|Abstract:||An accurate computational method for the calculations of flow and heat transfer in compact heat
exchangers is developed in collaboration with the National Center for Supercomputing Applications.
In this method, the unsteady Navier-Stokes and energy equations are solved. In the simulations
of flow and heat transfer over relatively simple heat exchanger fin geometries, a linearly scalable
performance of the code is achieved on the massively parallel CMS, demonstrating the
capability of this method to solve large scale heat transfer problems. Using this code, the heat transfer
enhancement mechanisms and performance of parallel-plate fin heat exchangers are studied extensively.
Geometry effects such as finite fin thickness and different fin arrangements (inline and
staggered) have been investigated. The time-dependent flow behavior due to vortex shedding has
been taken into consideration by solving the unsteady Navier-Stokes and energy equations. In the
unsteady regime, in addition to the time-dependent calculations, companion steady symmetrized
flow calculations have also been performed to clearly identify the effect of vortex shedding on heat
transfer and frictional loss. Additional comparisons have been made to the theoretical results for
fully developed flow between uninterrupted continuous parallel plates and those of restarted boundary
layers with negligible fin thickness, in order to quantify the role of boundary layer restart mechanism
as well as the geometry effects of finite fin thickness and fin arrangement.
It is shown in the current study that at higher Reynolds numbers, the additional effect introduced
by intrinsic three-dimensionality of the flow also plays an important role in determining the overall
heat exchanger performance. At sufficiently high Reynolds numbers, when the actual flow is threedimensional,
corresponding two-dimensional models overpredict overall heat transfer efficiency by
as much as 25%, while the overprediction of frictional loss is much less. More importantly, the overprediction
of rms fluctuations in heat transfer and frictional loss in two-dimensional models is much
larger, where the amplitude of fluctuations from two-dimensional models can be as much as 2 and
5 times of those from corresponding three-dimensional models for the heat transfer efficiency (Colburn j
factor) and frictional loss (friction factor), respectively. These differences are attributed to the strong coherence of spanwise vortices in two-dimensional simulations 'and the weakening of
spanwise vortices in the corresponding three-dimensional simulations due to the presence of streamwise
vortices. In two-dimensional simulations, the coherent spanwise vortices enhance mixing and
result in higher heat transfer efficiency. These span wise vortices at the same time lowers skin friction
on the fin surface. On the other hand, it has been well established that two-dimensional simulations
overpredict form drag due to higher Reynolds stresses in the wake. In current two-dimensional
simulations of flow over parallel-plate fins, the overprediction of form drag is nearly counter-balanced
by the underprediction of skin friction. Such mechanisms also shed light on enhancing heat
transfer while avoiding the normally associated increased pumping power penalty.
In the simulations of flow and heat transfer in more complex louvered fin geometries, current
numerical results clearly show the different flow regimes as the Reynolds number is increased,
which are generally in agreement with those observed in experimental flow visualizations. However,
at low Reynolds numbers, current interpretation of the flow characteristics is somewhat different.
At higher Reynolds numbers, the effect of flow unsteadiness is to increase overall heat transfer and
associated frictional loss.