Single Stream Heat Exchangers

In a single stream heat exchanger, one stream is assumed to have a uniform temperature and the other stream is modeled by specifying the heat exchanger enthalpy source.

This section defines what is meant by the heat exchanger enthalpy source in Simcenter STAR-CCM+, and describes how to activate this type of heat exchanger and specify its properties. The formulation of this heat exchanger is described Single-Stream Heat Exchanger Formulation.

Heat Exchanger Enthalpy Source

The heat exchanger enthalpy source is a device which introduces or removes a specified amount of heat flow in a region.

Local temperature with respect to a user-specified reference temperature establishes the distribution of this set amount of heat source or sink through the space of the region. In this way, the distribution of heat within the fluid is similar to what would be seen with a heat exchanger. It is associated with one fluid stream only and is a net source or sink with the model instead of a transfer from one continuum to another.

This device is a simple way to incorporate a known heat load into (or from) a model with a minimum of effort and a minimal computational burden. Sample applications include air conditioner evaporators, condensers, charge air coolers, radiators, electric heaters, and electronic devices.

The method is relatively insensitive to mesh size and utilizes resolution corresponding the grid fineness.

The Energy Source Option activates the heat exchanger enthalpy source.

Single-Stream Heat Exchanger Workflow

Select an energy model.
Do not select Eulerian multiphase models other than the Eulerian Multiphase Mixture and Volume of Fluid (VOF) Multiphase models; only these are compatible with heat exchanger simulation.
Select Regions > [region node] > Physics Conditions > Energy Source Option and set the Energy Source Option property to Heat Exchanger.
Select [region node] > Physics Conditions > Heat Exchanger Exit Temperature Specification and set the Outlet Temperature property to Inferred or Specified:
- Inferred means the outlet temperature $T_{o u t}$ is inferred. You must then specify the heat transfer rate $Q$ using the Heat Exchanger Total Energy Rate node under Physics Values.
- Specified means that you specify the outlet temperature $T_{o u t}$ using the Target Exit Temperature node under Physics Values. The heat transfer rate $Q$ is then inferred.
Set heat exchanger properties. Nodes for these properties become available under [region node] > Physics Values:
- Heat Exchanger Minimum Temperature Difference
- Heat Exchanger Boundaries
- Heat Exchanger First Iteration
- Heat Exchanger Temperature

Physics Values

Heat Exchanger Total Energy Rate

This node's Value sets the fixed rate of heat transfer,

Q_{t o t a l}

, from or to the device, typically measured in Watts. This quantity represents the total energy transfer for the heat exchanger enthalpy source.

Heat Exchanger Minimum Temperature Difference

This node's Value represents the minimum allowable temperature difference between the following:

The specified heat exchanger temperature and the maximum fluid temperature for a heater (that is, when the specified Heat Exchanger Total Energy Rate is positive)
The specified heat exchanger temperature and the minimum fluid temperature for a cooler (that is, when the specified Heat Exchanger Total Energy Rate is negative).

For a heater, the hot stream is assumed to have an infinite thermal capacitance, and the specified heat exchanger temperature $T_{h}$ refers to the constant temperature of the hot stream. Now, given the heat exchanger total energy rate $Q$ , the inlet cold stream temperature $T_{c i}$ , and the cold stream specific heat $c_{p c}$ , an energy balance could be used to predict the cold stream outlet temperature $T_{c o}$ as follows:

Figure 1. EQUATION_DISPLAY

T_{c o} = T_{c i} + \frac{Q}{{\dot{m}}_{c} c_{p c}}

(200)

where ${\dot{m}}_{c}$ is the mass flow rate of the cold stream. Physically, it is impossible to have $T_{h} < T_{c o}$ . However, if the heat exchanger temperature value is erroneously specified below the maximum cold stream temperature in the heat exchanger, the specified heat exchanger minimum temperature difference is used to correct the heat exchanger temperature. The heat exchanger temperature stays above the predicted maximum cold stream temperature. When this error occurs, a warning message appears on the console, which is an indication that the heat exchanger temperature specification is determined to be unphysical.

Similarly, for a cooler, the cold stream is assumed to have an infinite thermal capacitance, and the specified heat exchanger temperature $T_{c}$ refers to the constant temperature of the cold stream. Now, given the heat exchanger total Energy rate $Q$ , the inlet hot stream temperature $T_{h i}$ , and the hot stream specific heat $c_{p h}$ , an energy balance could be used to predict the hot stream outlet temperature $T_{h o}$ as follows:

Figure 2. EQUATION_DISPLAY

T_{h o} = T_{h i} - \frac{a b s (Q)}{{\dot{m}}_{h} c_{p h}}

(201)

where ${\dot{m}}_{h}$ is the mass flow rate of the hot stream. Physically, it is impossible to have $T_{c} > T_{h o}$ . However, if the heat exchanger temperature value is erroneously specified above the minimum hot stream temperature in the heat exchanger, the specified heat exchanger minimum temperature difference is used to correct the heat exchanger temperature. The heat exchanger temperature stays below the predicted minimum hot stream temperature. When this error occurs, a warning message appears on the console, which is an indication that the heat exchanger temperature specification is determined to be unphysical.

Heat Exchanger Boundaries

The properties of this region value node are used to specify the upstream and downstream interface boundaries that the Heat Exchanger (Single Stream) report uses:

Upstream Interface Boundary: Select the interface boundary through which fluid enters the heat exchanger region.
Downstream Interface Boundary: Select the interface boundary through which fluid leaves the heat exchanger region.

This node assumes that the heat exchanger region interfaces with two other regions as shown in the example below.

Heat Exchanger First Iteration

Use this node to set the First Heat Exchanger Iteration from which the heat exchanger enthalpy source is active and begins its contribution to the source term of the energy equation.

Ideally, activate the heat exchanger source term when the flow solution is beginning to conform to its final pattern (at least in terms of direction). Conformity could typically be achieved within the first 50 iterations of the flow solver for a moderately complex geometry.

Heat Exchanger Temperature

This region value node represents the reference temperature

T_{r e f}

of the heat exchanger enthalpy source. It is entered as a scalar profile.

The local fluid temperature at each cell is subtracted from the reference temperature $T_{r e f}$ to determine the relative amount of heat that is transferred from and/or to the cell. See Eqn. (225).

Target Exit Temperature

The properties of this node allow you to specify the temperature at the outlet, (the Target Outlet Temperature), while allowing Simcenter STAR-CCM+ to compute the overall Heat Transfer Rate (shown as a read-only property). You can optionally set the Under-Relaxation Factor, which governs the extent to which the new calculation supplants the old one as Simcenter STAR-CCM+ computes the heat transfer rate.

This node becomes available when the Outlet Temperature property of the Heat Exchanger Exit Temperature Specification region condition node is set to Specified.