Inlet Temperature Effects on Sensible Heat Transfer to Falling Liquid Films on Horizontal Round Tubes

The horizontal-tube, falling film heat exchanger has advantages over flooded bundles and finds widely use in vaporcompression and absorption systems for air-conditioning and refrigeration. The effects of the liquid feeding temperature on inter-tube falling-film heat transfer behavior are investigated experimentally for distilled water, pure ethylene glycol and Dowtherm 4000. It is found that the liquid feeding temperature significantly impacts the heat transfer coefficient for a fluid with a large viscosity (such as pure ethylene glycol and Dowtherm 4000), but has almost no effect on the heat transfer coefficient for a fluid with a small viscosity (such as water). It is also observed that the fluid thermophysical property changes associated with feeding temperature changes from about 20 to 45°C have a significant effect on the Nusselt number at a fixed Reynolds number. The Nusselt number increase is around 50% with a decreased liquid feeding temperature from 45 °C to 20 °C for water at Re ≈ 600. For pure ethylene glycol and Dowtherm 4000, the Nusselt number increases around 100% and 80% with a decreased liquid feeding temperature from 45 to 20°C at Re ≈ 30, respectively.