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Heat Transfer

Heat Transfer

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Heat Transfer

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  1. 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-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 the 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 • Q is the transfer of heat across the boundary of the system due to a temperature difference

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

  3. 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

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

  5. Transient Conduction: Lumped Capacitance Define: thermal time constant We can plot the normalizedsolution to the general problem • Notes: • The change in thermal energy storagedue to the transient process is: • General Transient Problem: Special Case negligible radiation, heat flux & heat generation