1. R.Žitný, J.Thýn Czech Technical University in Prague
Asymmetry of flow
Anomalies detected by RTD in Direct Ohmic heating – asymmetry of flow due to natural convection
R.Žitný, J.Thýn
Czech Technical University in Prague
Has not be presented yet (Nancy 2001)

2. Asymmetry of flow
Direct Ohmic Heating

3. Asymmetry of flow
Flow Visualisation

4. Flow visualisation Asymmetry of flow
Parallel flow instabilities have been observed experimentally in lateral channels of ohmic heater. One parallel stream is delayed and even stopped or reversed if the temperature increase is too high. H L Te
Warm and light liquid (Te) stands in the right channel.
p1-p0=0gL[1-(Te-T0)]
T0, p0, Q
This phenomenon can be explained by natural convection - theoretical analysis predicts two regimes:
Symmetric solution (flowrates and temperatures in lateral channels are equal)
Asymmetric solution exists within a certain range of flowrates and heating power.

5. Mathematical model Asymmetry of flow Integral model includes:
Ql1
Qci
Qli
Qc1
Qr1
Qri
Qli+1
Qci+1
Qln
Qcn
Wl1
Wr1
Wli
Wli-1
Wli+1
Wln-1 L1 Li Ln Hl Hc Hr p0
pl1
pli-1
pli+1
pli
pr1
pri-1
pri+1
prn
prn-1
pri
Tl0
Tl1
Tli-1
Tli
Tli+1
Tri+1
Tln
Tci
Thi+1
Tci+1
Thi
i-th cell
Integral model includes:
Mass balances
Heat transfer
Momentum(laminar flow)
Tracer balances

6. Mathematical model Stability
Asymmetry of flow
Mathematical model Stability
Asymmetry suppressed by:
Decreasing channel width
Viscosity increase
Cross-flow

7. Mathematical model RTD
Asymmetry of flow
Mathematical model RTD
Asymmetry of flow increases dead volume
ASYMMETRY without heating and for full electrodes

8. Mathematical model RTD
Asymmetry of flow
Mathematical model RTD
Effect of heating: high cross-flow at constant temperature
width of slits: 0.8, 1, mm

9. Mathematical model RTD
Asymmetry of flow
Mathematical model RTD
Effect of heating: the cross-flow is suppressed at heating

10. Mathematical model RTD
Asymmetry of flow
Mathematical model RTD
Effect of channels: Cross-flow high in narrow channel
width of lateral channel: 10 mm

11. Mathematical model RTD
Asymmetry of flow
Mathematical model RTD
Effect of channels: Cross-flow is low in a wide channel
width of lateral channel: 20 mm

12. RTD Experiments Asymmetry of flow
Tc99 is more suitable for measurement at heating (KCl injection has influence upon the power of heater)

13. RTD Experiments Tc99 Asymmetry of flow
Heating suppresses the cross flow. Experiments with radiotracers (Technetium 99)

14. RTD Experiments Tc99,KCl Asymmetry of flow
Comparison of experiments using conductivity method (KCl) and radiotracers (Tc99). Isothermal flow.

15. RTD Experiments Tc99 Asymmetry of flow
Comparison of experiments using radiotracers (Tc99) and integral model

16. Multiple detectors Tc99 Asymmetry of flow
Collimated detectors with different configuration of collimators.

17. Multiple detectors Tc99 Asymmetry of flow
Collimated detectors aimed towards the central heating section are able to detect the asymmetry of parallel flows

18. Conclusions Asymmetry of flow
Idea of spatially localized integral models has been applied for explanation of parallel flow asymmetry and instability.
Integral model is based upon mass, energy and momentum balances (natural convection).
Model predicts, that the instabilities depend upon flowrate, power, geometry of heater and can be suppressed by narrowing lateral channels.
Model predicts that the cross-flow is suppressed (or even reversed) at non-isothermal flow.
Theoretical prediction were confirmed by visualisation, RTD experiment and by measurement of temperatures.

19. CFD, RTD and Collimated Detectors Responses in Direct Ohmic Heating
R.Žitný, J.Thýn
Czech Technical University in Prague
Presented at ChISA 2000

20. CFD models CV-FLUENT Direct Ohmic FEM-COSMOS/M Heating Collimated
Detector
Direct
Ohmic
Heating
FEM-COSMOS/M
CV-FLUENT

21. RTD parallel and cross-flow
2.5 5
2.8 3 4 1 2 h
in mm
0.6
0.00
0.02
0.18
0.33
0.48
0.04

22. Comparison of RTD from CFD and experiments

23. Collimated detectors View factor method ("soft" radiation)
Irradiated area  h d
detector
collimator perfect absorption -rays
radiation source
in an element r
Single ray method ("hard" radiation)  h d
detector
collimator
imperfect absorber
radiation source

24. CFD and Collimated detectors
Point source
Cs-137

25. CFD new algorithms Cartesian "boxing” Interpolation inside an elementci
c(x,y,z) li

26. Czech Technical University in Prague
Experimental evaluation of algorithms of collimated detectors using point source Cs-137 and Tc-99
R.Žitný, J.Thýn
Czech Technical University in Prague
Has not been presented yet

27. Experimental setup Experiments with point sources
Cesium high energy (0.511 MeV)
Technetium-99 low energy (0.14 MeV)
Collimated detectors

28. Technetium One ray method
14mm

29. Technetium View factor

30. Cesium One ray method

31. Cesium View factor method

32. Comparison
View factor method
One ray method

33. Conclusions Results confirm working hypothesis, that the view factor method is more suitable for soft radiotracers, while the single ray method is better for hard radiation.
Correction of collimator thickness H
E is energy of radiation (in MeV), E0=0.4
Apology: Experiments performed with focused collimator in October 2000 have not been evaluated yet.

34. Acknowledgment IAEA Vienna for support of our projects, and to you for attention
Just for fun:
Monitoring pressure at a tracer injection