Heat exchanger heat transfer coefficient and CFD modelling

Heat exchanger performance assessment with a high precision level is one of the key success factors for OTEC. Evaluating heat transfer coefficient with high precision in OTEC working conditions - specifically low heat flux, low mass flux, and large dimensions – is not possible by applying existing methods, models, and heat transfer laws. Initial analysis shows that heat transfer is driven by thermal and hydraulic parameters, which are linked, and therefore require 2D or 3D numerical models to be accurately modelled.Naval Energies and Naval Group have therefore defined and applied a heat transfer qualification method to correlate a 3D CFD thermo-hydraulical model based on ANSYS Fluent code with reduced scale testing on a water/ammonia flooded shell & tube evaporator. The results meet the high expectations as an uncertainty of less than 10% was achieved on the shell side heat transfer coefficient of the flooded shell & tube evaporator in 2-phase flow conditions, lowering the uncertainty of Rankine Cycle electrical production to 2%.