Yahui Yang

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.

This paper focuses on the experimental analysis of convective heat transfer characteristics of pressure-driven gaseous flows through microtubes, which is frequently encountered in practical application of microfluidic devices accommodating gas flow, heat transfer and/or chemical reactions at micro scale. The present work has been carried out with the objectives to: (i) experimentally verify the applicability of conventional theory for the prediction of the internal forced convection heat transfer coefficient for tubes having an inner diameter lower than 1 mm and (ii) check the performance of some specific correlations proposed for the analysis of forced microconvection with gases in the last decades. Single commercial stainless steel microtubes are tested having an inner diameter between 0.17 and 1 mm. The most common thermal boundary conditions, namely uniform heat flux (H boundary condition) and uniform wall temperature (T boundary condition), have been experimentally obtained by applying Joule heating on the external surface of the microtubes (H b.c.) and by immerging the microtube in water with a fixed temperature by means of a thermostatic bath (T b.c.). The design of the test section has been made with care in order to ensure the appropriate experiment reliability and to improve the accuracy of microconvective tests at microscale. Experimental tests have been performed in both laminar and transitional flow regimes, with Reynolds number between 400 and 4000. The values of Nusselt number are experimentally determined and compared with both conventional theory and the prediction of the specific correlations developed for microchannels.

This paper deals with the analysis of the main features of forced microconvection of liquid and gas flows through microchannels. A critical overview of the main effects that tends to play an important role in the determination of Nusselt number in microchannels is presented. Some experimental data obtained at the Microfluidics Lab of the University of Bologna together with the main results which appeared recently in the open literature both for liquids and gases are used in order to highlight the peculiar characteristics of the convective heat transfer through microchannels and to suggest the guidelines for a physically based interpretation to the experimental results. By means of specific examples, it is shown that the thermal behavior at microscale of gas and liquid flows through microchannels in terms of convective heat transfer coefficients can be strongly affected by scaling and micro-effects but also by practical issues linked to the geometry of the test rig, the real thermal boundary conditions, the presence of fittings, position and type of the sensors, and so on. All these aspects have to be taken into account during the data post processing in order to obtain a correct evaluation of the Nusselt numbers. It is also highlighted how it is always useful to couple to the experimental approach a complete computational thermal fluid-dynamics analysis of the whole tested microsystem in order to be able to recognize “a priori” the main effects which can play an important role on the 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 the convective heat transfer coefficients (this is important for the development of new correlations); (ii) compare in a right way the conventional correlations with the experimental results.

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.

In this paper a double-layered microchannel gas-to-gas heat exchanger has been designed and experimentally investigated. The micro heat exchanger (micro HEX) core is based on 133 parallel microchannels machined into polished PEEK plate for both hot side and cold side. Each microchannel is 200 μm high, 200 μm wide and 39.8 mm long. The microchannel layers have been designed in order to be able to test the effect on the thermal performances of the micro heat exchanger of partition foils made in different materials and of various thicknesses. In addition, the device allows to test the layers under three different flow arrangements, namely, countercurrent flow, cocurrent flow and cross flow. Customized pressure and temperature sensors are integrated into the microHEX to enable in-situ measurements. Experimental tests have been performed for various mass flow rates of hot and cold currents. The experimental results are compared with the predictions of the classical theory for conventionally sized heat exchangers. The influence of flow arrangements and wall axial conduction on the thermal performance of the micro heat exchanger are analyzed by considering both the theory and the experimental results.

This paper presents the results of experimental and numerical investigation of forced convection of gas flows through stainless steel microtubes having inner diameters of 750 μm, 510 μm and 170 μm. The study covers both transitional and turbulent flow regimes (3000<Re<12000). In these regimes the flow is highly compressible, inducing conversion from thermal energy to kinetic energy inside microtubes. Moreover, reverse energy conversion takes place immediately after the fluid is vented to the outlet chamber where the measurement of fluid outlet temperature is performed. In this work the effects of fluid compressibility on the forced convection at microscale is quantitatively discussed by combining experimental data with numerical predictions. It is evidenced that compressibility effects can distinctively enhance convective heat transfer in terms of Nusselt number. This enhancement turns out to be more pronounced for microtubes with smaller inner diameter even at medium Reynolds numbers. In order to explore in-depth the heat transfer mechanism, the system is numerically simulated adopting the Arbitrary-Lagrangian-Eulerian (ALE) method and the Lam-Bremhorst Low-Reynolds number turbulence model to evaluate eddy viscosity coefficient and turbulence energy. The crossing of the numerical data, which provide the local value of pressure and temperature, with the experimental ones helps to explain the physical sense of the experimental results. In addition, the convective heat transfer coefficients obtained in the present work are compared with both classical correlations validated for conventional pipes and the correlations proposed for gas flows through 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.