You can find this out by looking at the IDF file that is generated when running a simulation. Open the IDF, search for HeatExchanger:FluidToFluid - docs, then look for the field "Heat Exchange Model Type". This will be one of the following options:
- CrossFlowBothUnMixed Specifies a single-pass, cross-flow heat exchanger. The effectiveness will be calculated using a cross-flow heat exchanger correlation for both streams unmixed.
- CrossFlowBothMixed Specifies a single-pass, cross-flow heat exchanger. The effectiveness will be calculated using a cross-flow heat exchanger correlation for both streams mixed.
- CrossFlowSupplyMixedDemandUnMixed Specifes a single-pass, cross-flow heat exchanger. The effectiveness will be calculated using a cross-flow heat exchanger correlation for flow mixed on the Loop Supply side and flow unmixed on the Loop Demand Side.
- CrossFlowSupplyUnMixedDemandMixed Specifes a single-pass, cross-flow heat exchanger. The effectiveness will be calculated using a cross-flow heat exchanger correlation for flow unmixed on the Loop Supply side and flow mixed on the Loop Demand Side.
- CounterFlow Specifies a counter-flow shell and tube heat exchanger. The effectiveness will be calculated using a counter-flow shell and tube heat exchanger correlation.
- ParallelFlow Specifies a parallel-flow shell and tube heat exchanger. The effectiveness will be calculated using a parallel-flow shell and tube heat exchanger correlation.
- Ideal Specifies an ideal heat exchanger. The effectiveness will be set to ‘1.0’ and the specified UA will be ignored. The heat transfer rate will be calculated as the maximum possible heat transfer rate.
Someone from the OpenStudio team can probably tell you what the default is (I've added the OpenStudio tag for you), but this shows you where you can find the answer to this and any similar questions for yourself.
