Sandro Salvigni

Experimental Analysis of Microconvective Heat Transfer in the Laminar and Transitional Regions

Abstract This article details experiments to determine the Nusselt number for laminar and transitional liquid flows (water and FC-72) through rough stainless-steel microtubes of 440-, 280-, and 146-μm inner diameter. Under laminar conditions, the average Nusselt number approaches the fully developed value for uniformly heated tubes as Reynolds decreases. For higher Reynolds, the region of thermal development increases the average convective heat transfer coefficient, which becomes a function of the Reynolds and Prandlt numbers and of the inner diameter-to-heated-length ratio. The effect of roughness is negligible in the laminar regime. Under transition, the average Nusselt steeply increases with Reynolds, more than for conventional pipes.

Laminar, Transitional and Turbulent Friction Factors for Gas Flows in Smooth and Rough Microtubes

Theoretical and experimental works on microscale transport phenomena have been carried out in the past decade in the attempt to analyse possible new effects and to assess the influence of scaling on the classical correlations which are used in macro-scale heat and fluid flow, following the need to supply engineers with reliable correlations to be used in the design of micro-scale devices. These results were sometimes in mutual contrast, as is the case for the determination of the friction factor, which has been found to be lower, higher or comparable to that for macroscopic channels, depending on the researchers. In this work the compressible flow of nitrogen inside circular microchannels from 26 μm to 508 μm in diameter and with different surface roughness (<1%) is investigated for the whole range of flow conditions: laminar, transitional and turbulence. Over 5000 experimental data have been collected and analysed. The data confirmed that in the laminar regime the agreement with the conventional theory is very good in terms of friction factors both for rough and smooth microtubes. For the smaller microchannels (<100 μm) when Re is greater than 1300 the friction factor tends to deviate from the Poiseuille law because the flow acceleration due to compressibility effect gains in importance. The transitional regime was found to start no earlier than at values of the Reynolds number around 1800–2000. Both smooth and sudden changes in the flow regime have been found, as reported for conventional tubes. Fully developed turbulent flow was attained with both smooth and rough tubes, and the results for smooth tubes seem to confirm Blasius’s relation, while for rough tubes the Colebrook’s correlation is found to be only partially in agreement with the experimental friction factors. In the turbulent regime the dependence of the friction factor on the Reynolds number is less pronounced for microtubes with respect to the prediction of the Colebrook’s correlation and the friction factor tends only to depend on the microtube relative roughness.Copyright

Thermal Performance of silicon micro heat-sinks with electrokinetically-driven flows

A heat sink consisting of microchannels of rectangular or trapezoidal cross-section through which a polar fluid is circulated by means of an electro-osmotic pump was studied numerically. The equivalent pressure head–volume flow rate curve was determined for both geometries and the influence of the aspect ratio was investigated. The dimensionless temperature profile was determined keeping also the effect of Joule heating into account. The cross-sectional Nusselt number was calculated for the above conditions and was found to be strongly influenced by the ratio of Joule heating to convective heat flux, Mz . The dependence of the Nusselt number on the dimensionless electro-osmotic diameter (kDh ) was also investigated for the two geometries and for increasing values of Mz , and a comparison with the values obtained analytically for slug flow under the same conditions was made. The value of the Nusselt number as a function of the aspect ratio was also calculated for increasing values of Mz . The numerical data presented in this paper can be useful to optimize the thermal performance of silicon micro heat-sinks.Copyright