Gian Luca Morini

Experimental Analysis of Gas Micro-Convection Through Commercial Microtubes

In this article, an experimental campaign devoted to analyzing the forced micro-convection features of heated gas flows through commercial stainless-steel microtubes having inner diameters of 172 μm and 750 μm is described. The experimental results obtained by heating the microtubes with an imposed uniform heat flux (H-boundary condition) at the external wall, in terms of Nusselt numbers, are compared to the predictions of the classical correlations validated for conventional pipes and to the correlations proposed for gas flows through microtubes under laminar and transitional conditions (100 < Re < 4,000). The cross-sections of the tested microtubes enabled the analysis of the effects of wall axial heat conduction on the Nusselt number. It was observed that the Nusselt number is strongly dependent on the Reynolds number in the laminar regime, and this fact is explained in the article with the effects of wall axial heat conduction and the difficulties in the experimental determination of the right exit bulk temperature of the gas flow, which cannot be ignored in the thermal analysis. The agreement between the Gnielinski correlation and the experimental Nusselt number is poor, especially for low Reynolds numbers, if one uses the average gas bulk temperature, obtained as the arithmetic mean between the inlet and outlet gas bulk temperature, in the definition of the experimental Nusselt number. On the contrary, the agreement with the Gnielinski correlation improves if the local wall-gas temperature difference near the exit of the microtube is used instead. The experimental results presented in the article demonstrate that the criteria for the design of accurate micro-convection tests can be quite different from those for the analysis of forced convection through conventional pipes.

The Effect on the Nusselt Number of the Nonlinear Axial Temperature Distribution of Gas Flows Through Microtubes

The characteristics of nitrogen convective heat transfer through commercial stainless-steel microtubes with inner diameters of 0.172 mm and 0.750 mm are investigated both experimentally and numerically. An analysis of the total uncertainty on the Nusselt number is carried out in order to quantitatively identify the contribution introduced by the different operative parameters in the experiments. The problems related to the estimation of the correct outlet temperature measurement for gas microflows and to the determination of the axial trend of the bulk gas temperature along the microtube are deeply discussed for both microtubes tested. It is highlighted that the axial local gas bulk temperature distribution is strongly nonlinear along the tube, especially for microtubes having a small inner diameter and a thick solid wall. It has been demonstrated that the trend of the experimental Nusselt numbers as a function of the Reynolds number can be considered in good agreement with that predicted by the conventional correlations if the average bulk temperature is calculated while taking into account the axial nonlinearity of the gas bulk temperature distribution and the accurate measurement of the outlet gas temperature. The results presented in this paper give a physical explanation of the unexpected low values of the Nusselt numbers determined in the previous experimental works on gas flows through microtubes.

Guidelines for the Determination of Single-Phase Forced Convection Coefficients in Microchannels

This paper deals with the analysis of the main features of forced microconvection of liq-uid and gas flows through microchannels. A critical overview of the main effects thattends to play an important role in the determination of Nusselt number in microchannelsis presented. Some experimental data obtained at the Microfluidics Lab of the Universityof Bologna together with the main results which appeared recently in the open literatureboth for liquids and gases are used in order to highlight the peculiar characteristics ofthe convective heat transfer through microchannels and to suggest the guidelines for aphysically based interpretation to the experimental results. By means of specific exam-ples, it is shown that the thermal behavior at microscale of gas and liquid flows throughmicrochannels in terms of convective heat transfer coefficients can be strongly affectedby scaling and micro-effects but also by practical issues linked to the geometry of the testrig, the real thermal boundary conditions, the presence of fittings, position and type ofthe sensors, and so on. All these aspects have to be taken into account during the datapost processing in order to obtain a correct evaluation of the Nusselt numbers. It is alsohighlighted how it is always useful to couple to the experimental approach a completecomputational thermal fluid-dynamics analysis of the whole tested microsystem in orderto be able to recognize “a priori” the main effects which can play an important role onthe convective heat transfer analysis. It is demonstrated in this paper that this “a priori”analysis is crucial in order to: (i) individuate the main parameters which influence theconvective heat transfer coefficients (this is important for the development of new corre-lations); (ii) compare in a right way the conventional correlations with the experimentalresults. [DOI: 10.1115/1.4024499]Keywords: microchannels, forced convection, scaling and micro-effects, Nusselt number

Design and Experimental Investigation of a Gas-to-Gas Counter-Flow Micro Heat Exchanger

In this work, a double-layered microchannel heat exchanger is designed for investigation on gas-to-gas heat transfer. The micro-device contains 133 parallel microchannels machined into a polished polyether ether ketone plate for both the hot side and cold side. The microchannels are 200 μm high, 200 μm wide, and 39.8 mm long. The design of the micro-device allows tests with partition foils in different materials and of flexible thickness. A test rig is developed with the integration of customized pressure and temperature sensors for in situ measurements. Experimental tests on the counter-flow micro heat exchanger have been carried out for five different partition foils and various mass flow rates. The experimental results, in terms of pressure drop, heat transfer coefficients, and heat exchanger effectiveness are discussed and compared with the predictions of the classic theory for conventionally sized heat exchangers.

