Fluidic

Convection mode

The tools available in the fluidic tab allow defining the parameters that drive the convection phenomenon in the end spaces, involving the surfaces of the frame (internal surface), the end cap (internal surface), the shaft, the rotor and stator ends, and the end winding or potting.

Two choices are available to define the convection occurring on the external surface of the frame and of the end caps: Natural or Forced.

By default, the convection mode is set to “Natural”.

Note: No inputs exist to define the convection in the airgap, as the convection in the airgap mainly depends on the rotation speed of the rotor and does not depend on the cooling strategy affecting the end spaces.

Table 1. Internal cooling – Definition of the convection mode

1	Natural convection
2	Forced convection

Natural convection

This convection mode models that no specific forced fluid flow exists in the end caps in addition to the natural fluid movement induced by the machine rotation speed. The modeled convection exchanges correspond to the sum of two phenomena:

The differences of fluid temperature existing in different volumes of the end spaces (giving a difference of fluid density) create some natural fluid swirling in the end spaces.
The fluid movement is induced by the rotation speed of the machine.

Our internal natural convection model is based on classical correlations for end spaces, considering different fluid velocities for the parts close to the rotating parts and far from the rotating parts.

Therefore, there is no user input to define in this mode.

Note: The natural convection mode is well adapted to model every enclosed machine without internal fans. When internal fans, or rotor fins exist, it is advised to switch to forced convection mode.

Forced convection.

This convection mode allows forcing the convection model to be used for every region of the end spaces.

It can be used to model:

Increased convection effects due to rotor fins of shaft mounted internal fans.
A fan internally forcing constant ventilation whatever the rotation speed of the machine.
Some forced convection coefficients in the end spaces.

Table 2. Internal cooling – Definition of the convection mode

0	The forced convection mode is selected.
1-4	Forced convection: The end spaces are divided into four areas, corresponding to four inputs the user must define in forced convection mode.

When selecting one of these four inputs, the corresponding exchange areas are highlighted in the axial view of the machine. See the illustrations below.

Table 3. Internal cooling – Definition of the forced convection parameters - Examples shown correspond to an SMPM IR machine

1	The « Upper » Connection Side region, corresponding to the Connection Side convection areas far from the rotating parts.
2	The « Lower » Connection Side region, corresponding to the Connection Side convection areas close to the rotating parts.
3	The « Upper » Opposite Connection Side region, corresponding to the Opposite Connection Side convection areas far from the rotating parts.
4	The « Lower » Opposite Connection Side region, corresponding to the Opposite Connection Side convection areas close to the rotating parts.

For all four represented regions, the convection can be modeled with three different input ways:

A tip speed ratio
A fluid speed
A convection coefficient

The user can select the input mode of each region using the dedicated arrow or clicking on the input mode to change it.

Table 4. Forced convection – Ways for defining the convection

1	The forced convection mode is selected.
2	Definition of the ratio between the rotor tip speed and the internal fluid velocity on the connection side, far from the rotor. See additional information below.
3	Definition of the fluid constant speed on the connection side, far from the rotor. The « Constant fluid speed » input mode can be used to model a constant ventilation speed.
4	Definition of the convection coefficient at reference internal fluid. The « Convection coefficient » input mode allows directly forcing a convection coefficient in the corresponding region.

Information about the « Tip speed ratio » input mode

The « Tip speed ratio » input mode allows setting a fluid velocity proportional to the rotor tip speed. This factor works as a divider, e.g., a tip speed ratio of 2 means that the fluid in this section is moving at half the speed relative to the rotor’s tip speed.

Figure 1. Definition of the fluid velocity proportional to the rotor tip speed

This can be used to model a shaft mounted fan of rotor fins.

The default values of tip speed ratios are 2 for the regions far from the rotor.

A tip speed ratio of 2 for an « Upper » region (meaning a region far from the rotor) corresponds to a shaft mounted fan, or rotor fins, that blows air to this region with an average efficiency.

For the « Upper » region of a side without a fan or fins, it is advised to set a rotor tip speed ratio of 5. This corresponds to theFluxMotor natural convection model.

The default value of tip speed ratio is 1.5 for regions close to the rotor.

In fact, for these regions, the considered fluid speed is the relative speed between the fluid velocity and the rotating parts speed, meaning that in these regions the convection is highly related to the rotation speed.

Note: For any chosen input mode, the end spaces are considered as totally enclosed. No fluid exchange exists between the end space fluid (the « internal fluid » and the « external fluid »). The cooling strategy corresponding to blow an external fluid at a fixed temperature into and through the machine cannot be modeled in the current version of FluxMotor.

Note: In both input modes « Tip speed ratio » and « Constant fluid speed », the fluid speed is applied to classical correlations depending on the nature of the sub-region (end winding, frame, rotor part…). In the « Convection coefficient » input mode, the same convection coefficient is applied in all sub-regions (end winding, end ring, rotor end, end cap, frame…) of the regions for which the coefficient is chosen.