Chapter 1. Essential Concepts

What is Heat Transfer ? Thermal Energy ?? Heat transfer is thermal energy in transit due to a temperature difference. When two objects are brought into thermal contact, heat flows from the object at the higher Temperature to that at the lower Temperature. Thermal energy is associated with the translation, rotation, vibration and electronic states of the atoms and molecules that comprise matter. It represents the cumulative effect of microscopic activities and is directly linked to the temperature of matter. Thermal Energy ??

What to do ???? Given system, Identify which mode of heat transfer governs the system (how is heat transferred?) Calculate the amount of energy being transferred (we need to understand rate equations)

3 Modes of Heat Transfer

Conduction: Heat transfer in a solid or a stationary fluid (gas or liquid) due to the random motion of its constituent atoms, molecules and /or electrons. Convection: Heat transfer due to the combined influence of bulk and random motion for fluid flow over a surface. Radiation: Energy that is emitted by matter due to changes in the electron configurations of its atoms or molecules and is transported as electromagnetic waves (or photons).

Visible or Invisible?? (Example of Thermal Radiation)

There are lots of units and Dimensions giving you hard time Thermal Energy: Energy associated with microscopic behavior of matter Temperature: A means of indirectly assessing the amount of thermal energy stored in matter Heat Transfer : Thermal energy transport due to temperature gradients Heat: Amount of thermal energy transferred over a time interval t  0 Heat Transfer Rate: Thermal energy transfer per unit time Heat Flux : Thermal energy transfer per unit time and surface area

Physical Origins and Rate Equations - Conduction - Heat transfer occurs across a stationary medium How to transfer ? Solid: Electron or Lattice wave Fluid: Molecule Examples Gases and liquids: when neighboring molecules collide, a transfer energy from the more energetic to the less energetic molecules must occur. Conducting solids: due to the translation motion of the free electrons Poor conducting solids: due to lattice waves induced by atomic motion

How much??? - Rate equation: Fourier’s Law - Applicable to one-dimensional, steady conduction across a plane wall of constant thermal conductivity:

Physical Origins and Rate Equations - Convection - Convection is the sum of energy transport by the random molecular motion (diffusion) and the bulk motion of fluid (advection) Natural (free) convection: flow causes by buoyant force Forced convection: flow caused by a mechanical device

How much??? - Rate equation: Newton’s Law of Cooling - h(W/m2K): convective heat transfer coefficient

Physical Origins and Rate Equations - Radiation - Energy emitted by matter that is at a nonzero temperature. Emission may be attributed to changes in the electron configurations of its atoms or molecules and is transported as electromagnetic waves (photons). Although radiation originates from matter, its transport does not require a material medium and occurs most efficiently in a vacuum.

Radiation vs Irradiation Case of small surface(Ts) exposed to large surroundings of uniform temp., Tsur For such a condition, irradiation may be approximated by emission from the black body Tsur

Relationship to Thermodynamics A thermodynamics is concerned with equilibrium states of matter, where an equilibrium state necessarily precludes the existence of a temperature gradient, unable to quantify the heat transfer rate. Nothing to do with the rate ? Yes it does !!!! For many heat transfer problems, the first law of thermodynamics provides a useful tool.

The First Law of Thermodynamics - The Conservation of Energy - The Conservation of Energy: An important tool in heat transfer analysis, often providing the basis for determining the temperature of a system. Time Basis: At an instant or Over a time interval Type of System: Control volume, Control surface

What if we take a control volume ? 1. At an Instant Time Surface Phenomena : rate of thermal and/or mechanical energy transfer across the control surface due to heat transfer, fluid flow and/or work interactions Volumetric Phenomena : Rate of thermal energy generation due to conversion from another energy form (e.g., electrical, nuclear, or chemical); energy conversion process occurs within the system. g : rate of change of energy storage in the system Conservation of Energy Each term has units of J/s or W. 2. Over a Time Interval Each term has units of J.

1) Transient Process for a Closed System of Mass (M) assuming heat transfer to the system (Inflow) and work done by the system (outflow). 2) Steady State for Flow through an Open System without Phase Change or Generation:

What if we take a control surface ? A special case for which no volume or mass is encompassed by the control surface. Conservation Energy (Instant in Time): Applicable for both steady-state and transient conditions

How to approach heat transfer problems ? 1.On a schematic of the system, represent the control volume(surface) by dashed line(s). 2. Choose the appropriate time basis. 3. Identify relevant energy transport, generation and/or storage terms by labeled arrows on the schematic. 4. Write the governing form of the Conservation of Energy requirement. 5. Substitute appropriate expressions for terms of the energy equation. 6. Solve for the unknown quantity.