The Generalized Lévêque Equation and its application to flat tubes without and with passive inserts

•Experimental investigation on heat and momentum transfer for flat tube flow with and without passive inserts for a wide range of Prandtl number (8 < Pr < 60) and Reynolds number values (40 < Re < 3500).•The results show that the Generalized Lévêque Equation is not applicable to flat tubes without passive inserts, yet becomes applicable with passive inserts, which are forcing a re-formation of boundary layers in the whole range of investigated Reynolds and Prandtl numbers. Searching for an analogy concept between heat transfer and pressure drop is anything but new, nevertheless it is still the subject of researches as it enables the prediction of heat transfer from pressure drop data, which helps reducing experimental effort for heat transfer measurements. The Generalized Lévêque Equation (GLE) is a widely-used analogy concept, which allows the prediction of heat transfer from pressure drop data, if hydrodynamic boundary layer is developed or still developing. This analogy concept has been successfully applied to flow situations with practical relevance like chevron-type plate heat exchangers, packed beds and in tube bundles, where the boundary layers are forced to reform continuously. In this contribution, the GLE is applied to flat tube flow without and with passive inserts, investigating the impact of passive inserts, which force the boundary layers to reform continuously, on the application of the GLE. This paper presents results of an experimental investigation on pressure drop and heat transfer in flat tubes without and with passive inserts, covering a wide range of Prandtl number (8 < Pr < 60) and Reynolds number values (40 < Re < 3500). These parameter ranges and the passive inserts represent the typical conditions and design used for the liquid side of single-phase flat tube heat exchangers used in the automotive industry. 238 experimental data points for heat transfer coefficients are compared to heat transfer coefficients calculated from pressure drop data with the GLE. The results show that the GLE is not applicable to flat tubes without passive inserts, yet becomes applicable with passive inserts, which are forcing a re-formation of boundary layers in the whole range of investigated Reynolds and Prandtl numbers.