Experimental Study of Free Convection Heat Transfer and Condensation of Vapor of Humid Air Over an Inclined Cold Tube Behnam Nabovati, Mahmood Yaghoubi, and Abdollah Avara Mechanical Engineering Department, Shiraz University, Shiraz, Fars, Iran In this study condensation heat transfer on a cold inclined circular cylinder due to natural convection for various conditions is investigated experimentally. The cylinder is placed in an isolated test room to permit pure natural circulation of ambient air. Ambient temperature and humidity of the test room are controlled by a refrigera- tion cycle and humidifying. The ambient relative air humidity changed in the range of 30 to 50% and temperature from 25 to 35 °C. The ethylene-glycol/water solution is used as a refrigerant to control and keep the temperature of the test section at a constant value. The cold surface temperature is varied from 2 to 6 °C. The condensation rate and heat flux are found to depend mainly on time, temperature difference between ambient air and cold surface, ambient relative humidity, and tube inclination. Results are plotted for various conditions with respect to time. The experimental results are used to propose a correlation to predict the condensate mass flow rate for free convection heat transfer. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21015 Key words: condensation heat transfer, humid air, natural convection, con- densation rate 1. Introduction The subject of condensation on outer surface of tubes is largely associated with various fields of engineering such as thermal power plants, refrigeration, air conditioning, and many chemical industries. Most of the evaporators in commercial plants, industrial plants, or residential appliances use circular tubes carrying cooling water or refrigerant. These tubes are exposed to humid air and their temperatures are for some cases less than the air dew point temperature. For such conditions, natural convection heat transfer with condensation occurs simultaneously. The rate of condensation heat transfer depends on important factors such as tube geometry, air temperature, air humidity, surface temperature, and tube inclination. Studies of dehumidification have attracted many re- searchers’ interest. The importance of this phenomenon in practical engineering applications led to the vast range of applied researches on different modes of condensation, vapor mixture flows, condensate flow regimes, various kinds of vapor mixtures, geometrical configurations, etc. © 2012 Wiley Periodicals, Inc. Heat Transfer—Asian Research 1