Creating a Two-Phase Fluid in Flownex
Is it possible to create two-phase fluids in Flownex?
If a user requires a specific two-phase fluid that is not available in Flownex®, it is possible to use either the CAPE-OPEN or the NIST Importer to import the required two-phase fluid, if the fluid is available from these databases.
This article will focus on using the NIST Importer to import two-phase fluids in Flownex® and provide the user with the necessary steps to import the fluid.
HOW TO IMPORT A FLUID USING THE NIST IMPORTER
Flownex® includes the capability to import fluids from NIST (National Institute of Standards and Technology) using the NIST Importer. The NIST Importer is available in the Import ribbon, as seen in Figure 1.
The NIST Importer allows users to import single-phase fluids, such as gasses or liquids, as well as two-phase fluids into Flownex®, as seen in Figure 2. This allows users to easily import fluids that are available in NIST and not included in the Flownex® library. It is important to note that NIST needs to be installed on the user’s computer to use the NIST importer.
STEP 1: SELECT THE FLUID AND DEFINE GENERATION OPTIONS
To import the two-phase fluid, the user should click on the Two-Phase tab and then select and load the two-phase fluid that needs to be imported from the “Fluids exported by Nist”-list.
The Generation Options can then be edited and specified for the different regions of the two-phase fluid before the fluid is generated. The different regions for a two-phase fluid (in this example, using two-phase water) are shown in Figure 4.
The Generation Options related to importing a two-phase fluid can be seen in Figure, 5 and each section will be described in more detail below.
NIST FLUID DATA
These properties are retrieved for the specific fluid from NIST’s database.
T triple point
T triple point is the temperature at the triple point of the fluid.
P triple point
P critical point is the pressure at the critical point of the fluid.
T maximum allowed
T maximum allowed is the maximum temperature at which the fluid is defined in the NIST library. No data can be generated for the fluid above this value.
OVERALL
This section is used to describe the limits of the data of the fluid.
Critical Offset
Critical Offset is the temperature difference between the critical temperature of the fluid and the maximum temperature at which a data point should be created in the two-phase region.
Minimum Pressure
Minimum Pressure is the lower limit for pressure at which the imported data should start for the subcooled and two-phase regions and must be greater than the P triple point.
Minimum Temperature
Minimum Temperature is the lower limit for temperature at which the imported data should start for the subcooled and two-phase regions and must be greater than the T triple point.
Maximum Temperature
The Maximum Temperature is the upper limit for temperature for the imported data points.
Maximum Pressure Factor (Pcrit*X)
The Maximum Pressure Factor is the factor that will be used to calculate the upper limit for pressure for the imported data points. The maximum pressure is equal to the specified factor (X) multiplied by the critical pressure (Pcrit) of the fluid.
SUBCOOLED
This section is used to describe the layout of the data to be imported for the subcooled region.
Temperature Increments
The Temperature Increments value refers to the number of temperature increments on a pressure line between the lowest and highest temperature in the subcooled region. The highest temperature in the subcooled region is equal to the critical temperature of the fluid.
Temperature Increment Factor
The Temperature Increment Factor value is the ratio between two consecutive temperature increments. Thus, the increment size between consecutive temperature data points will increase from the lowest to the highest temperature in this region.
Maximum Temperature Interval in Subcooled regime
The Maximum Temperature Interval in Subcooled regime refers to the upper limit for the increment size between temperature data points. The increment size will increase by the Increment Factor until it reaches the value specified in the maximum temperature interval field. Thereafter, the increment size will stay equal to this value.
TWO PHASE
This section is used to describe the layout of the data to be imported for the two-phase region.
Low to High Transition Quality
The Low to High Transition Quality refers to the quality value that distinguishes the low-quality and high-quality regions in the two-phase dome. A transition point is specified so that more increments can be specified in the low-quality region than in the high-quality region. Fluid properties change rapidly at low qualities, requiring more data points to better describe these properties in this region.
Quality Increments (Low Region)
The Quality Increments (Low Region) refer to the number of quality increments in the low-quality range of the two-phase region.
Quality Factor (Low Region)
The Quality Factor (Low Region) refers to the ratio between two consecutive quality data point increments; the increment size will increase from a quality of zero to the transition quality value.
Quality Increments (High Region)
The Quality Increments (High Region) refer to the number of quality increments in the high-quality range of the two-phase region.
Quality Factor (High Region)
The Quality Factor (High Region) refers to the ratio between two consecutive quality data point increments; the increment size will increase from the transition quality value to a quality of one.
The following three data entries define refinements to the temperature data point specification on the liquidus line close to the critical point. Some fluid properties (of particular interest is specific heat at constant pressure) change rapidly near the critical point, which requires more data points to better describe these changes.
