrugated tube as receiver, the maximum enhancement of overall heat transfer performance factor is 148% and the maximum restrain of von-Mises thermal strain is 26.8%.
•Asymmetric outward convex corrugated tube receiver is introduced as the metal tube of PTR.•Overall heat transfer performance factor is increased up to 148% by using ACPTR.•Regression equations are put forward for Nusselt number and Fanning friction factor.•Asymmetric outward convex corrugated tube can increase reliability of PTR effectively.
•Tube receiver with pin fin arrays inserting was introduced as tube receiver of PTR.•Average Nusselt number is increased up to 9.0% by using PFAI-PTR.•Overall heat transfer performance is increased ...up to 12.0% by using PFAI-PTR.
Tube receiver with pin fin arrays inserting was introduced as the absorber tube of parabolic trough receiver to increase the overall heat transfer performance of tube receiver for parabolic trough solar collector system. The Monte Carlo ray tracing method (MCRT) coupled with Finite Volume Method (FVM) was adopted to investigate the heat transfer performance and flow characteristics of tube receiver for parabolic trough solar collector system. To validate the feasibility of the developed MCRT and FVM combined method, the numerical results have been compared with experimental results conducted in the DISS test facility in Spain and the max relative error is less than 5%. The numerical results indicated that the introduction of absorber tube with pin fin arrays inserting design for the absorber tube of the parabolic trough receiver can effectively enhance the heat transfer performance. The average Nusselt number can be increased up to 9.0% and the overall heat transfer performance factor can be increased up to 12.0% when the tube receiver with pin fin arrays inserting was used.
•Defocus distance affected the adhesion width of the resin significantly.•The strength of TC4/CFRP-PEEK joint depended on the adhesion width of the resin.•CTi0.42V1.58 phase was identified at the ...joining interface.•Defects on the surface of TC4 gradually increased with the increase of defocus distance.
Laser joining of carbon fiber reinforced polyether ether ketone (CFRP-PEEK) to titanium alloy (TC4) was performed. The influence of defocus distance, also described as irradiance diameter of laser beam on interfacial joining was investigated. The appearance of welded joint and the microstructure at joint interface was observed. The fracture load was thereafter obtained. In addition, temperature field simulation at the joint interface during laser joining process was carried out to clarify the joining mechanism. The results indicated that good appearances were obtained with low defocused distance. The interfacial observation showed that mechanical bonding was generated at the joint interface since the melted resin was embedded in the rough surface of the TC4. Besides, a new phase formed at the interface between CFRP-PEEK and TC4, which was identified as CTi0.42V1.58. The content of the phase increased as the defocus distance increased. Shear force decreased when the defocus distance increased from −4 mm (irradiance diameter 0.84 mm) to 0 mm (irradiance diameter 0.6 mm), and then increased when the defocus distance increased from 0 mm to +8 mm (irradiance diameter 1.06 mm). This phenomenon was explained by simulation result of temperature field that the width of the range of melting temperature of the resin increased as the laser beam diameter increased. The simulation results were consistent with the changes of the width of resin adhesion in actual joint.
•Hybrid laser-MAG welding and laser welding were used to weld HG785D steel.•Different cooling rate led to difference of microstructure in both welded joints.•Grain size and microstructure types ...explain difference in mechanical properties.•Heat source melting efficiency of HLAW is larger compared to LBW.
Hybrid Laser-arc welding (HLAW) and laser beam welding (LBW) of HG785D steel were investigated by comparing microstructure, mechanical properties and heat source melting efficiency. Same microstructure of lath martensite (LM) was observed in fusion zone (FZ) of the two welded joints. However, coarser grain size and more random crystallographic orientations of FZ in HLAW joint were investigated via electron back-scatter diffraction (EBSD) analysis. M-A constituent and equiaxed ferrite (EF) were observed in finer-grain HAZ of HLAW joint, whereas fine martensite was obtained in LBW joint. More M-A constituent and granular bainite (GB) appeared in coarse-grain HAZ of HLAW joint. Higher heat input and lower cooling rate of HLAW compared to LBW resulted in presence of different microstructure. All the tensile specimen in both cases fractured at the base metal, while distinct difference in microhardness and low-temperature impact property was observed. Lower microhardness and higher impact energy of fusion zone were obtained in HLAW joint. Moreover, heat source melting efficiency of HLAW for 6-mm-thick HG785D steel plate could be improved by 26% compared to LBW.
