دانلود رایگان مقاله انگلیسی انتقال حرارت جریان در حال توسعه در لوله های افقی صاف در جریان انتقالی - الزویر 2018

abstract

Limited experimental work has been done specifically focussing on how the heat transfer characteristics of developing flow in the transitional flow regime changes along the tube length, and how it compares with fully developed flow. The purpose of this study was therefore to experimentally investigate the heat transfer characteristics of developing and fully developed flow in smooth horizontal tubes in the transitional flow regime at a constant heat flux. An experimental set-up was designed, built and validated against literature. Two smooth circular test sections with inner diameters of 4 mm and 11.5 mm, respectively, were used and the maximum length-to-diameter ratios were 1373 and 872, respectively. Heat transfer measurements were taken at Reynolds numbers between 700 and 10 000 at different heat fluxes. The boundaries of the different flow regimes were defined mathematically, and terminology to define the transition characteristics were presented. It was found that the Reynolds number at which transition started was independent of axial position, and transition occurred at the same moment in time along the whole tube length. However, the end of transition was dependent on axial position and occurred earlier as the flow approached fully developed flow. Free convection effects affected both the start and end of the transitional flow regime, and caused the Reynolds number range of the transitional flow regime to decrease. Correlations were developed to determine the start and end of the transitional flow regime for developing and fully developed flow for mixed convection conditions. The transitional flow regime of developing flow was divided into three regions. In the first region, the width of the transitional flow regime decreased significantly with axial position and free convection effects were negligible. In region 2, the width of the transitional flow regime decreased with both axial position and free convection effects. In the fully developed region, the width of the transitional flow regime was independent of axial position, but free convection effects caused it to decrease and even to become negligible. It was concluded that the heat transfer characteristics of developing flow in the transitional flow regime changes as the flow develops and differ significantly from fully developed flow.

7. Conclusions

Up to now, no experimental work has been done, specifically focussing on how the heat transfer characteristics of developing flow in the transitional flow regime changes along the tube length, how it compares with fully developed flow, and how it is affected by free convection effects. Furthermore, although the boundaries of the transitional flow regime have been described in literature, the descriptions were qualitatively and relied on visual observations, which are subjective. In most of the cases, no clear distinction between the transitional and quasi-turbulent flow regimes was made either. The purpose of this study was therefore to experimentally investigate the influence of axial position, free convection effects and Prandtl number on the heat transfer characteristics of developing and fully developed flow in smooth tubes in the transitional flow regime. The Reynolds number was varied between 700 and 10 000 to ensure that the transitional and quasi-turbulent flow regimes, as well as sufficient parts of the laminar and turbulent flow regimes, were covered. The boundaries between the flow regimes were defined mathematically, and terminology to define transitional flow characteristics were presented.