Tham khảo tài liệu 'new trends and developments in automotive system engineering part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 268 New Trends and Developments in Automotive System Engineering and the last curves differ up to 15 K in wall superheat ATsut Tw -Ts. It could be shown that the aging effect observed here is partly caused by a continuous flooding of the cavities on the surface which reduces the number of active nucleation sites. The other part could be attributed to depositions on the heated surface originating from the employed coolant liquid. The observed significant shift in the boiling curves strongly suggests that the aging conditions of the heated surface and the working fluid must not be overlooked in the interpretation of boiling flow measurements and in the specification of the model parameters based on such data. This caveat is particularly relevant for boiling of aqueous liquids on real technical surfaces. 7. Conclusions The enhancement of heat transfer rates based on a controlled transition from pure singlephase convection to subcooled boiling flow appears to be a promising approach for application in automotive cooling systems. A reliable and save thermal management requires a most comprehensive knowledge of how certain operation and system conditions may affect the boiling behaviour. Therefore we put our focus on a selection of engine relevant conditions and their possible impact on the modelling of the wall heat flux. This led us to the following resume. As for the influence of the mixing ratio of the two main components of the coolant water and ethylene-glycol the heat transfer rates in the boiling regime tend to decrease when the fraction of the more volatile water component is smaller. The tested wall heat flux model which basically assumes the coolant as an azeotropic mixture reflected the observed tendency very well. The effect of the mixing ratio can be evidently captured with sufficient accuracy in terms of the material properties of the mixture. For the considered range of engine relevant mixing ratios and subcooled boiling flow conditions non-azeotropic .
