FLUID116 Element Description

FLUID116 is a 3-D element with the ability to conduct heat and transmit fluid between its two primary nodes. Heat flow is due to the conduction within the fluid and the mass transport of the fluid. Convection may be accounted for either with additional nodes and convection areas or with surface elements SURF151 and SURF152. In both cases, the film coefficient may be related to the fluid flow rate. The element may have two different types of degrees of freedom, temperature and/or pressure.

The thermal-flow element may be used in a steady-state or transient thermal analysis. If the model containing the thermal-flow element is also to be analyzed structurally, the element should be replaced by an equivalent (or null) structural element. See FLUID116 in the Mechanical APDL Theory Reference for more details about this element.

Figure 116.1:  FLUID116 Geometry

FLUID116 Input Data

The geometry, node locations, and the coordinate system for this thermal-flow pipe element are shown in Figure 116.1: FLUID116 Geometry. The element is defined by two primary nodes, two additional nodes if convection is desired, several real constants (see Table 116.1: FLUID116 Element Real Constants), and the material properties. The length L of the element is determined from the two primary node locations.

The material properties can be input as numerical values or as tabular inputs evaluated as a function of pressure, temperature, velocity, time, and location. If temperature or pressure, you need to activate the appropriate pressure or temperature degrees of freedom. Tabular material properties are calculated before the first iteration (i.e., using initial values [IC]).

The fluid mass density ρ (Mass/Length³) is input as property DENS or computed following the ideal gas law if the real constant R_gas is present. If KEYOPT(2) = 2, 3, or 4, the convection film coefficient h_f (Heat/Length²*Time*Deg) is input by the options defined by KEYOPT(4). If KEYOPT(2) = 1, convection surfaces using FLUID116 velocities and other information are stored and can be used by SURF151 or SURF152 and optionally the user programmable feature USRSURF116 in order to determine film coefficients and bulk temperatures as a function of velocities and other parameters. The input tables are explained in detail in Table 116.2: FLUID116 Empirical Data Table (Optional). The thermal conductivity k_xx (Heat/Length*time*Deg) acts in the element longitudinal direction and is input as property KXX. The specific heat c_p (Heat/Mass*Deg or Heat*Length/Force*Time²*Deg) is input as property C. The fluid viscosity μ is input as property VISC. In an axisymmetric analysis, such as for annular flow, the flow area, the convection areas, and all other input should be on a full 360° basis.

KEYOPT(2) = 3 and 4 are variations of KEYOPT(2) = 2 used to avoid an artificial reduction of the change in temperature in the last element next to an inlet or outlet with no specified temperature. If such an inlet or outlet is at node I, use KEYOPT(2) = 3 and if it is at node J, use KEYOPT(2) = 4. All elements of a run of pipe should use the same KEYOPT, not just the end one. For networks where the usage of KEYOPT(2) is not obvious and the detailed temperature distribution is important, use KEYOPT(2) = 2 with a relatively fine mesh (small elements). The effect of KEYOPT(2) = 3 and 4 could be alternatively achieved by adjusting the convection areas (Real Constants 7 and 8) but it is not as convenient.

The coefficient of friction (input as property MU) is the starting value of the Moody friction factor (f). The friction factor for the first iteration is always assumed to be MU. The smooth-pipe empirical correlations are a function of Reynolds number (Re) and depend on whether the flow is laminar or turbulent (Re>2500). If a friction table is supplied (TB,FCON), the friction factor is recomputed each substep from the table (using linear interpolation where necessary). The table is also explained in detail in Table 116.2: FLUID116 Empirical Data Table (Optional).

The word PRES (or TEMP) should be input for the Lab variable on the D command and the pressure (or temperature) value input for the value. If a nodal heat (or fluid) flow rate is defined with the F command, input the word HEAT (or FLOW) for the Lab variable and input the flow rate for the value. If temperature is the only degree of freedom, (KEYOPT(1) = 1), you can input a known flow rate in units of mass/time via an SFE,,,HFLUX command (rather than F,,FLOW). Fluid weight effects are activated by specifying a nonzero acceleration and/or rotation vector [ACEL and/or OMEGA].

