Tree Configuration Thermosyphon Study - LEPTEN

More documents

Recommendations

Info

These results show that the vapor has a preferential path through the condensers located nearer the center of the evaporator. The vapor flow seems to search for the shorter way through the condenser. The forced convection tests (A and B) showed that for intense convective heat transfer external to the condensers, the system is overdimensioned. Only a small portion of the condensers is necessary to transfer the heat power input. The remaining portion did not even start-up. temperature [°C] 90 80 70 60 50 40 30 T21 T16 Evaporator T26 T11 condenser + cooling fluid 20 0 50 100 150 time [min] Figure 5. Temperatures as a function of time for test B Natural Convection Tests The effect of cooling by natural convection on the tree configuration thermosyphon was analyzed in tests C, D and E (see Table 2). Natural convection, especially with air as the cooling fluid, yields much less effective heat transfer. In test C, the heater power input was 80 W and the test lasted for a long time (almost 7 hours) in order to observe both the transient and steady state behaviors of the thermosyphon. In test D, the power input was 500 W, but the time of testing was much smaller, approximately 25 min. The thermal behavior of the thermosyphon when the heat power is turn off was also analyzed in this case. Finally, in test E, the power input was 150 W while stagnant water was confined externally to the condensers in order to obtain natural convection cooling. The evaporator temperatures as a function of time for test C are shown in Fig. 6. The graph shows that after approximately 150 min of test the temperatures reached steady state. The four parallel condensers present very similar behavior, indicating that the vapor is distributed evenly among the four condensers in this case, contrary to the forced convection tests presented previously. There are four important groups of curves in this graph. The first group correspond to the bottom half of the condensers and the evaporator, which are within 10°C of difference among them and is located in upper region of the graph (mean temperature of 85°C). The evaporator and the bottom half of the condensers are approximately isothermal. As for the group formed by T13, T18, T23 and T28 (mean temperature of 50°), and the group formed by T14, T19, T24 and T29 (mean temperature of 38°), which are located in the upper half of the condensers, the temperature levels are lower than the bottom half of the evaporator, especially for the tip of the condensers (T14, T19, T24 and T29), which are at a temperature only slightly higher than the cooling fluid (stagnant air). As the temperature of the condensers is higher than the cooling fluid in their entire length, one conclude that the entire length of the condensers are active, i.e., are exchanging heat with the external environment. However, the upper half of the condensers are not as effective as the bottom half, i.e, the upper half of the condensers are not fully started-up yet. The results of test D are very similar to test C, except that only T14, T19, T24 and T29 are cooler than the rest of the condenser. The power input in test D was much larger (500W) than in test C (80W) so the vapor had strength to advance further up into the condensers. Only the tips of the condensers were not fully started-up. As for test E (150W, water as cooling fluid) the results are also similar to test C: only the bottom of the condenser fully startedup, while the remaining parts are ineffective for heat transfer. 8 American Institute of Aeronautics and Astronautics
90 T6, T7, T8, T9 80 70 T12, T17, T22, T27 temperature [°C] 60 50 40 T10, T15, T20, T25 T11, T16, T21, T26 T13, T18, T23, T28 T14, T19, T24, T29 30 cooling fluid (stagnant air) 20 0 50 100 150 200 250 300 350 400 time [min] Figure 6. Temperatures as a function of time for test C Thermal resistances Several theoretical models and empirical correlations are available in the literature to estimate the vapor condensation resistance inside the condenser tube. Faghri 1 and Mantelli et al. 2 review some of these expressions and the results show differences of one order of magnitude in the predicted condensation heat transfer coefficient h c appearing in Eq. (8). I this work, Re f (defined by Eq. 6) ranges from 0.8 to 16. According to Faghri 1 , the falling liquid film is laminar for Re f < 30. Nusselt developed a theoretical model for laminar film flow and Faghri 1 presents a correlation for this model (Eq. 4). Kaminaga et al. 5 proposed a correlation from experimental data covering Re f in both the laminar and the turbulent ranges (Eq. 7). Figure 7 shows a comparison between the correlation of Kaminaga et al. 5 (Eq. 7) and the correlation from Nusselt’s model (Eq. 4) for a mean temperature of approximately 50°C. As one can see, Nusselt’s model yields conductance values one order of magnitude larger than Kaminaga’s correlation. Mantelli et al. 2 compared these two correlations along with many other correlations available in the literature against experimental data collected from a conventional two-phase thermosyphon made similarly to the condensers used in this work. Kaminaga’s correlation showed the best agreeement with the experimental data. Also, Kaminaga’s correlation yielded the smallest values of condensation conductance from all models tested that can be applied to laminar flow, while Nusselt’s model yields the largest values of h c . Appart from the large difference between the absolute values of the two models, they also present different behaviours with increasing power input: Nusselt’s model decrease with the heat transfer rate while Kaminaga’s correlation shows an increse of h c with the heat transfer rate. In this work, Kaminaga’s correlation is used in order to predict h c due to reasons to be explained later. Apart from the dependence on the heat transfer rate, the correlation of Kaminaga et al. 5 is also dependent on the mean temperature level. Figure 8 illustrates the dependence of the condensation resistance on both the mean temperature level and on the heat power input in the ranges of interest in this work. As one can see, the influence of the temperature is much smaller than the influence of the heat power input and can be neglected. As already mentioned, the objective of this work is to analyze the condensation process inside the parallel condensers. The condenser resistance is defined according to Eq. (10), i.e.: R c Tv − Tc R 6 + R = = 7 (11) q 4 9 American Institute of Aeronautics and Astronautics
Page 1 and 2: Tree Configuration Thermosyphon Stu
Page 3 and 4: Te −Tc Rt = (1) Q where T e and T
Page 5 and 6: low up, the rivulets are broken and
Page 7: (a) (b) Figure 4. Experimental setu
Page 11 and 12: prediction was obtained using Eqs.
Page 13: References 1 Faghri, A., Heat Pipe

Tree Configuration Thermosyphon Study - LEPTEN

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?