Computational fluid dynamics was used and a numerical simulation analysis of boiling heat transfer in microchannels with three depths and three cross-sectional profiles was conducted. The heat transfer coefficient and bubble generation process of three microchannel structures with a width of 80 μm and a depth of 40, 60, and 80 μm were compared during the boiling process, and the factors influencing bubble generation were studied. A visual test bench was built, and test substrates of different sizes were prepared using a micro-nano laser. During the test, the behavior characteristics of the bubbles on the boiling surface and the temperature change of the heated wall were collected with a high-speed camera and a temperature sensor. It was found that the microchannel with a depth of 80 μm had the largest heat transfer coefficient and shortest bubble growth period, the rectangular channel had a larger peak heat transfer coefficient and a lower frequency of bubble occurrence, while the V-shaped channel had the shortest growth period, i.e., the highest frequency of bubble occurrence, but its heat transfer coefficient was smaller than that of the rectangular channel.

With the rapid development of microelectronics, electronic devices continue to shrink, and the degree of integration continues to improve. However, as the calorific value per unit volume increases, the heat dissipation of these small devices cannot meet operating requirements, resulting in a device with a shorter life. At the same time, the continuous high-temperature conditions have led to a 3.8% decrease in reliability [

Research on boiling heat transfer originated from the saturated boiling heat transfer curve under standard atmospheric pressure proposed by Nukiyama [

Many studies have been conducted on boiling bubbles’ occurrence, growth, and detachment. Ranjan et al. [

Tiwari et al. [

Although considerable progress has been made in the enhancement of boiling heat transfer, numerical simulation and visual experiments have not been combined. There is little research on the effect of a certain size change on bubble behavior in fixed structures. This paper takes the copper boiling bottom plate as the research object. It uses numerical analysis and visual experiments to study the influence of structural factors (channel configuration, channel depth, and channel height-depth ratio) on boiling.

Fluid flow obeys the laws of the conservation of mass, momentum, and energy. These three basic fundamental principles correspond to three governing equations: the continuity equation expresses mass conservation, the N-S Equation expresses momentum conservation, and the energy equation expresses energy conservation.

Continuity equation:

Substituting the vector

When the basic parameters of fluid motion in the flow field are all three-dimensional incompressible fluid parameters, the density of the fluid is constant, and

Momentum equation:

If gravity is not considered and the physical force is simplified, _{x}_{y}_{z}

Energy conservation equation:

To make

If the fluid is an ideal gas, then:

The formation of bubbles must simultaneously meet two conditions: the core point of vaporization and the superheating of the liquid. Therefore, the density of the vaporization core point is a key factor affecting bubble formation. As

Since the liquid on the heated wall surrounding the vaporization core is constantly evaporating, the temperature of the surrounding superheated liquid will be higher than the temperature inside the bubble at the vaporization core point, causing the surrounding superheated liquid to continuously transfer heat to the bubble, causing the bubble to grow. The buoyancy of the bubble increases as its volume increases, exceeding the surface tension generated between it and the heating wall. The bubble will then detach from the heating wall, allowing surrounding liquid to flow into the now-vacant nucleation point. This process also disturbs the liquid near the wall, greatly reducing the thermal resistance and improving the boiling heat transfer performance.

Both homogeneous and heterogeneous nucleation occur during the nucleation process. This paper considers only heterogeneous nucleation, so homogeneous nucleation will not be introduced. During the boiling process, non-condensable gas in the liquid, or residual gas in the pits or cracks on the heated wall, will become a natural vapor nucleus.

As

As a result, all bubbles are generated on the wall. That is, before the pit can become the vaporization core, its radius must satisfy the

According to the Clausius-Clapeyron equation, the degree of superheating required to generate bubbles with radius

As superheating at the wall increases, the pressure difference

This paper studies the influence of rectangular channels with different depths on boiling heat transfer performance. The microstructure dimensions are shown in

The evaporation end uses a rectangular copper plate with external dimensions of 100 mm × 100 mm × 2 mm. A comparative experiment is performed on the surface of the copper plate with nine groups of channels of different sizes.

Name | Channel width ( |
Channel depth ( |
---|---|---|

Plain | 0 | 0 |

W60H40 | 60 | 40 |

W60H60 | 60 | 60 |

W60H80 | 60 | 80 |

W80H40 | 80 | 40 |

W80H60 | 80 | 60 |

W80H80 | 80 | 80 |

W100H40 | 100 | 40 |

W100H60 | 100 | 60 |

W100H80 | 100 | 80 |

Micro-nano laser marking technology was used to prepare the test base plate, and the surface topography structure was obtained through a series of experiments. Taking the test base plate with a width of 80 μm and a depth of 40 μm as an example (

The visual boiling test device consists of a heat source adjustment module, a temperature acquisition module, an image visualization module, and a boiling pool (

The average value of the readings of the thermal resistance temperature sensors on the test base plate is

In this experiment, the temperature only changes in a single direction-x-which can be obtained by integrating the expression of Fourier’s law:

The surface heat transfer coefficient,

For image interpretation, the distance between two adjacent channels (5 mm) was used as a reference scale. Using the bubble picture captured by the high-speed camera, the number of bubble diameter pixels and the actual number of pixels of the reference scale were compared using image processing software. To compare, the bubble diameter was also calculated using

To investigate whether bubble nucleation in the same site could be stable, we randomly selected the result of three groups to verify the robustness.

