Al_2O_3/R141 b + Span-80 nanorefrigerant for 0.05 wt.% to 0.4 wt.% is prepared by ultrasonic vibration to investigate the influence of nanoparticle concentrations on flow boiling heat transfer of Al_2O_3/R141 b + Span...Al_2O_3/R141 b + Span-80 nanorefrigerant for 0.05 wt.% to 0.4 wt.% is prepared by ultrasonic vibration to investigate the influence of nanoparticle concentrations on flow boiling heat transfer of Al_2O_3/R141 b + Span-80 in micro heat exchanger by direct metal laser sintering. Experimental results show that nanoparticle concentrations have significantly impact on heat transfer coefficients by homogeneity test of variances according to mathematical statistics. The heat transfer performance of Al_2O_3/R141 b+ Span-80 nanorefrigerant is enhanced after adding nanoparticles in the pure refrigerant R141 b. The heat transfer coefficients of 0.05 wt.%, 0.1 wt.%, 0.2 wt.%, 0.3 wt.%and 0.4 wt.% Al_2O_3/R141 b + Span-80 nanorefrigerant respectively increase by 55.0%, 72.0%, 53.0%, 42.3% and 39.9% compared with the pure refrigerant R141 b. The particle fluxes from viscosity gradient, non-uniform shear rate and Brownian motion cause particles to migrate in fluid especially in the process of flow boiling. This migration motion enhances heat transfer between nanoparticles and fluid. Therefore, the heat transfer performance of nanofluid is enhanced. It is important to note that the heat transfer coefficients nonlinearly increase with nanoparticle concentrations increasing. The heat transfer coefficients reach its maximum value at the mass concentration of 0.1% and then it decreases slightly. There exists an optimal mass concentration corresponding to the best heat transfer enhancement. The reason for the above phenomenon is attributed to nanoparticles deposition on the minichannel wall by Scanning Electron Microscopy observation. The channel surface wettability increases during the flow boiling experiment in the mass concentration range from 0.2 wt.% to 0.4 wt.%. The channel surface with wettability increasing needs more energy to produce a bubble. Therefore, the heat transfer coefficients decrease with nanoparticle concentrations in the range from 0.2 wt.% to 0.4 wt.%. In addition, a new correlation has been proposed by fitting the experimental data considering the influence of mass concentrations on the heat transfer performance. The new correlation can effectively predict the heat transfer coefficient.展开更多
Bio-based polyurethane foam has attracted increasing attentions due to eco-friendliness and fossil feedstock issues.However,the inherent flammability limits its application in different fields.Herein,we demonstrate a ...Bio-based polyurethane foam has attracted increasing attentions due to eco-friendliness and fossil feedstock issues.However,the inherent flammability limits its application in different fields.Herein,we demonstrate a green bio-based flame-retardant system to fabricate polyurethane foam composite with durable flame retardancy,smoke suppression,and thermal insulation property.In this system,the green bio-based polyol(VED)with good reactivity and compatibility plays a role of flame retardant and EG acts as a synergistic filler.As a result,the LOI value of foam composite increased to 30.5 vol.%and it achieved a V-0 rating in the UL-94 vertical burning test.Additionally,the peak heat release rate(pHRR)and the total smoke production(TSP)decreased by 66.1%and 63.4%,respectively.Furthermore,the foam composite maintained durable flame retardancy after accelerated thermal aging test,whose thermal-insulating property was maintained even after being treated in high-humidity environment with 85%R.H.for a week.This work provides a facile strategy for durable flame retardancy and long-term thermal insulation performance,and creates opportunities for the practical applications of bio-based foam composites.展开更多
The multi-channel discharge phenomenon for super-high-thickness(more than 1000 mm)cutting is studied in high-speed wire electrical discharge machining(HS-WEDM).In super-high-thickness cutting,the length of the wire el...The multi-channel discharge phenomenon for super-high-thickness(more than 1000 mm)cutting is studied in high-speed wire electrical discharge machining(HS-WEDM).In super-high-thickness cutting,the length of the wire electrode with a poor conductor is longer in the machining area,so the resistance of wire electrode cannot be ignored.It is found that the main reason for the formation of multi-channel discharge is that there is a high voltage between electrodes after dielectric breakdown due to the resistance characteristics of the wire electrode.When the conventional discharge circuit is used for