The concept of field synergy for fluid flow is introduced, which refers to the synergy of the velocity field and the velocity gradient field in an entire flow domain. Analyses show that the flow drag depends not only ...The concept of field synergy for fluid flow is introduced, which refers to the synergy of the velocity field and the velocity gradient field in an entire flow domain. Analyses show that the flow drag depends not only on the velocity and the velocity gradient fields but also on their synergy. The principle of minimum dissipation of mechanical energy is developed, which may be stated as follows: the worse the synergy between the velocity and velocity gradient fields is, the smaller the resistance becomes. Furthermore, based on the principle of minimum dissipation of mechanical energy together with conservation equa-tions, a field synergy equation with a set of specified constraints has been established for optimizing flow processes. The optimal flow field can be obtained by solving the field synergy equation, which leads to the minimum resistance to fluid flow in the fixed flow domain. Finally, as an example, the field synergy analysis for duct flow with two parallel branches is presented. The optimized velocity dis-tributor nearby the fork, which was designed based on the principle of minimum dissipation of me-chanical energy, may reduce the drag of duct flow with two parallel branches.展开更多
Fluid flow and heat transfer characteristics outside a vibrating tube were numerically simulated by the dynamic mesh method. The mechanism of heat transfer enhancement via periodic vibration of the tube was explored b...Fluid flow and heat transfer characteristics outside a vibrating tube were numerically simulated by the dynamic mesh method. The mechanism of heat transfer enhancement via periodic vibration of the tube was explored by using the field synergy principle. It is found that the field synergy angle between fluid velocity vector and temperature gradient vector for a periodically vibrating tube is significantly smaller than that for a stationary tube, and it changes approximately according to the sinusoidal law in a vibration period. The effect of time phase of the vibration on the field synergy angle and convective heat transfer coefficient were also discussed. Results indicate that the vibration can enhance heat transfer and this effect is more remarkable when time phase angle ranges between 50° and 1400 in a half period. Especially when the time phase angle is 90°, the average field synergy angle outside the tube reaches the minimum, which leads to the best heat transfer performance.展开更多
The main methods of single-phase convection heat transfer enhancement are analyzed in this paper, and the unity of contradiction between heat transfer enhancement and energy consumption(or exergy destruction)is expoun...The main methods of single-phase convection heat transfer enhancement are analyzed in this paper, and the unity of contradiction between heat transfer enhancement and energy consumption(or exergy destruction)is expounded.The thermodynamic relationship between heat(or exergy)transfer efficiency and energy consumption(or exergy destruction)as well as driving forces is established,and a general theoretical principle for single-phase convection heat transfer enhancement is further obtained. The principle shows that temperature gradient field distribution and velocity field distribution constrain each other,and that the optimum heat transfer efficiency can be obtained when they are synergetic.If the level of the synergy of temperature gradient field distribution with velocity field distribution is determined,the relative uniform temperature gradient is required,and vice versa.The principle also shows the relationship of relative temperature gradient with specific heat and coefficient of heat conductivity.The deduced results can be used as a theoretical guidance for single-phase convection heat transfer enhancement and optimum design of heat exchangers.展开更多
Conservation equations of sensible entarnsy and latent entransy are established for flue gas convective heat transfer with condensation in a rectangular channel and the entransy dissipation expression is deduced.The f...Conservation equations of sensible entarnsy and latent entransy are established for flue gas convective heat transfer with condensation in a rectangular channel and the entransy dissipation expression is deduced.The field synergy equation is obtained on the basis of the extremum entransy dissipation principle for flue gas convective heat transfer with condensation.The optimal velocity field is numerically obtained by solving the field synergy equation.The results show that the optimal velocity field has multiple longitudinal vortices,which improve the synergy not only between the veloctiy and temperature fields but also between the velocity and vapor concentration fields.Therefore,the convective heat and mass transfers are significantly enhanced.Flow with multiple longitudinal vortices close to the optimal velocity field can be generated by discrete double-inclined ribs set in the rectangular channel.The numerical results show that the total heat transfer rate in the discrete double-inclined rib channel increases by 29.02% and the condensing heat transfer rate increases by 27.46% for Re = 600 compared with the plain channel.展开更多
文摘The concept of field synergy for fluid flow is introduced, which refers to the synergy of the velocity field and the velocity gradient field in an entire flow domain. Analyses show that the flow drag depends not only on the velocity and the velocity gradient fields but also on their synergy. The principle of minimum dissipation of mechanical energy is developed, which may be stated as follows: the worse the synergy between the velocity and velocity gradient fields is, the smaller the resistance becomes. Furthermore, based on the principle of minimum dissipation of mechanical energy together with conservation equa-tions, a field synergy equation with a set of specified constraints has been established for optimizing flow processes. The optimal flow field can be obtained by solving the field synergy equation, which leads to the minimum resistance to fluid flow in the fixed flow domain. Finally, as an example, the field synergy analysis for duct flow with two parallel branches is presented. The optimized velocity dis-tributor nearby the fork, which was designed based on the principle of minimum dissipation of me-chanical energy, may reduce the drag of duct flow with two parallel branches.
