Introduction to Heat Transfer

TOPICS TO BE DISCUSSED Introduction Difference between Thermodynamics and Heat Transfer Heat Transfer Applications Modes of Heat Transfer Laws associated with conduction,convection and radiation Conservation of Energy principle in heat transfer

Relationship of Heat Transfer to Thermodynamics We learnt in thermodynamics that energy can be transferred by interactions between a system and its surroundings Heat and work are the only form of interactions between a system and its surroundings Thermodynamics is concerned with equilibrium end states and processes But does not provide information on the nature of the process or the rate at which energy is transferred Energy transfer rates are important in engineering process design and development

Heat Transfer Applications Boilers Nuclear reactors Heat Exchangers Condensers and cooling towers Electronic equipment cooling Cooling of Gas Turbine blades Jet Propulsion Combustion process

Modes of Heat Transfer There are three basic modes of heat transfer Conduction Convection Radiation We will first examine all three modes briefly and then examine them in more detail later

Conduction Conduction heat transfer is due to random molecular and atomic vibrational, rotational and translational motions High temperature and more energetic molecules vibrate more and transfer energy to less energetic particles as a result of molecular collisions or interactions The heat flux (a vector) qx´´ (W / m2) is characterized by a transport property know as the Thermal Conductivity, k (W / m · K) W = watts m = Meters K = temperature in Kelvin

Fourier Law of Heat conduction

Consider the heat flux through a slab of thickness, L X=L x L qx´´ T2 T1 For the one-dimensional plane, the heat flux or heat transfer rate is Fourier’s Law: The total heat transfer through a given cross-sectional area, A, is given by: where

Convection Convection heat transfer involves both energy transfer due to random molecular motions and by bulk motion of the fluid Convection heat transfer includes both forced convection and natural convection In convection heat transfer, the transfer of heat is between a surface and a moving fluid (liquid or gas), when they are at different temperatures. The rate of transfer is given by Newton’s Law of Cooling. Moving fluid T∞ Ts q’’ Ts > T∞

Different types of Convection and Phase change process

Typical values of convection heat transfer coefficient Process h (W / m2 K) Free Convection Gases 2 -25 Liquids 50 -1000 Forced Convection 35 -250 50 -20,000 with Phase Change Boiling or Condensation 2500 -100,000

Radiation All surfaces of finite temperature emit energy in the form of electromagnetic waves In the absence of an intervening medium, there is a heat transfer by radiation between two surfaces at different temperatures The maximum flux, E (W / m2), at which radiation may be emitted from a blackbody surface is given by: Stefan Boltzmann Law where Eb or E = Surface emissive power (W / m2) T = absolute temperature (K) σ = Stefan-Boltzmann constant = 5.67 x 10-8 (W / m2 ּ K4) Eb Ts

Conservation of Energy for a Control Volume or System Consider a control volume (C. V.) shown here: Energy Conservation or First Law of Thermodynamics requires that for the C.V. The rate of energy inflow (Ein) and rate of energy generation (Eg) must be balance by the rate of energy outflow (Eout) and energy storage (Est), Hence, For a short time interval Δt,

Conservation of Energy for a Control Surface For a surface illustrated below, there is no mass or volume, and consequently, Eg = 0, and Est = 0. For conservation of energy for the control surface under steady state or transient conditions: Surface Surroundings qcond” qrad” T1 Fluid qconv” Tsur U T T2 T T x

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