Differently sized channels have different effects on bubble detachment and boiling heat transfer. The variation of bubble detachment diameter (D) with the degree of wall superheating (Δ

As shown in

The variation of bubble detachment frequency (f) with the degree of wall superheating (Δ

As

The heating table temperature

As

In summary, for the scale of 40∼100 μm, the boiling heat transfer performance of the evaporation end bottom plate with the microchannel structure is greatly improved compared with that of the planar structure. For the microchannel evaporation end bottom plate, when the heat flux density is small, the larger the width and depth of the microchannel, the better the boiling heat transfer performance. When the heat flux density is high, however, increasing the width and depth of the microchannel has a negative impact on the boiling heat transfer performance.

As the volume fraction of the discrete phase in this simulation is greater than 10%, the Euler-Eulerian method was used for the simulation [

The VOF model is based on the immiscibility between phases in the flow field. That is, the sum of the volumes of each phase is 1, so the obtained volume fraction equation is:

Our boiling simulation only involves gas-liquid two-phase flow, so the above

Using the volume fractions of the gas-liquid two-phase flow in

Taking gravity and continuous body force into account, the momentum equation is as follows:

The mixed-phase density ρ and the mixed-phase dynamic viscosity μ can be calculated from the following

The energy equation is as follows:

Q is the source term, which is divided into two items according to the direction of energy transfer, as follows:

A two-phase flow was used in this simulation. The initial liquid phase was liquid water, which filled the entire 10 mm × 10 mm fluid domain. The heating wall’s solid material was copper, and its thickness was set to 2 mm.

ICEM CFD software was used for mesh division, as shown in

Density (kg/m^{3}) |
Viscosity (kg/m-s) | Gas-liquid surface tension (N/m) | |
---|---|---|---|

Water-liquid | 998.2 | 0.001003 | 0.06 |

Water-vapor | 0.5524 | 1.34 × 10^{–5} |

For the rectangular channel, numerical simulations were conducted for channels with the same width, but different depths and the heat transfer coefficient on the wall of the channel was monitored at the same time.

The gas phase volume fraction nephrogram can visually represent the bubble growth process, and the heat transfer coefficient can directly indicate the heat transfer performance of the microchannel.

As

Comparing

The heat transfer performance of three different shapes of microchannels–rectangular, V-shaped, and U-shaped (

The analysis showed that the effect of the bubble behavior on the surface heat transfer coefficient during the boiling heat transfer process is basically the same for the V-shaped and U-shaped microchannels as for the rectangular microchannel and will not be repeated here. The analysis was focused on the variation of the heat transfer coefficient with time for the three different shapes under the condition of a superheat of 6 K.

In this paper, the effect of microchannel structure on bubble detachment diameter, bubble detachment frequency, and boiling heat transfer performance was studied by a combination of simulation and experimental studies. The results of the study can be summarized as follows:

A single bubble will go through three stages in the process of nucleation of microchannels, growth on the substrate, bubble growth and detachment, and complete detachment. When compared to a flat bottom plate, the presence of microchannels causes bubbles to expand to a larger diameter before detaching.

The average level of bubble detachment frequency,

The depth of the microchannel greatly influences the frequency of bubble detachment. When the microchannel is deeper, the capillary force is stronger, which is more conducive to the growth of bubbles.

Under the same heat flux density, the degree of superheating of the surface of the evaporation end bottom plate with the microchannel structure is lower than that of the flat evaporation end bottom plate, which indicates that the microchannel structure significantly improves the boiling heat transfer performance.

When the heat flux density is small, for microchannels of the same width, the greater the depth, the better the boiling heat transfer performance. When the heat flux density is high, the reverse is true. The smaller the depth, the better the boiling heat transfer performance.

The peak value of the surface heat transfer coefficient of microchannels with different geometries follows the trend rectangular > V-shaped > U-shaped.

velocity components in x, y, z directions

velocity vector

velocity divergence

the pressure of the micro-body (Pa)

_{x}

_{y}

_{z}

body strength (N)

temperature (K)

_{p}

specific heat capacity (J/kg⋅K)

_{g}

saturation temperature under pressure inside the bubble (K)

_{s}

ambient temperature (K)

_{w}

wall temperature (K)

superheat (K)

_{ave}

average temperature of the temperature sensor (K)

_{cu}

copper block temperature (K)

heat flux (kW/m²)

_{sat}

saturated deionized water temperature (K)

surface heat transfer coefficient (W/m²⋅K)

_{b}

the actual diameter of the bubble (mm)

_{r}

reference actual diameter (mm)

_{b}

number of bubble pixels

_{r}

number of bubble pixels

volumetric mass transfer rate from gas phase to liquid phase (kg/m/^{3}⋅s)

volumetric mass transfer rate from liquid phase to gas phase (kg/m^{3}⋅s)

gravitational acceleration (m/s^{2})

body force (N)

_{lv}

latent heat transfer from liquid phase to gas phase (J/kg)

_{vl}

latent heat transfer from gas phase to liquid phase (J/kg)

the heat transfer coefficient of the fluid itself (W/m²⋅K)

_{T}

viscous dissipative term

gas molar constant

_{l}

liquid pressure outside the bubble (Pa)

_{s}

ambient pressure of boiling system (Pa)

_{g}

pressure inside the bubble (Pa)

liquid temperature (K)

phase volume fraction in the cell

liquid volume fraction

gas phase volume fraction

fluid density (kg/m³)

surface viscous stress

thermal conductivity (W/m⋅K)

Test base plate thickness (0.002 m)

mixed phase dynamic viscosity (Pa⋅s)

direction

copper

average

saturation

bubble

reference

liquid

vapor

This work was supported by the

The authors declare that they have no conflicts of interest to report regarding the present study.