Page 143 - Mechanical Engineer's Data Handbook

132 MECHANICAL ENGINEER’S DATA HANDBOOK

Thermal conductivity coefficients (W m- K- ’) at 20°C and 1 bar (continued)
Miscellaneous materials Insulating materials, cont.
Asphalt 1.26 Felt 0.04
Bitumen 0.17 Glass fibre quilt 0.043
Breeze block 0.10.20 Glass wool quilt 0.040
Brickwork: common 0.6-1 .o Hardboard 0.13
dense 1.6 Kapok 0.034
Carbon 1.7 Magnesia 0.07
Concrete: lightweight 0.1-0.3 Mineral wool quilt 0.04
medium 0.4-0.7 Plywood 0.13
dense 1 Sb1.8 Polyurethane foam 0.03
Firebrick (600°C) 1 .w Rock wool 0.045
Glass: crown 1.05 Rubber, natural 0.130
flint 0.84 Sawdust 0.06
Pyrex 1.30 Slag wool 0.042
Ice 2.18 Urea formaldehyde 0.040
Limestone 1.10 Wood 0.134.17
Mica 0.75 Wood wool slab 0.10.15
Cement 1.01
Paraffin wax 0.25
Porcelain 1.05
Sand 0.06
Sandstone 3.00
Slate 2.01

h Nu
3.14.6 Convection Stanton number St=--=-
pcC RePr
Convection is the transfer of heat in a fluid by the B9P2L30
mixing of one part of the fluid with another. Motion of Grashof number Gr = ~ P2
the fluid may be caused by differences in density due to where :
temperature differences as in ‘natural convection’ (or p=fluid density
‘free convection’), or by mechanical means, such as p = fluid viscosity
pumping, as in ‘forced convection’.
k =fluid conductivity
c = fluid specific heat
3.14.7 Dimensionless groups B = fluid coefficient of cubical expansion
C=fluid velocity
In the study of heat transfer by convection it is 9 =acceleration due to gravity
convenient to plot curves using dimensionless groups. L = characteristic dimension
Those commonly used are: h = heat transfer coefficient
PCL 0 =fluid temperature difference
Reynold’s number Re = -
P
3.14.8 Natural convection
hL
Nusselt number Nu=- Natural convection from horizontal pipe
k
CP
Prandtl number Pr = - Nusselt number Nu=- hL
k k