Thermo fluids 1

What is a refrigeration and heat pump cycle?

(Q)in is the energy transferred by heat into the system undergoing the cycle from the cold body, and (Q)out is the energy discharged by heat transfer from the system to the hot body. Net work in is required

What is (W)cycle for a power cycle?

(W)cycle= (Q)in-(Q)out

What is (W)cycle for a heat pump or refrigeration cycle?

(W)cycle= (Q)out- (Q)in

Irrversible Processess

- The system and its surroundings cannot be restored to exactly to initial state after process -All actual processes are irreversible

what is MEP

-Fictitious pressure that would act on the piston during the entire power stroke, to produce the same amount of net work as that would be produced during the actual cycle-------MEP= Net cycle work/ change in volume

What are the assumptions of a Rankine Cycle?

-Heat transfers outside of (Q)in and (Q)out are ignored -Change in PE and KE are neglected -Steady State

Ways to improve vapor power system performance

-Increase boiler pressure -decrease condenser pressure -superheat -reheat -regenerative feed water heating -supercritical cycle

Types of Gas Turbines

-Industrial gas turbines/ Gas turbine power plant -Aircraft type gas turbines/ Turbojet

4 strokes of an internal combustion engine

-Intake stroke -compression stroke -power stroke -exhaust stroke

What are the types of air- standard analysis gas power systems?

-Otto cycle -Diesel Cycle -Dual Cycle

Types of reciprocating Internal combustion engines

-Spark Ignition -Compression Ignition

What are assumptions of an ideal air- standard brayton cycle?

-Working fluid is air modeled as ideal gas -combustion replaced as heat transfer from surroundings

Exergy

-property that quantifies potential for use -Maximum theoretical value for W obtained from overall system as the closed system comes into equilibrium with surroundings (I.e. Dead state)

What is back work ratio for a vapor cycle

.01-0.1

What is the back work ratio for a brayton cycle

.30-.40

Describe the Dual Cycle processes

1-2: Isentropic Compression 2-3: Constant volume heat addition 3-4: Constant pressure heat addition (1st part of power stroke) 4-5: Isentropic expansion (power stroke) 5-1: constant volume heat rejection

Describe the Diesel Cycle processes

1-2: Isentropic compression 2-3: Constant pressure heat addition (1st part of power stroke) 3-4: Isentropic expansion (Power stroke) 4-1: Constant volume heat rejection (exhaust)

Describe the Otto cycle processes

1-2: Isentropic compression 2-3: Constant volume heat addition 3-4:isentropic expansion (Power stroke) 4-1: Constant volume heat rejection

Describe the processes of an ideal Rankine Cycle

1-2:isentropic expansion (turbine) 2-3: constant pressure heat transfer (cool down) (condensor) 3-4:isentropic compression (pump) 4-1: constant pressure heat transfer (heat up) (boiler)

Cold Air Standard Assumptions

1. Working fluid is air 2.All processes that make up the cycle are internally reversible 3.The combustion process is replaced by a heat-addition process 4.The exhaust process is replaced with a heat- rejection process 5.Working fluid is considered to be air

What is a system?

A quantity of matter of fixed identity, around which we can draw a boundary. Everything outside the boundary is the surroundings.

What is specific heat?

Amount of energy required to raise the temperature of a unit of mass of a substance by one degree at constant P or V

Why is Cp>Cv?

At constant pressure, the fluid must do work on its surroundings to maintain constant pressure whereas in constant volume the fluid does not do work on its surroundings.

What happens when M<0.3?

Compressibility can be ignored

Why does temperature increase when volume decreases?

Decreasing volume increases pressure, which is what increases temperature.

1st law of thermodynamics

Energy is conserved, cannot be created or destroyed. Transferred through work and heat

2nd law of thermodynamics

Every energy transfer or transformation increases the entropy of the universe.

What is intercooling?

For gas power systems, separates the work and heat interactions into separate processes by letting compression take place in stages with heat exchangers. By itself, intercooling compression does not increase thermal efficiency, but intercooling in conjunction with regeneration significantly increases efficiency.

How does compressibility impact speed of sound?

Increased compressibility decreases the speed.

What are mechanisms for entropy generation?

