The aim of this study is to analyze the exact region of mixed convection film condensation on a horizontal isothermal elliptical tube. Due to the practical importance of condensation in the design of condensers, air conditioning systems and many industrial process equipment, the problem of forced, mixed, and free convection heat transfer of pure vapor condenses on an elliptical tube has been studied by many researchers. They just estimated the value of convection term (F) and use an approximate value for each type of convection but no attempt has been performed to find the exact values of F which distinguish three different convection zones. So the main objective of this study is to analyze the mechanism of mixed convection on an elliptical tube encountered in film condensation heat transfer. The influential parameters and the way they affect the mixed convection area has been studied. The physical model is a condensate film flow on a horizontal elliptical tube with major axis in the gravitational direction. In order to investigate the effect of geometry on the process, parameter ellipticity (e) has been introduced. The boundary layer equations governed by the basis of conservation equations for a laminar, steady state condensate film with constant fluid properties are used as the working equations in this paper. A numerical approach has been used to investigate how parameters, including e, F, and the pressure gradient term (P) influence the limits of mixed convection. The results show that by increasing ellipticity, the lower limit of mixed convection shifts to the lower amount of F and cause a bigger zone of mixed convection. The pressure gradient term also affect the lower limit of mixed convection. it was seen that the influences of P on a circular tube is inverse of the effect of P on an elliptical one. The solution shows that F=0.001 can be used to define as the boundary of forced and mixed convection for all e and P. on the other side, it has been seen that the free convection occurs in lower amount of F as e increases. It is concluded that the upper limit of mixed convection zone which is the start point of free convection area where P=0 can be defined by F=1000 which can be applied for all e. Thus through this investigation the accurate and exact limits of mixed convection were found.

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