machining,the voltage between electrodes in the middle of the workpiece is much higher than that at the upper and lower ends,which is easier to generate multi-channel discharge.Multi-channel discharge has the characteristic of dispersing discharge energy,which results in inhomogeneous surface roughness on the whole section of the workpiece.To improve the distribution characteristics of the inter-electrode voltage after dielectric breakdown,a novel discharge circuit is proposed,which gradually increases the inter-electrode voltage of discharge points distributed along the thickness direction of the workpiece,and significantly improves the formation probability of multi-channel discharge on the whole cutting surface.A super-high-thickness workpiece with a thickness of 1000 mm is used for continuous cutting.The cutting speed of the two types of discharge circuit is basically the same.But the surface roughness machined by the rear-end parallel circuit is reduced by 15.8%,and the surface homogeneity is greatly improved.展开更多
Via material erosion in wire electrical discharge machining(WEDM),recast layers form on the surfaces of workpiece.In addition,ultra fine Wire-EDM can be usually cut once.To reduce the thickness of the recast layer as ...Via material erosion in wire electrical discharge machining(WEDM),recast layers form on the surfaces of workpiece.In addition,ultra fine Wire-EDM can be usually cut once.To reduce the thickness of the recast layer as much as possible,the wire electrical discharge-electrochemical machining(WEDCM)method was proposed,which is based on the micro conductivity of the dielectric and microelectrolytic characteristics by adjusting the no-load rate of the pulse in the machining process.Furthermore,a state discrimination and servo control system based on discharge current was designed.The experiment results of different no-load rates show that the electrolytic effects increase as the no-load rate increases,and the main machining process is spark discharge erosion with a no-load rate in the range of 10%to 80%.At 90%no-load rate,the amount of recast layer formation in the forward direction of the wire electrode is almost the same as that of electrolytic dissolution,and it can be practically processed without a recast layer.Compared with10%no-load rate,the kerf width only increases by 7.5%.展开更多
Compared with a copper wire electrode, molybdenum wire with a poor conductor is usually used as the electrode in high speed wire-cut electrical discharge machining(HSWEDM), so the resistance of an ultra-fine wire cann...Compared with a copper wire electrode, molybdenum wire with a poor conductor is usually used as the electrode in high speed wire-cut electrical discharge machining(HSWEDM), so the resistance of an ultra-fine wire cannot be ignored. To study the differences of discharge characteristics between the ultra-fine wire and the conventional diameter wire, the continuous discharge waveform of two kinds of wire electrodes was compared. It was found that there was a multichannel discharge phenomenon in the discharge waveform cutting by ultra-fine wire. Through the establishment of a discharge equivalent circuit model and the simulation analysis of the electrostatic field, it was found that the reason why ultra-fine wire is easy to form multi-channel discharge phenomenon is that the potential difference between the wire and the workpiece increased linearly along the axis of the wire. Besides, etching products like metal particles will distort the electric field between the electrodes. Both of them make it easy to form a multi-channel discharge in machining.The results show that the distributions of the equivalent resistance and the peak current are affected by the multi-channel discharge position. Multi-channel discharge can disperse energy and increase effective discharge frequency. Compared with the 0 mm spacing, at a spacing of 100 mm, the machining efficiency increases by 8.7%, the surface roughness decreases by 37.7%, and the average recast layer thickness decreases by 46.6% under the condition of ultra-fine wire-EDM.展开更多
