Papers by Author: Yun Bo Zhong

Abstract: Boiling is known to be a very efficient mode of heat transfer in earth gravity, however, in microgravity bubble behavior is different because the buoyancy effects are replaced by surface tension effects such as Marangoni convection. The modeling of nucleate boiling with the effect of Marangoni convection in 0 g is accomplished by using Phase Field Method. Numerical simulation is carried out of single nucleating vapor bubble on a heated wall with and without Marangoni convection. The results show that the flow field consists of a major vortex that recirculates colder fluid from the upper region, pulling it toward the hot surface to the point where the bubble meets the heated surface. This type of flow pattern has been observed in various experiments.

Abstract: Under reduced or micro gravity, bubble dynamic behaviors in liquid melts or liquid solutions are important and common problems on optimizing many processes in space science and technology. Due to limited availability experiments under reduced or micro gravity condition, the studies in this area is still quiet fragmentary. For this reason, we develop two pairs of Helmholtz-Maxwell (H-M) coils, which can produce a uniform gradient magnetic field for the ferrofluid filled in a closed Hele-Shaw cell, so as to achieve reduced or micro gravity condition. Afterwards, we establish the multi-physics mathematic model to calculate the magnetic field and the gravity compensation rate, which shows a 30mm×40mm well-distributed region for 90% gravity compensation rate exists.

Abstract: The problem of free surface deformation is involved in variable fields ranging from material processing to metallurgy. In order to investigate the transient evolution of fluid field and free surface deformation numerically, three numerical simulation methods are proposed among which one is based on level set method, the other two are based on moving mesh method. Afterwards, a benchmark problem of sessile droplet is chosen to test and verify each numerical method. A comparison of each numerical result and experimental result shows a good agreement between each other. Comparison and discussion of three numerical methods are made in the end.

Abstract: A direct current was imposed during the ageing treatment of cold worked Cu-Cr-Zr alloys. The effect of increasing current density and ageing time on the electrical conductivity and microhardness has been investigated. The electrical conductivity was found to increase by about 22% IACS to a maximum value of 90% IACS. Furthermore, the microhardness increased by 21 HV up to 176 HV. A number of Cr precipitates and Cr-rich clusters were observed after the ageing treatment with direct current. The direct current was found to significantly enhance the efficiency of precipitation which is most probably due to the electron wind force and the electromigration-induced mass transport.