THE MISSIONARY JOURNEY OF PAUL

Steam

NTATION STUDY OF A STEAM TURBINE POWER PLANT AT PART LOAD SETTINGSABSTRACT Power depletion extremely increases which is associated with the increasing of the industrial plants and daily using. Growing power demand can be provided with building up more efficient plants or optimized old power plants. One of the most important items of a power plant is steam turbine which is designed according to defined parameters (inlet pressure and temperature, flow rates, outlet pressure and power) which also effect the dimensions and performance of the turbine. Turbine loses and irreversibilities are minimum and so efficiencies and power generation are maximum at design conditions. However, power plants always have to operate at off-design or part- load conditions because of the changing of power demands and drop outs of the turbines and other items of the plants. In this study, it is aimed to analyses the isentropic efficiency of a high pressure steam turbine and thermal efficiency of power plant at different load conditions. Analyses showed that both steam turbines and power plant performance were reduced when the power plant operates at partial load conditions.INTRODUCTIONPower consumption highly increases which is related with the growing of the industrial plants and daily using. Increasing power demand can be supplied with building up more efficient plants or optimized old power plants. The thermal efficiency depends on the all equipment (turbines, boilers, pump, etc.) performance so how the equipment are efficient thermal efficiency and cost are optimum. The most important items of a power plant are steam or gas turbines therefore their design and operating conditions are very important.Steam turbine design is based on some characteristic features such as inlet pressure and temperature, flow rate, outlet pressure etc. and turbine geometry, dimensions and performance are defined with this characteristic features. Turbine loses and irreversibilities are minimum and performance and power generation are maximum at design conditions. However, a steam turbine does not always operate at design conditions because of changing of power demand and turbine loses and this means that it always operates lower efficiency. Estimating and defining of characteristics of steam turbines at off-design conditions have been studied since 1900 and today the studies are going on more different variables and modern tools.Figure 1. Produced power by plant [1] (1 Mtoe = 11630 GWh)Bresolin aimed to simulate the partial load characteristic with different control system (sliding pressure, throttling valve and nozzle valve control systems) of a steam turbine. In order to this, the inlet pressure which was calculated with respect to Schegliáiev and Stodola models where pressure is as a function of flow rate. The analyses showed that Schegliáiev and Stodola's models results were close and the operational modes by valves establishes theoretical limits of operation for steam turbines. Moreover, the best control system type is depended on turbine load and the sliding pressure control is better adapted to higher loads. An analytical approach was developed by Weir is expressed the compatibility between the enthalpy balances of cycle and flow characteristics of stage groups which is based on Stodola’s Ellipse Law and uses a rapidly convergent iterative technique to estimate of steam turbine performance at partial loads.Off-design steam pressures of a multistage turbine at a cogeneration plant model was investigated and criticized with respect to the Law of Ellipse by Cooke and this study has shown that the Constant Flow Coefficient is a special case of generalized Law of Ellipse.Petrovic and Riess presented a method which is based on finite element procedure and the through- flow theory for flow calculation in large steam turbines. The method was adapted to the three stages low pressure turbine in the 165 MW power plant which the flow field and performance over a wide range of mass flow and input pressure are calculated. The results showed that changing of flow field and performance of the first two stages are very little at operating conditions but the efficiency and internal work of the last stage are zero at 35% load.Herzog et al. have done a study to predict and explain the high pressure steam turbine behavior which helps the producer to take precautions to reduce both economic and technical risks because of the windage loses that occurs due to the bigger size of turbine blade and changing of the electricity demand. The study has analyzed the measurement of flow field at low Mach number for a four stages turbine in order to have better understanding about turbine aerodynamics and operation characteristics at partial load conditions. As a result experimental and numerical data has compared and a seven stage turbine was modeled.Gerolymos and Hanisch has developed a 3D flow calculation program which based on a multistage (four stages) turbine and its analysis with 3D averaged Favre-Reynolds-Navier-Stokes (RaNS) equations using the Launder–Sharma turbulence closure at partial loads to build up a flow model and explain turbo machinery characteristics at different conditions. The computation models the eight blade rows, including the tip clearance gaps of the rotors, using a 6x106 points grid, and computes the time-averaged flow using mixing planes between