Fluid Volumes in Thermal Liquid Networks

Fluid Volumes

Thermal Liquid components fall into two groups—those modeled with internal fluid volume and those not. A component has internal fluid volume if that volume is substantial and the internal fluid dynamics relevant. Components with fluid volume include pipes, tanks, and actuators. Components without fluid volume include valves, orifices, and sudden area changes. Fluid volumes enable you to:

  • Model internal component dynamics.

  • Satisfy the Simscape™ grounding rules.

  • Specify initial conditions in a model.

Component Dynamics

Components with fluid volume have internal states, defined as differential variables, that can evolve over time. The internal states capture the transient effects of dynamic compressibility, fluid inertia, and wall compliance, when supported and enabled. State changes occur gradually, over the course of characteristic response times determined from component geometries and fluid properties. Such components are called dynamic.

Dynamic Component

Components without fluid volume have no internal states. Algebraic equations constrain the states at the ports, but transient changes due to internal component dynamics are ignored. State changes occur instantaneously, without time lag. The components respond to dynamic changes just outside their ports, but so rapidly that they are, at each time step, at steady state with their neighbors. Such components are called quasi-steady.

Quasi-Steady Component

Grounding Rules

Dynamic components are required in Thermal Liquid models. These components have internal nodes at known pressure and temperature and therefore serve as state references in Thermal Liquid networks. You can satisfy the Simscape grounding rules by connecting at least one dynamic component to each Thermal Liquid network in your model.

Initial Conditions

To arrive at meaningful numerical solutions, the differential equations of dynamic components require initial conditions as inputs. The block dialog boxes of dynamic components provide initial condition parameters that you can set to change the initial configuration of your model. Quasi-steady components have no internal states and therefore no initial states to set.

The number of initial conditions should equal the number of degrees of freedom in your model. The number of degrees of freedom is the difference between the number of differential variables and the number of dynamic constraints. You can obtain both number using Simscape Statistics Viewer. You can view the initial conditions specified in a model using Simscape Variable Viewer.

Dynamic Components

The table lists the dynamic components that you can use, organized by library.

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