Pool boiling heat transfer outside horizontal tubes at higher heat flux

For the flooded evaporator in refrigeration or air conditioning systems, refrigerant is boiling on the shell side and water is flowing in the tube side. The pool boiling heat transfer coefficient of R134a outside one smooth tube and one reentrant cavity enhanced tube No.1 is investigated with an experimental approach. At the saturate temperature of 6, 10 and 16℃, the heat flux of 10-250 kW·m^-2, the heat transfer coefficient versus heat flux of smooth tube is investigated and compared with Cooper correlation. The external diameter of smooth tube and enhanced tube are 15.93 mm and 25.36 mm, respectively. At the heat flux of 10-250 kW·m^-2, it is found that the deviation of experimental result and Cooper correlation is within ±15%. the average value of m in h∝q^m, is 0.67. For the enhanced tube, the enhanced ratio is the largest at the heat flux less than 40 kW·m^-2. The enhanced ratio is decreasing as the increment of heat flux. At the heat flux larger than 250 kW·m^-2, the heat transfer coefficient of enhanced tube is approaching the smooth tube, and even smaller than smooth tube.