Electrodes of heater are perforated - cross flow from lateral channels should improve uniformity of temperatures in central channel. However, R.Žitný, J.Thýn Department of Process Engineering CTU in Prague, Faculty of Mechanical Engineering Acknowledgement: Research is subsidized by J04/98: EXPERIMENTAL SET-UP DIRECT OHMIC HEATER Problem: Parallel flows are typical for many apparatuses of process industries, e.g. shell&tube or plate heat exchangers, heaters, reactors. Sometimes instabilities or just only non-uniform distribution of flow in parallel channels occur if the apparatus operates at non-isothermal conditions. Parallel flow instabilities have been observed also in lateral channels of ohmic heater. One parallel stream is delayed or even stopped if the temperature increase is too high. Cross-flow through perforation of electrodes is also suppressed. These unpleasant effects are caused by natural convection. Mathematical description Theoretical analysis predicts two solutions of temperature and flow-fields:  Symmetric solution (flow-rates and temperatures in lateral channels are equal)  Asymmetric solution exists within a certain range of flow-rates and heating power. Asymmetric solution can be interpreted as a magnitude of disturbance causing instability of flow. Mathematical model enables to identify parameters having significant influence upon the stability limits, e.g. width of lateral channels. Experiments: Stimulus - response technique (injection of a tracer and measurement responses) has proved to be useful for detection of cross-flow. As tracers KCl (conductivity method), KMnO 4 (visualisation), Tc99 (radioisotope) were used. Conductivity method is superior at isothermal flows, while radioisotopes are better when the liquid is heated (KCl solution increases conductivity and therefore electric power). Asymmetries of flow are better observed by thermometers arranged along the lateral channels. Experiments were performed for 3 different widths of lateral channels (18, 10, 7.5 mm), full and perforated electrodes, flow-rates ml/s, heating power kW. Conclusions: Control volume model of natural convection can describe asymmetry and instability of parallel flow. Instabilities depend upon flow-rate, power, geometry and can be suppressed by narrowing lateral channels. Model and experiments predict that the cross-flow is suppressed (or even reversed) at non-isothermal flow. 15 x 21 Impulse response measured by conductivy method (KCl) radioisopes (Tc 99) Temperatures measured by 12 Pt100 thermometers Fast optical probes ReFlex H L Warm liquid (T e ) stands in the right channel if p 1 -p 0 =  0 gL[1-  (T e -T 0 )] T 0,p 0,Q Control volumes are characterised by: parabolic velocity profile linear increase of temperature residence times - serie of mixers Linear relationship between pressure drop and flow-rate through slits in electrodes Mass balances Heat transfer Momentum Tracer balances Narrow lateral channels increase cross-flow, but only at isothermal flow Cross-flow is lower for wide lateral channels.