International Journal Of Scientific Research And Education Volume 3 Issue 3 Pages March-2015 ISSN (e): Website:

Transcription

1 International Journal Of Scientific Research And Education Volume 3 Issue 3 Pages March-2015 ISSN (e): Website: Experimental Analyses for NDWCT Performance Using Trickle Fill Under the Effect of Cross Wind Authors Qasim Saleh Mahdi 1, Muwafaq Rahi Al-Hachami 2 1 Professor, Mechanical Engineering Dept., Al-Mustansiriyah University, Iraq 2 PhD Student, Mechanical Engineering Dept., Al-Mustansiriyah University, Iraq - qasim602006@yahoo.com, Muwafaq-phd@hotmail.com ABSTRACT In the present work, an experimental analyses for Natural Draft Wet Cooling Tower (NDWCT) performance was investigated using trickle fill under the effect of cross wind. A prototype design has been built to simulate NDWCT based on geometrical, dynamic and thermodynamic similarity. Experimental tests were done in summer season in Iraq (hot and dry weather) using (5 and 10) cm trickle fill. The mass flow rate of water has changed from (0.8 to 2.4) gpm, and cross wind has changed from (0 to 1) m/s. The obtained results clarify that cooling capacity as well as heat rejection and air enthalpy change will increase whenever fill thickness or even water flow rate are increased. However, increasing cross wind shows that a knee point is found at critical cross wind at bottom of the tower equal to (0.6) m/s. Keywords- Cooling Tower, Trickle Fill, Hot Weather, Cross Wind, Natural Draft Nomenclature d, dh top diameter (58.17m) v ref wind velocity at y ref dt throat diameter (53.5m) W vapor content in air (kgv/kga ) du base diameter (98m) y 10 m above ground level ref g gravity (9.81 kg/m 2 s) zu height from base to throat (m) M denotes for model zh height from throat to top (m) P denotes for prototype ρ density difference (kg/m3) R radius at any height Z (m) ρi inlet density (kg/m3) ν out wind velocity at outlet (m/s) INTRODUCTION Wet cooling towers are widely used all over the world due to the features of water over air. In hot weather mechanical towers are preferred where Natural Draft Wet Cooling Towers (NDWCT) s are ideal to be used Qasim Saleh Mahdi, Muwafaq Rahi Al-Hachami IJSRE Volume 3 Issue 3 March 2015 Page 2969

2 at cold and dry weather in big projects due to its high initial cost. Cold and dry weather is the best for NDWCT where hot and wet weather is the worse. The reason is, natural draft comes from heat and mass transfer between hot water (load) and air which is drafted from the gap between body of shell and earth level. Heat and mass transfer will decrease air density at higher levels of the shell than it at bottom which represents the main factor for draft force. Better heat and mass transfer, the bigger density difference. Increasing heat and mass transfer effected by many factors some of them out of control like weather conditions where other are under control like fill type and thickness, etc. Generally fills working in two ways either by scattering falling water drops into smaller ones as splash type or by making thin films of water like film fills. Trickle fill offered in many designs combining between dividing water into small drops and thin films so they have better performance comparing with other fill types. The present study done in hot and dry weather (Iraqi weather at summer as a sample). The possibility of drafting air naturally is critical. A test rig is built where load and cross wind are simulated using heaters and fans respectively. Trickle fill is used as a packing fill with (5) and (10) thicknesses to study tower parameters due to water mass flow rate change and cross wind effect. SURVEY Kloppers and Kröger [1,2] have make empirical correlations that figure out fill loss coefficient data and showed the effect of the Lewis relation, on the performance prediction of natural draft and mechanical draft wet-cooling towers.. Zhai and Fu [3] focused on the wind effect on cooling tower and tried to improve performance using wind-break walls placed at the lateral sides of cooling towers and they recover about 50% of the reduced cooling capacity. Al-Waked and Behnia [4] have used FLUENT to investigate: many design point related with falling water inside tower including nozzles number, design, drops size, and also water temperature Crosswinds velocity higher than (7.5) m/s enhanced the thermal performance of the NDWCT. Gao et al. [5], found that cross wind velocity is effective factor which may decrease or increase performance depend on velocity. When Froude number increase till (0.174), the performance decrease with increasing cross-wind velocity and after that performance will increase. Wang et al [6] monitored and experimented the thermal performance of a natural-draft wet cooling tower model with inlet airflow guiding channels under crosswinds conditions. Chen et al [7], proved that critical cross wind velocity raised due to increasing water flow rate and performance can be enhanced using cross walls controlling wind direction. EXPERIMENTAL WORK (NDWCT) rig is built in according thermo-dynamical and geometrical similarity with real tower built in Australia, where the tower height is (131) m and the fill base diameter is (98) m. This tower has been used as reference tower for many investigations before as like [8, 9]. Qasim Saleh Mahdi, Muwafaq Rahi Al-Hachami IJSRE Volume 3 Issue 3 March 2015 Page 2970

