部分旁通概念應用在裸管管陣熱交換器之性能探討

Abstract

為改善熱交換器的性能，本研究將部分旁通(partial bypass)概念導入傳統氣冷式裸管管陣熱交換器之設計，使得部分氣流流經熱交換器時不會接觸到前段熱交換器而直接流往後段熱交換器。其總熱傳性能預計可能不變或小幅度降低，然其總壓降將明顯降低。 為評估此性能，本研究藉由實驗的設計，分析傳統裸管熱交換器與含旁通設計之熱交換器之間性能的差異。結果顯示旁通效應除了明顯出現在低風速下之外，不論任何風速，越均勻的管陣排列方式越可以使流場更加均勻混合，性能會越好。而最重要的是，隨著幫浦功率越大，含旁通設計之熱交換器的熱阻也有接近傳統式熱交換器之熱阻的趨勢，這樣的趨勢指出抽管後的效益在幫浦功率很大的操作條件下還是可以顯現出來的，即使可能無法比得上傳統管陣的性能，但是卻可以非常接近，並且還可以節省一些管子材料。

The purpose of this research is to improve the performance of tube bundle subject to air-cooling. The tube bundle is consisted of multi-row bare-tubes. The improvement is made available by introducing the “partial bypass” concept which employs the heat transfer augmentation by increasing the effective temperature difference between the air side and tube side in the rear part of heat exchanger. The idea is to get rid of some tubes in the front part of the heat exchanger, namely the heat transfer area is reduced; thus, part of inlet stream directly bypasses the front part of heat exchanger. Under this circumstance, the total heat transfer performance comparing with traditional bare-tube heat exchanger may be about the same, but the total pressure drop will certainly be much lower. In order to investigate the performance of this innovative heat exchanger design, the experiment is carried out in this study to prove the proposed concept. The results show that the partial bypass idea is obviously effective under low airflow velocity. In addition, regardless of any airflow velocity, the more uniform arrangement of the tube bundle allows the flow field become more uniform mixing, and it makes the performance much better. The most important of all, with the greater the pumping power, the thermal resistance of the heat exchanger containing partial bypass design is slowly close to that of the conventional tube bundle. This trend shows that the benefits of partial-bypass designs can still be available under the operating condition of large pumping power. Even though the performance of the improved designs can never surpass that of the conventional tube bundle, they will become very close; moreover, we can also save some tube materials.