Thermimage (version 2.1)

Grashof: Determines the Grashof number for an object

Description

Determines the Grashof number for an object. The Grashof number is used in calculations of heat exchange.

Usage

Grashof(L = 1, Ts = 25, Ta = 20)

Arguments

L

Characteristic dimension of object in metres. Usually height, depending on object shape.

Ts

Surface Temperature of object, in degrees Celsius.

Ta

Air/Ambient Temperature surrounding object, in degrees Celsius.

Details

The Grashof number is a dimensionless number describing the ability of a parcel of fluid warmer or colder than the surrounding fluid to rise against or fall with the attractive force of gravity as follows: Gr=agL^3(Ts-Ta)/v^2 where L is the characteristic dimension, usually the vertical dimension. For reference, a cylinder's characteristic L would be its height, assuming it is standing on its end Units of L should be in metres This L should be the same L as is used for the convective coefficient calculation Ts is the surface temperature Ta is the ambient temperature v2 is the kinematic viscosity squared (calculated from airviscosity(Ta))

References

Blaxter, K. 1989. Energy Metabolism in Animals and Man. Gates, D. M. 2003. Biophysical Ecology. Dover Publications, Mineola, New York. 611 pp.

See Also

airviscosity

Examples

## The function is currently defined as
function (L = 1, Ts = 25, Ta = 20) 
{
    a <- 1/273
    g <- 9.81
    Gr <- a * g * L^3 * (Ts - Ta)/v^2
    Gr
  }

# Typical values for Grashof number range from 0.016 to 4.6e+09 if Ts-Ta varies from 
# 0.1 to 30oC

# Example calculation:
L<-1
Ts<-30
Ta<-20
Grashof(L, Ts, Ta)