A new homemade apparatus, i.e. vibration assisted extrusion equipment, is employed to extrude polypropylene. Vibration assisted extrusion is based on the application of a specific macroscopic shear vibration field. Re...A new homemade apparatus, i.e. vibration assisted extrusion equipment, is employed to extrude polypropylene. Vibration assisted extrusion is based on the application of a specific macroscopic shear vibration field. Reduction of apparent melt viscosity as a function of vibration frequency is measured at different screw speeds and die temperatures. The effect of the process is investigated by performing mechanical tests, differential scanning calorimetry studies, polarized light microscopy and wide-angle X-ray diffraction. It is found that, compared with conventional extrusion, vibration assisted extrusion could effectively improve the rheological properties of PP melt by incorporating an extra shear vibration field. Both the tensile strength and elongation at break increased under the shear vibration field. For vibration assisted extrusion samples, both the melting temperature and crystallinity increased, accompanied by remarkable grain refinement. Vibration assisted extrusion induced a significantly enhanced bimodal orientation with a high fraction of a^＊-oriented α-crystallites, while only a limited improvement in the flow direction orientation. A structural model, i.e. bimodal c-axis and a^＊-axis orientation of PP macromolecular chains, was adopted to explain the experimental results.展开更多
基金Supported by the National Natural Science Foundation of China[21276090]
文摘Al_2O_3/R141 b + Span-80 nanorefrigerant for 0.05 wt.% to 0.4 wt.% is prepared by ultrasonic vibration to investigate the influence of nanoparticle concentrations on flow boiling heat transfer of Al_2O_3/R141 b + Span-80 in micro heat exchanger by direct metal laser sintering. Experimental results show that nanoparticle concentrations have significantly impact on heat transfer coefficients by homogeneity test of variances according to mathematical statistics. The heat transfer performance of Al_2O_3/R141 b+ Span-80 nanorefrigerant is enhanced after adding nanoparticles in the pure refrigerant R141 b. The heat transfer coefficients of 0.05 wt.%, 0.1 wt.%, 0.2 wt.%, 0.3 wt.%and 0.4 wt.% Al_2O_3/R141 b + Span-80 nanorefrigerant respectively increase by 55.0%, 72.0%, 53.0%, 42.3% and 39.9% compared with the pure refrigerant R141 b. The particle fluxes from viscosity gradient, non-uniform shear rate and Brownian motion cause particles to migrate in fluid especially in the process of flow boiling. This migration motion enhances heat transfer between nanoparticles and fluid. Therefore, the heat transfer performance of nanofluid is enhanced. It is important to note that the heat transfer coefficients nonlinearly increase with nanoparticle concentrations increasing. The heat transfer coefficients reach its maximum value at the mass concentration of 0.1% and then it decreases slightly. There exists an optimal mass concentration corresponding to the best heat transfer enhancement. The reason for the above phenomenon is attributed to nanoparticles deposition on the minichannel wall by Scanning Electron Microscopy observation. The channel surface wettability increases during the flow boiling experiment in the mass concentration range from 0.2 wt.% to 0.4 wt.%. The channel surface with wettability increasing needs more energy to produce a bubble. Therefore, the heat transfer coefficients decrease with nanoparticle concentrations in the range from 0.2 wt.% to 0.4 wt.%. In addition, a new correlation has been proposed by fitting the experimental data considering the influence of mass concentrations on the heat transfer performance. The new correlation can effectively predict the heat transfer coefficient.
基金supported by the National Natural Science Foundation of China(Nos.22175123,52122302,and 51991351)the 111 Project(No.B20001)Fundamental Research Funds for the Central Universities,and Open and Innovative Fund of Hubei Three Gorges Laboratory(Nos.2022LF2021 and SC213011).
文摘Bio-based polyurethane foam has attracted increasing attentions due to eco-friendliness and fossil feedstock issues.However,the inherent flammability limits its application in different fields.Herein,we demonstrate a green bio-based flame-retardant system to fabricate polyurethane foam composite with durable flame retardancy,smoke suppression,and thermal insulation property.In this system,the green bio-based polyol(VED)with good reactivity and compatibility plays a role of flame retardant and EG acts as a synergistic filler.As a result,the LOI value of foam composite increased to 30.5 vol.%and it achieved a V-0 rating in the UL-94 vertical burning test.Additionally,the peak heat release rate(pHRR)and the total smoke production(TSP)decreased by 66.1%and 63.4%,respectively.Furthermore,the foam composite maintained durable flame retardancy after accelerated thermal aging test,whose thermal-insulating property was maintained even after being treated in high-humidity environment with 85%R.H.for a week.This work provides a facile strategy for durable flame retardancy and long-term thermal insulation performance,and creates opportunities for the practical applications of bio-based foam composites.
