Heat Transfer Options in Altair Flow Simulator
Altair EmployeeFlow Simulator has various ways to include temperature change in a flow model. These range from simply adding heat to a flow chamber to more complex methods of coupling the flow network to a thermal network.
Add Heat to Flow Chambers
Heat can be added (or removed) directly to a flow chamber. This heat can come from any source such as electronic components, rotating bearing losses, or combustion. The heat value can be a constant or a controller can be used to vary the heat with time or another model parameter (such as RPM for bearings).
Heat Transfer in Zero-Length Elements
Elements such as the conventional orifice and orifice plate have heat transfer options. One important consideration is where to apply the heat in a zero-length element. By default, all the heat is applied after the restriction. The fluid temperature change will affect the density and therefore the flowrate through these restrictions.
Heat Transfer in Tubes
Tube elements (standard, advanced, and incompressible) have several options for heat transfer. Tube elements are discretized by stations along the length of tube so heat transfer (fluid temperature change) can occur at each station. A common option is to apply convection heat transfer from the fluid in the tube to a specified wall temperature. This option assumes the wall temperature is known. There are several common correlations to calculate the tube Heat Transfer Coefficient (HTC) based on the fluid flow. These correlations account for laminar and turbulent flow. Entrance effects for developing boundary layers can also be applied.
If the tube wall temperature is not known, the tube element can be associated with thermal network resistors to simulate the heat transfer. Multiple conductor resistors can be used to model multiple tube layers (such as a metal tube wall covered with a layer of insulation). As mentioned before the tube element is discretized by stations. The length of tube between stations is called a segment. When associating a tube with a thermal network, each segment of the tube must be associated with a thermal node.
The ”Jacketed Pipe” domain creator can be used to quickly create the thermal network in this example. If needed, the resistors and thermal nodes can be modified after using the domain creator. For instance, the convection HTC to ambient can be modified.
The tubes and thermal network resistors can be used to model more complex situations such as a pipe within a pipe like this counter flow heat exchanger. For this example, the incompressible tube that represents the annulus flow is using 2 “circumferential” segments. One segment for the inner and one for the outer wall of the annulus.
Thermal Convector and Flow Chambers
Heat transfer between the thermal network and the flow network can also be modeled by attaching a convector directly to a flow chamber. There are several more HTC correlations besides the duct flow correlations available for the convector attached to a flow chamber. The HTC options include flat plate, body in crossflow, free convection, impingement, and thin rotating gap. Here are some examples of convectors attached to flow chambers.
Heat Exchanger Components
One final method to exchange heat between 2 fluid streams is the heat exchanger component. The Generic Heat Exchanger (GHX) has multiple options for pressure loss and heat transfer performance. The Plate Fin Heat Exchanger uses the heat exchanger geometry to calculate the heat transfer. The heat exchanger component has 4 connection points, an inlet and exit for the hot and cold streams.
