Study on Heat Transfer From Hot Water to Cold Air With Evaporation

Evaporation heat transfer from hot water flow to cold air flow in a horizontal duct was examined. Experiments were performed for turbulent condition. The test flow channel was rectangular. The width, height and length were 40 mm, 20 mm and 1,000mm. The length of heat transfer surface was 140 mm. In experiments, hot water was in the range of 40 °C ∼ 60 °C. Cold air was approximately at 20 °C. The air velocity was varied from 1.61 m/s ∼ 10.5 m/s. Heat transfer rate from hot water flow to cold air flow became large with an increase in air velocity. The higher the water temperature was, the larger the heat transfer rate was. When total heat flux from water to air flow was divided into two terms; the evaporation term and the forced flow convection term, the evaporation term dominate main part and that was about 60 ∼ 80 % of the total heat flux. Water surface temperature drop from bulk water temperature was confirmed. However, the temperature drop was less than 5 °C. The surface temperature drop had little effect on the evaporation heat flux and the convection heat flux. The measured evaporation heat flux was lower than the value predicted with the mass transfer correlation that was developed through analogy between heat transfer and mass transfer. The measured convection heat flux was higher, approximately twice, than the value that was calculated with the correlation for turbulent flow heat transfer probably because of bottom heating. The method to predict the heat transfer from the hot water flow to the cold air flow with the evaporation was developed by modifying the turbulent flow mass transfer correlation and the turbulent forced convection heat transfer correlation. Good results were obtained.