Thermodynamics Fundamentals Paper Example

Thermodynamics is primarily the study of heat and temperature in relation to work and energy. In general terms, thermodynamics can be defined as the study of energy transformations from one form to another with regard to matter. Thermal systems are analysed through the implementation of relevant equations of conservation, i.e., zeroth law of thermodynamics, the first law of thermodynamics (conservation of mass, conservation of energy), the second law of thermodynamics and the property relations law. In thermodynamics, energy is perceived as the ability to cause change. Thermodynamics is developed on the concept that heat energy corresponds to a definite amount of mechanical work.

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Zeroth law of thermodynamics: for two systems each experiencing a thermal balance with a third system, the initial two systems are in thermal symmetry with each other. This law gives meaning to the use of thermometers as an alternative system with a defined temperature gauge. Equally, thermal equilibrium between systems displays a transitive relation. For two systems to be in the relation of thermal equilibrium, they have to be separated by a wall that is only permeable to heat and should remain constant over time. The relation of thermal equilibrium is an equivalence relation on pairs of thermodynamic systems.

First law of thermodynamics: the difference between heat energy injected into a system from the external environment and work done by the system in its context is equal to the total internal energy change in that system. The first law of thermodynamics is also described as the law of conservation of energy. According to the first law of thermodynamics, a change in system energy equals the difference between heat added to a system and work done by the system. In many occurrences, basic types of energy (potential energy, kinetic energy and chemical energy are either constant or of least importance). The first law is often formulated as:

DU = Q - W

Change in internal energy DU of a closed system is equal to amount of heat supplied Q to the system less the work done.

The second law of thermodynamics: Heat flow from a colder region to a hotter region is not spontaneous, and equally, heat at a specific temperature cannot be entirely transformed into work. As a result, the entropy of a closed system (heat energy per unit temperature) continually increases over a given duration to a maximum value. Therefore, closed systems will tend to a steady state in which entropy is at maximum, and there is zero energy to perform work. The second law is mainly focused the direction of natural processes. According to this law, natural processes are unidirectional and irreversible (for example, heat will always flow from a hot region to a cold region and is irreversible)

Third law of thermodynamics: a perfect crystal's entropy in its most steady-state inclines to zero as the temperature nears absolute zero. At absolute zero, the system has to be in a state of minimum possible energy. Entropy is associated to the number of microstates that can be accessed. The ground state is typically unique and displays minimum energy, in such a case, entropy at absolute zero should be zero. Some finite entropy, however, exists as the system temperature is reduced to absolute zero if the system-order is not adequately defined. The reason for the existence of this entropy may either be because the system is locked into a configuration with an energy value that is not minimal or because the minimum energy state is not unique. The non-changing value is what we refer to them as residual entropy of the system.

These four laws of thermodynamics give a comprehensive description of all energy state changes in a system as well as its ability to do work in its immediate environment.

Dimensions and Units

The fundamental dimensions of thermodynamics and their units are:

Time - second (s)

Length -meter (m)

Mass - kilogram (kg)

Quantity - mole

The derived dimensions of classical thermodynamics are:

Force - F = ma Newton's (N)

Work - W joule (J or N.m)

Concepts and Definitions in Thermodynamics

Thermodynamic system - a thermodynamic system is a specific macroscopic region in space or within the universe that accommodates the eventuality of thermodynamic processes. Thermodynamic systems can either be open, closed, or isolated.

Macroscopic versus microscopic view (also known as a continuum) - the behaviour of matter can be studied at two levels, i.e. macroscopic level and microscopic level. The microscopic point of view sees matter as consisting of many small molecule atoms. From the microscopic perspective, a specific quantity of matter is taken without considering occurrences at the molecular level. From the microscopic point of view, the concern is on the effects of the initiative by many molecules these effects are not perceivable through human senses.

The concept of state - the thermodynamic state is defined by stating values of specific properties that are adequate to determine all other values. These properties are typically volume, pressure and temperature in fluid thermodynamic systems. Complex systems may require one to state more unique properties. Properties can either be extensive or intensive.

The concept of equilibrium - this is whereby the properties of a system remain definite and unchanged so long as external factors are not changed. A system in thermodynamic equilibrium must satisfy mechanical equilibrium (all forces must be balanced), thermal equilibrium (no temperature variations) and chemical equilibrium.

Surroundings - the external context of a system

Boundary - surface separating system from the surrounding.

Quasi-equilibrium processes - properties only define a state when a system is in equilibrium. Fr processes involving finite, unbalanced forces, the system can pass through states of no equilibrium which are impossible to treat. The most relevant idealisation is that only infinitesimal unbalanced forces exist to ensure a process is seen as occurring in a series of quasi-equilibrium states.

