Results 1 - 10 of 47
Results 1 - 10 of 47. Search took: 0.019 seconds
|Sort by: date | relevance|
[en] Highlights: • Field test was conducted on high-level cooling tower under crosswind conditions. • Inflow air uniformity coefficient decreases with the rising of crosswind velocity. • The uniformity coefficient decreases to 0.61 and 0.49 under θ1 = 5° and θ2 = 35°. • In 3.74 m/s, the ventilation rate reduces by 30.13% under 5° condition. • The ventilation performance is better under 5° condition. - Abstract: Field test was performed on the high level water collecting wet cooling towers (HWCTs) of a 1000 MW unit to investigate ventilation performance under crosswind conditions, the circumferential inflow air distribution rules and ventilation rate were analyzed in this paper. The test results manifest that crosswind destroys the uniformity of circumferential inflow air, increases the wind velocity in the windward side, and reduces wind velocity in the lateral and leeward side. Moreover, the uniformity coefficient of circumferential inflow air and ventilation rate continuously decrease with the increasing of crosswind velocity. In this study, θ represents the angle between cross walls and crosswind direction. When crosswind velocity reaches to 3.74 m/s, the uniformity coefficient decreases to 0.61 and 0.49 under θ1 = 5° and θ2 = 35°. Compared with 0.28 m/s condition, the ventilation rate reduces by 30.13% under θ1 = 5° and 34.36% under θ2 = 35°. Additionally, at the same crosswind velocity, the smaller the θ is, the better the ventilation performance becomes. Compared with θ2 = 35°, the uniformity of circumferential inlet air is better and the ventilation rate is larger than that under θ1 = 5° condition.
[en] A thermodynamic analysis of the counter flow wet cooling tower (CWCT) is performed in this paper. Both energy and exergy formulations are developed and validated for the system. Four types of exergy transfer processes occurring inside the CWCT are investigated schematically. A parametric study is conducted under various operating conditions in order to investigate the effects of thermal efficiency and water-to-air ratio on the exergy performance of the CWCT. Unlike past studies, the transiting exergy contained in the inlet and outlet water is not considered. It is found that the exergy efficiency is always less than 25%. The exergy parameters including evaporation water loss, exergy efficiency, exergy input, internal and external exergy losses are very sensitive to the thermal efficiency when it is very close to 1.0 at lower water-to-air ratios. - Research highlights: → We model counter flow wet cooling towers and make a detailed exergy analysis. → Four types of exergy transfer processes are investigated schematically. → Only a small part of exergy input, less than 25%, is effectively utilized.
[en] Highlights: • A half-cylindrical NDDCT CFD model is built and validated by a hot state test rig. • FLF is derived and verified to quantitatively describe flow characteristics effect. • Rear side flow separation area and main stream vortices are critical to ventilation. • The contributions of each flow field region on ventilation degradation is exhibited. • Flow convergence are transformed to be vortices barrier under high crosswind. - Abstract: The natural draft dry cooling tower (NDDCT) is a critical facility for an indirect dry cooling power plant in arid area for its merit of excellent water-saving. While crosswind degrades the performance of a NDDCT by changing the flow field inside and outside. In order to quantitatively study the influence of different flow characteristics on the performance of a NDDCT, hence to grasp the affecting mechanism of crosswind, a half-cylindrical computational fluid dynamics (CFD) model of a Heller type 660 MW NDDCT is developed and validated by a hot state modelling test rig. A flow loss factor (FLF) is derived and verified to linearly describe the effect of local flow field changing on the overall performance of a NDDCT. Based on the conjoint studies of the local FLF variation trends and the changing processes of correspondent flow characteristics in each specific flow segments, the critical factors influencing the performance of a NDDCT are identified under different crosswind conditions.
[en] In the present study, we have carried out an investigation into some of the environmental effects of high energy efficiency power generation plants that produce electrical and thermal energy. The use of natural gas fuels may represent a success for the replacement of liquid or solid fuels, because of drastic reductions of sulphur oxides, carbon dioxide and particulates. An additional problem involves the study of the effects of vapour emissions from the combustion of fuels and from the cooling towers of the condenser device. Particularly, the problem of the analysis of vapour and drift diffusion, because of emissions from the wet cooling devices, is faced in terms of modelling the plume rise and mass loss of drift in the atmosphere, because of evaporation, in order to predict the increment of rain and the humidity effect. The results obtained show that the amount of water emitted in the forms of vapour and drift, in terms of local scale diffusion, do not create problems of high increments of relative humidity or dangerous rainfall phenomena; while the effects of vapour emission are shown as insignificant problems because of the deposition of a sensible amount of drift, correlated with the type of of separator devices chosen can appear, but only on very limited areas. (author)
[en] Highlights: • Aerodynamic field around dry cooling tower is presented with numerical model. • Performances of cooling deltas are figured out by air inflow velocity analysis. • Setting angles of wind-break walls are optimized to improve cooling performance. • Optimized walls can reduce the interference on air inflow at low wind speeds. • Optimized walls create stronger outside secondary flow at high wind speeds. - Abstract: To get larger cooling performance enhancement for natural draft dry cooling tower with vertical cooling deltas under crosswind, setting angles of wind-break walls were optimized. Considering specific structure of each cooling delta, an efficient numerical model was established and validated by some published results. Aerodynamic fields around cooling deltas under various crosswind speeds were presented, and outlet water temperatures of the two columns of cooling delta were exported as well. It was found that for each cooling delta, there was a difference in cooling performance between the two columns, which is closely related to the characteristic of main airflow outside the tower. Using the present model, air inflow deviation angles at cooling deltas’ inlet were calculated, and the effects of air inflow deviation on outlet water temperatures of the two columns for corresponding cooling delta were explained in detail. Subsequently, at cooling deltas’ inlet along radial direction of the tower, setting angles of wind-break walls were optimized equal to air inflow deviation angles when no airflow separation appeared outside the tower, while equal to zero when outside airflow separation occurred. In addition, wind-break walls with optimized setting angles were verified to be extremely effective, compared to the previous radial walls.
