Flow boiling in microchannels has attracted wide attention due to its excellent heat transfer capability,but flow boiling instability is a huge challenge limiting its application.Instability can lead to a series of pr...Flow boiling in microchannels has attracted wide attention due to its excellent heat transfer capability,but flow boiling instability is a huge challenge limiting its application.Instability can lead to a series of problems,such as uneven flow distribution,temperature and pressure drop oscillations.This work proposes a novel asymmetric check microvalve(ACMV)structure,exhibiting high ratio of resistance between the reverse and forward flow.The results show the reverse pressure drop of the ACMV structure is 2.06 times that of the forward pressure drop,and the forward flow resistance of the ACMV structure is 16%smaller than that of the conventional inlet restrictor.In addition,bubble dynamics of an isolated bubble in the generated channel under dual outlet condition was numerically investigated.It is found that the bubble grows symmetrically in the rectangular channel upstream and downstream.The distance of bubble movement downstream in the microchannel with ACMV is three times that of the microchannel with inlet restrictor.The microchannel with ACMV can suppress the backflow of isolated bubble better than microchannel with inlet restrictor.Moreover,the growth of the bubble downstream extends the effective evaporation domain,which contributes to the enhanced bubble growth rate.The ACMV is expected to be a potential replacement for the conventional inlet restrictor,which provides a novel and efficient solution for future heat dissipation from high power devices.展开更多
Microchannel has demonstrated advantages in the thermal management of integrated chip. In this study, the topology optimization method is applied for designing a topological microchannel to optimize the performances o...Microchannel has demonstrated advantages in the thermal management of integrated chip. In this study, the topology optimization method is applied for designing a topological microchannel to optimize the performances of both heat dissipation and pressure drop. To validate the performance of the topological structure, the flow and heat transfer characteristics of topological microchannel under non-uniform heating flux are numerically studied. The topological structure is designed to cool a heating area of 10 mm×10 mm with 4 hotspots. Heat flux is 40 W/cm^2 in the hotspot area, while it is only 15 W/cm^2 in the rest heating area. The results of heat dissipation performance and pressure drop are compared with those of conventional straight microchannel. Numerical result shows that, compared to the straight microchannel, the hotspot temperature and pressure drop of topological microchannel can be reduced by 4 and 0.6 k Pa, respectively, under the flow rate of 2.2×10^-4 kg/s. The coefficient of performance(COP) of topological microchannel can be 16.1% better than that of straight microchannel, which can be attributed to the effects of optimized bifurcation and confluence structural of topological microchannel.展开更多
Visualization experiments are carried out to investigate the atomization characteristics of R1336mzz flash spray cooling.The influences of superheat,spray distance,and nozzle orifice diameter on spray cooling performa...Visualization experiments are carried out to investigate the atomization characteristics of R1336mzz flash spray cooling.The influences of superheat,spray distance,and nozzle orifice diameter on spray cooling performance are analyzed experimentally.As the superheat increases,finer droplets and thinner liquid film are observed;this is helpful to improve the two-phase heat transfer efficiency.Enlarging atomization angle under high superheat is also observed for flash spray cooling,and it benefits for reducing the spray distance.It can be found that when the inlet superheat is 19.8℃ and the spray distance is 6 mm,the critical heat flux(CHF)reaches 251 W/cm^(2) and the maximum heat transfer coefficient(HTC)reaches 37.4 kW/(m^(2)℃),which are 55%and 11.6%higher than those when the inlet subcooling is 6.9℃ and the spray distance is 12 mm,respectively.Using flash spray reduces the spray distance,which benefits for designing compact spray cooling device.In addition,the nozzle orifice diameter has great influence on the cooling performance of flash spray,and the choice of the nozzle depends on the superheat.This study provides a physical insight into the heat transfer enhancement in flash spray cooling.展开更多
Magnetophoresis is one of the most important separation methods in biological and chemical engineering. In this paper,a novel impact parameter on separation efficiency,i.e.,the angle between the vectors of magnetic fo...Magnetophoresis is one of the most important separation methods in biological and chemical engineering. In this paper,a novel impact parameter on separation efficiency,i.e.,the angle between the vectors of magnetic force and fluid velocity,was derived from the basic equation describing the motion of magnetic beads in microchannels. It is proposed that one of the most important approaches for separation efficiency enhancement is to improve the coordination of magnetic force field and fluid flow field. A T-shaped microchannel magnetophoretic separator was designed based on the angle. And then a two-dimensional dynamic model of magnetic beads moving in microchannels was established to study the separation efficiency of T-shaped microseparator by combined use of finite element method and Runge-Kutta method. The results show that the capture effi-ciency of T-shaped microseparator is much higher than that of the straight microseparator at the same conditions. For small magnetic beads at high fluid velocities,the designed T-shaped microseparator could still keep high separation efficiency whereas the conventional straight microseparator fails to separate the magnetic beads. Further analysis shows that the mechanism of separation efficiency enhancement lies in the synergy of magnetic force field and flow field,which directly leads to large deflected velocity of the magnetic beads from the main stream,and thus increasing the separation efficiency. It is anticipated that the results in this paper are theoretically helpful for the optimum design of highly efficient magnetophoretic separators.展开更多
基金support from the National Natural Science Foundation of China with Grant No.52276070the Fundamental Research Funds for the Central Universities with Grant No.30922010903。
文摘Flow boiling in microchannels has attracted wide attention due to its excellent heat transfer capability,but flow boiling instability is a huge challenge limiting its application.Instability can lead to a series of problems,such as uneven flow distribution,temperature and pressure drop oscillations.This work proposes a novel asymmetric check microvalve(ACMV)structure,exhibiting high ratio of resistance between the reverse and forward flow.The results show the reverse pressure drop of the ACMV structure is 2.06 times that of the forward pressure drop,and the forward flow resistance of the ACMV structure is 16%smaller than that of the conventional inlet restrictor.In addition,bubble dynamics of an isolated bubble in the generated channel under dual outlet condition was numerically investigated.It is found that the bubble grows symmetrically in the rectangular channel upstream and downstream.The distance of bubble movement downstream in the microchannel with ACMV is three times that of the microchannel with inlet restrictor.The microchannel with ACMV can suppress the backflow of isolated bubble better than microchannel with inlet restrictor.Moreover,the growth of the bubble downstream extends the effective evaporation domain,which contributes to the enhanced bubble growth rate.The ACMV is expected to be a potential replacement for the conventional inlet restrictor,which provides a novel and efficient solution for future heat dissipation from high power devices.
