Local heating effects on flow and heat transfer in microchannels

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.A series of numerical investigations was conducted to explore the effects of temperature-dependent viscosity and thermal conductivity on two-dimensional low Reynolds number convection of water in microchannels with locally heating. An emphasis was addressed on the fundamental characteristics of flow and thermal re-development at different localized heat fluxes and different inlet temperatures. The velocity field is highly coupled with temperature distribution and distorted through the variations of viscosity and thermal conductivity. The induced cross-flow velocity has a marked contribution to the convection. The heat transfer enhancement due to viscosity-variation is pronounced, though the axial convection introduced by thermal-conductivity-variation is insignificant unless for the cases of very low Reynolds numbers. The heat transfer enhancement is described by defining the peak value and location of relative Nusselt number distribution as ΔNu%max and Xmax. Strong nonlinear interaction mechanism prevails in the correlation of ΔNu%max and Xmax due to high heat flux condition and dramatic rise of liquid temperature.This study is supported by the National Natural Science Foundation of China (Grant No. 50636030)

Regime mapping and the role of the intermediate region in wall-coated microreactors

Operation of a wall-coated microreactor can occur in several mass transfer-reaction regimes. We define these regimes analytically in several planes of a multi-parametric map, taking into account the different degrees of concentration profile development, as well as the influence of non-unity orders of reaction and reactant inhibition in the kinetic law. It was found that the regions where conversion can be calculated from simplified mass transfer models are not discriminated by common results for entrance-length. We also illustrate the trade-offs that exist across this operating map concerning the catalyst design (costs associated with loading and volume) and overall system performance (evaluated in terms of reactant conversion, flow efficiency and microreactor effectiveness). It is shown that under certain conditions, the existence of moderate mass transfer resistance can be advantageous (even if internal limitations cannot be avoided), clarifying the role of the intermediate transport-reaction region

Rarefaction and thermal creep effects in square cross-section microchannels

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.Fluid flow and heat transfer in MEMS systems are numerically investigated in this study for potential rarefaction and thermal creep effects using our recently developed implicit, incompressible, hybrid (finite volume/finite element) flow solver. Rarefied flows characterized by the Knudsen number in the range of 0 ≤ Kn ≤ 0.1 are analyzed in detail for thermal creep effects in square cross-section microchannels with constant wall temperature boundary condition. Axial conduction becomes important for very low Reynolds number flows thereby enhancing the thermal creep effect. Three dimensional numerical simulations are conducted for simultaneously developing flows at very low Reynolds numbers in the range of 0.2 ≤ Re ≤ 5. Extended inlet boundary conditions are used to avoid entrance region singularity and also to account for axial heat conduction near the entrance. Friction coefficients are reduced with rarefaction in the slip flow regime. The reduction in friction coefficients was more pronounced due to thermal creep in the entrance region. Effects of rarefaction and thermal creep on heat transfer are studied for different gas-wall surface combinations as defined by the choice of momentum and thermal accommodation coefficients. It was found that heat transfer can increase or decrease with rarefaction. For very small values of gas-wall surface interaction parameter (β), it was observed that velocity slip dominates the temperature jump and heat transfer is enhanced with rarefaction. The opposite effect is observed for higher values of β. Nusselt number values increased slightly with thermal creep as Reynolds number was decreased

Absorption of spherical bubbles in a square microchannel

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.Microfluidics is a fast growing field in which the manipulation of bubbles in liquid phase is of utmost importance. In this paper, the absorption of spherical bubbles in a square microchannel is investigated for a bubbly flow. Numerical simulations of the gas-liquid two-phase flow and the mass transfer around spherical bubbles in a square microchannel are carried out. Correlations are established for the bubble velocity and the mass transfer rate. A model for the dissolution of spherical bubbles along a square microchannel is proposed in the case of the bubbly flow regime and validated using existing experimental data. This model can be used, for instance, for designing microabsorbers for lab-on-a-chip applications

Internal pressure measurements in microchannels of different shapes

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.This paper presents the experimental results of determining friction factors for two microchannels with circular cross-sections: rectilinear and curvilinear. The inner diameter of the channels is 68.9 and 70.3 μm. The Reynolds numbers ranged from 320 to 3215. Pressure measurements are carried out simultaneously in 16 locations along the straight microchannel and in 12 locations for the curved microchannel. The friction factor for the straight microchannel is in good agreement with the theoretical value for the round smooth tubes. For the curved microchannel, the friction factor value of the curved section is less than the reference value for smoothly curved tubes. The Reynolds number for the laminar-turbulent transition in a straight microchannel is 2300–2600. In the curved microchannel the transition is not observed. The length of the developing region was identified, and the inlet minor loss coefficient is calculated.This work was supported by the Siberian Branch of the Russian Academy of Sciences (integration project of SB RAS no. 110)

Simulation of copper-water nanofluid in a microchannel in slip flow regime using the lattice Boltzmann method with heat flux boundary condition

Laminar forced convection heat transfer of water–Cu nanofluids in a microchannel is studied using the double population Thermal Lattice Boltzmann method (TLBM). The entering flow is at a lower temperature compared to the microchannel walls. The middle section of the microchannel is heated with a constant and uniform heat flux, simulated by means of the counter slip thermal energy boundary condition. Simulations are performed for nanoparticle volume fractions equal to 0.00%, 0.02% and 0.04% and slip coefficient equal to 0.001, 0.01 and 0.1. Reynolds number is equal to 1, 10 and 50.The model predictions are found to be in good agreement with earlier studies. Streamlines, isotherms, longitudinal variations of Nusselt number and slip velocity as well as velocity and temperature profiles for different cross sections are presented. The results indicate that LBM can be used to simulate forced convection for the nanofluid micro flows. They show that the microchannel performs better heat transfers at higher values of the Reynolds number. For all values of the Reynolds considered in this study, the average Nusselt number increases slightly as the solid volume fraction increases and the slip coefficient increases. The rate of this increase is more significant at higher values of the Reynolds number

Performance analysis of non-circular microchannels flooded with CuO-water nanofluid

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.In this study the performances of various non-circular microchannel heat sinks (normalized with circular shape) have been comparatively analyzed for CuO-water nanofluid and baseline pure water flow. Nusselt number and Poiseuille number for each microchannel and thermo-physical properties of CuO-water nanofluid (viz., thermal conductivity, viscosity, specific heat and density) have been designed either empirically or from literature. Results for trapezoidal shape gives highest normalized pressure drops among all cases. Thermal performances for constant heat flux and constant wall temperature boundary conditions have been assessed in terms of normalized outlet wall temperature and normalized heat exchange rate. Results show that when thermal performance of any microchannel heat sink (MHS) improves, the hydraulic performance deteriorates. Trapezoidal microchannel gives best thermal performance in terms of normalized heat exchange rate specially with CuO-water nanofluids flow

Similarities and differences between flow boiling in microchannels and pool boiling

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Recent literature indicates that under certain conditions the heat transfer coefficient during flow boiling in microchannels is quite similar to that under pool boiling conditions. This is rather unexpected as microchannels are believed to provide significant heat transfer enhancement under single-phase as well as flow boiling conditions. This paper explores the underlying heat transfer mechanisms and illustrates the similarities and differences between the two processes. Formation of elongated bubbles and their passage over the microchannel walls have similarities to the bubble ebullition cycle in pool boiling. During the passage of elongated bubbles, the longer duration between two successive liquid slugs leads to wall dryout and a critical heat flux that may be lower than that under pool boiling conditions. A clear understanding of the similarities and differences will help in overcoming some of these limiting factors and in developing strategies for enhancing heat transfer during flow boiling in microchannels