Etiket Arşivleri: heat

Lab 7 – Heat and Temperature

Lab 7 – Heat and Temperature

Purpose:
• Observe the first law of thermodynamics: that energy is conserved.
• Investigate the transfer of heat.
• Determine the heat of fusion of ice
• Determine the heat of neutralization of acids and bases.
Theory:
1. Heat is another form of energy, and can be quantified with familiar units, joules.
2. The transfer of heat can be calculated by measuring a change in the temperature,
3. The heat capacity of a material tells you how much heat energy is required to cause a change in temperature. The heat capacity depends on how much the
material there is and is therefore referred to as the specific heat capacity.
Some specific heat capacities of materials:
water has a relatively high specific heat (4.184 J/°C*gram)
copper has a relatively low specific heat (0.378 J/°C *gram)
****To raise the temperature of water by 1 °C would take more than 11× the energy required to raise the temperature of the same mass of copper by 1 °C.
4. A calorie is the amount of heat required to raise the temperature of 1 gram of water by 1°C. The specific heat of water is equal to 1 calorie/°C*gram.
****We will make volumetric measurements using graduated cylinders, so the effective heat capacity for all our experiments is 1 calorie/°C*mL. (density of water is 1g/mL)
5. Heat of fusion – how much heat is required to melt ice; therefore the heat absorbed for the action of melting an amount of ice. (calories/gram)
6. Heat of neutralization – amount of heat given off in the reaction of an acid with a base.

HCl (aq) + NaOH (aq) Æ NaCl (aq) + H2O (l) + heat


Source: http://personal.monm.edu/asostarecz/CHEM%20100/Lab6_Heat.pdf

Temperature, Heat, and Thermal Energy

  • Temperature, Heat, and Thermal Energy

  • Heat

  • The transfer of thermal energy

  • Not a measure of energy but rather of energy transferred.

  • Measured in terms of joules or calories (cal).

  • Calories relate heat to changes in temperature.

  • Calorie is defined as the amount of heat needed to raise the temperature of one gram of water by one degree Celsius.

  • Specific Heat

  • The correlation between heat and temperature.

  • It measures how much heat is required to raise the temperature of a certain mass of a given substance.

  • Every substance has a different specific heat, but it is constant for that particular substance.

  • Temperature

  • A property of a material.

  • Depends on the material.

  • Measures the concentration of thermal energy in an object in much the same way that density measures the concentration of matter in an object.

  • A large object will have a lower temperature than a small object with the same amount of thermal energy.

  • Temperature

  • A measure of the average kinetic energy of the molecules that make up that material.

  • Solids are rigid because their molecules do not have enough kinetic energy to go anywhere.

  • The molecules in liquids have enough energy to move around one another but not enough to escape each other.

  • Gas molecules have so much kinetic energy that they disperse and the gas expands to fill its container.

  • Solids, Liquids, and Gases

  • When the temperature of a solid is raised, it will become a liquid. At an even higher temperature, it will become a gas.

  • This is explained by thermal energy.

–When the thermal energy of a solid is increases, the motion of the particles is increased and the temperature increases. This added thermal energy causes the particles to move rapidly so that their motion overcomes the forces holding the particles together.

–At this point the substance has changed from a solid to a liquid. This change occurs at the melting point.

–When a substance is melting, all of the added thermal energy goes to overcome the forces holding the particles together in the solid state.

–Once the solid is completely melted, there are no more forces holding the particles in the solid state and the added thermal energy again increases the particles motion and causes the temperature to rise.

–At the boiling point, further addition for energy causes another change of state where all the added thermal energy converts the material from the liquid state to the gas state.

  • Temperature is Measured in:

  • Celsius (oC)

–Water freezes at 0 oC and boils at 100 oC

  • Fahrenheit(oF)

–Temperature in the United States is measured in Fahrenheit

  • Kelvins (K)

–A measure of absolute temperature.

  • Thermal Energy

  • The energy created by moving particles inside a substance.

