Chemical Engineering and Processing 39 (20000) 471 – 483 A theoretical investigation of the influence of optical constants and particle size on the radiative properties and heat transfer involving ash clouds and deposits S.P. Bhattacharya * ,1 Cooperatie Research Centre for Clean Power from Lignite, 8 /677 Springale Road, Mulgrae, Vic., 3170, Australia Accepted 13 February 2000 Abstract This paper presents an analysis of the influence of optical constants and particle size on the radiative properties and heat transfer involving ash clouds and deposits. Previous reported studies are mostly based on grey values of complex refractive index, which fails to reflect the true nature of the ash cloud and ash deposits. The analysis indicates that for radiative heat transfer calculations, information on size distribution of the ash cloud and wall emittance are more important than the radiative properties of ash cloud. For prediction of emittance of an opaque particulate ash deposit, careful estimation of absorption index is more important than the real index, which may be taken to be constant at 1.5, unless the deposit material is weakly absorbing and composed of fine particles. In such cases knowledge of spectral values of both real and absorption index appears to be necessary. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Optical constants; Particle size; Radiative properties; Beat transfer; Ash clouds and deposits www.elsevier.com/locate/cep 1. Introduction The radiative heat transfer through flyash clouds and ash deposits depend on their radiative properties (emit- tance/absorbance) which in turn depend on the optical constants of the ash particles forming the wall deposits and flyash cloud. The optical properties of importance are real index (n ) and absorption index (k ); collectively these are called complex refractive index (m =n -ik ). This, together with the physical structure of the mate- rial determines the emittance/absorbance of wall de- posits [1]. Reliable spectral data of n and k of flyash particulate are rare [2], primarily because of the difficulties associated with their measurement. In fact, Goodwin’s [3] data is the only one reported in literature for spectral values of both n and k, although this is not for particulate ash, but for ash slags. Due to the lack of reliable and sufficient experimental data, most studies have assumed the optical constants n and k to be independent of wavelength, with n taken as 1.5 and k ranging from 0.005 to 0.05 [4–7]. The real index (n ) is assigned a constant value of 1.5 mainly because the constituent oxides have a real index of about 1.5 up to a wavelength of 5 m. However, based on the limited experimental data, complex refractive index of coal ash is known to have a spectral character [1], with n varying from 1.5 to 2, and k varying from very low values of 0.001 to about 1. As evident from limited industrial measurements [8] the emittance/absorbance of ash de- posits formed on furnace walls also have spectral char- acter. Reliable estimates of the absorption index (k ) may be made through carefully controlled application of the relatively simple techniques like transmission spectra with KBr pellets. Accurate determination of the real index (n ), how- ever, remains a difficult task, particularly for particles. Therefore, it is necessary to evaluate the effect of the assumption of the constant value of n on the radiative properties of ash cloud and deposits, and radiative heat transfer through them. No sensitivity study has been reported in literature that assesses the effect of non-grey optical constants on emittance of ash deposits and * Tel.: +61-3-92390813; fax: +61-3-95610710. E-mail address: spb@crc.powergen.com.au (S.P. Bhattacharya). 1 Senior research engineer, CRC-clean power from lignite. 0255-2701/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII:S0255-2701(00)00096-9