  Heat exchangers are devices that typically transfer heat between two fluids.
The performance of a heat exchanger such as heat transfer rate and pressure
loss strongly depends on the flow regime in the heat transfer system. In this
paper, we present a density-based topology optimization method for a two-fluid
heat exchange system, which achieves a maximum heat transfer rate under fixed
pressure loss. We propose a representation model accounting for three states,
i.e., two fluids and a solid wall between the two fluids, by using a single
design variable field. The key aspect of the proposed model is that mixing of
the two fluids can be essentially prevented without any penalty scheme. This is
because the solid constantly exists between the two fluids due to the use of
the single design variable field. We demonstrate the effectiveness of the
proposed approach through three-dimensional numerical examples in which an
optimized design is compared with a simple reference design, and the effects of
design conditions (i.e., Reynolds number, Prandtl number, design domain size,
and flow arrangements) are investigated.
