INTRODUCTION
The development of the advanced systems CAD, has allowed increasing the fields of application of Engineering of Simulation and, especially, Computational Fluid Dynamics (CFD). This method constitutes one of the branches of Mechanics of Fluids that uses numeric methods and algorithms to solve and to analyze problems on the flow of fluids. The fact that CFD has been developed later than other CAD, as those used for the analysis of thermal tensions, is due to the inherent difficulties that are presented in the equations that describe the flow of fluids. Nowadays, with the enormous possibilities of computers and the development of numeric methods, CFD is becoming a very practical and efficient tool for the analysis of situations that involve flowing and, consequently, in an invaluable analysis tool and design. The computers are used to carry out millions of calculations required to simulate the interaction of liquids and gases with complex surfaces projected by Engineering. All this means that CFD should no longer be contemplated as a tool used in universities and investigation centers by highly qualified specialists, but rather it is already used in numerous industries. Even with simplified equations and supercomputers of high yield, only approximated results can be reached in many cases. The continuous investigation, however, allows the software incorporation that increases the calculation speed and diminishes the error margin, at the time that allows analyzing more and more complex situations as transonic fluids and turbulent flows. The verification of the data obtained by CFD is usually carried out in tunnels of wind or other physical models to scale. This method discretizes a region of the space, creating what is known as a space mesh and dividing a region of the space in small control volumes. After that, the equations of conservation discretized are solved in each one of them, in the same way that an algebraic womb is solved in each cell in the reality in an iterative way until the residual is sufficiently small, according to Fujun (2004).
At national and international levels, multiple investigations related to CFD have been developed, among them are those carried out by Toneva et al. (2011), Chuanzhong et al. (2012) and Lisowski et al. (2012), that developed a hammer mill with sieve devices and the characterization of the flow in a phase, during the milling in a classifier mill of air. They also studied the movement of the material cut in the discharge outlet of the forage harvester with cutting unit of steering wheel, for mensuration with corn and numeric simulation, respectively. Also Coussirat et al. (2012), study by means of CFD the interaction rotor-stator in a centrifugal bomb with diffusor. Similarly, Lisiuk et al. (2007), carry out a theoretical analysis and for finite elements of load losses in an irrigation system of central pivot and Salcedo et al. (2017) introduce the computational fluid dynamics in the analysis of flows between porous. Finally, García et al. (2018) evaluate the process of sedimentation of discreet particles in turbulent flow.
On the other hand, Herrera et al. (2006, 2012, 2013, 2014, 2015) and Endalew et al. (2010), carry out investigations on the modelation and simulation by means of computational fluid dynamics of the current of air of a sprayer, analyzing the effect of the speed of the wind in the functioning of agricultural sprayers of fan. In addition, they analyze the aerodynamics of the sprayer fan ASS-800 used in fruit crops.
During the revision carried out it was verified that there are not works related with the application of the CFD in forage harvesters. Therefore, the objective of this work was to carry out a revision and analysis of the applications of Computational Fluid Dynamics, in the field of Engineering, but making bigger emphasis in Agricultural Engineering. It searched for establishing the necessary approaches for the diversification of their use in Agricultural Engineering, for their application in the improvement of a design of the discharge tower of one forage harvester. This work is associated to the investigation project titled “ Development of a module of machines for the production of animal food from different cultivations”, which belongs to the National Program of Animal Food.
DEVELOPMENT OF THE TOPIC
Possible Applications of Computational Fluid Dynamics (CFD)
CFD can be very useful in a wide spectrum of industries and it can represent a powerful help to the engineer of design, production and even of maintenance. It is being used in sectors so varied as the chemical, aerospace industry, metallurgical, electric, of metallic transformed etc.. It is also broadly used in the study of buildings, to predict the movement of air and comfort, as well as to analyze water heater - fluidic phenomena in the environment, such as dispersion of pollutants in the atmosphere, or of pouring in the rivers. Three the types of problems can be basically solved with that methodology, according to Barazal & Sala (2019),:
Simulation of an existent equipment, with the purpose of evaluating changes in their operation or in their design (energy savings, improvements in the quality of the product, environmental improvements), or to diagnose operational problems.
