MC 267 Module 2 Vol. 2.pdf

MC 267 Thermodynamics Lecture 02 Vol. 2 Dr. Clement A. Komolafe [email protected] Class: Tuesday (8-10 MC) (2.30-4.30 EL) Wednesday (6 am – 8 am)  Ground Rules The ground rules are : 1. Come to class promptly and listen attentively 2. Come to class...

MC 267 Thermodynamics Lecture 02 Vol. 2 Dr. Clement A. Komolafe [email protected] Class: Tuesday (8-10 MC) (2.30-4.30 EL) Wednesday (6 am – 8 am)  Ground Rules The ground rules are : 1. Come to class promptly and listen attentively 2. Come to class with a pen and paper and solve related problems copiously 3. Obtain and consult the recommended textbooks regularly 4. Make an acquaintance with your student handbook and adhere strictly to all its provisions 5. Do not be negatively influenced by peer pressure 6. Read, study and meditate on your books because that is your primary focus here 2  Learning Objectives The objectives are to:  know fundamental concept of a pure substance.  discuss the physics of phase-change processes.  demonstrate the use of the property tables. 3  WORKING SUBSTANCE QUALITY  Working fluid: This can be defined as the matter contained within the boundaries of a thermodynamic system. The working fluid in a thermodynamic system could be in solid, liquid or gaseous phase or a combination of the phases. Phase  Phase: refers to a quantity of matter that is homogeneous throughout in both chemical composition and physical structure.  Three common phases: solids, liquids and gases  Solids: molecules closely packed, high molecular forces, definite shape and volume  Liquids: molecules less closely packed, weaker molecular forces, fluid shape and definite volume  Gases: molecules move around, weakest molecular forces, no shape and container-defined volume. 4  WORKING SUBSTANCE QUALITY  Phase change types  Condensation: this is a process whereby gas changes into a liquid.  Vaporization: this is a process whereby liquid changes to gas.  Freezing: this is a process where liquid changes to solid.  Melting (fusion): this is a process where a solid changes into liquid.  Sublimation: this is a process where a solid changes to gas without becoming liquid.  Deposition: this is a process where a gas changes to solid without becoming liquid  Homogeneity in Physical Structure means that the matter is all solid, or all liquid, or all vapour. A system can contain one or more phases. 5  Pure Substances  A substance that has a fixed chemical composition throughout is called a pure substance. Water, nitrogen, helium, and carbon dioxide, for example, are all pure substances.  A mixture of two or more phases of a pure substance is still a pure substance as long as the chemical composition of all phases is the same.  At a given pressure, the temperature at which a pure substance changes phase is called the saturation temperature (Tsat).  Likewise, at a given temperature, the pressure at which a pure substance changes phase is called the saturation pressure (Psat).  Consider a piston–cylinder device containing liquid water at 20°C and 1 atm pressure being heated. 6  PureConcepts Fundamental Substances and Principles  Under these conditions, water exists in the liquid phase, and it is called a compressed liquid, or a subcooled liquid, meaning that it is not about to vaporize.  A liquid that is about to vaporize is called a saturated liquid.  Once boiling starts, the temperature stops rising until the liquid is completely vaporized. Any heat loss from this vapor will cause some of the vapor to condense. A vapor that is about to condense is called a saturated vapor.  A substance at state between saturated liquid and saturated vapor is referred to as a saturated liquid–vapor mixture since the liquid and vapor phases co-exist in equilibrium at these states. 7  Pure LAWS Substances OF THERMODYNAMICS  If the temperature of the vapor is, let us say, 300°C; and if we transfer some heat from the vapor, the temperature may drop somewhat but no condensation will take place as long as the temperature remains above 100°C (for P = 1 atm). A vapor that is not about to condense (i.e., not a saturated vapor) is called a superheated vapor. 8  Pure Substances  The degree of superheat is the difference between the actual temperature of a superheated vapor/steam and the saturation temperature at the pressure of the saturated vapor/steam. Properties of wet vapour and use of steam table  Dryness fraction (𝑥) is the condition or quality of a wet vapor. It is the mass of dry vapor (or saturated vapor) in 1 kg of the mixture of saturated liquid and saturated vapor. 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑑𝑟𝑦 𝑣𝑎𝑝𝑜𝑢𝑟 𝑥= 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑑𝑟𝑦 𝑣𝑎𝑝𝑜𝑢𝑟+𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑎𝑡𝑢𝑟𝑎𝑡𝑒𝑑 𝑙𝑖𝑞𝑢𝑖𝑑 Wetness fraction (x) is the mass of saturated liquid in 1kg of the mixture of saturated liquid and saturated vapor. It is denoted by 1−𝑥 9  Pure Substances Properties of wet vapour and use of steam table 10  Pure Substances LAWS OF THERMODYNAMICS Properties of wet vapour and use of steam table 11  Pure Substances Properties of wet vapour and use of steam table 12  Pure Substances Properties of wet vapour and use of steam table 13  Pure Substances Properties of wet vapour and use of steam table 14  Pure Substances Properties of wet vapour and use of steam table 15 Pure Substances 16  Pure Substances Properties of superheated vapour contd. Example 2.3 17  Properties of superheated vapour contd. Example 2.4 18  Properties of superheated vapour contd. Example 2.5 19  Classwork 1. Using the properties of ammonia in the steam table, calculate: 2. Calculate the internal energy for each of the states given below: (a) p = 20 bar, t = 250 oC 20