lecture6

lecture6 - 9/1/2010 Total energy, internal energy, heat...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

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 ...
View Full Document

Ask a homework question - tutors are online