Detailed Interpretations of CFD Results
CFD simulations of heat exchangers and related components provide a wealth of high resolution insights about thermal-hydraulic performance. A simulation provides several million times more pieces of information about the flow and temperature fields than a typical experiment.
Several post-processing techniques can offer valuable insight and possibly challenge our intuition:
- Streamlines, path lines, and streak lines
Distinctions among these three visual aids (Figure 1) are particularly important if the flow is unsteady.
Figure 1. Streamlines, path lines, and streak lines coincide only in steady flows
- Vortex identification and strength
Vortices are prominent flow features that can enhance mass, momentum, and energy transfer. Powerful techniques can be applied to a high resolution data set to find the location and strength of vortices, as well as the geometry parameters that most affect them. The position of vortices in a twisted tape flow (Figure 2) corresponds to areas of elevated heat transfer coefficient on the inner surface of the tube (Figure 3).
Figure 2. Strength of vortices identified with ?-2 technique in tube containing twisted tape insert
Figure 3. Heat transfer coefficient on ID of tube containing twisted tape insert
- Field-synergy principle
In addition to the Stanton number, alignment of the temperature gradient and the velocity gradient is a powerful indicator of the effectiveness of convective heat transfer, as presented by Tao et al.  (Figure 4).
Figure 4. Co-linear velocity and temperature gradients yield most effective convective heat transfer performance
Commercial CFD packages have many options for interrogating the CFD solution. In addition, custom scripts can be written, as we have done for the field-synergy analysis. It is important for HTRI to extract as much information as possible from our simulations to improve future designs and not to be satisfied with a few impressive color contours of temperature, velocity magnitude, or pressure. We have used CFD successively to improve flow distribution in shell-and-tube exchangers and air-cooled head exchangers, improve kettle reboiler circulation, optimize enhancement techniques, and solve tube vibration problems—both in the design stage and in addressing operational issues.
- W. Q. Tao, Y. L. He, Q. W. Wang, Z. G. Qu, and F. Q. Song, A unified analysis on enhancing single phase convection heat transfer with field synergy principle, Intl. J. Heat Mass Transfer 48 (9), 2871 – 4879 (2002).