.. _section-3.8:

Heat-transfer Time Step Control
-------------------------------

Each channel uses its own separate heat-transfer time step size, so that
channels in which temperatures are changing rapidly or in which boiling
is occurring can use small heat-transfer time steps while other channels
use larger steps.

After each heat-transfer time step, the size of the next step is
determined, based on number of criteria. Most of these criteria are
based on user-supplied values for maximum time step sizes and maximum
temperature changes per step. The time step used in the smallest of the
various criteria. If not other factor is more limiting, then the next
heat-transfer step size is set equal to the initial main time-step size.
This is usually in the range of .05-1.0 s. After the onset of boiling in
a channel, a maximum boiling heat-transfer time step size, typically
.01-.02 s, is used. In addition, a heat-transfer time step cannot go
past the end of a main time step. Also, if the channel has not started
boiling yet, then an attempt is made to end a heat-transfer time step
right at, or very close to, the time when the first bubble is formed.
For this purpose, the pre-boiling coolant routines make an estimate of
the boiling time at the end of each coolant time step. The boiling time
estimate is based on linear extrapolations in time for the coolant
temperature and the saturation temperature at each axial node. The other
criteria are based on the rate of change of the temperatures. The user
supplies values of the maximum change per time step for the fuel and
clad temperatures. Typical values are in the range of 30-50K. The rate
of change of the fuel center-line temperature, the fuel surface
temperature, and the clad mid-point temperature at each axial node are
used to determine maximum time step sizes. After the minimum of the
various time-step criteria has been found, the time step size is rounded
to eight decimal places to minimize differences in results caused by
different round-off errors on different computers. These criteria are
summarized below:

#. Initial time step size
#. Maximum boiling time step size, if boiling
#. End of a main time step
#. Initiation of boiling
#. Maximum fuel center-line temperature change
#. Maximum fuel surface temperature change
#. Maximum clad mid-point temperature change