  • More movement of particles=more thermal energy

  • More thermal energy=higher temperature

  • Less thermal energy=lower temperature

  • Always moves from hotter objects to cooler objects.

  • Conduction

  • The transfer of energy from one molecule to another.

  • This occurs when molecules hit against each other.

  • Takes place in solids, liquids, and gases, but works best in materials that have simple molecules that are located close to each other.

  • Metal is a good conductor.

  • Convection

  • The movement of heat by a liquid or gas.

  • The liquid or gas moves from one location to another, carrying heat along with it.

  • This movement of a mass of heated liquid or gas is called a current.

  • Radiation

  • Heat travels from the sun.

  • The transfer of heat by electromagnetic waves.

  • When infrared rays strike a material, the material’s molecules move faster.

  • Ocean Thermal Energy Conversion

  • The ocean’s thermal energy comes from the sun which alters the temperature of the ocean water.

  • Ocean thermal energy can produce electricity

–It does this three different ways

  • Closed-cycle

  • Open-Cycle

  • Hybrid System

  • Closed-cycle

  • This system relies on low-boiling point fluids. The warm ocean water is used to heat up an boil these liquids, which turn a turbine. This turbine produces electricity.

  • Open-Cycle

  • This system relies on a low-pressure environment to actually boil the ocean water and create steam. This stem will turn a turbine.

  • Hybrid System

  • This system combines both the open and closed cycle systems.

  • Commercial Applications for Thermal Energy

  • The thermal energy business deals with energy conservation, efficient conversion, and utilization of fossil fuels and other energy resources.

  • The main focus is on:

–Improving already existing technologies

–Manufacturing newer and better technologies

–Consulting

  • How Do Temperature, Heat, and Thermal Energy Relate

  • Temperature is the measure of heat.

  • Heat is thermal energy that flows from a warmer object to a cooler object.

  • More thermal energy means a higher temperature and less thermal energy means a lower temperature.

Double Pipe Heat Exchanger

Objective:

To study and evaluate the effects of hot and cold fluid flow rate and flow configurations on the rate of heat transfer through thin walled tubes. To determine the overall heat transfer coefficient for the double pipe heat exchanger for countercurrent flow and parallel (or co-current) flow.

System:

The heat exchanger consists of two thin wall copper tubes mounted concentrically on a panel. The flow of water through the center tube can be reversed for either countercurrent or parallel flow. The hot water flows through the center tube, and cold water flows in the annular region.

• Valves are used to set up desired flow conditions (rate and direction). Set the hot water valve in the correct position to achieve either countercurrent or parallel flow.

• Thermometers and thermocouples are placed near the entrance, midpoint and exit of each pipe. The thermometer should give coarse readings compared to the thermocouple. The thermocouples are connected to a selector switch on the front of the panel.

• The flow meter has a direct read scale in ft3 /min. The flow meter does not read zero at zero flow due to rubber offset. The flow meter can read either the cold or hot water flow rate by turning the appropria te valves.

• A synopsis of operation is as follows: Open or close the appropriate valves to set hot water flow at 0.2 ft3/min in countercurrent configuration. (All globe valves should be totally opened or totally closed.) The metering valves at the outlets should be used to control flow rates. Before beginning cold water flow, temporarily close valve#1 to conserve hot water. Set valve positions for cold water flow at 1.0 ft3/min, then resume hot water flow (open valve#1) Allow the system to reach steady state before taking measurements (1-2 min). Take at least three readings of temperature and cold water flow before changing to new cold water flow rate. Examine cold water flows of 0.8 ft3/min and 0.6 ft3/min. The two heat exchanger groups must work together once the flow has been initiated because the adjustment of flow in one group will affect the other team’s flows. You must communicate when you are ready to change flow rates.

Once you have taken readings for all three rates of cold water flow, reverse the direction of the hot water flow (to the parallel flow configuration) by opening and closing appropriate hot water valves. Collect parallel flow data at cold water flow rate of 0.6 ft3/min only.

• Continuing in parallel flow configuration and 0.6 ft3/min cold water flow, increase hot water flow to 0.4 ft3/min. Collect temperature data.

• Reverse direction of hot water flow (back to countercurrent flow configuration). Collect data at 0.6 ft3/min cold water flow. Take additional readings at cold water flow rates of 0.8 and 1.0 ft3/min.


Source: http://wwwcourses.sens.buffalo.edu/ce328/Spring05doublepipe

Heat and Temperature ( Ms. Lyons )

What is Heat??

Heat = Thermal Energy!!

Thermal Energy = the total energy of all of the particles in a material or object.

Throughout the ages people have invented a variety of devices to help create and capture heat for use.

Topic 1: Using Energy from Heat

What are some ways that we use heat?

Cook food

Warm buildings

Dry clothes

What are some ways Thermal Energy has been used throughout history?

Development of Heat Technologies

What heat technologies can you think of that have been developed through-out time? Why have they changed?

Examples….

Devices to generate, transfer, control or remove heat

Heat = Thermal energy

Can you think of any examples of devices that generate, transfer, control or remove heat?

Topic 2: Measuring Temperature

Thermometer: Mechanical or electrical device for measuring temperature. Early thermometer was invented by Galileo.

Scale: A series of equally measured sections that are marked and numbered for use in measurement.

Celsius Scale

Celsius Scale: Most commonly used in Canada. Unit of temperature is called a degree. Based on the boiling and freezing points of water.

Boiling Point: The temperature at which water boils.
100o C at sea level.

Freezing Point: The temperature at which water freezes. 0o C at sea level.

Another Scale…

Kelvin is another way of measuringtemperature.

Scientists use Kelvin to explain thebehaviour of gases.

“Absolute Zero” is measured in Kelvin – which is the coldest possible temperature

0 Kelvin = -273 ºC

Right Device for the Job

Each thermometer has a sensor – a material which is affected by changes in the environment (such as temperature)

The sensor produces a signal (information about temperature, such as an electrical current) which affects a responder (a pointer, light or other mechanism that uses the signal in some way)

The Thermocouple

Wires made up of two different metals are twisted together.

When the wire tips are heated, a small electric current is generated

The amount of current depends on the temperature.

They can measure higher temperatures than thermometers.

The electric current can be used to turn switches on or off if the temperature changes.

Used in kilns, diesel engines and industrial furnaces

Bimetallic Strip

Made of two different metals joined together

When the strip is heated one metal expands more than the other

Thus the strip coils more tightly

Movement of the strip can operate a switch that can control furnaces…commonly used in thermostats in homes

Your Brain…(extra)

Your brain has its own temperature sensor.

It monitors your own internal temperature. If the temperature outside changes, the sensor signals your brain to release chemicals that will help your body adjust to normal temperature (37°C)

Recording Thermometer: A bimetallic strip connected to a writing device and paper which records temperature fluctuations over time.

Infrared Thermogram: Records infrared radiation, (heat sensor) as different colors according to their temperature.

Lectures on Heat and Thermodynamics ( Michael Fowler )

Contents

HEAT

Feeling and seeing temperature changes

Classic Dramatic Uses of Temperature-Dependent Effects

The First Thermometer

Newton’s Anonymous Table of Temperatures

Fahrenheit’s Excellent Thermometer

Amontons’ Air Thermometer: Pressure Increases Linearly with Temperature

Thermal Equilibrium and the Zeroth Law of Thermodynamics

Measuring Heat Flow: a Unit of Heat

Specific Heats and Calorimetry

A Connection With Atomic Theory

Latent Heat

THERMAL EXPANSION AND THE GAS LAW

Coefficients of Expansion

Gas Pressure Increase with Temperature

Finding a Natural Temperature Scale

The Gas Law

Avogadro’s Hypothesis

EARLY ATTEMPTS TO UNDERSTAND THE NATURE OF HEAT

When Heat Flows, What, Exactly is Flowing?

Lavoisier’s Caloric Fluid Theory

The Industrial Revolution and the Water Whee

Measuring Power by Lifting

Carnot’s Caloric Water Wheel

How Efficient are these Machines?

