GGRA02H3 Lecture 3: GGRA01 Lecture Three.docx

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13 Mar 2012

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Lecture three: the global assembly line october 4th, 2011. From fordism to post-fordism, from spatially- fixed to spatially dispersed assembly line (test question* ) Global scale spatial separation of manufacturing operations (operations are not only occurring in one city but they are sourcing raw materials from everywhere ) Supply and commodity chains: the network of facilities and distribution capabilities and an enterprise uses to: supply chains are one side of the production system; finish product becomes commodity = part of commodity chain (being sold) Source (or procure raw materials (chemicals, ores, grains) or components. Transform the materials or assemble the components into products. Deliver the products to customers indirect through distributors or stores or directly to the purchaser.

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Tiny solid and liquid particles suspended in the atmosphere are called aerosols. Windblown dust, sea salts, volcanic ash, smoke from wildfires, and pollution from factories are all examples of aerosols. Depending upon their size, type, and location, aerosols can either cool the surface, or warm it. They can help clouds to form, or they can inhibit cloud formation. And if inhaled, some aerosols can be harmful to people's health.

These maps show average monthly aerosol amounts around the world based on observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite. Satellite measurements of aerosols, called aerosol optical thickness, are based on the fact that the particles change the way the atmosphere reflects and absorbs visible and infrared light. An optical thickness of less than 0.1 (palest yellow) indicates a crystal clear sky with maximum visibility, whereas a value of 1 (reddish brown) indicates very hazy conditions.

High aerosol amounts are linked to different process in different places and times of year. High aerosol amounts occur over South America from July through September. This pattern is due to land clearing and agricultural fires that are widespread across the Amazon Basin and Cerrado regions during the dry season. Aerosols have a similar seasonal pattern in Central America (March-May), central and southern Africa (June-September, and Southeast Asia (January-April).

In other cases, however, aerosol concentrations are not related to fires. For example, from May through August each year, aerosol amounts rise dramatically around the Arabian Peninsula and nearby oceans due to dust storms. Elevated aerosol amounts nestle at the foothills of the Himalaya Mountains in northern India in some months, and linger over eastern China for much of the year. These elevated aerosol amounts are due to human-produced air pollution.

What is the most likely source of the high concentration of particulates that appear off the coast of northwestern Africa?

What is the most likely source of the high concentrations of particulates over central Africa and central South America?

What is the most likely source of the high concentrations of particulates over eastern China?

apricottermite85

Materials fracture when the force per unit area, called STRESS,exceeds a critical value. In general, the critical stress dependson the direction as well as the magnitude of the force. Forexample, bones break differently when subjected to torsional(twisting) stress than when subjected to compressional (orsqueezing) stress. Let us consider compressional stress. Wet humanbone for 20-39 year olds has an ultimate compressive strength of ~16x104Ncm2 [8]. Consider a person who falls from a height to theground. Upon landing, the person's kinetic energy is converted towork. If the person decelerates over a distance h, we can estimatethe average force during the collision by

If a person lands stiff-legged on a hard surface and doesn'tbounce, the deceleration occurs over a very short distance, h ~ 1cm. Since the force is transmitted up the leg, the stress isgreatest where the cross-sectional area is least, i.e., the tibiajust above the ankle [8].
The following lists some different conditions that may occur duringlanding and physical explanations for why they make bone fracturefrom a fall more or less likely [8]:

(a) bounce [more likely. The impulse is twice as large for anelastic collision as for an inelastic collision.]
(b) bent knees [less likely. Bending the knees increases the timeand distance over which deceleration occurs, which reduces theforce.]
(c) landing in loose earth or sand [less likely. This increases hwhich reduces F.]
(d) turning the body to land on the side [less likely. Thisincreases h. It also distributes the force widely over the body(increases A), thereby reducing the stress.]

So, why can a properly executed karate kick break a concrete blockwithout fracturing bones [16]? First, bone is a very strongmaterial. Its ultimate compressive strength is approximately 40times larger than concrete. Second, contact is made with the edgeof the foot. This concentrates the force into a small area of thetarget and reduces the likelihood of bending a bone to the point offracture. Third, the collision with the target is essentiallyinelastic and extends over several milliseconds, so the peak force,though large, does not exceed the strength of the bone. If A(cross-sectional area of the tibia) ~ 3.3 cm2, compute how far a 69kg person can fall and land stiff-legged on both legs withoutbreaking a bone. Assume F is split evenly between two legs. (Hint:F<or =2(theta)A )

Vapor cycles
5EM - Thermodynamics
September 2010
1 The T-S diagram
The temperature-entropy diagram is an alternative to the well knownpressure-
volume diagram. Before using it to analyze different vapor cycles,we need
to understand it properly.
1. Plot the following transformations for ideal gases in a T-Sdiagram
(a) Isothermal expansion
(b) Isobaric expansion
(c) Isometric heating
(d) Reversible adiabatic expansion
(e) Irreversible adiabatic expansion
2. Repeat the last question, but this time do it for a realsubstance,
starting in the compressed liquid1 region and ending in thesuperheated
region, when possible. Indicate the boundaries of eachregion.
3. We know that under certain circumstances the area under theplot
in a P-V diagram represents the work performed during theprocess.
What does it represent in this case? Under what assumptions isyour
answer true? Compare examples 6.1 and 6.2.
2 Thermal machines
We understand by thermal machine a device capable of transformingther-
mal energy (heat) into mechanical energy (work). We already knowthat
these machines have a limited efficiency which, in the two-sourcesmodel, is
obtained by Carnotâs cycle.
(Compressed liquid is sometimes also called subcooled liquid)
2.1 Carnot cycle
This cycle, although efficient, is impractical. To begin with, theextreme
temperatures of the cycle should be within the sources temperaturesfor a
finite heat flow to exist.
1. Given the temperature of the sources, draw the cycle accordingto the
named limitation.
2. Show that this leads to lower efficiency than the theoreticallimit.
3. Assume that heat flow from/to the sources is proportional to thetem-
perature difference between them and the working fluid, anddemon-
strate that a higher power output is related to a decreasedefficiency.
2.2 Rankine cycle
Out of the several practical possibilities, Rankine cycle and itsvariations is
widely used, especially for electric power generation.
1. Draw the basic cycle in a T-S diagram and explain it.
2. What does the efficiency of the cycle depend on? Comparewith
Carnotâs.
3. What are the differences between the ideal cycle and the realone?
4. Describe how it is modified by superheating and/or reheating.Explain
te benefits.
2.3 Problems
After answering the previous questions solve the followingexercises from the
book: 8.2, 8.4, 8.5, 8.13, 8.19, 8.20
3 Frigorific machines
3.1 Reverse Carnot cycle
We have already studied Carnot cycle in detail as the ideal thermalmachine.
Now we will take is as the starting point to understand thecharacteristics
of frigorific machines.
1. Draw the ideal cycle in a T-S diagram and explain it.
2. Show that this cycle has a theoretical efficiency Ã =Tcold/Thot-Tcold
3. What are the difficulties in implementing such cycle in actualdevices?
3.2 Actual refrigerators
1. Why is the turbine replaced by an expansion valve in mostcases?
2. Plot the actual cycle and explain the differences with the idealcase.
3. How do cascade and multistage systems improve the performanceof
the refrigerator?
4. What aspects are important in the choice of a refrigerant fluid?Justify
3.3 Problems
After answering the previous questions solve the followingexercises from the
book: 10.2 10.7, 10.8, 10.9, 10.22, 10.23 10.1D

GGRA02H3 Lecture 3: GGRA01 Lecture Three.docx

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