Improvements in the equipment design, being able to evaluate a great variety of design options, to increase in this way, the possibilities for the technological innovation.
Simulation of processes, with or without phase transitions, interactions solid-fluid, etc.
Applications of CFD in Thermo Mechanical Equipment
The existent software of Computational Fluid Dynamics can be used for the numeric simulation of processes of fluid flow, blended, combustion, transport of heat and mass, etc.
The main applications of the CFD in equipment and systems are:
Diagnoses with prediction of flows of heat, fields of speeds and temperatures in conduits of gases, pipes of liquids and changers of heat.
Diagnoses with prediction of flows of heat, fields of speeds and temperatures in combustion cameras, boilers and ovens.
Analysis of flows in fans, separators, etc.
Design of components of plants of treatment for residual waters and clean waters.
Design of burners in general, burners of low NOx, valves and other auxiliary equipment.
Applications of CFD in Equipment and Electric Installations
It allows determining the fields of temperature and flows of heat with the purpose of optimizing the evacuation of heat in equipment, nets, spread and squares, as well as determining the repercussions derived from them. Its main fields of application are:
Electric squares and control closets and control.
Motors and alternators, with the possibility of a detailed knowledge of the flow of air, thermal balance and efficiency of the cooling of the machine.
Design of the system of ventilation of CT and local with high density of wiring and electric apparatuses.
Advising in thermal aspects in the design of equipment and electric installations.
Electric lines with responsibility.
Applications of CFD in Electronic Equipment and Assemblies
CFD is utilized to determine the fields of temperature and flows of heat, with the purpose of optimizing the evacuation of the heat in equipment and electronic components, as well as to value the repercussions derived from them, especially on the operation and duration. Its main applications are:
Applications of CFD in Industrial and Building Ventilation
CFD allows carrying out the analysis and simulation of the distribution of the air and its pollutants in commercial and residential buildings and in ships and industrial pavilions. Its main applications in this field are:
Design of ventilation systems.
Definition of the flow of air in the rooms and optimization of the disposition of the diffusors.
Studies of movement of the smoke in the event of fires and evaluation of the evacuation systems.
Studies of fire propagation and design of system against fires.
Advising in ventilation projects, heating and conditioned air.
Determination of the environmental contamination in the place of work.
Designing ventilation systems for machine rooms.
Definition of the quality of the interior air.
Applications of CFD in Turbo Machinery
The software of CFD is used to analyze the flow of liquid, gas or vapor in its movement through the blades and the different components of the turbo machine to diagnose operational problems or to improve its design. Among the main application fields are:
Optimized designs of pumps, considering the flow in 3D under non nominal conditions, with visualization of the cavitation areas.
Design of small hydraulic turbines, keeping in mind in each case their particular demands.
Improvements in the designs of turbines of vapor, optimizing the geometry of the blades, as well as of the carcasses, labyrinth closings.
Design of centrifugal and axial compressors.
Applications of CFD in Automobile Industry
CFD allows a direct access to data that cannot be measured directly, as well as to a visualization of the transitory structures of the flow. All that facilitates an analysis of the proposed modifications, before carrying out the project. The most interesting applications in this sector are:
External aerodynamics.
Ventilation system.
Refrigeration of the motor.
Valves.
Escape system.
Refrigeration of the controls.
Filters.
To these applications referred by the authors mentioned, their employment in agricultural machinery with similar possibilities to the automobile industry should be added, incorporating the equipment and agricultural implements with the usage of fluids in its technological process.
