Product: ABAQUS/Standard
This example illustrates the ABAQUS capability to solve heat transfer problems including cavity radiation. We simulate the effects of a fire condition on a plane finned surface. This problem was proposed by Glass et al. (1989) as a benchmark for thermal radiation. We compare their results with those obtained using ABAQUS.
The configuration shown in Figure 5.1.5–1 represents a plane wall with a uniform array of parallel rectangular fins attached. The problem represents three phases in a fire test. The first is the pretest, a steady-state condition where heat is transferred by natural convection from an internal fluid at a fixed temperature of 100°C to the plane inside wall. Heat is conducted through the wall and dissipated by radiation and natural convection from the outside wall and fin surfaces to the surrounding medium which is at a temperature of 38°C. The second phase is a 30-minute fire transient, where heat is supplied by radiation and forced convection from a hot external fluid at 800°C. After conduction through the fins and wall, heat is rejected by natural convection to the internal fluid. Finally, the third phase is a 60-minute cool down period, where heat absorbed during the fire transient is rejected to the surroundings by the same process as that used to establish the initial steady-state condition.
The finite element mesh used for the wall and fins is shown in Figure 5.1.5–2. By making use of the radiation periodic symmetry capability in ABAQUS, we are able to represent the array of fins while meshing only one fin and corresponding wall section.
The outside ambient is modeled with a single horizontal row of elements at some distance above the top of the fin (not shown in the figure). The varying ambient temperature is simulated by prescribing temperatures to the nodes of these elements. The elements representing the outside ambient are also assigned a surface emissivity of 1.0.
The thermal conductivity of the wall and fins is 50 W/m°C (k), their specific heat is 500 J/kg°C (c), and the density is 7800 kg/m3 (
). The surface emissivity of the wall and fins is 0.8, the Stefan-Boltzmann radiation constant is 5.6697 × 10–8 W/m2°K4, and the temperature of absolute zero is –273°C.
The natural convection between the internal fluid and the inside of the wall is modeled with a film boundary condition where the film coefficient is given as 500(
)1/3 W/m2°C, where 
is the inside wall temperature and 
is the temperature of the internal fluid. The film boundary condition user subroutine is used for this purpose since the film condition is temperature dependent.
The natural convection between the outside finned surface and its surroundings is modeled with a film boundary condition where the film coefficient is given as 2(
)1/3 W/m2°C, where 
is the temperature of the finned surface and 
is the outside ambient temperature. Again, the film boundary condition user subroutine is employed. The forced convection between the hot surroundings and the finned surface is modeled with a constant film coefficient of 10 W/m2°C.
The first simulation step is a steady-state heat transfer analysis to establish the initial pretest conditions. This is followed by a 30-minute transient heat transfer analysis during which time the ambient fire temperature is 800°C. Finally, a second transient heat transfer step is performed to simulate the 60-minute cool down period.
The integration procedure used in ABAQUS for transient heat transfer analysis procedures introduces a relationship between the minimum usable time increment and the element size and material properties. The guideline given in the User's Manual is
is the element size. This suggests that an initial time increment of 10 seconds is appropriate for the transient steps of this problem. Automatic time incrementation is chosen for the transient steps by setting DELTMX to 100°C. DELTMX controls the time integration by limiting the temperature change allowed at any point during an increment. A DELTMX of 100°C may seem rather coarse in comparison with temperature ranges of 800°C, but this is not expected to be the limiting factor; the strong nonlinearity of the radiation conditions is expected to dictate the time incrementation in the transient steps.The results published by Glass et al. include those obtained by a number of different heat transfer codes, all of which give similar results. Since the most details are given for the results obtained with the program TAU (Johnson, 1987), we have chosen to compare the ABAQUS results to those of TAU.
Figure 5.1.5–3 shows the history of the temperature at the top of the fin (point 1 in Figure 5.1.5–1). Figure 5.1.5–4 shows the histories of the temperature at the root of the fin (point 2 in Figure 5.1.5–1) and on the wall inside surface (point 3). In all cases the results obtained with ABAQUS match the TAU results quite well. In Figure 5.1.5–5 we show the temperature distribution around the fin perimeter (starting at point 1 and ending at point 2) at the end of the fire transient. Again, the ABAQUS and TAU results match closely. Finally, temperature contours at the end of the fire transient are shown in Figure 5.1.5–6.
Fire transient problem.
User subroutine FILM used in radiationfinnedsurf.inp.
Glass, R. E., et al., “Standard Thermal Problem Set,” Proceedings of the Ninth International Symposium on the Packaging of Radioactive Materials, pp. 275–282, June 1989.
Johnson, D., “Surface to Surface Radiation in the Program TAU, Taking Account of Multiple Reflection,” United Kingdom Atomic Energy Authority Report ND-R-1444(R), 1987.
