Gabriel Gamrat

An experimental study and modelling of roughness effects on laminar flow in microchannels

Three different approaches were used in the present study to predict the influence of roughness on laminar flow in microchannels. Experimental investigations were conducted with rough microchannels 100 to 300μm in height ( H ). The pressure drop was measured in test-sections prepared with well-controlled wall roughness (periodically distributed blocks, relative roughness k * = k /0.5 H ≈0.15) and in test-sections with randomly distributed particles anchored on the channel walls ( k * ≈0.04–0.13). Three-dimensional numerical simulations were conducted with the same geometry as in the test-section with periodical roughness (wavelength L ). A one-dimensional model (RLM model) was also developed on the basis of a discrete-element approach and the volume-averaging technique. The numerical simulations, the rough layer model and the experiments agree to show that the Poiseuille number Po increases with the relative roughness and is independent of Re in the laminar regime ( Re Po observed during the experiments is predicted well both by the three-dimensional simulations and the rough layer model. The RLM model shows that the roughness effect may be interpreted by using an effective roughness height k eff . k eff / k depends on two dimensionless local parameters: the porosity at the bottom wall; and the roughness height normalized with the distance between the rough elements. The RLM model shows that k eff / k is independent of the relative roughness k * at given k / L and may be simply approximated by the law: k eff / k = 1 − ( c (ϵ)/2π)( L / k ) for k eff / k >0.2, where c decreases with the porosity ϵ.

Modeling of Laminar Flows in Rough-Wall Microchannels

Numerical modeling and analytical approach were used to compute laminar flows in rough-wall microchannels. Both models considered the same arrangements of rectangular prism rough elements in periodical arrays. The numerical results confirmed that the flow is independent of the Reynolds number in the range 1-200. The analytical model needs only one constant for most geometrical arrangements. It compares well with the numerical results. Moreover, both models are consistent with experimental data. They show that the rough elements drag is mainly responsible for the pressure drop across the channel in the upper part of the relative roughness range.

Numerical Modelling of Heat Transfer in Rectangular Microchannels

The paper presents both three and two-dimensional numerical analysis of convective heat transfer in microchannels. The three-dimensional geometry of the microchannel heat sink followed the details of the experimental facility used during a previous research step. The heat sink consisted of a very high aspect ratio rectangular microchannel. Two channel heights, namely 1mm and 0.3mm (0.1mm), were used for 3D (2D) numerical model respectively. Water was employed as the cooling liquid. The Reynolds number ranged from 200 to 3000. In the paper, the thermal entrance effect is analyzed in terms of heat transfer efficiency. Finally, the comparison between measured and computed heat flux and temperature fields is presented. Contrary to the experimental work, the numerical analysis did not reveal any significant scale effect in heat transfer in microchannel heat sink up to the smallest size considered (0.1 mm).

Numerical and Experimental Investigations of Momentum and Heat Transfer in Microchannels

The paper presents the results of a research performed during recent years at LEGI Grenoble with joint participation of CzUT researchers. The special attention is devoted to experimental and numerical studies devoted to three cases involving various aspects of microchannel flow physics (range of microchannel sizes corresponds to the classification of Kandlikar [19]). The first aspect is related to hydraulic properties of a network of parallel triangular microchannels, where experimental investigations revealed the rapid increase of pressure drop for Re exceeding value of 10. The second aspect of the research was the influence of surface roughness, which was investigated both experimentally and numerically for periodically and randomly distributed surface elements. The third research case was devoted to the numerical modelling of heat transfer. As a result, experimental and numerical analyses showed that there was no scale effect for the microchannels considered, i.e. the relevance of the classical continuum flow model was confirmed.© 2008 ASME