Principles of Thermodynamics and Thermal Fluids (CHPE 203) Chapter 1 Introduction

What is thermal sciences? Thermal-fluid sciences involve the Transfer of energy Transport of energy and Conversion of energy, usually studied under the sub-categories of thermodynamics, heat transfer, and fluid mechanics.

Example of Thermal Fluids in Human Body The heart is constantly pumping blood to all parts of the human body, various energy conversions occur in trillions of body cells, and the body heat generated is constantly rejected to the environment.

Example of Thermal Fluid Applications Designing the radiator of a car involves: the determination of the amount of energy transfer from a knowledge of the properties of the coolant using thermodynamics, the determination of the size and shape of the inner tubes and the outer fins using heat transfer, and the determination of the size and type of the water pump using fluid mechanics.

What is Thermodynamics? Thermodynamics can be defined as the science of energy. words therme (heat) and dynamis (power), the ability to cause changes. The change in the energy content of a system is equal to the difference between the energy input and the energy output, and the energy balance is expressed as: ΔE = Ein - Eout

Thermodynamic Laws Zeroth Law of Thermodynamics First Law of Thermodynamics Second Law of Thermodynamics Third Law of Thermodynamics

Zeroth law of Thermodynamics: The Zeroth law states that “Two objects are in thermal equilibrium if both have the same temperature reading even if they are not in contact”.

First law of thermodynamics First law of thermodynamics is simply an expression of the conservation of energy principle, and it states that “Energy cannot be created or destroyed BUT it can be changed from one form to another”.

Second law of thermodynamics Second law of thermodynamics states that energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy. For example, a cup of hot coffee left on a table eventually cools to room temperature, but a cup of cool coffee in the same room never gets hot by itself.

Third Law of Thermodynamics Entropy is the measure of molecular disorder or randomness. As a system becomes more disordered, the position of the molecules becomes less predictable and the entropy increases. Entropy is the lowest in a solid because molecules are held in place and simply vibrate and highest in a gas where the molecules are free to move in any direction. Third Law of Thermodynamics States that: “Entropy of a pure crystalline substance at absolute zero temperature (zero Kelvin) is zero since the state of each molecule is known”.

Heat Transfer Energy that can be transferred from one system to another as a result of temperature difference, also, deals with the determination of the rates and mechanism of energy being transferred. Thermodynamics: deals with equilibrium states and changes from one equilibrium state to another. Heat transfer: deals with systems that lack thermal equilibrium(non-equilibrium phenomenon), and tell us the rate and mechanism of heat been transferred.

Fluid mechanics Is defined as the science that deals with the behaviour of fluids at rest (fluid statics) or in motion (fluid dynamics). fluid refers to a substance in the liquid or gas phase Hydro-dynamics deals with liquid flows in pipes and open channels. Gas dynamics deals with flow of fluids that undergo significant density changes, such as the flow of gases through nozzles at high speeds. Aerodynamics deals with the flow of gases (especially air) over bodies such as aircraft, rockets.

Unit Systems Any physical quantity characterized by dimensions called units: Primary dimensions: such as mass m, length L, time t, and temperature T Secondary dimensions: such as velocity u, energy E, and volume V are expressed in terms of the primary dimensions (derived from primary).

Units are still in common use today: 1- English system, which is also known as the United States Customary System (USCS), but this has no apparent systematic numerical base, and various units are related arbitrary to each other (12 in in 1 ft (foot), 16 oz in 1 lb, 4 qt in 1 gal, etc.). 2- Metric SI (which known as the International System). meter (m) for length, kilogram (kg) for mass, second (s) for time, degree Kelvin (°K) for temperature). 1 lbm (pound mass) = 0.45359 kg ft = 0.3048 m

Force: In SI system, N (Newton) force required to accelerate a mass of 1 kg at a rate of 1 m/s2. In the English system, the force unit is the pound-force (lbf ) and is defined as the force required to accelerate a mass of 32.174 lbm at a rate of 1 ft/s2. That is, 1 N = 1 kg · m/s2 1 lbf = 32.174 lbm · ft/s2   Unlike mass, weight W is a force. It is the gravitational force applied to a body, and its magnitude is determined from Newton’s second law, W = mg = (N) OR Force=mass x acceleration where m is the mass of the body, and g is the local gravitational acceleration (g is 9.81 m/s2 or 32.174 ft/s2 at sea level). The specific weight γ and is determined from γ =ρg, where ρ is density.

Energy energy unit is the Btu (British thermal unit) = energy required to raise the temperature of 1 lbm of water at 68 F by 1 F. In the metric system, the amount of energy needed to raise the temperature of 1 g of water at 15 oC by 1 oC is defined as 1 calorie (cal), and 1 cal= 4.1868 J. The magnitudes of the kilo-joule and Btu are almost identical (1 Btu=1.055 kJ). Work : a form of energy, defined as force times distance; therefore, it has the unit “Newton. meter (N ·m),” called a Joule (J), 1 J = 1 N · m and, the kilo-joule (1 kJ=103J).

Dimensional Homogeneity In engineering, all equations must be dimensionally homogeneous. That is, every term in an equation must have the same unit (apples and oranges do not add). E= 25 kJ + 7 kJ/kg Invalid Example: A tank is filled with oil whose density is ρ= 850 kg/m3. If the volume of the tank is V=2 m3, determine the amount of mass m in the tank. It is clear: m = ρ . V = (850 kg/m3) (2 m3) =1700 kg

Operations with Units Only add and subtract numbers with the same associated units 2 kg + 3 m Invalid If the dimensions are the same but the units differ, first convert to a common set of units 1 lb + 400 g Invalid 1 lb + 400 g 1 lb + 0.88 lb = 1.88 lb Valid You can multiply and/or divide unlike units, but you cannot cancel units unless they are the same Valid

A Conversion Factor in SI In SI, the conversion factor C used to transform the units of mass, length, and time, to the derived unit of force is obtained from: F = (mass) (acceleration) mass acceleration conversion factor g=9.81

1 in = 2.54 cm, 1ft=12 in

1 in = 2.54 cm, 1ft=12 in