Fla Frank Ganzevles

Marangoni convection in binary drops in air cooled from below

Marangoni convection in binary drops resting on a plate in quiescent air is studied. Water is used with low molar fractions of 2-butanol or ethanol. The droplets are homogeneously cooled from below with an initial temperature difference of 3.5 K. In the presence of air, the thermocapillary and concentration-gradients induced motions are mutually reinforcing. The latter motions last much longer than the time it takes for interfacial temperature gradients to become negligible. The Marangoni convection time increases with increasing concentration of alcohol if the volume concentration is less than 10 vol.%, levels off at higher concentrations and is higher for 2-butanol than for ethanol. The consequences for dropwise condensation in practical compact heat exchangers are investigated by measuring differences in temperature gradient histories during actual condensation processes.

Temperatures and the condensate heat resistance in dropwise condensation of multicomponent mixtures with inert gases

The temperature variations occurring in dropwise condensation at condenser plates of a compact, polymer heat exchanger are studied using instantaneous infrared temperature field recordings. An averaging procedure in time and an assessment of extreme values is proposed and carried out. With the results, the heat resistance of the condensate is quantified. It is found that mixing and convection in the condensate, caused by coalescence and drainage of drops, reduces the condensate heat resistance by a factor 4 as compared with purely conductive heat transfer. This reduction is comparable, both in nature and in magnitude, to the effect of enhanced mixing due to turbulence in the liquid film of filmwise condensation. A second condensable species has been added to the gas mixture in order to study the contribution of Marangoni convection due to concentration gradients to the condensate heat transfer resistance. No contribution is found.

The shape factor of conduction in a multiple channel slab and the effect of non-uniform temperatures

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Geometry adaptations to improve the performance of compact, polymer condensers

Practical, relatively simple methods are presented to enhance heat transfer with condensation in a compact, polymer plate heat exchanger. Tilting the exchanger, while adapting the spacers, increases the condensate and heat fluxes by 6–8%, as a function of the inlet mass fraction of the moist air at ambient conditions. Polymer inserts increase convective heat transfer and yield a total heat flux improvement of 20%, at the cost of increased pressure drop.

Heat and mass transfer from internal flows to hemispheres and flat parts in between

With the naphthalene sublimation technique, the mass transfer from hemispheres on a flat plate to air flowing between two parallel plates is measured. A length scale is introduced to render the correlation of the results independent of the drop size. Measured mass transfer from flat plates is compared to measured heat transfer to flat plates. With the aid of the findings, mean interface temperatures of condensate drops in a compact PVDF heat exchanger are predicted. The calculations are compared with temperatures measured with an infrared camera.