A laminar jet flow confined by a conical wall and an impinging plate was investigated. The Navier... more A laminar jet flow confined by a conical wall and an impinging plate was investigated. The Navier-Stokes equations were numerically solved by a finite difference technique and the results compared with laser Doppler anemometry data, the latter also covering the transition and turbulent flow regimes. Transition was found to start in the impingement region at a jet Reynolds number of around 1,600. The investigation concentrated on assessing the effects of nozzle-to-plate distance (always less than one nozzle diameter), jet Reynolds number, and nozzle outlet conditions. The shape of the velocity profile at the nozzle outlet determines the entire cell flow field, whereas nozzle-to-plate distance affects the flow in the expansion region. Under certain flow and geometric conditions a recirculation zone appears in the expansion region, in the vicinity of the plate.
The numerical prediction of mass transfer rates in the mass boundary layer over a membrane separa... more The numerical prediction of mass transfer rates in the mass boundary layer over a membrane separation requires the use of a very dense grid. In this work, a simple logarithmic variable transformation applied to the solute transport equation improves the well-known finite difference scheme, allowing the use of a larger grid spacing without loss of accuracy. This method is applied to solve laminar flow and solute transport equations in a parallel plate device with permeable walls. The concentration profiles along the membrane surface and in the mass boundary layer are predicted. For high permeate velocity, the grid spacing can increase four times without loss of accuracy. The method applied has several advantages comparatively to the optimized grid spacing method that can be used in alternative.
Nowadays, there is an attempt to develop new membrane separation cells to minimize the concentrat... more Nowadays, there is an attempt to develop new membrane separation cells to minimize the concentration polarization phenomenon. An efficient contact between the fluid and the membrane is fundamental to reach this objective. In the present study, the hydrodynamic characteristics of a liquid jet impinging perpendicularly to a flat and round shaped membrane are explored. The jet flow is confined by a conical wall extended from the jet nozzle to a short distance above the membrane. The momentum and mass transport equations in laminar regime are solved numerically by a finite difference scheme. The solution depends on Reynolds and Schmidth numbers and on two new dimensionless groups, Π v and Π π 0 ; Π v represents the ratio between the permeate velocity through a non-polarized membrane surface and the average jet velocity at the cell inlet, and Π π 0 the ratio between the osmotic pressure over a non-polarized membrane surface and the static pressure difference across the membrane. The concentration polarization is investigated in wide ranges of values of these groups and new indexes are defined to quantify the polarization level. For increasing values of Re, Sc, and Π v , the polarization level and the concentration at the membrane surface increase. For increasing values of Π π 0 the polarization level increases, but the concentration at the membrane surface tends to the bulk concentration. The suction effects on the velocity profiles in the layer over the membrane are also analyzed. A compact module of jet cells is proposed. : S 0 3 7 6 -7 3 8 8 ( 0 0 ) 0 0 5 7 2 -X
International Journal of Heat and Mass Transfer, 2001
Mass transfer from a soluble plate to an impinging liquid jet confined by a conical wall is inves... more Mass transfer from a soluble plate to an impinging liquid jet confined by a conical wall is investigated. The nozzle-to-plate distance is very short, less than one nozzle diameter, the flow regime is laminar, Re<1600, and the Schmidt number ranges from 960 to 50000. Navier–Stokes and solute transport equations are solved by a finite difference scheme. Numerical predictions of the
This paper introduces the Portuguese Web Archive initiative, presenting its main objectives and w... more This paper introduces the Portuguese Web Archive initiative, presenting its main objectives and work in progress. Term search over web archives collections is a desirable feature that raises new challenges. It is discussed how the terms index size could be reduced without significantly decreasing the quality of search results. The results obtained from the first performed crawl show that the Portuguese web is composed approximately at least by 54 million contents that correspond to 2.8 TB of data. The crawl of the Portuguese web was stored in 2 TB of disk space using the ARC compressed format.
A laminar jet flow confined by a conical wall and an impinging plate was investigated. The Navier... more A laminar jet flow confined by a conical wall and an impinging plate was investigated. The Navier-Stokes equations were numerically solved by a finite difference technique and the results compared with laser Doppler anemometry data, the latter also covering the transition and turbulent flow regimes. Transition was found to start in the impingement region at a jet Reynolds number of around 1,600. The investigation concentrated on assessing the effects of nozzle-to-plate distance (always less than one nozzle diameter), jet Reynolds number, and nozzle outlet conditions. The shape of the velocity profile at the nozzle outlet determines the entire cell flow field, whereas nozzle-to-plate distance affects the flow in the expansion region. Under certain flow and geometric conditions a recirculation zone appears in the expansion region, in the vicinity of the plate.
The numerical prediction of mass transfer rates in the mass boundary layer over a membrane separa... more The numerical prediction of mass transfer rates in the mass boundary layer over a membrane separation requires the use of a very dense grid. In this work, a simple logarithmic variable transformation applied to the solute transport equation improves the well-known finite difference scheme, allowing the use of a larger grid spacing without loss of accuracy. This method is applied to solve laminar flow and solute transport equations in a parallel plate device with permeable walls. The concentration profiles along the membrane surface and in the mass boundary layer are predicted. For high permeate velocity, the grid spacing can increase four times without loss of accuracy. The method applied has several advantages comparatively to the optimized grid spacing method that can be used in alternative.
Nowadays, there is an attempt to develop new membrane separation cells to minimize the concentrat... more Nowadays, there is an attempt to develop new membrane separation cells to minimize the concentration polarization phenomenon. An efficient contact between the fluid and the membrane is fundamental to reach this objective. In the present study, the hydrodynamic characteristics of a liquid jet impinging perpendicularly to a flat and round shaped membrane are explored. The jet flow is confined by a conical wall extended from the jet nozzle to a short distance above the membrane. The momentum and mass transport equations in laminar regime are solved numerically by a finite difference scheme. The solution depends on Reynolds and Schmidth numbers and on two new dimensionless groups, Π v and Π π 0 ; Π v represents the ratio between the permeate velocity through a non-polarized membrane surface and the average jet velocity at the cell inlet, and Π π 0 the ratio between the osmotic pressure over a non-polarized membrane surface and the static pressure difference across the membrane. The concentration polarization is investigated in wide ranges of values of these groups and new indexes are defined to quantify the polarization level. For increasing values of Re, Sc, and Π v , the polarization level and the concentration at the membrane surface increase. For increasing values of Π π 0 the polarization level increases, but the concentration at the membrane surface tends to the bulk concentration. The suction effects on the velocity profiles in the layer over the membrane are also analyzed. A compact module of jet cells is proposed. : S 0 3 7 6 -7 3 8 8 ( 0 0 ) 0 0 5 7 2 -X
International Journal of Heat and Mass Transfer, 2001
Mass transfer from a soluble plate to an impinging liquid jet confined by a conical wall is inves... more Mass transfer from a soluble plate to an impinging liquid jet confined by a conical wall is investigated. The nozzle-to-plate distance is very short, less than one nozzle diameter, the flow regime is laminar, Re<1600, and the Schmidt number ranges from 960 to 50000. Navier–Stokes and solute transport equations are solved by a finite difference scheme. Numerical predictions of the
This paper introduces the Portuguese Web Archive initiative, presenting its main objectives and w... more This paper introduces the Portuguese Web Archive initiative, presenting its main objectives and work in progress. Term search over web archives collections is a desirable feature that raises new challenges. It is discussed how the terms index size could be reduced without significantly decreasing the quality of search results. The results obtained from the first performed crawl show that the Portuguese web is composed approximately at least by 54 million contents that correspond to 2.8 TB of data. The crawl of the Portuguese web was stored in 2 TB of disk space using the ARC compressed format.
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Papers by João Miranda