Jinku Wang

Roughness and cavitations effects on electro-osmotic flows in rough microchannels using the lattice Poisson–Boltzmann methods

This paper investigates the effects of roughness and cavitations in microchannels on the electro-osmotic flow behaviors using the Lattice Poisson–Boltzmann methods which combined one lattice evolution method for solving the non-linear Poisson–Boltzmann equation for electric potential distribution with the other lattice evolution method for solving the Navier–Stokes equations for fluid flow. The boundary conditions are correctly treated for consistency between the both. The results show that for the electro-osmotic flows in homogeneously charged rough channels, the flow rate does not vary with the roughness height or the interval space monotonically. The flow rate varies slightly with the roughness height or even increases a little when the roughness is very small, and then decreases when the roughness height is larger than 5% channel width. The flow rate decreases first and then increase with the roughness interval space. An interval space at twice roughness width makes the flow rate minimum. For the heterogeneously charged rough channel, the flow rate increases with the roughness surface potential at a super-linear rate. For the electro-osmotic flows in microchannels with cavitations, the flow rate change little with the cavitations depth when the depth value is very low and decreases sharply when the depth is greater than 3% channel width. The flow rate trends to be a constant when the cavitations are very deep. The flow rate decreases with the cavitations width but increases with the cavitations interval.

Three-dimensional effect on the effective thermal conductivity of porous media

A three-dimensional mesoscopic method is developed for predicting the effective thermal conductivity of multiphase random porous media. The energy transport equations are solved by a lattice Boltzmann method for multiphase conjugate heat transfer through a porous structure whose morphology is characterized by a random generation-growth algorithm. Our numerical results show that the cell number in the third dimension influences the resulting effective thermal conductivity of three-dimensional porous media. The predicted effective thermal conductivity varies with the cell number in the third dimension following an exponential relationship, and it requires in the examples at least 10 cells along the third dimension before the predictions stabilize. Comparisons with the experimental data show that the effective thermal conductivities measured by the hot-probe and hot-wire techniques agree well with the predicted results by the two-dimensional model, whereas those measured by the transient comparative method agree more with the three-dimensional predictions.

LATTICE BOLTZMANN SIMULATIONS OF MIXING ENHANCEMENT BY THE ELECTRO-OSMOTIC FLOW IN MICROCHANNELS

The Lattice Boltzmann methods are used to study the mixing enhancements by the electro-osmotic flow in microchannel. Three sets of lattice evolution methods are performed for the fluid flow, for the electrical potential distribution, and for the concentration propagation. The simulation results show that the electro-osmotic flow induces y-directional velocity which enhances the mixing in microchannels. The mixing enhancement is related with the surface zeta potential arrangement and the external electric field strength.

Thermal conductivity measurement of an individual fibre using a T type probe method

A method was developed to measure the longitudinal thermal conductivity (TC) of an individual fibre using a T type probe. In the T type probe, a hot wire was supplied with a constant direct current, whose ends were connected to heat sinks to maintain the initial temperature. The test fibre was attached to the centre position of the hot wire at one end and the other end was connected to another heat sink. Based on a one-dimensional steady-state analysis of the heat conduction in the probe, the TC of the fibre and the thermal contact resistance at the junction between the fibre and the hot wire were simultaneously obtained, by changing the fibre length in the same contact condition at the junction. This method was verified by measuring the Pt wire as a reference sample, and good agreement was achieved between the measurement data and reference value. The TC of a pitch-based carbon fibre was obtained to be 490 W m −1 K −1 at room temperature, and the measurement uncertainty was analysed and compared with that of the previous T type probe method.