CFD SIMULATION OF COOLING TOWERS

Minor project onCFD SIMULATION OfNatural draft wet COOLING TOWERSBy:Sachin Sourabh(2k11/ME/124)Sagar Rastogi(2k11/ME/125)Sambhav Jain(2k11/ME/127)

ContentsAbstract IntroductionLiterature ReviewCooling Tower Basics Components of Cooling Tower TERMINOLOGIES USED IN COOLING TOWERSMerkel TheoryComputational fluid dynamics Definition History Fundamentals of CFD Work to be doneConclusionAbstractCooling towers are essentially heat removing devices that removes waste heat to atmosphere. They are widely used in a wide range of areas like oil refineries, petrochemical and other chemical plants, thermal power stations and HVAC systems for cooling buildings. A natural draft wet cooling tower uses natural air drift for movement of air. It is mainly used in power stations. A lot of research goes around too make these cooling towers more efficient. The importance of NDWCT efficiency can be drawn from the fact that a single degree rise in outlet water in a power plant substantially increases the power production cost.Computational fluid Dynamics offers a platform to perform simulation of a working cooling tower and checking its various parameters without actually modelling an actual one. Currently only 1d and 2d simulations of NDWCT is being used in cooling tower design. This study aims to study the various models that are currently used to design cooling towers and to do a literature research work that has been done in the CFD simulation of different cooling tower designs and to find out the prospects of future work that can be done in order to get a better, more efficient and commercially viable design. The main contribution of the project is to answer several important questions relating to natural draft wet cooling tower (NDWCT) modelling, design and optimisation.Specifically, the current work aims to conduct a detailed analysis of NDWCT and basic knowledge of Computational Fluid Dynamics (CFD). A general study of various cooling towers along with their common parts has been done. Followed by specific and detailed study of NDWCT along with its main parts like structure, fills and its types, drift eliminators, water basin and water spray system etc.IntroductionCooling towers are an integral part of many industrial processes. Their purpose is to reject waste heat. They are often used in power generation plants to cool the condenser feed-water as shown in Fig. Here, the cooling tower uses ambient air to cool warm water from the condenser in a secondary cycle.

classificationDry and Wet : In dry cooling towers the water is passed through finned tubes forming a heat exchanger so only sensible heat is transferred to the air. In wet cooling towers the water is sprayed directly into the air so evaporation occurs and both latent heat and sensible heat are exchanged. In a hybrid tower a combination of both approaches are used. In a hybrid tower a combination of both approaches are used.Forced or Natural Draft towers : Forced Draft towers tend to be relatively small structures where the air flow is driven by fan.In a natural draft cooling tower the air flow is generated by natural convection only. The draft is established by the density difference between the warm air inside the tower and the cool dense ambient air outside the tower.Counter-flow and Cross-flow cooling towers. In cross-flow configuration, the air flows at some angle to water flow.In counter-flow the air flows in the opposite direction to water flow.

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Natural draft wet cooling towers (NDWCT) in counter-flow configurationAir flow through this tower is produced by the density differential that exists between the heated (less dense) air inside the stack and the relatively cool (more dense) ambient air outside the tower.They are used extensively in the field of electric power generation, where large unified heat loads exist.In a NDWCT in counter flow configuration, there are three heat and mass transfer zones, the spray zone, the fill zone and the rain zone.