Laser penetration welding of magnesium alloys and pure titanium TA2 with unequal thickness was performed. Mg base metal with different Al content (AZ31B, AZ61A, AZ91D) was used to investigate the ...influence of Al element in microstructure and mechanical properties of Mg/Ti dissimilar joints. The results revealed that the change of Mg base metal did not influence the weld appearance of the joints. Three kinds of joint all presented the best mechanical property when the laser power was 3500 W. With the increase content of Al elements in Mg base metal, a reaction layer was observed which was identified as Ti3Al. The highest enrichment of Al element was obtained and its fraction reached 19.31 at% at the AZ91/TA2 interface. The chemical potential gradient of Al from AZ91 to Ti alloy was higher than that from the other two base metals based on thermodynamic calculation. The maximum fracture load reached 3597 N when AZ61 was employed as the base metal and the fracture position was the Ti base metal. AZ31/TA2 joints failed at the weld seam without necking due to the rapid propagation of cracks at the Mg/Ti interface. The AZ91/TA2 joint failed inside the Mg fusion zone with necking at the middle area of the weld, which resulted from the precipitation of brittle phases such as Mg–Al, Ti–Al phases in the fusion zone of Mg alloys.
Fabrication of dense aluminum (Al-1100) parts (>99.3% of relative density) by our recently developed laser-foil-printing (LFP) additive manufacturing method was investigated as described in this ...paper. This was achieved by using a laser energy density of 7.0 MW/cm
to stabilize the melt pool formation and create sufficient penetration depth with 300 μm thickness foil. The highest yield strength (YS) and ultimate tensile strength (UTS) in the LFP-fabricated samples reached 111 ± 8 MPa and 128 ± 3 MPa, respectively, along the laser scanning direction. These samples exhibited greater tensile strength but less ductility compared to annealed Al-1100 samples. Fractographic analysis showed elongated gas pores in the tensile test samples. Strong crystallographic texturing along the solidification direction and dense subgrain boundaries in the LFP-fabricated samples were observed by using the electron backscattered diffraction (EBSD) technique.
Photocatalysts currently in use are only able to utilize very small part of the solar spectrum that arrives at the earth's surface (mainly ultraviolet light). Most of the photons that are not ...absorbed by the photocatalysts are converted to heat. However, there is no consensus on the effect of reaction temperature on photo-thermo chemical synergetic catalysis, which has been studied herein using experimental investigations combined with thermodynamic analysis. An elaborate photo-thermo chemical reaction test rig was initially designed and set up that can test experimental variable while the other influence factors were kept constant. The effects of ultrasonic and operation temperature on Pt/TiO2 particle cluster distribution during the photo-thermo chemical synergetic catalytic water splitting process were analyzed by an upright microscope for the first time. The results indicated that the H2 production rate varies with reaction temperature, and 55 °C is the optimum temperature for the photo-thermo chemical synergetic catalytic water splitting process studied here. A maximum H2 production rate of 11.934 mmol/(h g) could be achieved using a classical Pt/TiO2 catalyst, when operating under the optimum reaction conditions.
•Elaborate photo-thermo chemical reaction test rig was designed and established.•Experiment with theoretical analysis is used to study the effects of temperature on photocatalysis.•H2 with temperature variation is studied by experiment with theoretical analysis.•H2 production rate can reach 11.934 mmol/h/g for Pt/TiO2 with optimum condition.