3 According to similarity theory and to simulate cooling tower, the geometry proportion of model to prototype tower is 1:100. The dimensions of the cooling tower are (58.17 cm 98 cm 131 cm) (top outlet diameter bottom diameter height). Air velocity similarity has to follow either Reynolds number or Froude number where air velocity in test rig will be higher than it at real tower following Reynolds while it will vary directly with the square root of the model scale following Froude. Experiment thermal state showed that the velocity has to achieve Fr for both the prototype and the model [5, 10], where: Δρ Δρ ΔFr vout/ gl vout/ gl (1) ρ i ρ i P M From equation (1) wind velocity has to be 1/10 times of real wind velocity. The wind velocity scale between model tower and prototype tower must be also equal, hence [10]: v v out top P v v out top M (2) Where ν top is the top level wind velocity of model tower and it is found that ν top is about double of (ν bottom ). Furthermore, v v y ref y ref Hyperbolic tower is built in real dimensions according to the following equations [11]: 0.2 (3) 4R 2 / d 2 T Z 2 / b 1 (4) Shell used in test rig is built following same equation (4) so as to be fully match according similarity. The full rig design is shown in figure (1). 2 Figure 1. Schematic diagram for experimental rig. Qasim Saleh Mahdi, Muwafaq Rahi Al-Hachami IJSRE Volume 3 Issue 3 March 2015 Page 2971

4 Hot water is used to simulate load, its temperature controlled at (50 C) by using two electrical heaters. Two fans are fixed as shown in figure (1) to simulate wind. The upper fan supply cross wind with velocity two times as the bottom fan so wind velocity is changed as (0, 0.8, 1.2, 1.6, and 2) m/s and ( 0, 0.4, 0.6, 0.8, and 1) m/s respectively. Water flow rate is change as (0.8, 1, 1.2, 1.6, 2, and 2.4) gallon per minute. Nozzles with diameter of (2) mm are used in these tests where trickle fills with (5 and 10) cm are studied. Trickle fill is available in many shapes like: cylindrical, splash or honey cells. In these tests cylindrical trickle shape is simulated using polyethylene grid with square holes of (1x1) cm, as shown in figure (2). Figure 2. (a) Cylindrical shape of trickle fills. (b) Simulated fill. (c) Polyethylene grid of trickle fill (c) RESULTS AND DISCUSSIONS Results are presented into two categories in order to study the performance of NDWCT with the variation of water mass flow rate and cross wind velocity. Many parameters of cooling towers are effected by the ratio of water mass flow rate and that is cleared in figures (3, 4, 5, 6 and 7). In (NDWCT) air is drafted due to the change in density of air at inlet and outlet. Static pressure difference between bottom and top sides of tower effects natural draft also. While water mass flow rate is controlled, air drafted naturally which means that air flow rate is not controlled and so far m w ma it is clear that is not controlled also as shown in figures (3). Different behaviors noted using different thicknesses, m w m a increases by increasing water mass flow rate at (10) cm thickness where it is fluctuated using (5) cm. Increasing fill thickness may obstruct air flow rate. Increasing water flow rate normally decreases tower range (the difference between water temperatures at inlet and outlet) if air flow is fixed but when air flow rate is not controlled, it is noticed that increasing water flow rate effecting air flow rate due to evaporation increment and all together will change tower range and effectiveness by the same way as shown in figures (4 and 5). Increasing fill thickness means increasing contact area and more contact time and so this will effect tower range behavior. Tower range maximum values are (10.8 and 11) C using (5 and 10) mm thickness respectively. It is clear from figure (6) that any increasing in water flow rate will increase humidity of air outlet. Maximum target for relative humidity is 100%, it is recorded that maximum relative humidity reached in this work are (95.5% and 97%) using (5 and 10) cm thickness respectively which give an idea about the good behavior for trickle fill in natural draft cooling tower. Air enthalpy increased by increasing Qasim Saleh Mahdi, Muwafaq Rahi Al-Hachami IJSRE Volume 3 Issue 3 March 2015 Page 2972

