### 12.2.1. Table Interpolation

When the flow solver needs to interpolate a value of at a given temperature, , and pressure, , (for example, the common calculation of ) there are three possible regions for the interpolation to happen: supercritical (point 1), vapor (point 2) and liquid (point 3).

For any of these regions there are three possible interpolation options available in the flow solver available by setting the following expert parameter:

    EXPERT PARAMETER:
       prop interp option = 1, 2, or 3 (default)
    END

Option 1 tells the flow solver to use no saturation clipping, option 2 always uses saturation clipping assuming that saturation data has been extended along the critical isotherm, and option 3 (Default) uses no saturation clipping above the critical pressure.

: Interpolation in the supercritical region

For this region there is a slight difference between option 2 and option 3 interpolation.

Option 2: Interpolation for vapors or liquids is performed the same as if the point were below the critical point.

Option 3: First, the solver checks if and . If this condition is met, then the procedures given below are used. If not, then a standard bilinear interpolation is used from the four table values that enclose the desired point. This would correspond to point 1 on the figure.

: Vapor side interpolation below the critical point

Option 2 and option 3 are equivalent in this case.

If and then the flow solver detects if the interpolated value must be clipped to the saturation value. This is done by first looking up the table location where the value of is located and then calculating and . If then the solver does not need to clip to saturation and standard bilinear interpolation is used. If then the solver simply sets the interpolated value of to .

: Liquid side interpolation below the critical point

Important: This feature is unsupported for .rgp files and the Real Gas equation of state, but is fully supported for the IAPWS equation of state. It can be accessed only when using an additional expert parameter with the Redlich Kwong equation of state or an .rgp file and some custom flow solver CCL that forces the flow solver to read the two dimensional tables for a liquid. In a future release this feature will be superseded.

For liquids the recipe is almost the same as for a vapor. First the solver looks up the table location where the value of is located and calculates and . If then the solver does not need to clip to saturation and standard bilinear interpolation is used. If then the solver simply sets the interpolated value of to .

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