查看更多>>摘要：The performance of a direct methanol fuel cell greatly depends on the two-phase flow behaviors in the anode gas diffusion layer and flow channel. However, complex and opaque nature of gas diffusion layer poses a grand challenge for deep insight into the two-phase flow behaviors. Therefore, a homogeneous permeable network integrated with a microchannel was put forward to simulate the anode gas diffusion layer and microchannel in the present study, by which the dynamic two-phase flow behaviors were visually studied with the help of adaptive image enhancement method. The influence of the position and number of the gas inlet, and the gas and liquid flow rates on the two-phase flow behavior were analyzed. The results showed that the liquid saturation decreased and the number of the position for bubble generation increased with increasing the gas flow rate in case the gas was suppled from only one gas supply orifice. When two gas supply orifices simultaneously supplied the gas, the liquid saturation was between the cases using a single one gas supply orifice. For given liquid and gas flow rates of 0.1 mL/min and 0.5 mL/min, the liquid saturation was about 84%, which was between 83% and 92% for the single-one cases. Besides, the two-phase flow exhibited strong periodicity at low gas flow rates. The obtained results provide a deep understanding of the two-phase flow behaviors in such a simulated structure, helping better design and operation of direct methanol fuel cells.
查看更多>>摘要：During quenching, the Leidenfrost effect deteriorates the heat transfer at high temperatures before the arrival of the minimum film boiling point. Accurate prediction of the minimum film boiling temperature (T-min) is significant in assessing cooling efficiency and providing a reference for optimization design. The experimental investigation on the T-min is carried out in multi-narrow rectangular channels containing internal and external coolant channels. The effects of initial surface temperature and wall heat flux on T(min )are discussed in detail. The experimental results indicate that T(min )increases with the increase of initial wall temperature and wall heat flux, by an average of 6.5% and 7.7%, respectively. Under high wall temperature and heat flux conditions, the ratio of T(min )to the initial wall temperature at the inlet is much larger and then remains nearly constant with the propagation of the quench. Furthermore, an improved dimensionless prediction model of T-min is proposed considering the effects of high initial wall temperature and wall heat flux. The comparison results illustrate good agreement with relative errors within & PLUSMN; 15%. Meanwhile, the current model predicts most of the published experimental data with an uncertainty of +/- 15%.
查看更多>>摘要：Space nuclear reactors, a mobile power supply equipment that converts nuclear energy to electric energy, have become significant for space exploration power supply, due to its long life and high power density. A detailed and accurate dynamic model is necessary to study the dynamic characteristics and design an appropriate control system. A dynamic model, including a reactor core, heat dissipation, and thermoelectric conversion models, was derived based on the conservation equations. Detailed development process is presented. MATLAB and Simulink were used to implement the model. To verify the developed model, simulations, including the steady state and startup process, were performed. The calculation results agreed well with the design parameters and the results obtained using other transient code. The dynamic characteristics were obtained by introducing the reactor power and reactivity disturbances. The space nuclear reactor can be concluded to have positive reactivity feedback, strong nonlinearity, and a large time delay. In this study, we provide research foundation for future control system design.
查看更多>>摘要：Computational methods in heat transfer and fluid flow have gained significant progresses and stimulated strong interest and continued efforts in academic and industrial communities for a broad range of thermal engineering. The topics included in this special issue cover computational methods from microscopic to macroscopic modellings and related applied researches from single-phase to multi-phase flow phenomena, as well as an introduction of Prof. Wen-Quan Tao's 50-year academic career in heat transfer. The research efforts documented in the papers clearly reflect the most recent advances in computational heat transfer and fluid flow. The vision of the paper shows some of the key achievements and main challenges which still remain in applied thermal engineering. Finally, some future perspectives in employing modern methods are given to highlight pathways of ongoing researches in understanding and predicting complex physical phenomena and processes in heat transfer and fluid flows.
Ates, AysenurBenam, Behnam ParizadYagci, VedatMalyemez, Muhammed Caglar...
12页
查看更多>>摘要：Flow boiling in heat sinks has been rigorously exploited in the heat transfer community to achieve high heat removal rates for a variety of applications. In this regard, a higher cooling performance could be possible with surface modifications including the use of micro pin fins and tip clearance. The aim of this study is to reveal the effects of pin fins and tip clearance as well as distribution pin fins, which served for more uniform flow distribution, on flow boiling. For this, experiments were conducted on five different heat sinks including a reference configuration (plain channel) and different elliptical pin fin and tip clearance configurations at mass fluxes of 125kg/m2s, 225kg/m2s, and 325kg/m2s within the heat flux range of 18 - 175W/cm2 using de-ionized water as the working fluid. Experimental flow boiling heat transfer results and performances of each heat sink were presented with the help of flow visualization. Accordingly, the maximum heat transfer performance was achieved with a pin fin heat sink with distribution pin fins and no tip clearance on the heating area. It was also proven that streamlined pin fin configurations with uniform flow distribution are helpful in enhancing the heat transfer performance in flow boiling.
NSTL
Elsevier