The heat transfer in a fiber assembly is generally performed by a conduction, convection and radiation. Any of these three types of the heat transfer is important. However, up to date, few articles have dealt with the problems on the convective heat transfer in a fiber assembly. Therefore, from the point of view mentioned above, the effective thermal conductivity of a unidirectionally oriented fiber assembly heated from a lower surface is, in this paper, measured by an experimental apparatus based on the steady comparative method, and the free convective heat transfer generating in a fiber assembly is estimated experimentally. The following results have been obtained :
(1) The effective thermal conductivity of a fiber assembly heated from a lower surface is higher than that heated from an upper surface in the region of a lower volumetric ratio.
(2) The critical modified Rayleigh's number beginning to generate the free convection is about 0.18 for polyester fibers and 0.19 for cotton fibers. The free convective heat transfer is given by the following experimental expressions :
(i) for polyester fibers;
N
u*=1.21R
a*0.113 (R
a*>0.18)
N
u*=1 (R
a*≤0.18)
(ii) for cotton fibers;N
u*=1.20R
a*0.112 (R
a*>0.19)
N
u*=1 (R
a*≤0.19)
where, N
u* and R
a* are the modified Nusselt's number and the Rayleigh's number respectively .
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