Fundamentals of Thermal-Fluid Sciences PDF

Fundamentals of Thermal-Fluid Sciences Yunus A. Çengel, John M. Cimbala, Robert H. Turner McGraw-Hill HEAT ENGINES  Objectives Introduce the second law of thermodynamics. Identify valid processes as those that satisfy both the first and second laws of thermodynamics....

Fundamentals of Thermal-Fluid Sciences Yunus A. Çengel, John M. Cimbala, Robert H. Turner McGraw-Hill HEAT ENGINES  Objectives Introduce the second law of thermodynamics. Identify valid processes as those that satisfy both the first and second laws of thermodynamics. Discuss thermal energy reservoirs, reversible and irreversible processes, heat engines, refrigerators, and heat pumps. Describe the Kelvin–Planck and Clausius statements of the second law of thermodynamics. Apply the second law of thermodynamics to cycles and cyclic devices. Describe the Carnot cycle. Idealized Carnot heat engines, refrigerators, and heat pumps. Determine the expressions for the thermal efficiencies and coefficients of performance for reversible (and irreversible) heat engines, heat pumps, and refrigerators.  RE-CAP: FIRST LAW OF THERMODYNAMICS Conservation of energy : energy can be neither created nor destroyed during a process; it can only change forms. Energy cannot be created or destroyed; it can only change forms.  INTRODUCTION TO THE SECOND LAW: (Inadequacies of the first law) Transferring heat to a paddle wheel will not cause it to rotate. A cup of hot coffee does not get hotter in a cooler room. These processes cannot occur Transferring heat to even though a wire will not generate electricity. they are not in violation of the first law!!!  INTRODUCTION TO THE SECOND LAW: (Inadequacies of the first law) Work can always be converted to heat directly and completely, but the reverse is not true. So processes occur in a certain direction, and not in the reverse direction. A process must satisfy both the first and second laws of thermodynamics to proceed.  INTRODUCTION TO THE SECOND LAW OF THERMODYNAMICS The 2nd law may be used to identify the direction of processes. The 1st law is concerned with the quantity and transformations of energy from one form to another with no regard to its quality. The 2nd law also asserts that energy has quality as well as quantity. The 2nd law provides the necessary means to determine the quality as well as the degree of degradation of energy during a process.  THERMAL ENERGY RESERVOIRS A hypothetical body with a relatively large thermal energy capacity (mass x specific heat) that can supply or absorb finite amounts of heat without undergoing any change in temperature is called a thermal energy reservoir, or just a reservoir. In practice, large bodies of water such as oceans, lakes, and rivers as well as the atmospheric air can be modeled accurately as thermal energy reservoirs because of their large thermal energy storage capabilities or thermal masses.  THERMAL ENERGY RESERVOIRS Bodies with relatively large thermal masses can be modeled as thermal energy reservoirs. THERMAL ENERGY RESERVOIRS A source supplies energy in the form of heat, and a sink absorbs it. HEAT ENGINES ? The devices that convert heat to work: 1. They receive heat from a high-temperature source (steam, solar energy, oil furnace, nuclear reactor, etc.). 2. They convert part of this heat to work (usually in the form of a rotating shaft.) 3. They reject the remaining waste heat to a low-temperature sink (the atmosphere, rivers, etc.). 4. They operate on a cycle. Heat engines and other cyclic devices usually involve a fluid to and from which heat is transferred while undergoing a cycle. This fluid is called the working fluid (water/steam). HEAT ENGINES Energy balance: Part of the heat received by a heat engine is converted to work, while the rest is rejected to a sink. Example of a Heat Engine: Steam Power Plant A portion of the work output of is consumed internally to maintain continuous operation. Thermal Efficiency Some heat engines perform better than others (convert more of the heat they receive to work) – Heat Engine 2 is better. Thermal Efficiency: Even the most efficient heat engines reject almost one-half of the energy they receive as waste heat.  Can we save Qout? In a steam power plant, the condenser is the device where large quantities of waste heat is rejected to rivers, lakes, or the atmosphere. Can we not just take the condenser out of the plant and save all that waste energy? A heat-engine cycle cannot be completed without rejecting some heat to a low-temperature sink. The answer is, unfortunately, a firm no for the simple Every heat engine must waste some energy reason that without a heat by transferring it to a low-temperature rejection process in a reservoir in order to complete the cycle, even condenser, the cycle cannot under idealized conditions. be completed. The 2nd Law of Thermodynamics: Kelvin–Planck Statement It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work. No heat engine can have a thermal efficiency of 100 percent, or as for a power plant to operate, the working fluid must exchange heat with the environment as well as the furnace. The impossibility of having a 100% A heat engine that efficient heat engine is not due to violates the Kelvin– friction or other dissipative effects. It is a Planck statement of the limitation that applies to both the second law. idealised and the actual heat engines. REFRIGERATORS AND HEAT PUMPS In a household refrigerator, the freezer compartment where heat is absorbed by the refrigerant serves as the evaporator, and the coils usually behind the refrigerator where heat is dissipated to the kitchen air serve as the condenser. Basic components of a refrigeration system and typical operating conditions. REFRIGERATORS AND HEAT PUMPS The transfer of heat from a low- temperature medium to a high- temperature one requires special devices called refrigerators. Refrigerators, like heat engines, are cyclic devices. The working fluid used in the refrigeration cycle is called a refrigerant. The most frequently used refrigeration cycle is the vapor- compression refrigeration cycle.  