1. -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

2. -2- 1. Introduction Motivation examples in energy and process technology thermal engines and refrigerating machines chemical industry and process technology

3. -3- 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

4. -4- advantages of evaporation  heat transfer and emission isobaric/isothermal (high thermodynamic efficiency)  high heat transfer coefficient 1. Introduction Motivation

5. -5- 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

6. -6- aim: Þ shift of boiling curve to lower  T Þ avoidance of hysteresis effects qualitative illustration of boiling curve 1. Introduction Motivation

7. -7- 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/q o ) F WR F WM separation of parameters empirical calculation method:  1. Introduction Parameters

8. -8- Ideal smooth surface with ideal potential nucleation sites conic reentrant cavity 2. Analysis of the microstructure Method

9. -9- Real rough surface with real potential nucleation sites y 500 µm 0 x 500 µm 2. Analysis of the microstructure Method

10. -10- deterministic structures: emery ground R a = 0.53 µm stochastic structures: sandblasted R a = 0.25 µm z = 4.57 µm x = 500 µm y = 445 µm z = 7.84 µm x = 500 µm y = 500 µm 2. Analysis of the microstructure Method

11. -11- 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

12. -12- three-dimensional envelope surface method (R k = 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

13. -13- emery ground fine sandblasted R k = 2500 μm local distribution of potential nucleation sites on heating surface 2. Analysis of the microstructure Method

14. -14- size distribution of the cavity-parameter P5* 2. Analysis of the microstructure Method

15. -15- standard apparatus for pool boiling 3. Heat transfer and bubble formation Apparatus condenser evaporator test tube

16. -16- horizontal copper tube Propane p* = 0,1 T s = -3,5°C, p s = 4,247 bar 3. Heat transfer and bubble formation

17. -17- active nucleation sites: simultaneous and accumulated 3. Heat transfer and bubble formation

18. -18-  = 262°  = 284° 4.5 x 4.5 mm visualization of bubble formation by high speed video sequences Propane p* = 0.1, q = 20 kW/m², fine sand blasted copper tube on horizontal centre line 3. Heat transfer and bubble formation

19. -19- 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

20. -20- temporal sequence of activation Propane, p* = 0.1, q = 20 kW/m², horizontally fine sandblasted copper tube 3. Heat transfer and bubble formation Results

21. -21- local distribution of accumulated and simultaneous (figure 150) aktive nucleation sites: Propane p* = 0,1, q = 20 kW/m², N/A k = 622 ( = 30 / mm²)  = 280°  = 257° 3. Heat transfer and bubble formation Results

22. -22- emery ground fine sandblasted 3. Heat transfer and bubble formation Results

23. -23- Propane p* = 0,1, q = 5 kW/m², fine sandblasted copper tube on horizontal centre line model based evaluation of image sequence 42 4. Bubble movements

24. -24- Propane p* = 0,1, q = 5 kW/m², fine sandblasted copper tube on horizontal centre line 4. Bubble movements

25. -25- 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