Instructor: M.T. Esfidani HEAT TRANSFER Instructor: M.T. Esfidani

Heat transfer Heat transfer is the exchange of thermal energy between physical systems, depending on the temperature and pressure, by dissipating heat. The fundamental modes of heat transfer are: conduction  convection  radiation. Heat transfer always occurs from a region of high temperature to another region of lower temperature.   Thermal equilibrium is reached when all involved bodies and the surroundings reach the same temperature.

Definitions Mechanisms of Heat (Thermal Energy) Transfer: Conduction: transmission of heat across matter, due to direct physical contact, e.g. in solids, liquids and gases. Radiation: heat transfer due to electromagnetic radiation across a space, even in a vacuum. Convection: heat transfer by “currents” in a gas or liquid, due to temperature differences or forced flow, an important mechanism of energy transfer between a solid surface and a liquid or a gas. All mechanisms may be involved in practical heat transfer, but the dominant mechanism differs in different cases. (Source of illustrations: http://sol.sci.uop.edu/~jfalward/ heattransfer/heattransfer.html)

Kelvin = Degree Celsius (oC) + 273 Definitions Concepts & Terminologies: Heat CAPACITY (of a system): Quantity of ENERGY required to heat a whole system by 1 K (Kelvin). Unit: J·K-1 SPECIFIC HEAT (or SPECIFIC HEAT CAPACITY): Quantity of ENERGY required to heat a unit mass of a system by 1 K. Unit: J·kg-1·K-1 Kelvin = Degree Celsius (oC) + 273 J= joule

(error in handout “m-3” ) Definitions T1 T2 Q A d Temperature profile THERMAL CONDUCTIVITY, k : a measurement of heat flow through a body. It is the heat transmitted in unit time, in a direction normal to a surface of unit area, through a distance, d, across a unit temperature difference over the distance. Q, heat flow per unit time (Js-1) × d, distance (m) A, area (m2) × (T1-T2) temperature difference (K) k = (Eq. 6a) (Eq. 6a) Unit: Wm-1K-1 (or Wm-1 °C-1) (error in handout “m-3” ) (analogous to electrical conductivity or hydraulic permeability) ( note: 1W = 1 J s-1)

Definitions Why are diamonds so “cool”? Brick Styrofoam Thermal conductivity of other common materials: Definitions Why are diamonds so “cool”? Material Thermal conductivity (298 K), W·m-1·K-1 Diamond 895-2300 ( the highest k) Carbon Nanotubes 1400 Silver 429 Copper 386 Gold 317 Aluminium 237 Iron 80.2 Brick Wood Wool Styrofoam (for building insulation) 0.15 – 0.6 0.12 -0.04 0.04 (very low k) 0.01 Why dose wool feel so “warm”?

conversion Home Work: capacity of your heater and cooler? Thermal conductivity of other common materials: conversion Home Work: capacity of your heater and cooler?

Phase change T-H Diagram Thermal conductivity of other common materials: Phase change T-H Diagram