V.S. Duryodhan

A simple and novel way of maintaining constant wall temperature in microdevices

Constant wall temperature /homogeneity in wall temperature is the need of various lab-on-chip devices employed in biological and chemical investigations. However method to maintain this condition does not seem to be available. In this work, a novel and simple way of maintaining constant wall temperature is proposed. A diverging microchannel along with conjugate effects is utilized towards this end. Both measurements and three dimensional numerical simulations are undertaken to prove the design. The investigation has been carried out over a large parameter range (divergence angle: 1–8°; length: 10–30 mm; depth: 86–200 μm; solid-to-fluid thickness ratio: 1.5–4.0, and solid-to-fluid thermal conductivity ratio: 27–646) and input conditions (mass flow rate: 4.17 × 10−5 −9.17 × 10−5 kg/s, heat flux: 2.4–9.6 W/cm2) which helped in establishing the finding. It is observed that a nearly constant wall temperature condition can be achieved over a large parameter range investigated. A model to arrive at the design parameter values is also proposed. The method is further demonstrated for series of microchannels where we successfully maintain each station at different temperature within ±1 °C. The finding is therefore significant and can be employed in both single and multi-stage processes such as PCR requiring different constant wall temperature with a fine resolution.

Liquid flow through converging microchannels and a comparison with diverging microchannels

Diverging and converging microchannels are becoming an important part of microdevices. In this work, an experimental study of liquid flow through converging microchannels is performed and analyzed using results from 3D numerical simulations. Converging microchannels of various configurations: hydraulic diameter (118–177 µm), length (10–30 mm) and convergence angle (4°–12°) are used to measure the pressure drop for a volume flow rate range of 0.5–5 ml min−1 (8.33 10−6–8.33 10−5 kg s−1) using deionised water as the working fluid. It is observed that the pressure drop in a converging microchannel varies non-linearly with the volume flow rate, and inversely with the convergence angle and hydraulic diameter. An equivalent hydraulic diameter is introduced in order to predict the overall pressure drop through the converging microchannel using the established theory for straight microchannels. The equivalent hydraulic diameter of the converging microchannel lies at 1/3.6th of the total length from the narrowest width of the microchannel; compared with 1/3rd for the diverging microchannel. A comparative analysis of flow through diverging and converging microchannels is also performed. It is shown that fluidic diodicity varies asymptotically with the angle and length of microchannels, whereas it increases with the volume flow rate. A theoretical expression for diodicity is also derived. The maximum fluidic diodicity is found to lie between 1.2 and 1.3. The data presented in this work is of fundamental importance and can help in optimizing the design of various microdevices.

Effect of Cross Aspect Ratio on Flow in Diverging and Converging Microchannels

Aspect ratio is an important parameter in the study of flow through noncircular microchannel. In this work, three-dimensional numerical study is carried out to understand the effect of cross aspect ratio (height to width) on flow in diverging and converging microchannels. Three-dimensional models of the diverging and converging microchannels with angle: 2-14 deg, aspect ratio: 0.05-0.58, and Reynolds number: 130-280 are employed in the simulations with water as the working fluid. The effects of aspect ratio on pressure drop in equivalent diverging and converging microchannels are studied in detail and correlated to the underlying flow regime. It is observed that for a given Reynolds number and angle, the pressure drop decreases asymptotically with aspect ratio for both the diverging and converging microchannels. At small aspect ratio and small Reynolds number, the pressure drop remains invariant of angle in both the diverging and converging microchannels; the concept of equivalent hydraulic diameter can be applied to these situations. Onset of flow separation in diverging passage and flow acceleration in converging passage is found to be a strong function of aspect ratio, which has not been shown earlier. The existence of a critical angle with relevance to the concept of equivalent hydraulic diameter is identified and its variation with Reynolds number is discussed. Finally, the effect of aspect ratio on fluidic diodicity is discussed which will be helpful in the design of valveless micropump. These results help in extending the conventional formulae made for uniform cross-sectional channel to that for the diverging and converging microchannels.