Bubble nucleation and jetting inside a millimetric droplet

In this work, we present experiments and simulations on the nucleation and successive dynamics of laser-induced bubbles inside liquid droplets in free-fall motion, i.e. a case where the bubbles are subjected to the influence of a free boundary in all directions. The droplets of a millimetric size are released from a height of around 20\,cm and acquire a velocity of around 2\,m/s at the moment the bubble is nucleated. Within this droplet, we have investigated the nucleation of secondary bubbles induced by the rarefaction wave that is produced when the shock wave emitted by the laser-induced plasma reflects at the drop surface. Interestingly, three-dimensional clusters of cavitation bubbles are observed. Their shape is compared with the negative pressure distribution computed with a CFD model and allows us to estimate a cavitation threshold value. High-speed recordings of the drop/bubble dynamics are complemented by the velocity and pressure fields simulated for the same initial conditions. The effect of the proximity of a curved free surface on the jetting dynamics of the bubbles was qualitatively assessed by classifying the cavitation events using a non-dimensional stand-off parameter which depends on the drop size, the bubble maximum radius and the relative position of the bubble inside the drop. Here, we found that the curvature of the free surface does not play a determinant role on the jet dynamics, being the distance to the surface the dominant parameter. The oscillation of the laser-induced bubbles promote the onset of Rayleigh-Taylor and Rayleigh-Plateau instabilities, observed on the drop's surface. The specific mechanisms leading to the destabilisation of the droplet surface were identified through a careful inspection of the high speed images

A Numerical Simulation of Turbulent Flow through a Curved Duct

This paper presents the comparison the results of an experimental work with a numerical work keeping the geometry of the test duct and inlet boundary conditions unaltered. The numerical simulation is validated with the experimental results based on the wall y+ approach for different turbulence models suited for this type of geometry. The experimental work is carried out at mass averaged mean velocity of 40m/s with the measurement of total pressure by a pre-calibrated multi-hole pressure probe and the results presented in the form of a pressure contours in 2-D. For validation of the numerical results Standard k-ε, k-ω and Reynolds Stress Model (RSM) are used to solve the closure problem. The turbulence models are investigated in the commercial CFD code of Fluent using y+ value as guidance in selecting the appropriate grid configuration and turbulence model. Based on the wall y+ values for different turbulence models, it is concluded in the present study that the mesh resolving the fully turbulent region is sufficiently accurate in terms of qualitative features and RSM turbulence model predicts the best results while comparing with the experimental results

Effects of different orientations of winglet arrays on the performance of plate-fin heat exchangers

Numerical investigations pertaining to heat transfer enhancement of a plate-fin heat exchanger using two rows of winglet type vortex generators (VG) have been performed in this work. Five different strategic placements of the VG, namely, common-flow up in series (CFU CFU), common-flow down in series (CFD–CFD), combined (CFD–CFU), inline rows of winglet (IRW) and staggered rows of winglet (SRW), were considered. Performance parameters in terms of Nusselt number and quality factor were evaluated from the velocity and temperature data obtained from the solutions of full Navier–Stokes and energy equations. The Reynolds number was varied in the range of 250–1580. Results show that amongst the different types of arrangements of the VG, performance of CFU–CFU configuration is best in terms of heat transfer as well as quality factor