AME 60634 Int Heat Transfer Heat Transfer is

AME 60634 Int. Heat Transfer • Heat Transfer is the transport of thermal energy due to a temperature difference across a medium(s) – mediums: gas, liquid, solid, liquid-gas, solid-liquid, solid-solid, etc. – Thermal Energy is simply the kinetic energy (i. e. motion) of atoms and molecules in the medium(s) • Atoms/molecules in matter occupy different states – translation, rotation, vibration, electronic – the statistics of these individual molecular-level activities will give us thermal energy which is approximated by temperature • Heat Transfer, Thermal Energy, and Temperature are DIFFERENT. DO NOT confuse them. • Heat generation (electrical, chemical, nuclear, etc. ) are not forms of heat transfer Q but forms of work W D. B. Go – Q is the transfer of heat across the boundary of the system due to a temperature difference 1

AME 60634 Int. Heat Trans. Definitions Quantity Thermal Energy Meaning Symbol/Units Energy associated with molecular behavior of matter U [J] – extensive property u [J/kg] – intensive property Temperature Means of indirectly assessing the amount of thermal energy stored in matter T [K] or [°C] Heat Transfer Thermal energy transport due to a temperature gradient (difference) various Heat Rate/Heat Flow Heat Flux D. B. Go Thermal energy transferred over a time interval (Δt > 0) Thermal energy transferred per unit time per unit surface area Heat Transfer 2

AME 60634 Int. Heat Trans. Modes of Heat Transfer • Conduction & convection require a temperature difference across a medium (the interactions of atoms/molecules) • Radiation transport can occur across a vacuum D. B. Go 3

AME 60634 Int. Heat Trans. Conservation of Energy: Fundamental Law First law of thermodynamics: Conservation of Energy Heat transfer perspective: for a Volume heat transfer across boundary & mass transfer across boundary (advection) time basis time interval Work! Heat transfer perspective: for a Surface time instant D. B. Go 4

AME 60634 Int. Heat Transient Conduction: Lumped Capacitance • General Transient Problem: Special Case negligible radiation, heat flux & heat generation Define: thermal time constant We can plot the normalized solution to the general problem Notes: • The change in thermal energy storage due to the transient process is: D. B. Go 5