Unit 13 Competency 2 - Examine thermal systems in engineering
Suggested Objective a: Investigate the laws of thermodynamics as related to heat engines, refrigerators, and thermal efficiency
Heat Engine Cycle
A heat engine typically uses energy provided in the form of heat Links to an external site. to do work Links to an external site. and then exhausts the heat which cannot be used to do work. Thermodynamics is the study of the relationships between heat and work. The first law Links to an external site. and second law of thermodynamics Links to an external site. constrain the operation of a heat engine. The first law is the application of conservation of energy to the system, and the second sets limits on the possible efficiency of the machine and determines the direction of energy flow.
Heat engines are typically illustrated on aPV diagram Links to an external site. |
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Heat engines such asautomobile engines Links to an external site.operate in a cyclic manner, adding energy in the form of heat in one part of the cycle and using that energy to do useful work in another part of the cycle. |
Information copied from http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heaeng.html Links to an external site. on December 12, 2014.
Heat Engines and Second Law of Thermodynamics
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Heat Engines
We need to think of entropy in a new way, though it is yet the same fundamentally as before. Entropy cannot build up indefinitely in a system. If it is introduced accompanying some heat input, it must eventually be released from the system.
This restriction does not affect the conversion of work into work, however. A plant that converts the rush of a river into electricity does not have to worry about entropy. Similarly, conversion of work into heat does not lead to a buildup of entropy. Conversion of heat to work, however, the basic process of a heat engine, must be done carefully to avoid buildup of entropy.
In fact, heat cannot be completely converted into work. Some heat must also be outputted as heat, to carry the entropy back out of the system. We can rewrite part of the thermodynamic identity as: σ in= Q in/τ in . We want some of the input heat Q in to be converted into work, so we know that Qout will be less than Q in .
We want all of the entropy to be extracted, however, and so we want σ in = σ out . The only way to accomplish such a feat is to have τ in > τ out . For this reason, we replace all of the "in" subscripts by "h", standing for "high temperature", and the "out" subscripts by "l", to indicate "low temperature".
Information copied from http://www.sparknotes.com/physics/thermodynamics/heat/section1.rhtml Links to an external site. on December 12, 2014.
The second law of thermodynamics
The second law of thermodynamics comes in more than one form, but let's state in a way that makes it obviously true, based on what you've observed from simply being alive.
The second law states that heat flows naturally from regions of higher temperature to regions of lower temperature, but that it will not flow naturally the other way.
Heat can be made to flow from a colder region to a hotter region, which is exactly what happens in an air conditioner, but heat only does this when it is forced. On the other hand, heat flows from hot to cold spontaneously.
Heat engines
We'll move on to look at heat engines, which are devices that use heat to do work. A basic heat engine consists of a gas confined by a piston in a cylinder. If the gas is heated, it expands, moving the piston. This wouldn't be a particularly practical engine, though, because once the gas reaches equilibrium the motion would stop. A practical engine goes through cycles; the piston has to move back and forth. Once the gas is heated, moving the piston up, it can be cooled and the piston will move back down. A cycle of heating and cooling will move the piston up and down.
A necessary component of a heat engine, then, is that two temperatures are involved. At one stage the system is heated, at another it is cooled.
In a full cycle of a heat engine, three things happen:
- Heat is added. This is at a relatively high temperature, so the heat can be called QH.
- Some of the energy from that input heat is used to perform work (W).
- The rest of the heat is removed at a relatively cold temperature (QC).
The following diagram is a representation of a heat engine, showing the energy flow:
An important measure of a heat engine is its efficiency: how much of the input energy ends up doing useful work? The efficiency is calculated as a fraction (although it is often stated as a percentage):
Work is just the input heat minus the rejected heat, so:
Note that this is the maximum possible efficiency for an engine. In reality there will be other losses (to friction, for example) that will reduce the efficiency.
The third law of thermodynamics states this : it is impossible to reach absolute zero. This implies that a perpetual motion machine is impossible, because the efficiency will always be less than 1.
Refrigerators, air conditioners, etc.
A device such as a refrigerator or air conditioner, designed to remove heat from a cold region and transfer it to a hot region, is essentially a heat engine operating in reverse, as the following energy flow diagram shows:
A refrigerator, consisting of a fluid pumped through a closed system, involves a four-step process. An air conditioner works the same way.
- Step 1 - The fluid passes through a nozzle and expands into a low-pressure area. Similar to the way carbon dioxide comes out of a fire extinguisher and cools down, the fluid turns into a gas and cools down. This is essentially an adiabatic expansion.
- Step 2 - The cool gas is in thermal contact with the inner compartment of the fridge; it heats up as heat is transferred to it from the fridge. This takes place at constant pressure, so it's an isobaric expansion.
- Step 3 - The gas is transferred to a compressor, which does most of the work in this process. The gas is compressed adiabatically, heating it and turning it back to a liquid.
