Fundamentals of Thermodynamics II Energy and Heat Transfer

References Required Introduction to Naval Engineering – (Ch 2 pg. 9-14, 18, and 22-28) Optional Principles of Naval Engineering – (pg )

Objectives Comprehend stored energy and transitional energy and give examples of each. Comprehend thermal and mechanical energy and give examples of each. Apply the concepts of work, heat, and power. Apply the definition of a system and correctly identify various types of systems. Comprehend the relationship between temperature and pressure in a working substance. Comprehend the state of a working substance with respect to saturated conditions. Apply the Ideal Gas Law. Comprehend the mechanisms of heat transfer and give examples of each.

Introduction Why are we looking at thermodynamics? Naval ships use either fossil fuel or nuclear fuel as energy for operation Definition of energy? Types of energy/sources

Thermodynamics Definition: science concerned with the interrelationship between thermal energy and mechanical energy Energy conversion of greatest significance on ship is: stored thermal mechanical Heat transfer: science that deals with methods by which thermal energy is able to translate

Classifications of Energy Stored (contained in) Transitional Mechanical Energy associated with large bodies or objects Potential Kinetic Work Thermal Energy associated primarily with systems of molecules Potential Kinetic Heat Potential Energy – energy associated with an object’s position or elevation relative to a reference energy level Kinetic Energy – energy associated with an object’s motion.

Stored Energy Mechanical Energy – the energy associated with relatively large bodies – Typically derived from a source outside the object – This is the ultimate goal for the systems we will study

Stored Energy Gravitational Potential Energy (Mechanical) Energy stored in a system due to relative positions m = mass g = gravitational acceleration

Potential Energy Elastic Potential Energy (Mechanical) Energy stored in an elastic object that is deformed under tension or compression x= change in position k = spring constant

Kinetic Energy Mechanical Kinetic Energy Energy stored in a system by virtue of the relative velocities of the component parts of the system

More Stored Energies Chemical Energy – The energy associated with the arrangement of atoms or molecules and the forces that bind them together – E.g. Burning fossil fuels in a gas turbine or diesel engine.

More Stored Energies Nuclear – Energy associated with the arrangement of and bonds between nucleons in the nucleus of an atom. – e.g. fission, fusion, decay of radioactive materials

More Stored Energies Electrical – Energy associated with interactions with an electric field

More Stored Energies Thermal – Energy associated with the force of attraction between molecules and molecular motion – Internal Energy (U) – the total stored thermal energy within a substance. – Thermodynamics is interested in CHANGES in U, rather than absolute quantities of U.

Stored Energy Thermal Potential Energy Energy associated with the force of attraction between molecules Similar to gravitational potential energy Highest in Solids Lowest in Gases

Stored Energy Thermal Kinetic Energy Energy stored in a system due to the motion of the molecules Thermal kinetic energy is proportional to the temperature of a substance

Stored Energy Flow Work (displacement energy) – The amount of stored energy required to maintain the continuous steady flow of working fluid – Applicable to open systems – Related directly to the pressure required to move a unit volume of the substance across the system boundary

Stored Energy

Energy in Transition Work—Mechanical Energy in transition – Units: ft-lb or Joules (Newton-meter) – The result of a tangible force acting through a tangible distance (displacement) – Work is independent of the time it takes to do it – Work is relative to frame of reference Is there any work done in “working out”

Thermal Energy - Heat

Power

Power

Sign Convention Thermodynamics is concerned with changes in energy rather than absolute quantities of energy. Since changes are relative to previous states, it is necessary to define a sign convention – Heat in or Work out – positive – Heat out or Work in -- negative

Systems A quantity of matter contained within a prescribed boundary (boundaries need not be physical) Systems are the most basic unit of study in thermodynamics Three types of systems based on how energy and mass move into and out of the system – Open – Closed – Isolated

Systems Open System – Both energy and mass may cross the boundary – e.g. An open leg of piping. Both water and heat can enter and exit the system.

Systems Closed System – energy may cross the boundary, but mass is constant – e.g. A soda bottle with the lid on tightly. Energy may be removed (soda cools in the fridge), but the mass of soda is constant

Systems Isolated system – Neither energy nor mass may cross the boundary – e.g. The universe (as we understand it) is an isolated system. – These systems are useful for analyzing closed systems in which energy losses can be considered negligible.

Working Substances Any substance (or fluid) that undergoes changes to affect a change in the sytem Examples?

Phase and State State – Determined by two independent properties of the working fluid (water) e.g. Temperature and pressure – Using steam tables and two independent properties, it is possible to determine all other properties of the water.

Phase and State Phase – Not to be confused with state. – Phase refers to the physical arrangement of the substances molecules. – Examples of phases are: solid, liquid, and gas. – The working fluid can exist in multiple states and multiple phases with in the boundaries of the system.

Phase and State Example: – A cylinder contains water at 300F and 1014 psig. What is the phase and state of the water? The temperature and pressure of the water are two independent properties that define the state of the water. The saturation temperature of water at 1014 psig is ~555F. The phase of the water is liquid ( T < T SAT for the given P)

Pressure/Temperature Relationships Ideal Gases – An ideal gas is a theoretical gas composed of randomly moving, non-interacting point particles – Under standard temperature and pressure conditions (STP), most real gases behave like an ideal gas.

Pressure/Temperature Relationships

Pressure/Temperature Relationships Saturation Conditions – Saturation – a condition in which a mixture of vapor and liquid can exist together at a given temperature and pressure. – Also known as the boiling point Saturation Temperature – the temperature, for a given pressure, at which a fluid boils Saturation Pressure – the pressure, for a given temperature, at which a fluid boils

Terms and Definitions Subcooled liquid – A liquid at a temperature below the saturation temperature for a given pressure Superheated vapor – A vapor at a temperature above the saturation temperature for a given pressure

Terms and Definitions Sensible Heat: – The energy necessary to heat (or cool) a fluid from a subcooled (saturated) state to saturation (a subcooled state). – Heat absorbed or rejected with a corresponding change of temp, but no change in phase

Terms and Definitions Latent Heat: – The energy necessary to change the phase of a fluid. – Heat absorbed or lost with a change in phase. – A change in latent heat doesn’t not result in a change in temperature of the substance. – This energy is used to break the bonds between molecules (i.e crystal structure of ice to liquid)

Terms and Definitions Latent Heat of Vaporization: – the amount of heat necessary to change a liquid to a vapor with out a change in temperature. Latent Heat of Fusion: – That amount of heat that must be added to a solid to melt it or must be removed from a liquid to solidify it.

Definitions and Terms Saturation Temperature(Pressure): – The temperature(pressure) for a corresponding pressure(temperature) at which a liquid boils Saturated liquid/saturated vapor – a liquid(vapor) a the temperature corresponding to the boiling point, at a given pressure Subcooled liquid – A liquid at a temperature below the saturation temperature for a given pressure Superheated vapor – A vapor at a temperature above the saturation temperature for a given pressure

T-s Diagram Subcooled liquid Saturated liquid/vapor Superheated vapor

Mechanisms of Heat Transfer Heat Transfer: heat flow from one body, region or substance to another Energy moves from higher temp -> lower temp Convention: – Higher temp: heat source – Lower temp: heat sink or receiver – Types: conduction, radiation, and convection

Conduction Definition: transfer of thermal energy when source and sink are in physical contact Energy transfer occurs layer to layer General Conduction Equation:

Radiation Definition: electromagnetic radiation generated by thermal motion of charged particles in all matter. All objects, whose temp is above absolute zero (including those that do not produce visible light), radiate thermal energy

Convection Not really a transfer, but a transport Definition: transportation or movement of some portions of a fluid within a mass of fluid – Can be due to density differences caused by temperature differences – Physical contact between a portion of the fluid mass is required, however it is not considered conduction (convection is specific to fluids)

Convection Natural circulation: due to density only Forced circulation: mechanical device (fan)

Take Aways List and describe the various types of potential/kinetic energy (gravitational, elastic, thermal) Describe and apply Mechanical work and it’s relationship to stored energy Define the mechanisms of heat transfer and give examples of each. Or, given an example, describe the mechanism employed (justify your answer)

Take Away Define sensible heat and latent heat. Define saturation temperature, saturation pressure, saturated liquid/vapor, subcooled liquid, superheated vapor Indicated the above conditions on a T-s diagram Apply the ideal gas law to various situations Describe the mechanisms of heat transfer and give examples.

Homework