Characteristics of gas-liquid counterflow in inclined ducts with particular reference to reflux condensers

Zapke, Albert (Stellenbosch : Stellenbosch University, 1997-12)

Thesis

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.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/18141
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