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12 TRƯỜNG ĐẠI HỌC SƯ PHẠM KỸ THUẬT - ĐẠI HỌC ĐÀ NẴNG

13.34%, 13.55% respectively with Reh = 5000. Thus,
the thermal effectiveness in case 1 has the smallest
value in three cases with Reynolds numbers of the hot
fluid flow Reh from 5000 to 17000. For example, at
Reh = 17000, the thermal effectiveness in case 2, case
3 are higher than that in case 1 by 4.76%, 5.17%
respectively.
5. CONCLUSION

In this study, CFD simulations were conducted to
Fig.8. Effects of Reynolds numbers of the hot fluid investigate heat transfer process in a double pipe
flow on mean Nusselt numbers of the hot fluid flow for longitudinal fined heat exchanger with the aim to shed
the three cases light on design of an efficient double pipe heat
exchanger. Longitudinal rectangular fins of inner pipe
were designed by changing the ratio of the fin height
to the thickness of the space between the two pipes
(annulus) but must ensure that the heat transfer area of
the inner tube remained constant. These configurations
consist of case 1 (x =75%), case 2 (x=50%) and case 3
(x=37.5%) respectively. After validating the CFD
predictions using analytical data, effects of
longitudinal rectangular fin geometric parameters on
temperature distribution, heat transfer rate,
effectiveness were studied. The main findings are:
Fig.9. Effects of Reynolds numbers of the hot fluid - Temperature at outlet of the hot and cold fluid
flow on mean Nusselt numbers of the cold fluid flow flow increases with Reynolds numbers of the hot fluid
for the three cases flow increases in all cases. The maximum temperature
C. Effectiveness at outlet of the cold fluid flow for case 1, 2, 3 are
294.55K, 294.62K, 294.63K respectively with Reh =
17000 and the minimum temperature at outlet of the
hot fluid flow for case 1, 2, 3 are 330.95K, 330.88K,
330.84K respectively with Reh = 5000. Additionally,
the hot and cold fluid flow temperature and velocity
distribution are evenly distributed concentrically.
- Friction coefficient of the hot fluid flow
decreases with Reynolds numbers of the hot fluid flow
increases in all cases. For Reh < 9000, the friction
coefficient of the hot fluid flow in case 3 has the
largest value, but at Reh > 9000 the friction coefficient
of the hot fluid flow in case 3 has the smallest value in
Fig.10. Effects of Reynolds numbers of the hot fluid three cases.
flow on thermal effectiveness for the three cases • Mean Nusselt numbers of the hot fluid flow
Fig. 10 shows effects of Reynolds numbers of the increase with Reynolds numbers of the hot fluid flow
hot fluid flow on thermal effectiveness for the three increases in all cases, but mean Nusselt numbers of the
cases. It can be seen that the thermal effectiveness cold fluid flow has decreased trend. In there, the
increases with Reynolds numbers of the hot fluid flow Nu and Nu of case 1 have smallest value in three
increases in all cases. This is because the increase of h c
Reynolds numbers of the hot fluid flow leads to mean cases. The Nu and Nu of case 1 have smallest value
c
h
heat transfer rate Q increases while maximum mean in three cases. For example, at Reh = 17000, Nu in
h
heat transfer rate Qmax remains unchanged, so the case 2, case 3 are higher than that in case 1 by 11.89%,
thermal effectiveness increases (Based on Eq. (7)). As
seen in Fig. 10, the maximum thermal effectiveness 6.16% respectively, and Nu in case 2, case 3 are
c
for case 1, 2, 3 are 15.54%, 16.28%, 16.35% higher than that in case 1 by 0.75%, 4.84%
respectively with Reh = 17000 and the minimum respectively.
thermal effectiveness for case 1, 2, 3 are 12.92%,
ISBN: 978-604-80-9779-0