Chapter 1 - Introduction 2012 - Web

Chapter 1 - Introduction 2012 - Web - WHAT IS HEAT...

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W HAT IS H EAT T RANSFER ? There are three principal laws upon which engineering studies are derived: Conservation of Mass (Mass Transfer, Reactor Design) Conservation of Momentum (Fluid Mechanics, Mass Transfer) Conservation of Energy (Thermodynamics, Heat Transfer )
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C ONSERVATION OF E NERGY Thermodynamics – the study of the transfer of work and energy between a system and its surroundings – in equilibrium Key thermodynamic functions (enthalpy, entropy) are state functions – neither rate or pathway influence values How we achieved equilibrium is not of concern in thermodynamics BUT is important for designing heating or cooling devices Heat Transfer – determination of the rate of energy transferred from one place to another resulting in a temperature change
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E XAMPLE Consider a can of cola you want to cool down in a refrigerator. 20 o C How much heat needs to be removed from the can? THERMODYNAMICS If you leave it in the fridge, what will the temperature of the can be as a function of time? HEAT TRANSFER How long will it take to cool down? HEAT TRANSFER Surrounding Air, T = 4 o C REFRIGERATOR
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C OOLING THE C AN Question: How much heat needs to be transferred to cool the can? First Law of Thermodynamics Deals with the conservation of energy At steady state, we know between the system (can) and surroundings (refrigerator) We have a can ( closed system ), filled with 355 mL (~355g) of (mostly) water (+ 39 g of sugar!) Q: Sitting in the fridge, what kind of energy does the can possess? Potential energy? Kinetic energy? Other types of energy (like)? 0 E
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I NTERNAL E NERGY Internal energy is the sum of sensible (molecular spin, translation, vibration, etc.), latent (intermolecular forces phase changes), chemical (bond energy), and nuclear (fundamental particle) components in the absence of a phase change, only sensible energy is typically of concern in heat transfer Internal energy is defined by the variable , the specific internal energy of a system (J/kg) relative to water at triple point ( = 0). Typically, we are interested in changes in internal energy between two conditions (i.e. comparing U at T 1 , P 1 vs. T 2 , P 2 ) U ˆ U ˆ
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the correlation between heat flow and the heat capacity ( c P ) of the system. For a closed system a ssuming sensible energy is the only contributor to heat generation/loss , the change in internal energy is: c p = ability of a material to store heat For water, from Appendix A, Table A6: c P (water, 20°C)=4.182 kJ/kg K c P (water, 4°C)=4.207 kJ/kg K Average c P = 4.194 kJ/kg K U = -23.8 kJ NOTE: For gases or large T , c P = f(T) T c m U m U Q P ˆ ) 16 )( 194 . 4 )( 355 . 0 ( K kgK kJ kg T c m U P 2 1 T T P dT c m U
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Chapter 1 - Introduction 2012 - Web - WHAT IS HEAT...

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