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Aastha_tandukar
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heat transferthermodynamics
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    sonal_shahi Beginner

    First law of thermodynamics states that during any cycle that a system undergoes, the cyclic integral of the heat equals cyclic integrals of the work. The first law, however, places no restriction on the direction of flow of heat and work. A cycle in which a given amount of heat is transferred fromRead more

    First law of thermodynamics states that during any cycle that a system undergoes, the cyclic integral of the heat equals cyclic integrals of the work. The first law, however, places no restriction on the direction of flow of heat and work. A cycle in which a given amount of heat is transferred from the system and an equal amount of work is done on the system satisfies the first law just as well as a cyclic in which the flow of heat and work are reversed. However, we know from our experience that a proposed cycle that does not violate the first law does not ensure that the cycle will actually occur. It is a kind of experimental evidence that led to the formulation of the second law of thermodynamics. Thus a cycle will occur only if both the first law and second law of thermodynamics are satisfied.

    Consider a hot water glass left in a cooler room that eventually cools off. This process satisfies the first law of thermodynamics as the amount of energy lost by hot water equals amount of heat gained by the surroundings. If we consider a reverse process, i.e., hot water getting even hotter in a cooler room as a result of heat transfer from the room air. It is obvious that this process never takes place. Furthermore, this process does not violet first law of thermodynamics as long as amount heat lost by air equals the amount of heat gained by hot water

    Gasoline is  used as a car drives up hill, but the fuel in gasoline tank cannot be restored to it’s original level when car coasts down the hill. However, this process as well does not violate first law of thermodynamics.

    These arguments clarify that processes proceed in a certain directions and not in the reverse direction. The first law of thermodynamics places no restriction on the direction of a processes and satisfying the first law of thermodynamics does not ensure that the process can actually occur. This inadequacy of the first law of thermodynamics to identify whether a process can take place in remedied by introducing another general principle, called second law of thermodynamics. The reversed processed discussed in above examples violet the second law of thermodynamics.

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sonal_shahi
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heat transferthermodynamics
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    Aastha_tandukar Beginner

    Similarities i) Work transfer and heat transfer are transient phenomena. Systems do not possess heat or work. Whenever systems undergo changes of state, they can cross the system boundary. ii) They are observed only at the boundaries of the system. iii) They are path functions and inexact differentiRead more

    Similarities

    i) Work transfer and heat transfer are transient phenomena. Systems do not possess heat or work. Whenever systems undergo changes of state, they can cross the system boundary.

    ii) They are observed only at the boundaries of the system.

    iii) They are path functions and inexact differentials.

    iv) They are not a property of a system.

     

    Differences:

    i) Work transfer is defined as the way by which a system may exchange energy with its surroundings. Work is said to be transferred if a certain system is displaced by a force in the direction of it.

    Heat transfer is defined as the form of energy that is transferred across the boundary of a system at a given temperature to another system or the surrounding at a lower temperature by virtue of the temperature difference between the two systems.

    ii) In thermodynamics, the work done by the system is considered as positive work transfer and work done on the system is considered as negative work transfer whereas heat transferred to a system is considered as positive heat transfer and heat transfer from a system is considered as negative heat transfer.

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Aastha_tandukar
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heat transferthermodynamics
  1. sonal_shahi Beginner

    The perpetual motion was originally conceived as a purely mechanical contrivance when once set in motion would continue to run forever. Such a machine would be merely a curiosity of no practical value and we know that the presence of friction makes it impossible. One would be of immense value is a mRead more

    The perpetual motion was originally conceived as a purely mechanical contrivance when once set in motion would continue to run forever. Such a machine would be merely a curiosity of no practical value and we know that the presence of friction makes it impossible. One would be of immense value is a machine producing a continuous supply of work without absorbing energy from the surroundings, such as a machine is called a perpetual motion machine of the first kind.

    It is always possible to devise a machine to deliver a limited quantity of work without requiring a source of energy in the surroundings. For example, a gas compressed behind a piston will expand and do work at the expense of the internal work of the gas. Such a device cannot produce work continuously, however, for this to be achieved the machine must be capable of undergoing a succession of cyclic processes. An equation  ΣδQ=ΣδW states that if a net amount of heat is not supplied by the surroundings during a cycle, no net amount of work can be delivered by the system. Hence, the first law of thermodynamics implies that a perpetual motion machine of the first kind is impossible.