Count Rumford

Rumford’s Theory of Heat

THE DISCOVERY OF ENERGY CONSERVATION: MAYER AND JOULE

Robert Mayer and the Color of Blood

James Joule

But Who Was First: Mayer or Joule?

The Emergence of Energy Conservation

INETIC THEORY OF GASES: A BRIEF REVIEW

Bernoulli’s Picture

The Link between Molecular Energy and Pressure

Maxwell finds the Velocity Distribution

Velocity Space

Maxwell’s Symmetry Argument

What about Potential Energy?

Degrees of Freedom and Equipartition of Energy

Brownian Motion

IDEAL GAS THERMODYNAMICS: SPECIFIC HEATS, ISOTHERMS, ADIABATS

Introduction: the Ideal Gas Model, Heat, Work and Thermodynamics

The Gas Specific Heats C and C

V P Tracking a Gas in the (P, V) Plane: Isotherms and Adiabats

Equation for an Adiabat


Kaynak

Heat Transfer Sample Question Paper

Q.1 (a) Attempt any THREE of the following:

a) Mention two modes of heat transfer with examples.
b) Draw a sketch and describe the principle of convection as a mode of heat transfer.
c) Define perfect Black body.
d) Name two heat transfer equipments where latent heat is exchanged.

Q.1 (b) Attempt any ONE of the following:

a) Derive rate equation for heat transfer through a thick walled cylinder.
b) Draw the diagram and describe the concept of optimum thickness of insulation with a neat diagram.

Q.2 Attempt any TWO of the following:

a) A furnace is insulated with 230 mm thick fire brick, 115 mm of insulating brick, 230 mm thickness of building brick. The inside temperature of furnace is 1213 K and outside temperature is 318 K. The thermal conductivities of fire brick, insulating brick, building bricks are 6.047, 0.581 and 2.33 W/m.K Find out
1) Heat loss per unit area.
2) Temperatures at interfaces
b) Water is flowing in a tube of 16 mm diameter at a velocity of 3 m/s. The temperature of tube is 297 K and the water enters at 353 K and leaves at 309 K.
Data: Properties of water 12204
1) Density of water = 984.1 kg/m3
2) Specific heat of water = 4.187 KJ/kg.k
3) Viscosity of water = 485 x 10 -6 Pa.s
4) Thermal conductivity of water = 0.657 W/m.K
Calculate the heat transfer coefficient.
c) Cold fluid is flowing through a double pipe heat exchanger at a rate of 15 m3/hr. It enters at 303 K and is to be heated to 328 K. Hot thermic fluid is available at the rate of 21 m3/hr. & at 383 k.
Data: 1) Specific heat of thermic fluid = 2.72 KJ/kg.K
2) Density of water = 1 gm/cm3
3) Density of thermic fluid = 0.95 gm/cm3
4) Specific heat of water = 4.187 KJ/kg.k
Find out the log mean mean temperature difference for counter current type of flow by the following steps:
i) Outlet temperature of hot fluid
ii) Temperature difference at two ends
iii) LMTD

Q.3 Attempt any FOUR of the following: 

a) Draw a neat labelled diagram of 1-2 pass heat exchanger.
b) Mention any four characteristics of solutions to be considered before selecting the evaporator?
c) Differentiate evaporation and drying on two points.
d) What is dropwise condensation and filmwise condensation?
e) Mention any four dimensionless groups used in heat transfer and give significance of each group.

Q.4 (a) Attempt any THREE of the following:

a) Draw a neat diagram to indicate heat transfer from bulk of a hot fluid to bulk of cold fluid flowing across a metal surface and show the temperature profile.
b) Write down equation to calculate Nusselt number in laminar flow and explain all the terms.
c) Explain why heat transfer rate is more in dropwise condensation? Give two reasons.
d) Draw a graphical diagram indicating co-current and counter current heat exchange and give expression for LMTD in both the cases.

