-1- Microstructure of solid surfaces – characterization and effects on two phase flows...

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-1- Microstructure of solid surfaces – characterization and effects on two phase flows ______________________________________________________________________________________ 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

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

___________________________________________________________________________________________

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