การเพิ่มประสิทธิภาพการผสมในห้องที่มีการระบายอากาศโดยการปรับเปลี่ยนทางออกของช่องปล่อยอากาศ / ทศพล สถิตย์สุวงศ์กุล = Mixing enhancement in a ventilated chamber by means of manipulation of a nozzle exit / Tosapole Stitsuwongkul
Two main issues regarding the distribution of temperature in a ventilated chamber are investigated. Firstly, the effects of pyramid-like lobed nozzles varying in period and gap in combination with the size of end-wall opening are investigated. Secondly, the effects of supply-air velocity also in combination with the size of end-wall opening in the case of nozzle without lobes are investigated. The dimensions of the test chamber are WxLxH = 50x100x50 cm[superscript 3]. The nozzle exit and the end-wall opening are rectangular with the width spanning the whole width of the chamber. The height of the nozzle exit (h) is 4 cm while that of end-wall opening is adjustable. Two series of lobed nozzles are studied : one with the period of 2h, and the other 4h. In each series, the gap between neighboring lobes is varied at 0h (no gap), 1h, 2h, and 4h interval, and the amplitude and the depth of lobes are fixed at 2h and 1h respectively. The experiment is conducted using heated supply air. The temperature difference between that of the supply air and that of ambient air is 40 ํC. The Reynolds numbers based on the height of the nozzle exit and supply-air velocity, at low and high velocities, are 1,000 and 8,800 respectively. The results reveal many interesting characteristics. For the effects of lobed nozzle and the size of end-wall opening, it is found that the lobes help promote mixing and distribution of supply air inside the chamber and, thus, cause the temperature in the chamber to become more uniform, particularly in the lower zones and for cases of small opening (opening ratio less than or 0.24). For cases of large opening, however, the lobes have little effect except near the nozzle exit. The average temperature and the temperature distribution inside the chamber depend on both the period and the gap in complex and coupled manner. In case of the lobed nozzles with 2h period, the 4h gap causes the highest increase in average temperature, while, those of the 4h period, the 0h gap causes the highest increase. Detailed examination reveals that the lobes cause the temperature profiles to become increasingly fuller towards the lower zones as the flow develops downstream in comparison with those of the case of nozzle without lobes. For the effects of supply-air velocity in the case of nozzle without lobes, it is found that the variations in average temperature with the end-wall opening in the upper and lower zones display different characteristics. Namely, 1) inthe upper zones, the variation in average temperature with end-wall opening is relatively independent of the supply-air velocity. Specifically, 1.1) the average temperature depends upon the closing only in the range of closing from 0 (full opening) to triangle, where triangle is the wall jet thickness at the far end of the chamber determined from the case of full opening. On the contrary, 1.2) in the range of closing beyond triangle, the average temperature in the upper zones is approximately constant, independent of the closing. In contrast, 2) in the lower zones, the variation in average temperature with end-wall opening strongly depends upon the supply-air velocity. Namely, 2.1) in the case of high velocity, the average temperature varies withthe end-wall opening in basically the same manner as that is found in the upper zones. That is, it varies with the closing only in the range of closing from 0 to triangle but is approximately constant, in dependent of the closing, beyond the closing of triangle. In striking contrast, 2.2) in the case of low velocity, the average temperature exhibits linear dependency on the closing throughout the mid 60% range of closing, even when the closing is beyond triangle. 3) The difference in the characteristics of variation of average temperature with end-wall opening at different velocities is attributed to the difference in degree in which the direction of high- and low-momentum wall jets can be changed ; high momentum jet is less likely to change its direction than low momentum one. As a result, wall jets of different strength in momentum cause recirculations of different strength. Finally, a time scale tau, defined as the lapse time required for the jet to travel from the nozzle to the far end wall, is suggested to be used for the correlation of these results.