COURSE INTRODUCTIONHeat TransferPemindahan HabaKM30403

Prerequisite: KM21102 Engineering ThermoKM30803 Applied ThermoPn. Fadzlita Mohd Tamirifadzlita@ums.edu.my

COURSE DESCRIPTIONThis course discusses the fundamentals of heat transfer including three modes: conduction, convection and radiation. In conduction mode, steady-state and unsteady state heat transfer are covered for one and two dimensions. For a convection mode, it is divided into categories; free and forced convection heat transfer applied in laminar and turbulent flow either external or internal flows. Radiation heat transfer includes the equations for overall emissitivity and view factor (F) for simple planes that are in common geometric relationships with each other. Different types of heat exchangers are also discussed including their designs.

COURSE OBJECTIVESStudents learn how to use the concepts of thermodynamics and formulate engineering problems in the three modes of heat transfer and obtain mathematical solutions using a variety of techniques in calculus and differential equations. In teams, students learn how to solve heat transfer problems through assignments / small projects.

COURSE OUTCOMESAbility to demonstrate knowledge on the basic concept and mechanism of heat transfer, which are conduction, convection and radiationAbility to develop thermal resistance networks for practical heat transfer problemsAbility to identify the related equations to heat transfer mechanisms and problemsAbility to analyse and solve engineering problems and applications using all of the above.Ability to apply knowledge to enable the analysis and design of heat exchangers.

REFERENCESYunus A. Cengel , Afshin A. Ghajar (2011), Heat and Mass Transfer: Fundamentals and Applications, 4th Edition, McGraw-Hill. Yunus A. Cengel (2008), Introduction to Thermodynamics and Heat Transfer, 2nd Edition, McGraw-Hill Higher EducationDr. Harimi Mohamed (2007/2008), Heat Transfer KM4313: Elective I.J.P. Holman (2002), Heat Transfer, 9th Edition, McGraw-Hill Education, Singapore.Frank Kreith, Raj M. Malik & Mark S. Bohn (2011), Principles of Heat Transfer, 7th Edition, Cengage Learning.

EVALUATION SYSTEMQuizzes5%Assignment25%Test 110%Test 210%Final Exam50%Total100%

Attendance is COMPULSORY

COURSE SCHEDULE

WeeksTopics / Contents1-2Introduction Overview of Heat Transfer3-5Conduction (Steadystate)Conduction (Unsteady-state)6-8Convection Heat Transfer9-11Radiation Heat Transfer12-13Heat Exchangers14Revision

Thermodynamics and Heat TransferThe science of thermodynamics deals with the amount of heat transfer as a system undergoes a process from one equilibrium state to another, and makes no reference to how long the process will take.The science of heat transfer dealswith the determination of the rates of energy that can be transferred from one system to another as a result of temperature difference.

Thermodynamics deals with equilibrium states and changes from one equilibrium state to another. Heat transfer, on the other hand, deals with systems that lack thermal equilibrium, and thus it is a nonequilibrium phenomenon.Therefore, the study of heat transfer cannot be based on the principles of thermodynamics alone.However, the laws of thermodynamics lay the framework for the science of heat transfer.

DEFINITIONHeat or Thermal energy is related to temperature of matterFor a given material & mass, the higher the temperature, the greater its thermal energy.Heat Transfer is a study of the exchange of thermal energy through a body or between bodies which occur when there is a temperature difference.Heat transfers from the one with higher temperature to one with lower temperature (hot to cold)

Heat TransferThe basic requirement for heat transfer is the presence of a temperature difference.The second law requires that heat be transferred in the direction of decreasing temperature.

The temperature difference is the driving force for heat transfer.The rate of heat transfer in a certain direction depends on the magnitude of the temperature gradient in that direction.The larger the temperature gradient, the higher the rate of heat transfer.

Application Areas of Heat Transfer

Units & Conversion Factors For Heat Measurement

SI UnitsUSCS UnitsThermal Energy1 J9.4787x10-4 BtuHeat Transfer Rate1 J/s or 1 W3.4123 Btu/hrHeat Flux1 W/m20.3171 Btu/h ft2

Physical PropertiesDensityMass of fluid contained in a unit volume=m/VDynamic ViscosityProperty of a fluid, resistance to shear deformation= du/dyKinematic ViscosityRatio of dynamic viscosity to mass density=/

Thermal ConductivityMeasure of the ability of a material to conduct heatFouriers law of conductionSpecific HeatAmount of heat that is required to raise the temperature of a unit mass by one degree in a constant pressure processCoefficient of Thermal ExpansionChange in the density as a function of temperature at constant pressureThermal DiffusivityRatio of heat conducted through the material to the heat stored per unit volume

Modes of Heat TransferConductionTransfer of heat through solids or stationery fluidsConvectionUses the movement of fluids to transfer heatRadiationUses electromagnetic radiation emitted by an object for exchanging heat , does not require a mediumOr a combination of them

ConductionLattice vibrationParticle collisionSolids with free electrons (metals): Hot side electrons moves faster than cool side electronsFaster electrons give energy to slower electronsEquilibrium: electrons moving at the same average velocityConduction through electrons collision more effective than lattice vibrationMetals are better heat conductors than ceramic

Fluids: conduction occurs through collisions between freely moving molecules.Thermal conductivity, K: measure the effectiveness of heat transferred through a material.

Negative (-) Q:heat flowing out of bodyConduction

Thermal conductivity, K [W/mK]1 W/mK = 0.578 Btu/hr ftFK at 300K (540R):Conduction

MaterialK (W/mK)K(Btu/hr ftF)Copper399231Aluminum237137Carbon steel 1%C4325Glass0.810.47Plastic0.2-0.30.12-0.17Water0.60.35Air0.0260.02

Problems:One surface of a 2-cm-thick copper plate is maintained at 300K and the other surface at 250K. Calculate the rate of heat transfer per unit area through the plate (K=401 W/mK).

Calculate the rate of heat transfer through a glass window (K=0.81 W/mK) 1m high, 0.5m wide, 0.5cm thick, if the outer-surface To=24C and the inner-surface Ti=24.5C.Conduction

Uses the motion of fluids to transfer heat.Natural / free convection: fluid movement is created by the warm fluid itself. Density of fluid decrease as it is heated, hot fluids rises, replaced by cool fluids circulation of air.Forced convection: fluid movement by external means (eg. wind or fans)Convection coefficient, h: measure how effectively a fluid transfers heat by convection.Convection

Convection coefficient, h [W/m2K]Determined by factors such as the fluid density, viscosity & velocity.Higher fluid velocity, increase the h.Convection

Problems:Calculate the rate of heat transfer by natural convection between a shed roof of area 20m x 20m and ambient air, roof Troof=27C, air Tair= -3C, and h=10W/m2K.Convection

RadiationDoes not require mediumUses electromagnetic radiation (photons)Radiative heat transfer occurs when emitted radiation strikes another body and is absorbedGamma rays, x-rays, UV, visible light, IR, microwaves, radio wavesShorter wavelength: more energetic, contains more heatLonger wavelength: can penetrate through thicker solids

Emitted T > 0 K

Radiation

F, shape factor: percentage of the emitted radiation reaching the surfaceFor an object in an enclosure, radiative exchange between the object and the wall, F=1

Radiation

Combination of Modes of Heat Transfer

Dimensionless Numbers Used In Heat TransferReynolds Number, ReNusselt Number, NuPrandtl Number, PrGrashof Number, GrRayleigh Number, RaBiot Number, BiFourier Number, Fo

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