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Unformatted text preview: 9/1/2010 Total energy, internal energy, heat transfer
ME 200 Thermodynamics I Lecture 6 September 3rd, 2010 Purdue University , Dr. Tim Pourpoint – timothee@purdue.edu Last Lecture
• Concepts of work, specifically:
– Moving boundary work – Concepts of:
• Non‐quasi static work And: • Cyclic work ME 200 2 1 9/1/2010 This Lecture
• Today’s agenda
– Heat transfer (Q) • Objectives
– Define the concept of heat (Q) and the terminology associated with energy transfer by heat – Introduce the three mechanisms of heat transfer:
• Conduction • Convection • Radiation ME 200 3 Heat Transfer
• Recall mass cannot cross the boundaries of a closed system, but energy can • Energy can cross the boundaries of a closed system by two modes:
– Heat, Q – Work, W ME 200 4 2 9/1/2010 Heat Transfer
• Heat is a form of energy that is transferred between a system and its surroundings by virtue of a temperature difference in a direction of higher T to lower T ME 200 5 Heat Transfer
• Heat is energy in transition and is only recognized as it crosses the boundary • Heat is form of energy so units are kJ or Btu (1 kJ=0.94782 Btu) • Heat flux: q
– Rate of heat transfer per unit area – Units: W/m2 or Btu/(ft2‐h) • Heat transfer rate:
– Based on heat flux
ENERGY ONLY RECOGNIZED AS “HEAT TRANSFER” AS IT CROSSES BOUNDARY ME 200 6 3 9/1/2010 Adiabatic Process
• Adiabatic process: no heat transfer • There are two situations when this can occur:
– Insulated system – No temperature difference (no driving force) IN THEORY ME 200 IN PRACTICE 7 Quantifying Heat Transfer
• Amount of heat transferred during a process between two states 1 and 2 is (notation, tbd): • Heat transfer rate • Heat transfer occurs in a particular direction so we have sign convention
– Heat transfer to a system is positive – Heat transfer from a system is negative • Q is either given, zero or is only unknown left in equations
ME 200 8 4 9/1/2010 In summary so far… (1/2) far… Heat is the form of energy transferred between two systems or between a system and its surroundings owing to temperature difference Heat is energy in transition and observed as a boundary phenomenon System does not possess heat, i.e., heat is not a property of a given system Heat is relevant to a process not a state Units: kJ (SI) or Btu (English)
9 ME 200 In summary so far… (2/2)
• Heat transfer is a directional quantity i.e. both magnitude and direction
– Heat transfer to system is positive i.e. Q > 0 – Heat transfer from system is negative i.e. Q < 0 – Adiabatic system (no heat transfer) i.e. Q = 0 • We can consider heat addition (Qin) and heat rejection (Qout) with only magnitudes because direction is specified via appropriate subscripts • Very important to follow consistent sign convention ME 200 10 5 9/1/2010 Heat Transfer: Conduction
• Molecular interactions • Medium required • Derivative of T dT Qcond qcond A k A dx
Fourier’ Law ME 200 Incropera and Dewitt, Fundamentals of Heat and Mass Transfer, 1996. 11 Heat Transfer: Convection
• Fluid motion • Medium required • T difference Qconv qconv A h A Tsurface T ME 200 Incropera and Dewitt, Fundamentals of Heat and Mass Transfer, 1996. 12 6 9/1/2010 Convective Heat Transfer
Convection coefficient: W/m2‐K Free convection: 5 to 12 W/m2‐K Forced convection: 10 to 300 W/m2‐K Incropera and Dewitt, Fundamentals of Heat and Mass Transfer, 1996. ME 200 13 Conductive and Convective HeatTransfer Incropera and Dewitt, Fundamentals of Heat and Mass Transfer, 1996. ME 200 14 7 9/1/2010 Heat Transfer: Radiation
• Electromagnetic nature • No medium required • T4
T2, q2” 4 Qrad qrad A A Tsurface T4 T1, q1” Net radiation heat exchange between two surfaces With: = Stefanp‐Boltzmann constant = 5.67x10‐8 W/(m2K4) = emissivity of surface, 0 < < 1 ME 200
Incropera and Dewitt, Fundamentals of Heat and Mass Transfer, 1996. 15 Radiative Heat Transfer Incropera and Dewitt, Fundamentals of Heat and Mass Transfer, 1996. ME 200 16 8 9/1/2010 More on Heat and Work
• Direction of work and heat depicted via arrows or subscripts in and out • For given system, both Q and W depend on:
– Process path – Initial and final states • These are called path functions • They have inexact differentials: Q, W dQ, dW • Point functions only depend on initial/final state • They have exact differentials: dE, dV
17 ME 200 Application ME 200 18 9 9/1/2010 Example 6.1
• A potato initially at room temperature (25C) is being baked in an oven that is at 200C. • Is there any heat transfer in this process?
– Yes – No – Depends ME 200 19 Example 6.2
• A poorly insulated home with 1300 ft2 of ceiling is located in a region with an 8‐month heating season. • The average outdoor temperature during the heating season is 40F and the inside temperature is fixed at 70F. • The owner will spend $850 to increase the R‐value of the insulation in the ceiling from 11 to 40 (hr‐ft2‐F/Btu). • The house is heated with electricity that costs 10 cents/kWh. How much energy will the owner save each year? How long will it take for the saved energy to pay for the new insulation?
ME 200 20 10 9/1/2010 Questions / Train Yourself
• In what forms can energy cross the boundaries of a closed system? • When is the energy crossing the boundaries of a closed system heat and when is it work? • What is an adiabatic process? • A gas in a piston‐cylinder device is compressed and as a result its temperature rises. Is this a heat or work interaction? • What are point and path functions? (Give some examples) ME 200 21 Watch Energy Units
• Changes in KE and PE have SI units of kg‐m2/s2 1 N = 1 kg‐m/s2 1 N‐m = 1 kg‐m2/s2 1 J = 1 N‐m 1 kJ = 1000 J = 1000 N‐m • Therefore, to convert changes in KE and PE to kJ you need to divide by 1000 • In English units, you need to (a) convert from lbm to lbf via 32.2 lbm‐ft/s2/lbf and then convert from ft‐lbf to Btu by dividing by 778 ft‐lbf/Btu
ME 200 22 11 9/1/2010 Summary and Conclusions
• In this lecture, we discussed:
– The concept of heat (Q) and the terminology associated with energy transfer by heat – Three mechanisms of heat transfer:
• Conduction • Convection • Radiation ME 200 23 Next Week
• Energy balance for closed systems (Wednesday) • Energy analysis of cycles (Friday) ME 200 24 12 ...
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