Q.4 (b) Attempt any ONE of the following:
a) Draw a neat diagram of a plate heat exchanger and show the types of flow in it. Give only one advantage of this type of heat exchanger.
b) What are surface extended heat exchangers? What is their specific application in chemical industry?

Q.5 Attempt any TWO of the following:

a) Derive a relation between overall and individual heat transfer coefficient in convection.
b) Water is to be heated from 298 K to 313 K at a rate of 30 kg/s. Hot water is available at 353 K at the rate of 24 kg/s for heating in a counter-current heat exchanger. Calculate the required heat transfer area if overall heat transfer coefficient is 1220 W/ m2K.
c) An evaporator at atmospheric pressure is fed at the rate of 10,000 kg/hr of 4% concentration of caustic soda. Thick liquor leaving evaporator contains 20% caustic soda. Find:
i) Capacity of evaporator.
ii) If 9000 kg of steam is fed. What will be the economy of an evaporator.

Q.6 Attempt any FOUR of the following: 
a) Differentiate between Natural convection and Forced convection on the following points. i) rate of heat transfer ii) how the currents are generated?
b) What is Dittus-Boelter equation? Write it down and give it’s use in heat transfer.
c) State and explain Stefan Boltzmann law of radiation.
d) Explain the terms absoptivity, reflectivity.
e) How economy of an evaporator can be increased? Name methods and explain any one of them?

1st Law of Thermodynamics Heat Transfer

1st Law of Thermodynamics
Heat Transfer
Lecture 6 October 14, 2009
Review

GOES:  Geostationary Operational Environmental Satellite
Maintain constant altitude (~36,000 km) over a single point, always over the equator   Imagery is obtained approximately every 15 minutes  Generally has poor spatial resolution but good temporal resolution

•POES: Polar Operational Environmental Satellites
–circular orbit moving from pole to pole closer to the Earth (879 km) than GOES
–Sees the entire planet twice in a 24 hour period.
–Good Spatial Resolution: Lower altitude results in higher resolution images
–Poor Temporal Resolution: Over any point on Earth, the satellite only captures two images per day.

•Visible
–Measures visible light (solar radiation, 0.6 mm) which is reflected back to the satellite by cloud tops, land, and sea surfaces.
– Thus, visible images can only be seen during daylight hours!

•Infrared (IR)
–Displays infrared radiation (10 to 12 mm) emitted directly by cloud tops, land, or ocean surfaces.
– Wavelength of IR depends solely on the temperature of the object emitting the radiation
–Advantage: You can always see the IR satellite image

•Water Vapor (WV)
–Displays infrared radiation emitted by the water vapor (6.5 to 6.7 mm) in the atmosphere
–Can determine dry layers from moist layers in the atmosphere

RADAR
Radar uses electromagnetic radiation to sense precipitation.
Sends out a microwave pulse (wavelength of 4-10 cm) and listens for a return echo.
If the radiation pulse hits precipitation particles, the energy is scattered in all directions
The intensity of precipitation is measured by the strength of the echo, in units of decibels
Doppler Radar: can determine velocity as well as reflectivity

Energy
Energy is the ability or capacity to do work on some form of matter Work is done on matter when matter is either pushed, pulled, or lifted over some distance

Potential energy – how much work that an object is capable of doing PE = mgh

Kinetic energy – the energy an object possesses as a result of its motion KE = ½ mv2

Laws of Thermodynamics

1st Law of Thermodynamics – Energy cannot be created or destroyed.   Energy lost during one process must equal the energy gained during another

2nd Law of Thermodynamics – Heat can spontaneously flow from a hotter object to a cooler object, but not the other way around.  The amount of heat lost by the warm object is equivalent to the heat gained by the cooler object