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kirpathapa
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heat transferthermodynamics
  1. lidiya thapa Beginner

    Merits of a two stroke engine over four stroke engine i) A two stroke engine has twice the number of power stroke than a four stroke engine at the same speed. Hence theoretically a two stroke engine develops double the power per cubic meter of the swept volume than the four stroke engine. ii) The weRead more

    Merits of a two stroke engine over four stroke engine

    i) A two stroke engine has twice the number of power stroke than a four stroke engine at the same speed. Hence theoretically a two stroke engine develops double the power per cubic meter of the swept volume than the four stroke engine.

    ii) The weight of two stroke engine is less than four stroke engine because of lighter flywheel due to more uniform torque on crankshaft.

    iii) The scavenging is more complete in low -speed two stroke engines, since exhaust gases are not left in the clearance volume as in the four stroke engine.

    iv) Since there are only two stroke in the cycle, the work required to overcome the friction and the exhaust strokes is saved.

    v) Since there are no mechanical valves and valve gears, construction of two strokes engine is simple which reduces its initial costs.

    vi) A two stroke engine can be easily reversed by a simple reversing gear mechanism.

    vii) A two stroke engine can be easily started than a four stroke engine.

    viii) A two stroke engine occupies less space.

    ix) A lighter foundation will be sufficient for two stroke engine.

    x) A two stroke engine has less maintenance cost since it requires only few parts.

    Demerits of two stroke engine compared with four stroke engine

    i) Since the firing takes place in every revolution, the time available for cooling will be less than in a four stroke engine.

    ii) Incomplete scavenging results in mixing of exhaust gases with the fresh charge which will dilute it, hence lesser power output.

    iii) Since the transfer port is kept open only during a short period, less quantity of the charge will be admitted into the cylinder which will reduce the power output.

    iv) Since both the exhaust and the transfer port are kept open during the same period, there is possibility of escaping of the fresh charge through exhaust port which will reduce thermal efficiency.

    v) For a given stroke and clearance volume, the effective compression stroke is less in a two stroke engine than in a four stroke engine.

    vi) In a crankcase compressed type of two stroke engine, the volume of charge down in the crankcase is less due to the reduction in the crankcase volume because of rotating parts.

    vii) A fan scavenged two stroke engine has less mechanical efficiency since some power is required to run the scavenged fan.

    viii) A two stroke engine needs better cooling arrangements because of high operating temperature.

    ix) A two stroke engine consumes more lubricating oil.

    x) The exhaust in a two stroke engine is noisy due to sudden release of the burnt gases.

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Aastha_tandukar
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heat transferthermodynamics
  1. sonal_shahi Beginner

    The characteristics of steady state steady flow process are; i) A steady state process is defined as a process during which a fluid flows through  a control volume steadily. ii) At a steady state of a system, any thermodynamic property will have a fixed value at a particular location and will not alRead more

    The characteristics of steady state steady flow process are;

    i) A steady state process is defined as a process during which a fluid flows through  a control volume steadily.

    ii) At a steady state of a system, any thermodynamic property will have a fixed value at a particular location and will not alter with time. Therefore, volume, mass, and total energy content of the control volume remain constant during a steady state process.

    iii) During steady state process, no intensive or extensive properties within the control volume change with time.

    iv) The boundary work transfer is zero for steady state system and the total mass or energy entering a control volume must be equals total mass or energy leaving it.

    v) The fluid properties at an inlet or an outlet remain constant during a steady state process. The properties may be different at different inlets and outlets. They might vary over the cross  section of an inlet or an outlet. However, all properties including velocity and elevation must remain constant with time at a fixed point at an inlet or an outlet. It follows that mass flow rate of fluid at an opening must remain constant during a steady state process.

    vi) The fluid properties at an opening are usually considered to be uniformed over the cross section. Thus, the  fluid properties at an inlet or an outlet may be specified by the average single values.

    vii) The heat and work interactions between a steady state process and its surroundings do not change with time. Hence, power delivered by a system remains constant during a  steady state process.

