Showing papers by "Vasudevan Raghavan published in 2006"

Thermal analysis of the vortex tube based thermocycler for fast DNA amplification: Experimental and two-dimensional numerical results

Abstract: In this article, experimental and numerical analyses to investigate the thermal control of an innovative vortex tube based polymerase chain reaction (VT-PCR) thermocycler are described. VT-PCR is capable of rapid DNA amplification and real-time optical detection. The device rapidly cycles six 20μl 96bp λ-DNA samples between the PCR stages (denaturation, annealing, and elongation) for 30cycles in approximately 6min. Two-dimensional numerical simulations have been carried out using computational fluid dynamics (CFD) software FLUENT v.6.2.16. Experiments and CFD simulations have been carried out to measure/predict the temperature variation between the samples and within each sample. Heat transfer rate (primarily dictated by the temperature differences between the samples and the external air heating or cooling them) governs the temperature distribution between and within the samples. Temperature variation between and within the samples during the denaturation stage has been quite uniform (maximum variation a...

Effects of forced convection and surface tension during methanol droplet combustion

Abstract: A numerical investigation of surface tension and forced convection effects on moving and suspended methanol droplets burning in a zero-gravity, low-pressure air environment is presented. Simulations were conducted using a predictive, transient, axisymmetric model for an initial droplet diameter of 0.5 mm, an ambient temperature of 1200 K, and initial Reynolds numbers (Re 0 ) in the range of 1-100. Results indicate that, for moving droplets, due to the presence of an envelope flame at some stage during the droplet lifetime, surface tension is important over the range of Ke 0 considered; the extinction diameter decreases with increasing Re 0 . For suspended droplets, when transition or envelope flame is present (Ke 0 less than approximately 15), surface tension is important; when an envelope flame is present (Re 0 less than approximately 10), the extinction diameter increases with Re 0 . Both for suspended and moving droplets, the droplet lifetime is weakly sensitive to surface tension. The variation of droplet lifetimes with Re 0 is much stronger for suspended droplets than for moving droplets. Depending on the Reynolds number, results on methanol droplet lifetimes and extinction diameters measured through suspended droplet experiments may not be applicable to moving droplets.

CFD analysis of thermal control in a vortex tube based polymerase chain reaction chamber

Abstract: In this paper, numerical analys is to investigate the thermal control of an innovative vortex tube based polymerase chain reaction thermocycler (VT -PCR) is described. VT PCR is capable of rapid DNA amplification and real -time optical detection. The device rapidly cycles six 20 µL 96 bp �-DNA samples between the PCR stages (denaturation, annealing and elongation) in approximately 6 mi nutes. Two -dimensional numerical simulations have been carried out using CFD software FLUENT v.6.2.16. Heat transfer rate (primarily dictated by the temperature differences between the samples and the external air heating or cooling them) governs the tempe rature distribution between and within the samples. Temperature variation between and within the samples during the denaturation stage has been quite uniform (maximum variation around ±0.5 uC and 1.6uC, respectively). During cooling, by adjusting the cold r elease valves in the VT -PCR during some stage of cooling, the heat transfer rate has been controlled. Improved thermal control, which increases the efficiency of the PCR process, has been obtained by slightly decreasing the rate of cooling. Thus, almost un iform temperature distribution between and within the samples (within 1 uC) has been attained for the annealing stage as well.

The Role of Surface Tension Effects During Methanol Droplet Combustion

Abstract: A numerical investigation of methanol droplet combustion in a zero-gravity and low-pressure convective environment is presented. Simulations have been carried out using a predictive, transient and axisymmetric model, which includes droplet heating, liquid-phase circulation and water absorption. First, a suspended droplet (constant relative velocity) burning in an ambient of air at 300K is considered. A nearly quiescent environment (initial Reynolds number Re0 =0.01) is used to impose a weak gas-phase convective flow, introducing a deviation from spherical symmetry. The resulting weak liquid-phase circulation is greatly enhanced due to surface tension effects, which create a complex, time-varying, multicellular flow pattern within the liquid droplet. The complex flow pattern, which, in the presence of surface tension, results in nearly perfect mixing, causes increased water absorption within the droplet, leading to larger extinction diameters. Surface tension effects are shown to be dominant in causing water absorption, even at initial Reynolds numbers as high as 5. Results for combustion in a nearly quiescent environment (Re0 =0.01) with varying initial droplet diameters, (d0 = 0.16 to 1.72 mm), show that predictions of droplet extinction diameters, although they are still below the experimental data, do improve substantially when surface tension effects are included. Next, results for suspended droplets and for moving droplets burning in an ambient of air at 1200K, for a range of initial Reynolds numbers that are of interest in spray combustion (Re0 =1-100) are presented. It is shown that, for moving droplets, due to the presence of an envelope flame at some stage during the droplet lifetime, surface tension is important over the entire range of Re0 considered; the extinction diameter decreases with increasing Re0 . Extinction is not observed for a moving droplet when surface tension effects are neglected. For suspended droplets, when transition or envelope flame is present, which corresponds to Re0 less than approximately 15, surface tension is important; when an envelope flame is present (Re0 less than approximately 10), the extinction diameter increases with Re0 . The variation of droplet lifetime with Re0 is much stronger for suspended droplets than for moving droplets. Depending on the Reynolds number, results on methanol droplet lifetimes and extinction diameters measured through suspended droplet experiments may not be applicable to moving droplets.Copyright © 2006 by ASME