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

Abstract

In this thesis, turbulent forced convective heat transfer downstream of blockages with elongated holes in a rectangular channel was studied. The rectangular channel has a width-to-height ratio of 12:1. The blockages have the same cross section as that of the channel. The diameter of all elongated holes of the blockages is three quarters of the channel height. The blockages are classified into two different types with two different hole-to-blockage area ratios (ratio of total crosssectional area of holes to cross-sectional surface area of the blockage) of 0.5 or 0.6. For each hole-to-blockage area ratio, the blockages are again subdivided into three different cases using three different aspect ratios (hole-width-to-height ratio) which are determined by the number of holes four, six, and eight holes per blockage. Experiments for total six different cases of blockages were performed under a uniform wall temperature condition (50C). The experiments were conducted at three different Reynolds numbers of about 7,000, 12,000, and 17,000, respectively. Three copper plate heaters with twenty one embedded thermocouples were used to measure the average heat transfer on the surface of channel walls between two consecutive blockages. Results from this study showed that the blockages with elongated holes enhance the average heat transfer by up to 5.06 and 4.08 times that by fully developed turbulent flow 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 by fully 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-toblockage area 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 an alternative when more weight should be put on the heat transfer enhancement than TP value.