Components of Cooling TowerThe basic components of an evaporative tower are: Frame and casing, fill, cold water basin, drift eliminators, air inlet, louvers, nozzles and fans. Frame and casing: Most towers have structural frames that support the exterior enclosures (casings), motors, fans, and other components. With some smaller designs, such as some glass fiber units, the casing may essentially be the frame. Fill: Most towers employ fills (made of plastic or wood) to facilitate heat transfer by maximising water and air contact. Fill can either be splash or film type. With splash fill, water falls over successive layers of horizontal splash bars, continuously breaking into smaller droplets, while also wetting the fill surface. Plastic splash fill promotes better heat transfer than the wood splash fill. Film fill consists of thin, closely spaced plastic surfaces over which the water spreads, forming a thin film in contact with the air. These surfaces may be flat, corrugated, honeycombed, or other patterns. The film type of fill is the more efficient and provides same heat transfer in a smaller volume than the splash fill.Components of Cooling TowerCold water basin: The cold water basin, located at or near the bottom of the tower, receives the cooled water that flows down through the tower and fill. The basin usually has a sump or low point for the cold water discharge connection. In many tower designs, the cold water basin is beneath the entire fill. Drift eliminators: These capture water droplets entrapped in the air stream that otherwise would be lost to the atmosphere. Air inlet: This is the point of entry for the air entering a tower. The inlet may take up an entire side of a towercross flow design or be located low on the side or the bottom of counter flow designs. Louvers: Generally, cross-flow towers have inlet louvers. The purpose of louvers is to equalize air flow into the fill and retain the water within the tower. Many counter flow tower designs do not require louvers. Nozzles: These provide the water sprays to wet the fill. Uniform water distribution at the top of the fill is essential to achieve proper wetting of the entire fill surface. Nozzles can either be fixed in place and have either round or square spray patterns or can be part of a rotating assembly as found in some circular cross-section towers. TERMINOLOGIES USED IN COOLING TOWERSDry-bulb temperature.Dry-bulb temperature (tdb) usually referred to as the air temperature is the property of air that is most commonly used. When people refer to the temperature of the air, they are normally referring to its dry-bulb temperature. The dry-bulb temperature is an indicator of heat content and is shown along the bottom axis of a psychometric chart. The vertical lines extending upward from this axis are constant-temperature lines.Wet-bulb temperature.Wet-bulb temperature (twb) is the reading when the bulb of a thermometer is covered with a wet cloth, and the instrument is whirled around in a sling. The wet-bulb temperature is the lowest temperature that can be reached by evaporation of water only.Relative humidity (RH). RH is the ratio of the partial pressure of water vapor in air over the saturation vapor pressure at a given temperature. When the relative humidity is 100%, the air is saturated and therefore, water will not evaporate further. Therefore, when the RH is 100% the wet-bulb temperature is the same as the dry-bulb temperature, because the water cannot evaporate any more.Range.The range is the difference in temperature of inlet hot water (t2) and outlet cold water (t1),t2t1. A high cooling-tower range means that the cooling tower has been able to reduce the water temperature effectively.TERMINOLOGIES USED IN COOLING TOWERSApproach.The approach is the difference in temperature of outlet cold water and ambient wet-bulb temperature,t1tw. The lower the approach, the better the cooling tower performance. Although both range and approach should be monitored, the approach is a better indicator of cooling tower performance.

Cooling tower capability.The capability of the cooling tower is a measure of how close the tower can bring the hot water temperature to the wet-bulb temperature of the entering air. A larger cooling tower (that is, more air or more fill) will produce a closer approach (colder outlet water) for a given heat load, flowrate and entering air condition. The lower the wet-bulb temperature, which indicates either cool air, low humidity or a combination of the two, the lower the cooling tower can cool the water. Capability tests are conducted per the ATC-105 Code of the Cooling Tower Institute. The thermal performance of the cooling tower is thus affected by the entering wet-bulb temperature. The entering air dry-bulb temperature has an insignificant effect on thermal performance.

Effectiveness.A cooling towers effectiveness is quantified by the ratio of the actual range to the ideal range, that is, the difference between cooling water inlet temperature and ambient wet-bulb temperature. It is defined in terms of percentage.

TERMINOLOGIES USED IN COOLING TOWERSLiquid-to-gas ratio (L/G).The L/G ratio of a cooling tower is the ratio of the liquid (water) mass flow rate (L) to gas (air) mass flow rate (G). Cooling towers have certain design values, but seasonal variations require adjustment and tuning of water and air flow rates to get the best cooling tower effectiveness.Number of transfer units (NTU).Also called the tower coefficient, the NTU is a numerical value that results from theoretical calculations based on a set of performance characteristics. The value of NTU is also representative of the degree of difficulty for the cooling process. The NTU corresponding to a set of hypothetical conditions is called therequired coefficientand is an evaluation of the problem. The same calculations applied to a set of test conditions is called theavailable coefficientof the tower involved. The available coefficient is not a constant but varies with operating conditions. The operating characteristic of a cooling tower is developed from an empirical correlation that shows how the available coefficient varies with operating conditions.Cooling capacity.The cooling capacity of a tower is the heat rejected [kcal/h or TR (refrigeration tons; 1 TR = 12,000 Btu/h = 3,025.9 kcal/h)], and is determined by the product of mass flow rate of water, times the specific heat times the temperature difference.

Local Cooling Tower Theory

Heat is transferred from water drops to the surrounding air by the transfer of sensible and latent heat

Merkel theoryThe early investigators of cooling tower theory grappled with the problem presented by the dual transfer of heat and mass. The Merkel theory overcomes this by combining the two into a single process based on enthalpy potential.Dr. Merkel developed a cooling tower theory for the mass (evaporation of a small portion of water) and sensible heat transfer between the air and water in a counter flow cooling tower. The theory considers the flow of mass and energy from the bulk water to an interface, and then from the interface to the surrounding air mass. The flow crosses these two boundaries, each offering resistance resulting in gradients in temperature, enthalpy, and humidity ratio.Merkel demonstrated that the total heat transfer is directly proportional to the difference between the enthalpy of saturated air at the water temperature and the enthalpy of air at the point of contact with water. Merkel theory

The main assumptions of Merkel theory are the following:The saturated air film is at the temperature of the bulk water.The saturated air film offers no resistance to heat transfer.The vapor content of the air is proportional to the partial pressure of the water vapor.The heat transferred from the air to the film by convection is proportional to the heat transferred from the film to the ambient air by evaporation.The specific heat of the air-water vapor mixture and the heat of vaporization are constant.The loss of water by evaporation is neglected.The force driving heat transfer is the differential enthalpy between the saturated and bulk air.

Merkel equation

The cooling characteristic, a degree of difficulty to cooling is represented by the Merkel equation:Where:K= overall enthalpy transfer coefficient, lb/h-ft2a= Surface area per unit tower volume, ft2/ft3V= Effective tower volume, ft3L= Water mass flow rate, lb/h

Merkel Equation basically says that at any point in the tower, heat and water vapor are transferred into the air due (approximately) to the difference in the enthalpy of the air at the surface of the water and the main stream of the air. Thus, the driving force at any point is the vertical distance between the two operating lines. And therefore, the performance demanded from the cooling tower is the inverse of this difference. Solution to merkel equationThe solution of the Merkel equation can be represented by the performance demand diagram. TheKaV/Lvalue is equal to the area under the curve, and represents the sum of NTUs defined for a cooling tower range.

Psychrometric chartThis cooling process can best be explained on a psychometric chart, which plots enthalpy versus temperature. The process is illustrated in the so-called driving-force diagram shown. The air film is represented by the water operating line on the saturation curve. The main air is represented by the air operating line, the slope of which is the ratio of liquid (water) to air (L/G).

Loss of WaterEvaporation Rate is the fraction of the circulating water that is evaporated in the cooling process. A typical design evaporation rate is about 1% for every 12.5C range at typical design conditions. It will vary with the season, since in colder weather there is more sensible heat transfer from the water to the air, and therefore less evaporation. The evaporation rate has a direct impact on the cooling tower makeup water requirements.

Loss of Water (contd.)Drift is water that is carried away from the tower in the form of droplets with the air discharged from the tower. Most towers are equipped with drift eliminators to minimize the amount of drift to a small fraction of a percent of the water circulation rate. Drift has a direct impact on the cooling tower makeup water requirements.Recirculation is warm, moist air discharged from the tower that mixes with the incoming air and re-enters the tower. This increases the wet bulb temperature of the entering air and reduces the cooling capability of the tower. During cold weather operation, recirculation may also lead to icing of the air intake areasThe water is introduced into the tower through spray nozzles approximately 10m above the basin. The primary function of the spray zone is simply to distribute the water evenly across the tower. The water passes through a small spray zone as small fast moving droplets before entering the fill.There are a range of fill types. Generally they tend to be either a splash bar fill type or film fill type. The splash bar type acts to break up water flow into smaller droplets with splash bars or other means. A film fill is a more modern design which forces the water to flow in film over closely packed parallel plates. This significantly increases the surface area for heat and mass transfer.As the water leaves the fill and enters the rain zone, the water film breakup into droplets again before it is finally collected in the basin below the tower.The air enters the tower radially through the rain zone where it initially flows in a part counter flow part cross flow manner before being drawn axially into the fill and up into the tower. The air leaving the fill is generally supersaturated. Drift eliminators are placed above the spray nozzles to recover entrained water spray droplets in the flow.

Value of PerformanceCooling tower performance is important as inefficient operation can place serious limitations on plant performance.An underperforming cooling tower will have an increased cooling water outlet temperature and therefore increase the condenser back-pressure. This has the effect of decreasing the turbine performance and station electrical generation output.A one degree Kelvin rise in water outlet temperature may be equivalent to a 5kPa increase in condenser back-pressure (depending on operating point) and a 0.3% change in turbine heat rate.For a 660MW(e) unit to generate the same power output under these conditions, it would require an additional 5, 200 tonnes of coal per annum, which at a price of $35AUD per tonne is about $180,000AUD per annum.This provides strong motivation to improve the heat and mass transfer characteristics of power station cooling towers and produce reliable methods to optimise and design them to specificationFlow DescriptionCooling tower theory relies on the simplification of a complex air/water flow interaction to a simple one dimensional volumetric heat and mass balance to which empirical correlations can be applied.In a counter flow wet cooling tower, water falls vertically down through the fill in a liquid film or as droplets falling through air. Air, driven by tower draft or fan, rises vertically in the opposite direction. Heat and mass is exchanged between the two phases at the interface as shown in Fig.Both evaporation and sensible heat transfer cool the water causing the air temperature and humidity to increase with height through the fill or heat transfer zones.The models developed here are limited to one dimension and the following simplifications are made.The temperature gradient within the liquid film is ignored and the temperature is taken as the bulk average value (Tw) at each vertical location.Similarly, the air temperature and the species concentration of water vapour within the air are assumed to be at their bulk average values so that horizontal temperature and species concentration gradients are ignored.At the interface of the two phases there is assumed to be a thin vapour film of saturated air at the water temperature.

Computational fluid dynamicsThe governing equations for incompressible steady fluid flow can be written in general form as:

where is the air density (kg/m3), u is the fluid velocity (m/s), is the flow variable (u, v, w, k, , T, ) and is the diffusion coefficient for and S the source term. These equations can be expanded into the individual momentum and transport equations which, together with the continuity equation give the Navier-Stokes Equations. These equations can be solved numerically enabling fluid flow to be simulated forming the basis for CFD.

