Fundamentals of Thermodynamics Chapter 1 Introduction Prof. Siyoung Jeong Thermodynamics I MEE2022-01
Thermodynamics : Science of energy and entropy - Science of heat and work and properties related to heat and work Basis for diverse scientific disciplines - Physics, chemistry, biology, etc. Applied to various fields - Mechanical engineering, chemical engineering, aerospace engineering, etc. 2
3 1.1 A Thermodynamic system and the control volume Schematic diagram of a steam power plant
4 Schematic diagram of a refrigerator
Thermodynamic system - System / Surroundings - Control mass (no mass flow) - Control volume (more general) - Open / closed systems - System boundary / Control surface - Isolated system : No influence from surroundings Control mass Control volume 5
1.2 Macroscopic versus microscopic points of view Macroscopic vs. Microscopic 10^20 atoms 25 mm Atmospheric P, T Continuum 6*10^20 equations! Classical Thermodynamics Statistical Thermodynamics 6
7 1.3 Properties and state of a substance Phase : gas, liquid, solid A substance may exist in in various states (T, p, etc.). Thermodynamic property : observable & macroscopic - Intensive : pressure, temperature, density, etc. - Extensive : mass, volume, etc. - Specific property : property/mass, intensive property, ex) specific volume Thermodynamic equilibrium - Thermal equilibrium : T - Mechanical equilibrium : P - Chemical equilibrium : μ (chemical potential)
1.4 Processes and cycles Process : Path of state change States during a process? - Quasi-equilibrium process : Defined ideally but applied to many practical cases - Non-equilibrium process Iso-X process : Constant X - Isothermal : T=const. - Isobaric (isopiestic) : P=const. - Isochoric : V=const. Cycle : Initial state = Final state 8
1.5 Units for mass, length, time, and force SI units : Metric International System - Time : second [s] - Length : meter [m] - Mass : kilogram [kg] cf) mole [mol] : 12 g of carbon-12 - Force : Newton [N] English Engineering Units - ft, in, lbm, lbf, 9
10 Ex. 1.1 What is the weight of a 1 kg mass at an altitude where the local acceleration of gravity is 9.75 m/s 2?
11 1.6 Specific volume and density v, Specific volume = volume / mass [m 3 /kg] r, Density = mass / volume [kg/m 3 ] Continuum concept : V m Molal property : [m 3 /kmol], r [kmol/m 3 ] v v lim V V } Both intensive properties
12 Ex. 1.2 A 1 m 3 container, shown in Fig. 1.9, is filled with 0.12 m 3 of granite, 0.15 m 3 of sand, and 0.2m 3 of liquid 25 water; the rest of the volume, 0.53 m 3, is air with a density of 1.15 kg/m 3. Find the overall (average) specific volume and density.
13 1.7 Pressure Continuum concept P lim A A F n A Unit : 1 Pa = 1 N/m 2 1 bar = 10 5 Pa = 0.1 MPa 1 atm = 101325 Pa ~ 1 bar
14 Absolute vs. gauge pressure P P B B A P P 0 P P A mg B 0 rgh P 0 rgh P A ragh 0
15 Fluid with a constant density: P P P rgh Fluid with a varying density: P P P dp r( z) gdz B B 0 0 0 H z 0 P P 0 z z dz P P dp z H P P B
16 Ex. 1.3 The hydraulic piston/cylinder system shown in Fig. 1.11 has a cylinder diameter of D = 0.1 m with a piston and rod mass of 25 kg. The rod has a diameter of 0.01 m with an outside atmospheric pressure of 101 kpa. The inside hydraulic fluid pressure is 250 kpa. How large a force can the rod push with in the upward direction?
17 Ex. 1.4 A mercury barometer located in a room at 25 has a height of 750 mm. What is the atmospheric pressure in kpa? Ex. 1.5 A mercury (Hg) manometer is used to measure the pressure in a vessel as shown in Fig. 1.13. The mercury has a density of 13,590 kg/m 3, and the height difference between the two columns is measured to be 24 cm. We want to determine the pressure inside the vessel.
18 Ex. 1.6 What is the pressure at the bottom of the 7.5 m tall storage tank of fluid at 25, shown in Fig. 1.15? Assume that the fluid is gasoline with atmospheric pressure 101 kpa on the top surface. Repeat the question for the liquid refrigerant R-134a when the top surface pressure is 1 MPa.
19 Ex. 1.7 A piston/cylinder with a cross-sectional area of 0.01 m 2 is connected with a hydraulic line to another piston/cylinder with a cross-sectional area of 0.05 m 2. Assume that both chambers and the line are filled with hydraulic fluid of density 900 kg/m 3 and the larger second piston/cylinder is 6 m higher up in elevation. The telescope arm and the buckets have hydraulic piston/cylinders moving them, as seen in Fig. 1.16. With an outside atmospheric pressure of 100 kpa and a net force of 25 kn on the smallest piston, what is the balancing force on the second larger piston?
1.8 Energy Microscopic view - Intermolecular (potential) energy - Molecular kinetic energy - Intramolecular energy 20
21 1.9 Equality of temperature Sense of hotness or coldness However, difficult to rigorously define temperature Two objects in thermal contact for a long time No change thermal equilibrium
22 1.10 The zeroth law of thermodynamics Zeroth law T A = T thermometer T B = T thermometer T A = T B Basis of temperature measurement
1.11 Temperature scales Fahrenheit, Celsius - Celsius: formerly know as centigrade Fahrenheit Celsius Ice pt 32 0 Steam pt 212 100 Absolute temperatures K = C + 273.15 R = F + 459.67 23
24 1.12 Engineering applications
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28 Home Work - 2017 1.42 1.60 1.78 1.81