Influence of ambient conditions and water flow on the performance of pre-cooled natural draft dry cooling towers

A simplified heat and mass transfer model in cellulose medium was developed to predict the air outlet temperature and humidity after evaporative cooling. The model was used to simulate the operation of pre-cooled Natural Draft Dry Cooling Towers (NDDCTs) by a validated MATLAB code. The effects of supplied water flow rate to the media, ambient temperature and humidity on the performance of pre-cooled NDDCTs were investigated. It was found that the effect of the selected water flow rates on tower performance is negligible. Both ambient temperature and humidity affect the tower performance

Numerical and experimental study on the spray characteristics of full-cone pressure swirl atomizers

Numerical and experimental studies have been performed to investigate the macroscopic spray structure and spray characteristics of sprays generated by a full-cone pressure swirl atomizer. The simulation employs Eulerian-Lagrangian scheme to account for the multiphase flow and the linearized instability sheet atomization model to predict film formation, sheet breakup and atomization. Reynolds-Averaged Navier–Stokes (RANS) equations are solved for turbulent gas flow. The model predictions show great consistency with the experimental measurements of the spatial variation of the droplet size and velocity obtained from Phase Doppler Particle Analyser (PDPA). The robustness of this model makes it useful to predict the structures and characteristics of co-flow sprays produced by pressure-swirl atomizers. This particular spray is quite important in spray cooling application but is not extensively studied. The study reveals that the entrainment effect and intense central-region atomization cause small droplets to concentrate on the spray axis and large droplets to dominate in the peripheral region of the spray. This finding is consistent with the observation that turbulence kinetic energy of air is maximum near the nozzle exit, where atomization is intense and momentum exchange is strong, and gradually decreases in both radial and axial directions. Moreover, the drops inside the full cone are relatively small, and evaporate more easily than their large counterparts in the peripheral region, hence removing substantial sensible heat from surrounding air and creating low-temperature region in the central of the spray

Water spray for pre-cooling of inlet air for Natural Draft Dry Cooling Towers: experimental study

This paper deals with an experimental investigation of inlet air pre-cooling with water sprays aimed to enhance the performance of Natural Draft Dry Cooling Towers during high ambient temperature periods. An open-circuit wind tunnel with a test section of 1 × 1 m cross section and length of 5.2 m was employed to represent an inlet flow area section in a Natural Draft Cooling Tower. Experimental measurements of droplet evaporation and air cooling are presented. Nine high pressure, hollow cone nozzles were tested at various droplet sizes, air velocities (1, 2, 3 m/s), and injection rates under different ambient conditions. The water spray was characterized using a Phase Doppler Particle Analyser (PDPA). The effects of drop size distribution and air velocity on droplet evaporation, cooling effectiveness, and coverage area were investigated. The data shows clear trends of cooling enhancement with low air velocity or small droplet size distribution. It was found that the spray cooling efficiency, to a large extent, is dependent on spray coverage area. The experimental findings will benefit optimizing spray cooling performance in Natural Draft Dry Cooling Towers and nozzle arrangement

Parametric study on spray cooling system for optimising nozzle design with pre-cooling application in natural draft dry cooling towers

Pre-cooling of inlet air with water spray is proposed for performance enhancement of natural draft dry cooling towers (NDDCTs) during high ambient temperature periods. Previous experiments showed promising results on cooling enhancement using spray cooling. Spray nozzles characteristics are an essential element for designing spray cooling systems. A parametric analysis is conducted on the effect of different spray characteristics parameters and various hollow-cone droplet size distribution patterns on a spray cooling system performance for nozzle design optimisation. A validated Eulerian-Lagrangian 3-D model with a novel nozzle representation method for hollow-cone nozzles was used to perform the analysis. The results can help prepare design principles of effective spray nozzles for spray cooling systems. The results showed a clear trend of increasing spray cooling efficiency as droplet velocity increases due to better spray dispersion. 15% improvement in cooling performance was achieved by increasing droplet velocity from 20 to 80 m/s. A spray nozzle with high injection velocity, small droplet size distribution, large cone angle and a droplet size pattern of mean droplet size increasing towards the spray periphery was found to be the best for sprays with D ≤ 50 μm. Complete evaporation was successively achieved within 5 m distance using a spray with droplet size distribution of D = 20 μm and droplet velocity of 120 m/s at an air velocity of 1 m/s

Pre-cooling with Munters media to improve the performance of Natural Draft Dry Cooling Towers

This paper presents a simulation study on pre-cooling of inlet air using Munters media to enhance the performance of a Natural Draft Dry Cooling Tower (NDDCT) on hot and dry days. The ambient humidity was kept constant at 30% while the ambient temperature ranged from 10° to 50 °C during simulation. Three Munters media (CELdek7060, CELdek7090 and CELdek5090), each with three thicknesses, were employed to pre-cool the tower inlet air. Results show that NDDCT can benefit from pre-cooling with wetted media when the ambient air is hot and dry. The trade-off between the air pressure drop across the media and the benefit of evaporative pre-cooling of the inlet air was also examined. It was found that there is a critical ambient temperature below which pre-cooling application does not benefit NDDCT performance. The value of this critical temperature is a function of the tower design and the selection of wetted media. Amongst the studied media, only the pre-cooling with 0.10 m and 0.15 m CELdek7060 is to be useful in the studied case. The dependence of heat rejection improvement of the proposed tower on medium thickness is weak. Besides, thicker medium does not necessarily bring more benefit