Numerical simulation of counter-flow wet-cooling towers (original) (raw)
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Performance characteristics of counter flow wet cooling towers
Energy Conversion and Management, 2003
Cooling towers are one of the biggest heat and mass transfer devices that are in widespread use. In this paper, we use a detailed model of counter flow wet cooling towers in investigating the performance characteristics. The validity of the model is checked by experimental data reported in the literature. The thermal performance of the cooling towers is clearly explained in terms of varying air and water temperatures, as well as the driving potential for convection and evaporation heat transfer, along the height of the tower. The relative contribution of each mode of heat transfer rate to the total heat transfer rate in the cooling tower is established. It is demonstrated with an example problem that the predominant mode of heat transfer is evaporation. For example, evaporation contributes about 62.5% of the total rate of heat transfer at the bottom of the tower and almost 90% at the top of the tower. The variation of air and water temperatures along the height of the tower (process line) is explained on psychometric charts.
A critical investigation into the heat and mass transfer analysis of counterflow wet-cooling towers
Numerical Heat Transfer, Part A: …, 2004
This study gives a detailed derivation of the heat and mass transfer equations of evaporative cooling in wet-cooling towers. The governing equations of the rigorous Poppe method of analysis are derived from first principles. The method of Poppe is well suited for the analysis of hybrid cooling towers as the state of the outlet air is accurately predicted. The governing equations of the Merkel method of analysis are subsequently derived after some simplifying assumptions are made. The equations of the effectiveness-NTU method applied to wet-cooling towers are also presented. The governing equations of the Poppe method are extended to give a more detailed representation of the Merkel number. The differences in the heat and mass transfer analyses and solution techniques of the Merkel and Poppe methods are described with the aid of enthalpy diagrams and psychrometric charts. The psychrometric chart is extended to accommodate air in the supersaturated state.
A comprehensive design and performance evaluation study of counter flow wet cooling towers
International journal of refrigeration, 2004
Fouling of cooling tower fills is one of the most important factors affecting its thermal performance, which reduces cooling tower effectiveness and capability with time. In this paper, the fouling model presented in an earlier paper using the experimental data on fill fouling, is used to investigate the risk based thermal performance of the cooling tower. It is demonstrated that effectiveness of the cooling tower degrades significantly with time indicating that for a low risk level ðp ¼ 0:01Þ; there is about 6.0% decrease in effectiveness for the given fouling model. The sensitivity analysis of the cooling tower is investigated for both rating and design calculation for different values of mass flow rate ratios. The effect of atmospheric pressure on the thermal performance of the cooling tower is also demonstrated. q Etude permettant d'évaluer la conception et al performance des tours de refroidissement humides à contre courant
Thermodynamic study of wet cooling tower performance
International Journal of Energy Research, 2006
An analytical model was developed to describe thermodynamically the water evaporation process inside a counter-flow wet cooling tower, where the air stream is in direct contact with the falling water, based on the implementation of the energy and mass balance between air and water stream describing thus, the rate of change of air temperature, humidity ratio, water temperature and evaporated water mass along tower height. The reliability of model predictions was ensured by comparisons made with pertinent experimental data, which were obtained from the literature. The paper elaborated the effect of atmospheric conditions, water mass flow rate and water inlet temperature on the variation of the thermodynamic properties of moist air inside the cooling tower and on its thermal performance characteristics. The analysis of the theoretical results revealed that the thermal performance of the cooling tower is sensitive to the degree of saturation of inlet air. Hence, the cooling capacity of the cooling tower increases with decreasing inlet air wet bulb temperature whereas the overall water temperature fall is curtailed with increasing water to air mass ratio. The change of inlet water temperature does not affect seriously the thermal behaviour of the cooling tower.
Thermal performance analysis of a closed wet cooling tower
Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering, 2007
A detailed model was developed and employed to examine the thermal performance of a closed wet cooling tower. The model is capable of predicting the variation of air thermodynamic properties, sprayed and serpentine water temperature as well as heat transfer rates exchanged between air and falling water stream inside the indirect wet cooling tower. The reliability of simulations was tested against experimental data obtained from the literature. A parametric study was conducted to evaluate the thermal behaviour of the indirect cooling tower under various air mass flowrates, serpentine water mass flowrates and inlet temperatures. The results of the theoretical investigation revealed an increase in cooling capacity and percentage loss of sprayed water due to evaporation, with increasing air mass flowrate. On the other hand, the increase of serpentine water mass flowrate resulted in slight increase in the overall temperature reduction of serpentine water. The effect of variable serpentine water inlet temperature on thermal performance of the indirect wet cooling tower was insignificant compared to other cases. Thermal performance analysis of a closed wet cooling tower Proc. IMechE Vol. 221 Part E: J. Process Mechanical Engineering JPME119 © IMechE 2007 Thermal performance analysis of a closed wet cooling tower JPME119
Applied Thermal Engineering, 2012
A thermodynamic model was developed and used to assess the sensitivity of thermal performance characteristics of a closed wet cooling tower to inlet air conditions. In the present study, three cases of different ambient conditions are considered: In the first case, the average mid-winter and mid-summer conditions as well as the extreme case of high temperature and relative humidity, in Athens (Greece) during summer are considered according to the Greek Regulation for Buildings Energy Performance. In the second case, the varied inlet air relative humidity while the inlet air dry bulb temperature remains constant were taken into account. In the last case, the effects on cooling tower thermal behaviour when the inlet air wet bulb temperature remains constant were examined. The proposed model is capable of predicting the variation of air thermodynamic properties, sprayed water and serpentine water temperature inside the closed wet cooling tower along its height. The reliability of simulations was tested against experimental data, which were obtained from literature. Thus, the proposed model could be used for the design of industrial and domestic applications of conventional air-conditioning systems as well as for sorption cooling systems with solid and liquid desiccants where closed wet cooling towers are used for precooling the liquid solutions.