Non-equilibrium process - states are undefined during the process. Only the initial and final states can be defined.

Steady flow - conditions remain unchanged within a system during the process

Heat - energy transferred only due to differences in temperature.

Work - Any other means that could change the energy of a system. In thermodynamics work maybe, electric and magnetic, push-[u; mechanical work, chemical work, surface tension work, elastic work etc. Work is defined as positive when done in the external context of a system. Work done on the system is negative.

Forms of energy

• Sensible energy - these are kinetic energies within molecules.
• Latent energy - this is the phase of a system.
• Chemical energy - is found within atomic bonds in a molecule of a system.
• Nuclear energy - these are strong bonds within the nucleus of the atom.
• Static energy - energy stored within the system.
• Dynamic energy - energy interactions at the boundary of a system.

Single Phase Heat transfer

Single phase heat transfer is the most straightforward heat transfer concept. In this form of heat transfer, all the media remain in their original state (liquid or gas) throughout the process. The simple design of a single phase heat exchanger has two fluids at varying heats and separated by a conducting material. One fluid flows through metal cylinders while the former fluid flows around the metal cylinders. Heat transfer on both sides of the tube is through convection while heat transfer through the tube walls is by conduction. Heat exchangers can be alienated into several classes:

• Ordinary heat exchanger: two fluids at varying temperatures flow through paths created by tube walls. Heat is transferred through conduction and convection through the walls. A regular heat exchanger can either be a single phase (no change in original state of fluid) or two-phase (either of the fluids may change its state towards the end of the process).
• Regenerators: This is a type of heat exchanger that uses the concept of heat regeneration. Heat is obtained from the hotter fluid and is alternatingly deposited in a thermal storage medium, and then it is transported to the cold fluid.
• Cooling towers: This is a special type of heat exchanger which brings liquids and gases into direct contact with the aim of reducing the temperature of the liquid. During this process, a small amount of water evaporates thus reducing the temperature of water circulating through the tower.

Single phase heat transfer is typically conducted in either tube and shell systems i.ee., the heat exchanger is composed of a set of tubes inside a shell as in Figure 1 below. A tube-sheet separates the fluid in the tube from the fluid in the shell at the ends of the heat exchanger.

Two-phase heat transfer

In a two-phase heat exchange process, one or both of the fluids (cold or hot) in the system experiences a phase change. Phase change in a two-phase heat exchanger is achieved either by exceeding the boiling point of a liquid such that it vaporises into a super-heated gas (for example in an evaporator of a cooling system) or condensing a superheated gas to its sub-cooled liquid form (for example in a condenser).

The heat gained or lost during temperature changes in a particular phase is known as sensible heat while heat gained or lost during a change of phase is known as latent heat. The latent heat involved during phase changes between liquids and gases is many times greater than latent heat in the liquid phase.

The turbulence of the materials primarily determines transfer of heat in both liquid and gaseous components. For the efficiency of heat transfer, high turbulence is desired. With regards to the media used, a film of laminar flow of gaseous or liquid materials exists near a plane wall. Laminar flow is not desirable for heat transfer since heat transfer by convection is generally non-existent. However, achieving higher turbulence makes the insulating film of laminar flow thinner, increases convectional heat transfer and as a result facilitates efficient transport of heat.

To define the state of a fluid (regarding turbulent or laminar flow), we use a dimensionless number called Reynold's (Re) number.

Equation 1 below defines the Reynold's number

(1). Re = v.X / u

Where : u = /p

= dynamic viscosity [Cp = kg/m. s]

u = kinematic viscosity [St = cm2/s]

p = fluid density [kg/m3]

v = fluid velocity [m/s]

X = hydraulic diameter [m]

To achieve high turbulence, disturbances in flow have to be increased. Therefore, a rough surface results in higher turbulence than a smooth surface. Accurate comparison between Reynold's numbers with different passages requires that their geometries be exactly the same. Factors that result in high turbulence include:

• A herringbone pattern on plates
• A channel with a small cross-sectional area
• Using a fluid with low viscosity

Two-phase cooling

Effective heat regulation is an essential component in numerous modern day applications. Applications range from human comfort, powered electronics, photovoltaic arrays, etc. two-phase cooling systems utilise boiling fluids to transfer heat from its source to the operational fluid. The advantage that two-phase cooling systems have over single-phase cooling systems is that a single-phase system is reliant on temperature differential for heat transfer while the two-phase system utilises boiling which has a higher efficiency achieved by utilising latent heat of vaporisation.

The various applications in which two-phase cooling systems are implemented in modern-day applications include power electronics, data centres, avionics, radars, lasers etc. Most of these applications have recent...