The carrier excitation, relaxation, energy transport, and conversion processes during light‐nanocrystal (NC) interactions have been intensively investigated for applications in optoelectronics, ...photocatalysis, and photovoltaics. However, there are limited studies on the non‐equilibrium heating under relatively high laser excitation that leads to NCs sintering. Here, the authors use femtosecond laser two‐pulse correlation and in‐situ optical transmission probing to investigate the non‐equilibrium heating of NCs and transient sintering dynamics. First, a two‐pulse correlation study reveals that the sintering rate strongly increases when the two heating laser pulses are temporally separated by <10 ps. Second, the sintering rate is found to increase nonlinearly with laser fluence when heating with ≈700 fs laser pulses. By three‐temperature modeling, the NC sintering mechanism mediated by electron induced ligand transformation is suggested. The ultrafast and non‐equilibrium process facilitates sintering in dry (spin‐coated) and wet (solvent suspended) environments. The nonlinear dependence of sintering rate on laser fluence is exploited to print sub‐diffraction‐limited features in NC suspension. The smallest feature printed is ≈200 nm, which is ≈¼ of the laser wavelength. These findings provide a new perspective toward nanomanufacturing development based on probing and engineering ultrafast transport phenomena in functional NCs.
Conventional thermal sintering of nanoparticles occurs on a nanosecond scale or longer. In this article, laser nanocrystal sintering process with characteristic time on a picosecond scale is reported. The process enables direct, polymer‐free and high‐precision metal printing which has been challenging to achieve up to date. The smallest feature printed is ≈200 nm, which is ≈¼ of the laser wavelength.
Currently, light-based three-dimensional (3D) printing with submicron features is mainly developed based on photosensitive polymers or inorganic-polymer composite materials. To eliminate ...polymer/organic additives, a strategy for direct 3D assembly and printing of metallic nanocrystals without additives is presented. Ultrafast laser with intensity in the range of 1 × 1010 to 1 × 1012 W/cm2 is used to nonequilibrium heat nanocrystals and induce ligand transformation, which triggers the spontaneous fusion and localized assembly of nanocrystals. The process is due to the operation of hot electrons as confirmed by a strong dependence of the printing rate on laser pulse duration varied in the range of electron–phonon relaxation time. Using the developed laser-induced ligand transformation (LILT) process, direct printing of 3D metallic structures at micro and submicron scales is demonstrated. Facile integration with other microscale additive manufacturing for printing 3D devices containing multiscale features is also demonstrated.
•Revealing effect of processing window on laser welding-brazing of Al/ 22MnB5 steel.•Fe2 (Al, Zn)5 and scattered distributed FeZn10 was identified at the interface.•The fracture modes changed with ...the increase of defocused distance.•Effects of welding thermal cycle on HAZ of aluminum alloys were analyzed.
Laser welding-brazing of AA6061 alloys and press-hardened 22MnB5 steel in lap configuration was carried out with Zn-Al15 filler metal. The processing window in different defocused distances was investigated and its influence on microstructure and mechanical properties of Al/steel dissimilar joints was explored. The results revealed that good appearances could be acquired in different processing windows with variation of defocused distances. The processing window was enlarged with increase of laser spot diameters, from laser power of 2.1–2.3 kW in defocused distance of +20 mm (laser spot diameter of 1.4 mm) to 2.8–3.6 kW in defocused distance of +40 mm (laser spot diameter of 2.4 mm). The thickness of interfacial intermetallic layer increased gradually from 0.47 to 1.86 μm for defocused distance of +20 mm to 3.76–11.03 μm of +40 mm. The newly formed phase at the interface at defocused distance of +20 mm was identified as Fe2(Al, Zn)5 and those were identified as the mixture of Fe2(Al, Zn)5 and FeZn10 when the defocused distances were +30 mm and +40 mm. The tensile-shear test of laser welded-brazed joints produced in various processing windows showed two types of fracture modes occurred. The interfacial failure appeared with the defocused distance less than +30 mm. The maximum fracture load reached 2793 N, representing 84.6% joint efficiency in regard to aluminum alloys. However, when the defocused distance increased to +40 mm, the maximum tensile-shear force decreased to 2242 N and failure occurred in the softening region of HAZ in aluminum alloy caused by effect of large laser irradiation. At larger defocused distance, the weak position of the dissimilar joints would transfer from the interface to the HAZ of the aluminum alloy, resulting in a bottleneck to improve mechanical property of joints.
PDF
