Session: 05-07-02: CFD Development and V&V

Paper Number: 140411

140411 - Flow Maldistribution in Heat Exchangers and Role of Computational Fluid Dynamics in Predictions 

Heat exchanger performance is important when optimizing the efficiency of energy intensive processes. Many industries are exploring how to take advantage of this equipment to lower carbon footprint amid growing environmental concerns and regulations. Most heat exchanger designs assume uniform flow distribution of process fluids, even though it is maldistributed to some degree during actual operation. Maldistribution in flow can occur because of the mechanical design, heat transfer processes inside the exchanger, two-phase flow, or fouling. Maldistribution can lead to considerable reduction in the effectiveness by decreasing the heat transfer coefficient and mean temperature difference.

To tackle these challenges, the use of computational fluid dynamics (CFD) has proven instrumental. CFD enables detailed simulation of flow inside the heat exchanger, inherently predicting the maldistribution. The predictions facilitate corrective measures at the design phase. CFD can also be used to troubleshoot and improve existing exchangers by simulating possible solutions, some of which may be applied in retrofit.

This presentation highlights HTRI (Heat Transfer Research, Inc) proprietary research and draws insight from literature case studies of flow maldistribution observed in industrial-scale test rigs.

One of the most common examples occurrences of maldistribution is in air-cooled heat exchangers (ACHE). In the first case study, single phase and two phase tubeside flow distribution in an industrial scale ACHE bundle is analyzed. It is shown that tubeside flow in ACHE can possibly be severely maldistributed between rows of tubes if phase change occurs. The second case study investigates maldistribution on the airside of the ACHE. The CFD results of the ACHE intake air flow with surrounding structures is shown. It is found that the fans can produce inherent maldistribution due to its swirling flow profile. Airside maldistribution can further be exacerbated by ambient wind coming from different directions, and surrounding equipment/structure blockages.

In shell and tube heat exchangers (STHE), maldistribution can be due to design or heat transfer process conditions. In another case study, tubeside maldistribution due to axial nozzle is investigated. It is found that most of the flow goes into the central core of tubes. On the shellside, viscous laminar flow can become maldistributed. This maldistribution can be studied by Richardson number. In another CFD study of a shellside flow in a no-tube-in-window (NTIW) exchanger, the ratio of window flow velocity and cross flow velocity is found to be the most important parameter in keeping track of maldistribution. It is stated that as the ratio increases, flow uniformity decreases linearly, but performance decreases at non-linear, steeper gradients.  Shellside maldistribution also occurs due to leakage streams and bypass streams. Typical fractions of these streams are discussed, and their effect on heat transfer is analyzed.

The aforementioned case studies reflect real challenges in optimizing heat exchanger design and operation, to achieve efficient performance. In this presentation maldistribution predictions, strategies to improve heat exchanger effectiveness, are summarized.

Presenting Author: Syed Mohammad Hameed Ul Haq Heat Transfer Research, Inc.