IV. RESULTADOS
4.1. Sistema estructural:
The visualization window between the 1st and 2nd heat exchanger allows to record the flow
pattern using a Photron FASTCAM Mini UX100 high speed camera (CMOS sensor with Bayer color filter array). The high speed camera has been coupled with a 100 mm Tokina macro lens and a LED illumination system is used as light source. During a condensation test at the same mass velocity, different images have been recorded changing the vapor quality in the glass tube; the vapor quality at the inlet of the glass tube is settled changing the water conditions in the 1st heat exchanger. The images reported from Figure 4.19 to
Figure 4.44 have been recorded at 2000 fps except for the visualization at mass velocity equal to 200 kg m-2 s-1 that have been recorded at 3200 or 4000 fps depending on the vapor
quality. In the figures below for each mass velocity and vapor quality there are several images to better appreciate the flow pattern.
One of the most famous flow regimes definition is that reported in Nema et al. [31], which summarized the works of Coleman and Garimella [32-34]. They investigated flow regime during condensation of R134a in both circular channel, inner diameter of 4.91 mm, and rectangular channels with an hydraulic diameter between 1 and 4.8 mm. The four major flow regimes categories observed are: annular, wavy, intermittent and dispersed; each category is then divided in subcategories (Figure 4.18).
As known the annular flow regime is characterized by a liquid film at the tube wall and a vapor core in the channel. They subdivided the annular low regime in:
- mist flow: a very thin liquid film at the wall and a vapor core with significant liquid entrainment;
- annular ring: mist flow with periodic appearance of ring pattern;
- wave ring: similar to the annular ring pattern except that the liquid rings become noticeably thicker at the bottom compared to the top;
- wave packet: wave packets occurring periodically in the flow and waves confined to the bottom of the tube;
- annular film: noticeably thick and approximately uniform liquid film on the tube wall. In the wavy flow regime there are liquid and vapor layers with liquid flowing on the bottom of the tube and vapor on the top. In this regime they considered only two sub-categories: - discrete waves: the liquid-vapor interface is clearly distinguishable with the dominant wave pattern being of large wavelength and amplitude. Increasing the wave intensity this flow regime can be further classified;
- disperse waves: the liquid-vapor interface is indistinguishable and there are multiple waves typically with smaller wavelengths and amplitudes along the dominant waves.
In the intermittent flow pattern they considered:
- slug flow pattern: vapor slug move through the liquid often accompanied by discrete waves. The slug can be followed by trailing bubbles;
- plug flow pattern: solitary plug without trailing bubbles.
In the dispersed flow pattern they placed the bubbly flow, characterized by flow of small vapor bubbles in the liquid.
Figure 4.18. Flow regime classification. Image from Nema et al. [31]
In the light of the foregoing, the flow pattern visualized during the convective condensation of R134a at a saturation temperature of about 40°C in the test section with an inner diameter of 3.38 mm, are the following:
- at G=200 kg m-2 s-1 and vapor quality ranging between 0.88 and 0.69 the flow seems to be
annular, the liquid film on the bottom of the channel is recognizable and slightly thicker than the film on the top even at this vapor quality (Figure 4.19-Figure 4.21); it can also be supposed that for vapor qualities higher than 0.88 the flow is annular. If the vapor quality is in the range 0.6<x<0.44 (Figure 4.22-Figure 4.24) the flow pattern is in a transition between annular ring and discrete waves (stratified-wavy). So for this mass velocity the two macro-categories of annular flow and wavy flow are present;
- at G=150 kg m-2 s-1 the annular flow and wavy flow are still the two predominant flow
regimes. For x=0.81 (Figure 4.25) or higher the flow is mainly annular ring with the liquid film that start to increase on the bottom of the channel; for 0.69<x<0.58 the flow is in the transition between annular and discrete waves (Figure 4.26 and Figure 4.27); for 0.52<x<0.41 the discrete waves flow pattern has been visualized (Figure 4.28 and Figure 4.29);
- at G=100 kg m-2 s-1 the discrete wave flow pattern can be recognized in a wide range of
vapor qualities, from x=0.81 to x=0.27 (Figure 4.30-Figure 4.37). The wave intensity and amplitude is changing in the vapor quality range but the liquid-vapor interface is always clearly distinguishable. At x=0.21 waves at the interface are able to wash the upper part of the tube (Figure 4.37);
- at G=50 kg m-2 s-1 in the vapor quality range between 0.81 and 0.45 the flow is completely
stratified and the interface between the liquid-vapor is completely smooth without waves (Figure 4.38-Figure 4.42). At x=0.37 and x=0.26 the there some liquid plug (Figure 4.43 and Figure 4.44).
Figure 4.45 summarizes in a plot mass velocity versus vapor quality the visualizations recorded.
1) Mass velocity G = 200 kg m-2 s-1
Figure 4.19. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 200 kg m-2 s-1 and vapor quality of 0.88
Figure 4.20. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 200 kg m-2 s-1 and vapor quality of 0.79
Figure 4.21. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 200 kg m-2 s-1 and vapor quality of 0.69
Figure 4.22. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 200 kg m-2 s-1 and vapor quality of 0.60
Figure 4.23. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 200 kg m-2 s-1 and vapor quality of 0.52
Figure 4.24. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 200 kg m-2 s-1 and vapor quality of 0.44
2) Mass velocity G = 150 kg m-2 s-1
Figure 4.25. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 150 kg m-2 s-1 and vapor quality of 0.81
Figure 4.26. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 150 kg m-2 s-1 and vapor quality of 0.69
Figure 4.27. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 150 kg m-2 s-1 and vapor quality of 0.58
Figure 4.28. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 150 kg m-2 s-1 and vapor quality of 0.52
Figure 4.29. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 150 kg m-2 s-1 and vapor quality of 0.41
3) Mass velocity G = 100 kg m-2 s-1
Figure 4.30. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 100 kg m-2 s-1 and vapor quality of 0.81
Figure 4.31. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 100 kg m-2 s-1 and vapor quality of 0.68
Figure 4.32. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 100 kg m-2 s-1 and vapor quality of 0.59
Figure 4.33. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 100 kg m-2 s-1 and vapor quality of 0.46
Figure 4.34. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 100 kg m-2 s-1 and vapor quality of 0.41
Figure 4.35. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 100 kg m-2 s-1 and vapor quality of 0.37
Figure 4.36. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 100 kg m-2 s-1 and vapor quality of 0.27
Figure 4.37. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 100 kg m-2 s-1 and vapor quality of 0.21
4) Mass velocity G = 50 kg m-2 s-1
Figure 4.38. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 50 kg m-2 s-1 and vapor quality of 0.81
Figure 4.39. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 50 kg m-2 s-1 and vapor quality of 0.73
Figure 4.40. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 50 kg m-2 s-1 and vapor quality of 0.65
Figure 4.41. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 50 kg m-2 s-1 and vapor quality of 0.58
Figure 4.42. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 50 kg m-2 s-1 and vapor quality of 0.45
Figure 4.43. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 50 kg m-2 s-1 and vapor quality of 0.37
Figure 4.44. Flow pattern during condensation of R134a at 40°C saturation temperature, mass velocity equal to 50 kg m-2 s-1 and vapor quality of 0.26
Figure 4.45. Visualization performed during the condensation of R134a at 40°C saturation temperature in a circular minichannel with an inner diameter of 3.38 mm. Each point
corresponds to a recorded flow pattern.
0 50 100 150 200 250 300 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 M A SS VEL O C IT Y [k g m -2s -1] VAPOR QUALITY [\] G200 G150 G100 G50