Investigations were conducted with brazed plate heat exchangers and consisted of defining the thermal and hydrodynamic performances of these types of heat exchanger and comparing them in terms of the hydraulic diameter parameter. The hydrodynamic characteristics are presented in the turbulent and laminar-transitional flow regions. An equation of a type a Rebhas been used to accurately fit the isothermal pressure drop data in both the viscous and turbulent flow regions. A distinctive kink of the hydrodynamic characteristics occurred at a critical Reynolds number indicating the start of the turbulent flow regime. Moreover, in the turbulent part of flow, two linear zones can also be defined, and at the junction between them, some flow disturbances have been measured due mainly to the flow change between the quasi- and fully turbulent flow regimes. Thermal investigations were also conducted with regard to the hydraulic diameter parameter, and the thermal and hydrodynamic correlations are then coupled by only one equation predicting the global performance.
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