[en] Highlights: • A mass transfer coefficient correlation is developed for ceramic foam packing. • The correlation is evaluated by experimental data. • The effects of pressure and inlet conditions on heat and mass transfer are analyzed. - Abstract: The cooling tower model is simple but has necessary precision for the packing design and outlet conditions forecast of the humidifier, especially under the pressure and temperature level of micro humid air turbine. This research focuses on developing a mass transfer correlation of the novel ceramic foam packing, which has the potential to be used in humidification. The heat and mass transfer properties of the packing are investigated with the mass transfer coefficients calculated from 105 groups of experimental data, and the effects of pressure, water/air mass flow ratio, inlet water temperature and inlet air enthalpy are analyzed. It is shown that the mass transfer coefficient increases with the increase of water/air mass flow ratio, but decreases with the increase of inlet water temperature and inlet air enthalpy. The effect of pressure on heat and mass transfer is related with air mass flow. A dimensionless group correlation of mass transfer coefficient is developed and evaluated. It is shown that the deviations between the predicted and experimental values are estimated within ±12% for 75% experimental conditions. The developed correlation can be used to predict the packing height and the water/air outlet conditions of the ceramic foam packing humidifier.
[en] This paper presents an experimental investigation of the thermal performances of a forced draft counter flow wet cooling tower filled with an 'VGA' (Vertical Grid Apparatus) type packing. The packing is 0.42 m high and consists of four (04) galvanised sheets having a zigzag form, between which are disposed three (03) metallic vertical grids in parallel with a cross sectional test area of 0.0222 m2 (0.15 m x 0.148 m). This study investigates the effect of the air and water flow rates on the cooling water range as well as the tower characteristic, for different inlet water temperatures. Two operating regimes were observed during the air water contact, a pellicular regime (PR) and a bubble and dispersion regime (BDR). These two regimes can determine the best way to promote the heat transfer. The BDR regime seems to be more efficient than the pellicular regime, as it enables to cool larger water flow rates. The comparison between the obtained results and those found in the literature for other types of packing indicates that this type possesses very interesting thermal performances
[en] Due to limited space and/or improper placement of evaporative cooling towers, discharge recirculation likely occurs in practical applications. The air recirculation may adversely affect energy efficiency of the chilling plants and increase the potential of visible plume around the towers. In this study, the amount of recirculation in a counter-flow cooling tower is evaluated by computational fluid dynamics (CFD) simulation tests under different enclosure structures and crosswind conditions. Then the effects of recirculation in cooling towers on energy performance of a chilling plant and plume potential are investigated. The evaluation is conducted on a dynamic simulation platform using the weather data in a typical meteorological year of Hong Kong. Results show that crosswind can enhance recirculation in cooling towers under lower air flow rate conditions. The recirculation ratio can reach up to 15%. Results also reveal that air recirculation in cooling towers could result in the increase of overall chilling plant energy consumption by over 1.5%. The recirculation also results in significant increase of plume occurrence frequency, particularly in spring season. - Highlights: ► Discharge recirculation in a cooling tower is evaluated by CFD modeling. ► The recirculation ratio can reach up to 15%. ► The recirculation can increase the overall chilling plant energy consumption by over 1.5%. ► The recirculation can significantly increase the plume occurrence frequency.
[en] Absorption refrigeration systems are an alternative to vapor compression ones in cooling and refrigeration applications. In comparison with single effect absorption units, double effect systems have improved performance. Also, they are more available commercially than the other multi effect absorption cycles. An important challenge in the operation of such systems is the possibility of crystallization within them. This is especially true in developing air-cooled absorption systems, which are attractive because cooling tower and associated installation and maintenance issues can be avoided. Therefore, distinguishing the working conditions that may cause crystallization can be useful in the design and control of these systems. In this paper a computational model has been developed to study and compare the effects of operating parameters on crystallization phenomena in three classes of double effect lithium bromide-water absorption refrigeration systems (series, parallel and reverse parallel) with identical refrigeration capacities. It is shown that the range of operating conditions without crystallization risks in the parallel and the reverse parallel configurations is wider than those of the series flow system. - Highlights: → We study crystallization of double effect absorption refrigeration systems. → We consider series, parallel and reverse parallel cycles. → We study the effect of operating conditions on crystallization. → We choose optimum distribution ratio for parallel and reverse parallel systems. → Crystallization possibility is low in parallel and reverse parallel cycles.
[en] In this paper a thermodynamic assessment and a preliminary cost evaluation are given for an evaporative gas turbine (EvGT) cycle packed humidifier. Both background theory and simulation results are included. Two different approaches were used for the humidifier system modelling: the full integration of the mass-energy balance and mass transfer equations (called SAT model), and an atmospheric cooling tower-based model (called CT model). Both approaches were used to perform component thermodynamic analyses and to determine the humidifier packing design. Within these approaches, two simulation cases are discussed: a test case, with experimental results from the pilot-plant of the University of Lund, and a case study of the saturators for the optimised HAT (humid air turbine) cycles of a plant with a 50 MW power output. The two cases presented consider two different operating conditions for the saturator: the first being a 'non-optimised' saturator, and the later the 'optimal' configuration with reduced exergetic losses. For the case study, the saturator design and cost evaluation are also included. All simulation results were performed with the in-house SAT (SATurator simulation tool) code