THERMODYNAMICS & HEAT TRANSFER

0th Law of Thermodynamics

If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other.

1st Law of Thermodynamics (Law of Energy Conservation)

Energy cannot be created or destroyed, only transformed from one form to another

2nd Law of Thermodynamics

Heat cannot spontaneously flow from a colder body to a hotter body. The entropy of an isolated system always increases

3rd Law of Thermodynamics

As temperature approaches absolute zero (0 K), the entropy of a perfect crystal approaches zero

Kelvin-Plank Statement

It is impossible to construct a device that operates in a cycle and converts all the heat absorbed from a single thermal reservoir entirely into work.

Clausius Statement

It is impossible to construct an a device that operates in a cycle and produces no effect other than the transfer of heat from a cooler body to a hotter body.

Therme

meaning heat

Dynamis

meaning power or force

Thermodynamics

the branch of engineering science that deals with energy, its transformation, and the relationship among heat, work, temperature, and matter

System

portion of the universe chosen for observation and analysis

Surroundings

everything outside the defined system that can interact with it

Boundary

the real or imaginary surface that separates a thermodynamic system from its surroundings

Closed System

allows energy to cross its boundary, but does NOT allow mass to cross

Open System

both mass and energy may cross the boundary

Isolated System

exchanges neither mass nor energy with its surroundings

Fixed Boundary

the boundary cannot change position or volume, such as a rigid, closed pressure cooker

Moving Boundary

the boundary can expand or compress, such as the piston in an internal combustion engine

Boundary work

the mechanical energy transferred across this moving boundary during expansion or compression

Extensive property

depends on the amount of matter present in the system; varies directly with the mass i.e. total volume (V), mass (m), Internal Energy (U), Enthalpy (H)

Intensive property

its value is independent of the size or extent of a system; does not depend on the amount of matter i.e. pressure, temperature and density

Specific Properties

divide an extensive property by mass to remove the effect of size

Temperature

indicates thermal state and a measure of the internal energy of a body

Absolute temperature

the number of degrees above absolute zero expressed in Kelvins or Rankine

Pressure

the normal (perpendicular) force exerted by a fluid per unit area against which the force is exerted

Absolute pressure

the measure of pressure above zero

Gage pressure

measured above existing atmospheric pressure. It is the excess of the absolute pressure over the atmospheric pressure

101.325 kPa (1 Pa = 1 N/m2)

Standard atmospheric pressure

Density

mass per unit volume

Specific volume

volume per unit mass, reciprocal of density

Specific Heat

the quantity of energy required to raise the temperature of a unit mass by 1 K

1.005 kJ/ kg.K

specific heat of dry air

4.184 kJ/ kg. K

specific heat of water

Specific internal energy

represents a substance's total microscopic energy per unit mass

Specific enthalpy

total enthalpy of a system divided by its mass

Energy

the capacity to do work or produce a change

Internal Energy

the energy stored within the molecules of a substance

Kinetic Energy

the energy possessed by an object because of its motion

Potential Energy

the energy possessed by an object because of its position relative to a reference level

Flow Energy

When a fluid enters or leaves an open system, pressure pushes the fluid across the system boundary

Enthalpy

To simplify calculations for flowing fluids, internal energy and flow energy is combined

Heat

energy transferred because of a temperature difference

Work

energy transferred when a force acts through a distance or when a system exerts a force causing motion

thermodynamic process

the change of a system from one equilibrium state to another due to heat transfer, work transfer, or both

Isobaric Process (Constant Pressure)

For constant pressure, boundary work

Isochoric Process (Constant Volume)

Since the volume cannot change, there is no movement of the boundary

Isothermal Process (Constant Temperature)

Since the temperature does not change, the internal energy of an ideal gas also remains constant

Adiabatic Process (No Heat Transfer)

one in which no heat enters or leaves the system

Gas Laws

describe the relationship between pressure (P), volume (V), temperature (T), and amount of gas (n)

Boyle's Law (Isothermal Process)

states that the pressure (P) of a given mass of an ideal gas is inversely proportional to its volume (V), provided the temperature remains constant

Charles's Law (Isobaric Process)

states that the volume of a given amount of dry gas is directly proportional to its absolute temperature (in Kelvin) when pressure remains constant

Gay-Lussac's Law (Isochoric Process)

states that the pressure (P) of a given mass of gas is directly proportional to its absolute temperature (T) in Kelvin) when the volume (V) is held constant.

Combined Gas Law

merges Boyle's Law, Charle's Law, and Gay-Lussac's Law to describe the relationship between pressure (P), volume (V), and temperature (T) for a fixed amount of gas

Ideal Gas Law

also known as the general gas equation, is an equation of the state of a hypothetical ideal gas

sensible heat

Amount of heat added or removed measured in change of temperature without changing state

Latent Heat

The heat exchange when a material changes its state of molecular aggregation without change in temperature

Latent Heat of Fusion (Melting/Freezing)

Energy to change between solid and liquid

334 kj/kg

latent heat of melting of ice

Latent Heat of Vaporization (Boiling/Condensation)

Energy to change between liquid and gas.

2260 kj/kg

latent heat of boiling of water

Latent Heat of Sublimation (Sublimation/Deposition)

Energy to change directly between solid and gas

Psychrometry

It refers to the measurement of physical properties of gas mixture

moist air

Psychrometry is the study of the properties of?

Psychrometrics

is the study of the physical and thermodynamic properties of moist air—a mixture of dry air and water vapor.

Dry Bulb Temperature

In psychrometric chart, these represented by the x-axis

Humidity ratio

In psychrometric chart, these represented by the y-axis

Wet-bulb temperature

In the psychrometric chart, what does the diagonal line represents?

Sensible heating or cooling

refers to a rate of heat transfer attributable only to a change in dry-bulb temperature of the air

Humidification

may be adiabatic or with addition of heat

Cooling and dehumidification

results in a reduction of both the dry-bulb temperature and the humidity ratio

Chemical dehumidification process

the water vapor from the air is absorbed or adsorbed by a hygroscopic material

Dry bulb temperature of the air

temperature as measured by an ordinary dry bulb thermometer

Wet bulb temperature of the air

temperature as measured by a wet bulb thermometer

wet bulb thermometer

an ordinary thermometer whose bulb is enclosed in a wetted cloth sac or wick

Dew Point Temperature

the temperature at which the water vapor in the air is saturated

Humidity

the water vapor in the air

Absolute humidity or vapor density

the mass of water vapor per unit volume of air

Relative humidity

ratio of the actual partial pressure exerted by the water vapor in any volume of air to the partial pressure that would be exerted by the water vapor if the water vapor in the air is saturated at the temperature of the air.

Humidity Ratio

also called as specific humidity, is an expression of the mass of water vapor per unit mass of dry air

Saturation ratio

also called as percentage humidity, is the ratio of the mass of water vapor in the air per unit mass of dry air to the mass of water vapor required for saturation of the same air sample

Enthalpy

represents the total heat content of moist air, combining sensible heat (temperature) and latent heat (moisture).

wet-bulb depression

The difference between the dry-bulb and wet-bulb temperatures

condensation

When air becomes saturated, additional cooling will cause?

Degrees of superheat

difference between the dew point and dry bulb temperature of air

Saturation line

the condition of the mixture lies on the ____, the air is said to be saturated, meaning that any decrease in temperature will result in condensation of the water vapor into liquid.

Thermal conductivity

The amount of heat transferred per unit temperature per unit length

Conduction

the transfer of thermal energy through direct contact between molecules, acting primarily within solid materials.

Convection

the transfer of heat by the actual physical movement of liquids or gases

Radiation

the transfer of heat through electromagnetic waves, which does not require a medium to travel through

Fourier's Law

states that the rate of heat transfer across any plane normal to the x direction, Qx, is proportional to the wall area, A, and the temperature gradient in the x direction, dT/dx

conductivity

The relative capacity of a material to conduct heat

density differences

In natural convection, the movement of the fluid is primarily caused by?

convection currents

Heat transfer by convection occurs when heat moves from one place to another by means of currents that are set up within some fluid medium. These currents are known as

Newton's Law of Cooling

states that the rate of change of an object's temperature is directly proportional to the difference between its own temperature and the ambient (surrounding) temperature

Natural (Free) Convection

Caused by density differences due to temperature

Forced Convection

Fluid motion caused by external devices

Stefan-Boltzmann Law

The primary law governing blackbody radiation that quantifies the intensity of radiation emitted by unit surface area into a fixed direction from the blackbody as a function of wavelength for a fixed temperature

radiant energy

Heat energy transmitted by wave motion is called?

Blackbody

hypothetical body capable of absorbing all radiation at the maximum rate for a given temperature

Emissivity of a Surface

expressed as the ratio at which the given surface emits radiation to the rate of radiation of a black body at the same temperature

Thermal Conductivity

the amount of heat transmitted in unit time across unit area through unit thickness for unit temperature change