基金supported by the National Natural Science Foundation of China(Grant No.51706102)the Fundamental Research Funds for the Central Universities(Grant No.30917011325)
文摘Microchannel has demonstrated advantages in the thermal management of integrated chip. In this study, the topology optimization method is applied for designing a topological microchannel to optimize the performances of both heat dissipation and pressure drop. To validate the performance of the topological structure, the flow and heat transfer characteristics of topological microchannel under non-uniform heating flux are numerically studied. The topological structure is designed to cool a heating area of 10 mm×10 mm with 4 hotspots. Heat flux is 40 W/cm^2 in the hotspot area, while it is only 15 W/cm^2 in the rest heating area. The results of heat dissipation performance and pressure drop are compared with those of conventional straight microchannel. Numerical result shows that, compared to the straight microchannel, the hotspot temperature and pressure drop of topological microchannel can be reduced by 4 and 0.6 k Pa, respectively, under the flow rate of 2.2×10^-4 kg/s. The coefficient of performance(COP) of topological microchannel can be 16.1% better than that of straight microchannel, which can be attributed to the effects of optimized bifurcation and confluence structural of topological microchannel.
基金supported by the National MCF Energy R&D Program(Grant No.2018YFE0312300)the National Natural Science Foundation of China(Grant No.51706102).
文摘Visualization experiments are carried out to investigate the atomization characteristics of R1336mzz flash spray cooling.The influences of superheat,spray distance,and nozzle orifice diameter on spray cooling performance are analyzed experimentally.As the superheat increases,finer droplets and thinner liquid film are observed;this is helpful to improve the two-phase heat transfer efficiency.Enlarging atomization angle under high superheat is also observed for flash spray cooling,and it benefits for reducing the spray distance.It can be found that when the inlet superheat is 19.8℃ and the spray distance is 6 mm,the critical heat flux(CHF)reaches 251 W/cm^(2) and the maximum heat transfer coefficient(HTC)reaches 37.4 kW/(m^(2)℃),which are 55%and 11.6%higher than those when the inlet subcooling is 6.9℃ and the spray distance is 12 mm,respectively.Using flash spray reduces the spray distance,which benefits for designing compact spray cooling device.In addition,the nozzle orifice diameter has great influence on the cooling performance of flash spray,and the choice of the nozzle depends on the superheat.This study provides a physical insight into the heat transfer enhancement in flash spray cooling.
基金supported by the National Natural Science Foundation of China (Grant No.50925624)the National Basic Research Program of China ("973" Project) (Grant No.2012CB720404)the Science and Technology Commission of Shanghai Municipality (Grant No.11XD1403100)
文摘Magnetophoresis is one of the most important separation methods in biological and chemical engineering. In this paper,a novel impact parameter on separation efficiency,i.e.,the angle between the vectors of magnetic force and fluid velocity,was derived from the basic equation describing the motion of magnetic beads in microchannels. It is proposed that one of the most important approaches for separation efficiency enhancement is to improve the coordination of magnetic force field and fluid flow field. A T-shaped microchannel magnetophoretic separator was designed based on the angle. And then a two-dimensional dynamic model of magnetic beads moving in microchannels was established to study the separation efficiency of T-shaped microseparator by combined use of finite element method and Runge-Kutta method. The results show that the capture effi-ciency of T-shaped microseparator is much higher than that of the straight microseparator at the same conditions. For small magnetic beads at high fluid velocities,the designed T-shaped microseparator could still keep high separation efficiency whereas the conventional straight microseparator fails to separate the magnetic beads. Further analysis shows that the mechanism of separation efficiency enhancement lies in the synergy of magnetic force field and flow field,which directly leads to large deflected velocity of the magnetic beads from the main stream,and thus increasing the separation efficiency. It is anticipated that the results in this paper are theoretically helpful for the optimum design of highly efficient magnetophoretic separators.
