The 14th IFToMM World Congress, Taipei, Taiwan, October 25-30, 2015 DOI Number: 10.6567/IFToMM.14TH.WC.PS20.007 Water distribution below a single spray nozzle in a natural draft wet cooling tower ZHANG Guofang 1, a *, ZHENG Yuan 2, b, CHEN Qian 2, c 1 College of Mechanical & Electrical Engineering, Hohai University, Changzhou 213022, Jiangsu Province, China 2 College of Energy and Electric Engineering, Hohai University, Nanjing 210098, Jiangsu Province, China a zhang_gf@hhu.edu.cn, b zhengyuan@hhu.edu.cn, b chen15365080835@163.com Keywords: Natural draft wet cooling tower; Numerical simulation; Water distribution Abstract: The water distribution in a natural draft wet cooling tower (NDWCT) has great effect on the performance of cooling tower. This paper presents a three-dimensional two-phase simulation of flow field below a single spray nozzle with particular emphasis on the water distribution. The standard - model has been utilized in the CFD model. The simulation has adopted the Eulerian approach for both the air phase and water phase. The results show the non-uniform profile of water distribution across the radius of computational domain. The structures of the spitting device should be improved to get better distribution of sprayed water. Introduction Natural draft wet cooling towers (NDWCT) are commonly used in many industrial processes especially in thermal power stations. In NDWCT condenser feed water is sprayed on the top of fill to increase the area in direct contact with cooling air. Due to the combination of heat and mass transfer inside NDWCT, the air is warmed up and moisturized. The effect of air temperature increase and partial evaporation of water induces buoyancy which is the main force driving the air to flow upward through the towers. Meanwhile, the water will be cooled and collected at the basin of NDWCT. The energy released is discharged to the environment by air. The thermal performance of NDWCT plays an important role in the total cycle efficiency of power plants. Even a small reduction of water temperature at outlet can lead to a great saving in fuel costs. Therefore it is necessary to study the operation of the cooling tower and to optimize its design parameters. The main design parameters which influence the thermal performance of NDWCT include geometric dimensions (i.e. fill depth, tower inlet height), operating parameters (i.e. water flow rate, air flow rate) and ambient conditions. The basis of simulation is the Merkel method[1] and its modifications, the e-ntu[2] and Poppe method[3]. Being low-dimensional models of heat and mass transfer, they are not able to take more complex factors into account, such as shape of the tower fill, flow non-uniformities and the effect of cross-wind. Many efforts have been made over the past decades to improve the performance of wet cooling systems, including mathematical model development [4, 5], exergy research[6, 7], drop size distribution below fills[8], influence of cross-wind[9-11] and so on. In this paper a three-dimensional model is developed and the flow characteristics of the field near a single spray nozzle under various conditions are analyzed and numerically simulated. Mathematical models In the multiphase model two phases are distinguished. The primary phase is air and the secondary
phase is water. The governing equations are conservation equations of continuity, momentum for each phase and phase volume fractions. The standard - model is used to describe the turbulence and requires solving 2 additional transport equations for and. The volume fraction of each phase must satisfy, where i is the number of phases. For air-water two-phase flow (1) where and are the volume fraction of air phase and water phase respectively. The effective density of a single phase is defined as (2) where is the physical density of phase i. The continuity equations can be written respectively for the primary phase and the secondary phase as follows ( ) ( ) (3) (4) where and are the velocity vectors of air phase and water phase. The momentum equation is ( ) ( ) (5) where p is the pressure, is the stress-strain tensor, g is the vector of gravitational acceleration, is an interaction force between phases and are additional forces acting on phase i. The stress-strain tensor of phase i is defined as ( ) ( ) (6) where and are shear and bulk viscosity of phase i respectively. Boundary Conditions The cylindrical numerical domain shown in Fig. 1 has a height of 1.4m and a radius of 3m. A nozzle with an impeller is located in the center of the domain. The water sprayed from the nozzle is reflected when in contact with the plate and pushes continuous rotating of the impeller. (a) Fig. 1. General view of the current numerical domain: (a) boundary conditions, (b) spitting device structure. (b)
