Experimental And Theoretical Research On Single-Phase Flow And Heat Transfer Characteristics In Multi-Port Extruded Tubes

The micro- and mini-channel heat exchangers have been widly applicated in the field of electronics cooling, automobile air conditioning, aerospace, etc. Thus, it is of important significance to investigate the friction and heat transfer characteristic of flowing in micro- and mini-chanel, which is applied to the optimizing design of small-sized heat exchangers. However, up to now some published experimental results of pressure drop and heat transfer deviated from the prediction values of classical theories, and there is no generally accepted theory for the micro- and mini-scale flow and heat transfer characteristics. Therefore, a further study was performed for single-phase flowing and heat transfer in micro- and mini-channels.Scaling effects and friction and heat transfer theories of flowing in micro- and mini-channels were summarized in detail, which were necessary for analysis and comparision of the experimental phenomenon. An experimental setup was designed and built. In addition, the experimental uncertainties of the instruments were also conducted to guarantee the tested results accuracy and reliable. Then the friction factor and heat transfer coefficients were tested for water, ethanol and acetone flowing through two aluminum multi-port extruded tubes with the geometries of rectangular and circular, and the diameters of 0.715 and 0.86 mm, respectively. In addition, the flow flux, inlet temperature and heat flux were varied to meet different working conditions.The pressure drop experimental values were compared with the classical theories, and it was found that the friction characteristics in mini-channels were in agreement with the conventional theories at low Reynolds numbers. Whereas, with the Reynolds values increase, the influence of surface roughness was significant as the tested friction factors were much higher than the theoretical predictions. The friction factors under heating conditions were lower than those for isothermal conditions. Moreover the effect of heating was also delay the transition of laminar to turbulent flow. The frictional factors increased with the decrease of inlet temperatur and heat flux. Based on the variation of thermophysics properties, the test friction factors were corrected.The scalling effects of entrance effects, axial heat conduction and varied properities had significant influence on the convective heat transfer for flow through the tested tubes, which resulted the experimental results deviated from the predictions of conventional theories. The impact of axial heat conduction was obviously at low Reynolds numbers, while the entrance effects was dominant for lager values of Reynolds number. Besides, the Nusselt numbers increased with the decrease of inlet temperature and heating flux dut to the varied properities. The Nusselt numbers were also corrected on the base of the above mentioned three kinds of scaling effects with high prediction accuracy.The pressure drop and convective heat transfer data were measured simultaneously in the experiment and the characteristics were compared with each other. The results showed that the lower critical Reynolds numbers were both near the value of 2000, whereas, the hydraulic upper critical point was higher compared with that of thermal one. The relationship between friction pressure drop and heat resistance was also analisized, and the results suggested that the pressure drop increased as the inlet temperatue and heat flux decreased when the heat resistence was fixed constant, which was influenced by the temperature dependent viscosity. Fruthermore, it was also found that the relationship between the pressure drop and heat resistence of transition flow regime was different form that of laminar flow.