-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

Introduction Motivation examples in energy and process technology thermal engines and refrigerating machines chemical industry and process technology

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

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

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

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

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

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

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

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

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

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

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

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