Heat transfer and aerodynamics of complicated profiled heat transfer surfaces
In tubular recuperative heat exchangers such as "gas-gas", "gas-liquid" are mainly used heat transfer of circular tubes. However, circular tubes, as part of the heat transfer surface, in terms of improving their heat and aerodynamic performances to date have exhausted themselves. Now, technically and economically the most accessible and reasonable by reducing weight and improving energy efficiency and resource saving heat exchange equipment is application in heat exchangers surfaces that have high heat and aerodynamic efficiency, low cost and ease of production. Such requirements are met complicated profiled tubes of flat-oval form that have some significant advantages over tubes of circular profile and above all have a 2-3 times lower aerodynamic drag at practically the same intensity of heat transfer. It should also be noted that the study of the effect on the heat and aerodynamic properties of geometric parameters arrays tubes of flat-oval form in a wide range of change in the cross-flow in Ukraine and in the world does not hold.
Within the performed research work was produced the experimental models of flat-oval tubes ranging relative elongation profile d2/d1=2-5.
The study of convective heat transfer and aerodynamic drag in single complicated profiled tubes and their arrays carried out in wide variation of their geometrical characteristics and of the flow velocities. The influence on the heat transfer and drag surface geometrical characteristics and operational factors was determined. On the basis of the research proposed new generalized dependencies for calculation convective heat transfer and aerodynamic drag surfaces with complicated profiled flat-oval tubes that take into account the impact on the intensity of heat transfer and drag the geometrical characteristics of tubes, steps between them and operational factors in a wide range of its change.
The structure of the flow in the form of flow lines on the surface of flat-oval tube was investigated. The results of flow visualization showed that in combination of semicircle and straight sections of flat-oval tube reveal two field flow separation to form on the surface of the closed vortex and the surface of the tube round shape observed only one region tear, which later gradually transformed into a vortex feeding should.
According to experimental results obtained for the first time the engineering methods of thermal and aerodynamic calculations surfaces with complicated profiled flat-oval tubes that allow you to develop new types of tubular recuperative heat exchangers on new hardware components in a wide range of geometrical parameters of tubes and their ranges of elongation profile tube 2 to 5, the relative transverse and longitudinal steps of tubes, respectively, from 2 to 3.5 and 2 to 5.5 in the range of changing flow velocity from 2 m/s to 30 m/s.
The results of heat and aerodynamic calculations showed that the air heaters, made of flat-oval tubes dimensions and weight specifications for (20-30%) lower than the air heater, made of tubes traditional circular shape with the same thermal power units.