Simulated behaviour of the SSIG

Having validated the model, it can be used to simulate and understand the general behaviours of the SSIG. Another main advantage of having an analytical model is its ability of calculating some immeasurable parameters such as the ice particle size and the axial variation of the fluid temperature within the SSIG. In this section, the validated model is used to generate data to demonstrate some operating characteristics of the binary ice system.

109 Figure 4.10 shows the simulated axial temperature profiles of the fluid along the SSIG under different loads when the compressor is running; the left hand side represents the inlet to the SSIG. In general, 3 different modes can be expected.

Mode 1: No ice is produce in the SSIG. The SSIG inlet liquid temperature is

high enough that the liquid cannot be cooled down to its freezing point in the ice generator. Therefore the SSIG essentially operates as an ordinary liquid chiller with a significant difference between inlet and outlet temperatures.

Mode 2: Initial part of the scraped heat exchanger acts as a liquid chiller

bringing the fluid temperature down towards its freezing point, while the rest produces ice. The profile can be divided into two sections as discussed previously. The temperature gradient of the first section is much larger than the second section and the boundary between the two sections moves with the load conditions.

Mode 3: Ice is generated along the entire length of the SSIG. In this case, the

fluid going into the ice generator already contains some ice in it. The temperature of the fluid drops slightly along the SSIG.

Among the three operation modes, mode 2, particularly with a small length of Section I, is the most desirable one. In Mode 1, no ice can be produced, suggesting the cooling capacity is too small when compared to the required load. It should be avoided through proper design and sizing of the equipment. Binary ice systems normally incorporate with control mechanisms (Guilpart et al. 2005) to ensure the appropriate amount of ice is generated to avoid ice blockage. When the load is too low (Mode 3), the system will be cycled-off by the control monitoring the inlet temperature. Therefore, in practice, Mode 2 is the most likely to occur.

110 (a)

(b)

Figure 4.11 Local fluid temperature and ice concentration along the SSIG

Figure 4.11 illustrates the binary ice temperature and ice concentration profiles along the SSIG under Mode 2 with a fixed evaporating temperature. It can be seen that there are two distinct gradients of temperature profile along the length of the SSIG, with relatively much larger temperature drop in Section I; in fact, over 70% of the overall temperature drop takes place in the first 20% of the length in Figure 4.11a. As expected, Section II experiences a much smaller temperature drop (no more than 1 °C), due to involvement of the latent heat. The ice concentration increases steadily in a slightly non-linear manner to ~6.3%.

As the load changes, both the gradients and the lengths of the two sections will change accordingly. Figure 4.11b demonstrates the binary ice temperature and ice

Section II Section I

111 concentration with a higher inlet temperature but with all other working conditions remained unchanged. Section I becomes longer when the difference between the inlet temperature and the freezing point is larger. At the outlet of the ice generator, the ice concentration (~5.4%) is smaller thus less ice is produced when compared to Figure 4.11a. Unless the length of the SSIG exceeds an estimated length of 2.3m based on the design working conditions of the test rig, it is rather unlikely that Section III will occur.

Figure 4.12 shows the maximum thickness of the ice layer between two scraping actions along the SSIG, which is thicker at the initial part of Section II, suggesting the predicted ice particle sizes are between 6 to 8 µm. The reason is that when water is taken out from the solution, solution concentration goes up and the freezing point of the remained liquid decreases, and this leads to a smaller difference between the solution freezing point and the wall surface temperature, assuming the evaporating temperature remains constant; the generated ice layer becomes thinner. A limitation of the model is that it can only predict a sudden jump of ice thickness between section I and II; however as in reality, the thickness increases from zero thickness over a short distance.

Figure 4.12 Maximum ice layer thickness between two scrapings along the SSIG

To make full use of the SSIG, the length of Section I should be kept as short as possible during operation. The length is determined by the many parameters such as the fluid inlet temperature, the flowrate, the solution freezing point and the evaporation temperature. In practice, if the same solution concentration is used, then the freezing point is a fixed value. The evaporation temperature cannot be controlled

112 directly. Therefore only the influences of the first two parameters are addressed here. The length of Section I against the inlet fluid temperature Tfluid,in under three different flowrates is shown in Figure 4.13 Apparently, when the inlet temperature is higher (i.e. a higher load), more heat needs to be taken out from the solution in order get to the freezing point. This leads to a longer section without producing ice. A larger flowrate also increases the length of section I.

Figure 4.13 Variation of the length of Section I against the inlet fluid temperature Tfluid,in under three different flowrates

The following figures (Figure 4.14 - Figure 4.16) present the variations of some of the model outputs due to changes in working conditions that could be caused by occurrence of a fault. Y-coordinate on the right is the temperature of binary ice at the SSIG outlet and the left hand is the cooling capacity of the SSIG, for a range of evaporating temperatures, flowrates and initial solution concentrations, while the inlet temperature is kept constant at -5 °C. Depending on the combination of various parameters, individual data points can fall into one of the three possible operation modes described earlier.

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Figure 4.14 Binary ice temperature and cooling capacity at ice generator outlet against evaporating temperature (design Te = -20 °C, flowrate fixed at 9L/min, initial solution

concentration at 15% by mass)

The effects of evaporating temperature are shown in Figure 4.14. A lower evaporating temperature increases the temperature difference between the binary ice and the refrigerant under the same initial solution concentration and the inlet temperature, resulting in a lower flow temperature at the outlet and a higher cooling capacity. It is worth noting that, in a system, one would normally expect the cooling capacity increases with increasing Te, as the refrigerant mass flow rate increases due to higher suction vapour densities at higher Te. However, the presented results are for the SSIG alone in which the refrigerant mass flow rate is assumed constant. Therefore the capacity decreases as the Te increases. Faults such as binary ice flow reduction and cooling water flow reduction would change the evaporating temperature.

Figure 4.15 Variations of outlet flow temperature and cooling capacity with flowrate (design flowrate = 9 Litre/min, design Te = -20 °C, initial solution concentration at 15% by mass)

114 Flowrate could decrease due to flow restriction, which is a common fault in secondary system. Figure 4.15 indicates that a lower solution flowrate would lead to a lower outlet flow temperature. Though this results in a higher ice concentration, the cooling capacity reduces due to the reduced flow rate.

The output parameters will deviate from the expected values when an incorrect solution concentration is assumed in the model, which can be interpreted as a fault. Figure 4.16 shows that both cooling capacity and outlet temperature drop when the initial solution concentration is increased. A fluid with a higher solution concentration has a lower freezing point. Although the fluids enter the heat exchanger at the same temperature, the temperatures at the end of Section I (or at the beginning of Section II) are at the freezing points corresponding to individual initial concentrations. For a fluid with a lower freezing point (i.e. with a higher initial solution concentration), its temperature difference with the refrigerant is smaller (as observed from the measurements), resulting in a lower cooling capacity. In addition, a higher initial solution concentration also leads to a decrease in the length of Section II. As the cooling capacity is much higher in Section II than in Section I due to the involvement of latent, a shorter Section II will reduce the overall cooling capacity too.

Figure 4.16 Outlet binary ice temperature and cooling capacity against initial solution concentration (weight of ethanol/total solution weight) (design flowrate = 9 Litre/min, design Te =

115