Thermo-Hydraulic Performance and Energy Efficiency of Hybrid Nanofluids in Heat Exchangers: A Comprehensive Review

Abstract

The study of hybrid nanofluids — which are a mixture of two or more dissimilar nanoscale solids in the conventional liquid — is underway as a tunable heat-transfer fluid for compact and high duty heat exchangers. Unlike the base fluid they are engineered to have higher thermal conductivity, but more important, a controllable ratio of conductivity, heat capacity, viscosity, stability, surface interaction and cost. This review critically assesses that balance in terms of thermo-hydraulic and energy-efficiency. The literature is structured on the preparation and stability, models for thermophysical properties, flow and heat transfer mechanisms, configuration of heat exchanges, penalties on pressure drop, efficiency criteria, first- and second-law efficiency, numerical modelling, machine learning and scale-up. The evidence suggests that hybridization may be useful for providing a valuable thermal synergy, especially if a highly conductive carbonaceous phase is mixed with a stable metal or metal-oxide phase. Nevertheless, energy requirement is not necessarily lower when there is an increase in conductivity. However, this can be offset by viscosity increase, aggregation, fouling, lack of property relations and incomplete accounting for pumping power. The best performance is typically obtained when the particle loading is low to moderate (heat transfer improvement outweighs hydraulic penalty) in all types of exchangers: across plate, double-pipe, shell-and-tube, compact finned, spiral, coil and triple-tube. This review demonstrates the invalidity of comparisons when the Reynolds number is equal or when the flow rate is equal and suggests that comparisons should be made at the same heat duty, pumping power and exchanger volume. It also works to create a reporting system that connects colloidal characterization to impacts on heat-exchanger performance and effect on the life cycle. The main finding is that hybrid nanofluids should be designed as system working fluid rather than just based on thermal conductivity. Long duration stability tests, uncertainty-aware correlations, validated multiphase models, standardized thermo-hydraulic metrics, corrosion and erosion data, safe handling protocols and demonstration under realistic duty cycles at pilot scale are needed to pave the way for future progress.