基金supported by the National Natural Science Foundation of China(No.51975290)。
文摘The multi-channel discharge phenomenon for super-high-thickness(more than 1000 mm)cutting is studied in high-speed wire electrical discharge machining(HS-WEDM).In super-high-thickness cutting,the length of the wire electrode with a poor conductor is longer in the machining area,so the resistance of wire electrode cannot be ignored.It is found that the main reason for the formation of multi-channel discharge is that there is a high voltage between electrodes after dielectric breakdown due to the resistance characteristics of the wire electrode.When the conventional discharge circuit is used for machining,the voltage between electrodes in the middle of the workpiece is much higher than that at the upper and lower ends,which is easier to generate multi-channel discharge.Multi-channel discharge has the characteristic of dispersing discharge energy,which results in inhomogeneous surface roughness on the whole section of the workpiece.To improve the distribution characteristics of the inter-electrode voltage after dielectric breakdown,a novel discharge circuit is proposed,which gradually increases the inter-electrode voltage of discharge points distributed along the thickness direction of the workpiece,and significantly improves the formation probability of multi-channel discharge on the whole cutting surface.A super-high-thickness workpiece with a thickness of 1000 mm is used for continuous cutting.The cutting speed of the two types of discharge circuit is basically the same.But the surface roughness machined by the rear-end parallel circuit is reduced by 15.8%,and the surface homogeneity is greatly improved.
基金the National Natural Science Foundation of China(Nos.51575271 and 51975290)。
文摘Via material erosion in wire electrical discharge machining(WEDM),recast layers form on the surfaces of workpiece.In addition,ultra fine Wire-EDM can be usually cut once.To reduce the thickness of the recast layer as much as possible,the wire electrical discharge-electrochemical machining(WEDCM)method was proposed,which is based on the micro conductivity of the dielectric and microelectrolytic characteristics by adjusting the no-load rate of the pulse in the machining process.Furthermore,a state discrimination and servo control system based on discharge current was designed.The experiment results of different no-load rates show that the electrolytic effects increase as the no-load rate increases,and the main machining process is spark discharge erosion with a no-load rate in the range of 10%to 80%.At 90%no-load rate,the amount of recast layer formation in the forward direction of the wire electrode is almost the same as that of electrolytic dissolution,and it can be practically processed without a recast layer.Compared with10%no-load rate,the kerf width only increases by 7.5%.
基金co-supported by the National Natural Science Foundation of China (Nos. 51575271 and 51975290)。
文摘Compared with a copper wire electrode, molybdenum wire with a poor conductor is usually used as the electrode in high speed wire-cut electrical discharge machining(HSWEDM), so the resistance of an ultra-fine wire cannot be ignored. To study the differences of discharge characteristics between the ultra-fine wire and the conventional diameter wire, the continuous discharge waveform of two kinds of wire electrodes was compared. It was found that there was a multichannel discharge phenomenon in the discharge waveform cutting by ultra-fine wire. Through the establishment of a discharge equivalent circuit model and the simulation analysis of the electrostatic field, it was found that the reason why ultra-fine wire is easy to form multi-channel discharge phenomenon is that the potential difference between the wire and the workpiece increased linearly along the axis of the wire. Besides, etching products like metal particles will distort the electric field between the electrodes. Both of them make it easy to form a multi-channel discharge in machining.The results show that the distributions of the equivalent resistance and the peak current are affected by the multi-channel discharge position. Multi-channel discharge can disperse energy and increase effective discharge frequency. Compared with the 0 mm spacing, at a spacing of 100 mm, the machining efficiency increases by 8.7%, the surface roughness decreases by 37.7%, and the average recast layer thickness decreases by 46.6% under the condition of ultra-fine wire-EDM.
基金financially supported by the National Natural Science Foundation of China(Nos.51033004 and 51121001)China Postdoctoral Science Foundation(No.2013M540711)
文摘A new homemade apparatus, i.e. vibration assisted extrusion equipment, is employed to extrude polypropylene. Vibration assisted extrusion is based on the application of a specific macroscopic shear vibration field. Reduction of apparent melt viscosity as a function of vibration frequency is measured at different screw speeds and die temperatures. The effect of the process is investigated by performing mechanical tests, differential scanning calorimetry studies, polarized light microscopy and wide-angle X-ray diffraction. It is found that, compared with conventional extrusion, vibration assisted extrusion could effectively improve the rheological properties of PP melt by incorporating an extra shear vibration field. Both the tensile strength and elongation at break increased under the shear vibration field. For vibration assisted extrusion samples, both the melting temperature and crystallinity increased, accompanied by remarkable grain refinement. Vibration assisted extrusion induced a significantly enhanced bimodal orientation with a high fraction of a^＊-oriented α-crystallites, while only a limited improvement in the flow direction orientation. A structural model, i.e. bimodal c-axis and a^＊-axis orientation of PP macromolecular chains, was adopted to explain the experimental results.
