# Example Calculations

You are encouraged to look at all of the Two Phase examples used in the Quality Assurance tests. These can be found in the "[FluidFlow3 folder]\QA 2-Phase Flow" folder. Example 1. An air water two phase model. Constant quality. Two phase can be specified at a boundary, for a single fluid, or we can mix gas and liquid streams in order to make a two phase mixture. We will use the second method in this example. Make a model as shown below: The model shows two known flows (one fluid air(2), one fluid water(1)) combining and being heated via a plate exchanger, then flowing to a separation vessel (5). The red dot on the Knock Out Pot (separator) represents the liquid outlet and the yellow dot represents the vapor outlet. You can load this example directly from the "[FluidFlow3 folder]\QA 2-Phase Flow\Separating\Knock Out Pot with Constant Quality.ff3" or better enter the following data yourself. Node 1 (water inlet) conditions are: Node 2 (air inlet) conditions are: Node 6 (the plate exchanger) conditions are: Pipe information is: Your node numbers and pipe numbers may be different, this just depends on the connection order. Pipes (-1 and -2 connecting the known flows to the connector) 0.5 m in length 2" sch 40 pipe. Pipe (-6 from the connector to the plate exchanger) 60 m in length and an inside diameter of 50.8 mm Pipe (-3 connecting the plate exchanger to the KO Pot) 60m in length and an inside diameter of 50.8 mm. Pipe (-4 vapor outlet from KO Pot) 5m and 6" sch 40 pipe. Pipes (-5 and -7 liquid outlet from the KO Pot) 10m and 2" sch 40 pipe. Before you make the calculation, change the two phase correlation that is to be used via the Calculation Option Dialog (F2 key or via the menu Options | Calculation) to use the Beggs Brill method. Overview of Results: This is an example of two phase flow with constant quality. This means that the vapor mass fraction is constant and there is no mass transfer between the phases. It does not mean that the pressure loss per unit length is constant or that the velocity between the two phases is constant. In the first pipe section after mixing (pipe -6) you can see that the gas superficial velocity increases from the start to the end of pipe -6. For 60m of pipe -6, the total pressure loss is 146790 Pa, but the friction loss is 145349 Pa. Since the pipe is horizontal the difference is the acceleration loss. After the exchanger the mixture has experienced a temperature increase of 30 °C. The total pressure loss in the pipe after the exchanger (-3) is 208628 Pa (pipe -3 is identical in length and diameter to -6). This is because gas volume and velocity as well as other fluid properties have changed with the increase in temperature in the outlet pipe. You can get a feel for the differences by displaying the Beggs Brill flow pattern map. To do this click on the pipe, then click on the chart tab in the data palette and then use the Settings drop down to select the flow pattern map. As a final exercise before leaving this example, try another two phase correlation. For example MSH and note the differences in pressure losses (total system losses have increased by about 9%). If you use the Whalley criteria, FluidFlow selects the Friedel method and in this case total system pressure losses reduce by 17.5%. For this example MSH, Whalley and Beggs Brill are probably the most appropriate methods. You can see that the total system pressure loss estimation can vary considerably. Example 2. A Refrigerant system with changing quality. You can open the file "[FluidFlow3 folder]\QA 2-Phase Flow\Condensing\Elevation Test with change in quality across exchanger due to heat addition.ff3". In this example we have a single fluid, R-152a flowing through the system shown above, from a known pressure (node 1) at 1.16 atm and the fluid is at its saturation temperature of -19.9°C. We have specified a vapor quality (vapor mass fraction of 0.18) at the inlet and we are carrying out the calculation using the Whalley criteria. Overview of Results: This is an example of two phase flow with changing quality. This means that the vapor mass fraction is not constant and there is mass transfer between the phases. You can see this in the results of all flowsheet elements, examine the vapor quality entering and leaving each node or pipe, and you will see that quality is increasing as we flow through the system. This is because the pressure falls as the fluid flows down a pipe (or across a bend) and so some of the liquid boils to form more vapor. This is called flashing, and FluidFlow assumes that instantaneous isenthalpic flashing occurs. You should also notice that velocities are increasing and that mixture densities are decreasing. At the heat exchanger we are adding some 30,000 Watts of heat and this has the effect of vaporizing additional liquid. Across this element the quality increases from 0.19623 to 0.4578. As an exercise, change the heat loss model at the exchanger from "Fixed Transfer Rate" to "Ignore Heat Loss". What do you expect to happen to the vapor quality leaving the exchanger? It should decrease. You should also note that system flow will increase because of this.