Papers by Author: Shu Sheng Zhang

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Authors: Wei Chang, Shu Sheng Zhang, Can Zhao, Yun Li Zhang
Abstract: To investigate new features of flow patterns transition during flow boiling in confined vertical narrow rectangular channels, both experiments and theoretical analysis were carried out in the present study. When the channel size was smaller than the bubble departure diameter, a new flow pattern was observed and defined as “confined bubbly flow”. According to the relative size between bubble departure diameter and channel size, two groups of flow pattern transition criteria were developed by using modified drift flux model, with taking features of flow boiling and narrow confinement into account. Satisfactory agreement was obtained by compare model prediction with experiment results. However, further verification and modification are still needed for wider applications.
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Authors: Lei Guo, Shu Sheng Zhang, Ya Qun Chen, Lin Cheng
Abstract: In this paper, the boiling phenomena of steam boiler under atmospheric pressure are simulated by using the UDF program of CFD software. Characteristics including pressure, temperature and vapor fraction respectively for bubble, slug and annular flow regimes are extracted as the input characteristic vectors of the BP neural network and Elman neural network for the purpose of identifying the boiling two-phase (vapor/liquid) flow regimes within wall tubes. It reveals that the rate of recognition accuracy of flow regimes with BP neural network is up to 95.24%, as well as 100% with Elman neural network within the groups taken into consider. By analyzing relations between flow regimes, wall temperature and wall heat transfer coefficient, it is found that changes in flow regimes will cause drastic variation in heat transfer coefficient of the wall surface, and the coefficient reduces rapidly as the wall temperature increases and eventually converge to a minimum. It is a very effective method of using numerical simulation to extract the signal under poor experimental conditions, and is good reference for the further research.
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Authors: Lei Guo, Shu Sheng Zhang, Ya Qun Chen, Lin Cheng
Abstract: Boiling heat transfer in vertical rectangular mini-channels with a width of 1mm and 0.1mm is studied in this paper. The course of bubble generation, growth and departure is numerically simulated and analyzed, and the influence of the movement of phase interfaces on pressure difference and average surface heat transfer coefficient is investigated by using geometry reconstruction and interface tracking. All the calculation is conducted taken into account the gravity, surface tension and wall adhesion. This paper presents that the width of channels plays a significant role in the course of bubble generation, growth and motion, which also leads to changes of critical heat flux. In addition, it is found that surface tension is much more important than gravity in the process of boiling heat transfer. As the channel size is decreased, the boiling heat transfer coefficient increases significantly, which proves that mini-channels can enhance the heat transfer. However, the boiling heat transfer coefficient obtained through numerical simulation is higher than the existing experimental data due to the ideal assumptions adopted in simulation.
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Authors: Wei Chang, Shu Sheng Zhang, Shuai Tian, Meng Jia Huo
Abstract: Based on the establishment of a two-dimensional model, a numerical simulation was conducted in this paper to study the flow boiling and heat transfer characteristics of ethanol in a corrugated mini-channel. User defined functions were employed to describe the key processes of heat and mass exchange at the phase interface. Bubble growth profile was monitored over time and its influence on system pressure drop and heat transfer coefficient was also analyzed. The simulation result shows that the nucleation sites tend to distribute near the internal peaks of the heating wall due to the enhanced local turbulence. The system pressure drop increases over the heating time and fluctuates within a certain range. The heat transfer coefficient decreases with increasing quality, and this trend is consistent with the result of similar experimental studies.
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Authors: Yan Chen, Wei Chang, Shu Sheng Zhang, Chun Sheng Guo
Abstract: An ammonia-stainless steel steady state modeling of loop heat pipe was established, the impacts on the LHP operating temperature induced by alterable heat loads under 3 operating cases (the different position between evaporator and condenser, the changing of ambient temperature and the changing of heat sink temperature) were analyzed and conclusions were made. Changing the position between evaporator and condenser has a significant influence on the LHP operating temperature. Anti-gravity operation will reduce the performance of the LHP, this phenomenon is obviously in low heat load range. Further more, increasing of fluid pressure drop in the loop will induce decreasing of the LHP performance. The temperature difference between ambient and heat sink will influence the transition heat load (from variable conductance mode to fixed conductance mode), the bigger the temperature difference the higher the transition heat load.
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Authors: Lei Guo, Shu Sheng Zhang, Lin Cheng
Abstract: Two different types of channels are investigated which have I- and Z-shaped cross-sections with a width of 2mm. Using the numerical simulation method, the influence of wall contact angle to the process of bubble generating and growth up is studied, and the relationship between different channel shapes and pressure drop is also investigated. In the calculation process, the effects of gravity, surface tension and wall adhesion are taken into account. It is found that wall contact angle has a great influence to the morphology of bubbles. The smaller the wall contact angle is, the rounder the bubbles are, and the shorter the bubbles take to departure from the wall, otherwise, the bubbles are more difficult to depart. The variation of contact angle also has effect upon the heat transfer coefficient, the greater the wall contact angle is, the larger bubble-covered area is, thus the wall thermal resistance gets higher, and the heat transfer coefficient becomes lower. The role of surface tension in the process of boiling heat transfer is much larger than the gravity in narrow channels. The generation of bubbles dramatically disturbs the boundary layer, and the bubble bottom micro-layer can enhance the heat transfer. The heat transfer coefficient of Z-shaped channels is larger than that of I-shaped channels, while the pressure drop of the former is obviously higher.
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