Thermal-hydraulic performance of TiO2-water nanofluids in an offset strip fin heat exchanger

The flow and heat transfer characteristics of the TiO2-water nanofluid assuming as a single-phase in the rectangular offset strip fin structure for different Reynolds number (500-1000) and TiO2 nanoparticle volume concentration values (0%-4%) were investigated numerically under three-dimensional, steady state and laminar flow conditions. Simulations were also performed for 1% and 4% nanoparticle volume concentrations of Al2O3-water nanofluid, and the results were compared with those of TiO2-water nanofluid. Results show that when the TiO2-water nanofluid is used, the heat transfer rate, heat transfer coefficient and Nusselt number increase with increasing both Reynolds number and nanoparticle volume concentration, and parallel to these, both pressure loss and pumping power increase. Considering the values of the performance evaluation criteria (PEC) number, it is clear that the use of TiO2-water nanofluid in offset strip fin structure at all Reynolds numbers examined between 1%-4% volume concentration values is quite advantageous. It is observed that TiO2-water nanofluid is much superior to Al2O3-water considering the PEC number. When the Reynolds number is 1000 and the volume concentration value of the TiO2 nanoparticle is 4%, the PEC number value is found to be 1.19, that is, there is a 19% increase compared to water. It is considered that the results of this study can be used as important data on the design of automobile radiators, air-conditioning and defense.

Flow Field Measurements in a Metal Additively Manufactured Offset Strip Fin Array Using Laser Doppler Velocimetry

Metal additive manufacturing (AM) of heat exchanger enables custom and conformal designs for a wide range of applications. However, one challenge with metal AM is the resultant surface roughness formed when using this process which is non-existent during traditional manufacturing processes. The goal in this study is to explore how this roughness impacts the pressure drop and flow field of a commonly used heat exchanger surface called an offset strip fin (OSF). Two OSF of the same geometry are tested: one with an average fin roughness of 34 µm from metal AM and the other with an average fin roughness 2.5 µm, used as a baseline. The roughness from the metal AM process increased pressure losses and transitioned the flow to turbulent-like behavior at lower Reynolds numbers when compared with the smooth fin. Laser Doppler Velocimetry (LDV) measurements captured the row number in the fin array where transition from laminar to turbulent-like flow occurred. The location of transition from low to high turbulence levels occurred earlier in the fin array as the Reynolds number was increased for the smooth and rough fins. Wake profiles of time-averaged axial velocity were similar between the rough and smooth fins, with the rough fins having higher levels of turbulence intensity and less symmetric wake profiles. Overall, this study indicates that a pressure loss penalty is associated with using metal AM OSF due to the resultant surface roughness and an earlier transition to turbulent-like flow.