Analysis of Developed Investigations Applying CFD in Engineering
Several investigations have been carried out in this field. Lisiuk et al. (2007), make a theoretical analysis and for finite elements of the load losses in an irrigation system of central pivot with swan neck. In Figure 1a, speeds in this system of watering are shown in a detailed way. There, it is possible to observe that the flow accelerates when entering to the conduit on the front base, while an area of low speed takes place in the anterior part of the neck base. This gradient of speeds produces a concentration of viscous effects generating big load losses in the entrance area of the swan neck, the profile of speeds in traverse form in that area. It is significant that in a small section of 1,9 cm, the speed varies between 0,2 to 1,2 m/s. It is also relevant that the profile of speeds in the exit of the swan neck is not uniform, but rather it is displaced toward the external wall.
In Figure 1b, the distribution of pressures is appreciated along the whole conduit. It is noticed that a small area of high pressure when the flow impacts on the entrance border of the swan neck. In turn, it is observed how a center of low pressure takes place in the anterior part of the neck entrance, area of removed flow, transforming, in this area, pressure energy in kinetic energy. The pressures increase toward the external face of the neck curve due to the centrifugal efforts.
Oro (2012) and Rubiano (2018), carry out investigations applying CFD in centrifugal pumps. These authors develop the interaction rotor-stator in a centrifugal pump with and without diffusor, with their rational design and aimed at identifying and studying the possible amplification of pressure fluctuations inside the machine (RSI). In Figure 2, the application of CFD in centrifugal pumps is shown. a) Static pressure in the bun and area of the water-cutter (nominal flow) b) Distribution of pressure.
Herrera et al. (2006, 2012, 2013, 2014, 2015), carry out investigations on the modelation and simulation by means of the computational fluid dynamics of the air current of sprayers. These authors analyze the effect of the wind speed in the acting of the agricultural sprayers of fan. On the other hand, they also carry out analysis of the aerodynamics of the fan of the sprayers ASS-800 used in fruit-bearing. The profile of speed oscillated from 4 to 34 m/s.
In Figure 3, the modeling air current by computational fluid dynamics in the program ANSYS 5.3 is shown. There, it is shown that at the distance of 0,8 m of the exit of the fan, a small deviation of the current occurs, this is due to the non-uniformity in the distribution of air speed at the exit of the fan. The speed is concentrated on a cone from 0,8 to 1,6 m of maximum longitude, the cone wears away, being this a transition area where the speed distribution is Gaussian. When surpassing the 1,6 m of distance, it rectifies the direction and the wearing of the central cone of speed of the current finishes and it presents another profile type.
In Figure 4, the graphs of the air current are observed. They were modeled in CFD for the different work regimes to study. In graph (a), the sprayers without movement and from graph (b) to (d), the spraying machine to a speed of 3,6 km/h and a wind of speed 5 m/s in direction contrary to the movement of the sprayer, forming angles of 30o, 45o and 60o with regard to this, respectively.
Legend: a) without movement; b) with speed of the sprayer of 3,6 km/h and a speed of the wind of 5 m.s-1 forming angles of 30o; c) with same speeds, but forming angles of 45o; d) with same speeds, but forming angles of 60o.
In Figure 5, the behavior of the air flow of sprayer is shown in different directions and speeds of the wind: a) the air flow of sprayer with wind at 90o and speeds of 1, 3 and 5 m·s-1 and b) air flow of sprayer with speed of the wind 5 m.s-1 and direction of 45o, 90o, 135o and 180o.
Salcedo et al. (2017), introduce computational fluid dynamics in the modelation of a fluid crossing a porous medium. These authors propose a practical case by means of the simulation of an air flow produced by the fan of a hydraulic sprayer assisted by air that crosses a porous medium (vegetation). The work consists on endowing of the necessary tools to configure a model CFD for, later on, to adjust the resistance from the porosity to the current passing using experimental data. The adjustment contemplates three cases: 1 - considering only inertial losses equal between the different porous bodies, 2 - considering these inertial losses and also the viscous losses and 3 - considering only inertial losses different between the different porous bodies, according to Figures 6a, b and c, respectively.
