A study of heat transfer in simultaneously developing flow through rectangular tubes is presented in this paper. Heat transfer coefficients were measured for three different tube sizes and shapes (Dh = 2.21 mm, α = 0.050; Dh = 3.02 mm, α = 0.108; and Dh = 1.74 mm, α = 0.029), which correspond to typical dimensions used in automotive heat exchangers. For each of these tubes, several different tube lengths were tested to measure the effect of developing flow on the Nusselt number. The study primarily focussed on the laminar and transition regimes, with some data in the turbulent regime, which is typical of the operating conditions for many automotive heat exchangers. The results demonstrate that developing flow enhances Nusselt numbers, especially for the short tubes used in heater cores, although for the geometry range studied, the effect of aspect ratio was not very significant. Heat transfer correlations were developed from the data, with excellent agreement between the data and the values predicted by these correlations. These correlations accounted for the effects of Reynolds number (118 < Re < 10671) Prandtl number (6.48 < Pr < 16.20), and bulk-to-wall property variations (0.243 < μbw < 0.630), and geometric features such as tube length, hydraulic diameter, and aspect ratio.

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