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Gradual enlargement or contraction
The Gradual Area Change block represents a local hydraulic resistance, such as a gradual cross-sectional area change. The resistance represents a gradual enlargement (diffuser) if fluid flows from inlet to outlet, or a gradual contraction if fluid flows from outlet to inlet. The block is based on the Local Resistance block. It determines the pressure loss coefficient and passes its value to the underlying Local Resistance block. The block offers two methods of parameterization: by applying semi-empirical formulas (with a constant value of the pressure loss coefficient) or by table lookup for the pressure loss coefficient based on the Reynolds number.
If you choose to apply the semi-empirical formulas, you provide geometric parameters of the resistance, and the pressure loss coefficient is determined according to the A.H. Gibson equations (see [1] and [2]):
$${K}_{GE}=\{\begin{array}{ll}{K}_{cor}{\left(1-\frac{{A}_{s}}{{A}_{L}}\right)}^{2}\xb72.6\mathrm{sin}\frac{\alpha}{2}\hfill & \text{for0}\alpha \text{=}{45}^{o}\hfill \\ {K}_{cor}{\left(1-\frac{{A}_{s}}{{A}_{L}}\right)}^{2}\hfill & \text{for}{45}^{o}\text{}\alpha \text{}{180}^{o}\hfill \end{array}$$
$${K}_{GC}=\{\begin{array}{ll}{K}_{cor}\xb70.5{\left(1-\frac{{A}_{s}}{{A}_{L}}\right)}^{0.75}\xb71.6\mathrm{sin}\frac{\alpha}{2}\hfill & \text{for0}\alpha \text{=}{45}^{o}\hfill \\ {K}_{cor}\xb70.5{\left(1-\frac{{A}_{s}}{{A}_{L}}\right)}^{0.75}\xb7\sqrt{\mathrm{sin}\frac{\alpha}{2}}\hfill & \text{for}{45}^{o}\text{}\alpha \text{}{180}^{o}\text{}\hfill \end{array}$$
where
K_{GE} | Pressure loss coefficient for the gradual enlargement, which takes place if fluid flows from inlet to outlet |
K_{GC} | Pressure loss coefficient for the gradual contraction, which takes place if fluid flows from outlet to inlet |
K_{cor} | Correction factor |
A_{S} | Small area |
A_{L} | Large area |
α | Enclosed angle |
If you choose to specify the pressure loss coefficient by a table, you have to provide a tabulated relationship between the loss coefficient and the Reynolds number. In this case, the loss coefficient is determined by one-dimensional table lookup. You have a choice of three interpolation methods and two extrapolation methods.
The pressure loss coefficient, determined by either of the two methods, is then passed to the underlying Local Resistance block, which computes the pressure loss according to the formulas explained in the reference documentation for that block. The flow regime is checked in the underlying Local Resistance block by comparing the Reynolds number to the specified critical Reynolds number value, and depending on the result, the appropriate formula for pressure loss computation is used.
The Gradual Area Change block is bidirectional and computes pressure loss for both the direct flow (gradual enlargement) and return flow (gradual contraction). If the loss coefficient is specified by a table, the table must cover both the positive and the negative flow regions.
Connections A and B are conserving hydraulic ports associated with the block inlet and outlet, respectively.
The block positive direction is from port A to port B. This means that the flow rate is positive if fluid flows from A to B, and the pressure loss is determined as $$p={p}_{A}-{p}_{B}$$.
Fluid inertia is not taken into account.
If you select parameterization by semi-empirical formulas, the transition between laminar and turbulent regimes is assumed to be sharp and taking place exactly at Re=Re_{cr}.
If you select parameterization by the table-specified relationship K=f(Re), the flow is assumed to be turbulent.
Resistance small diameter. The default value is 0.01 m.
Resistance large diameter. The default value is 0.02 m. This parameter is used if Model parameterization is set to By semi-empirical formulas.
The enclosed angle. The default value is 30 deg. This parameter is used if Model parameterization is set to By semi-empirical formulas.
Select one of the following methods for block parameterization:
By semi-empirical formulas — Provide geometrical parameters of the resistance. This is the default method.
By loss coefficient vs. Re table — Provide tabulated relationship between the loss coefficient and the Reynolds number. The loss coefficient is determined by one-dimensional table lookup. You have a choice of three interpolation methods and two extrapolation methods. The table must cover both the positive and the negative flow regions.
Correction factor used in the formula for computation of the loss coefficient. The default value is 1. This parameter is used if Model parameterization is set to By semi-empirical formulas.
The maximum Reynolds number for laminar flow. The transition from laminar to turbulent regime is assumed to take place when the Reynolds number reaches this value. The value of the parameter depends on the geometrical profile. You can find recommendations on the parameter value in hydraulics textbooks. The default value is 350. This parameter is used if Model parameterization is set to By semi-empirical formulas.
Specify the vector of input values for Reynolds numbers as a one-dimensional array. The input values vector must be strictly increasing. The values can be nonuniformly spaced. The minimum number of values depends on the interpolation method: you must provide at least two values for linear interpolation, at least three values for cubic or spline interpolation. The default values are [-4000, -3000, -2000, -1000, -500, -200, -100, -50, -40, -30, -20, -15, -10, 10, 20, 30, 40, 50, 100, 200, 500, 1000, 2000, 4000, 5000, 10000]. This parameter is used if Model parameterization is set to By loss coefficient vs. Re table.
Specify the vector of the loss coefficient values as a one-dimensional array. The loss coefficient vector must be of the same size as the Reynolds numbers vector. The default values are [0.25, 0.3, 0.65, 0.9, 0.65, 0.75, 0.90, 1.15, 1.35, 1.65, 2.3, 2.8, 3.10, 5, 2.7, 1.8, 1.46, 1.3, 0.9, 0.65, 0.42, 0.3, 0.20, 0.40, 0.42, 0.25]. This parameter is used if Model parameterization is set to By loss coefficient vs. Re table.
Select one of the following interpolation methods for approximating the output value when the input value is between two consecutive grid points:
Linear — Uses a linear interpolation function.
Cubic — Uses the Piecewise Cubic Hermite Interpolation Polinomial (PCHIP).
Spline — Uses the cubic spline interpolation algorithm.
For more information on interpolation algorithms, see the PS Lookup Table (1D) block reference page. This parameter is used if Model parameterization is set to By loss coefficient vs. Re table.
Select one of the following extrapolation methods for determining the output value when the input value is outside the range specified in the argument list:
From last 2 points — Extrapolates using the linear method (regardless of the interpolation method specified), based on the last two output values at the appropriate end of the range. That is, the block uses the first and second specified output values if the input value is below the specified range, and the two last specified output values if the input value is above the specified range.
From last point — Uses the last specified output value at the appropriate end of the range. That is, the block uses the last specified output value for all input values greater than the last specified input argument, and the first specified output value for all input values less than the first specified input argument.
For more information on extrapolation algorithms, see the PS Lookup Table (1D) block reference page. This parameter is used if Model parameterization is set to By loss coefficient vs. Re table.
Parameters determined by the type of working fluid:
Fluid density
Fluid kinematic viscosity
Use the Hydraulic Fluid block or the Custom Hydraulic Fluid block to specify the fluid properties.
[1] Flow of Fluids Through Valves, Fittings, and Pipe, Crane Valves North America, Technical Paper No. 410M
[2] Idelchik, I.E., Handbook of Hydraulic Resistance, CRC Begell House, 1994
Elbow | Local Resistance | Pipe Bend | Sudden Area Change | T-junction