5 temperature, water content, or both. Increasing relative humidity due to the increase of water flow rate causes increasing in air enthalpy also as show in figure (7). As shown in figure (8), tower cooling capacity is increased with the increasing of water flow rate. Tower capacity is total heat lost from water through cooling tower; it is calculated as [12]: cp w Tower capacity (kw) mw Tin 1 mw T (5) out 1000 Where m is water consumed by evaporation which calculated by: w 2 ( d ) v W W (6) m w out 4 out in Where (W) is vapor content in air (kgv/kga) and (d) is top diameter. Figure (9) shows that heat rejected by air is increased due to the increase of tower cooling capacity. Because of exergy difference for water and air from side and losses from other side, heat rejected to air is less than tower cooling capacity for same values of water mass flow rates. Changing of Wind Velocity Results revealed that tower range as well as effectiveness, tower cooling capacity, and heat rejection are effected by wind velocity as shown in figures (10, 11, 12 and 13). Cross wind effects at both top and bottom of tower. Its direction and velocity are the more effective parameters. The effect of wind at bottom started by increasing air flow inside tower from the side where wind strike, at same time it decreases air flow to tower from the opposite side. This kind of effect destroy regularity in water falling system and in air drafting system which supposed to be symmetrical so far causing different water to air mass flow rate ratios across tower shell. Above variables effected externally by wind velocity, direction, harmony due to adjacent terrains or building. The effect of wind velocity at tower is changeable due to some tower parameters changes like shell size, fill type, fill thickness, and water flow rate. It is observed that minimum effectiveness was occurred at (0.6) m/s and this as long with that mentioned by Gao et al [5] which showed that knee point is at (0.45 m/s) for a rig has the dimensions (37 cm 68 cm 85 cm), while Al-Waked [13] showed, through a CFD analyses on tower of (129.8) m height and (95.2) m base diameter, that knee point was at (5) m/s (which equal to 0.5 m/s in terms of similarity). Cooling capacity and heat rejection showed decreasing till (0.6) m/s using (5) cm thickness where they are continue decreasing till (0.8) m/s using (10) cm thickness and then both will increase, as shown in figures (12) and (13). Qasim Saleh Mahdi, Muwafaq Rahi Al-Hachami IJSRE Volume 3 Issue 3 March 2015 Page 2973

6 Figure 3. The effect of water flow rate on the ratio of water to air flow. Figure 4. The effect of water flow rate on the tower range. Figure 5. The effect of water flow rate on effectiveness. Figure 6. The effect of water flow rate on air relative humidity. Qasim Saleh Mahdi, Muwafaq Rahi Al-Hachami IJSRE Volume 3 Issue 3 March 2015 Page 2974

