-1- Microstructure of solid surfaces – characterization and effects on two phase flows...
Transcript of -1- Microstructure of solid surfaces – characterization and effects on two phase flows...
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Microstructure of solid surfaces –characterization and effects on two phase flows
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1) Introduction - Motivation
2) Analysis of the microstructure of the heated surface
3) Heat transfer and bubble formation
4) Bubble movements
5) Conclusion
apl. Prof. Dr.-Ing. Andrea Luke
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1. Introduction
Motivationexamples in energy and process technology
• thermal engines and refrigerating machines • chemical industry and process technology
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1. Introduction
Motivation
• cooling of electronic devices • heat recovery in machine tools
examples in electronics and production technology
extruder
wall
capillary structure
condenser
adiabatic transport zone
evaporator
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advantages of evaporation
heat transfer and emission isobaric/isothermal
(high thermodynamic efficiency)
high heat transfer coefficient
1. Introduction
Motivation
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disadvantages of evaporation
heat transport mechanisms are complexer, compared to single phase heat transfer:
movement of phase interface, non-equilibrium effects and interactions between the phases
mechanisms are not yet clarified in detail
design of evaporators by means of empirical equations
consideration of various boiling mechanisms
1. Introduction
Motivation
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aim:
shift of boiling curve to lower T avoidance of hysteresis effects
qualitative illustration of boiling curve
1. Introduction
Motivation
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parameters:
• thermophysical properties
• operating parameters:
- pressure
- temperature
- heat flux
• properties of heating surface
• orientation of heating surface
• convection effects ...
1 / o = F(p*) F(q/qo) FWR FWMseparation of parameters
empirical calculation method:
1. Introduction
Parameters
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Ideal smooth surface with ideal potential nucleation sites
conic reentrant cavity
2. Analysis of the microstructure Method
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Real rough surface with real potential nucleation sites
y
500 µm
0 x 500 µm
2. Analysis of the microstructure Method
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deterministic structures: emery ground
Ra = 0.53 µm
stochastic structures: sandblasted
Ra = 0.25 µm
z = 4.57 µm
x = 500 µmy = 445 µm
z = 7.84 µm
x = 500 µmy = 500 µm
2. Analysis of the microstructure Method
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determination of potential nucleation sites on the microstructure of the heating surface
• local distribution of cavities
• distribution of distances between neighbouring potential nucleation sites
• size distribution of newly defined cavity-parameters
2. Analysis of the microstructure Method
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three-dimensional envelope surface method (Rk= 2500 μm)
y = 500 μm
z = 7,84 μm
x = 500 μm
Topography
example of a cavity and the parameter P5*
determination of single cavities
y = 500 μm
z = 4,00 μm
x = 500 μm
2. Analysis of the microstructure Method
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emery ground fine sandblasted Rk= 2500 μm
local distribution of potential nucleation sites on heating surface
2. Analysis of the microstructure Method
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size distribution of the cavity-parameter P5*
2. Analysis of the microstructure Method
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standard apparatus for pool boiling
3. Heat transfer and bubble formation Apparatus
condenser
evaporator
test tube
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horizontal copper tubePropane p* = 0,1 Ts= -3,5°C, ps= 4,247
bar
3. Heat transfer and bubble formation
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active nucleation sites: simultaneous and accumulated
3. Heat transfer and bubble formation
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= 262°
= 284°
4.5 x 4.5 mm
visualization of bubble formation by high speed video sequencesPropane p* = 0.1, q = 20 kW/m², fine sand blasted copper tube on horizontal centre line
3. Heat transfer and bubble formation
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simultaneous active nucleation sites
Propane p* = 0,1, q = 20 kW/m², horizontally fine sand blasted copper tube
after t=150 ms, ( N/A ) M = 7 t=151 ms, ( N/A ) M = 9
= 280°
= 257°
3. Heat transfer and bubble formation
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temporal sequence of activationPropane, p* = 0.1, q = 20 kW/m², horizontally fine sandblasted copper tube
3. Heat transfer and bubble formation Results
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local distribution of accumulated and simultaneous (figure 150) aktive nucleation sites:
Propane p* = 0,1, q = 20 kW/m², N/Ak = 622 ( = 30 / mm²)
= 280°
= 257°
3. Heat transfer and bubble formation Results
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emery ground
fine sandblasted
3. Heat transfer and bubble formation Results
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Propane p* = 0,1, q = 5 kW/m², fine sandblasted copper tube on horizontal centre line
model based evaluation of image sequence
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4. Bubble movements
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Propane p* = 0,1, q = 5 kW/m², fine sandblasted copper tubeon horizontal centre line
4. Bubble movements
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5. Conclusion
method: local measurements and analysis of microstructure, heat transfer and bubble formation
aim: short term: improvement of empirical equations to calculate the heat transfer in boiling
long term: development of an universally valid theory of the heat transfer in boiling by modelling the transport processes during evaporation on the heating surface