The thermal behavior of Fe84Nb7B9 nanocrystalline powders were analyzed by DTA, XRD and TEM measurements. The results show that nanocrystalline powders with grain size of less than 10 nm can be prepared by mechanical ...The thermal behavior of Fe84Nb7B9 nanocrystalline powders were analyzed by DTA, XRD and TEM measurements. The results show that nanocrystalline powders with grain size of less than 10 nm can be prepared by mechanical alloying. After annealed below 770 ℃, the obtained powders remain in the scale of nanometer without new phases formed, exhibiting good thermal stability. On its DTA curve, a wide temperature interval of 450 ℃ between its two exothermic peaks can be observed. The powders in as-milled state has bcc structure of supersaturated solid solution type, and nanocrystalline together with amorphous phases can be observed in the powder annealed at 350 ℃, which is beneficial for the good soft magnetic properties.展开更多
The uniformity principle of temperature difference field is very useful in heat exchanger analyses and optimizations.In this paper, we analyze some other heat transfer optimization problems in the thermal management s...The uniformity principle of temperature difference field is very useful in heat exchanger analyses and optimizations.In this paper, we analyze some other heat transfer optimization problems in the thermal management system of spacecrafts,including the cooling of thermal components, the one-stream series-wound heat exchanger network, the volume-to-point heat conduction problem, and the radiative heat transfer optimization problem, and have found that the uniformity principle of temperature difference field also holds. When the design objectives under the given constraints are achieved, the distributions of the temperature difference fields are uniform. The principle reflects the characteristic of the distribution of potential in the heat transfer optimization problems. It is also shown that the principle is consistent with the entransy theory. Therefore, although the principle is intuitive and phenomenological, the entransy theory can be the physical basis of the principle.展开更多
The entransy theory is widely used and found to be effective in thermal analyses and optimizations.Some researchers considered the entransy variation due to viscous heating as part of entransy dissipation and analyzed...The entransy theory is widely used and found to be effective in thermal analyses and optimizations.Some researchers considered the entransy variation due to viscous heating as part of entransy dissipation and analyzed convective heat transfer based on the differential relationship between entropy and entransy.However,it has been pointed out that the derivation of the differential relationship between entropy and entransy is incorrect.In this paper,the convective heat transfer processes with viscous heating is reconsidered and analyzed from the viewpoint of the energy conservation and the entransy balance equation.It is shown that the influence of the viscous heating is equivalent to that of an inner heat source.Therefore,the contribution of viscous heating to system entransy should not be treated as part of entransy dissipation,but entransy flow into the system.Two-stream parallel and counter flow heat exchangers with viscous heating and a thermal insulation transportation problem of heavy oil are taken as examples to verify the theoretical analyses intuitively.In the examples,the numerical results show that the entransy dissipation rates could be negative when the influence of the viscous heating on the system entransy is treated as part of the entransy dissipation.This is obviously unreasonable.Meanwhile,when the entransy contribution from the viscous heating to the system entransy is treated as entransy flow into the system,it is shown that the entransy dissipation rate is always positive,and the heat transfer processes can be well explained with the entransy theory.展开更多
基金Project(2001BA310A03-1) supported by the National Key Technologies R & D Program of China during the 10th Five-year Plan Period Project(05JJ30095) supported by the Natural Science Foundation of Hunan Province, China
文摘The thermal behavior of Fe84Nb7B9 nanocrystalline powders were analyzed by DTA, XRD and TEM measurements. The results show that nanocrystalline powders with grain size of less than 10 nm can be prepared by mechanical alloying. After annealed below 770 ℃, the obtained powders remain in the scale of nanometer without new phases formed, exhibiting good thermal stability. On its DTA curve, a wide temperature interval of 450 ℃ between its two exothermic peaks can be observed. The powders in as-milled state has bcc structure of supersaturated solid solution type, and nanocrystalline together with amorphous phases can be observed in the powder annealed at 350 ℃, which is beneficial for the good soft magnetic properties.
基金Project supported by the Science Fund for Creative Research Groups of National Natural Science Foundation of China(Grant No.51621062)
文摘The uniformity principle of temperature difference field is very useful in heat exchanger analyses and optimizations.In this paper, we analyze some other heat transfer optimization problems in the thermal management system of spacecrafts,including the cooling of thermal components, the one-stream series-wound heat exchanger network, the volume-to-point heat conduction problem, and the radiative heat transfer optimization problem, and have found that the uniformity principle of temperature difference field also holds. When the design objectives under the given constraints are achieved, the distributions of the temperature difference fields are uniform. The principle reflects the characteristic of the distribution of potential in the heat transfer optimization problems. It is also shown that the principle is consistent with the entransy theory. Therefore, although the principle is intuitive and phenomenological, the entransy theory can be the physical basis of the principle.
基金the Science Fund for Creative Research Groups of the National Natural Science Foundation of China(Grant No.51621062)。
文摘The entransy theory is widely used and found to be effective in thermal analyses and optimizations.Some researchers considered the entransy variation due to viscous heating as part of entransy dissipation and analyzed convective heat transfer based on the differential relationship between entropy and entransy.However,it has been pointed out that the derivation of the differential relationship between entropy and entransy is incorrect.In this paper,the convective heat transfer processes with viscous heating is reconsidered and analyzed from the viewpoint of the energy conservation and the entransy balance equation.It is shown that the influence of the viscous heating is equivalent to that of an inner heat source.Therefore,the contribution of viscous heating to system entransy should not be treated as part of entransy dissipation,but entransy flow into the system.Two-stream parallel and counter flow heat exchangers with viscous heating and a thermal insulation transportation problem of heavy oil are taken as examples to verify the theoretical analyses intuitively.In the examples,the numerical results show that the entransy dissipation rates could be negative when the influence of the viscous heating on the system entransy is treated as part of the entransy dissipation.This is obviously unreasonable.Meanwhile,when the entransy contribution from the viscous heating to the system entransy is treated as entransy flow into the system,it is shown that the entransy dissipation rate is always positive,and the heat transfer processes can be well explained with the entransy theory.