Che-205: Heat Transfer Fundamentals(Spring 2017)

Dr. Muhammad Wasim Tahir

Department of Chemical EngineeringUniversity of Engineering & Technology Lahore

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Course outline

1. Introduction to heat transfer

2. Application and importance of heat transfer

3. Modes of heat transfer

4. Steady state one dimensional conduction including heat sources and convective boundary conditions

5. Extended surfaces

6. Transient conduction: lumped capacity method

7. Free and forced convection under various flow patterns

8. Dimensional analysis

9. Importance of temperature in mechanism of heat transfer

10. Calculation of caloric and wall temperature

11. Momentum and heat transfer analogies

12. Boiling and condensation

13. Principles of radiation heat transfer

14. Introduction to heat transfer equipment and their configurations

Marks allocation

1. Final examination 40 %

2. Mid-term examination 30 %

3. Quiz 20 %

4. Attendance/Conduct 10 %

Suggested reading material

Text Book:

RAJPUT, R. K., Heat and Mass Transfer, S. Chand & Company Ltd., Delhi, 2006

1. Introduction to heat transfer

What is heat?

o Particles in a system are in continuous motion (vibration, rotation, translation)

Solid Liquid Gas

o Particle motion attributes to K.E. for each particle referred to as heat or thermal energy

o Sum of K.Es. of all the particles in a body is referred to as heat

o Faster the particles in a body move hotter the body gets

o Heat is denoted by Q

o Measured in Joules (J) in MKS system

1. Introduction to heat transfer

Temperature

o Every particle in a system moves with its own energy

o Particles interact (collide) with each other and change energies

o Averaging thermal energies of particles gives temperature of the system

o Temperature is measure of the average kinetic energy of a system

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1. Introduction to heat transfer

Temperature scales

Centigrade (°C), Kelvin (K), Fahrenheit (°F), Rankine (°R)

http://www.citycollegiate.com/gaslaws.htm

1. Introduction to heat transfer

Transfer of heat

o Temperature gradient within a system or between systems drives the transfer of heat

o Heat is always transferred from a point of higher T to that of the lower

o Heat transfer can be defined as heat or thermal energy in transit

Thermodynamics & heat transfer

o Thermodynamics deals with the end or final state

o Heat transfer deals with the rate of energy transport

1. Introduction to heat transfer

Transfer of heat

o Temperature gradient within a system or between systems drives the transfer of heat

o Heat is always transferred from a point of higher T to that of the lower

o Heat transfer can be defined as heat or thermal energy in transit

Thermodynamics & heat transfer

o Thermodynamics deals with the end or final state

o Heat transfer deals with the rate of energy transport

1. Introduction to heat transfer

Energy conservation o Total energy of the system is conserved

o Energy can be transferred to or from a system by heat, work, mass flow

Ein – Eout = ΔE

Ėin – Ėout = dE/dt

At steady state Ėin = Ėout

Total energyentering the

system

Total energyleaving the

system

Change in total Energy of the

system̶ =

2. Application & importance of HT

In general o Maintaining body temperature

o In life support (from sun to earth)

o Cooking (electric & gas ranges)

o Cooling & keeping warm

o Relaxation & relieving muscle pain

o Weather formations

2. Application & importance of HT

In engineering o Steam engines, turbines, boilers

o Nuclear & solar energy

o power generation

o Chemical process industry

o Electronic device cooling

o Insulation designs on pipes & walls use HT analysis

2. Application & importance of HT

Engineering designo Boilers & heat exchangers

o Electric and hybrid vehicle battery cooling

2. Application & importance of HT

Example 1: Water heating in electric pot1.2 kg of liquid water initially at 15°C is to beheated to 95°C in a teapot equipped with a1200-W electric heating element inside (Fig. 1–18).

The teapot is 0.5 kg and has an average specificheat of 0.7 kJ/kg · °C. Taking the specific heat ofwater to be 4.18 kJ/kg · °C and disregarding anyheat loss from the teapot, determine how longit will take for the water to be heated.

3. Modes of heat transfer

Conduction:

HT due to particle inreaction alone. No bulk movement

Convection:

HT due to actual movement of bulk of fluid

Radiation:

HT by electromegnatic waves (photons). Require no medium

3. Modes of heat transfer

Conduction

Fourier‘s law of heat conduction:

The rate of heat conduction through a plane layer is proportional to the temperature difference across the layer and the heat transfer area, but is inversely proportional to the thickness of the layer.

That is,

Heat conduction rate α (A·ΔT)/(Δx)

Where, ΔT = (T2 ̶ T1)

3. Modes of heat transfer

Thermal conductivity (k):Rate of heat transfer through a unit thickness of material per unit area per unit degree rise in temperature.

o k is a measure of material‘s ability to conduct heat

e.g. for water, k = 0.608 (W/m·°C) at room T

and for iron, k = 80.2 (W/m·°C) at room T

o Higher value of k indicative of good condutor

o Lower value of k is indicative of poor conductor or good onsulator

3. Modes of heat transfer

Thermal conductivity (k):

Data derived from

3. Modes of heat transfer

Thermal diffusivity (α):Thermal diffusivity of a material can be viewed as the ratio of theheat conducted through the material to the heat stored per unitvolume.

α = (Heat conducted)/(Heat stored)

α = k/(ρ CP)

Larger value of thermal diffusivity indicates faster propogation of heat through the material

3. Modes of heat transfer

Example 3.1:

3. Modes of heat transfer

Example 3.2:

3. Modes of heat transfer

Convection

Rate of convective heat transfer between a solid surface and adjacent fluid is given by Newton’s law of cooling.

Heat convection rate = hA (Ts ̶ Tf)

Where, h = Heat transfer or film coefficient given in (W/m2·°C)

3. Modes of heat transfer

Convection

Rate of convective heat transfer between a solid surface and adjacent fluid is given by Newton’s law of cooling.

Heat convection rate = hA (Ts ̶ Tf)

Where, h = Heat transfer or film coefficient given in (W/m2·°C)

3. Modes of heat transfer

Example 3.3: