Lawson 1996
Lawson 1996
Lawson 1996
C . D . ZlESLER
British Gas pic
1. Introduction
There is a pressing need for practical simulations of heat transfer in high-temperature
industrial processes which can successfully model the mechanisms controlling thermal
performance. Among the benefits of such simulations is that they enable more effective
and efficient original plant designs to be produced in shorter time. They also have
the advantage that they allow the identification of improvements to existing plant
to be made more readily, helping the assessment and implementation of new
technologies, such as regenerative burners, or novel control methods, in suitable
applications.
Zone models have been used successfully in the field of metal reheating furnaces
for some time, since these furnaces are geometrically simple; by contrast, they have
not been applied with any great success in situations like potteries or kilns where
the radiative heat transfer is difficult to evaluate. This restriction has been due entirely
to the limitations of the available radiation submodels. The submodel is responsible
for calculating the exchange areas required by the zone model from the geometric
and thermophysical information. Provided that the radiation is accurately character-
ized, heat-transfer analyses can, in most cases, be performed with the use of relatively
simple models of the other physical processes.
RADEX is a new program developed for evaluating exchange areas in radiative heat
transfer. It was designed to be compatible with previously developed zone models.
This paper describes this radiation program, the principles upon which it is based,
and its application in a design study.
109
£ Oxford Univenily Preii 19%
110 D. A. LAWSON AND C. D. ZIESLER
ZONE 2
ZONE 1
BACK VWkLL
HEARTH
4. Verification of RADEX
A detailed verification study has been performed which can be illustrated by the
following examples. In the first study, the view factors produced by RADEX for a
112 D. A. LAWSON AND C. D. ZFESLER
TABLE 1
Comparison of RADEX with analytic values
a(8F)
Ray density Mean error Max. error
(m- 2 ) (%) (%) rms (<5F)
The use of radiant tubes for heat treatment and reheating is becoming an increasingly
attractive option for a range of processes where stock quality is the primary concern.
TABLE 2
RADEX compared with RAD3D
TABLE 3
Input conditions and results from radiant tube model
1.25
- 1.15
- 1.05
900
700
500
100
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Normalized Time
and their lateral positioning (designs 1, 2, 3, and 6) have little influence on the
efficiency, but cause a marked deterioration in the uniformity. The parameter that
demonstrates an improved efficiency is the enclosure shape which increases it by 3.5
percentage points in this study. The lack of a marked effect on efficiency of the
variations made in this study is, in part, due to the constraint of constant enclosure
size imposed at the start in order to make the results directly comparable. Figure 4
shows the heating curve for the load, along with the temperature difference between
the top and bottom of the load for designs 1 and 3.
The main conclusion which can be drawn from this study is that only the alteration
of the enclosure shape leads to an improved efficiency without undue deterioration
in the uniformity of the temperature across the load.
6. Conclusions
A program, RADEX, has been implemented and tested. This program can calculate the
exchange areas pertinent to a wide variety of zone-model and heat-transfer applica-
tions. It use, and its ability to enhance the value of existing programs, has been
demonstrated in a parametric study of a radiant tube batch furnace. It has been
designed not only for computational efficiency, but also for ease of use and to display
the high level of input flexibility demanded of contemporary applications.
REFERENCES
CHAN, S. H., 1987. Numerical methods for multidimensional radiative transfer analysis in
participating media. Annual Review of Numerical Fluid Mechanics & Heat Transfer 1,
305-50.
116 D. A. LAWSON AND C. D. ZIESLER
EMERY, A. P., JOHANSSON, O., LOBO, M., & ABROUS, A., 1991. A comparative study of methods
for computing the diffuse radiation viewfactors for complex structures. J. Heat Transfer
113,413-22.
GOULD, H. & TOBOCHNIK, H., 1988. Computer simulation methods (part 2). Addison-Wesley,
Reading, Mass.
GUILBERT, P. W., 1989. Computer program RAD3D for modelling thermal radiation.
Technical report AERE-R 13534, Harwell.
HOTTEL, H. C , & SAROFIM, A. F., 1967. Radiative transfer. McGraw-Hill, New York.
NOBLE, J. J., 1975. The zone method: explicit matrix relations for total exchange areas. Int. J.
Heat & Mass. Transfer 18, 261-9.
OMORI, T., TANIGUCHI, H., & KUDO, K.., 1990. Monte Carlo simulation of indoor environment.
Int. J. Numer. Meths Eng 30, 615-27.
PALMER, M. R., 1989. A practical computer package for the thermal design of high temperature
industrial plant. Proceedings of the 1989 International Gas Research Conference, Tokyo.