The ground is defined as outflow boundary condition and the velocity boundary condition is used to specify the velocity of the cooling water flow at the inlet of the physical domain. The plate and the impeller are all set to be the reflect boundary to simplify the calculation, although water may attach to the wall in actual situation. Other surfaces are defined as no-slip wall condition. Fig. 2. Computational grids The quality of the mesh greatly influences both the accuracy and the convergence rate of the iterative solution. The computational domain consists of 1,216,100 structured and unstructured mesh elements with cell size ranging between 0.0015m and 0.1m. The grid is refined in the region close to the nozzle. The finite-volume approach is utilized to discrete the governing equations and the SIMPLE algorithm is adopted to compute. Results and discussion As known to all, for the same water flow rate, the total surface area of droplets with smaller diameters is much bigger, which leads to better heat and mass transfer between water and air. Water droplets inside NDWCT have different sizes depending on the structure and characteristics of the spray nozzle. And the distribution of the sprayed water on top of fills is also an important aspect of tower performance. In the following section, the distribution of the cooling water at various height and location under different operating conditions are discussed in detail. Water distribution For example, when the velocity of water at inlet is 1.484m/s and the speed of impeller is 2 rad/s, contours of water volume fraction in the plane x=0m (symmetrical plane) is shown in Figure 3. It can be seen that the spitting device with rotating speller leads to large scope of spayed water, but the water distribution in the central region is so poor that can result in a reduction below the design performance of NDWCT. Fig. 3. Contours of water volume fraction in the plane x=0m
0.00 0.44 0.78 1.16 1.53 1.92 2.29 2.78 3.17 3.51 3.93 4.39 4.75 5.14 5.56 5.88 Volume fraction of water The water volume fraction contours, shown in Figure 4, depicts the non-uniformity in water distributions in the planes of y=-0.5m and y=-0.8m. It can be seen from Figure 3(a) and 3(b) that the sprayed water concentrates in several areas around the center at different heights. Along the radial direction, the concentration of water increases to the top and then decreases. The structure of spitting device should be accountable for the non-uniformity of water distribution in NDWCT which leads to non-uniform heat and mass transfer. (a) y=-0.5m (b) y=-0.8m Fig. 4. Contours of water volume fraction in the planes: (a) y=-0.5m, (b)y=-0.8m The diametric profile of water volume fraction on the central lines at 3 different heights is depicted in Figure 5. It can be found that the volume fraction of water is lower in the center, then increases while away from the center. After reaching the top values, the volume fraction decreases rapidly being close to zero. 0.002 0.0015 0.001 0.0005 0 y=-0.2m y=-0.5m y=-0.8m Length in diameter direction Fig. 5 Diametric profile of volume fraction of water on central lines at 3 different heights Velocity Magnitude Figure 6 shows the velocity magnitude of water in the plane x=0m. Figure 7 is the detail of part 1 and part 2 in Figure 6 with enlarged scale. It can be seen from Figure 6 especially from Figure 7 that there are obvious eddy in some region. The reasons for this phenomena are the restraint of the boundary condition and the difference of pressure in the domain.
Fig. 6 The velocity magnitude of water in the plane x=0m (Unit: m/s) (a) Part 1 (b) Part 2 Fig. 7 The detail of part 1 and part 2 in Fig. 5 with enlarged scale Pressure field As shown in Figure 8, the static pressure on the front blade is more well-distributed and higher than the static pressure on the back blade. Therefore, the blade is given forces from back to front which pushes impeller rotating. The simulation results of the pressure field agree with hydrodynamics laws and verify the validity of the model and the method.
(a) back (b) front Fig. 8. Pressure distribution of impeller blade (Unit: Pa) Conclusion The purpose of research in this article is to appraise the performance of a certain spitting device under the criteria of design conditions. The results show that the range of sprayed water is large but the distribution is non-uniform. The structure of the device especially the three support legs have resulted in the non-uniformity and should be improved to avoid negative influence on the performance of heat and mass transfer in NDWCT. The proposed approach gave insight to the process revealing the spatical distributions of the water and could be applied in other research of the cooling towers. References [1]. Merkel F. Verdunstungskühlung[J]. VDI-Zeitchrift. 1925,70:123-8. [2]. Jaber H, Webb R. Design of Cooling Towers by the Effectiveness-NTU Method[J]. Journal of Heat Transfer. 1989,111:837-43. [3]. Poppe M, Rögener H. Berechnung von Rückkühlwerken[J]. VDI-Wärmeatlas. 1991,111:1-15. [4]. Kloppers JC, Kröger DG. A critical investigation into the heat and mass transfer analysis of counterflow wet-cooling towers[j]. International Journal of Heat and Mass Transfer. 2005,48(3-4):765-77. [5]. Asvapoositkul W, Treeutok S. A simplified method on thermal performance capacity evaluation of counter flow cooling tower[j]. Applied Thermal Engineering. 2012,38:160-7. [6]. Wang L, Li N. Exergy transfer and parametric study of counter flow wet cooling towers[j]. Applied Thermal Engineering. 2011,31(5):954-60. [7]. Muangnoi T, Asvapoositkul W, Wongwises S. An exergy analysis on the performance of a counterflow wet cooling tower[j]. Applied Thermal Engineering. 2007,27(5-6):910-7. [8]. Terblanche R, Reuter HCR, Kröger DG. Drop size distribution below different wet-cooling tower fills[j]. Applied Thermal Engineering. 2009,29(8 9):1552-60. [9]. Goodarzi M, Ramezanpour R. Alternative geometry for cylindrical natural draft cooling tower with higher cooling efficiency under crosswind condition[j]. Energy Conversion and Management. 2014,77:243-9. [10]. Gao M, Sun F-z, Turan A. Experimental study regarding the evolution of temperature profiles inside wet cooling tower under crosswind conditions[j]. International Journal of Thermal Sciences. 2014,86:284-91.
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