Experimental analysis of heat transfer between a heated wire and a rarefied gas in an annular gap with high diameter ratio

In this paper a first experimental attempt is performed to measure heat conduction through rarefied air at rest contained between two concentric cylinders. The heat transfer between a heated platinum wire having a diameter (d) of 0.15 mm, disposed along the axis of a cylindrical shell in stainless steel having an inner diameter (D) of 100 mm, and a surrounded rarefied gas has been studied experimentally and numerically. The ratio between the outer and inner diameter of the annular region filled by the gas is large (D/d=667). In the annular region filled with air the pressure was varied by using a vacuum pump from atmospheric value down to 10−3 mbar. Temperature differences between the wire and the external stainless steel wall in the range 50-125 K were imposed and the heat power transferred from the wire to the surround was measured as a function of the gas pressure starting from air at atmospheric conditions down to 10−3 mbar. The experimental results obtained in these tests were compared with the numerical results obtained by using the linear and nonlinear Shakhov kinetic models.

Use of the μPIV technique for an indirect determination of the microchannel cross-section passage geometry

In this work the possible use of the μPIV technique for the experimental determination of the microchannel cross-section geometry has been investigated by means of a blind test in which a series of experimental measurements obtained using glass microchannels having a declared rectangular cross-section with a depth of 100 μm and width of 300 μm and a square microchannel with a 300 μm side have been compared with the direct SEM visualisation of the real cross section of the microchannels. For the (oPIV measurements water is used as working fluid. The laminar fully developed 2D velocity profile has been reconstructed by moving the focal plane of the microscope objective from the bottom to the top of the microchannel. The results shown in this paper demonstrate that the real cross section geometry of the microchannel can be predicted by minimizing the difference between the theoretical and the experimental 2D velocity profiles. When the right passage geometry is determined, the average difference between the theoretical and the experimental velocity is within 4-6%.

Single-Phase Laminar Forced Convection in Microchannels With Rounded Corners

This work investigates the frictional and heat transfer behavior of laminar, fully developed flow in microchannels with trapezoidal and rectangular cross-section and rounded corners under boundary conditions of uniform heat flux along the channel length and perimeter and uniform temperature on the heated perimeter of each cross section. The equations of momentum and energy are solved numerically using the least square method, and the results are validated with analytical data, when available. The runs have been carried out for different aspect ratios and nondimensional radii of curvature, with either all sides or three sides heated, one short side adiabatic for rectangular geometries and three sides heated, the longest one adiabatic for trapezoidal geometries. The Poiseuille and Nusselt numbers are reported and show, for the rectangular cross-section heated on all sides, a maximum increase for the highest value of the aspect ratio with increments in the Poiseuille and Nusselt numbers of about 11% and 16%, respectively, for values of the nondimensional radius of curvature of 0.5, increasing as the geometry approaches the circular duct (12.5% and 21%). The increase is less pronounced as the aspect ratio decreases and also when only three sides are heated (maximum increase of the Nusselt number around 10% for the latter); in the case of trapezoidal geometry the effects of rounding the corners are almost negligible (a maximum increase in the Nusselt number of around 2%).

Experimental uncertainties analysis as a tool for friction factor determination in microchannels

The constant growth of the studies on microchannel flows has brought under question the validity of the relations for heat transfer and fluid flow, which are usually employed at the macroscales. Rarefied flows in the slip-flow region have attracted much attention and solutions have been developed using first- and second-order boundary conditions. These models need to be experimentally validated through careful test in order to be able to use them for more complex problems and engineering applications. In the current work the error propagation analysis is applied to a set of error-free measurements artificially generated in order to assess the influence of the uncertainty on each of the measured quantities on the determination of the Poiseuille number for rarefied flows: it is shown that the most limiting factor is the accuracy on the tube diameter, while flowrate and pressure drop errors can be kept contained provided the measurement ranges for the transducers are suitably chosen. The total uncertainty is also calculated and the limit of the investigable Reynolds numbers defined. The possibility of experimentally evidencing the differences between first- and second-order boundary conditions is investigated and it is concluded that this is the case only for highly rarefied flows (Kn > 0.5).

Electro-osmotic flows inside triangular microchannels

This work presents a numerical investigation of both pure electro-osmotic and combined electro-osmotic/pressure-driven flows inside triangular microchannels. A finite element analysis has been adopted to solve the governing equations for the electric potential and the velocity field, accounting for a finite thickness of the electric double layer. The influence of non-dimensional parameters such as the aspect ratio of the cross-section, the electrokinetic diameter and the ratio of the pressure force to the electric force on the flow behavior has been investigated. Numerical results point out that the velocity field is significantly influenced by the aspect ratio of the cross section and the electrokinetic diameter. More specifically, the aspect ratio plays an important role in determining the maximum volumetric flow rate, while the electrokinetic diameter is crucial to establishing the range of pressures that may be sustained by the electro-osmotic flow. Numerical results are also compared with two correlations available in the literature which enable to assess the volumetric flow rate and the pressure head for microchannels featuring a rectangular, a trapezoidal or an elliptical cross-section.

The Rules of Single-Phase Forced Convection in Microchannels

This paper deals with the experimental analysis of forced micro-convection features of liquid and gas flows through microchannels. An overview of the main effects that tends to play an important role on the determination of the Nusselt numbers in microchannels is presented. Some experimental data obtained at the Microfluidics Lab of the University of Bologna together with the main results appeared recently in the open literature are used in order to highlight the characteristics of the convective heat transfer through microchannels with inner diameter from 0.75 mm down to and 0.15 mm. It is shown that the behavior of gas and liquid flows through microchannels in terms of convective heat transfer coefficients can be strongly affected by scaling and micro-effects as by practical issues linked to the geometry of the test rig, fittings, position of the sensors and so on. It is demonstrated that the comparison with the conventional correlations for the prediction of the convective heat transfer coefficients gives good results only if one has verified beforehand that the main scaling and micro-effects are negligible.Copyright