rows. After validating the method at different operating conditions a computation was run at reduced mass-flow conditions at the design speed and the results were compared to understand the multistage flow structure at these conditions. The calculations revealed that bigger flow separation has been occurred because of radial vortex over the concave side of the last stator at partial loads operating conditions. The radial vortex drug up to flow and so the flow inside of the channels was blockaged and turbine case damaged.He studied the effect of unsteady flow because of partial load conditions on the aerodynamic performance of multistage turbines and also has developed a 3D Navier-Stokes method and has made a comparison between the variable forces over the blades and experimental results for a single-stage for partial expansions.Stastny published a report examining the relationship between turbine blade and flow field at partial load conditions. At off-design conditions, changing of velocity vectors at inlet section were calculated along the operation time and the results were compared with experimental data. The results showed that flow separation on the leading edge occurs at off-design conditions and this situation would lead to an increase of kinetic energy losses. Moreover, distribution of pressure over the profile surface gives some information about flow separation.Mandal has developed software to predict the performance of steam turbines at partial load conditions. In developing this software, some of the parameters that cause partial load conditions (flow rate, steam initial parameters, the condenser pressure and turbine speed change) have been considered however, pressure and flow changes turbine that may occur in extraction turbines and quality of steam at back stages of turbine were not considered. Although this software gives actual results for both high and intermediate pressure turbine, cannot give good results for low pressure turbines.Bhattacharya [11] investigated the causes of partial load conditions and the effects of steam extracting from interstage of turbines and actual operating conditions have occurred in all types of turbine and has developed Mandal's study by adding steam quality, turbine loses, adding sub-programs and optimization methods.Many experimental and numerical studies have done to analyze of steam turbine isentropic efficiency at partial load conditions. In this study, effects of different flow rate on steam turbine isentropic efficiency and power plant performance have analyzed.THERMODYNAMIC MODEL AND OFF-DESIGN CONDITIONSSteam power plants are run by Rankine cycle which includes steam boilers, steam turbines, condensers, heat exchangers, pumps and valves. Net produced work from the cycle and thermal efficiency highly depend on the chemical values of the burned fuel, boiler efficiency, the consuming energy on the condenser and pump(s) and isentropic efficiencies of steam turbine(s) and pump(s). Any pressure, temperature and flow rates changing at the inlet and/or outlet conditions will affect the both turbine isentropic efficiency and thermal efficiency.The case power plant is located in Marmara region as two units. It produces 160 MW per unit with 37% efficiency. It has 4 LP FWHs and 2HP FWHs, 3 impulse steam turbines which have 13, 10 and 5 stages, a boiler, condenser, degasifier, pumps in a unit, Figure 2.Steam turbine is a heat engine that converts heat energy of the high temperature and pressure steam to kinetic energy and then kinetic to mechanical energy in two stages and/or electrical energy with alternators.The maximum efficiency conditions, which the specific heat for each produced power is minimum, are also corresponding the most economical condition is called turbine operating conditions or turbine design conditions. Therefore, the maximum thermal efficiency conditions have to take into account while steam turbines design. In order to increase the efficiency, steam turbine capacities have to determine wide range of values between full load and zero load conditions Turbines are designed and manufactured taking into consideration some special conditions. Turbine blade profile and the geometric dimensions of each section are designed to take into consideration some parameters. Such as, turbine inlet and outlet parameters (pressure and temperature), specific steam flow rates, the current enthalpy drops, specific condenser pressure, specific extraction steam rates and the grid frequency are determined for specific conditions. In addition, the rotor diameter, height of the first and last blades, the number of stages and the thermodynamic cycle of system are also used to determine the design conditions.Off-Design conditions can be created by many internal and external factors. Although the internal factors are generally controllable, it is very difficult to control the external factors. Some conditions that cause the off- design conditions and reduce the turbine performance are changing of the inlet properties of the steam turbine, grid frequency, the condenser pressure and the performance of the feed water heater; partial arc, deformation of steam path and operating at partial loads Although the internal and external losses are minimum at design conditions, while operating time, it is almost impossible to work with design parameters at real working conditions which occur out of control.Figure 2. Case power