Natural convection in rectangular cavities at high Rayleigh number

Natural convection in rectangular two-dimensional cavities with differentially heated side walls is a standard problem in numerical heat transfer. Most of the existing studies has considered the low Ra laminar regime. The general thrust of the present research is to investigate higher Ra flows extending into the unsteady and turbulent regimes where the physics is not fully understood and appropriate models for turbulence are not yet established. In the present study the Boussinesq approximation is being used, but the theoretical background and some preliminary results have been obtained[1] for flows with variable properties

Forced convection in micro-channel with synthetic jet : effect of operating frequency

A three-dimensional computational model has been developed to investigate the cooling effect on the microchip of synthetic jet interacting with a cross-flow in a micro-channel. The conjugate problem is solved by determining the temperature distributions in a heated solid and the fluid flowing in the micro-channel which cools it, thereby simulating the application to a microchip. A parametric study was performed on a fixed geometry by using 1 MWm -2 heat flux at the surface of the silicon wafer to investigate the effect of frequency of the jet at a constant Reynolds number, that is the amplitude is reduced in proportion to the increase in frequency. The hot region in the silicon wafer resulting from the use fluid flowing undisturbed in a micro-channel, are removed when the synthetic jet is switched on thereby significantly lowering the maximum temperature in the wafer. Contrary to the two-dimensional case, there is little difference in the cooling performance when the jet was driven at different frequencies in three-dimensional configuration. This is illustrated by the fact in the end of the simulations at a jet Reynolds number of 40, the maximum temperature in the substrate was 0.5 K lower at 1120 Hz than at 560 Hz and 1 K lower than at 280 Hz.10 page(s

Three-dimensional modelling of heat transfer in micro-channels with synthetic jet

An in-house computer code is developed and applied to investigate the effect of a synthetic jet on heat transfer rates in forced convection of water in silicon microchannels etched in the rear side of the silicon substrate. To account for the deflection of the membrane located at the bottom of the actuator cavity, a moving mesh technique to solve the flow and heat transfer is purposefully adopted. The governing equations are transformed into the curvilinear coordinate system in which the grid velocities evaluated are then fed into the computation of the flow in the cavity domain thus allowing the conservation equations of mass, momentum and energy to be solved within the stationary computational domain. The fully three-dimensional model considers the SIMPLE method to link the pressure and velocity. A heat flux of 1 MW/m 2 is applied at the surface of the top of the silicon wafer and the resulting complex, conjugate heat transfer through the silicon substrate is included. The hydrodynamics feature of the flow is validated against existing experimental results and verified against numerical results from commercial package ANSYS CFX 11.0. Good agreement has been achieved. To track the development of the flow and heat transfer when the actuator is switched on, numerical results of 20 full cycles of the actuator are simulated. When the actuator is switched on, noticeable temperature drop is observed at all points in the substrate from those which existed when there has been a steady water flow in the channel. At the end of 20th cycle of actuation, the maximum temperature in the wafer has reduced by 5.4 K in comparison with the steady flow values. In comparison with the two-dimensional study which account for 17K reduction, it indicates that synthetic jet has only smaller beneficial cooling and has been over-estimated in the previous two-dimensional study.10 page(s

The Impact of Blood Rheology on Drug Transport in Stented Arteries: Steady Simulations

Background and Methods It is important to ensure that blood flow is modelled accurately in numerical studies of arteries featuring drug-eluting stents due to the significant proportion of drug transport from the stent into the arterial wall which is flow-mediated. Modelling blood is complicated, however, by variations in blood rheological behaviour between individuals, blood’s complex near-wall behaviour, and the large number of rheological models which have been proposed. In this study, a series of steady-state computational fluid dynamics analyses were performed in which the traditional Newtonian model was compared against a range of non-Newtonian models. The impact of these rheological models was elucidated through comparisons of haemodynamic flow details and drug transport behaviour at various blood flow rates. Results: Recirculation lengths were found to reduce by as much as 24% with the inclusion of a non-Newtonian rheological model. Another model possessing the viscosity and density of blood plasma was also implemented to account for near-wall red blood cell losses and yielded recirculation length increases of up to 59%. However, the deviation from the average drug concentration in the tissue obtained with the Newtonian model was observed to be less than 5% in all cases except one. Despite the small sensitivity to the effects of viscosity variations, the spatial distribution of drug matter in the tissue was found to be significantly affected by rheological model selection. Conclusions/Significance: These results may be used to guide blood rheological model selection in future numerical studies. The clinical significance of these results is that they convey that the magnitude of drug uptake in stent-based drug delivery is relatively insensitive to individual variations in blood rheology. Furthermore, the finding that flow separation regions formed downstream of the stent struts diminish drug uptake may be of interest to device designers