해양운송연구부 김충만 연구원

선박의 항주 중 저항을 감소시키기 위해서는 침수표면적을 줄여주는 방법이 있으며 , 이에 대한 연구로 탄성피막이나 공기주입을 하여 저항감소를 유도하거나 유동의 조직적인 구조 (riblet) 를 바꾸어 주는 방법 등이 사용되고 있다 . 그 외에 고분자 폴리머 (Polymer) 용액 등 각종 첨가제 (additives) 를 이용한 방법이 있으나 이 외에는 효과적인 마찰저항 감소방법이 알려져 있지 않다 . 특히 저항감소는 선박의 전체저항에서 마찰저항이 차지하는 비중이 매우 높은 탱커 (tanker) 나 컨테이너 (container) 선박 등에 적용하여 선박의 고속화 및 동력 절감을통한 선박 운용 경제성 향상에 직접적인 요구에 대응할 수 있고 , 기존 화석연료의 소모를 저감시킴으로써 대기오염 및 지구 온난화 방지에 기여할 수 있다 .

저항감소에 대한 심화된 여러 연구 중 공기 주입을 이용한 마찰저감방법으로 공기윤활법에 관한 연구가 Denmark DK group 및 미국 Washington State University 등에서 활발히 이루어지고 있으며 , 미소기포주입법으로 micro bubble 을 난류경계층에 주입하여 난류마찰저항 감소를 유도하는 연구가 미국 등 각국에서 진행되고있다 또한 초발수성도료를 선저에 도포하여 물과의 접촉각을 줄임으로써 마찰저항저감의 효과를 유도하는 방법등이 미국 및 일본에서 연구된 바 있고 이와 더불어 고분자 중량체 (polymer) 용액이나 계면활성제 등의 첨가제를 주입하여 길이가 매우 긴 분자구조의 합성을 통한 난류 마찰저항 감소 연구가 수행된 바 있다.

저항감소를 위한 새로운 연구로 , 나노입자들을 생성하고 이러한 나노입자들을 포함하는 버블을 몰수체 표면에 형성하여 버블 속의 나노입자들이 몰수체의 경계층으로 유입되도록 함으로써 난류강도 및 마찰저항을 감소시키는 연구를 수행하였다 . 친수성 (Hydrophilic) 입자는 NaCl 을 사용하고 소수성 (Hydrophobic) 입자는 DEHS(Di-Ethyl-Hexyl-Sebacat) 를 0.1% Isopropyl alcohol 에 희석시켜 다분산입자 및 500nm 근처의 단분산 입자를 발생시켜 효과를 알아보았다 .

Figure 1. Schematics of the submerged body       Figure 2. Schematic diagram of a DMA 

그림 1 은 MOERI 의 중형캐비테이션에서 실험한 몰수체의 도면과 장착 모습이고 , 그림 2 는 단분산 나노입자 생성을 위한 DMA(Differential Mobility Analyzer) 장비이다 .

  Figure 3. Schematic diagram and experimental setup for the measurement of drag reduction using nano-sized particles

그림 3 은 입자를 발생시키고 몰수체에 다분산 / 단분산입자로 주입되기까지의 입자생성을 위한 장치의 개념도이고 , 실제 실험을 위해 설치된 장비의 모습이다 . 현재 진행중인 실험이라 마찰저항 감소는 1% 근처에서 균일하게 나타나지만 , 나노입자를 포함한 버블이 난류경계층에 효과적으로 주입될 수 있는 몰수체의 최적화 된 설계와 기작에 대해서 심화된 연구가 수행 된다면 저항감소의 효과는 충분한 가능성이 있다고 본다 .

- 참고문헌 -

Adachi, M., Y. Kousaka and K. Okuyama (1985). "Determination of particle size distribution of ultrafine aerosols using a differential mobility analyzer." J. Aerosol Sci. 16: 109-123.

Hoppel, W. A. (1978). "Determination of the aerosol size distribution from the mobility distribution of the charged fraction of aerosols." Journal of Aerosol Science 9: 41-54.

Kodama, Y., A. Kakugawa, T. Takahashi, S. Nagaya, K. Sugiyama (2002).     "Microbubbles: Drag Reduction Mechanism and Applicability to Ships", 24th Symposium on Naval Hydrodynamics, Fukuoka, Japan.

Yoon, H. S., Y. H. Park, S. H. Van, H.T. Kim, D.S. Kim, and W.J. Kim (2003). “Experimental Investigation on the Effect of Polymer Solution and Micro-bubble on the Drag Reduction of an Axi-symmetric Body”, Proc. of the Annual Spring Meeting of Society of Naval Architecture of Korea, Korea.

2010-12-09