an ENGINEERING Education Makes A WORLD OF DIFFERENCE: HEAT TRANSFER

Heat transfer ...you are getting warmer...!
Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy and heat between physical systems. As such, heat transfer is involved in almost every sector of the economy.^[1] Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation, and transfer of energy by phase changes. Engineers also consider the transfer of mass of differing chemical species, either cold or hot, to achieve heat transfer. While these mechanisms have distinct characteristics, they often occur simultaneously in the same system.
Heat conduction, also called diffusion, is the direct microscopic exchange of kinetic energy of particles through the boundary between two systems. When an object is at a different temperature from another body or its surroundings, heat flows so that the body and the surroundings reach the same temperature, at which point they are in thermal equilibrium. Such spontaneous heat transfer always occurs from a region of high temperature to another region of lower temperature, as described by the second law of thermodynamics.
Heat convection occurs when bulk flow of a fluid (gas or liquid) carries heat along with the flow of matter in the fluid. The flow of fluid may be forced by external processes, or sometimes (in gravitational fields) by buoyancy forces caused when thermal energy expands the fluid (for example in a fire plume), thus influencing its own transfer. The latter process is often called "natural convection". All convective processes also move heat partly by diffusion, as well. Another form of convection is forced convection. In this case the fluid is forced to flow by use of a pump, fan or other mechanical means.

Note to Teachers

Heat and temperature are among the most misunderstood concepts in science.
Temperature is a physical state, based on the molecular activity of an object. If you cut an object in half, each half will have the same temperature.
Heat is a transfer of energy, which might change the state of temperature. Heat can be transferred without a change in temperature during a phase change (latent heat)
There is no such concept as the amount of heat IN an object – heat is an energy transfer

ü Did you know that HEAT is a form of ENERGY? Yep, this form of energy
affects everything from fuel to all of the parts in a racecar. And, we're going
to look at a few experiments to learn how this form of energy acts and why we
need to know all about it. Benjamin Thompson (1753 - 1814), known in Europe
as "Count Rumford", was the first person to show that HEAT is a form of ENERGY

The heat energy of a substance is determined by how active its atoms and molecules are. A hot object is one whose atoms and molecules are excited and show rapid movement. A cooler object's molecules and atoms will be less excited and show less movement. When these guys are in the excited state, they take up a lot of space because they're moving around so fast. When the atoms and molecules settle down, or cool down, they take up less space.

$Description: C:\Users\Krishna\Desktop\heat_e1.gif$

If a HOT high-energy atom comes into contact with a cool low-energy atom, the excited atom will loose some of its energy to the cool atom. The two atoms will settle into an energy level that's between where they each started out. That level is called Thermal Equilibrium. Did you know a lot of racecar drivers will let their engines cool down while idling before shutting it off after a hard race? They do this to let all of the internal engine parts come to Thermal Equilibrium.

$Description: C:\Users\Krishna\Desktop\red_cup.gif$

Now that you have an idea of what heat is, let's look at some of the effects this form of energy. Here's another Experiment. Blow up a balloon, but not all the way. Measure its circumference with a tape measure. Put the balloon in a refrigerator for about an hour, then take it out and measure it again. Did it shrink? Why?

It shrank because the air molecules inside the balloon slowed down and are taking up less space. Now put the balloon in direct sunlight. What happened after it warmed up? Why?

What do you think happens to all of the parts inside of an engine when they heat up?

From the above experiment, we learned a very important fact - heat causes things to "grow".
We call this Thermal Expansion. If things "grow" when heated, what do you think happens
to them when they get cold?

If you get a chance to check out some railroad tracks, notice the gaps between the rails. (Don't get run over by a train!). Thermal expansion and contraction is why those gaps are intentionally placed there, and why expansion joints are place in long highway bridges. You've probably felt expansion joints on old concrete highways. Remember the "thump - thump" sound as you rode over them?

Description: http://www.powermasters.com/images/R_Rail.gif

Most materials expand (grow) and contract (shrink) when heated or cooled. Water is most dense at 4° C. When it gets colder than that, it actually expands! Expansion and contraction occur in all directions but they can be broken down into the following categories:
Linear Expansion, for objects such as connecting rods.
Area Expansion, for objects such as pistons.
Volume Expansion, for substances such as fuel in a fuel cell.

Description: http://www.powermasters.com/images/expansion_rods.gif

Another example of linear expansion is shown by the illustration directly above. Say a steel pushrod starts out cold in the morning at 32° Fahrenheit (T1). The engine warms up to 200° Fahrenheit; a difference (what engineers call Delta T) of 168 degrees. Each inch of the pushrod will expand a certain amount for every degree of temperature rise. This amount is known as the coefficient of linear expansion. Each material has it own unique "coefficient". For example, Aluminum has about twice the expansion rate as mild steel. Now you can see why engine builders need to know their stuff. They have to know how much clearance to build into engine assemblies to allow for thermal expansion and contraction!

Description: http://www.powermasters.com/images/Piston_to_Wall.gif

It's extremely important to understand material expansion due to the heat that occurs in many areas of an internal combustion engine. One measurement that is critical is the piston-to-wall clearance. This is the difference between the diameter of the piston and the diameter of the cylinder wall.

Engine builders must know how much certain materials will expand when heated. For example, if an engine block is cast iron and a piston is aluminum, the piston and cylinder will expand differently. If the block and piston are made of the same material, the piston-to-wall clearance will expand at the same rate and the clearances will be different.

Another example of linear expansion can be found in piston-ring end gap. This is the space between the ends of a piston ring when installed in the cylinder. The ring undergoes linear expansion as the engine heats up, which causes the end gap to get smaller (the ring tries to grow together as it gets hot). If there is not enough end gap when the engine is put together, the ends of the ring may butt together when the engine gets hot and this could seriously damage the engine.

Description: http://www.powermasters.com/images/calorimiter.gif

Racecars use different fuels such as gasoline, nitromethane and alcohol. These fuels produce different amounts of heat (and power) when burned.

Scientists use test devices such as the calorimeter to measure how much heat a particular fuel will produce.

How the calorimeter works: Water flows through a calorimeter at a certain rate (for example, 1 gallon per minute). A measured amount of fuel is burned and gives up its heat to the water. The water temperature is measured as it enters and leaves the calorimeter. The difference in temperature is the amount of heat contained in the fuel.

Did you know that the alcohol used in race engines has only about half the heat energy as gasoline for
a given amount? This is why engines burning alcohol must run bigger jets in the carburetor than those for gasoline engines.

Did you also know that diesel fuel has more heat energy than gasoline? Another fact about diesel fuel is that it burns slower than gasoline. We'll investigate this further in another lesson.

The table at the right shows the "air/fuel ratios" required for engines using these types of fuels. You will learn more about this in another lesson.

Fuel Type	Air/ Fuel Ratio
Gasoline	14.6:1
Methanol (a form of alcohol)	6.46:1
Ethanol (a form of alcohol)	8.94:1

Heat Transfer

Heat travels from a hot object to a cold object.
You might say that heat flows down a temperature hill.
So how does heat travel from one object to another?
By Conduction, Convection and Radiation.

Liquids and gases do not conduct heat very well, but they can transfer heat by CONVECTION. Look at the engine at the left. Water carries heat from a hot engine through a pump and delivers it to a radiator, whose duty is to give up heat to the air. Remember, with convection, heat is transferred from one place to another by the motion of the gas or liquid.

CONDUCTION is the main way for heat to transfer through solid materials. For example, the outside surface of an engine gets hot because heat energy is transferred from the combustion chamber through the metal to the outside surface.

All solid materials conduct heat, but some do a better job than others. Generally, metals are good conductors while porous materials are not. Styrofoam is an example of a poor conductor. It is a great insulator. Good conductors of electricity are usually good conductors of heat.

Description: http://www.powermasters.com/images/Radiation_sun.gif

The easiest way to explain heat transfer by RADIATION is by the heat we feel from the sun. Even though the sun is about 93 million miles away from the earth, we still feel some of its heat. It travels to earth through the vacuum (no air) of space by way of rays!

Description: http://www.powermasters.com/images/Light_Bulb_Convection.gif

If you hold your hand near a light bulb (but not too close!), much of the heat that you feel is from RADIATION. How else is the light bulb transferring heat to your hand. Would you guess CONVECTION or CONDUCTION?

Description: http://www.powermasters.com/images/prof_owl.gif

Well, that's a quick look at HEAT ENERGY! We'll cover some of the same material in the lessons on Thermodynamics, Temperature, Heat Transfer and the Properties of Materials.

Talk to your teacher about how this subject applies to your racecar and what you can do to improve performance by applying the things you learned in this lesson. Remember, HEAT is what makes your racecar go!


6.Thermal conductivity of air at room temperature in kcal/m hr �C is of the order of
0.002
0.02
0.01
0.1
0.5.
ANSWER : : 0.02

an ENGINEERING Education Makes A WORLD OF DIFFERENCE

tab

scrolling

Friday 7 February 2014

HEAT TRANSFER

No comments: