Heat Transfer
Conduction, convection and radiation are three different physical mechanisms that all end up as a W/m² — and picking the wrong one is the most expensive mistake in the subject. These topics animate the mechanism before the correlation.
Start from the beginning →8 topics you can watch now, 14 still to come.
The three modes
Three mechanisms, three laws, one unit.
- Fourier's law and thermal conductivity
- Newton's law of cooling
- Stefan-Boltzmann law
- The general heat conduction equation
Conduction
Energy moving through a material that stays where it is.
- Conduction and Thermal ResistanceFourier's law rearranged is Ohm's law, so the whole of circuit analysis carries over — including the case where adding insulation increases the heat loss.
- Fins and Extended SurfacesThe only term in Q = hA·ΔT that is free to change, and it costs nothing to run. Then two different measures of whether the fin was worth fitting.
- Transient ConductionSteady state says where a body ends up and nothing about how long it takes. One dimensionless number decides which of two entirely different methods applies.
- Conduction through cylinders and spheres in full
- Heisler charts and numerical methods
Convection
Energy carried away by fluid that is itself moving.
- Convectionh is not a property of anything — it is a result, spanning four orders of magnitude. Which is exactly why the dimensionless groups exist.
- Boiling and CondensationThe highest coefficients in the subject, and a cliff at the end of them. Whether that cliff is catastrophic depends on which variable you control.
- Forced convection correlations over plates and in tubes
- Natural convection in enclosures
Radiation
Energy crossing a vacuum, with no material involved at all.
- Shape factor algebra
- Radiation networks and the radiosity method
- Gas and solar radiation
Heat exchangers
Where all three modes are put to work on purpose.
- Shell-and-tube configuration and correction factors
- Compact and plate exchangers
- Regenerators and recuperators
About Heat Transfer
Thermodynamics tells you how much energy moves and in which direction. Heat transfer tells you how fast, which is usually the question an engineer actually has to answer. A design that works thermodynamically and takes a week to reach temperature has not worked.
Three modes carry all of it. Conduction moves heat through material by contact. Convection moves it by carrying fluid away, which is why this subject depends on fluid mechanics. Radiation needs no medium at all and dominates once things are hot enough.
The idea that ties the subject together is thermal resistance — treating a heat path like an electrical network of resistances in series and parallel. It converts a differential-equation problem into circuit arithmetic, and it is why fins, walls and exchangers can be analysed with the same method.
What to know first
- Engineering Thermodynamics, at least the first law
- Fluid Mechanics for the convection material
Where it gets used
- Sizing a heat sink so a component stays within its rating
- Understanding why insulation thickness has diminishing returns
- Reading heat-exchanger performance and knowing what limits it