First Law of Thermodynamics

Heat
Heat is a form of energy and is the total internal energy of a substance Therefore the 1st law states that heat is really energy in the process of being transferred from a high temperature object to a lower temperature object.
Heat transfer changes the internal energy of both systems involved
Heat can be transferred by:
Conduction
Convection
Advection
Radiation
Specific Heat
Heat capacity of a substance is the ratio of heat absorbed (or released) by that substance to the corresponding temperature rise (or fall) The heat capacity of a substance per unit mass is called specific heat.  Can be thought of a measure of the heat energy needed to heat 1 g of an object by 1ºC Different objects have different specific heat values 1 g of water must absorb about 4 times as much heat as the same quantity of air to raise its temperature by 1º C This is why the water temperature of a lake or ocean stays fairly constant during the day, while the temperature air might change more Because of this, water has a strong effect on weather and climate Latent Heat Latent heat is the amount of energy released or absorbed by a substance during a phase change

Example
1: Getting out of a swimming pool
•In the summer, upon exiting a swimming pool you feel cool. Why?
•Drops of liquid water are still on your skin after getting out.
•These drops evaporate into water vapor.  This liquid to gas phase change causes energy to be absorbed from your skin.

Example
2: Citrus farmers
•An orange crop is destroyed if temperatures drop below freezing for a few hours.
•To prevent this, farmers spray water on the orange trees. Why?
•When the temperature drops below 32oF, liquid water freezes into ice.
•This liquid to solid phase change causes energy to be released to the fruit.
•Thus, the temperature of the orange remains warm enough to prevent ruin.

Example
3: Cumulus clouds
•Clouds form when water vapor condenses into tiny liquid water drops.
•This gas to liquid phase change causes energy to be released to the atmosphere.

Types of Heat Transfer

Heat can be transferred by:
Conduction
Convection
Advection
Radiation

Conduction
Conduction is the transfer of heat from molecule to molecule within a substance Molecules must be in direct contact with each other. The measure of how well a substance can conduct heat depends on its molecular structure.  Air does not conduct heat very well This is why, in calm weather, the hot ground only warms the air near the surface a few centimeters thick by conduction!

Convection
Convection is the transfer of heat by the mass movement of a fluid (such as water and air) in the vertical direction (up and down) Convection occurs naturally in the atmosphere On a sunny day, the Earth’s surface is heated by radiation from the Sun. The warmed air expands and becomes less dense than the surrounding cold air.  Because the warmed air is less dense (weighs less) than cold air, the heated air rises. As the warm air rises, the heavier cold air flows toward the surface to replace the rising air. This cooler air becomes heated in turn and rises. The cycle is repeated. This vertical exchange of heat is called convection and the rising air parcels are known as thermals
The warm thermals cool as they rise.  In fact, the cooling rate as a parcel rises can be calculated
If the thermal consists of dry air, it cools at a rate of ~10°C/km as it rises. This is called the lapse rate.
Convection is one process by which clouds can form.  As the temperature of the thermal cools, it may reach a point where it reaches saturation (the temp. and dewpoint are the close to the same) Thermals condense and form a cloud.

Advection
Advection is the transfer of heat in the horizontal direction.   The wind transfers heat by advection Happens
frequently on Earth Two types:
Warm air advection (WAA): wind blows warm air toward a region of colder air
Cold air advection (CAA): wind blows cold air toward a region of warmer air

Radiation
All things with a temperature above absolute zero emit radiation Radiation allows heat to be transferred through wave energy These waves are called electromagnetic waves The
wavelengths of the radiation emitted by an object depends on the temperature of that object (i.e., the sun mainly emits radiative energy in the visible spectrum, and the earth emits radiative energy in the infrared spectrum). Shorter wavelengths carry more energy than longer wavelengths
A photon of ultra-violet radiation carries more energy than a photon of infrared radiation.  The shortest wavelengths in the visible spectrum are purple, and the longest wavelengths are red.

Kirchoff’s Law
Good absorbers of a particular  wavelength are good emitters at that wavelength and vice versa Our atmosphere has many selective absorbers Carbon Dioxide, Water Vapor, etc… These gases are good at absorbing IR radiation but not solar radiation Thus these gases are called greenhouse gases due to the fact they help to absorb and reemit IR radiation
back toward the Earth’s surface thus keeping us warmer then we would otherwise be
Solar Radiation Budget
Earth-Atmosphere
Energy Balance