Presión de entrada a la bomba (PIP)

Refrigerants are initially grouped to show the possible VT entry regions that could produce liquification in the nozzle or/and VT main body, and for their potential VT applications in cooling and heating. This grouping is then used to assist in establishing the approach to identify VT entry state, as described in Section 3.2.2.

The Step 1 involves examining the T-s diagram (or the saturated vapour entropy data as a function of temperature) of each individual refrigerant. Based on their T-s diagram, the refrigerants can be broadly divided into two groups.

One group is for the refrigerants that behave in such a way that the refrigerant could have more than 1 saturated temperature for a given entropy on the right hand side of the critical point. This corresponds to their T-s diagrams having both positive and negative gradients on the saturated vapour line, as shown in Figure 3.4a. On the other hand, when the line always has negative gradients, as shown in Figure 3.4b, they belong to the second group. In other words, for the second group, their saturated vapor entropy is increasing with decreasing temperature on the right hand side of the critical point. The refrigerants in the second group are likely to encounter liquification as they have a higher chance of entering the 2-phase zone during the temperature reduction

process within the VT. R134a is an example in this group. It is more likely that in the first group they stay in the superheated region, e.g. R600a.

In this study, various refrigerants in individual groups will be analysed thoroughly, including R227ea, R236fa, R245ca, R245fa, R218, R600 (natural fluid) and R600a (natural fluid) in Group 1, and R125, R134a, R143a, R152a, R290 (natural fluid), R32, R41, R717 (natural fluid) for Group 2.

a b

Figure 3.4 Sketch of refrigerants in two groups: Group 1 (a); Group 2 (b) In order to identify what regions (on the T-s diagram) might be considered as suitable for the VT entry state, the diagram is divided into 3 regions, A, B & C, Figure 3.5a (for Group 1) and Figure 3.5b (for Group 2). Region A represents sub-cooled liquid; this region has little relevance for VT operation. B is the two-phase region, consisting of a mixture of saturated liquid and saturated vapour. C is the superheated region. As seen in Figure 3.5a, there is an inflection point P8. In addition, the saturated liquid and vapour lines themselves can also be treated as two individual sub-regions. For Group 1, Region C can be sub-divided into three areas, I, II and III. Sub-area I is bounded by the saturated vapour line, the vertical line passing through P and the critical pressure line (constant Cr pressure line). Sub-area II is on the right side of the vertical line passing through P. Sub-area III is below point P, and between the saturated vapour line and the vertical line passing through P.

8 _{At this point, the rate of change of entropy with respect to the change in temperature (i.e. dT/ds) is}

equal to infinity T s Cr T s Cr

a b

Figure 3.5 Regions in two groups and some possible flow processes from the VT nozzle entry to the cold end: (a) Group 1 and (b) Group 2 (Points 1, 3, 5 and 7 are the

possible VT nozzle entry positions; Points 1′, 3′, 5′ and 7′ are respectively the possible VT nozzle outlet positions; Points 2, 4, 6 and 8 are respectively the fluid

state at the cold end

In Group 1, when the refrigerant, at a state either on or close to the saturated vapour line, enters the nozzle from the Sub-area I (e.g. Point 1 and 5, Figure 3.5a), liquid formation may occur as the flow enters into region B (e.g. 1 to 2, and 5 to 6, Figure 3.5a). However, when the entry point is located in the Sub-areas II & III, then liquid formation within either the nozzle or in the VT main body is rather unlikely (e.g. 3 to 4, Figure 3.5a). It is useful to note that in certain locations within Sub-area I, though liquification inside the nozzle can be avoided (5 to 5′), it is still possible for it to occur in the cold end (5′ to 6).

On the other hand, for Group 2, the entry state to the nozzle could be at, say, location 1, 3 or 5, as shown in Figure 3.5b. Points 1 and 5 could be considered either on or close to the saturation line. Once again, the refrigerant could enter the 2-phase region, B, during its transit through the nozzle (1 - 1′, Figure 3.5b) or the main VT body towards the cold end (5′- 6, Figure 3.5b). However, if the entry point is moved further away from the saturation line into the superheated region C, such as location 3, then the VT should be able to operate without the presence of liquid either in the nozzle or in the VT main body.

Based on the above discussion, it can be concluded that:

 to purposely achieve a two-phase fluid at the cold exit for cooling purpose, as exemplified in Figure 2.19, for Group 1 refrigerants, the entry point must be located in the area I of region C and at the same time must avoid liquid

4’{T4’, III 2 X B T s Cr P A C II I 3 6 5 1′ 1 5′ 4 3′ 7 7′ 8 B C T s Cr A 6 5′ 5 1 1′ 2 3 3′ 4

formation within the nozzle. For Group 2 refrigerants, the entry point should be close to the saturated vapour line in the region C, and also must ensure no liquid forms inside the nozzle.

 to ensure only vapour phase at the cold exit, as exemplified in Figure 2.22, for Group 1 refrigerants, the entry point should locate in the Sub-areas II & III of region C where liquid formation is not possible. When the entry point situates in the Sub-area I of C, liquid formation must be prevented inside both the nozzle and the VT main body (e.g. 7 to 8, Figure 3.5a) by adding a suitable degree of superheat. For Group 2 refrigerants, the entry point should be far away from the saturated vapour line to eliminate any possibilities of liquification in the VT.

Therefore, author is proposing that the refrigerant state point at the nozzle entry could be used as an initial selection criterion for their applications in VT.