Numerical Investigation of fan performance in a forced-draft air-cooled heat exchanger.

[摘要] ENGLISH ABSTRACT: This study aims to develop an accurate and reliable numerical model of anAir-Cooled Condenser (ACC) using Computational Fluid Dynamics (CFD).Simplified methods for modelling the axial ow fan and heat exchanger areused to limit the complexity of the computations. The actuator disk and extended actuator disk model is presented and validated using two fans with different physical characteristics. The A-fan is an axial flow fan commonlyused in industrial cooling applications and the B2a-fan is an axial ow fan developedat Stellenbosch University. The heat exchanger model is based on theA-frame heat exchanger typically used in ACCs. Validation is performed withrespect to heat exchanger mechanical losses and heat transfer. The operatingpoint of each combined fan and heat exchanger unit is determined analyticallyand numerically under ideal operating conditions. The results are validatedby comparing the kinetic energy recovery coeficient to an experimental designfrom literature. A numerical recovery coeficient of 0.527 was measured comparedto 0.553 measured experimentally. The axial ow fan, heat exchangerand ACC model are successfully validated.A 30 fan ACC bank in a 6x5 configuration is analysed with regard to performanceunder cross-wind conditions using three different fan configurations.The ACC is subjected to four different wind speeds along five directions. Comparisonsare drawn between the volumetric, thermal and overall performanceusing the heat-to-power ratio. The so-called A-fan ACC, B2a-fan ACC andCombined ACC are considered. Major findings indicate that the performance of an ACC decreases with increasing cross-wind speed. Superior overall performanceis measured for the B2a-fan ACC resulting from a 19 % increase in performance to the A-fans on the upstream periphery. Higher thermal performanceis also measured as well as 6-10 % lower power consumption than the A-fan ACC and Combined ACC. The A-fan ACC exhibits the highest sensitivity to increasing cross-wind speeds along with the highest power consumption.Heat-to-power performance is measured 9-10 % lower than the B2a-fan ACC and 7 % lower than the Combined ACC as a result.A comparative study between the 6x5 and 3x10 ACC layout is also presented.Wind directions leads to volumetric, thermal and overall performancedifferences up to 23 % for the A-fan ACC in 3x10 layout for a constant windspeed. The 6x5 layout measured differences up to 5 %. The 3x10 layout istherefore considered to exhibit a higher sensitivity to wind direction. This is attributed to the asymmetrical nature of the configuration. The A-fan ACC in 3x10 layout consumes 9.70 % more power compared to the 6x5 layout. TheB2a-fan ACC consumes 6.95 % more power with similar power consumption measured for the Combined ACC.An on-site measurement methodology for determining the fan volumetricow rate using the measured fan power consumption and resultant blade loading along with the characteristic curves to determine the flow rate is presented.This is discussed, analysed and applied to the 30 fan ACC under cross-windconditions. Differences in predicted volumetric ow rate up to 6.45 %, measuredto the numerical results, are noted for fans not subjected to distorted inflow. Predictions for upstream periphery fans shows poor correlation with differences up to 22.81 % measured. The results do indicate that the use of theblade loading for fans subject to distorted in flow gives more accurate results for flow rate predictions compared to the power consumption.

[发布机构] Stellenbosch University