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401.
A study was carried out to observe bubble nucleation and site deactivation mechanisms under equilibrium pool boiling of liquids on single sites. The experiments were conducted with benzene, ethanol, and their mixtures on single sites made of glass. These mechanisms were filmed using a cine camera as the temperature of the boiling liquid was decreased in programmed steps. The experiments showed that the deactivation of a bubble nucleation site in the surface occurred by progressive condensation of vapor within it until the volume of vapor became equal to or smaller than the volume of a spherical bubble of diameter equal to that of the site. Based upon these experimental observations, a site deactivation mechanism is proposed for heterogeneous pool boiling and stability criteria are developed for single cylindrical cavities. The effect of relevant parameters on depth-to-diameter ratio of the sites is also determined. 相似文献
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403.
Bubble dynamics is important in flow boiling of minichannel, and ultrasonic field effects bubble behaviors. However, flow boiling bubble movements in minichannels under ultrasonic field have received little research attention and are still poorly understood. In this paper, the effects of ultrasonic field on bubble dynamics are experimentally studied by capturing the bubble motion behaviors of the flow boiling bubbles. The ultrasonic frequencies are set to 23, 28, 32, and 40 kHz. Bubble tracking algorithm, which studies the growth, trajectories, velocities, and traveled distances for bubbles, is created to qualitatively describe bubble motion behavior of flow boiling in minichannel. It is found that after the application of ultrasound, the detachment frequency, velocity, and travel distance of the bubbles significantly increases, and the growth behavior and trajectory are extremely complex, the two-phase gas-liquid flow is extremely unstable. The bubbles gain kinetic energy as the ultrasound frequency increases. Finally, numerical simulations are used to quantitatively investigate the mechanism of bubble motion in microchannels under ultrasonic fields. 相似文献
404.
This paper deals with the study of the dynamics of net vapor generation point in the boiling channel of the steam generator of Kaiga‐1 nuclear power plant. The dynamics has been studied by perturbing liquid velocity at the inlet of boiling channel with a step function and heating rate with a ramp function. Both finite volume method (FVM) and finite difference method (FDM) have been applied to solve the model equations that have been developed to predict boiling boundary. The effect of thermal non‐equilibrium conditions on subcooled boiling has been taken into consideration. A comparative study of the two methods has been carried out based on the analytical solution of the equations. The study shows that at higher system frequency, increasing number of computational grids increase the accuracy of numerical solutions using FVM while FDM fails to achieve the same. The superiority of FVM over FDM for the problem has also been confirmed by grid convergence analysis. An attempt has also been made to find the analytical solution of the effect of change of heat input on boiling boundary, which is an essential part of computations for the simulation of startup and shutdown of the steam generator. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
405.
在前文[1]实验研究的基础上,利用带液池的多孔壁微热虹吸蒸发段模型,对颗粒多孔薄层在真空状态下的沸腾两相流与传热进行了分析,综合实验数据,建立了传热关联式。 相似文献
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407.
本文利用动态微液层模型对双组分混合物的沸腾换热现象进行了理论预测。本模型认为沸腾换热的机理主要是由于在气泡的周期生长过程中所形成微液层的蒸发。模型中考虑了气泡生长过程中液体传质的影响,给出了气泡生长过程中传热面上气-液-固接触的动态构造,计算结果与实验结果能够较好的符合。 相似文献
408.
Dynamic instabilities of two-phase flow associated with refrigerant R-11 in a uniformly heated horizontal in-tube boiling system were experimentally investigated. An experimental setup was designed and built to work in a wide range of mass fluxes G [75–1050 kg/(m2 s)], heat fluxes q (0–100 kW/m2), and fluid inlet temperatures Tinlet (2–24°C). Dynamic instability data were obtained under various working conditions. The dependence of oscillation amplitude and period on system parameters is discussed, and the boundaries of various oscillations are located on the steady-state characteristic curves. 相似文献
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