# (584s) Study on Solidification Rate during the Phase Transformation Process of Pure Ethanol in the Annular Channel

- Conference: AIChE Annual Meeting
- Year: 2017
- Proceeding: 2017 AIChE Annual Meeting
- Group: Fuels and Petrochemicals Division
- Session:
- Time:
Wednesday, November 1, 2017 - 3:15pm-4:45pm

There is a variety of latent heat thermal

energy storage systems researches focusing on phase change materials (PCMs) in

recent years. The most commonly used is paraffin wax, which can be used to

store waste heat. As for cool storage system, water is most suitable and

practical. However, these researches on phase change problems has been carried

out within the temperature range 273 K to 373 K. It is of great necessity to

find proper PCMs on cryogenic regions (< 173K).

Choosing a suitable material is one of the

most critical aspects for designing a thermal energy storage system. Comparing

with n-propanol, n-butane, and taking into consideration of density specific

heat, phase transition temperature and Std enthalpy change of fusion, ethanol (C2H6O)

is regarded as the most suitable material under such a low temperature. The freezing

point is 159K and melting point is the same.

A typical heat exchange system containing a

hollow cylinder of PCM with a heat transfer fluid (HTF) flowing inside the

inner tube is depicted as the figure 1. Inner circle diameter is 20 mm, and outer

circle diameter is 90 mm with a length of 1000 mm.

Figure 1 A typical heat exchange system

Based on the system above, this paper

conducts the numerical study to simulate different conditions via ANSYS Fluent and

redesigns a special visualized phase change experimental channel to observe

ethanol solidification process. Physical properties of PCM are related to the

temperature, pressure, especially temperature. Changes of some parameters are negligible,

but others cannot be ignored. Hence, the following assumptions are made for the

numerical analysis for the convenience of calculation.

(1) Ignore the effect of PCM volume expansion from solid to liquid.

(2) Physical properties of PCM are kept constant over small temperature

ranges.

Those factors affecting the solidification

rate during the phase transformation process of the pure ethanol at really low temperature,

including the initial temperature of the PCM, the wall temperature, and

temperature and velocity of HTF, are investigated to identify the primary one.

For numerical study, two simplified

physical model are used to figure out the main reasons. First, to treat the

inner wall temperature as constant, thus, the model is more simplified. In this

case, there is only PCM, which releases heat and gradually freeze. The outer

wall is regarded as adiabatic boundary, but the inner wall temperature is set

119 K, 124 K or 129 K, and the initial PCM temperature (in the solid phase) is

set 164K

or 169k, respectively. HTF flows into the inner tube with different velocities

and different temperatures to absorb heat from the PCM in the other model. Here,

the initial PCM temperature is also set 164K or 169k. Propane is used as HTF. And

inlet velocity of HTF are 0.1 m/s, 1.0 m/s, and Reynolds Number are around 900,

9000, which indicate that the flow states are laminar, turbulent, respectively.

This model is much closer to the experiment. Non-steady calculation method is

used for the both cases to acquire the liquid fraction of computational domain

over time, whose initial value is 1. Proportion of liquid PCM is tracked every 10

minute.

Another model similar to the literature

(Yoshihiro, 1986) is built to validate the accuracy of numerical

simulation. By comparing the Nusselt numbers along the axis in the condition of

the same Reynolds number, the results agree well with the literature. The dimensionless

temperature *¦È* is defined as

where:

*T _{0}*is the initial temperature of PCM

*T _{f}*is the freeing point

*T _{w}* is the wall temperature

In the case 1, the conclusions

are as follows:

(1) it takes almost the same time for the liquid fraction of PCM to fall

to just over half, at the different *¦È*,

(2) The time when the liquid fraction reduces to the same low level

(<5%) monotonically increases with the *¦È*.

As there is great deviation to regard the

wall temperature *T _{w}* as

constant in the case 1, the boundary conditions do not conform to the reality.

Therefore, the case 2 is built to simulate the actual situation, where HTF takes

away the heat from PCM. It is also concluded that time for the liquid fraction

of PCM reduces to half are the same when considering the different wall temperature

and initial temperature. Another significant finding is that inlet velocity of

HTF makes little difference on solidification rate of PCM. This can be explained

that primary thermal resistance of heat transfer process is from PCM side. The

effects from the thermal conductivity of PCM far overweigh the convective heat

transfer coefficient. Both cases prove that in the early stage, boundary temperature

has little influence on the solidification rate in a certain range. However, considering

the flow of HTF, the inlet velocity does not contribute to solidification, but

inlet temperature has an accelerating effect on it.

A special experimental pipeline consists of

20 mm diameter stainless steel tube and 90 mm diameter PMMA (Polymethylmethacrylate)

tube, with a length of 1000 mm, is designed to visually present the

solidification process of ethanol. The whole circulatory system uses liquid

nitrogen to provide cold energy and uses propane as HTF to carry it, while the PMMA

tube is filled with pure ethanol. The experimental conditions are controlled as

close to the simulation conditions as possible. For numerical study, it is easy

to gain the percentage of liquid PCM over time, while it is difficult to figure

out how much liquid PCM is left in the tube. To overcome this problem, it

records the time when half and ninety-five percent of PCM freezes. The most

results are accordant with the numerical predictions.