CFD simulation of wet cooling towers
Applied Thermal Engineering, 2006
Heat and mass transfer inside a natural draft wet cooling tower (NDWCT) have been investigated numerically under different operating and crosswind conditions. The three-dimensional CFD model has utilized the standard k-e turbulence model as the turbulence closure. The current simulation has adopted both the Eulerian approach for the air phase and the Lagrangian approach for the water phase. The film nature of the water flow in the fill zone has been approximated by droplets flow with a given velocity. The required heat and mass transfer have been achieved by controlling the droplet velocity. At that specific droplet velocity, effects of the following operating parameters on the thermal performance of the NDWCT have been investigated: droplet diameter, inlet water temperature, number of nozzles, water flow rate and number of tracks per nozzle. As a result, the effect of crosswind velocity on the thermal performance has been found to be significant. Crosswinds with velocity magnitude higher than 7.5 m/s have enhanced the thermal performance of the NDWCT. (M. Behnia). 1 Tel.: +61 2 9036 9518; fax: +61 2 9036 9519; Mobile: +61 414 369 518. www.elsevier.com/locate/apthermeng Applied Thermal Engineering 26 (2006) 382-395
Performance Assessment of a Counter Flow Cooling Tower – Unique Approach
Energy Procedia, 2017
Cooling tower is one of the most suitable system for evaporative cooling of hot water in comparison with other types of evaporative cooling systems. In the present study, a finite difference model is developed for predicting the characteristics of coupled heat and mass transfer processes occurring in a counter flow forced draft cooling tower. This model consists of thermal effectiveness, height of the tower and moisture effectiveness as variable parameters, and provides a correlation for heat and mass transfer coefficients in order to obtain the desired performance parameters. The predicted results for evaporative cooling process are in good agreement with the experimentally measured data. The mathematical model developed in this study can be used as a tool for predicting the cooling tower performance characteristics.
Numerical model of evaporative cooling processes in a new type of cooling tower
International Journal of Heat and Mass Transfer, 2005
A numerical model for studying the evaporative cooling processes that take place in a new type of cooling tower has been developed. In contrast to conventional cooling towers, this new device called Hydrosolar Roof presents lower droplet fall and uses renewable energy instead of fans to generate the air mass flow within the tower. The numerical model developed to analyse its performance is based on computational flow dynamics for the two-phase flow of humid air and water droplets. The Eulerian approach is used for the gas flow phase and the Lagrangian approach for the water droplet flow phase, with two-way coupling between both phases. Experimental results from a full-scale prototype in real conditions have been used for validation. The main results of this study show the strong influence of the average water drop size on efficiency of the system and reveal the effect of other variables like wet bulb temperature, water mass flow to air mass flow ratio and temperature gap between water inlet temperature and wet bulb temperature. Nondimensional numerical correlation of efficiency as a function of these significant parameters has been calculated.
A complete model of wet cooling towers with fouling in fills
Applied thermal engineering, 2006
A cooling tower basically consists of three zones; namely, spray zone, packing and rain zones. In cooling towers, a significant portion of the total heat rejected may occur in the spray and rain zones. These zones are modeled and solved simultaneously using engineering equation solver (EES) software. The developed models of these zones are validated against experimental data. For the case study under consideration, the error in calculation of the tower volume is 6.5% when the spray and rain zones are neglected. This error is reduced to 3.15% and 2.65% as the spray and rain zones are incorporated in the model, respectively. Furthermore, fouling in cooling tower fills as well as its modeling strategy is explained and incorporated in the cooling tower model to study performance evaluation problems. The fouling model is presented in terms of normalized fill performance index (g F,norm ) as a function of weight gain due to fouling. It is demonstrated that the model is asymptotic, which is similar to typical asymptotic fouling model used in conventional heat exchangers.