Continuity and Momentum Equations

Solution ProcedureAn overview of the heat and mass transfer procedure and water flow representation is as follows:The water enters the tower through the spray nozzles. The water mass flow rate and temperature are specified and droplet spray trajectories are initiated at approximate nozzle locations across the tower.The spray droplets pass through the spray zone and upon reaching the top surface of the fill, they are terminated and their temperature is recorded and used as an input to the fill model.In the fill an external procedure is used to determine the change in water temperature and mass through the fill. This procedure is implemented in subroutines, written in C programing language and compiled directly into FLUENT through its USER-DEFINEDFUNCTION (UDF) capabilities. The same procedure then calculates the energy and mass source terms to couple the energy and mass transfer with the continuous phase. Additionally, the procedure determines the momentum source terms representing the flow resistance the gas phase experiences through the fill.At the bottom of the fill the new water temperature and mass flow rate are used to initiate the droplet flow in the rain zone.The droplets pass through rain zone with heat, mass and momentum coupled with the gas phase. On reaching the basin, the trajectories are terminated and the temperature and mass of the droplets are recorded.GeometryIn the first step geometry is created in 2 D using reference data providing different parts of cooling tower considering important details. The structure of whole model imagined in advance, because the possibilities in the subsequent steps depended on the composition of different geometrical shapes .Assumptions were made to take into account the main features of real construction.2-D symmetry model is developed, fix the fill corresponding to real arrangement.Inlet and outlet space is created at bottom and top of the tower Cooling tower shell is considered as a wall with zero thickness and its profile is formed by curve by three point including throat.Assuming symmetrical thermal and flow field in the model, only one half of the cooling tower is modeled with a symmetry boundary condition

MeshingAfter geometry mesh is generated. During mesh generation much attention to be paid with mesh quality requirement recommendation in FLUENT . In order to have an appropriate resolution of the flow field inside the cooling tower the computational domain is discretised into a large number of finite volume cells.Different parts is meshed with different element sizing .Fill zone must be fine meshed.By using mapped face meshing mesh the model with appropriate element sizing.After mesh generation create name of different parts of cooling tower.The inner and outer surface of the wall inside the model have identical shapes but are disconnected, so the mesh sizes on the two sides of the walls can be different.

Boundary ConditionsVelocity inlet boundary condition is used to define the inlet velocity and other properties of air. Velocity magnitude of air takes normal to the boundary of inlet. Turbulence is taken as intensity and length scale. Thermal condition and species in mole fraction is defined. Pressure out let is defined at out let of air .Other zone also define likewise.

Cell zone conditionIn cell zone surface body is considered as a fluid. The operating pressure is atmospheric in upstream from the center line of cooling tower. The gravitational acceleration is 9.81 m/s2.Operating temperature is 300 K and operating density is 1.22 kg/m3.

1Tower Height130 m2Tower base diameter98 m3Tower top diameter68 m4Ambient Pressure 101 kPa5Hot water temperature318 K6Cold water temperature 305 K7Dry Bulb temperature of air300 K8Cooling range 10 K9Relative humidity of ambient air 30 %10Approach5 K11Water flow rate50 kg/sec

Results

Conclusion A detailed study of Natural Draft Wet Cooling Tower was done along with understanding of Computational Fluid Dynamics and its principles. A Two Dimensional CFD model of natural draft wet cooling tower has been developed. Geometry of the model was completed in Gambit 6.2 and Meshing and Results were developed in Fluent 6.3.FUTURE PROSPECT OF THE WORKAn advanced 3-D CFD model of a NDWCT and further understanding of heat and mass transfer processes in the tower and how they are coupled with the air flow field can be performed and to provide designers with immediate conclusions on how tower performance is related to key design parameters.

Work to be doneThe accuracy to which the flow field is computed has improved as turbulence models have advanced and computational power has increased. Unfortunately the availability of the data to validate the models has not progressed. No models to date achieve more detailed validation than a simple comparison of the tower water outlet temperature with manufacturers data or full scale measurements.Currently, one dimensional models are usually employed to design cooling towers. There are a number of deficiencies with their use however, that have not been addressed.In addition, despite the number of numerical NDWCT models in literature, few examine the detail of the heat and mass transfer in the tower and provide designers with immediate conclusions and recommendations to produce better cooling towers. There has been no optimisation study in literature that considers two dimensional effects and the possibility of radially varying the fill depth and water distribution.