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)

Asymmetry of flow Direct Ohmic Heating

Asymmetry of flow Flow Visualisation

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.

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

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

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

Mathematical model RTD Asymmetry of flow Mathematical model RTD Effect of heating: high cross-flow at constant temperature width of slits: 0.8, 1, 1.2 1.4 mm

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

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

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

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

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

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

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

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

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

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.

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

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

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

Comparison of RTD from CFD and experiments

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

CFD and Collimated detectors Point source Cs-137

CFD new algorithms Cartesian "boxing” Interpolation inside an element ci c(x,y,z) li

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

Experimental setup Experiments with point sources Cesium-137 high energy (0.511 MeV) Technetium-99 low energy (0.14 MeV) Collimated detectors

Technetium One ray method 14mm

Technetium View factor

Cesium One ray method

Cesium View factor method

Comparison View factor method One ray method

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.

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