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

[摘要] ENGLISH ABSTRACT: An experimental investigation on gas-liquid counterflow in inclined rectangular ducts isconducted. The pressure drop across the sharp-edged gas inlet and the pressure gradientinside 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 to150 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 numberrelated while it becomes dependent on the superficial densimetric gas Froude number asthe gas flow is increased. According to experiment the hydraulic diameter is the requiredlength dimension in the gas Reynolds number while the duct height becomes thecharacteristic dimension in the Froude number regime.Flooding curves are generated for duct inclinations from close to the horizontal tothe vertical. The data correlate in terms of the phase Froude numbers and adimensionless liquid property parameter containing the hydraulic diameter, density,surface tension and the viscosity. The flooding gas velocity is found to be stronglydependent on the duct height, the phase densities and the duct inclination. The liquidviscosity has a stronger effect than the surface tension. Both these properties howeverplaya secondary role. Flooding is not related to the gas Reynolds number.A theoretical model, based on the phenomenological findings of the adiabaticcounterflow investigation, is derived to evaluate the performance of an air-cooled refluxcondenser. Field tests are conducted on a full scale reflux condenser and the measuredperformance is compared to the model prediction. The reflux condenser is found toachieve only 60% of the predicted heat rejection rate due to the existence of so-calledcold or dead zones. Indications are that excessive entraiment in the bottom header andthe subsequent accumulation of condensate in the finned tubes causes a maldistributionof the steamside flow. In the process noncondensable gases accumulate and form deadzones, causing ineffective performance. Flooding as found in single-ducts does notappear to contribute to the formation of the dead zones.

[发布机构] Stellenbosch University