Development of a High-Performance Heat Exchanger Using Fine Tube and a Couple of Turbulence Promoters

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(1) A new-type compact heat exchanger has been suggested, and the fundamental heat transfer performance has been studied. The design concept is based on the new principles for heat transfer enhancement, namely, "reducing the size of heat transfer surfaces" and "arranging a couple of turbulence promoters to cause drastic change in turbulence structure". To satisfy such requirements, the heat exchanger consists of fine tubes(o. d. =1mm)and woven threads(d. =O. 3mm). The maximum heat transfer coefficient obtained in the experiment is fourteenfold larger than that around a cylinder without threads. The heat transfer coefficient per unit projected area is about 5x1O^3W/(m^2K), and that per unit volume reaches 3x1O^6W/(m^3K).(2) Heat-transfer mechanism of the fine-tube heat exchanger has been studied on the basis of the flow and heat-transfer experiiients. To clarify the effect of the woven threads on turbulence promotion, a test section scaled up 30 times as large as the actual heat-transfer element was made of transparent Pyrex glass, and the flow field around fine tubes was measured by laser Doppler velocimeter using castor oil as a working fluid. On the other hand, the distributions of Nusselt number around the fine tube were measured using silicon oil. The heat-transfer enhancement and its mechanism were discussed in the light of the turbulence behavior which was highly augmented by the threads.(3) Using steam as a working fluid, heat-transfer characteristics of the fine-tube condenser have been measured at atmospheric pressure. Since the threads intensify the vapor shear and act as fins, heat transfer is enhanced markedly ; the maxinuin heat transfer coefficient reaches 2OOkW/(m^2K) (the maximum heat flux is 2000 kW/m^3).