Heat transfer enhancement for turbulent flow through blockages with elongated holes in a rectangular channel

摘要

In this thesis, turbulent forced convective heat transfer downstream of blockageswith elongated holes in a rectangular channel was studied.The rectangular channel has a width-to-height ratio of 12:1. The blockages havethe same cross section as that of the channel. The diameter of all elongated holes of theblockages is three quarters of the channel height. The blockages are classified into twodifferent types with two different hole-to-blockage area ratios (ratio of total crosssectionalarea of holes to cross-sectional surface area of the blockage) of 0.5 or 0.6. Foreach hole-to-blockage area ratio, the blockages are again subdivided into three differentcases using three different aspect ratios (hole-width-to-height ratio) which aredetermined by the number of holes four, six, and eight holes per blockage.Experiments for total six different cases of blockages were performed under a uniformwall temperature condition (50C). The experiments were conducted at three differentReynolds numbers of about 7,000, 12,000, and 17,000, respectively. Three copper plateheaters with twenty one embedded thermocouples were used to measure the average heattransfer on the surface of channel walls between two consecutive blockages.Results from this study showed that the blockages with elongated holes enhancethe average heat transfer by up to 5.06 and 4.08 times that by fully developed turbulentflow through a smooth channel at the same Reynolds numbers for small and large holeto-blockage area ratios, respectively. The friction factor ratios for small and large holeto-blockage area ratios of the blockages reached 345 and 89 times, respectively, that byfully developed turbulent flow through a smooth channel at the same Reynolds numbers.TP (Thermal Performance) values varied from 0.65 to 1.11 depending on cases.According to the results, Case L-2, which has six elongated holes and hole-toblockagearea ratio of 0.6, is the best option from the TP point of view. But Case S-2,which also had six elongated holes and hole-to-blockage area ratio of 0.5, can be analternative when more weight should be put on the heat transfer enhancement than TPvalue.