Posts Tagged ‘forensic engineering’

Heat Transfer in Mechanical Engineering, Part I, Conduction

Sunday, February 14th, 2010

     Last week we finished up our series on fluid mechanics with a look at fluid dynamics, which considers fluids that move.  This week we’ll talk about heat transfer, which is the study of how heat moves through vacuums, gases, liquids, and solid objects.

     Understanding heat transfer is important when designing insulating materials, because they’re responsible for conserving energy by keeping heat contained inside things, things like pipes, boilers, and steam turbines.  Done in reverse, the concepts of heat transfer can also be used to determine how to dissipate excess heat, like in automobile engines and electrical equipment, to keep them from overheating.

     In the most basic of terms, heat transfer takes place because heat always wants to travel from a place of higher temperature to a place of lower temperature.  Heat will continue to flow in this direction until temperatures reach equilibrium, and this is true whether we’re considering heat moving through gases, liquids, or solids.  For example, if you pull the plug on your refrigerator, the heat from the air in your kitchen will begin to flow through the walls of the refrigerator where it will get absorbed by the cold food and ice cubes.  Eventually the heat will stop flowing when the temperature of the stuff inside of the refrigerator equals the temperature of the air in the kitchen.

     Now, there are different means by which heat can be transferred.  These include conduction, convection, and radiation.  Heat transfer analysis can get complicated, especially if it involves a combination of these means.  For now, let’s focus on conduction.

     As its name implies, heat transfer by conduction occurs when heat is conducted through a material.  Let’s consider the simple conductive heat transfer problem shown in Figure 1. 

Figure 1 – Heat Flow Through A 3-Centimeter Thick Copper Plate

 

    Figure 1 shows a 3 centimeter (cm) copper plate.  One side of the plate has a temperature of 400 degrees Centigrade (°C), the other side that of 100°C.  For this example let’s say that the plate is 30cm high and 20cm wide.  So, how do you calculate the heat flow?

    Now remember, heat always flows from hot to cold, so we know that it’s going to flow from the 400°C side to the 100°C side of the plate.  But let’s get into a little more detail.  Conductive heat flow depends on this temperature difference and some other things, like the ability of the material itself to conduct heat.  As with other materials and processes commonly used, scientists have performed lab experiments to measure heat conducting ability for all sorts of materials and recorded them in resource materials under the heading, “thermal conductivity,” so this element of information is readily available to us if we only seek out the reference manuals.  But in addition to this thermal conductivity factor, heat flow also depends on the area of the material that it is flowing through, as well as the material’s thickness.  So with this said, we can calculate the heat flow in Figure 1 to be:

        Heat Flow  =  (The Thermal Conductivity of Copper) x

                                   (The Area That The Heat Is Flowing Through) x

                                   (The Difference In Temperature) / (The Plate Thickness)

     From reference manuals, we know that the thermal conductivity of copper is 370 Watts/meter °C.  The second part of the equation asks us to calculate the area through which our heat will be flowing, and we calculate this to be 20cm x 30cm, or 600cm2.  Now in order for the units to match those of the thermal conductivity constant, we have to convert the area and thickness from centimeters (cm) to units of meters (m).  Therefore, 600cm2 becomes 0.06m2, and 3cm becomes 0.03m.  So our completed equation becomes:

Heat Flow = (370 W/m °C) x (0.06m2) x (400°C – 100°C) / (0.03m)

= 222,000 Watts

     This means that 222,000 Watts of heat will flow from the 400°C side to the 100°C side.   “Watts” don’t just apply to light bulbs, as we can see in this example.  Here, the number of Watts represent how much heat energy is flowing in a given amount of time.  This heat energy flow will stop when the temperature on each side of the copper plate becomes the same.  

     That wraps things up for our discussion of heat transfer via conduction.  Next week we’ll consider heat transfer by convection, and the next time someone talks about their convection oven, you’ll know what they’re talking about.

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Fluid Mechanics in Mechanical Engineering, Part V, Fluid Dynamics Continued

Sunday, February 7th, 2010

     Last week we talked about Daniel Bernoulli and his famous Bernoulli Principle, which is the cornerstone of fluid dynamics.  As we’ll see in this week’s installment, the Bernoulli Principle doesn’t just apply to water flowing inside pipes.  Let’s consider another instance in which it is instrumental, that of an airplane wing. 

     Figure 1 shows the side view of a wing with arrows indicating direction of air flow as the plane moves through the air.

Figure 1 – A Side View of an Airplane Wing

 

     Even though he lived more than 100 years before the first airplane, Bernoulli’s Principle can be used to explain why such a contraption can fly. You see, when comparing air flowing above and beneath a wing, its very shape makes the air flow want to travel faster along the top than it does on the bottom.