When using the rotational speed and slip factor real constants (real constants 7-10 in Table 116.1: FLUID116 Element Real Constants), you can specify either numerical values or table inputs. If specifying table inputs, enclose the table name in % signs (for example, %tabname%). Also, if using table inputs for rotational speed, either both real constants 7 and 8 should have the same table name reference, or real constant 8 should be unspecified. Similarly, if using table inputs for slip factor, either both real constants 9 and 10 should have the same table name reference, or real constant 10 should be unspecified. Both rotational speed and the slip factor can vary with time and location.

If tabular real constants are used, then any node in a FLUID116 network must refer to a single table name. For correct results, at any node, the table names from different elements must all be the same, and a table name cannot be used along with any numerical real constant from a different element. Note that the table primary variables (for example, X, Y, Z, or ELEM) refer to the coordinates and element numbers of attached SURF151 or SURF152 elements (KEYOPT(2) = 1).

See Steady-State Thermal Analysis in the Thermal Analysis Guide for more information on using table inputs.

Element loads are described in Nodal Loading. Element body loads may be input as heat generation rates at the nodes. The node J heat generation rate HG(J) defaults to the node I heat generation rate HG(I).

KEYOPT(8) is used for inputting flow losses (see Table 116.1: FLUID116 Element Real Constants). Momentum losses in pipes due to bends, elbows, joints, valves, etc., may be represented by a fictitious (equivalent) length of pipe L_a.This equivalent length may be input directly or calculated from an input constant K, the hydraulic diameter D, and the friction factor f.

You can connect FLUID116 elements to the pressure nodes of hydrostatic fluid elements (HSFLD241 or HSFLD242) to model the following two scenarios: fluid flow between two fluid volumes modeled by hydrostatic fluid elements, or fluid flow through an orifice between a fluid volume (hydrostatic fluid elements) and the environment. In either case, a single FLUID116 element is recommended. For the case of flow between two fluid volumes, the FLUID116 element uses the current density of the fluid having a larger pressure among the two hydrostatic pressure nodes. For the case of fluid exchange with the environment, the FLUID116 element uses the current density of the fluid modeled by the hydrostatic fluid elements. If more than one FLUID116 elements are used, the elements that are not directly connected to hydrostatic fluid elements use the density defined by the FLUID116 material property. For FLUID116 elements that are directly or indirectly connected to a hydrostatic fluid element, you must set KEYOPT(1) = 3 to convert the fluid element mass flow rate to volume change (for compatibility with the hydrostatic fluid elements).

A summary of the element input is given in "FLUID116 Input Summary". A general description of element input is given in Element Input.

FLUID116 Input Summary

Nodes

I, J or I, J, K, L (see KEYOPT(2))

Degrees of Freedom

PRES, TEMP if KEYOPT(1) = 0

TEMP if KEYOPT(1) = 1

PRES if KEYOPT(1) = 2 or 3

Real Constants

See Table 116.1: FLUID116 Element Real Constants

Material Properties

MP command: KXX, C, DENS, MU, VISC, HF

Surface Loads

You can specify imposed mass flow via an SFE,,,HFLUX command. Valid only when KEYOPT(1) = 1.

Body Loads

Heat Generations -- 

HG(I), HG(J)

Special Features

Nonlinearity

KEYOPT(1)

Pressure and temperature degrees of freedom:

0 -- 

PRES and TEMP degrees of freedom

1 -- 

TEMP degrees of freedom only

2 -- 

PRES degrees of freedom only. This option is not valid when FLUID116 is connected to HSFLD241 or HSFLD242.

3 -- 

PRES degrees of freedom only. This option is valid only when FLUID116 is directly or indirectly connected to HSFLD241 or HSFLD242.

