Boiling heat transfer (BHT) is the basis for thermal management in several industries including refrigeration, power systems and electronics. Among a number of strategies to enhance boiling heat transfer, surface modi?cation has proven to be an attractive means to improve the boiling heat transfer coef?cient (HTC). Accurate prediction of critical heat ?ux (CHF) is very important for design and operation of various heated equipments such as evaporator, fuel cell cooling system, nuclear reactor, etc. It will cause a drastic reduction of heat transfer coef?cient when CHF occurs. As heated uniformly, the wall temperature of boiling heat transfer tubes increases abruptly, causing the material of heated facilities to be sometimes destroyed, which is called boiling crisis. In the present research, pool boiling on nano structured surfaces was studied. Nano coating was performed on the aluminum surface specimens and its boiling heat transfer characteristics were compared with those of bare surface of copper and aluminum. Also in this experimental study critical heat flux and boiling heat transfer coefficient were evaluated. To make the nano structure coating on the surface of aluminum alloy specimens, the anodizing process was performed. Chromic acid and sulfuric acid solution were utilized as electrolyte. Prior to anodizing, mechanical and chemical treatment, i.e. sandblasting and etching were applied. 50 gr/l of NaOH solution at 60 degrees Celsius was used for two etching durations, i.e. 20, and 180 seconds. In the sulfuric acid solution, the coating was performed for three different durations, i.e. 15, 30, and 45 minutes. The same treatment was done for the chromic acid solution but coating continued for 25, 35, and 45 minutes. Maximum critical heat flux occurred on the samples which were weakly etched and anodized for 30 minutes in sulfuric acid. Critical heat flux increased by 8 percent compared with that of untreated aluminum alloy surface and increased by 90% compared with that of copper surface. Also, 159 present increases in boiling heat transfer coefficient were observed for weakly etched samples. In order to investigate the surface characteristics, contact angle and AFM pictures were studied. Also, surface roughness and real area of 6 samples were measured using an atomic force microscope (AFM). The AFM pictures indicated that surface roughness increases by anodizing time. The change in roughness is more affected for the surfaces treated in chromic acid solution compared with surfaces treated in sulfuric acid solution. In one hand, copper is widely used in industry and its thermo-physical properties is very desirable for many applications, and in the other hand critical heat flux of aluminum is higher than that of copper, aluminizing process was used to coat the copper surface with aluminum. The critical heat flux of aluminized copper sample increased by 37 present compared with that of copper surface without coating; meanwhile the boiling heat transfer coefficient did not change significantly. In order to make the aluminized coating, 88% Al 2 O 3 , 10% Al and 2% NH 4 Cl were used. Aluminizing was done at 850 degrees Celsius for 3 hours. Key Words: pool boiling, nano structure coating, critical heat flux, heat transfer coefficient, surface roughness
