Abstract An experimental investigation of the convective heat transfer on a flat surface in a multiple-jet system and hole channel is described. The system consists of free jets with an undisturbed flow of the air. Principle arrays were considered as in-line as well as staggered with different spacing distance in X- and Y-directions. For measuring the heat transfer, a metal sheet made of nickel alloy of size 200 mm × 170 mm was heated electrically with direct current supply up to 400 A and 7 V. The top side was cooled with air from the nozzle array. The temperature distribution on the black coated bottom side was recorded with an infrared thermo camera. Because of the sheet thickness was only 0.1 mm, the temperature on both sides can be assumed as equal. The resolution of the temperature difference is 0.15 mm/pixel, thus it is possible to determine the local heat transfer with a high accuracy. Varied parameters were the jet inner diameter with d, of 5.8 mm and 8 mm, the jet Reynolds number in the range from 1400 to 41400, the normalised distance nozzle to sheet H/d from 1.0 to 10.0, and the normalised nozzle spacing S/d from 2.0 to 10.0. The geometrical arrangement of nine jets arrays (3x3) was tested. The profile of the local and average heat transfer coefficient for multiple free jets system were discussed and compared to those of a single free jet. The results have shown that the multiple jet system enhances the heat transfer over the entire range than those for the hole channel and a single nozzle. A maximum of the heat transfer was found for the normalised spacing S/d = 6.0 for multiple jets system. This is because the interference between adjacent jets is reduced. But for the hole channel the normalised spacing S/d = 4 provides the maximum heat transfer. The normalised distance H/d had nearly no effect on heat transfer in the range 2 < H/d < 5 for both multiple jets system and hole channel arrays. The Reynolds number exponent m for the multiple free jets arrays at optimum spacing distance of is found approximately 0.7. While for the hole channel array the exponent of Reynolds number for maximum average Nusselt number is found 0.66. Because of the difference between exponents of Reynolds number for these two cases, the crossflow is limited for the multiple free jets system in comparison to the hole channel array. In addition, the uniformity of the heat transfer is examined in this work. The experimental results show that the uniformity depends strongly on the type of the jet array. Therefore the heat transfer is more uniform over the impinging area for the staggered array than the other arrays.
Free Tube Jet - Impingemenet - Heat Transfer - Arrary - Infrared Techuique - Hole Channels - Heat Transfer Uniformaty