Air Distribution through Heat Exchangers

One of the most important parameters governing the performance of finned-tube heat exchangers is the distribution of the air passing through them. Detailed knowledge of the air distribution is necessary to design a highly efficient heat exchanger because it alerts the design engineer to sections that are air-starved and provides the necessary information to maximize the output of high-flow portions. Although designing a heat exchanger for a specific air flow distribution pattern is a difficult task by conventional means, it is now feasible with intelligent computer programs such as ISHED.

The current research effort seeks to provide a thorough understanding of the air distribution patterns and their causes in finned-tube heat exchangers. The laboratory effort is based on Particle Image Velocimetry (PIV) measurements to characterize the air flow distribution. This highly accurate technique employs a high-speed, double-pulsed camera to track the motion of particles in the flow that are illuminated with laser emission. Preliminary measurements were taken so far for airflow without heat transfer and moisture removal. The figure below shows a data file image of laser-illuminated particles in the flow field and the corresponding vector representation deduced from the data files for the indicated inlet region.

The laboratory effort is complimented by Computational Fluid Dynamics (CFD) models, which are formulated to replicate the laboratory results. The goal of the computational effort is to establish a reliable and accurate prediction method for determining the air flow distribution associated with various installation configurations. CFD is a much more desirable method of determining the air flow distribution because it involves less time and equipment investment than laboratory measurements. The figure below shows some of the initial CFD results for the air flow distribution passing through one half of an A-shaped coil (left), and a portion of the heat exchanger inlet velocity profile and low-flow region caused by air recirculation (right).

David Yashar
HVAC&R Equipment Performance Group
(301) 975-5868
dyashar@nist.gov