ES 202 Fluid and Thermal Systems Lecture 14: Phase Change (1/14/2003)

Lecture 14ES 202 Fluid & Thermal Systems2 Assignments Homework: –3-2, 3-4, 3-6, 3-9 in Cengel & Turner Reading assignment –Section 3-1 to 3-4, 3-6 in Cengel & Turner

Lecture 14ES 202 Fluid & Thermal Systems3 Announcements A note on Lab 2 write-up –due date –one write-up per lab group (higher expectation) –Contents: water wall, Torricelli experiment, pipe friction –Advice: start early, discuss with me if you need advice Homework policy for Problem –not an easy problem –contains many concepts in a complete solution –Goal of homework: facilitate your learning Feedback on Exam 1 from you

Lecture 14ES 202 Fluid & Thermal Systems4 Road Map of Lecture 14 Background quiz Phase change –example: the boiling process –difference in molecular structure –State Principle –representation of P-v-T surface –two dimensional projection of P-v-T surface –terminology and definition –models: ideal gas incompressible substance

Lecture 14ES 202 Fluid & Thermal Systems5 Background Quiz How many phases does a substance have? –solid –liquid –gas Zeroth law of thermodynamics –Statement: If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. –Significance: Introduce the concept of temperature First law of thermodynamics –Significance: Conservation of energy Second law of thermodynamics –Significance: Entropy cannot be destroyed

Lecture 14ES 202 Fluid & Thermal Systems6 Background Quiz (Cont’d) Energy balance for an open system: Entropy balance for an open system:

Lecture 14ES 202 Fluid & Thermal Systems7 Background Quiz (Cont’d) Simple substance: –there is only one reversible work mode which can change the energy of the system think of work mode as an independent control knob (apart from heat transfer) which can change the energy of a system State Principle: –Two independent, intensive thermodynamic properties are sufficient to fully determine the thermodynamic state of a simple substance. –For example: –Example from ideal gas model:

Lecture 14ES 202 Fluid & Thermal Systems8 The Boiling Process Exercise: Sketch the temperature variation of a pot of water from the moment that you turn on the stove Question: The temperature does not change even though you keep adding heat to the system. Where does the energy go? Question: What happens to the water during the boiling process, i.e. its internal structure? Question: What is the surrounding pressure on the water?

Lecture 14ES 202 Fluid & Thermal Systems9 P-v-T Surface State Principle: –Two independent, intensive thermodynamic properties are sufficient to fully determine the thermodynamic state of a simple substance. One can visualize the thermodynamic state of a simple substance on a 3D surface Show the P-v-T surface –identify the solid, liquid, vapor regions –identify the two-phase dome

Lecture 14ES 202 Fluid & Thermal Systems10 Two-Dimensional Representation The 3D surface can be represented by multiple contour lines on a 2D plane, for example: –P-v plane (direction of increasing temperature) –T-v plane (direction of increasing pressure) –P-T plane Sketch these above planes for: –an ideal gas –an incompressible substance Where are the ideal gas model and incompressible substance state located on the P-v-T surface relative to the two-phase dome?

Lecture 14ES 202 Fluid & Thermal Systems11 Terminology and Definition Pure substance Compressed liquid Saturated liquid Saturated vapor Superheated vapor The two-phase (vapor) dome Saturated liquid line Saturated vapor line Critical point Triple point Quality Trace the boiling process on the T-v plane –effect of imposed pressure on boiling point