A could be introduced to monitor the temperature

A relationship to the temperature difference
was hard to deduce for both the heat transfer coefficient and the heat flux for
the initial data collected as shown by Figures 2 and 3 respectively. This was a
result of the large temperature difference between the heating element and the liquid.

The initial data was collected whilst the boiling mechanism was seen to be convective
as little to no ebullition was observed, however convection currents were
visible at the heating element surface. The large disparity in temperature
readings for the heating element and the liquid may have been due to the limited
heat transfer as well as the distance between the thermometer and the heated
liquid. Error was introduced in the liquid temperature reading as the
thermometer was not able to record an average temperature for the first few
results due to the poor mixing preventing the increase in thermal energy being
observable. An electric heat sensor could be introduced to monitor the
temperature of the liquid as the thermometer became difficult to view as the
bubble formation increased and became more vigorous.

The change in boiling regime to nucleate
boiling was marked by the formation of small vapour bubbles. The first few nucleation
sites were located around surface imperfections such as where the surface of
the heating element was disturbed by protruding wires. Despite knowledge on the
analytic prediction of the formation of nucleation sites being limited, the
location of the nucleation sites has been found to be in pits and grooves along
surfaces which corresponds to the results of this experiment (Eckert &
Drake, 1972).

Figure 2 showed the change to nucleate boiling corresponded with a large
increase in heat flux whilst Figure 3 revealed a rapid increase in heat
transfer coefficient which was expected due to similar results being derived in
literature (Foust, 1980).

The increase in the heat transfer coefficient may have been caused by vapour
bubbles which condensed before reaching the surface of the liquid therefore
releasing energy into the bulk liquid (Roshesnow, 1964). As the temperature difference was
increased, the bubble formation became more vigorous leading to turbulence which
caused the mixing within the fluid to increase (Roshesnow, 1964). The increase in
heat flux may have been a result of the combination of the increased mixing and
the condensation of vapour bubbles within the bulk fluid. After the heat
transfer coefficient reached a maximum of, Figure 2 shows the value
of the coefficient gradually decreased as the temperature difference increased
at a rate which was lower than the initial increase. This may have been due to
the additional bubble formation and growth reducing the heating elements
contact with the liquid (Foust, 1980). As vapour generally
has a lower heat transfer coefficient compared to liquid, the bubbles caused a
resistance to heat transfer, which in turn reduced the total observable heat
transfer coefficient (Coulson
JM, 1964).

The effects of the increased bubble production were also revealed in Figure 3,
as a decrease was observed in the rate at which the heat flux increased at a
temperature difference slightly less than 20 ?C, which was the same point at
which the heat transfer coefficient reached a maximum and began decreasing. Whilst data was being collected, a minimal change in
the temperature of the liquid was observed which may have contributed to the
low heat transfer coefficient calculated. The small changes in temperature may
have been due to the poor insulation provided by the glass cylinder causing
heat to be transferred to the surroundings. To improve the experiment an
insulated material could be used for the container with a small viewing glass
to allow the regimes of boiling to be observed to reduce heat dissipated to the
surroundings.

The experimental critical heat flux was found to
be  however the theoretical value was higher at  which gave a percentage error of 4.8% between
the two values. Due to the small error margin the theoretical critical heat
flux could be used as a prediction however the experiment provides a basis for
the suitability of the approximation. An experiment could be run on a pilot
scale to verify whether the approximation would be safe to use for an
industrial scale plant. Operating at the critical heat flux can be dangerous as
it can cause the heating element to increase above a safe working temperature
which can lead to the equipment being damaged (Collier,
1981).

Therefore when boiling, the aim is to operate at a high heat flux which is
still lower than the critical heat flux to enable an efficient but safe
process. The example for finding the time taken to boil the CFC was an example
of this as the operational heat flux was 80%, which was a compromise between a
high heat flux and a safe value in respect to the critical value. When scaling
up to industrial scale operating at 80% of the critical heat flux would be
advisable, therefore a temperature difference of about 30 ?C should be
implemented. The selected temperature difference has the added benefit of a
reasonably high heat transfer coefficient. An attempt to induce the nucleate
boiling regime faster could be made by using a heating element and pre-heater
surfaces rough therefore reducing the time taken to boil the CFC and the energy
cost of running the heater.