plant [12]Part Load ConditionsPartial load conditions may occur energy demand variations, plant maintenances and repair so it is important to control steam flow. Partial load control can be arranged with two ways. The first is throttling governing which controls the system by changing turbine inlet pressure. If steam pressure decreases the steam flow rate and temperature are also decrease so in this system loses will be much more. The second system is nozzle governing which changes the flow rate at constant turbine inlet pressure and temperature. The second control method is much more efficient and so more common.The steam flow rate is arranged by governors or check valves at different loads. While turbine inlet flow rate decreases the turbine inlet pressure also decreases (Ellipse Law). However, thermal efficiency and turbine isentropic efficiencies are highly depend on the pressure drops, that’s way turbine inlet pressure keep constant even if flow rate changes.Produced power is proportional with the steam flow rate and heat drop of steam turbine so this make possible to arrange power by changing this parameters [15] and sliding pressure boilers can also be used to governed the load.RESULTS AND DISCUSSIONPerformance calculations at partial loads have done with guaranteed pressure, temperature and flow rates values for 100%, 80% and 60% loads which were obtained from the plant manufacturer.Pressure and Temperature VariationsDesign inlet pressure of high pressure turbine does not change while the load is changing but extraction and outlet pressures of back pressure, extraction and condensing turbines vary in proportion with the load. Therefore, this will reduce the turbine and plant efficiencies. Temperature and pressure variations at a HP turbine stages for different loads are shown in Figure 4.Figure 3. Partial load conditions at turbine blade Figure 4. Temperature & Pressure variations in turbine stages at partial loadsSpecific Enthalpy Drops and Power VariationsSpecific enthalpy values change with load variation at HP turbine stages which can be seen in Figure 5. The amount of enthalpy drop and also theoretical work of HP turbine increases because of Inlet pressure and temperature generally keep constant but outlet pressure changes in proportion with load variation. However, amount of theoretical enthalpy drop and work of IP turbine and LP turbine keep almost constant, except for very low load.The power generated from the plant depends on the enthalpy drop and flow rate of turbine are main parameters to determine the plant load. Despite the increasing amount of the specific work, the generated power decreases because of the decreasing of flow rate with variation to load at the high pressure turbine. In addition to this, the enthalpy drops of the intermediate and low pressure turbines are nearly constant but generated power is also decreasing with the reduction of flow rate. The generated power from the turbines at partial loads are shown in the Figure 6.? = (ℎ?? − ℎ???) (1)? = ?̇ ? (2)Figure 5. Specific enthalpy variation at turbine stages at partial loadsFigure 6. Specific work and turbine power variations at partial loadsFigure 7. Isentropic efficiency of turbine and plant thermal efficiency variations at partial loadsInternal Efficiency of a HP Steam TurbineHow the performance of a steam turbine has changed at partial load conditions can be explained on the basis of the relationship between steam pressure, temperature and flow rate. Power output rate, the relationship between the enthalpy drop and the pressure ratio, Law of Ellipse or Stodola’s Cone and Schegliaev model can describe this relationship.Turbine indicated efficiency is an important performance parameter which can be calculated from turbine inlet and outlet parameters that guaranteed by turbine producer or the difference between theoretical enthalpy drops and loses for each stage. In this study, high pressure turbine indicated efficiency is calculated with turbine producer data, Figure 7??= ℎ??−ℎ??? ℎ??−ℎ???,?Thermal Efficiency of Power PlantThermal efficiency, which describes in Eq. 1 the ratio of net work to heat, of a system does not only depend on an equipment performance, it depends on the performance of all the equipment that create the plant such as boiler, steam turbines, condenser, pumps and heat exchangers for case plant.??= ???????The results show that thermal and produced power decrease as the load decreases, Figure 7. This means that unit electricity costs will increase and the economic criteria will get worse. Moreover, most of the system equipment will also operates off design conditions that damage them.CONCLUSIONIn the first part of this study, a short review about turbine performance at partial loads and off design conditions has been done and then general information about power plants was given. In third part of the study, operation principles, design and off design conditions of steam turbines have investigated/introduced and then performance analysis of a high pressure steam turbine and a power plant have done at off design conditions for a real plant. Performance calculations at partial loads have done with guaranteed pressure, temperature and flowrates values which were obtained from the plant manufacturer. Analyzes showed that both turbine and the power plant performances reduce at partial load conditions. In order to obtain exact results, a comprehensive