Questions
6–8 questions in major PSU papers
Difficulty
Medium-Hard
Importance
High yield for HPCL/NTPC/ONGC
Overview
Heat Transfer is a fundamental subject in mechanical and chemical engineering exams, focusing on the mechanisms of thermal energy transport and exchange systems. Understanding how heat flows through solids, fluids, and across interfaces is critical for solving thermodynamics-based problems in industrial plant operations. Mastery of the governing equations for conduction, convection, radiation, and exchanger efficiency is mandatory for high-scoring aspirants.
Modes of Heat Transfer
Heat transfer occurs via conduction in solids, convection in fluids, and radiation in any medium or vacuum. In PSU exams, numerical questions often require equating these modes to find unknown interface temperatures or heat flux rates.
- Fourier's Law: Q = -kA(dT/dx)
- Newton's Law of Cooling: Q = hA(Ts - Tinf)
- Stefan-Boltzmann Law: Eb = sigma * T^4
- Thermal Resistance Analogy: R_cond = L/kA; R_conv = 1/hA
- Critical thickness of insulation: r_critical = k/h (cylinder), 2k/h (sphere)
Convection and Dimensionless Numbers
Convection analysis relies on dimensionless numbers to correlate Nusselt number with flow conditions. Aspirants must know the physical significance of each number as these are frequent targets for theory-based MCQ questions.
- Nusselt Number (Nu) = hL/k (convective/conductive resistance)
- Reynolds Number (Re) = rho*v*D/mu (inertial/viscous forces)
- Prandtl Number (Pr) = mu*Cp/k (momentum/thermal diffusivity)
- Grashof Number (Gr): used for natural convection
- Biot Number (Bi): indicates internal vs external resistance
Heat Exchangers: LMTD and NTU
Heat exchangers are a high-yield area where you apply LMTD for performance analysis and NTU-Effectiveness for design optimization. Differentiating between counter-flow and parallel-flow configurations is essential for calculating temperature profiles correctly.
- LMTD = (Theta1 - Theta2) / ln(Theta1/Theta2)
- Q = U*A*LMTD
- Effectiveness (epsilon) = Q_actual / Q_max
- NTU = UA/C_min
- Counter-flow exchangers always yield higher effectiveness than parallel-flow
Formula Sheet
Q = -kA(dT/dx)
Q = hA(Ts - Tinf)
Q = sigma * A * F1-2 * (T1^4 - T2^4)
R_total = sum(R_th)
Q = U*A*LMTD
Q = C_h(Th,in - Th,out) = C_c(Tc,out - Tc,in)
epsilon = (1 - exp(-NTU(1+Cr))) / (1 + Cr) for specific cases
r_critical = k/h
Nu = f(Re, Pr)
Exam Tip
Always identify if the problem is steady-state or transient first; transient problems often involve the lumped parameter analysis (if Bi < 0.1).
Common Mistakes
- Confusing the Biot number (internal resistance) with the Nusselt number (surface resistance) in numerical problems.
- Neglecting to convert temperatures to Kelvin when applying the Stefan-Boltzmann law for radiation.
- Incorrectly identifying the C_min fluid in NTU effectiveness calculations, leading to wrong efficiency values.
More Revision Notes
Ready to test yourself?
Play topic-wise Heat Transfer questions in Aspirant Arcade — gamified MCQ practice.
Download Free