Refrigerator: Coefficient of Performance The efficiency of a refrigerator is expressed in terms of the coefficient of performance (COP). The objective of a refrigerator is to remove heat (QL) from the refrigerated space. COPR value is greater than unity  Heat Pumps The objective of a heat pump is to supply heat QH into the warmer space. The work supplied to a heat pump is used to extract energy from the cold outdoors and carry it into the warm indoors. for fixed values of QL and QH  Most heat pumps in operation today have a seasonally averaged COP of 2 to 3. Most existing heat pumps use the cold outside air as the heat source in winter (air-source HP). In cold climates their efficiency drops considerably when temperatures are below the freezing point. In such cases, geothermal (ground-source) HP that use the ground as the heat source can be used. Such heat pumps are more expensive to install, but they are also more efficient. Air conditioners are basically refrigerators whose When installed refrigerated space is a room or a building instead of the backward, an air food compartment. conditioner functions The COP of a refrigerator decreases with decreasing as a heat pump. refrigeration temperature. Therefore, it is not economical to refrigerate to a lower temperature than needed. The 2nd Law of Thermodynamics: Clasius Statement It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower- temperature body to a higher-temperature body. It states that a refrigerator cannot operate unless its compressor is driven by an external power source, such as an electric motor. To date, no experiment has been A refrigerator that conducted that contradicts the second violates the law, and this should be taken as Clausius statement of the second law. sufficient proof of its validity. REVERSIBLE AND IRREVERSIBLE PROCESSES Reversible process: A process that can be reversed without leaving any trace on the surroundings. Irreversible process: A process that is not reversible. All the processes occurring in nature are irreversible. Why are we interested in reversible processes? (1) they are easy to analyse and (2) they serve as idealised models (theoretical limits) to which actual processes can be compared. Some processes are more irreversible than others. We try to approximate reversible processes. Two familiar Reversible processes deliver the most and consume the reversible processes. least work. Effect on surroundings – figure 9(b).  The factors that cause a process to be irreversible are called irreversibility. Friction They include friction, unrestrained expansion, mixing renders a of two fluids, heat transfer across a finite temperature process difference, electric resistance, inelastic deformation of irreversible. solids, and chemical reactions. The presence of any of these effects renders a process irreversible. Irreversibilities (a) Heat transfer through a temperature difference is irreversible, and Irreversible (b) the reverse compression process is and impossible. expansion processes. 24 THE CARNOT CYCLE – Theoretical Thermodynamic Cycle P-V diagram of the Carnot cycle P-V diagram of the reversed Carnot cycle The Reversed Carnot Cycle: The Carnot heat-engine cycle is a totally reversible cycle. Therefore, all the processes that comprise it can be reversed, in which case it becomes the Carnot refrigeration cycle. THE CARNOT CYCLE Execution of the Carnot cycle in a closed system. Reversible Isothermal Expansion (process 1-2, TH = constant) Reversible Adiabatic Expansion (process 2-3, temperature drops from TH to TL) Reversible Isothermal Compression (process 3-4, TL = constant) Reversible Adiabatic Compression (process 4-1, temperature rises from TL to TH26) THE CARNOT PRINCIPLES The Carnot principles. 1. The efficiency of an irreversible heat engine is always less than the efficiency of a reversible one operating between the same two reservoirs. 2. The efficiencies of all reversible heat engines operating between the same two reservoirs are the same.  This temperature scale is called the Kelvin scale, and the temperatures on this scale are called absolute temperatures. For reversible cycles, the A conceptual experimental setup to heat transfer ratio QH /QL can determine thermodynamic be replaced by the absolute temperatures on the Kelvin scale by temperature ratio TH /TL. measuring heat transfers QH and QL.  THE CARNOT HEAT ENGINE The Carnot heat engine is the most efficient of all heat engines operating between the same high- and low-temperature reservoirs. No heat engine can have a higher efficiency than a reversible heat engine operating between the same high- and low-temperature reservoirs. Any heat engine Carnot heat engine  The Quality of Energy The higher the temperature The fraction of heat that can be converted to of the thermal energy, the work as a function of source temperature. higher its quality. THE CARNOT REFRIGERATOR AND HEAT PUMP Any refrigerator or heat pump Carnot refrigerator or heat pump No refrigerator can have a higher COP How do you increase the than a reversible refrigerator operating COP of a Carnot between the same temperature limits. refrigerator or heat pump? The COP of a reversible refrigerator or heat pump is the maximum theoretical value for the specified temperature limits. Actual refrigerators or heat pumps may approach these values as their designs are improved, but they can never reach them. The COPs of both the refrigerators and the heat pumps decrease as TL decreases. That is, it requires more work to absorb heat from lower-temperature media. Example - A reversible heat engine operates between a source (hot reservoir) at 1000K and a sink (cold reservoir) at 300K. The heat engine is supplied with heat at a rate of 800 kJ/min. Evaluate: (a) the thermal efficiency and (b) the power output of the engine. 34  Summary Introduction to the second law Heat engines Thermal efficiency The 2nd law: Kelvin-Planck statement Refrigerators and heat pumps Coefficient of performance (COP) Reversible and irreversible processes Irreversibility, reversible processes The Carnot cycle The reversed Carnot cycle The Carnot heat engine The quality of energy The Carnot refrigerator and heat pump Tomorrow's Tutorials will be on ‘Energy Transfer’ 36