measurement system and standards must be improved for all type of turbine and control systems.ACKNOWLEDGEMENTThis paper was compiled from presented master thesis by Karakurt at Graduate School of Natural and Applied Sciences of Yıldız Technical University. Authors thank to Prof. Bahri ŞAHİN for his advice and support.NOMENCLATUREη EfficiencyFWH Feed water heaterh EnthalpyHP High pressureIP Intermediate pressureLP Low pressureQ HeatP PowerPr PressureT TemperatureW Specific work Subscriptsi Indicatedin Inletout OutletIdealThermalT TurbineTot TotalREFERENCES“BP Energy Outlook 2035,” British Petroleum, Statistical Review, 2015.C. S. Bresolin, P. S. Schneider, H. A. Vielmo, and F. H. R. França, “Applications of Steam Turbines Simulation Models in Power Generations Systems,” Eng. Térmica Therm. Eng., vol. 5, no. 1, pp. 73–77, Jul. 2006.C. D. Weir, “An Analytical Approach to the Estimation of the Performance of Steam Turbine Cycles Off- Design,” Proc. Inst. Mech. Eng. Part J. Power Energy, vol. 199, no. 1, pp. 33–43, Feb. 1985.D. H. Cooke, “Modeling of Off-Design Multistage Turbine Pressures by Stodola’s Ellipse.” Bechtel Power Co., Nov-1983.M. Petrovic and W. Riess, “Off-design flow analysis of low-pressure steam turbines,” Proc. Inst. Mech. Eng. Part J. Power Energy, vol. 211, no. 3, pp. 215–224, May 1997.N. Herzog, M. Binner, J. R. Seume, and K. Rothe, “Verification of Low-Flow Conditions in a Multistage Turbine,” pp. 563–574, Jan. 2007.G. A. Gerolymos and C. Hanisch, “Multistage three-dimensional Navier-Stokes computation of off- design operation of a four-stage turbine,” Proc. Inst. Mech. Eng. Part J. Power Energy, vol. 213, no. 4, pp. 243– 261, Jun. 1999.L. He, “Computation of unsteady flow through steam turbine blade rows at partial admission,” Proc. Inst. Mech. Eng. Part J. Power Energy, vol. 211, no. 3, pp. 197–205, May 1997.M. Šťastný, P. Šafařík, I. Hořejší, and R. Matas, “Flow around the sections of rotor blading of a turbine stage with relatively long blades at off-design conditions,” Proc. Inst. Mech. Eng. Part J. Power Energy, vol. 211, no. 3, pp. 207–213, May 1997.A.. Mandal, “Off design performance prediction of steam turbines,” Department Of Mechanical Engineering Indian Institute Of Technology, Delhi, Project Report, 2005.S. Bhattacharya, “Prediction Of Off-Design Performance Of Steam Turbines and Extraction Steam Flow Optimization,” Department Of Mechanical Engineering Indian Institute Of Technology, Delhi, Project Report, 2007.Z. Oktay, “Investigation of coal-fired power plants in Turkey and a case study: Can plant,” Appl. Therm. Eng., vol. 29, no. 2–3, pp. 550–557, Feb. 2009.P. Shlyakhin and P. N. Shl’iakhin, Steam Turbines: Theory and Design. University Press of the Pacific, 2005.“Turbine and Auxiliaries.” CANTEACH, Jul-1994.A. Özge, Marine Steam Turbines and Plants

RS TO COMMUNITY PARTICIPATION IN CRIME PREVENTION IN LOW INCOME COMMUNITIES ABSRACTCommunity participation in crime prevention has been embraced by anti-crime organizations as a panacea for crime problems. This approach gained its preeminent status after governments realised that law enforcement alone cannot reduce crime without involving communities. This paper provides insight into challenges facing community participation in one of the Cape Town townships. The study employed qualitative method and participants such as ordinary citizens and representatives of anti-crime organizations operating in Khayelitsha were purposively selected. Data was collected using in- depth face-to-face interviews. Key findings show that Khayelitsha residents patrol streets during the night under a neighborhood watch project; and by reporting committed crimes to police or providing police with information on potential crimes, this same community patrol helps decrease potential criminal activities. Community participation in Khayelitsha however, faces some impediments such as poverty among the community residents, and ineffective police response to crimes.INTRODUCTIONCommunity participation in crime prevention activities is consistently gaining global ascendancy following high crime rates in many parts of the world engendered by ineffectiveness of law enforcement. The involvement of citizens in crime prevention is widely regarded as an ideal approach towards crime prevention and crime reduction. Community participation in crime prevention is grounded on the tenet that the traditional law enforcement cannot fight crime effectively without support from local communities who know their areas (Liebermann and Coulson 2004). It is in this context governments are actively mobilizing and integrating local communities into their crime prevention programmes in attempts to build strong collaboration between police and ordinary civilians.Community participation in crime prevention regained its popularity in the 1970s (Newham 2005) and it is not a new phenomenon in community policing. Community participation in crime prevention has a long history. Literature indicates that the involvement of civilians in maintaining peace and security in their areas has been in practice since the time of the settlement of America where local communities were the first peace officers patrolling streets as volunteers. But this participation of civilians in crime prevention arena lost its hegemony in the mid-nineteenth century after the introduction of formal police officers (Ren et al. 2006) which rendered civilians