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Chapter 7 Thermal and Energy Systems is prepared by: Dr. Roozbeh Vaziri MENG&MECT190 Purpose of This Chapter Calculate various energy, heat, work, and power quantities that are encountered in mechanical engineering, and express their numerical values in the SI and USCS. Describe how heat is transferred from one location to another by the processes of conduction, convection, and radiation. Apply the principle of energy conservation for a mechanical system. Explain how heat engines operate and understand the limitations on their efficiency. • Energy is needed to accelerate an object, stretch it, heat it, and elevate it. • In the internal-combustion engine of Figure, diesel fuel is burned to release thermal energy. • The engine converts thermal energy into the rotation of its crankshaft and ultimately into the motion of a vehicle. • Gravitational Potential Energy Near the surface of the Earth, the acceleration of gravity is given by the standard acceleration values Gravitational potential energy is associated with changing the elevation of an object within a gravitational field, and it is measured relative to a reference height, for instance the ground or the top of a workbench. • Elastic Potential Energy Elastic potential energy is stored by an object when it is stretched or bent in the manner described by Hooke’s law Hooke's law is a principle of physics that states that the force F needed to extend or compress a spring by some distance X is proportional to that distance. That is: F = kX, where k is a constant factor characteristic of the spring: its hardness, and X is small compared to the total possible deformation of the spring. For a spring having stiffness k, the elastic potential energy stored within it is given by • DL is the spring’s elongation, defined as the distance that it has been stretched or compressed. • Kinetic Energy • Kinetic energy is associated with an object’s motion. • As forces or moments act on a machine, they cause its components to move and store kinetic energy by virtue of velocity. • The motion can be in the form of Vibration (the cone of a stereo speaker) Rotation (the flywheel attached to an engine’s crankshaft) Translation (the straight-line motion of the piston in an engine or compressor). • Work of a Force The work of a force is illustrated in Figure in the context of a piston that slides horizontally in its cylinder. The force F is applied to the piston to compress the gas in the cylinder while the piston moves to the right • If the gas already has been compressed to a high pressure and the piston moves to the left the force F can be applied to resist that expansion. • The work W of the force as the piston moves through the distance is defined by • Power • When a force performs work during the interval of time , the average power is • As work is performed more rapidly becomes smaller, and the average power increases accordingly • Heating Value • When a fuel is burned, the chemical reactions that take place release thermal energy and waste products including water vapor, carbon monoxide, and particulate material. • Mechanical engineers design machinery that manages the release of energy stored in chemical form, and that energy is subsequently converted to more useful forms. • In a furnace, power plant, or gasoline engine, the energy output of the combustion process is measured by a quantity called the heating value H. • Heating values are the amount of energy released per unit mass of fuel burned. • In calculations involving the combustion of a fuel, the heat Q that is released by burning mass m is given by • Specific Heat • As heat flows into an object, its temperature rises from an initial value T0 to T according to • The specific heat, the parameter c is a property that captures how materials differ with respect to the amount of heat they must absorb to raise their temperature. • The physical property that quantifies the amount of heat that must flow into or out of a material to produce a phase change is called the latent heat. • Transfer of Heat • heat as energy that is being transferred from one location to another because of a temperature difference. • The three mechanisms for heat transfer are known as conduction, convection, and radiation • Heat Conduction The quantity of heat that flows along the rod during a time interval Δt is given by This principle is known as Fourier’s law of heat conduction • Thermal conduction is the transfer of heat (internal energy) by microscopic collisions of particles and movement of electrons within a body. • The rate at which energy is conducted as heat between two bodies is a function of the temperature difference (temperature gradient) between the two bodies and the properties of the conductive medium through which the heat is transferred. • Thermal conduction was originally called diffusion. • Heat convection • Heat can also be transferred by a fluid that is in motion; that process is known as convection. • The cooling system of an automobile engine, for instance, operates by pumping a mixture of water and antifreeze through passageways inside the engine’s block. • Excess heat is removed from the engine, transferred temporarily to the coolant by convection, and ultimately released into the air by the vehicle’s radiator. • Because a pump circulates the coolant, heat transfer is said to occur by forced convection. • A liquid or gas can circulate on its own, without the help of a pump or fan, because of the buoyancy forces created by temperature variations within the fluid. • As air is heated, it becomes less dense, and buoyancy forces cause it to rise and circulate. The rising flow of warm fluid(and the falling flow of cooler fluid to fill its place) is called natural convection. • Heat radiation • The third mechanism of heat transfer is radiation, which refers to the emission and absorption of heat without direct physical contact. • Radiation occurs when heat is transmitted by the long infrared waves of the electromagnetic spectrum. ENERGY CONSERVATION AND CONVERSION • With the concepts of energy, work, and heat in mind, we will now explore the conversion of energy from one form to another. • The chemical energy stored in the fuel (be it gasoline, jet fuel, or natural gas) is released as heat, which in turn is converted into mechanical work. • The principles of energy conservation and conversion are built on the concept of a system, which is a collection of materials and components that are grouped together with respect to their thermal and energy behavior. • System is imagined to be isolated • All forces that crossed an imaginary boundary drawn around the body were included on the diagram, and other effects were disregarded. • The first law of thermodynamics states that these three quantities balance according to • Heat Q is positive when it flows into the system; W is positive when the system performs work on its surroundings; and DU is positive when the system’s internal energy increases. HEAT ENGINES AND EFFICIENCY • The heat engine sketched schematically in Figure represents any machine that is capable of converting the heat supplied to it into mechanical work. • The sources of energy, or heat reservoirs, are large enough so that their temperatures do not change as heat is removed or added. • In the context of an automotive engine, Qh represents the heat released by burning fuel in the engine’s combustion chamber; W is the mechanical work associated with the rotation and torque of the engine’s crankshaft; Ql is the heat that warms the engine block and is expelled from the exhaust • The second law of thermodynamics: No machine can operate in a cycle and only transform the heat supplied to it into work without also rejecting part of the supplied heat. • The ideal Carnot efficiency of the heat engine is given by