RAWINTS3 December 2017
- Condensation on Fins
- The enhanced heat transfer tubes for vapour condensation is widely used, e.g. in refrigeration, air conditioning, petroleum and food industry etc. A generally used intensification method is the finning of the tube surface. The enhancement is provided not only by increasing the contact area between the vapour and the colder surface but also by the action of surface tension forces, which help to move a condensate from the fin additionally to other forces (gravitational, shear stress, …). Any curvature variation along the interface induces a pressure inhomogeneity within the liquid which leads to fluid redistribution. This pressure variation is known as the surface-tension pressure gradient (STPG). The fin shape influences the condensate film flowing along the fin surface. This leads to the arising of STPG, which effects on the film flow. It is presented in the condensate flowing along any fin, because each fin shape has corners and/or rounding. This fact reveals two main problems in the theoretical description of vapour condensation on the finned surface. First of all, the fin shape becomes one of the parameters in the modelling that complicates the simulation, considering the big variety of possible profiles. Secondly, there are some difficulties with the validation of existing models.
The local data about the condensate behavior on the fin and the liquid flow between fins are very important for improvement of existing models and validation of the approaches made. To perform such kind of investigations the gravitational force has to be negligibly small in comparison with the capillary forces. To satisfy these conditions on earth, the height of the fin has normally to be smaller than 2 mm, depending on the liquid properties. This makes local measurements to be very difficult. The estimation shows that under microgravity conditions the lateral length of the fin may be increased by a factor 30-60. That allows using of modern measurement techniques without any limitations.
- Reliable data on condensate behavior on the curved surface.
- Validation and improvement of existing models.
- Development of new way of condensation enhancement.
- Identify critical parameters for finning selection
Condensate film (blue) on the fin (grey) at different gravity levels.
HFE -7100, Pv=635 hPa; Tsat=47.5 oC; Tbase=35 oC; g0=9.8 m/s2
ESA Parabolic Flight Campaigh 61, Record N° 9458