Toneva et al. (2011) and Chuanzhong et al. (2012), develop a mill of hammers with device of sifting. Figures 7a and 7b show the design and construction of the sifting and the simulation of the distribution of air flow with the application of CFD, to determine the appropriate assembly curve of the sieve and the numeric simulation of air flow in the tube of transport of the particles.
Brazee et al. (1998), study the losses caused by hydropneumatic sprayers in fruit culture, with the analysis of the effect of width variation of exit diffusor of fan in the aerodynamics of air flow during the work process to different speeds of movement of the sprayer. Evaluations were carried out with widths in the exit of diffusor of the fan of 115 and of 150 mm. It was considered, the sprayer without movement and speeds of work of 2,18; 4,5 and 6,35 km/h. As variable answers, the deviation and reach of air flow will be analyzed through the interpretation of the graphics of speed. Similar studies were carried out by Boné et al. (2014) during the characterization of air flow generated by hydropneumatic sprayers.
Endalew et al. (2010), developed an integrated model of computational fluid dynamics in 3D of air flow from a sprayer of orchard of crossed flow assisted by air of two fans through orchard pear trees without leaves, of 3 m of average height.
Falcinelli (2004), carries out the simulation for CFD of the action of winds on the tanks for different topographies. In this work, a numeric analysis is presented by means of CFD of the movement field around a metallic tank with the objective of obtaining the distribution of pressures of wind on it.
Lisowski et al. (2012), analyze the movement of chopped material in the discharge outlet of the forage harvester with a cut unit of steering wheel, carrying out the application of the computational fluid dynamics, in an outlet configuration, using the software Fluent v. 6.2. The work consisted on the characterization of the air movement and of the material cut in the discharge outlet of a forage harvester. The speed was determined with changes in the form, the speed of rotation of the cutting bolster, the number of blades and the quantity of vegetable material of corn fed to the unit of cut of the steering wheel.
Cao & Li (2011) analyzed by means of CFD, the field of air flow of three-dimensional in a mill, using the computation program Fluent v. 6.3. Then, the simulated results of the speeds of air flow were compared with the ones obtained by tests to validate the results of the numeric simulation.
On the other hand, several authors have studied the flow simulation in greenhouses (Flores et al., 2011; Shah et al., 2013; Espinal et al., 2015 and Villagran et al., 2018a, 2018b). These authors have carried out investigations on the thermal behavior in space greenhouses in Colombian under conditions of day and night climate, as well as in others built in hillside. They have also analyzed the design and climatic evaluation in greenhouses for conditions of intertropical mountain climate. Moreover, these investigators carried out climatic analysis of zenithal greenhouses of three houses without cultivation and with ventilation forced, respectively, in order to obtain the patterns of thermal distribution inside the greenhouses, under the established meteorological conditions, for the appropriate development of the cultivations.
García (2018) evaluates the process of sedimentation of discreet particles in turbulent flow. Mini-hydroelectric, without equipment to eliminate sand, frequently present obstructed nozzles and for that, a device is created to be connected to the high pressure pipe. In this investigation, the Solidworks is used to simulate the behavior of the speed profile of the current lines in the water flow, according to the Runge Kutta 2 for the simulation of particle movement (stones)(Figure 8). It is demonstrated that the device avoids the obstruction of the nozzles by the stones and it is concluded that, the sedimentation process in turbulent flow, happens in particles of diameter ≥ 2 mm. Also Nieto et al. (2004), carried out a study directed to the application of computational fluid dynamics in thermal regenerators.
CONCLUSIONS
During the review the following issues were verified:
The high potentialities and diversity in the application of CFD that constitutes a tool that can solve problems of high complexity, with the prediction of pressure, speeds profile, and studies of the aerodynamics, with multiple uses in Engineering, in general and in Agricultural Engineering, in particular. CFD is a valuable tool for analysis and simulation, to improve the design of dissimilar machines and equipment. It is remarkable its application in determining the effect of wind on the air flow of agricultural sprayers and in climate studies in greenhouses.
That there are not works related to the application of CFD in forage harvesters.