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ABSTRACT
To explore the mechanism of flow boiling in microchannels, the processes of a single-vapor bubble evaporating and two lateral bubbles merging in a 2D microchannel are investigated. The temperature recovery model based on volume of fluid method is adopted to perform the flow boiling phenomena. The effects of wall superheat, Reynolds number, contact angle, surface tension, and two-bubble merger on heat transfer are discussed. Wall superheat dominates the bubble growth and is roughly proportional to wall heat flux. The update of velocity and temperature fields’ distribution in the channel increases with increasing inflow Reynolds number, which improves the wall heat flux markedly. Besides, the area of thin liquid film between the wall and the bubble is enlarged by reducing the contact angle, thus, expanding the wall heat flux several times compared with the single-phase cross section. However, variation of surface tension (0.0589, 0.1 N/m) is found to be insignificant.
Nomenclature
| A | = | area of the bottom wall (mm2) |
| cp | = | specific heat capacity at constant pressure (kJ/kg · K) |
| f | = | volumetric body force (kg/m2 · s2) |
| g | = | gravitational acceleration (9.81 m/s2) |
| i | = | specific enthalpy (kJ/kg/K) |
| k | = | thermal conductivity (W/m · K) |
| n | = | normal to surface |
| p | = | pressure (Pa) |
| qlv | = | volumetric evaporation rate (W/m3) |
| r | = | initial bubble diameter (mm) |
| Re | = | Reynolds number |
| s | = | grid spacing (µm) |
| T | = | temperature (K) |
| t | = | time (s) |
| u | = | instantaneous velocity (m/s) |
| v | = | volumetric generation rate (s) |
| x | = | coordinate in the x-direction |
| y | = | coordinate in the y-direction |
| α | = | volume fraction of liquid |
| μ | = | dynamic viscosity (kg/m · s) |
| ρ | = | density (kg/m3) |
| σ | = | surface tension (N/m) |
| Superscript | = | |
| T | = | transposition |
| Subscripts | = | |
| int | = | interface |
| i, j | = | indices |
| l | = | liquid |
| lv | = | phase change |
| sat | = | saturation |
| v | = | vapor |
| w | = | wall |
Nomenclature
| A | = | area of the bottom wall (mm2) |
| cp | = | specific heat capacity at constant pressure (kJ/kg · K) |
| f | = | volumetric body force (kg/m2 · s2) |
| g | = | gravitational acceleration (9.81 m/s2) |
| i | = | specific enthalpy (kJ/kg/K) |
| k | = | thermal conductivity (W/m · K) |
| n | = | normal to surface |
| p | = | pressure (Pa) |
| qlv | = | volumetric evaporation rate (W/m3) |
| r | = | initial bubble diameter (mm) |
| Re | = | Reynolds number |
| s | = | grid spacing (µm) |
| T | = | temperature (K) |
| t | = | time (s) |
| u | = | instantaneous velocity (m/s) |
| v | = | volumetric generation rate (s) |
| x | = | coordinate in the x-direction |
| y | = | coordinate in the y-direction |
| α | = | volume fraction of liquid |
| μ | = | dynamic viscosity (kg/m · s) |
| ρ | = | density (kg/m3) |
| σ | = | surface tension (N/m) |
| Superscript | = | |
| T | = | transposition |
| Subscripts | = | |
| int | = | interface |
| i, j | = | indices |
| l | = | liquid |
| lv | = | phase change |
| sat | = | saturation |
| v | = | vapor |
| w | = | wall |