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heat engine //موضوع كطريق استدلالي عن المكائن الحرارية

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  1. [11]
    حسن هادي
    حسن هادي غير متواجد حالياً
    عضو متميز
    الصورة الرمزية حسن هادي


    تاريخ التسجيل: Nov 2006
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    قانون الغاز المثالي

    Gases have various properties that we can observe with our senses, including the gas pressure p, temperature T, mass m, and volume V that contains the gas. Careful, scientific observation has determined that these variables are related to one another, and the values of these properties determine the state of the gas.
    If we fix any two of the properties we can determine the nature of the relationship between the other two. You can explore the relationship between the variables at the animated gas lab. If the pressure and temperature are held constant, the volume of the gas depends directly on the mass, or amount of gas. This allows us to define a single additional property called the gas density r, which is the ratio of mass to volume. If the mass and temperature are held constant, the product of the pressure and volume are observed to be nearly constant for a real gas. The product of pressure and volume is exactly a constant for an ideal gas. This relationship between pressure and volume is called Boyle's Law in honor of Robert Boyle who first observed it in 1660. Finally, if the mass and pressure are held constant, the volume is directly proportional to the temperature for an ideal gas. This relationship is called Charles and Gay-Lussac's Law in honor of the two French scientists who discovered the relationship.
    The gas laws of Boyle and Charles and Gay-Lussac can be combined into a single equation of state given in red at the center of the slide:
    p * V / T = n * Rbar
    where * denotes multiplication and / denotes division. To account for the effects of mass, we have defined the constant to contain two parts: a universal constant Rbar (on the figure, an R with a bar drawn over the top) and the mass of the gas expressed in moles n. Performing a little algebra, we obtain the more familiar form:
    p * V = n * Rbar * T
    A three dimensional graph of this equation is shown at the lower left. The intersection point of any two lines on the graph gives a unique state for the gas.
    Engineers use a slightly different form of the equation of state that is specialized for a particular gas. If we divide both sides of the general equation by the mass of the gas, the volume becomes the specific volume, which is the inverse of the gas density. We also define a new gas constant R, which is equal to the universal gas constant divided by the mass per mole of the gas. The value of the new constant depends on the type of gas as opposed to the universal gas constant, which is the same for all gases. The value of the equation of state for air is given on the slide as .286 kilo Joule per kilogram per Kelvin. The equation of state can be written in terms of the specific volume or in terms of the air density as
    p * v = R * T
    p = r * R * T
    Notice that the equation of state given here applies only to an ideal gas, or a real gas that behaves like an ideal gas. There are in fact many different forms for the equation of state for different gases. Also be aware that the temperature given in the equation of state must be an absolute temperature that begins at absolute zero. In the metric system of units, we must specify the temperature in Kelvin (not degrees Celsius). In the Imperial system, absolute temperature is in Rankine (not degrees Fahrenheit).

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  2. [12]
    حسن هادي
    حسن هادي غير متواجد حالياً
    عضو متميز
    الصورة الرمزية حسن هادي


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    Thermodynamics is a branch of physics which deals with the energy and work of a system. Thermodynamics deals only with the large scale response of a system which we can observe and measure in experiments. Small scale gas interactions are described by the kinetic theory of gases. There are three principal laws of thermodynamics which are described on separate slides. Each law leads to the definition of thermodynamic properties which help us to understand and predict the operation of a physical system. We will present some simple examples of these laws and properties for a variety of physical systems, although we are most interested in the thermodynamics of propulsion systems and high speed flows. Fortunately, many of the classical examples of thermodynamics involve gas dynamics.
    In our observations of the work done on, or by a gas, we have found that the amount of work depends not only on the initial and final states of the gas but also on the process, or path which produces the final state. Similarly the amount of heat transferred into, or from a gas also depends on the initial and final states and the process which produces the final state. Many observations of real gases have shown that the difference of the heat flow into the gas and the work done by the gas depends only on the initial and final states of the gas and does not depend on the process or path which produces the final state. This suggests the existence of an additional variable, called the internal energy of the gas, which depends only on the state of the gas and not on any process. The internal energy is a state variable, just like the temperature or the pressure. The first law of thermodynamics defines the internal energy (E) as equal to the difference of the heat transfer (Q) into a system and the work (W) done by the system.
    E2 - E1 = Q - W
    We have emphasized the words "into" and "by" in the definition. Heat removed from a system would be assigned a negative sign in the equation. Similarly work done on the system is assigned a negative sign.
    The internal energy is just a form of energy like the potential energy of an object at some height above the earth, or the kinetic energy of an object in motion. In the same way that potential energy can be converted to kinetic energy while conserving the total energy of the system, the internal energy of a thermodynamic system can be converted to either kinetic or potential energy. Like potential energy, the internal energy can be stored in the system. Notice, however, that heat and work can not be stored or conserved independently since they depend on the process. The first law of thermodynamics allows for many possible states of a system to exist, but only certain states are found to exist in nature. The second law of thermodynamics helps to explain this observation.
    If a system is fully insulated from the outside environment, it is possible to have a change of state in which no heat is transferred into the system. Scientists refer to a process which does not involve heat transfer as an adiabatic process. The implementation of the first law of thermodynamics for gases introduces another useful state variable called the enthalpy which is described on a separate page.

