Experimental study of forced and free convective heat transfer in the thermal entry region of horizontal concentric annuli

• Main Authors: Mohammed H.A., Campo A., Saidur R.
• Other Authors: 15837504600
• Format: Article
• Published: 2023
• Subjects: Forced free convection; Horizontal annulus; Thermal entry length problem; Forced convection; Natural convection; Nusselt number; Pipe flow; Reynolds number; Stainless steel; Tubes (components); Vehicular tunnels; Axial distance; Combined convection; Concentric annuli; Constant wall heat flux; Convective heat transfer; Dimensionless variables; Empirical correlations; Entry length; Entry region; Experimental setup; Experimental studies; Free convections; Heated lengths; Inner tubes; Laminar airflow; Local Nusselt number; Long tubes; Radius ratio; Re numbers; Short tube; Surface temperature distribution; Thermal entrance; Heat exchangers

Forced and free convective heat transfer for thermally developing and thermally fully developed laminar air flow inside horizontal concentric annuli in the thermal entrance length has been experimentally investigated. The experimental setup consists of a stainless steel annulus having a radius ratio of 2 and an inner tube with a heated length of 900mm subjected to a constant wall heat flux boundary condition and an adiabatic outer annulus. The investigation covers Reynolds number range from 200 to 1000, the Grashof number was ranged from 6.2�105 to 1.2�107. The entrance sections used were long tube with length of 2520mm (L/Dh=63) and short tube with length of 504mm (L/Dh=12.6). The surface temperature distribution along the inner tube surface, and the local Nusselt number distribution versus dimensionless axial distance Zt were presented and discussed. It is inferred that the free convection effects tended to decrease the heat transfer at low Re number while to increase the heat transfer for high Re number. This investigation reveals that the Nusselt number values were considerably greater than the corresponding values for fully developed combined convection over a significant portion of the annulus. The average heat transfer results were correlated in terms of the relevant dimensionless variables with an empirical correlation. The local Nusselt number results were compared with available literature and show similar trend and satisfactory agreement. � 2010.