점탄성 특성을 가진 폴리머용액의 난류유동 열적입구길이에 관한 실험적 연구

An experimental study on the thermal entrance lengths for viscoelastic polymer solutions in turbulent tube flow

본 연구에서는 내경이 각각 8.5mm와 10.3mm이며 무차원길이가 각각 710과 1158인 두 개의 시험관을 사용하는 유동장치를 제작하여 시험관 입구에서부터 유체역 학적 경계층(hydrodynamic boundary layer)과 열적 경계층(thermal boundary layer)이 동시에 발달하기 시작하는 경계조건을 형성하고 관벽에서 일정한 열 플럭스(constant heat flux)를 발생하는 조건을 부여하였다. 퇴화현상(degradation)에 대하여 비교적 안정성을 가진 폴리아크라마이드(polyacrylamide) Separan Ap273을 수도물에 용해하여 제조한 폴리머용액으로 유동특성과 열전달특성을 실험하여 열적입구길이와 열전달특성 을 규명하고자 한다.

Thermal entrance lengths of turbulent tube flow for viscoelastic polymer solutions are investigated experimentally in the recirculating flow system with tubes of inside diameters 8.5mm(L/D=710) and 10.3mm(L/D=1158), respectively. In the present system, the hydrodynamic and thermal boundary layers develop simultaneously from the beginning of the test section. To provide the boundary condition of constant heat flux at the wall, the test tubes are heated directly by electricity. The polymer solution used in the current study is 1000 wppm aqueous solution of polyacrylamide(Separan AP-273). The apparent viscosity of the polymer solutions circulating in the flow system are measured by the capillary tube viscometer at regular time intervals. Thermal entrance lengths vary due to the rate of degradation. The entrance lengths of degraded polymer solutions are about 500~600 times the diameter. However, the entrance lengths of fresh polymer solutions are greater than the lengths of the test tubes used in this study suggesting that thermal entrance lengths for viscoelastic polymer solutions are greater than 1100 tube times the diameters. Friction factor is almost insensitive to the degradation, but the heat transfer $j_{H}$-factor is affected seriously by degradation. Based on the present experimental data of fresh solutions a correlation for the heat transfer $j_{H}$-factor is presented.ted.