KEYOPT(2) (used only if KEYOPT(1) = 0 or 1)

0 -- 

2 nodes and no convection surface or convection information

1 -- 

2 nodes and convection information passed to SURF151 or SURF152

2 -- 

4 nodes and convection surface logic included with this element, convection area shared between nodes I and J

3 -- 

4 nodes and convection surface logic included with this element, convection area only at node I

4 -- 

4 nodes and convection surface logic included with this element, convection area only at node J

KEYOPT(4) (used only if KEYOPT(2) = 2, 3, or 4)

Film coefficient (h_f) definition

0 -- 

Use MP,HF

1 -- 

Use real constants 9 thru 12 (see Table 116.1: FLUID116 Element Real Constants)

2 -- 

Use TB,HFLM for h_f as a function of temperature and average velocity

3 -- 

Use TB,HFLM for h_f as a function of temperature and Reynold's number

4 -- 

Use TB,HFLM for Nu as a function of temperature and Reynold's number (h_f = K_xx*Nu/diam)

5 -- 

Use call to User116Hf

KEYOPT(5) (used only if KEYOPT(4) = 0, 2, 3, 4, or 5)

Evaluation of film coefficient:

0 -- 

Average fluid temperature (TI + TJ)/2

1 -- 

Average wall temperature (TK +TL)/2

2 -- 

Average film temperature (TI + TJ + TK + TL)/4

3 -- 

Differential temperature (TI + TJ)/2 - (TK + TL)/2

KEYOPT(6) (used only if KEYOPT(1) = 0 or 2)

Fluid conductance coefficient definition:

0 -- 

Use conductance formula

1 -- 

Use real constant C

2 -- 

Use TB,FCON as a function of temperature and average velocity

3 -- 

Use TB,FCON as a function of temperature and Reynold's number

4 -- 

Use call to User116Cond

KEYOPT(7) (used only if KEYOPT(6) = 0)

Friction factor calculation:

0 -- 

Use smooth pipe empirical correlations

1 -- 

Use MP,MU

2 -- 

Use TB,FCON with friction factor being a function of temperature and average velocity

3 -- 

Use TB,FCON with friction factor being a function of temperature and Reynold's number

KEYOPT(8) (used only if KEYOPT(6) = 0)

Flow losses specified by input:

0 -- 

Use real constant L_a as the additional length

1 -- 

Use real constant K as loss coefficient

KEYOPT(9)

Discretization scheme:

0 -- 

Upwind difference linear shape function (default). This scheme has lower order accuracy than the other schemes.

1 -- 

Central difference linear shape function. This scheme has higher order accuracy but it can lead to oscillations near bends.

2 -- 

Upwind difference exponential shape function. This scheme has high accuracy and does not produce oscillations near bends.

Table 116.1:  FLUID116 Element Real Constants

(Given in the order required for input in the real constant table)
No.	Name	Definition	Units
1	D	Hydraulic diameter.	Length
2	A	Flow cross-sectional area.	Length2
3	Nc	Number of flow channels (defaults to 1). If greater than 1, real constants and element output are on a per channel basis.
4-6		not currently used
7	(An ) I	If KEYOPT(2) = 1, angular velocity associated with node I. If KEYOPT(2) = 2, 3, or 4, convection area between nodes I and K. Defaults to πDL/2 if KEYOPT(2) = 2, defaults to πDL if KEYOPT(2) = 3 where: L = element length	Length2
8	(An)J	If KEYOPT(2) = 1, angular velocity associated with node J. Defaults to value at node I. If KEYOPT(2) = 2, 3, or 4, convection area between nodes J and L. Defaults to πDL/2 if KEYOPT(2) = 2, defaults to πDL if KEYOPT(2) = 4	Length2
9	SLIPFAI	If KEYOPT(2) = 1, slip factor at node I.
10	SLIPFAJ	If KEYOPT(2) = 1, slip factor at node J. Defaults to value at node I.
9-12	N1, N2, N3, N4	(Used if KEYOPT(4) = 1 and KEYOPT(2) = 2, 3, or 4) Nu = N1 + N2 Re N3 PrN4 where: Re = Reynolds number (WD/ µA) Pr = Prandtl number  (Cpμ/KXX) Cp = specific heat For example, the Dittus-Boelter correlation for full-developed turbulent flow in smooth pipes may be input with N1 = 0.0, N2 = 0.023, N3 = 0.8, and N4 = 0.4 (heating).
13	Pp	Pump pressure.	Force / Length2
14		Used to compute conductance coefficient C where: Δp = pressure drop
	Cr	If KEYOPT(6) = 1, conductance coefficient is used to calculate flow. Hence,
	La	If KEYOPT(6) = 0, and KEYOPT(8) = 0, additional Length of pipe to account for flow losses (for example, valves, orifices, etc.) Hence, where: ρ = DENS F = friction coefficient
	K	If KEYOPT(6) = 0 and KEYOPT(8) = 1, this real constant is the loss coefficient K. Hence,
15-18		not currently used
19	Rgas	Gas constant in ideal gas law (ρ = p/(RgasTabs)), where Tabs is the absolute temperature and p = average pressure. If zero, use ρ as specified by the DENS material property.	Length2 / Deg * Time2
20	VDF	Viscous damping multiplier. Default 0.0
21	Cver	Units conversion factor for viscous damping. Default = 1.0 Qv = VDFCverFπVISC(VELOC)2L = viscous heating for element, with F = 8.0 for laminar and 0.21420 for turbulent flow.

