Thermodynamics - Heat

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Heat and EnergyHeat transfer mechanicsFuels and combustionThermal expansion

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HAET AND ENERGYHeatHeat is a form of energy which is transferred from one

body to another body at law temperature, by virtue ofthe temperature difference between the bodies.TemperatureTemperature is a scientific quantity which corresponds

to primary sensations of hotness and coldness

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System A system may be defined as a collection of matter within prescribedand identifiable boundaries which are not necessarily inflexible. Thereare three types of systems isolated system , closed system and opensystem in which there is a transfer of mass across the boundaries.

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body

boundary

surrounding

A body in a system existsin three states.1. Solid systems2. Liquid systems3. Gases systems


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Zeroth law of thermodynamics When two systems are in thermal contact and there is no flow of heatbetween them, they are said to be in thermal equilibrium.

when two bodies A and B are each in thermal equilibrium with thirdbody C, then A and B are also in thermal equilibrium with each other.

This is called the zeroth law of thermodynamics.

The zeroth law leads to the conclusion that temperature is welldefined physical quantity.

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A B

C


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Temperature scalesThermodynamic temperature scale (Absolute temperature scale)Thermodynamic temperature is denoted by the symbol T and ismeasured in Kelvin, symbol K. Kelvin is the SI unit of temperature or oftemperature change.The thermodynamic temperature scale uses a fixed point, the triplepoint of water. This is the temperature at which saturated water vapor,pure water and melting ice are all in equilibrium. The TPT is defined as273.16 K.Using the constant volume gas thermometer,for example, the pressure P tr is measured at triplepoint of water, 273.16 K. if the pressure is p at an

unknown temperature T on the thermodynamicscale,

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Temperature scales cntd..The Celsius and Fahrenheit Scales The Celsius temperature, symbol , is defined by , where T is thethermodynamic temperature. The ice point is a Celsius temperature of 0 0Cand the steam point, the temperature of steam at 760 mmHg pressure or1.013105 Pa is 1000C.

The Fahrenheit scale, used in USA but a smallerdegree than the Celsius scale and deferentzero of temperature. The relationship betweenthe Celsius and Fahrenheit scales is

Where is Fahrenheit temperature. The freezingand boiling temperature of water in Fahrenheit is320F and 2120F

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Heat Capacity, Specific Heat Capacity The heat capacity of a object, is the quantity of heat required to raise thetemperature by 1 degree. It is expressed in joules per Kelvin (JK -1) in SI units.The specific heat capacity of a substance is the heat required to increasethe temperature of 1 kg of it by 1 degree or in other words, it is the heatcapacity per kg of the substance. Specific heat capacity is expressed in joules per kilogram per Kelvin and the symbol is c. specific heat capacity of water is 4200 J kg -1 K-1.

heat capacity, C = mass specific heat capacitySensible heat due to temperature changes, of a system can be expressed as

Where m is the mass of the object, c itsspecific heat capacity and its temperaturechange. A common way of measuring specificheat capacities of solid or liquids is the methodof mixtures.

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Heat Transfer MechanicsThermal energy transfer occurs through three mechanisms:

1. Conduction2. Convection3. Radiation

Conduction occurs primarily in solids and to a lesser degree in fluids as warmer, more energetic molecules transfer their energy to cooleradjacent molecules. Convection occurs in liquids and gases, andinvolves the mass movement of molecules such as when stirring ormixing is involved.

The third way that heat is transferred is through electromagnetic

radiation of energy. Radiation needs no medium to flow through and,therefore, can occur even in a vacuum. Electromagnetic radiation isproduced when electrons lose energy and fall to a lower energy state.Both the wavelength and intensity of the radiation is directly related tothe temperature of the surface molecules or atoms.

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Fuels and Combustions FuelsFuel is any material that stores energy that can later be extracted to perform

mechanical work in a controlled manner. T he vast majority of fuels are based oncarbon, hydrogen or some combination of Carbon and hydrogen are calledHydrocarbonsCombustionCombustion is the process of chemical reaction between fuel and oxygen (reactants). Theprocess releases heat and produces products of combustion. The main elements whichare burn are CARBON, HYDROGEN and SULPHER. The Oxygen used in combustionprocess normally comes from the atmosphere and this brings nitrogen normally doesnothing in the process but makes up the bulk of gases remaining after combustion.

Combustion chemistry

In the case of fuels, the combustion of each element should be done separately. Theimportant rule is that the constant number of atoms of each substance before and afterthe process.

C + O2 CO2 mass ratio: 12 + 32 = 44Hence 1kg of C needs 32/12 kg of Oxygen and makes 44/12 kg of CO 2.

2H2 + O2 2H2O mass ratio : 4 + 32 = 36Hence 1kg of H2 needs 8kg of O 2 and makes of 9 kg of water.

S + O2 SO2 mass ratio : 32 + 32 = 64Hence 1 kg of S needs 1 kg of O2 and makes 2kg of SO 2.

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Fuels and Combustions cntd The constituents of air are given as percentage composition by mass and volume. Neglecting allother gases but Oxygen and Nitrogen, the composition is given in the table.

Calorific value of a fuel The calorific value of a fuel is the quantity of heat produced during complete combustion ofunit mass of that substance and it is usually expressed in (MJ/kg) mega-joules per kilogram

Heat transfer = Mass of fuel Calorific value

The combustion process generates water vapor and certain techniques may be used to recoverthe quantity of heat contained in this water vapor by condensing it.

The Higher Calorific Value (or Gross Calorific Value - GCV) suppose that the water ofcombustion is entirely condensed and that the heat contained in the water vapor is recovered.

The Lower Calorific Value (or Net Calorific Value - NCV) suppose that the products ofcombustion contains the water vapor and that the heat in the water vapor is not recovered.

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Substance By mass By volume

Oxygen 23 % 21 %Nitrogen 77 % 79 %


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Thermal expansion The effect of the temperature to change the dimension of material isgenerally considered as the thermal expansion. Most of the metalsexpand when they are heated and contract when they are cooled, theamount of expansion per degree rise of temperature differs withdifferent materials.Linear expansion

The amount of linear expansion is expressed by the coefficient of linearexpansion. The increase in length per unit length per degree increasein temperature is called the coefficient of linear expansion of the metal.

Let the final length of the rod is , L and the final length can be writtenas

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Thermal expansion cntd Superficial expansion

The coefficient of the superficial expansion is the increase in area perunit area per degree increase in temperature. A = A

A = A (1+ ) A - Increase in area - Coefficient of superficial expansion A - Initial area A - Final area

Consider a square has a unit areaincrease in area A = A A = (1+ )(1+ ) 1 = 2

is very small quantity for any metal therefore can be neglectedfrom the expression and expansion of metal is simplified to 2 . This value is equal to the coefficient of superficial expansion.

= 2

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1

11 (1+ )(1+ )


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Thermal expansion cntd Cubical expansion

The coefficient of cubical expansion is the increase in volume per unit volume perdegree increase in temperature.Let original volume V 0 and increase in temperature then

V= V 0 Considering a block of metal that has unit volume, it can be easily shown that = 3

Expansion of liquidsThere are no linear dimensions for liquids therefore the coefficient of cubicalexpansion of a liquid is an independent quantity.

Increase in volume, V = V

Apparent volume expansion

The difference between the volumetric expansion of the liquid and volumetricexpansion of its container is called the apparent or relative increase in volume ofliquid.Relative increase in volume of liquid = volume increase of liquid volume increaseof container

V = ( - )V

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End of the chapter

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Thermodynamics - Heat

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