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ME POST: Thermodynamics and Heat Transfer

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Thermodynamics and Heat Transfer

Thermodynamics and Heat Transfer tests the laws of thermodynamics, property relations, power and refrigeration cycles, and conduction, convection, and radiation.

Core rules and concepts

  • First law for a steady-flow control volume: Q-W=delta H+delta KE+delta PE; state the sign convention explicitly.
  • Second law: entropy generation is nonnegative; Carnot efficiency eta=1-T_L/T_H uses absolute temperatures.
  • Conduction: Fourier law q=-kA dT/dx; plane-wall resistance R=L/(kA). Convection: q=hA(T_s-T_inf). Radiation: q=epsilon sigma A(T_s^4-T_sur^4).
  • Heat exchangers: LMTD q=UA delta T_lm; counterflow is more effective than parallel flow for the same terminal temperatures.
  • Cycle efficiency is net work divided by heat input; heat rate is the inverse of efficiency with a unit conversion.

Worked example

A plane wall has k=0.5 W/(m K), area 10 m^2, thickness 0.1 m, and faces at 30 C and 10 C. Thermal resistance is R=L/(kA)=0.1/(0.5 x 10)=0.02 K/W. Heat rate q=delta T/R=20/0.02=1000 W. The temperature difference must be in kelvin, which has the same magnitude as Celsius; adding a second layer would add resistances in series.

Traps

Using Celsius in Carnot or radiation equations, reversing the first-law sign convention, ignoring fouling or contact resistance, and confusing LMTD with arithmetic mean temperature difference are frequent errors.