  Topology optimization (TO) is a powerful method to design innovative
structures with improved heat transfer performance. In the present study, a
multi-fidelity TO method with a delicately defined objective function is
developed for flow boiling heat transfer in microchannels. Low-fidelity TO is
conducted for the reduced-order process of single-phase laminar convective heat
transfer, which generates a set of structure candidates for subsequent
high-fidelity evaluation of flow boiling heat transfer. To avoid the possible
iteration between the low-fidelity TO and high-fidelity evaluation which leads
to inefficient solution of the multi-fidelity TO, distributions of velocity,
temperature and two-phase in microchannels with single-phase and/or flow
boiling heat transfer are investigated and compared in detail, based on which a
new objective function is delicately defined, which can be employed in the
low-fidelity TO yet can stand for the performance of the high-fidelity problem.
With the help of the new objective function, the efficiency of the
multi-fidelity TO is significantly improved and TO structures are designed with
hot spots eliminated, thermal resistance reduced and temperature uniformity
improved. The present work provides a new method for TO of complicated heat and
mass transfer problems. Keywords: topology optimization, flow boiling,
multi-fidelity optimization, microchannels, convective heat transfer
