| A natural-draft-counter-flow cooling tower (NDCFCT) is a heat rejection device which extracts waste heat to the atmosphere. The performance of a NDCFCT is constrained by the tower inlet air flow. Unsatisfied intake air leads to high-temperature cooling water, which means low cooling tower performance. Reduced cooling tower performance lowers the efficiency of the thermal power plants they are serving. A novel concept of high-level water collecting cooling towers (HWCCTs) improves the NDCFCT performance by reducing the flow resistances and thereby improving the air flow rate passing through the tower. In a HWCCT, there is no rain zone which is replaced by a high-level water collector. The HWCCT uses the potential energy of cooling water to reduce the water pumping head up to 15 m. This is a substantial benefit for power plants, e.g., it saves electricity of 0.12 billion KWh per year for an AP1000 nuclear power plant with two generator units. This contributes to the low-carbon scenario.In the present study, a computational model was developed to simulate the operation of a HWCCT. The model of the water collector was validated by experimental study and the HWCCT model was validated by a rigorous comparison with measured cooling tower performance. The validated HWCCT model was then used to simulate the operation of the HWCCT under no crosswind condition. The variations of the air flow distribution, temperature distribution, pressure distribution, air flow rate, cooling distribution and the water outlet temperature with air speeds were reported based on the numerical simulations. The study on the heat-acceptance and winter conditions found that the main cooling of a HWCCT is due to the cooling at water distribution zone and the fill zone. The water outlet temperature increases with the increase in air speed within the range of 0-10 m/s while it decreases with the increase in the air speed at 10-40 m/s.Beyond the above, the model of a traditional cooling tower was developed to do performance comparison with the HWCCT, i.e., the comparison on air flow distribution, temperature distribution, air flow rate and water outlet temperature. The reasons why the differences exist were discussed. The comparative study found that the main zone for heat and mass transfer in a HWCCT is the top section of the tower, especially the upper part of the fills, which is due to the directing effect of the flow pass produced by the water collector. The air speed at the center of the tower is the highest while it is quite small near the tower shell. For the traditional tower, the highest air speed occurs at the 2/3 of the tower radius. The air flow in the radial direction of the HWCCT is more uniform when compared with the traditional tower. The dismantling of the rain zone in the HWCCT results in no flow resistance at rain zone and thereby contributing to high air flow rates. The cooling loss in the rain zone is however compensated by the enhancement at the fill and water distribution zones. Consequently, the cooling performance of the HWCCT is higher when compared with that of the traditional cooling tower at low crosswind speeds. For high crosswind speeds, there exist vortexes at the upper section of the air intake on the windward side, which impairs the cooling effect of the fills on the windward side and as a result, the cooling performance of the HWCCT is lowered significantly.Moreover, the performance of the HWCCT was investigated when the water collector was at three installing angles (i.e.,40°,45° and 50°) and three installing heights of 8,9 and 10 m. The optimized installing height and angle of the water collector were obtained. Finally, the effects of introducing cross wall and wind deflector were discussed. |
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