into passive participants in finding solutions to crime problems in their communities.In the early days of the introduction of formal police, it seemed that police was primed to become a panacea to crime problems since such formal police controlled and kept crime rates low. The situation changed unexpectedly when formal police started using technological devices in their operations: telecommunication devices such as radios, 911 emergency telephone systems, and vehicle patrols as a way of responding to people who were asking for help resulted increased crime rates. The high crime rates were attributed to the fact that police were handling crime problems from their offices in lieu of policing communities. This working method eroded the relationship between police officers and communities as police offers became estranged from troubled communities (Fleissner and Heinzelmann 1996).It is argued that the participation of local communities in crime prevention activities is justifiable since they know their crime problems and localities better than outsiders from their communities. In fact, community participation is grounded on this tenet of local communities’ familiarity (Liebermann and Coulson 2004). It is only in this way that police could become productive if they collaborate with local communities. Without this collaboration, police officers are clueless strangers about major criminal activities taking place in specific communities. In the same vein, Friedman (1998) postulates that it was ordinary citizens who contributed to the decrease in crime high rates in the 1990s in the United States of America. It is therefore argued here that police performance in deterring criminal activities correlates with the level at which local communities are involved and participate in crime prevention activities.producers of public safety along with the police. This makes the implementers of community participation model in crime prevention activities assume that the model stimulates an environment in which both ordinary citizens and police share responsibilities of improving and maintaining public safety (Pattavina et al. 2006). On the other hand, Zhao et al. (2002) posit that governments regard the participation of local communities in policing as an effective way of compensating for the scarcity of financial and human resources given that this approach puts emphasis on voluntarism. Still on the benefits of community participation in crime prevention, Botterill and Fisher (2002) also point out that involving local communities in crime prevention programmes mobilises more human resources than could be done by government alone. Although community participation contributes to minimising expenditure on crime prevention programmes, local communities still have to be fully empowered with relevant skills and resources that would enable them to participate effectively.The level at which community members are willing to participate in crime prevention programmes is influenced by context and social organization. Communities that share common understandings and values are more willing to achieve common interests, including maintaining safety and order (Carcach and Huntley 2002, Fagan and Meares, 2008). According to Sampson and Groves (1989), social organization is reflection of the capacity for a community to self- regulate. It could be argued therefore that social organization is a prerequisite for a community to achieve meaningful participation in crime prevention programmes.However, social organization does not always yield effective community participation as the willingness of community members to participate in crime prevention programmes may be hindered byfactors other than social disorganization. These factors include fear of crime among community members, demographic profile of the community and community members’ perceptions of local government agencies (Ren et al. 2006). Yet, studies carried out by Sherman (2002) and Kane (2005) also reveal that community members’ perceptions of local police and other government institutions in their communities also have colossal influence on community members’ decisions to volunteer in crime prevention programmes. Poor relationships with police and lack of trust in the police impinge on community participation in crime prevention. Poor community participation based on lack of trust in the police, however, may be understandably and justifiable because no-one would be willing to collaborate with untrusted partners. In line with this argument, one study revealed that positive perceptions of community policing were strong correlated to increases in crime prevention behaviour (Scheider et al. 2003).In the South African context, the participation of local communities in crime prevention is in its infancy because the approach only became popular in the country after the demise of a*******d in 1994. During a*******d, crime rates were very high across the black and colored communities but fighting and preventing crime in non-white occupied communities was not on the government’s agenda. The a*******d government was committed to preventing criminal activities in white-occupied communities, and it concentrated resources and police in mainly these communities to ensure they remain unaffected by criminal activities which were prevalent in black-occupied communities (Shaw 2002).In post-a*******d South Africa, the promotion and support