luokai0 Heat Transfer Expert
Before Starting
- Which mode? (conduction, convection, radiation, or combined)
- Steady state or transient?
- Internal or external convection?
Core Expertise Areas
Conduction
Fourier law: q = negative k A dT over dx, heat flows down temperature gradient. Thermal resistance: R = L over kA for flat wall, ln(r2 over r1) over 2 pi k L for cylinder. Composite walls: resistances in series and parallel, total R determines heat flux. Fins: extended surfaces increase heat transfer area, fin efficiency eta_f. Biot number: Bi = hL over k, ratio of convection to conduction resistance.
Convection
Newton cooling law: q = h A delta T, h is convection coefficient. Nusselt number: Nu = hL over k, dimensionless convection coefficient. Forced external: flat plate Nu correlations, cylinder in cross flow. Forced internal: Dittus-Boelter for turbulent pipe flow, Sieder-Tate with viscosity correction. Natural convection: buoyancy driven, Rayleigh number Ra = Gr times Pr.
Radiation
Stefan-Boltzmann: q = epsilon sigma A T4, radiation from surface at T. View factor: F_12 is fraction of radiation from 1 intercepted by 2. View factor reciprocity: A1 F12 = A2 F21. Radiation network: analogous to electrical resistance for enclosures. Gray body: emissivity constant with wavelength, simplifies radiation calculations.
Heat Exchangers
LMTD method: Q = UA times LMTD, log mean temperature difference. Effectiveness-NTU: useful when outlet temperatures unknown. Parallel flow vs counterflow: counterflow achieves closer temperature approach. Fouling: additional thermal resistance from deposits, reduces performance over time.
Best Practices
- Identify dominant heat transfer mode before detailed analysis
- Check Biot number to determine if lumped capacitance is valid
- Use appropriate correlation for flow geometry and regime
- Account for fouling in heat exchanger design with fouling factors
Common Pitfalls
| Pitfall | Fix |
|---|---|
| Lumped capacitance when Bi greater than 0.1 | Use spatial analysis or numerical method |
| Wrong correlation for flow regime | Check Re and geometry match correlation assumptions |
| Ignoring radiation at high temperature | Radiation scales as T4, dominates above 500 C |
| LMTD undefined for equal inlet-outlet temperatures | Use effectiveness-NTU method instead |
Related Skills
- fluid-mechanics-expert
- thermodynamics-mech-expert
- manufacturing-processes-expert