A Study of boiling parameters under conditions of laminar non-Newtonian flow with particular reference to massecuite boiling.
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Date
1985
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Abstract
Crystallization is done in the sugar industry using
natural circulation vacuum evaporative crystallizers
known as vacuum pans. the fluid which is known as
massecuite consists of a suspension of crystals in concentrated
molasses. It is highly viscous and slightly
non-Newtonian, and laminar conditions prevail in the
apparatus.
Research on forced convection boiling heat
transfer, pressure drop and vapour holdup has been done
mostly in turbulent flow under pressures higher than
atmospheric, but no studies have been made when boiling
viscous fluids under vacuum.
This thesis describes a series of experiments which
were undertaken with the following objectives:
(a) to determine the influence of the pertinent
variables on heat transfer, friction losses and vapour
holdup while boiling under laminar conditions
(b) to produce a method for the calculation of the evaporation
and circulation rates in vacuum pans, as
this would make possible the optimization of this
type of equipment.
The apparatus used consisted of a single tube steam
heated forced circulation evaporator. The void fraction,
pressure and centerline temperature were measured
along the tube. The fluids used were syrup, molasses
and massecuite covering a thousandfold change in viscosity.
The tests were conducted under different conditions
of vacuum and steam pressures with varying tube
inlet velocities. The experimental results show that the boiling heat
transfer coefficient can be correlated as a function of
the two phase Reynolds number and dimensionless density
ratio and that it is inversely proportional to the tube
length to the power of one third. The pressure drop can
be estimated using the equation of Oliver and Wright (1964)
for bubbly flow. Equations are proposed for calculating the
void fraction in the highly subcooled region and point of
bubble departure. These equations form the basis of a computer
program which by a stepwise and iterative method simulates
the boiling process along the tube.
Measurements taken on a natural circulation pan
with tubes of different length show that this method
predicts the effect of the tube length with reasonable
accuracy. The limitations of this study are that the
experiments were done with a single diameter tube so
that the effect of diameter has not been established
with certainty. Only sugar products were used in the
experiments, and caution is necessary if this method is
applied to other fluids.
Description
Thesis (Ph.D.)-University of Natal, Durban, 1985.
Keywords
Sugar--Manufacture and refining., Laminar flow., Viscous flow., Crystallization., Vacuum technology., Theses--Chemical engineering.