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