Marco Lorenzini

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.

A Criterion for the Experimental Validation of the Slip-Flow Models for Incompressible Rarefied Gases Through Microchannels

This paper is devoted to analyzing the friction factor of incompressible rarefied gas flow through microchannels. A theoretical investigation is conducted in order to underline the conditions for experimentally evidencing rarefaction effects on the pressure drop. It is demonstrated that for a fixed geometry of the microchannel cross section it is possible to calculate the minimum value of the Knudsen number for which the rarefaction effects can be observed experimentally, taking into account the experimental uncertainties on the evaluation of the friction factor.Copyright

Assessing Uncertainties in Friction Factor Measurement as a Tool in Devising Experimental Set-Ups

The promising performance of microchannels has given rise to intensive research on pressure drop and heat transfer characteristics of flows at the small- validate new ones, experiments need to be conducted, which are particularly difficult given the characteristic dimensions involved and the magnitude of the fluxes to be measured. Although more care has been devoted lately to the design of experiments in terms of control of geometry and boundary conditions, the uncertainties which inevitably affect each measurement do not seem to have been given the proper consideration. Correctly calculating uncertainties not only allow to a correct assessment of the experimental data obtained, but can also be used to decide which measurements need to have the highest precision to achieve a certain accuracy, thus saving money on the others. In this paper, a quantitative criterion is given to assess the accuracy achievable in the determination of the friction factor in the laminar regime for the flow of a fluid in a circular microtube. The influence of the six quantities (pressure drop, outlet pressure, temperature, length, pressure and volume flow rate) measured to determine f in the laminar regime are studied separately and when combined. It is found that at low Reynolds numbers flow rate and pressure drop measurements are determinant for the final value of the uncertainty, while at larger Reynolds numbers the influence of the accuracy in measuring the hydraulic diameter prevails and also limits the minimum value that the total uncertainty can take.Copyright

Optimization of Metallic Multi-Microchannel Array Evaporators

Pulsation and maldistribution effects (hot spot formation) with characteristic frequencies below 100 Hz occurring in electrically powered microchannel array heat exchangers used as evaporators for water were investigated primarily with the aid of visualisation techniques. Pulsation at subaudio frequencies was found to be dominated by the consequences of boiling in the inlet plenum, creating large vapour bubbles that intermittently entered the microchannel array, pushing liquid water at velocities too high to achieve complete evaporation. A new design minimising the residence time in the inlet and comprising an intermediate void was found to produce a two phase mixture that could be evaporated in an array of 68 microchannels, each 200 μm wide, 100 μm deep, and 20mm long, at a mass flux of 60 kgm−2 s−1 at an average surface temperature of 220°C. The redesign led to a change in characteristic flow patterns in the microchannel arrays from plug and slug flow to film flow and drops moving along the walls of the microchannels, as evident from high speed (103 fps) video sequences. By means of infrared thermography of the surface of a metallic device comprising four layers of microchannel arrays, maldistribution between the microchannel array layers leading to hot spot formation was observed when the device was operated in constant power mode. The formation of these hot spots could be avoided by the use of simple temperature control electronics operating at characteristic frequencies below 10−1 Hz.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

Experimental Study of Innovative Heat Sinks for High Power Density Electronic Components

In this work, some simple air-cooled high-effectiveness heat sinks are proposed for the cooling of electronic devices such as microprocessors for personal computers. The performance of the heat sinks are experimentally investigated. In particular the temperature of the heat sink surface in contact with the devices to be cooled is measured together with the air temperature and flow, by varying the air flow rate. The pressure drop in the air through the heat sinks is also measured. The thermal resistance of the disspators is then calculated and compared to that of commercially available heat sinks.Copyright