- Step 4 - The hot liquid passes through coils on the outside of the fridge, and heat is transferred to the room. This is an isobaric compression process.
A refrigerator is rated by something known as the coefficient of performance, which is the ratio of the heat removed from the fridge to the work required to remove it:
The P-V graph for a refrigerator cycle
The P-V (pressure-volume) graph is very useful for calculating the work done. For any kind of heat engine or refrigerator (reverse heat engine), the processes involved form a cycle on the P-V graph. The work is the area of the enclosed region on the graph. The diagram for a refrigerator is a little more complicated than this because of the two phase changes involved, but this is basically what it looks like:
Information copied from http://physics.bu.edu/~duffy/py105/Heatengines.html Links to an external site. on December 12, 2014.
Introduction
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The Second Law of Thermodynamics
Links to an external site. states that heat will spontaneously always flow from a hot region to a cold region. By itself it never flows the other way, but can be made to do so under the influence of an external agency. The Second Law of Thermodynamics
Links to an external site. also states that this outside influence must do some work.
In a kitchen refrigerator the inside of a closed box is to be kept cool by removing heat from the inside and depositing it on the outside. Because the heat will not move freely from the cold inside to the hot outside it must be made to do so using an intermediate fluid which absorbs heat on the inside, then carries outside of the box and releases the heat to the air (see figure 1.) This fluid circulates in a pipe which passes in and out of the back of the refrigerator, kept moving by a compressor driven by an electric motor. It is the work done by this compressor (using electrical energy from the household electricity supply) that makes the refrigerator work without violating the Second Law of Thermodynamics.
Refrigerators and the First Law
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Any refrigerator takes in energy from the region to be cooled (or kept cold) and deposits heat energy into some region outside of the refrigerator, such as your kitchen (see figure 2.) In order to work there has to be some work done by the compressor and its electric motor. Using the First Law of Thermodnamics we can write
| QC - QH = -W |
(Note: since work in done one the refrigerator by another device, the compressor, rather than by the refrigerator itself, according to the sign convention which is part of the first law, the work done is negative.)
Suppose that 2.4 MJ of work is used to remove 5.2 MJ of heat from the inside of the refirgerator, then an amount of heat QH = QC + W = 5.2 MJ + 2.4 MJ = 7.6 MJ must be added to the kitchen.
Efficiency of a refrigerator
The efficiency of a refrigerator (known as the coefficient of performance, COP) is defined as
| COP = | Amount of heat removed from the inside of the refrigerator | = | QC |
| Work done to operate the refrigerator | W |
For example, if 20 MJ are removed from the inside of the refrigertor by doing 7.5 MJ of work, then the coefficient of performance is equal to 20/7.5 = 2.67.
Information copied from http://physics.csustan.edu/Ian/HowThingsWork/Topics/Temperature/ThermoLaws/Refrigerators.htm Links to an external site.on December 12, 2014.
Physics - Thermodynamics: (11 of 14) The Refrigerator: How It Works
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Suggested Objective b: Apply the laws of thermodynamics to careers in engineering
Job Description: Perform engineering duties in designing, constructing, and testing aircraft, missiles, and spacecraft. May conduct basic and applied research to evaluate adaptability of materials and equipment to aircraft design and manufacture. May recommend improvements in testing equipment and techniques.*A job as a Thermodynamics Engineer falls under the broader career category of Aerospace Engineers.
What Thermodynamics Engineers Do:
- Write technical reports or other documentation, such as handbooks or bulletins, for use by engineering staff, management, or customers.
- Direct or coordinate activities of engineering or technical personnel involved in designing, fabricating, modifying, or testing of aircraft or aerospace products.
- Formulate conceptual design of aeronautical or aerospace products or systems to meet customer requirements.
- Analyze project requests, proposals, or engineering data to determine feasibility, productibility, cost, or production time of aerospace or aeronautical products.
- Review performance reports and documentation from customers and field engineers, and inspect malfunctioning or damaged products to determine problem.
- Evaluate product data and design from inspections and reports for conformance to engineering principles, customer requirements, and quality standards.
- Plan or conduct experimental, environmental, operational, or stress tests on models or prototypes of aircraft or aerospace systems or equipment.
- Develop design criteria for aeronautical or aerospace products or systems, including testing methods, production costs, quality standards, and completion dates.
- Plan or coordinate activities concerned with investigating and resolving customers' reports of technical problems with aircraft or aerospace vehicles.
- Maintain records of performance reports for future reference.
- Formulate mathematical models or other methods of computer analysis to develop, evaluate, or modify design, according to customer engineering requirements.
- Evaluate and approve selection of vendors by studying past performance or new advertisements.
- Direct research and development programs.
Information copied from http://www.mymajors.com/career/thermodynamics-engineer/ Links to an external site. on December 12, 2014.