Transport Phenomenon

Brief Explanation of Transport Phenomenon

Transport phenomena are all irreversible statistical processes that result from the random continuous motion of molecules, which is usually seen in fluids. The term “transport phenomena” refers to a variety of mechanisms by which particles or quantities move from one place to another. Transport phenomena in thermodynamics is a branch of science that examines how mass, momentum, and energy move between distinct system phases or regions. These phenomena are fundamental to many physical, chemical, and engineering processes, including heat transfer, mass transfer, and fluid flow.

The study of transport phenomena is the methods and principles governing the movement of mass, momentum, and energy between various phases or areas of a system. It involves a grasp of the physical and chemical characteristics of the substances involved as well as the variables such as temperature, pressure, concentration gradients, and surface area that affect the rate and effectiveness of the transport processes. These variables can be changed to optimize the transport procedures for certain applications.

The three main types of transport phenomena are:

1.     Viscosity (Transfer of momentum).

2.     Conduction (Transfer of energy)

3.     Diffusion (Transfer of mass)

1. Viscosity (Transfer of momentum)

Viscosity is a fundamental transport phenomenon that characterizes a fluid’s resistance to flow. It measures the internal friction of a fluid which results from the interaction between the molecules of the fluid.

Figure 1: Coefficient of Viscosity

The fluid’s viscosity and the rate of shear, or the rate of change of velocity with respect to the distance which is in uniform y-direction inside the fluid, determine how much momentum is transferred as a result of viscosity and is given by

  

The transmission of momentum will be higher and the fluid will be more resistant to changes in velocity or direction as viscosity increases. 

Hence, the coefficient of viscosity from the figure 1 is given by,

where, η– the coefficient of viscosity.

m- mass of molecules

n- number of density molecules

c- average velocity of gas molecules 

-mean free path

Example of transfer of momentum.

Figure 2: Transfer of momentum Example

The above figure depicts the viscosity-induced momentum transfer taking place at the molecular level. The molecules in a fluid are constantly interacting with one another, colliding and passing momentum from one molecule to the next. The molecules in the lower layers of a fluid that is moving are slowed down by friction with the stationary layers of fluid above them. The fluid resists any changes in velocity or direction as a result of this momentum transfer between adjacent fluid layers.

2. Conduction (Transfer of energy)

Conduction is the direct process of energy transfer that takes place when two objects are in direct contact. It happens as a result of energy shifting from the warmer object to the colder object due to the object’s different temperatures.

 Figure 3: Coefficient of Thermal Conductivity

The rate of heat transfer through a material is proportional to the temperature gradient across the material and the area (A) normal to the direction of heat flow which is given

Mathematically, the coefficient of thermal conductivity from the figure 3 is given as,

Where, k -coefficient of thermal conductivity.

 m- the mass of molecules

 n- number of density molecules

c- average velocity of gas molecules,  

 – mean free path and

- Specific heat

         Example of Conduction.

 Figure 4: Example of Heat Conduction 

In this figure, the transfer of energy through conduction takes place at the atomic or molecular level where kinetic energy is transferred from one particle to another. When a warmer object’s particle collides with a cooler object’s particle, some of the warmer objects kinetic energy is transferred to the cooler object, causing it to gain energy and warm up.

3. Diffusion (Transfer of mass)

Diffusion transfer of mass is the process of mass transfer in which a substance moves from an area of high concentration to an area of low concentration. The random motion of molecules is responsible for this transfer. The substance will continue to diffuse until it achieves equilibrium, at which point the concentration is uniform throughout the system.

 Figure 5:Coefficient of Diffusion

The rate of mass transfer by diffusion is proportional to the concentration gradient, which is the difference in concentration between two points which is given by:

Mathematically, the coefficient of diffusion from the figure 5 is given as,

Where, D- Coefficient of diffusion

 c- average velocity of gas molecule

 – mean free path

Example of Transfer of mass (Diffusion)

  

Figure 6: Example of Transfer of mass    

In this diagram, mass transfers take place that is the movement of mass move from one place to another due to a concentration gradient. There needs to be a force pushing molecules in the direction of the mass transfer by diffusion. A concentration gradient that is kept in place by the constant passage of molecules in both directions provides the driving force. The concentration gradient is eventually at the equilibrium when the substance’s concentration is balanced across the system because molecules move from high-low concentration locations as they move from one to another.

Applications of Transport Phenomenon

The basic idea of transport phenomena has many real-world applications in a variety of disciplines such as material science, chemical engineering, biomedical engineering, and environmental engineering. It includes:

1. Mass transfer: It is a main role in the design and optimization of the separation process used in the chemical industry such as distillation, absorption, and extraction.

 2.  Heat Transfer: Vital in many industrial and engineering processing including the design of heat exchangers, refrigeration systems, and combustion engines.

3.Material Science: The transport phenomenon is used to understand the diffusion of atoms and molecules in solids and liquids which is crucial for the creation of new materials with certain qualities, such as increased conductivity, strength, and durability.

  4.   Fluid mechanics: used in the designs of pumps, turbines, and other fluid-handling machinery. The study of blood flow and the flow of air is similarly governed by these principles.

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