Influence of heat transfer and wetting angle on condensable fluid flow through nanoporous anodic alumina membranes.

The flow of isobutane and of freon 142b (1-chloro-1,1-difluoro-ethane) through anodic alumina membranes with pore diameters between 18 and 60 nm in a capillary condensation regime is experimentally and theoretically explored. The capillary condensation effect increases the membrane permeance for condensable gases from 25 to 150 m3(STP) m-2 bar-1 h-1 at certain conditions. To describe the experimental results, a model is suggested accounting for heat transfer from the condensing to the evaporating meniscus, different boundary conditions for the heat transfer between the environment and the membrane, and wettability of the pore wall. The proposed model indicates a large influence of heat supply from the environment to the membrane on the permeance in the capillary condensation regime and a moderate influence of condensate contact angle in the range of 0-60°. Measuring the temperature of the permeate side of the membrane allows to find a suitable boundary condition to describe heat transfer. The obtained boundary condition yields an excellent fit of experimental results of condensate flow through membranes with different pore diameters for the two utilized fluids. Also, confocal Raman spectroscopy gave evidence on the fraction of pores filled with condensate.

Mass flow and momentum flux in nanoporous membranes in the transitional flow region.

An experimental study of momentum transfer in nanoporous polymeric track-etched membranes with pore diameters ranging from 100 to 1300 nm and nanochannel lengths of 12-20 µm was performed using He, N2, CO2, and SF6 propellants in a wide range of plenum and background pressures. Mass flux through the membranes was elaborated as a combination of Knudsen diffusion and viscous flow at Knudsen numbers above 0.1 and become choked at lower Knudsen numbers. The discharge coefficient for the membranes attained was 0.6, making the permeation rate similar to that of thin orifices. The effect is attributed to the mirror reflection of the molecules from the pore walls at low angles of incidence. The exhaust gas velocity is found to be dependent on the plenum to background pressure ratio and channel length-to-diameter ratio, reaching 0.9 of the velocity of the gas expanded to vacuum (up to 2 M). Close to an isothermal expansion occurs in nanochannels of all sizes. A general quantitative description for gas expansion in nanochannels is provided. The highest thrust is generated in the choked flow regime with the SF6 propellant and a value of 4.5 N cm-2 is attained at a propellant consumption of 0.165 kg (cm2 s)-1 for the membranes with 1300 nm nanochannels. The specific impulse of 138 s is reached for helium. The results show the prospects of the utilization of nanoporous membranes in cold gas propulsion systems.

Venous air embolism through central venous access.

An 25-year-old man was buried by an avalanche during off-slope skiing. He was rescued by his companions and resuscitated by mouth-to-mouth ventilation. The emergency physician from a helicopter based emergency medical service placed two venous lines in both external jugular veins and secured the airway with a tracheal tube. When transferred to the emergency department an additional central venous catheter was inserted via his right femoral vein. The subsequent computed tomography scan revealed several small air bubbles adjacent to the endothelium of the brachiocephalic vein. In an experimental setting, it was shown that air could enter the circulation via a central venous catheter within a few seconds, but measured values of embolising air were smaller than the calculated values when applying the law of Hagen-Poiseuille. Nevertheless, it is important to keep the lumens of a central venous catheter filled with saline before any manipulation in order to prevent or attenuate venous air embolism.

The potential of venous air embolism ascending retrograde to the brain.

A bench study was performed to investigate the potential of air bubbles entering a central vein via a central venous catheter to ascend retrograde to the brain. The results support the hypothesis that air bubbles may rise retrograde against the venous blood flow, depending on bubble size, central vein diameter and cardiac output. A review of radiological findings in published case reports indicates that the occurrence of retrograde cerebral air embolism is underestimated.