基金the National Basic Research Program of China (973 Program, Grant No. 2007CB206903)the New Century Excellent Talents in University (Grant No. NCET-05-0583).
文摘Fluid flow and heat transfer characteristics outside a vibrating tube were numerically simulated by the dynamic mesh method. The mechanism of heat transfer enhancement via periodic vibration of the tube was explored by using the field synergy principle. It is found that the field synergy angle between fluid velocity vector and temperature gradient vector for a periodically vibrating tube is significantly smaller than that for a stationary tube, and it changes approximately according to the sinusoidal law in a vibration period. The effect of time phase of the vibration on the field synergy angle and convective heat transfer coefficient were also discussed. Results indicate that the vibration can enhance heat transfer and this effect is more remarkable when time phase angle ranges between 50° and 1400 in a half period. Especially when the time phase angle is 90°, the average field synergy angle outside the tube reaches the minimum, which leads to the best heat transfer performance.
基金Supported by the National Natural Science Foundation of China(Grant No.20376078)the National Basic Research Program of China("973"Project)(Grant No.2004CCA03100)
文摘The main methods of single-phase convection heat transfer enhancement are analyzed in this paper, and the unity of contradiction between heat transfer enhancement and energy consumption(or exergy destruction)is expounded.The thermodynamic relationship between heat(or exergy)transfer efficiency and energy consumption(or exergy destruction)as well as driving forces is established,and a general theoretical principle for single-phase convection heat transfer enhancement is further obtained. The principle shows that temperature gradient field distribution and velocity field distribution constrain each other,and that the optimum heat transfer efficiency can be obtained when they are synergetic.If the level of the synergy of temperature gradient field distribution with velocity field distribution is determined,the relative uniform temperature gradient is required,and vice versa.The principle also shows the relationship of relative temperature gradient with specific heat and coefficient of heat conductivity.The deduced results can be used as a theoretical guidance for single-phase convection heat transfer enhancement and optimum design of heat exchangers.
基金supported by the National Basic Research Program of China (2007CB206901)
文摘Conservation equations of sensible entarnsy and latent entransy are established for flue gas convective heat transfer with condensation in a rectangular channel and the entransy dissipation expression is deduced.The field synergy equation is obtained on the basis of the extremum entransy dissipation principle for flue gas convective heat transfer with condensation.The optimal velocity field is numerically obtained by solving the field synergy equation.The results show that the optimal velocity field has multiple longitudinal vortices,which improve the synergy not only between the veloctiy and temperature fields but also between the velocity and vapor concentration fields.Therefore,the convective heat and mass transfers are significantly enhanced.Flow with multiple longitudinal vortices close to the optimal velocity field can be generated by discrete double-inclined ribs set in the rectangular channel.The numerical results show that the total heat transfer rate in the discrete double-inclined rib channel increases by 29.02% and the condensing heat transfer rate increases by 27.46% for Re = 600 compared with the plain channel.
