Download - Thermo I.ppt
Energy and Heat Energy and Heat TransferTransfer
Thermodynamics 1Thermodynamics 1
ObjectivesObjectivesComprehendComprehend
• Forms of energy• Energy conversion• Heat transfer processes• Principles of operation of various heat
exchangers• Boundary layers
ReferencesReferences• INE,
– Chapter 1, pp. 3-5– Chapter 2, pp. 9-14, 16-28 (22-28)***
• PNE, – Chapter 8, pp. 8-1 - 8-14 ***
• HW: LG #1
Energy ClassificationEnergy Classification
• Mechanical– Stored– Transitional
• Thermal– Stored– Transitional
Mechanical EnergyMechanical Energy
• Stored– Potential energy– Kinetic energy
• Transitional– Work– Power
Thermal EnergyThermal Energy
• Stored– Internal potential energy– Internal kinetic energy
• Transitional– Heat (energy in transition)– BTU as a unit of measurement
Mechanisms of Heat Mechanisms of Heat TransferTransfer
• Conduction• Radiation• Convection
ConductionConduction
• Heat transfer - Hot Cold• Physical contact between regions • Iron Bar Example
General Conduction General Conduction EquationEquation
Q = ktA (T1-T2)/Lwhere
Q : Quantity of heat (Btu or cal)k : Coefficient of thermal conductivity (Btu/((hr)(F)(ft))t : Time (hr)T1 : Temp at hot end (F)T2 : Temp at cold end (F)L : Distance between the two ends (in)A : Cross sectional area (sq ft)
RadiationRadiation
• Heat transfer - Hot Cold• NoNo physical contact between regions• Heat Sink:
– Absorbs radiant energy (Dark/Opaque)– Reflects radiant energy (White/Shiny/Polished)
• Sun warms the earth
ConvectionConvection
• Combined conduction/radiation• Occurs in liquids and gases• Transfer of thermal energy through a fluid
due to the actual movement of the fluid• Types
– Natural– Forced
Natural ConvectionNatural Convection
• Movement caused by differences in fluid density
• Density differences usually caused by differences in temperature
T hot T hot
Forced ConvectionForced Convection
• Movement caused by mechanical device– Pump– Fan
DefinitionsDefinitions• Sensible Heat
– When heat added results in the change in temperature (changing Temp, 1 phase)
• Latent Heat– When the heat added results in a physical change of the substance (2
phases, constant Temp)
• Saturation Temperature/Pressure– Psat/Tsat– The point at which liquid and vapor may exist in equilibrium contact
with each other
Steam DomeSteam Dome
Definitions (cont)Definitions (cont)• Saturated Liquid/Vapor
– A liquid/vapor at a specified pressure which is at Tsat for the pressure
• Subcooled Liquid– A liquid at that specified pressure which is below the Tsat
• Superheated Vapor– A vapor that has been raised above Tsat for a given
pressure• Quality (x) (opposite of moisture (m))
– Ratio of saturated vapor to total mass in a saturated mixture of liquid/vapor (x=1-m)
Definitions (cont)Definitions (cont)
• Latent Heat of Vaporization– Amount of heat necessary to change a mass
of liquid to vapor without changing the temperature
• Latent Heat of Fusion– Amount of heat that must be
added/removed to a unit mass to melt/solidify it
Steam DomeSteam Dome
Mollier DiagramMollier Diagram
Heat ExchangersHeat Exchangers
• Counter flow • Parallel flow • Cross flow• Single pass• Multi-Pass
Counter FlowCounter Flow
Parallel FlowParallel Flow
Cross FlowCross Flow
Heat ExchangersHeat Exchangers
• Single pass - One fluid passes the other fluid only once.
• Multi-Pass - One fluid passes the other more than once via the arrangement of the tubes, inlet/outlet ports, or by baffles used to guide a fluid through a specific path.
Boundary LayersBoundary Layers
• Boundary Layer - Thin layer of stagnant fluid adjacent to heat exchanger wall. Poor at transferring heat.
Boundary Layers (cont)Boundary Layers (cont)
• Effect on heat transfer– Velocity– Turbulent Flow
• Scale/Chemical deposits• Soot/Dirt buildup
SummarySummary