Understanding thermostat options for steady state analyses

If you run a steady state solution or solution step, and your model includes thermostats, you must also select a Thermostat option to control how the software treats any Thermostat modeling objects in the solution.

Sink to Average Temperature

Sink to Average Temperature constrains the heater elements at the average of the set point temperatures specified on the corresponding sensor.

For example, Tavg = 0.5 × (Tcut-in+Tcut-off).

The physical interpretation of this option is that the heaters are thermally close to the sensor and will be near or between the set point temperatures. Sink to Average Temperature is the simplest and most robust of the Thermostat options.

Note:

With this option, the heater elements are treated as temperature sinks. That is, heat can flow into or out of the heater elements. If you select Sink to Average Temperature, check the solution results to verify that heat is flowing out of the heater elements. If heat is flowing into the heater elements, it means that the heater element temperatures would normally be above the set point, even if the heater was disabled. In such a case, you should not use the Sink to Average Temperature option.

Proportional Heat Load

Proportional Heat Load creates a linear ramped heat load between the set point temperatures. At the top of the dead band, the heat load is the maximum heater output. At the bottom of the dead band, the heat load is zero. With this option, the solution algorithm iterates to find the correct heat load and temperature combination to achieve the time averaged steady-state solution. In a true transient simulation, heaters can cycle on and off. In this case, the result is a time averaged picture. Use a large relaxation factor when using this option.

Equivalent Heat Load

Equivalent Heat Load instructs the solver to make multiple steady-state iterations to determine the influence of each heater in the model on all of the sensors. It then solves a linear system of equations which determines the heat load required at each heater in order for the sensor temperature to be at the mid point of each dead band. For nonlinear models, such as those with strong radiation or temperature dependent conductivities, multiple outer iterations are automatically performed to find the target temperatures. The heat loads required on each heater are reported at the end of the analysis.

This option is useful for heater sizing studies. It is possible that this option can return negative heat loads on the heaters, which indicates that those heater locations must be cooled for all the sensors to be at their target temperatures in the steady-state condition.

Note:

This option is not supported for coupled thermal-flow solutions when you select the Solve Flow check box. For coupled thermal-flow solutions, use the Sink to Average Temperature option instead.

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