Flow boiling is an effective heat transfer mechanism, which is important in many industrial applications including in nuclear power plants. A unique flow boiling experiment has been constructed in Thermal-Hydraulics Experimental Laboratory for Multiphase Applications (THELMA) at Reactor Engineering Division of Jožef Stefan Institute. The experiment consists of a custom-designed heat exchanger, which allows visual observation of the boiling surface. Heat flux at the boiling surface is controlled with the temperature and flow rate of the two fluids involved. The present design provided accurate measurements for low and medium heat flux magnitudes, however, modifications are needed for the flow boiling studies at high heat fluxes.

Numerical simulations provide better understanding of the complex devices as well as enable their design optimization. The main objective of the present study is optimization of the heat exchanger, which is used for flow boiling experiments. In particular, previous studies have shown that up to 50% of the total heat transfer in our experiment takes place in the inlet and outlet manifolds, i.e. the heads of the heat exchanger. In order to increase the heat transfer in the test section itself, all heat losses need to be reduced to the minimum, including the heat transfer in the heads of the heat exchanger. For that reason, a computational fluid dynamics (CFD) model has been constructed for the present heat exchanger, which includes conjugate heat transfer in the primary and secondary fluid flow as well as several solid domains that are made of different solid materials. Results revealed the most critical parts of the device with severe heat leakages, which need improvements. Thus, modifications have been proposed in the geometry as well as in the selection of more appropriate material properties. Comparison between the present and the optimized design has shown significantly better heat isolation of the fluids inside the heads, which will hopefully allow experiments at much higher heat fluxes up to the critical heat flux (CHF).