of community participation in anti-crime activities was manifested in numerous policies formulated towards crime prevention, and thesepolicies require collaboration between police and local communities. The establishment of community policing forum (CPF) in 1995 for example, sent a strong message to South Africans and the international community that the post-a*******d government was committed to embracing and consolidating community participation in crime prevention programmes. The formation of the CPF aims at enhancing police visibility in the community, full community participation, and creating a sense of ownership. This organization is a legislative body formed in accordance with the South African Police Service Act 68 of 1995. The Act requires police stations to work closely with communities through community policing forums (RSA 1995).The participation of local communities was also endorsed in other anti-crime policies that succeeded the establishment of the CPF. In the National Crime Prevention Strategy developed in 1996 (RSA 1996), community participation was one of the four-pillar approach to crime prevention strategy (Newham 2005). Community participation was also placed at the core of the South African Police Service’s (SAPS) own in-house policy on crime prevention which is known as the national crime combating strategy (NCCS) developed in 2000 in response to crime which was increasing at alarming rates in the country (SAPS 2002). In this policy, the SAPS acknowledged the role of local communities in fighting and preventing crime and placed them at the forefront in the policy implementation which intended to achieve the following broad objectives:(i) to restore public confidence in the police and encourage community participation; and (ii) to reduce crime in particular areas though policing (SAPS 2002).Although the South African government implemented these comprehensive policies on crime prevention and has consistentlybeen increasing expenditure on fighting crime (RSA 1998, Mbola 2009, Leuvennink, 2015), there is substantial evidence that the crime situation in South Africa is still frightening (Centre for the Study of Violence and Reconciliation (CSVR) 2009, Leuvennink 2015, OSAC 2015). This failure of the crime prevention policies is linked to factors such as long-standing allegations of an ineffective and corrupt South African Police Service (Shaw 1996, Singh 2005, Faull 2007, Faull 2011), including scarce resources in major anti-crime organizations such as the SAPS and CPF (Pelser 1999). However, crime rates are very high in big cities such as Cape Town, Durban and Johannesburg (Lancaster 2013, OSAC 2014) but the most affected communities are those occupied by black people such as Khayelitsha (Achmat 2014). This paper provides insight into challenges impeding community participation in crime prevention in low income communities Cape Town metropolitan.MethodologyThe study was conducted in Khayelitsha, one of the townships in Cape Town metropolitan jurisdiction.1 The selection of this community was motivated by its current high crime situation. This community is among the crime hot-spots in Cape Town (City of Cape Town 2009) and in the country. Khayelitsha is characterised by overcrowded and unplanned informal houses, few and narrow streets which some have poor and often limited street lights whilst others have no lights at all.This study used a qualitative research design and data was collected from various categories of participants from different anti-crime organizations and community residents. The participants were 451 In South Africa, township is a suburb or a small town officially designed by a*******d for black Africans.including: 6 community leaders (2 ward councilors and 4 street committee members), 33 community residents, and 6 representatives of anti-crime organizations such as Khayelitsha Security and Safety Forum, Khayelitsha Development Forum (KDF), Community Policing Forum (CPF), the South African National Civic Organization (SANCO), and the South African Police Service (SAPS). This sample size was arrived at through data saturation. Data saturation suggests that researchers stop collecting data when the point of data saturation is reached. The point of data saturation is reached when there is evident redundancy or replication in information collected from participants in research (Simon 2011, Marshall et al. 2013). The point of data saturation in this study was reached at the 45th participant.The selection of participants was done using a purposive sampling method. The preference of purposive sampling method was based on its advantage which is to allow a researcher to choose participants who are assumed to possess and provide invaluable information to the research question (Guarte and Barrios 2006). In addition, the method provides a researcher with the flexibility in determining the kind of information to be known and how to get this information (Bernard 2002). This implies that the researcher in this study had the flexibility in selecting participants who assumed to possess invaluable and relevant information to address research questions.The data collection tool used in this study was in-depth face-to-face interviews with all 45 participants. The purpose and objectives of this research were explained to each participant so that they take informed decisions on whether to participate in the study or not. Participa.