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sonal_shahi
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heat transferthermodynamics
  1. Aastha_tandukar Beginner

    It has been observed that when the experiments are performed at relatively low pressure and temperatures, most of the real gases obey Boyle’s law and Charles’s law appreciably. However, behavior of real gases deviates considerably at elevated pressures and low temperatures. The ideal gas equation caRead more

    It has been observed that when the experiments are performed at relatively low pressure and temperatures, most of the real gases obey Boyle’s law and Charles’s law appreciably. However, behavior of real gases deviates considerably at elevated pressures and low temperatures. The ideal gas equation can be derived analytically using kinetic theory of gases with the assumptions.

    i) A finite volume of gas contains large number of molecules.

    ii) The size of the molecules is much smaller than average distance between the molecules. Therefore, they do not interact with each other or with the walls of the container except during collision.

    iii) The collision between the molecules or their collision with the walls is perfectly elastic.

    iv) The time duration of collision is negligibly small in comparison to time duration between two successive collisions of a molecule.

    As long as these assumption are valid, the behavior of a real gas approaches nearly that of ideal gas.

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Aastha_tandukar
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heat transferthermodynamics
  1. sonal_shahi Beginner

    A system which is homogeneous in composition, homogeneous in chemical aggregation and invariable in chemical aggregation is said to be pure substance. Homogeneous composition refers the composition of each part of the system is same as the composition of every other part. Homogeneous  chemical compoRead more

    A system which is homogeneous in composition, homogeneous in chemical aggregation and invariable in chemical aggregation is said to be pure substance. Homogeneous composition refers the composition of each part of the system is same as the composition of every other part. Homogeneous  chemical composition refers chemical element must be combined chemically in the same way in all parts of the system. Invariable chemical aggregation refers the state of chemical combination of system does not change with time.

    A pure substance can have multiple phases. An  ice-water mixture is a pure substance. Air is being composed of a mixture of nitrogen, oxygen and other gases, formally not a pure substance. However, experience show that we can often be treated air as a pure substance with little error. An air-steam mixture is not a pure substance.

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Aastha_tandukar
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energyenergy transferquestionthermodynamics
  1. sonal_shahi Beginner

    The ability to perform work is said to be energy of a system. Alternatively, energy of a system can be defined as capacity of system to exert force for a certain interval. If a system has high energy content, it can provide higher magnitude of force for longer interval of time. Exchange of energy acRead more

    The ability to perform work is said to be energy of a system. Alternatively, energy of a system can be defined as capacity of system to exert force for a certain interval. If a system has high energy content, it can provide higher magnitude of force for longer interval of time.

    Exchange of energy across the boundary of a system due to the intensive property difference between system and surrounding is said to be energy transfer. Energy transfer occurs as heat transfer or work transfer.

    A form of energy which constitutes total energy of a system, remains within the system boundary as inherent property of a system is said to be stored energy. Internal energy, potential energy, kinetic energy,etc., are examples of stored energy. Stored energy has unique value for each equilibrium state and is independent of path. Hence, it is a thermodynamic property.

    A form of energy that can cross the boundary of a system during thermodynamic process is said to be transient energy. Heat transfer and work transfer are examples of transient energy. Transient energy is a a dynamic form of energy and do not have unique value for each equilibrium state. It depends on properties of system as well as properties of surrounding. It is dependent on path. Thus, it is not thermodynamic property.

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Aastha_tandukar
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heat energyquestionthermodynamics
  1. sonal_shahi Beginner

    If a process can be carried out in such a way that the effects produced by it on the system as well as the surroundings can be completely restored to their initial state and no changes are left in any of the systems taking part in the process or to the surroundings, then the process is said to be aRead more

    If a process can be carried out in such a way that the effects produced by it on the system as well as the surroundings can be completely restored to their initial state and no changes are left in any of the systems taking part in the process or to the surroundings, then the process is said to be a reversible process. In this process, the system passes through a succession of equilibrium state or states that differ from equilibrium only infinitesimally. Such a process must occur slowly in a controlled manner. If a process does not satisfy these conditions, it is said to be irreversible process, i.e., the processes which produce a permanent change in the thermodynamic state of the system and cannot be retraced in the opposite manner, are known as irreversible process.

    Examples of reversible processes are an infinitesimally slow expansion and compression of ideal gas at constant pressure, all the mechanical processes taking place under the action of conservative force, etc.

    Decay matter, flowing of current through a conductor, rusting of iron, etc. is examples of irreversible processes.

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