bubble behaviour 调研

文献阅读笔记

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fictitous boiling

根据文献综述[1]，彭晓峰和他的老师王补宣自1993年开始研究了methanol和deionized water在长方形微通道中受到三面加热的沸腾情况，第四面使用Pyrex玻璃覆盖。实验中他们发现了一种没有可见气泡的沸腾模式[2][3][4]。他们认为存在一个气泡成核的最小空间，称之为evaporating space，如果空间小于这个值，就无法形成可见的气泡。此时的沸腾模式被称为fictitious boiling。1998年，他们基于以下假设：

“internal evaporation and bubble growth have not yet been realized or there may exist countless microbubbles with in the liquid that cannot be visualized by ordinary means.

使用 thermodynamic phase stability theory，给出了一个无量纲参数[5]：

$$\begin{equation} N_{mb} = \frac{h_{fg} \cdot a_v}{c\cdot\pi\cdot(\nu''-\nu')\cdot q \cdot d_h} \end{equation}$$
其中c是经验常熟。当这一无量纲数大于1时，fictitious boiling可能发生。此时

液体处于一种高度非平衡的状态，具有很大的吸收，传递和输运热能的能力。

也有其他人将这一现象解释为相间传递和重叠（interphase propagation and superposition model） [6]（实际上还是这伙人自己）

2000年，他们给fictitious boiling的发生做出了更准确地预测[7]：

They proposed that the pressure wave from the initial growth of a cluster could be reflected in the walls of a microchannel and thereby suppress the growth of the emerging bubble. A criterion for the development of fictitious boiling would then be

$$\begin{equation} L < 0.5 \cdot c \cdot \tau_c \end{equation}$$

where L is the scale of the microchannel, c is the speed of sound,and $\tau_c$ is the time required for the initial development phase of the bubble embryo(~10μs).

boiling in porous media

以下内容来自彭晓峰 ch8 Boiling in Micro-Structures and Porous Media

Abstract

• the dynamic bubble behavior was significantly affected by the bead-packed structure
• Intensive（强烈的） wetting of the liquid replenishment（补充） protected heated surface from full dryout
• The bubble shape and primary bubble interface were described by using a force balance on the bubble
• An introductory model was proposed to perform a theoretical analysis and explore the dryout process（干燥过程） inside the pore structure.
• The contribution of interfacial effects on boiling heat transfer is extremely distinct in porous media, where unique phenomena, or problems, may arise because the dynamic behavior of the bubble interface is significantly affected by the porous structure during boiling.

Test Apparatus

• glass beads: 0.5~7mm
• 3 main areas
  
  • cavity
  • narrow corner zone
  • bridge channel

Experment details

• Heat flux
  
  • low: 16.1kW/m2, 36.8kW/m2, 45.2kW/m2
  • moderate: 67.6kW/m2
  • High
• 气泡生长过程
  
  • generated in corners
  • penetrated in cavities
  • enlongated
  • trancated(snip-off)
  • escaped

bubble behaviour: Boiling process

bubble behaviour: Static Description of Primary Bubble Interface

Replenishment and Dynamic Behavior of Interface

Replenishing Liquid Flow

Dynamic Behavior of Bubble Interface

Interfacial Heat and Mass Transfer at Pore-Level