Thesis (PhD)--Stellenbosch University, 1997 / ENGLISH ABSTRACT: An experimental investigation on gas-liquid counterflow in inclined rectangular ducts is
conducted. The pressure drop across the sharp-edged gas inlet and the pressure gradient
inside the duct are measured. Combinations of water, methanol, propanol, air, argon,
helium and hydrogen are tested. The duct height and width are varied from 50 mm to
150 mm and 10 mm to 20 mm respectively. The emphasis is on high void fraction flow,
i.e. low liquid flow rates as encountered in air-cooled reflux condensers.
At low to moderate gas flow rates the pressure gradient is gas Reynolds number
related while it becomes dependent on the superficial densimetric gas Froude number as
the gas flow is increased. According to experiment the hydraulic diameter is the required
length dimension in the gas Reynolds number while the duct height becomes the
characteristic dimension in the Froude number regime.
Flooding curves are generated for duct inclinations from close to the horizontal to
the vertical. The data correlate in terms of the phase Froude numbers and a
dimensionless liquid property parameter containing the hydraulic diameter, density,
surface tension and the viscosity. The flooding gas velocity is found to be strongly
dependent on the duct height, the phase densities and the duct inclination. The liquid
viscosity has a stronger effect than the surface tension. Both these properties however
playa secondary role. Flooding is not related to the gas Reynolds number.
A theoretical model, based on the phenomenological findings of the adiabatic
counterflow investigation, is derived to evaluate the performance of an air-cooled reflux
condenser. Field tests are conducted on a full scale reflux condenser and the measured
performance is compared to the model prediction. The reflux condenser is found to
achieve only 60% of the predicted heat rejection rate due to the existence of so-called
cold or dead zones. Indications are that excessive entraiment in the bottom header and
the subsequent accumulation of condensate in the finned tubes causes a maldistribution
of the steamside flow. In the process noncondensable gases accumulate and form dead
zones, causing ineffective performance. Flooding as found in single-ducts does not
appear to contribute to the formation of the dead zones. / AFRIKAANSE OPSOMMING: Die teenvloei van gas en vloeistof in reghoekige skuins buise is eksperimenteel ondersoek.
Die drukverlies oor die skerp gasinlaat en die drukval in die buis is gemeet vir verskillende
kombinasies van water, propanol, metanol, lug, argon, helium en waterstof. Buishoogtes en
breedtes van 50 mm tot 150 mm en 10 mm tot 20 mm respektiewelik is getoets. Die klem
van die ondersoek is op lae vloeistofvloeitempos soos teenwoordig tydens kondensasie van
stoom in lugverkoelde teenvloeikondensors.
Vir lae tot matige gasvloeitempos is die drukval afhanklik van die gas Reynolds-getal
terwyl die densimetriese gas Froude-getal die heersende parameter word soos die gasvloei
toeneem. Die hidrouliese diameter verteenwoordig die dimensie in die Reynolds-getal maar
die buishoogte word die karakteristieke dirnensie in die Froude-getal gebied.
Vloedingskurwes is vir 'n reeks van buishoeke gegenereer. Die vloedingdata korreleer
in terme van die Froude-getal en 'n dimensielose parameter bestaande uit die hidrouliese
diameter, oppervlakspanning, vloeistofdigtheid en die vloeistofviskositeit. Die
vloeidingsnelheid is primêr van die buishoogte, vloeierdigthede en die buishoek afhanklik.
Die vloeistofviskositeit-effek is sterker as die van die oppervlakspanning. Beide die
eienskappe speel egter 'n sêkondere rol. Die gas Reynolds-getal beïnvloed nie die
vloeidingsproses nie.
Die fundamentele bevindinge van die teenvloeiondersoek is toegepas om die
werkverigting van 'n lugverkoelde teenvloeikondenser teoreties te modelleer.
Werkverigtingstoetse is uitgevoer op 'n volskaal teenvloeikondenser. Die toetsresultate
word vergelyk met die teoretiese voorspelling. Die teenvloeikondensor behaal slegs sowat
600% van die voorspelde warmteoordrag omdat van die gevinde buise gedeeltelik by
omgewingstemperatuur is. Hierdie verskynsel heet koue of dooie sones. Dit blyk dat die
kondensaat in die onderste spruitstuk nie vrylik kan dreineer nie en in die vorm van
druppels deur die stoom opgesleur word. Gevolglik versamel kondensaat binne die buise en
sodoende kan nie-kondenseerbare gasse nie effektief uit die teenvloeikondensor verwyder
word nie. Soos die gasse versamel word koue sones gevorm. Dit blyk dat vloeding soos
waargeneem in enkelbuise nie tot die vorming van koue sones bydra nie.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/18141 |
| Date | 12 1900 |
| Creators | Zapke, Albert |
| Contributors | Kroger, D. G., University of Stellenbosch. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering, Jacobs, Ivan |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 1 v. (various pagings) : ill. |
| Rights | Stellenbosch University |
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