Thermodynamics Laws

• Thermodynamics and heat transfer are fundamental in thermal engineering. Thermodynamics concerns energy, work, heat, and their relationships. Heat transfer focuses on thermal energy exchange between systems.

Laws of Thermodynamics

• These are fundamental principles governing energy and matter behavior.

Zeroth Law

• If two systems are in thermal equilibrium with a third, they are in thermal equilibrium with each other, meaning thermal equilibrium is transitive.
• This law defines temperature as a fundamental property determining thermal equilibrium.

First Law

• Embodies the principle of energy conservation.
• Energy transforms but is neither created nor destroyed.
• Expressed as ΔU = Q - W, where ΔU is the change in internal energy, Q is heat added, and W is work done.

Second Law

• Introduces entropy.
• The total entropy of an isolated system increases or remains constant in ideal cases.
• Spontaneous processes are irreversible, increasing the universe's entropy.
• Heat does not spontaneously flow from cold to hot bodies.
• It limits the efficiency of heat engines and other thermal processes.

Third Law

• As temperature nears absolute zero, processes cease, and entropy approaches a minimum or zero for perfect crystalline substances.
• It provides a reference for determining entropy.
• Absolute zero is unattainable in a finite number of steps.

Heat Transfer Mechanisms

• Includes conduction, convection, and radiation.

Conduction

• It is heat transfer through direct contact within a material, driven by a temperature gradient, flowing from high to low temperature regions.
• Fourier's Law of Conduction: q = -k * A * (dT/dx), where q is the heat transfer rate, k is the thermal conductivity of the material, A is the area through which heat is transferred, and dT/dx is the temperature gradient.
• Thermal conductivity (k) measures a material's ability to conduct heat.

Convection

• It is heat transfer via fluid movement (liquid or gas), involving combined conduction and fluid motion effects.
• Includes natural (free) and forced convection.
• Natural convection arises from density differences due to temperature variations.
• Forced convection uses external sources like fans or pumps for fluid motion.
• Newton's Law of Cooling: q = h * A * (Ts - T∞), where q is heat rate, h is the coefficient, A is the surface area, Ts is surface temperature, and T∞ is fluid temperature.
• The convective heat transfer coefficient (h) relies on fluid properties, flow conditions, and surface geometry.

Radiation

• It is heat transfer through electromagnetic waves that doesn't need a medium, occurring even in a vacuum.
• All objects emit thermal radiation based on temperature and surface properties.
• Stefan-Boltzmann Law: q = ε * σ * A * (Ts^4 - Tsurr^4), where q is heat rate, ε is emissivity, σ is 5.67 x 10^-8 W/m^2K^4, A is the surface area, Ts is surface temperature, and Tsurr is surroundings temperature.
• Emissivity (ε) measures a surface's ability to emit radiation, ranging from 0 to 1.
• Blackbody: Perfect emitter and absorber (ε = 1).

Combined Heat Transfer

• Real situations combine conduction, convection, and radiation.
• Analysis involves considering thermal resistances of each mode and using appropriate equations.