Simulation of a cross flow wind aided evaporator

At present, mechanical evaporators are being used by the textile and dyeing units in Tirupur, South India, to further concentrate the discharge from Reverse Osmosis (RO) units. A large amount of wood is burnt as fuel, leading to air pollution and destruction of the natural habitat. To circumvent this problem, the evaporation of water in a cross flow tower configuration has been experimentally studied recently [L. Philip, Reddy, K. S., B. Kumar, B. S. Murty, A. Kannan, Performance evaluation of a solar and wind aided cross-flow evaporator for RO reject management, Desalination 317 (2013) 1–10]. A rigorous mathematical modeling and process simulation approach is now demonstrated for performance analysis and design of wind aided evaporators. Experimental conditions involving different packing configurations including sticks sourced from natural vegetation were simulated. The simulation predictions were fitted to the experimental results and the Number of Transfer Units (NTU) was identified. Under conditions of low relative humidity and high water inlet temperature, significant evaporation rates could be achieved. In drier places, the proposed concept offers considerable promise. Numerical simulation enables quick design and performance evaluation of wind aided evaporation schemes that may be incorporated in different industries facing RO reject management problems. © 2014 Elsevier B.V. All rights reserved.

An in house numerical simulation scheme of the mathematical model equations describing the novel wind aided evaporator was developed and tested under different operating conditions such as air relative humidity, water inlet temperature and packing internals. Numerical simulations serve a viable predictive tool for both design and rating of the wind aided evaporator. It also provided valuable insight into the various facets of tower operation such as internal mapping of its performance, identification of linear correlation between inlet and exit relative humidities of the air and sensitivity to variation in operating conditions. The feasibility of wind aided evaporation involving cross flow packed configuration NTU = 0.375 NTU = 0.4 36.10 32.33 28.30 4.33 35.32 32.53 28.90 4.45 34.6 32.7 29.37 4.55 revealed that considerable evaporation rates of water could be achieved under favorable conditions of high water inlet temperature and low inlet air relative humidity. Arranged packing media involving structured packing provided good contact between the water and air and enabled high evaporation rates. The Number of Transfer Units NTU indicated a simple scaling relation with the depth of the tower up to 1 m in the air flow direction as long as the air could flow freely through the tower and the wet bulb temperature limit was not reached. Inexpensive tower internals such as sticks sourced locally from natural habitat may be used in dry climates. The wind aided evaporator option may be applied as an inexpensive option in dry climes and large open spaces. Considerable scope of process enhancement exists for effective utilization of the wind aided evaporation tower volume. It is recommended to conduct experiments using the wind aided evaporator under controlled inlet conditions. This will facilitate the development of correlations for the tower characteristic parameter NTU for different packing media. Nomenclature a interfacial area for mass transfer and heat transfer per unit volume of packing (m2/m3) AARD Absolute Average Relative Deviation expressed as a percentage B width of the cross flow wind aided evaporator (m) Cp specific heat at constant pressure (J/kg·K) Cs humid heat (J/kg·K) ˙ ev energy flux across the interface (J/m2·K) G mass flux (kg/m2 s) G* dimensionless mass flux (−) H enthalpy (J/kg) h heat transfer coefficient (W/m2·K) k mass transfer coefficient (kg/m2·K) LPH liters per hour Le Lewis number (−) NTU Number of Transfer Units (−) q variable value t temperature (K) x x coordinate y y coordinate z z coordinate X dimensionless distance along the x-direction Y dimensionless distance along the y-direction Z dimensionless distance along the z-direction Subscripts a air expt. experimental G gas phase i interface p pressure sim. simulation v water vapor w water at 0 °C Greek λ ω latent heat of vaporization humidity (kg water vapor/kg dry air) Acknowledgments This research was financially supported by the Department of Environment and Forest, Tamil Nadu, India. The authors are also grateful to the Tamil Nadu Pollution Control Board, Green Textile Movement (GTM), Tamil Nadu and Free Look Fashions, Plot No. R7, State Industries Promotion Corporation of Tamil Nadu (SIPCOT), Perundurai, Erode (Dist.), Tamil Nadu for all the help provided.