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  3. [13]
    شكرى محمد نورى
    شكرى محمد نورى غير متواجد حالياً
    مشرف


    تاريخ التسجيل: Mar 2006
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    وسام الاشراف

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    الأخ حسن هادي .

    اجمل المنى .

    عطاء دائم وموفق بأذنه تعالى .

    [COLOR="Purple"]نثمن جهود ومثابرتك على اطروحاتك المتجددة .[/COLOR]

    مع التحية والسلام .

    البغدادي .

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  4. [14]
    حسن هادي
    حسن هادي غير متواجد حالياً
    عضو متميز
    الصورة الرمزية حسن هادي


    تاريخ التسجيل: Nov 2006
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    نرجوا ان نكون قد وّفقنا في وضع الروابط المناسبة مع التوضيح الخاص بها لمنعفة الاخوة الاعضاء والله ولي التوفيق وندرج لكم في هذه المشاركة دورة كارنوت الحرارية وبامكانكم متابعة الدورات الاخرى في الروابط المدرجة في المشاركة المرقمة #5 مع التقدير *
    ********************************
    Carnot cycle

    From Wikipedia, the free encyclopedia


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    The Carnot cycle is a particular thermodynamic cycle, modeled on the hypothetical Carnot heat engine, proposed by Nicolas Léonard Sadi Carnot in the 1820s and expanded upon by Benoit Paul Émile Clapeyron in the 1830s and 40s.
    Every thermodynamic system exists in a particular state. A thermodynamic cycle occurs when a system is taken through a series of different states, and finally returned to its initial state. In the process of going through this cycle, the system may perform work on its surroundings, thereby acting as a heat engine.
    A heat engine acts by transferring energy from a warm region to a cool region of space and, in the process, converting some of that energy to mechanical work. The cycle may also be reversed. The system may be worked upon by an external force, and in the process, it can transfer thermal energy from a cooler system to a warmer one, thereby acting as a refrigerator rather than a heat engine.
    The Carnot cycle is a special type of thermodynamic cycle. It is special because it is the most efficient cycle possible for converting a given amount of thermal energy into work or, conversely, for using a given amount of work for refrigeration purposes
    .************************************************* **
    The Carnot cycle
    The Carnot cycle when acting as a heat engine consists of the following steps:
    1. Reversible isothermal expansion of the gas at the "hot" temperature, TH (isothermal heat addition). During this step (A to B on diagram) the expanding gas causes the piston to do work on the surroundings. The gas expansion is propelled by absorption of heat from the high temperature reservoir.
    2. Isentropic (Reversible adiabatic) expansion of the gas. For this step (B to C on diagram) we assume the piston and cylinder are thermally insulated, so that no heat is gained or lost. The gas continues to expand, doing work on the surroundings. The gas expansion causes it to cool to the "cold" temperature, TC.
    3. Reversible isothermal compression of the gas at the "cold" temperature, TC. (isothermal heat rejection) (C to D on diagram) Now the surroundings do work on the gas, causing heat to flow out of the gas to the low temperature reservoir.
    4. Isentropic compression of the gas. (D to A on diagram) Once again we assume the piston and cylinder are thermally insulated. During this step, the surroundings do work on the gas, compressing it and causing the temperature to rise to TH. At this point the gas is in the same state as at the start of step 1.

    Figure 1: A Carnot cycle acting as a heat engine, illustrated on a temperature-entropy diagram. The cycle takes place between a hot reservoir at temperature TH and a cold reservoir at temperature TC. The vertical axis is temperature, the horizontal axis is entropy.




    [edit] Properties and significance


    [edit] The temperature-entropy diagram


    A generalized thermodynamic cycle taking place between a hot reservoir at temperature TH and a cold reservoir at temperature TC. By the second law of thermodynamics, the cycle cannot extend outside the temperature band from TC to TH. The area in red ΔQC is the amount of energy exchanged between the system and the cold reservoir. The area in white Δ W is the amount of work energy exchanged by the system with its surroundings. The amount of heat exchanged with the hot reservoir is the sum of the two. If the system is behaving as an engine, the process moves clockwise around the loop, and moves counter-clockwise if it is behaving as a refrigerator. The efficiency of the cycle is the ratio of the white area (work) divided by the sum of the white and red areas (total heat).


    The behavior of a Carnot engine or refrigerator is best understood by using a temperature-entropy (TS) diagram, in which the thermodynamic state is specified by a point on a graph with entropy (S) as the horizontal axis and temperature (T) as the vertical axis. For a simple system with a fixed number of particles, any point on the graph will represent a particular state of the system. A thermodynamic process will consist of a curve connecting an initial state (A) and a final state (B). The area under the curve will be:
    which is the amount of thermal energy transferred in the process. If the process moves to greater entropy, the area under the curve will be the amount of heat absorbed by the system in that process. If the process moves towards lesser entropy, it will be the amount of heat removed. For any cyclic process, there will be an upper portion of the cycle and a lower portion. For a clockwise cycle, the area under the upper portion will be the thermal energy absorbed during the cycle, while the area under the lower portion will be the thermal energy removed during the cycle. The area inside the cycle will then be the difference between the two, but since the internal energy of the system must have returned to its initial value, this difference must be the amount of work done by the system over the cycle. Mathematically, for a reversible process we may write the amount of work done over a cyclic process as:
    Since dU is an exact differential, its integral over any closed loop is zero and it follows that the area inside the loop on a T-S diagram is equal to the total work performed if the loop is traversed in a clockwise direction, and is equal to the total work done on the system as the loop is traversed in a counterclockwise direction.

    [edit] The Carnot cycle


    A Carnot cycle taking place between a hot reservoir at temperature TH and a cold reservoir at temperature TC.


    Evaluation of the above integral is particularly simple for the Carnot cycle. The amount of energy transferred as work is
    The total amount of thermal energy transferred between the hot reservoir and the system will be
    and the total amount of thermal energy transferred between the system and the cold reservoir will be
    . The efficiency η is defined to be:
    where
    ΔW is the work done by the system (energy exiting the system as work), ΔQH is the heat put into the system (heat energy entering the system), TC is the absolute temperature of the cold reservoir, and TH is the temperature of the hot reservoir. This efficiency makes sense for a heat engine, since it is the fraction of the heat energy extracted from the hot reservoir and converted to mechanical work. It also makes sense for a refrigeration cycle, since it is the ratio of energy input to the refrigerator divided by the amount of energy extracted from the hot reservoir.

    [edit] Carnot's theorem

    Main article: Carnot's theorem (thermodynamics)
    It can be seen from the above diagram, that for any cycle operating between temperatures TH and TC, none can exceed the efficiency of a Carnot cycle.

    A real engine (left) compared to the Carnot cycle (right). The entropy of a real material changes with temperature. This change is indicated by the curve on a T-S diagram. For this figure, the curve indicates a vapor-liquid equilibrium (See Rankine cycle). Irreversible systems and losses of heat (for example, due to friction) prevent the ideal from taking place at every step.


    Carnot's theorem is a formal statement of this fact: No engine operating between two heat reservoirs can be more efficient than a Carnot engine operating between the same reservoirs. Thus, Equation 3 gives the maximum efficiency possible for any engine using the corresponding temperatures. A corollary to Carnot's theorem states that: All reversible engines operating between the same heat reservoirs are equally efficient. Rearranging the right side of the equation gives what may be a more easily understood form of the equation. Namely that the theoretical maximum efficiency of a heat engine equals the difference in temperature between the hot and cold reservoir divided by the absolute temperature of the hot reservoir. To find the absolute temperature in kelvins, add 273 degrees to the Celsius temperature. Looking at this formula an interesting fact becomes apparent. Lowering the temperature of the cold reservoir will have more effect on the ceiling efficiency of a heat engine than raising the temperature of the hot reservoir by the same amount. In the real world, this may be difficult to achieve since the cold reservoir is often an existing ambient temperature.
    In other words, maximum efficiency is achieved if and only if no new entropy is created in the cycle. Otherwise, since entropy is a state function, the required dumping of heat into the environment to dispose of excess entropy leads to a reduction in efficiency. So Equation 3 gives the efficiency of any reversible heat engine.

    [edit] Efficiency of real heat engines

    Carnot realized that in reality it is not possible to build a thermodynamically reversible engine, so real heat engines are less efficient than indicated by Equation 3. Nevertheless, Equation 3 is extremely useful for determining the maximum efficiency that could ever be expected for a given set of thermal reservoirs.
    Although Carnot's cycle is an idealisation, the expression of Carnot efficiency is still useful. Consider the average temperatures,
    at which heat is input and output, respectively. Replace TH and TC in Equation (3) by <TH> and <TC> respectively.
    For the Carnot cycle, or its equivalent, <TH> is the highest temperature available and <TC> the lowest. For other less efficient cycles, <TH> will be lower than TH , and <TC> will be higher than TC. This can help illustrate, for example, why a reheater or a regenerator can improve thermal efficiency.
    See also: Heat Engine (efficiency and other performance criteria) الروابط فعالة مع الود والاحترام

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  5. [15]
    حسن هادي
    حسن هادي غير متواجد حالياً
    عضو متميز
    الصورة الرمزية حسن هادي


    تاريخ التسجيل: Nov 2006
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    اقتباس المشاركة الأصلية كتبت بواسطة شكرى محمد نورى مشاهدة المشاركة
    الأخ حسن هادي .

    اجمل المنى .

    عطاء دائم وموفق بأذنه تعالى .

    [COLOR="Purple"]نثمن جهود ومثابرتك على اطروحاتك المتجددة .[/color]

    مع التحية والسلام .

    البغدادي .
    حياك الله يا مشرفنا العزيز وتقبل تحياتنا
    تحياتي اخوكم حسن العراقي *

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  6. [16]
    سنان عبد الغفار
    سنان عبد الغفار غير متواجد حالياً
    عضو فعال جداً


    تاريخ التسجيل: May 2006
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    مشكور كثيرا على ابداعاتك ايها العضو الاكثر من المتميز جدا

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  7. [17]
    حسن هادي
    حسن هادي غير متواجد حالياً
    عضو متميز
    الصورة الرمزية حسن هادي


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    اقتباس المشاركة الأصلية كتبت بواسطة سنان عبد الغفار مشاهدة المشاركة
    مشكور كثيرا على ابداعاتك ايها العضو الاكثر من المتميز جدا
    اعتز بمداخلتك يا اخي الكريم سنان وحياك الله اخوكم حسن عراق

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  8. [18]
    مهندس نورس
    مهندس نورس غير متواجد حالياً
    عضو فعال


    تاريخ التسجيل: Dec 2006
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    عزيزي حسن هادي .

    اطلعت على اغلب مواضيعك التي كتبتها ووجدتها هي بمثابة ملحمة هندسية رائعة وثرية ولم اجد لها

    مثيل يذكر حقا انك انسان ومهندس منقطع النظير .

    استمر على هذا النمط ويباركك الله على جهودك .

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  9. [19]
    حسن هادي
    حسن هادي غير متواجد حالياً
    عضو متميز
    الصورة الرمزية حسن هادي


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    اقتباس المشاركة الأصلية كتبت بواسطة مهندس نورس مشاهدة المشاركة
    عزيزي حسن هادي .

    اطلعت على اغلب مواضيعك التي كتبتها ووجدتها هي بمثابة ملحمة هندسية رائعة وثرية ولم اجد لها

    مثيل يذكر حقا انك انسان ومهندس منقطع النظير .

    استمر على هذا النمط ويباركك الله على جهودك .
    حياك الله اخي العزيز يا مهندس نورس وبارك الله فيك اخي الكريم وجعلنا الله من المؤهلين لنيل رضاه ورضاكم والله ولي التوفيق اخوكم حسن هادي

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