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Title: Investigation on thermal performance enhancement of self-rotating twisted tapes in a double-pipe heat exchanger
Authors: Zhang, Shaojie
Degree: Ph.D.
Issue Date: 2019
Abstract: With the rapid population increase and industrial development, global energy demand is rising significantly in recent decades. In order to achieve energy-saving benefit, heat transfer enhancement techniques become crucial to promote the thermal efficiency of heat exchanger system. Among various heat transfer techniques of passive method, twisted tape have been considered as a promising alternative for convective heat transfer due to the caused extra swirl flow. Nowadays, a novel type of twisted tapes, namely self-rotating twisted tapes (SRTTs), has been widely commercialized in many practical applications. It is defined as the device that can rotate automatically when fluids flow through the tube with high water velocity. Considering the existence of rotating behavior, SRTTs appear to perform greater performance in terms of inducing stronger turbulence and reducing pressure drop. However, limited studies are found to comprehensively investigate the thermal characteristic of the heat exchanger tube fitted with SRTTs. Hence, this study aims to investigate and compare the thermal performance enhancements of SRTTs and stationary twisted tapes (STTs) in a double-pipe heat exchanger, and also to examine as the effect of various fabrications (e.g. twist ratios, perforated ratio and length ratio) for different Nusselt numbers, friction factors and thermal performance factors. In addition, experiments on measuring the rotational speed of SRTTs at given Reynolds number were conducted to explore the effect of rotational speed on thermal performance. Firstly, STTs and SRTTs were prepared with the same geometry (e.g. twist ratio, thickness and tapes width). The experiment was carried out in the turbulence flow from 12000-45000. The experimental results reported that STTs performed similar thermal performance as SRTTs when SRTTs keep stationary. Also, it is found that the tube fitted with SRTTs could increase the average heat transfer rate by 18.8% and reduce the average friction factor by 12.1% in rotating condition. Interesting findings were observed that the thermal performance factor of SRTTs increases considerably when SRTTs switch from stationary to rotating conditions. Then, SRTTs with different twist ratios (Y=2.2, 3, 4 and 6) were fabricated to study the influences of twist ratios on rotational speed and thermal performance. The results indicated that twist ratio has less effects on the start of rotating behavior and lower twist ratios were found to have considerably increased the rotational speed as compared with higher twist ratios. In addition, the tube fitted SRTTs yield higher Nusselt number by 22.2%, 16.2%, 12% and 7.4% and friction factor of 101%, 116%, 126% and 137% for twist ratio of 2.2, 3, 4 and 6, respectively. It was observed that the maximum thermal performance factor of 1.03, 0.944, 0.924 and 0.898 could be obtained at the start of rotating behavior for twist ratios of 2.2, 3, 4 and 6, respectively. The valuable findings from the experiment could be further recommended for proper application of SRTTs. Also, some correlations varying with twist ratios were proposed to predict the Nusselt number and friction factor within 10% deviation.
Afterwards, the influences of perforated SRTTs with different perforated ratios (PR=0, 1.61, 3.63, 6.46 and 10.1%) on thermal behavior were studied experimentally. Perforated ratio is defined as the ratio of perforated area to twisted tapes area. Smaller perforated ratio was found to function in rising rotational speed and start to rotate at relatively lower Reynolds number. Our results revealed that the increase of perforated ratio could play positive role in enhancing heat transfer rate, friction factor and thermal performance factor. Interestingly, the thermal performance factor increases obviously from 0.862 to 0.924, 0.987 to 1.025, 1.04 to 1.078, 1.084 to 1.101 and 1.042 to 1.055 for PR=0, 1.61, 3.63, 6.46 and 10.1%, respectively. Comparison of perforated twisted tapes and perforated SRTTs was conducted based on thermal performance factor. In addition, some correlations associated with perforated ratios were established to estimate the heat transfer and friction factor characteristics of the perforated SRTTs in double-pipe heat exchangers. According to the previous researches, fabrication of short length to twisted tapes was proposed for mitigating the pressure drop. Therefore, in this project, an experiment was carried out to explore the effects of length ratio (LR) on the thermal characteristics. The definition of length ratio is the length of SRTTs to the tube length. The experimental results reported that SRTTs with larger length ratio seem to rotate faster than those with smaller length ratio. Heat transfer rate can be increased by 57-66%, 61-114%, 62-127% and 95-160% for LR=0.3, 0.45, 0.6 and 1 respectively. The tube equipped with full-length SRTTs (LR=1) gave higher heat transfer performance, pressure drop and thermal performance factor as compared with those equipped with lower length ratio (LR=0.3, 0,45 and 0.6). Therefore, full-length SRTTs were suggested as alternatives to be employed in the heat exchanger system. Finally, two correlations regarding length ratio were formulated to evaluate the thermal performance in form of Nusselt number and friction factor. To conclude, this thesis introduced a novel type of twisted tapes, namely self-rotating twisted tapes (SRTTs), into heat exchanger system, and experimentally investigated the thermal performance of SRTTs in a double-pipe heat exchanger with various configurations (e.g. twist ratio, perforated ratio and length ratio). Accordingly, the findings and results gave that the optimal configurations for proper use of SRTTs are lower twist ratio (Y=2.2), large perforated ratio (PR=10.1%) and full length (LR=1). Additionally, some correlations associated with these fabricated parameters were suggested for the prediction of Nusselt number and friction factor. The experimental correlations were validated by experimental values within acceptable level and could be widely and easily used for thermal performance evaluation of heat exchanger systems with SRTTs.
Subjects: Hong Kong Polytechnic University -- Dissertations
Heat exchangers
Heat -- Transmission
Mass transfer
Pages: xxii, 149 pages : color illustrations
Appears in Collections:Thesis

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