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Note:  Real constants 7 through 12 and 20 and 21 are used only if KEYOPT(1) = 0 or 1 and real constants 13 through 19 are used only if KEYOPT(1) = 0 or 2.

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The data in Table 116.2: FLUID116 Empirical Data Table (Optional) is entered in the data table with the TB commands. The curves are initialized by using the TB command. The temperature for the first curve is input with the TBTEMP command, followed by TBPT commands for up to 100 points. Up to 20 temperature-dependent curves (NTEMP = 20 maximum on the TB command) may be defined in this manner. The constants (X, Y) entered on the TBPT command (two per command).

Table 116.2:  FLUID116 Empirical Data Table (Optional)

Constant	Meaning
Film Coefficient The film coefficient table is initialized with the TB,HFLM command. The TBPT data are:
X	Velocity (Length/Time)
Y	Film Coefficient (Heat/(time*area*temp) The velocity may be replaced with the Reynold's number, and the film coefficient may be replaced with the Nusselt number, depending on KEYOPT(4).
Fluid Conductance/Friction Factor The fluid conductance/friction factor is initialized with the TB,FCON command. The TBPT data are:
X	Velocity (Length/Time)
Y	Corresponding friction factor value (Dimensionless)
The velocity may be replaced with the Reynold's number, and the friction factor may be replaced with the fluid conductance, depending on KEYOPT(6) and KEYOPT(7).

FLUID116 Output Data

The solution output associated with the element is in two forms:

The fluid flow rate is expressed in units of Mass/Time and is positive from node I to node J. In an axisymmetric analysis these flow rates and all other output are on a full 360° basis. The fluid flow rate and the heat flow rate at the nodes may be printed with the OUTPR command. A general description of solution output is given in Solution Output. See the Basic Analysis Guide for ways to view results.

The following notation is used in Table 116.3: FLUID116 Element Output Definitions:

The Element Output Definitions table uses the following notation:

A colon (:) in the Name column indicates that the item can be accessed by the Component Name method (ETABLE, ESOL). The O column indicates the availability of the items in the file Jobname.OUT. The R column indicates the availability of the items in the results file.

In either the O or R columns, “Y” indicates that the item is always available, a number refers to a table footnote that describes when the item is conditionally available, and “-” indicates that the item is not available.

Table 116.4: FLUID116 Item and Sequence Numbers lists output available through the ETABLE command using the Sequence Number method. See The General Postprocessor (POST1) in the Basic Analysis Guide and The Item and Sequence Number Table in this reference for more information. The following notation is used in Table 116.4: FLUID116 Item and Sequence Numbers:

FLUID116 Assumptions and Restrictions

FLUID116 Product Restrictions

When used in the product(s) listed below, the stated product-specific restrictions apply to this element in addition to the general assumptions and restrictions given in the previous section.

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