Experimental Investigation onfluid mechanics of different micro heat transfer devices

The future technological horizons for engineering applications (Automotive, Energy, Bioengineering) will require the use of small size heat exchangers with high heat exchange efficiency. In the present work, the thermo-hydrodynamic behaviour of eight different micro heat exchangers is investigated experimentally using thermal measurements and µPIV (micro Particle Image Velocimetry), in order to detail and improve the thermal performance of micro devices. To test the heat transfer properties and the detailed fluid flow behaviour, this research was extended from typical range of Reynolds number in the laminar regime, from 50 to 1500, to the turbulent regime with Reynolds number up to 4000. Moreover, different cooling fluids are employed to analyse the heat transfer capability of micro-devices under different working conditions. Results are evaluated first in term of the Nusselt-Reynolds diagram and then through the efficiency of the cells performed by relating the average Nusselt number to the Fanning factor. On the base of the thermal analysis, the µPIV measurements were employed to detail the observed global heat exchange performances of each micro-cell configuration. In order to make a link with results highlighted by the thermal measurements the attention was focused on local fluid recirculation and acceleration as related to the specific geometry and to the different flow rates. The main result of this investigation is that the standard serpentine micro cell attains the highest efficiency regardless of flow regime, getting high Nusselt numbers combined with low pressure losses, as derived by the observation of quite high local velocities and few recirculation regions. In fact, PIV results highlighted that the main reason for the increasing Nusselt number is only dependent on local high intensity accelerations and not on recirculation regions, that appear to contribute only to pressure loss increments. Also in case of working fluids with high glycerin contents, e.g. 30 % water and 70 % glycerin, the results suggested high thermal exchange properties, mainly related to the glycerin physical properties and not only to local high intensity fluid accelerations.