Improving the heat transfer characteristics of a Spiky Central Receiver Air Pre-heater (SCRAP) using helically swirled fins

Thesis (MEng)--Stellenbosch University, 2019.ENGLISH ABSTRACT: The purpose of this project is to investigate the effect of helically swirled fins on the heat transfer coefficient and pressure drop in the spiky central receiver air pre-heater (SCRAP). First a theoretical analysis was done to investigate the effects of a swirl on the heat transfer coefficient and pressure drop. After this, the effects of swirled fins were simulated numerically and investigated. Experiments were then conducted to provide validation of the numerical model. Empirical correlations indicated that there is an increase of 20%- 30% in the heat transfer coefficient at low Reynolds numbers (Re=6600 andm˙ =0.03 kg/s), while maintaining pressure losses below 20 %. Higher (Re larger than 22 000 and m˙ >0.1 kg/s) Reynolds numbers introduced large pressure drops, surpassing a 100% increase, accompanied by mediocre heat transfer coefficient increases of 34 %. For the straight duct the average difference between analytical and numerical results was 14.8% for the heat transfer coefficient and 7.5% for the pressure drop. The curved duct simulation deviates from the analytical results by 14.3% for the heat transfer coefficient. For the pressure drop a maximum of 11.2% deviation is experienced, with a accuracy of 4% within the region of 5000<Re<27 500. A test section was designed, containing 24 symmetrical helically swirled fins, making one full rotation within 200 mm. Due to its complex geometry the test section was manufactured using selective laser sintering. Tests were then conducted at a constant temperature of a nominally 100 ◦C on the outside of the section, provided through a steam chamber at ambient pressure. Compressed air then flowed through the inner curved ducts. An over-prediction of 10% is experienced in the heat transfer coefficient for the numerical simulation compared to the experimental results. This overprediction is due to the numerical simulation being an ideal situation with no external factors influencing the outcome. Further for the curved duct, analytical pressure drop predictions, at flow rates of 5000<Re<22 000, compared well to both the experimental and numerical results. These results stay within 10% from each other. In conclusion, it was found that with the implementation of swirled fins the heat transfer coefficient can be increased with 21%- 29 %. All the same the pressure drop increases remarkably from 8 %, at 21% increase in heat transfer coefficient, to almost 300 %, at a heat transfer coefficient increase of 29 %, for the investigated case. The implementation of curved ducts is thus an option at low flow rates where the pressure drop will not be increased greatly. It was found that with a flow rate of less than 0.035 kg/s the heat transfer coefficient increase will be larger than the pressure drop increase. At the design point, which is 0.0326 kg/s, the heat transfer coefficient increases by 23% with an increase in pressure drop of 20 %. However, the manufacturing of these complex fins are very expensive, making the implementation of swirled fins challenging.AFRIKAANSE OPSOMMING: Die doel van hierdie projek is om die effek van helies gekrulde vinne in die puntige sentrale ontvanger lug-voorverwarmer (PSOLV) op die warmte-oordragskoëffisiënt en drukval te ondersoek. Teoretiese analise was gedoen om die effekte van ’n krul op die warmteoordragskoëffisiënt en drukval te ondersoek. Daarna is die effekte van gekrulde vinne numeries gesimuleer en ondersoek. Eksperimente was toe uitgevoer om die numeriese model te bevestig. Die empiriese korrelasies dui ’n 20%- 30% toename in warmte-oordragskoëffisiënt, gepaard met aanvaarbare drukverliese van laer as 20% by lae vloeitempo’s (Re=6600 en m˙ =0.03 kg/s). Hoër vloeitempo’s (Re> 22 000 en m˙ >0.1 kg/s) lei tot groot druk verliese van groter as 100%, gepaardgaande met middelmatige verbetering in die warmte-oordragskoëffisiënt van 34 %. Vir die reguit kanaal was die gemiddelde verskil tussen die teoretiese en simulasie resultate 14,8% vir die warmte-oordragskoëffisiënt en 7,5% vir die drukval. Die gekrulde kanaal simulasie verskil van die teoretiese resultate met 14,3% vir die warmte-oordragskoëffisiënt. Vir die drukval was ’n maksimum van 11,2% afwyking waargeneem, met ’n akkuraatheid van 4% binne die vloeistreek van 5000<Re<27 500. ’n Toets afdeling was ontwerp, wat bestaan uit 24 simmetriese helies gekrulde vinne, wat een volle rotasie binne 200mm maak. Die toetsafdeling is vervaardig deur selektiewe laser sintering. Hierdie metode was gekies as gevolg van die toetsafdeling se komplekse meetkunde. Toetse was gedoen om so ver as moontlik ’n konstante temperatuur van 100 ◦C op die buitekant van die toets deel te implementeer en lug dan deur die binne gekrulde kanale te laat vloei. Die warmte-oordragskoëffisiënt word oorvoorspel met n persentasie van 10% vir die simulasie in vergelyking met die eksperimentele resultate. Hierdie oorvoorspelling is as gevolg van die simulasie kondisies wat ideaal is en geen eksterne faktore ’n invloed daarop het nie. Verder vir die gekrulde kanaal het die teoretiese drukvalvoorspellings by lae vloeitempo’s van 5000<Re