Numerical analogues of non-fractal porous cometary dust particles are generated using a recently introduced model of open-pore solid particles. The refractive index of amorphous silicate is considered and the particle sizes follow a Sekanina-Hanner distribution within the size-range (0.01 mu m-1 mu m). Using the Discrete Dipole Approx-imation (DDA) method, we calculate the thermal re-emission spectrum of a population of porous particles in the spectral region 0.2-39.8 mu m and the equilibrium temperature of the particles (when placed at 1 AU distance from Sun). Porosity tends to reduce the temperature of the particles because porous particles radiate heat more effectively than dense particles. A characteristic particle size is determined for which the particle temperature is maximum regardless of the porosity. The temperature of a cloud of polydisperse particles is more homogeneous for highly porous particles. These results are compared with those generated by Mie theory where the Bruggeman mixing rule is applied (EMT method). When agreement between the DDA and the EMT results is achieved, simple approximations are derived from the EMT to analyze and make clear the various behaviors of the spectral thermal re-emission revealed in the numerical data. In particular, the reason why the emission peak around 10 mu m is depressed by porosity and shifted to the smaller wavelength by a steeper particle-size distribution is explained.
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