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In a counter-flow double pipe heat exchanger, oil (m = 2 kg/s, C${_p} $= 2.1 kJ/kg.$^{o}$C) is cooled from 90$^{o}$C to 40$^{o}$C by water (m = 1 kg/s, C${_p}$ = 4.2kJ/kg$^{o}$C.) which enters the inner tube at 10$^{o}$C. The radius of the inner tube is 3 cm and its length is 5 m. Neglecting the wall resistance, the overall heat transfer coefficient based on the inner radius, in kW/m$^{2}$.K, is

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