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

(1)提出了一种新型紧凑型换热器，并对其基本换热性能进行了研究。该设计理念是基于强化换热的新原理，即“减小换热面尺寸”和“布置几个湍流促进器以引起湍流结构的剧烈变化”。为了满足这些要求，换热器由直径为1 mm的细管和直径为0.3 mm的编织螺纹组成。实验测得的最大换热系数是无螺纹圆柱体的14倍。单位投影面积换热系数约为5×10~(-3)W/(m^2K)，单位体积换热系数达到3×10~(-6)W/(m~(^3K))。(2)在流动和传热实验的基础上，对细管换热器的传热机理进行了研究。为了弄清编织丝线对湍流的促进作用，用透明的聚四氟乙烯玻璃制作了一个比实际换热元件大30倍的试验段，并以蓖麻油为工质，用激光多普勒测速仪测量了细管周围的流场。另一方面，用硅油测量了细管周围的努塞尔数分布。针对螺纹高度强化的湍流特性，讨论了强化换热的机理。(3)以水蒸气为工质，测量了常压下细管冷凝器的换热特性。由于螺纹管强化了蒸汽切变，起到了翅片的作用，强化换热效果显著，最大换热系数达到20OKW/(m^2K)(最大热流密度为2000kW/m^3)。

(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).