     Bernoulli’s principle comes into play with the airplane wing just as it did in last week’s water pipe flow example.  That is, the total energy of flow is the same at all points as the air flows above and below the wing.  Now, if air flow speeds up on top of the wing, then the flow’s kinetic energy increases along with it.  And remember last week’s analogy of change for $100?  Well, something has to give, so in this example the increase in kinetic energy is accomplished at the expense of pressure energy, but the total energy remains the same.  This decrease in pressure energy then translates into a drop in pressure on top of the wing.  The higher pressure beneath the wing overcomes the lower pressure above the wing.  This imbalance is what provides the plane’s lift, enabling it to get off the ground once it achieves a high enough speed on its race down the runway.

     The Wright brothers, men ahead of their time, were actually among the first aeronautical engineers.  They possessed remarkably advanced knowledge of mathematics and mechanical engineering principles.  They also understood what Bernoulli taught, and they used his Principle to design and test the shapes of wings on their gliders and planes.  They met with success when they determined that the wing’s shape was crucial to supplying lift.  In fact, they determined that, depending on the wing’s shape, it would provide the plane with the most lift for the least amount of air speed, allowing them to use a lighter engine to drive the propellers.  Weight is always a factor when flying, and the ability to use a lighter engine went a long way towards getting their first plane off the ground.

     That’s it for Fluid Mechanics.  Next week we’ll continue with a discussion of heat transfer, which is the study of how heat moves through vacuums, gases, liquids, and solid objects.

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Fluid Mechanics in Mechanical Engineering, Part IV, Fluid Dynamics Continued

Sunday, January 31st, 2010

     Last week we began our discussion on fluid dynamics.  We saw how it’s used to determine flow and velocity of water within a pipe.  This week we’ll continue our discussion, exploring in some detail the Bernoulli Principle and what it has to say on the subject.

     Daniel Bernoulli was a Dutch born mathematician who studied fluid dynamics during the 18th Century.  He analyzed the flow of water and determined that as fluid flow speeds up, its pressure goes down, and vice versa.  In 1738, he came up with what is now known as the Bernoulli Principle.  This Principle is based on the First Law of Thermodynamics, which you will remember teaches us that energy cannot be created or destroyed. 

     One of the conclusions that can be drawn from the Bernoulli Principle is that for fluid flowing steadily, say water within a pipe, or even air flowing over a pitcher’s curve ball in flight, the total energy of the flow remains constant.  By “total energy,” I mean the sum of three types of energy:  pressure energy, kinetic energy, and potential energy.  Total energy will remain constant all along the flow, although its three parts can change.

     The “pressure energy” part of the total energy is due to the pressure of the fluid flow.  For example, pressure energy can be added by a pump to make water flow through a pipe more readily.  The “kinetic energy” part of total energy is due to the speed of the fluid flow.  And as its name implies, kinetic energy is the energy of movement.  The “potential energy” part of the total energy is related to a change in elevation from one end of the fluid flow to the other, like you’d have on a pipe running downhill.  It can be said that water at the top of the hill has high potential energy because gravity wants to make it flow down to the bottom of the hill. 

     So how does this Principle help us today?  Well, Bernoulli’s Principle is the very foundation upon which fluid dynamics is built, and it’s consistently used to solve complex problems involving fluid flow.  To illustrate Bernoulli’s Principle, let’s take a look at Figure l.  Here water is flowing through a level pipe with three sections:

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Figure 1 – Water Flowing Through a Pipe With Three Sections

 

     According to Bernoulli, the total energy of the flowing water is the same from one end of the pipe to the other, and the total energy is equal in each of the three sections of pipe.  As the water flows through the pipe from Section 1 to the narrower Section 2, it speeds up as it squeezes through, so its kinetic energy increases.  Since the total energy must remain the same and the pipe is level, (this is significant because it means that potential energy is zero), the kinetic energy increases at the expense of pressure energy.  This results in a pressure drop in Section 2.  

     Not following?  Well, it’s like making change for a hundred dollar bill.  Let’s say pressure energy is represented by $20 bills and kinetic energy is represented by $10 bills.  Let’s also say that you have $100 worth of these bills in your wallet. The $100 represents the total energy.  Now, pretend that you are water flowing into Section 1 of the pipe. While in Section 1, you look in your wallet and you find that you have four $20 bills and two $10 bills, which add up to $100.  Okay.  Now, when you move into Section 2, you check your wallet again.  You discover that your wallet now has three $20 bills and four $10 bills.  So you now have fewer $20 bills, more $10 bills, but you still have a total of $100.  Fewer $20 bills means lower pressure, and more $10 bills means higher speed.

     Okay, getting back to the water, what do you think is going to happen when it flows from narrow Section 2 into wide Section 3?  Well, the flow will slow down as it fills the extra space present in Section 3.  Since the Bernoulli Principle tells us that the total energy of the flow must remain the same, the pressure energy must increase at the expense of the kinetic energy.  This in turn causes the pressure within the pipe to go up and the flow’s speed to go down.  

     Thanks to Bernoulli, if we can calculate the total energy in one section of the pipe, then we can calculate the speed of the water flow in another section if the pressure within that section is known.  Again, this is possible because we know that the total energy must remain constant all along the flow.

     Next week we’ll see how the Bernoulli Principle applies to the other type of fluid, air. 

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Thermodynamics In Mechanical Engineering, Part I

Sunday, December 6th, 2009

     Last week we followed Dorothy through the forest and watched Scarecrow transform from a fire trap to a robust fire-retardant fiberglass composition with the help of materials science.  This week we’ll explore the magical world of thermodynamics, and nobody knows thermodynamics like the Great and Powerful Oz.  In fact, he’s a real “Wiz” at it! 

     But seriously, thermodynamics is one of those out-of-sight, out-of-mind things that we take for granted in our daily lives.  Without thermodynamics we wouldn’t have modern conveniences like electricity, air conditioning, or anything with a motor, like the cars we can’t seem to do without.  The world would essentially be in the Dark Ages again. 

     Often referred to as “thermo” among mechanical engineers, thermodynamics is the science that deals with heat and work in processes used in power plants, refrigeration compressors, and engines.  Thermo also deals with the properties of substances that absorb and release heat energy, things like water (steam), refrigerants, and fuels (coal, gasoline, natural gas, etc.).

     In thermodynamics there are basically two laws that must be obeyed.  The first law states that energy cannot be created or destroyed, it can only be transformed from one form into another.  An example of this principle at work would be when you gas up your car.  According to the first law of thermodynamics, the chemical energy that is released when gasoline is burned by the engine must add up to the work energy put out by the engine to move all its parts and accelerate the car.  The first law sets up an energy accounting system, so to speak.  This principle makes it possible to analyze and design engines, refrigeration equipment, etc. 

     The second law of thermodynamics states that it is impossible to build something that is 100% efficient.  So, going back to the car example above, the second law tells us that we must also account for things like the heat energy lost to the atmosphere from the hot engine parts and the fumes leaving through the exhaust pipe.  This heat energy essentially wastes gasoline and doesn’t do any useful work, but it is a real phenomenon which must be dealt with when doing engineering design work.

     Thermodynamics can be broken down into different subsets, including power cycle analysis, refrigeration cycle analysis, stoichiometry, and psychrometrics. We’ll begin exploring these next time.

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Forensic Engineering Focus On Electrical Fires

Sunday, October 4th, 2009

     Property damage and loss of lives, these are often the result of fires.  But did you know that one of the leading causes of fire is electricity?  Residential electrical fires claim the lives of nearly 500 Americans each year and injure another 2300.  Annually, these fires result in over $800 million in property losses.

     Approximately one third of the nearly 70,000 home electrical fires that occur each year are traceable to design and manufacturing defects in electrical products.  The rest are caused by the misuse and poor maintenance of electrical products, overloaded circuits and extension cords, and incorrectly installed wiring.

     The three components that must be present in order for a fire to manifest and sustain itself are well known.  These components make up the “Fire Triangle,” a potentially lethal combination of heat, fuel, and oxygen.  If any one of these three components is missing from the triangle, a fire can’t be started or sustained.  In the case of an electrical fire, it’s electricity that creates the heat component of the Fire Triangle.

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The Fire Triangle

     How does electricity contribute to fires?  One example would be an overloaded extension cord.  Homeowners are sometimes unaware that extension cords must be sized appropriately for their ultimate usage.  If not, they can overheat, particularly if they are damaged.  Damage to cords can result from a myriad of factors, from factory production errors to kinking when heavy furniture is carelessly placed on top of them.

     The same principle holds true for electrical products.  If their internal wiring or a component is insufficiently sized or damaged, overheating can result.  If things get hot enough and there is sufficient airflow (oxygen) and combustible material (fuel) in the vicinity, then the fire triangle is complete.  The fire starts internally and can soon spread to other objects in the area.

     Electrical arcing can occur when an energized electrical circuit is broken.  For example, suppose a wire carrying current is suddenly broken in two.  If the voltage is high enough, the electricity will want to continue to flow through the air across the break to form an electrical arc.  If the power flowing through the arc is great enough, heat can once again complete the Fire Triangle, resulting in fire.

     Forensic engineering analysis of evidence collected from a fire scene often yields telltale signs of overheating due to overloaded electrical circuits or damaged wiring in components.  Under close examination by an experienced professional, even the smallest strand of wire can point to the cause of an electrical fire.

     CSI skills aren’t only employed at crime scenes.  Forensic engineers also use similar techniques to get to the true story of cause and effect.

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Machine Safety, Operator Safety, And Keeping Those Fingers

Sunday, September 27th, 2009

     Crushed fingers, amputations, burns, blindness, these are all too common undesirable occurrences involving moving machinery.  Eliminating the risk of such accidents is an integral part of the engineering design process, where risk assessment goes hand and hand with industry standards in order to provide adequate machine safeguards and protection to operators as well as bystanders.

     Machine safeguards fall into three basic categories: Guards, Devices, and Distance.

     Guards are physical barriers that are added to machines with the goal of keeping body parts, clothing, etc., separated from potentially hazardous areas.  An example would be a metal cage surrounding drive belts and pulleys.  Guards can also serve to keep material fragments and debris from flying out of machines while in operation, such as when an enclosure is built around the grinding wheel of a bench grinder.

     Devices can consist of automatic controllers, often connected to sensors on machine componets.  These controllers use a form of “safety interlock logic” to monitor the operating state of machinery.  They must act quickly and automatically to stop the normal operation of a machine if they sense that an undesirable object, say a person’s forearm, is in danger of entering a hazardous area.

     Controllers can be in the form of hard-wired electromechanical relays, embedded microprocessors, or programmable logic controllers (PLCs).  Their sensors can include electrical switches embedded in floor mats or mounted on movable guards, incorporated into control handle grips, or linked to an access door latch.  Still other sensors are more elaborate, using more sophisticated methods to maintain safety, such as photoelectric devices known as laser curtains.  These act by spreading beams of light across an opening which may be a gateway to a dangerous area.  If the beam is broken by an object, the controller takes appropriate action and renders the machinery inoperable.

     Distance safeguards operate as you would infer them to, by designing machinery so that hazardous areas are kept a great enough distance from body parts, etc., so as to eliminate any danger of them being drawn into an unsafe area.  An example of this factor at work would be when machinery is developed so that moving gears and other potential hazards are kept far out of the reach of someone by virtue of their overall design. 

      Sometimes even the best machine safeguard designs can be rendered ineffective after a piece of machinery is put into actual operation.  The reasons for this are varied, from poor maintenance of equipment, to lack of training for operating personnel, to inadequate supervision of workers, or perhaps the machine has been modified to operate outside the parameters of its design capacity.  Whatever the reason, people can be put at risk for serious injury and even death if machine safeguards are bypassed, eliminated, and defeated.

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Forensic Engineering, Mom’s Way

Sunday, September 20th, 2009

     When you were a child, were you lucky enough to have someone who inspired you?  Someone who filled you with the wonder and passion needed to pursue a fulfilling career?  Was it a parent, a teacher, a friend?  For me, it was my mother.

      Mom grew up on a farm in west-central Wisconsin back in the 1930s.  The Depression was on, and she and her siblings learned how to be resourceful at a young age.  That’s how you survived in the days before rural electrification, high speed communication networks, interstate highways, big box stores, and many other things that we take for granted in the United States today. 

      When I was a kid back in the ’60s, my mom was the plumber, carpenter, mechanic, and general fix-it person of the family.  No, my father wasn’t dead or absent, he just wasn’t handy, nor did he desire to be.  Unlike many housewives, Mom didn’t call for professional help when the vacuum cleaner or washing machine was on the fritz.  Instead she learned how they ticked, determined what she had to do to get them working again, then got busy fixing them herself.  Whether it was a lack of money to pay a professional for their services or a genuine appreciation of the subject matter that motivated her, I don’t know.  I just know that Mom, whether she realized it or not, approached technical challenges in our home much like a systems engineer.  She had an eye for improving product design and used forensic engineering skills to get to the heart of her dead appliance’s problem.

      Mom’s lack of formal technical training didn’t hold back her fix-its, and the basement workbench in our home was similar to a product development laboratory.  As her apprentice on household repair projects, Mom would get me involved.  It didn’t take long before I, too, understood how the timer in her washing machine made valves open and close, how the motor in her sewing machine made the needle move, how the toaster turned on and off, and how to fix a clock that wasn’t keeping time. 

      The education that my mother gave me formed a real world technical foundation for my future studies of engineering in college.  Mom’s school gave me a practical understanding of the workings of machines and other devices that many of my classmates lacked.  And the desire to keep things hands-on has stayed with me through my career, where the sanitary conditions of an office environment were often supplemented by activities that would continue to get my hands dirty. 

      I still love to take things apart, problem solve, and innovate.  Thanks to Mom, I’m the engineer that I am today.  My writing skills I had to pick up elsewhere…

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