Convection

Discussion

the basics

Convection is the transfer of internal energy into or out of an object by the physical movement of a surrounding fluid that transfers the internal energy along with its mass. Although the heat is initially transferred between the object and the fluid by conduction, the bulk transfer of energy comes from the motion of the fluid. Convection can arise spontaneously (or naturally or freely) through the creation of convection cells or can be forced by propelling the fluid across the object or by the object through the fluid.

spontaneous convection

Spontaneous convection is driven by buoyancy for the most part and surface tension to a lesser extent.

• exposed surface area
• viscosity
• density
• conductivity (when conductivity is high, there is no need for convection)
• acceleration due to gravity

Examples

• General
  • boyancy driven convection — bénard cells — Rayleigh-Bénard convection
  • surface tension driven convection — surface downwelling — Marangoni effect — Rayleigh-Bénard-Marangoni convection
• atmospheric circulation
  • local
    • updrafts — anabatic winds
      • thermals
      • cumulus clouds: stratoculumus, altocumulus, cirrocumulus, and big daddy cumulonimbus
    • downdrafts -- katabatic winds
      • downbursts, microbursts
      • mountain winds: chinook, foehn (föhn), santa ana, diablo, viento zonda, bergwind
  • global — hadley cells (tropical), ferrel cells (temperte - less well defined), polar cells (polar, obviously)
    • polar high
    • polar easterlies
    • polar front
    • prevailing westerlies — temperate zone — zone of mixing
    • horse lattitudes — subtropical deserts
    • trade winds (ex. alizé)
    • doldrums — intertropical convergence zone
      • ITCZ follows the Sun (thermal equator)
      • monsoons
• ocean currents are driven by a combination of temperature and salinity gradients (thermohaline circulation) in the deep ocean, by winds near the surface, and by topography everywhere water touches land
  • gulf stream
    • the historical first reported ocean current
    • keeps eurpoe warmer than north america at the same lattitude
  • deep ocean return current and its effect on global climate
    • submarine rivers
    • ocean's conveyor belt
    • affected by salinity of surface waters near Greenland, lower salinity means lower density and less likely to sink, the end of the world is nigh
• geologic
  • mantle convection drives plate techtonics
  • outer core convection (along with charge separation) generates Earth's magnetic field
• solar
  • core — where the action takes place
  • radiative zone — the Sun is a good enough conductor (high density, high conductivity) that energy flows by radiation (why not conduction?)
  • convection zone
  • photosphere — convection cells show up as granules (granule implies small, but small is a relative word)
  • corona — material is thrown out gradually in the solar wind or explosively in solar prominances or coronal mass ejections

forced convection

The heat loss due to the forced convection is given by the following equation called Newton's Law of Cooling…

where…

P = dQ/dt is rate at which heat is transferred
h = convection heat-transfer coefficient (or film coefficient or film conductance)
A = exposed surface area
T = temperature of the immersed object
T₀ = temperature of convecting fluid

There is currently no general theoretical model for analyzing forced convection problems. The heat-transfer coefficients h are can only be described by equations based on empirical analysis. For example, the h of air is approximately equal to…

h = 10.45 − v + 10√v

where v is the relative speed of the object through the air; that is, the speed of the object through the air or the speed of the air around the object. This equation is valid for speed from 2 to 20 m/s. Outside of this range h is… ?

wind chill

Which leads us to the topic of wind chill. Q. What is it and why do we need it? A. To insipe fear and get you to watch the weather report on the local news.