LAMINAR FLOW INVERTORS FOR RTD IMPROVEMENT AND HEAT TRANSFER
ENHANCEMENT
ZITNY,R.; STRASAK,P.; SESTAK,J.
KEYWORDS:
Flow invertor, Residence time distribution, Secondary flow
1. PRINCIPLES OF FLOW INVERTORS
The flow invertor is a unit, installed between two straight pipes, which transfers the
portion of slowly moving fluid at the tube wall towards the tube axis (and vice versa). This
inversion should improve the residence time distribution characteristics especially in laminar
flow (so that the residence times of different fluid particles will be approximately the same).
Basic areas of possible applications are:
- Tubular reactors; conversion of chemical reactions depends upon the reaction time
and the maximum yield is achieved when the residence times are uniform.
- Tubular heat exchangers; heat transfer intensity depends upon the radial temperature
gradient at the tube wall and therefore decreases with the growing distance from the
entry; the flow invertor transports yet unheated fluid from the central region towards
the tube wall, thus increasing temperature gradient and heat transfer, Nauman [7].
- Thermal treatment of foods; the flow inversion could improve the uniformity of
exposition times and product quality e.g. in the milk pasteurisation process; consider
the fact, that too long exposition times cause denaturation of the processed food (and
fouling), while too short exposition decreases lethality factor.
A number of principles how to narrow the RTD in a pipe has been suggested (Zhang
[5], Nauman [4]), usually by using fixed or moving mechanical internals (static mixers,
twisted tape, flow dividers, oscillating baffles, series of rotational mixers along tube axis...),
and it is undoubtable a great challenge for CFD specialists to find out an optimal geometry
of such apparatuses. Very interesting possibility is to employ secondary (transversal) flows
induced by centrifugal forces in coiled pipes, see Nigam, Saxena [6]. They assume several
coils in different geometrical arrangements, e.g. as two perpendicularly oriented coils which
exhibits remarkably good efficiency. Similar principle makes use of a sequence of rectangular
bends, see Crookes [7], Cassaday [8].
2. RTD MODELS FOR LAMINAR INVERTORS
3. HEAT TRANSFER ENHANCEMENT PREDICTED BY INVERTOR MODELS
4. INVERTORS BASED UPON A LOCAL BENDING OF A TUBE
5. EXPERIMENTAL RESULTS
The invertor behaviour was studied by stimulus response method using the following
apparatuses setup: A plastic tube (inner diameter 1 cm, length 1.3 m) was fixed at a
supporting plate, so that its central part could be arbitrarily shaped (the length of straight
sections was approximately 60 cm and curved section 10 cm). Two conductivity probes,
consisting of two parallel platinum wires (diameter 0.35 mm, distance 1.5 mm, length 6 mm),
were attached to the ends of the measuring section. A fine wire screen was installed upstream
each probe, so that a sufficient mixing of tracer was ensured. The tracer (3% solution of KCl)
was injected manually by a syringe at the tube centre near the first screen. A small disc
(diameter 3 mm), which improves distribution of the tracer, was inserted among the needle
orifice and the screen. The tracer amount (cca 3 ml), the injection speed and the geometry of
screens are the most important factors affecting accuracy of impulse response identification
and their optimal values were found by a special experimental program.
Responses of the detectors were amplified, A/D converted and evaluated by using PC.
Subsequent data treatment consists of background level change correction, impulse response
identification (using FFT) and averaging of repeated responses. Selected results are given on
fig.11: it is obvious that experiments confirm the inversion effect, but the optimal angle of
straight pipes seems to be greater than the estimate based upon the theoretical analysis.
6. CONCLUSIONS
The residence time distribution of laminar flow invertor, based upon the Dean
secondary flow in a curved pipe, had been analyzed and experiments confirmed predicted
20% increase of first appearance time for optimal geometry design.
Rather general analytical models of invertors were suggested and their parameter
was correlated to the Reynolds number and to the invertor geometry. The models have just
one parameter, which can be interpreted as an efficiency (or a measure of nonideality) of
particular invertor. Thus the model parameter can be used for comparison of different
invertors design.
REFERENCES:
1. Nauman, E.B.: On residence time and trajectory calculation in motionless mixers,
Chem. Eng. Journal, 47 (1991), pp. 141-148
2. Nauman, E.B.: The residence time distribution for laminar flow in
helically coiled tubes, Chemical Engineering Science, 32 (1977), pp. 287-293
3. Dean, W.R.: Note on the motion of fluid in a curved pipe, Phil.Mag.Ser.7, Vol.4,
No.4 (1927), pp.208
4. Nauman, E.B.; Buffham, B.A.: Mixing in continuous flow systems, John Wiley &
Sons, N.Y., 1983
5. Zhang, G.T.; Wannenmacher,N.; Haider,A.; Levenspiel,O.: How to narrow the RTD
of fluids in laminar flow in pipes, Chem. Eng. Sci., 50 (1990), pp. 43-48
6. Saxena, A.K.; Nigam, K.D.P.: Coiled configuration for flow inversion and its effect
on residence time distribution, AIChE J., 30 (1984), pp.363-368
7. Nauman, E.B.: Enhancement of Heat Transfer and Thermal Homogenity with
Motionless Mixers, AIChE J., 25 (1979), pp. 246-258
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