7 Figure 7. The effect of water flow rate on air enthalpy change. Figure 8 The effect of water flow rate on tower cooling capacity. Figure 9. The effect of water flow rate on heat rejected to air. Figure 10 The effect of wind velocity on effectiveness. Qasim Saleh Mahdi, Muwafaq Rahi Al-Hachami IJSRE Volume 3 Issue 3 March 2015 Page 2975

8 Figure 11 The effect of wind velocity on tower range. Figure 12. The effect of wind velocity on tower cooling capacity. Figure 13. The effect of wind velocity on heat rejection. CONCLUSIONS Increasing fill thickness increases relative humidity up to saturation line faster. Increasing water mass flow rate, increases cooling capacity, heat rejected to air, and air enthalpy change also. Qasim Saleh Mahdi, Muwafaq Rahi Al-Hachami IJSRE Volume 3 Issue 3 March 2015 Page 2976

9 Cross wind effects tower parameters. Knee point is appear in effectiveness at cross wind about (0.6 m/s) for (5 and 10) cm fill thickness and it is not fixed at specific wind velocity but it changes depending on many factors including water mass flow rate, fill type, and fill thickness. REFERENCES [1] Kloppers J. C. and Kroger D. G., Loss Coefficient Correlation for Wet-Cooling Tower Fills, Applied Thermal Engineering, vol.23, pp , [2] J.C. Kloppers, D.G. Kröger, The Lewis factor and its influence on the performance prediction of wetcooling towers, International Journal of Thermal Sciences 44 (2005) [3] Zhai Z. and Fu S., Improving Cooling Efficiency of Dry-Cooling Towers Under Cross-Wind Conditions by Using Wind-Break Methods, Applied Thermal Engineering, vol.26, pp , [4] Al-Waked R. and Behnia M., CFD Simulation of Wet Cooling Towers, Applied Thermal Engineering, vol.26, pp , 2006 [5] Gao M., Sun F., Wang K., Shi Y. and Zhao Y., Experimental Research of Heat Transfer Performance on Natural Draft Counter Flow Wet Cooling Tower Under Cross-Wind Conditions, International Journal of Thermal Sciences, vol.47, pp , 2008 [6] Kai Wang, Feng-zhong Sun, Yuan-bin Zhao, Ming Gao, Lei Ruan, Experimental research of the guiding channels effect on the thermal performance of wet cooling towers subjected to crosswinds Air guiding effect on cooling tower, Applied Thermal Engineering 30 (2010) [7] Chen Y., Sun F., Wang H., Mua N. and Gao M. Experimental Research of the Cross Walls Effect on the Thermal Performance of Wet Cooling Towers under Cross Wind Conditions, Applied Thermal Engineering vol.1, pp.1-7, [8] Williamson, Behnia and S. W. Armfield, Thermal optimization of a natural draft wet cooling tower, International Journal of Energy Research, 2008; 32: [9] Williamson N., Behnia M. and Armfield S., Comparison of a 2D Axisymmetric CFD Model of a Natural Draft Wet Cooling Tower and a 1D Model, International Journal of Heat and Mass Transfer, vol.51, pp , [10] Z.-G. Zhao, Cooling Tower, China Water-Power Press, Beijing, [11] Gould Ph. L. and Wilfried B. Krätzig, Cooling Tower Structures, Structural Engineering Handbook, CRC Press LLC, [12] Lemouari M. and Boumaza M., Experimental Investigation of the Performance Characteristics of a Counter Flow Wet Cooling Tower, International Journal of Thermal Sciences, vol.49, pp , [13] Rafat Al-Waked, Crosswinds Effect on the Performance of Natural Draft Wet Cooling Towers, International Journal of Thermal Sciences, vol.49, pp , Qasim Saleh Mahdi, Muwafaq Rahi Al-Hachami IJSRE Volume 3 Issue 3 March 2015 Page 2977

10 Qasim Saleh Mahdi, Muwafaq Rahi Al-Hachami IJSRE Volume 3 Issue 3 March 2015 Page 2978