Study of the effects of geometrical parameters of a spiral corrugated convective tube on the heat transfer rate

Schmittinger, Elianne (2017-03)

Thesis (MEng)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: The single start spiral indentation heat transfer enhancement technique is investigated on a boiler convective tube. It improves the heat transfer rate and increases the pressure drop significantly. This increases the thermal efficiency of the boiler and leads to reduced fuel costs. Current empirical correlations for calculating the heat transfer rate of the spiral tube require correction factors in order to obtain accurate results. A CFD model is constructed to eliminate the need of a correction factor and to investigate the effects that the geometrical parameters of the spiral indentation have on the heat transfer rate and pressure drop. The fluid flow characteristics in the CFD model reflect what is expected from theory where higher fluid velocities cause boundary layer thinning and increased pressure drag. The spiral depth is increased to 2.8 mm. The heat transfer rate increases by an average of 1.4 % and the pressure drop by 31.7 %. The increased spiral depth promotes secondary swirl flow inside the tube, increasing the pressure drag over the indentations and thinning the boundary layer at the wall. The spiral pitch is decreased to 20 mm and it is found that the pressure drop increases by an average of 5.9 % and the heat transfer rate by 0.3 %. The CFD model results show that a pulse like flow is promoted down the length of the convective tube. This pulse like flow increases the heat transfer rate and pressure drop over the tube length. It is the result of interference between the indentations in the separation and reattachment zone of the boundary layer. The spiral pitch is decreased to 22 mm and the depth is increased to 2.4 mm. The heat transfer rate increases by an average of 0.7 % and the pressure drop by 24.0 %. At higher inlet velocity conditions the pulse like flow is promoted again. It is deduced that at the right combination of spiral depth, pitch and fluid velocity, the pulse like flow is promoted. The spiral pitch-to-depth ratio is investigated and it is found that at ratios below 10, the pressure drop and Nusselt number increases drastically. As the ratio increases beyond 14, the pressure drop and Nusselt number decreases gradually. It is recommended that the spiral depth of the current spiral tube be increased to 2.4 mm to achieve a spiral pitch-to-depth ratio of 10. This will improve the heat transfer rate by an average of 1.0 % at an acceptable increased pressure drop of an average of 20.7 %. It is concluded that by changing the geometrical parameters the heat transfer rate is improved and can be used to improve the thermal efficiency of a firetube boiler.

AFRIKAANSE OPSOMMING: die aanbring van 'n enkel-begin, spiraal-indentasie word ondersoek. Dit verbeter die warmteoordrag maar verhoog die drukval aansienlik. Die termiese rendement van die ketel word verhoog en dit lei tot 'n vermindering in brandstofkoste. Huidige numeriese korrelasies vir die berekening van die warmteoordrag van die spiraal-buis gebruik korreksie-faktore wat meer akkurate resultate verseker. ‘n Berekenings-Vloei-Meganika (BVM) model is geskep om die gebruik van die korreksie-faktor te elimineer en om die gevolge van verskillende geometriese parameters van die spiraal-indentasie op die warmteoordrag-tempo en drukval van die buis te ondersoek. Die vloei-eienskappe in die BVM-model weerspieël die teoretiese verwagting waar hoër vloei-snelhede die grenslaag verdun en die drukval verhoog. Die spiraal diepte is vermeerder tot 2.8 mm. Die warmteoordrag-tempo verhoog met 'n gemiddeld van 1.4 % en die drukval met 31.7 %. Die verhoogde spiraaldiepte bevorder sekondêre vorteks-vloei binne die buis. Dit verhoog die drukafhanklike sleurkrag oor die indentasies en verdun die grenslaag aan die wand. Die spiraal-spasiëring is verminder na 20 mm en daarword bevind dat die drukval verhoog met 'n gemiddeld van 5.9 % en die warmteoordrag-tempo met 'n gemiddeld van 0.3 %. Die resultate van die BVM-model dui daarop dat die vloei pulseer in die lengte van die konveksie-buis. Hierdie pulserende vloei verhoog die warmteoordrag-tempo en drukval oor die buis. Dit is die gevolg van die interaksie tussen die indentasies in die skeiding- en vashegtingsone van die grenslaag. Die spiraal-spasiëring is verminder na 22 mm en die diepte verhoog tot 2.4 mm. Die warmteoordrag-tempo verhoog met 'n gemiddeld van 0.7 % en die drukval met 24.0 %. Met hoër inlaat-snelhede word die pulserende vloei verder bevorder wat daarop dui dat met die regte kombinasie van spiraal-diepte, spasiëring en vloeistof-snelheid die pulserende vloei bevorder kan word. Die spiraal-spasiëring tot diepte-verhouding is ondersoek. Die resultate dui daarop dat met ‘n waarde van minder as 10 die drukval en Nusselt-getal drasties verhoog. Met verhogings bo 14, verminder die drukval en Nusselt-getal geleidelik. Dit word aanbeveel dat die spiraal-diepte van die huidige spiraal-buis verhoog word tot 2.4 mm om 'n spiraal-spasiëring tot diepte-verhouding van 10 te verkry. Dit sal die warmteoordrag-tempo met 'n gemiddeld van 1.0 % verhoog met 'n aanvaarbare drukval verhoging van om en by 20.7 %. Die gevolgtrekking is dus dat die warmteoordrag-tempo verhoog kan word deur die verandering van die geometriese parameters om sodoende die termiese rendement van 'n ketel te verbeter.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/101447
This item appears in the following collections: