ME 216 Exam 1

University of Alabama ME 216 Shaon Talukdar

What are thermal-fluid sciences?

the physical sciences that deal with energy and the transfer, transport, and conversion of energy

System

whatever we want to study

Surroundings

everything external to the system

Boundary

distinguishes system from its surroundings

Therme

heat

Dynamis

power

Microscopic Approach

aims to characterize by statistical means the average behavior of the particles making up a system and use this info to describe the overall behavior of the system

Macroscopic Approach

describes system behavior in terms of the gross effects of the particles making up the system, specifically, effects that can be measured by instruments such as pressure gages and thermometers

Heat

form of energy that can be transferred from one system to another as a result of temp difference

Heat Transfer

deals with the determination of the rates of such energy transfers and variation of temp

Why are units important?

dimensional homogeneity

Closed System

always contains the same matter

Can anything transfer across the boundary of a closed system?

energy transfer is permitted, however mass cannot transfer

Open System

a given region of space through which mass flows

Can anything transfer across the boundary of an open system?

mass and energy can cross the boundary

State

the condition of a system as described by its properties

Process

a transformation from one state to another

Equilibrium

a state of balance

Thermal Equilibrium

if the temp is the same throughout the entire system

Mechanical Equilibrium

if there is no change in pressure at any point of the system with time

Phase Equilibrium

if a system involves two phases and when the mass of each phase reaches an equilibrium level and stays there

Chemical Equilibrium

if the chemical composition of a system does not change with time, that is, no chemical reactions occur

The State Postulate

the state of a simple compressible system is completely specified by two independent, intensive properties

Simple Compressible System

if a system involves no electrical, magnetic, gravitational, motion, and surface tension effects

Extensive Property

depends on the size or extent of a system

ex. mass, volume, energy

How do you find the value of an extensive property for an overall system?

sum of its values for the parts into which the system is divided

Intensive Property

independent of the size or extent of a system

ex. pressure, temperature

Are intensive properties additive like extensive properties?

no

Density

mass/volume

Specific Volume

1/density

intensive property

Absolute Pressure

pressure with respect to the zero pressure of a complete vacuum

MUST be used in thermo relations

What is a change in energy of a system composed of?

1. kinetic energy

2. potential energy

3. internal energy

Sensible Energy

the portion of the internal energy of a system associated with the kinetic energies of the molecules

Latent Energy

the internal energy associated with the phase of a system

Chemical Energy

the internal energy associated with the atomic bonds in a molecule

Nuclear Energy

the tremendous amount of energy associated with the strong bonds within the nucleus of the atom itself

What are the two forms of energy interactions associated with a closed system?

1. heat transfer

2. work

What’s the difference between heat transfer and work?

an energy interaction is heat transfer if its driving force is a temp difference; otherwise it’s work

What are energy transfers by heat induced by?

a temperature difference between the system and its surroundings

What direction does net energy transfer by heat occur in?

decreasing temperature

Adiabatic

if a system undergoes a process involving no heat transfer with its surroundings

Heat transfer into a system is…

positive

Conduction

the transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interaction between particles

Convection

the transfer of energy between a solid surface and the adjacent fluid that is in motion; combined effects of conduction and fluid motion

Radiation

the transfer of energy due to the emission of electromagnetic waves (photons)

Work

energy from a force acting through a distance

Work done by a system is…

positive

Heat and work are both ____ functions

path

Heat and work NOT states, they’re…

processes

Systems possess energy, but not…

heat or work

First Law of Thermodynamics

energy cannot be created or destroyed, it can only change forms

Efficiency Equation

desired output/required input

Heating Value of the Fuel

the amount of heat released when a unit amount of fuel at room temp is completely burned and the combustion products are colled to the room temp

Lower Heating Value (LHV)

when the water in the combustion gases is a vapor

Higher Heating Value (HHV)

when the water in the combustion gases is completely condensed and thus the heat of vaporization is also recovered

Generator

a device that converts mechanical energy to electrical energy

Phase

a quantity of matter that is homogeneous throughout in both chemical composition and physical structure

Pure Substance

a substance that has a fixed chemical composition throughout

What’s a mixture of several things but still considered a pure substance?

air

Compressed Liquid

a substance that is not about to vaporize

where temp is lower than the saturation temp corresponding to the pressure at the state

Saturated Liquid

a liquid that is about to vaporize

temp increases considerably while the specific volume increases slightly

Saturated Liquid-Vapor Mixture

the state at which the liquid and vapor phases coexist in equilibrium

Saturated Vapor

a vapor that is about to condense

Superheated Vapor

a vapor that isn’t about to condense (that isn’t a saturated vapor)

P-V-T Surface Single-Phase Regions

• solid

• liquid

• vapor

P-V-T Two-Phase Regions

• liquid-vapor

• solid-vapor

• solid-liquid

P-V-T Vapor Dome

dome-shaped region composed of the two-phase liquid-vapor states

Saturation State

a state at which a phase changes, begins, or ends

P-V-T Saturated Liquid and Saturated Vapor Lines

lines bordering the vapor dome

Saturation Temperature

the temperature at which change takes place at a given pressure

Phase Diagram

projections of the P-V-T surface onto the pressure-temp plane

Saturation Pressure

designates the pressure at which a phase change takes place at a given temp

When are pressure and temperature not independent? (P-V-T)

two-phase regions

P-V Diagram

projection of the P-V-T surface onto the pressure-specific volume plane

T-V Diagram

projection of the P-V-T surface onto the temperature-specific volume plane

Quality (x)

for a two-phase liquid-vapor mixture, the ratio of the mass of vapor present to the total mass of the mixture

ranges 0 to 1

What’s the quality of a saturated liquid?

x = 0

What’s the quality of a saturated vapor?

x = 1

Property Diagrams for Phase-Change Processes

How can you find the specific volume of a two-phase liquid-vapor mixture?

use the saturation tables and quality, x

Equations Used to Fix the State in a Two-Phase Liquid-Vapor Region

When is linear interpolation used?

when a state does not fall exactly on the grid of values provided by property tables

Closed System Energy Balance

the change in the amount of energy contained within a closed system during some time interval equals net amount of energy transferred in and out across the system boundary by heat and work during the time interval

Thermodynamic Cycle

a sequence of processes that begins and ends at the same state

ex. power cycles, refrigeration cycles, heat pump cycles

How is the performance of a system undergoing a power cycle evaluated?

thermal efficiency