R.

1
Acknowledgement Research is subsidized by
J04/98 212200008
Parallel flow asymmetries
due to non-uniform temperatures, investigated by
tracer techniques

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

Theoretical prediction
Problem Nonuniform distribution of flow into
parallel channels of heaters or heat exchangers
in the case of buoyancy opposing flow.
Z can be expressed in terms of Grashof and
Reynolds numbers as ZGr/Re
?-heat transfer coefficient, cp-heat capacity, H
x B -cross section of channel
G W/m3-volumetric heat source
f1 in a wide channel
Experimental verification
Coloured tracer (KMnO4 velocities in parallel
channels)
Predicted instability
Predicted symmetry
Active volume is reduced if a channel is blocked
Mean residence time (conductivity method KCl)
Temperatures in lateral channels measured by
Nortech Reflex 4 (optical fibres)
PT21
PT21
Conclusions Buoyancy has undesirable effects in
heaters with downwards oriented parallel flows
(non-uniform distribution of flow), which can be
suppressed by decreasing the width of channel H
(this is the most efficient way), or by
increasing viscosity ?. This theoretical
prediction was tested experimentally using tracer
techniques (solution of KCl and KMnO4 ) and by
measurement of temperature profiles.
Thýn, J., Žitný, R., Kluson, J.,
Cechák,T.,Analysis and Diagnostics of Industrial
Processes by Radiotracers and Radioisotope Sealed
Sources, Ed. CVUT, Prague (1999) Žitný,R., Thýn,
J.,Parallel flow asymmetries in continuous
heaters, CVUT Workshop 2001, Praha, Joye D.D.,
Wojnovich M.J. Aiding and opposing mixed
convection heat transfer in a vertical tube loss
of boundary condition at different Grashof
numbers, Int.J.Heat and Fluid Flow, Vol.17, No.5,
1996, 468-473