Inelastic deformation Electric current through a resistor Spontaneous chemical reaction Friction In general- irreversible processes

Entropy Statement of the 2nd Law

It is impossible for a system to operate in any way that entropy is destroyed

Kelvin-Plank statement of the 2nd law of thermodynamics

It is impossible for any system to operate in a thermodynamic cycle and deliver a net amount of energy by work to its surroundings while receiving energy by heat transfer from a single thermal reservoir

Clausius Statement

It is impossible for any system to operate in such a way that the sole result would be an energy transfer by heat from a cooler to a hotter body.

Equation for Mach number

M=V/c -ratio of velocity of fluid to speed of sound

What is the refrigeration capacity?

Measure of the effective cooling power of a refrigerator (Qin)

Why is the back work ratio for a brayton cycle much higher than that of a rankine cycle?

Much more energy is needed to compress gas than liquid. Vapor cycles compress liquids, which has a lower specific volume and takes less energy to compress.

Why is a four stroke engine not a thermodynamic cycle?

No control volume or control mass, working fluid is exchanged with surroundings. Why we must use air- standard assumptions

Adaibatic

No heat transfer, only work

What is the fundamental difference between gas power plant and jet engine?

No shaft work, no separation between compressor and turbine

What is the compression ratio?

Ratio of the maximum volume to the minimum volume.

What is the cutoff ratio?

Ratio of the volume after combustion to the volume before combustion.

What does radiation depend on?

Surface Emissivity, Surface Area and Temperature.

Vapor cycle

System/ device that relies on converting liquid to vapor in order to rotate turbine(s) to produce power.

What is a power cycle?

Systems undergoing cycles that deliver a net work transfer of energy to their surroundings during each cycle.

What is the physical meaning behind the Tds Equations

Tds-- Q du-- U dh-- W pdv-- W vdp-- Q

What is a state?

The condition of a system as described by its properties. Solid is defined by having molecules that vibrate around fixed location. Liquid is defined by having molecules that vibrate within a fixed volume. A gas has molecules that are not bounded by location or volume.

Why does gas in a constant volume heat up when given heat energy?

The energy cannot go to expanding the gas or doing work on surrounding, all energy is transferred to internal energy or kinetic energy of the particles which is what defines temperature.

Corollaries of the 2nd Law for Power Cycles

The thermal efficiency of a an irreversible power cycle is always less than the thermal efficiency of a reversible power cycle when each operates between the same two thermal reservoirs.

What is internal energy?

The total energy stored by the particles that make up a system.

Where does M=1 occur?

Throat of a nozzle

What is the first law of thermodynamics in equation form?

U= Q - W

What is the difference between an open and closed feedwater heater?

Used in ideal regenerative Rankine cycles, in an open feedwater heater, extracted steam is mixed with feed water at the same pressure, while in a closed feedwater heater, steam does not mix with feed water. Feed water is essentially compressed water from the pump.

Zeroth Law of thermodynamics

When two objects are in thermal equilibrium with a third object, they are in thermal equilibrium

Exergy equation

X= (U-Uo)+Po(V-Vo)-To(S-So) +KE +PE

intensive property

a physical property that remains the same no matter how much of a substance is present

extensive property

a property that depends on the extent, amount of matter or size of a system

isothermal process

a thermodynamic process that takes place at constant temperature

What are the types of heat transfer?

conduction, convection, radiation

Isentropic

constant entropy

Which is larger, cp or cv?

cp

Binary Vapor Cycle

discharge from one cycle is input to another cycle

exergy efficiency

e= n((1-(to/tu))/(1-(to/ts))

What is ideal gas

gas that obeys the ideal gas law, composed of many fast moving particles that interact in perfectly elastic collisions

Clausius Inequality

is expressed as the cyclic integral of Q/T is always less than or equal to zero. This inequality is valid for all cycles, reversible or irreversible. if =0: reversible if >0: irreversible if <0: impossible

What is thermal efficiency for a power cycle?

n=1- (qout)/(qin)

define entropy change as equation

s2-s2=int(Q/T) ds=(Q/T)

Cutoff ratio

the ratio of the cylinder volumes after and before the combustion process (heat addition process)

What is a thermodynamic cycle?

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

Describe the Carnot cycle

the system undergoes four internally reversible processes: Adiabatic compression, isothermal expansion, adiabatic expansion, isothermal compression