Saturation Temperature Increments
The Saturation Temperature Increments value refers to the increment size between the saturation temperature lines in the two-phase region.
First Temperature Increment Below Critical Point
The First Temperature Increment Below Critical Point is the difference between the critical temperature and the first saturation temperature below the critical temperature.
phase region.
Number of Refined Temperatures (Below Critical Point)
The Number of Refined Temperatures (Below Critical Point) refers to the number of temperature lines below the critical point with small increments between each consecutive line.
SUPERHEATED
This section is used to describe the layout of the data to be imported for the superheated and gas-only regions.
Temperature Increments
The Temperature Increments value refers to the number of temperature increments on a pressure line between the vapour saturation temperature and the specified maximum temperature.
Temperature Increment Factor
The Temperature Increment Factor refers to the ratio between two consecutive temperature data point increments; the increment size will increase from the vapour saturation temperature on a pressure line to the maximum temperature.
Pressure Increments for Gas Only
The Pressure Increments for Gas Only value refers to the number of pressure lines from the triple point pressure and the lowest pressure specified in the Minimum Pressure for Gas Only field. The gas-only region refers to the region where the pressure is below the triple point pressure and where the fluid exists only as a gas. The lowest temperature for these constant pressure lines is the sublimation temperature of the fluid.
Minimum Pressure for Gas Only
The Minimum Pressure for Gas Only refers to the lowest pressure line to be imported for the gas-only region.
Pressure Increment Factor Gas Only
The Pressure Increment Factor Gas Only refers to the ratio between two consecutive pressure line increments; the increment size will increase from the triple point pressure to the lowest pressure line specified.
SUPERCRITICAL
This section is used to describe the layout of the data to be imported for the supercritical region.
Pressure Increment Factor
The Pressure Increment Factor refers to the ratio between two consecutive pressure line increments; the increment size will increase from the critical pressure to the maximum allowable pressure.
SATURATED BOILING
This section is used to define saturated boiling fluid properties to be used in calculating the boiling heat transfer coefficient.
The boiling heat transfer coefficient is calculated by using the correlation obtained from Steiner and Taborek (1992) [1]. The Boiling Fluid Reference Flux and Boiling Fluid Reference Heat Transfer Coefficient are retrieved from the Nucleate Flow Boiling Heat Transfer Coefficients at Normalized Conditions table in Steiner and Taborek (1992) [1]. It is not necessary to change any of these values. If the imported fluid is not listed in the table from Steiner and Taborek (1992) [1], then the value for boiling fluid reference flux will be assigned according to its category (fluids in the same category have the same reference flux value), and the average of the heat transfer coefficients for all the fluids in the same category will be assigned to the fluid’s heat transfer coefficient. If the average is used, then the negative of the value is specified in the config file. This will result in a warning being issued to alert the user to this averaging assumption. The user has the option to find and specify the heat transfer coefficient that is appropriate for the fluid they are using.
Four options are available to choose from for the Boiling Fluid Category: Inorganic, Hydrocarbon, Refrigerant, and Cryogenic.
Radiation Model
Three options are available to choose from for the Radiation Model: None, Water, and Carbon dioxide. The None option can be selected if the fluid will not participate in radiation.
STEP 2: GENERATE THE FLUID
After all the relevant inputs have been specified, click the Generate button to generate data points for the fluid. An example of water generated as a two-phase fluid is shown in Figure 6.
Figure 7 below shows a zoomed-in view of the region around the critical point of an imported two-phase water example. This illustrates the dense pressure line distribution near the critical point resulting from the “Number of Redefined Temperature Lines” specification under the Two-Phase section (as shown in Figure 5). The change in increment size between consecutive pressure lines can also be observed in Figure 7. This increase in increment size is also applied to the temperature data points on each pressure line.
STEP 3: SAVING THE IMPORTED FLUID
When satisfied with the data shown
on the T-S diagram, the user can click the save button and specify the name of
the new category that should be created in the Project Database as well as the
fluid name, then the fluid can be saved.
After the fluid has been saved, the fluid will be available in the Project Database as shown in Figure 9.
STEP 4: ASSIGNING THE IMPORTED FLUID TO A NETWORK
The imported fluid can then be assigned to a network as shown in Figure 10.
REFERENCES
[1] Steiner, D., Taborek, J. 1992. Flow Boiling Heat Transfer in Vertical Tubes Correlated by an Asymptotic Model, Heat Transfer Engineering, 13:2, 43-69, DOI: 10.1080/01457639208939774.
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[1] Steiner, D., Taborek, J. 1992. Flow Boiling Heat Transfer in Vertical Tubes Correlated by an Asymptotic Model, Heat Transfer Engineering, 13:2, 43-69, DOI: 10.1080/01457639208939774.