R.

1
Acknowledgement Research is subsidized by
J04/98 212200008
Parallel flow asymmetries
in continuous heaters
15 x 21

• R.Žitný, J.Thýn
• Department of Process Engineering
• CTU in Prague, Faculty of Mechanical Engineering
• E-mail zitny_at_fsid.cvut.cz

1. INTRODUCTION Parallel flows are typical for
many apparatuses e.g. shelltube or plate heat
exchangers, heaters, reactors. Sometimes
instabilities or 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 direct ohmic heater.

• 5. CONTROL VOLUME MODEL
• Cross-flow through perforation has been respected
  in a simple integral model, based upon
• Mass balances
• Heat transfer
• Momentum
• Tracer balances
• of control volumes characterised by
• parabolic velocity profile (experimentally
  evaluated umax/umean1.73 at 30 ml/s, theory
  1.744)
• linear increase of temperature
• residence times - serie of mixers
• Responses to tracer injection will be compared
  with experiments.

2. DIRECT OHMIC HEATER Volumetric heat source
enables in principle more uniform heating,
important e.g. for food processing
(sterilization). Problems are with overheating at
walls, e.g. at the surface of electrodes which
must be cooled. The current design makes use
processed liquid flowing in lateral channels for
the electrodes cooling. Electrodes of heater are
perforated - cross flow from lateral channels
should improve uniformity of temperatures in
central channel and uniformity of residence
times. This design is suitable for very viscous
liquids (sauces, juices,....), i.e. for creeping
flow (Reltlt1) when natural convection can be
neglected.
6. 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), KMnO4 (visualisation), Tc99
(radioisotope) were used. Asymmetries of flow are
better observed by thermometers arranged along
the lateral channels..

• 3. PROBLEMS - BUOYANCY
• There are problems with heating of low viscous
  liquids (e.g. water, milk), associated with
  influence of buoyancy and natural convection.
• ASYMMETRY of parallel flows in lateral
  channels. If one of parallel flows slows down its
  temperature increases, and liquid in this channel
  becomes lighter. This creates driving force
  (pressure difference) promoting circulation from
  fast channels to the slow channel thus
  increasing initial small disturbance. One
  parallel stream is delayed or even stopped if the
  temperature increase is too high.
• CROSS-FLOW suppression. Warmer liquid in the
  central channel is lighter than liquid in lateral
  channels and corresponding pressure profile
  forces liquid to flow through perforation in
  opposite direction (out of the heating zone).

• 4. MATHEMATICAL DESCRIPTION
• Theoretical analysis which does not take into
  account cross-flow through perforation 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 identifies parameters
  having significant influence upon the stability
  limits, e.g. width of lateral channels.
• More details can be found in
• http//www.fsid.cvut.cz/en/u218/peoples/zitny/imag
  ohm/instabil/instabil.doc

• CONCLUSIONS
• Asymmetry and instability of parallel flows can
  be explained by natural convection.
• Stability of flow can be improved by increasing
  friction losses, e.g. by narrowing lateral
  channels.
• Model and experiments predict that the cross-flow
  is suppressed (or even reversed) at
  non-isothermal flow.