UBC Theses and Dissertations
Membrane deflection in energy exchangers Aminian, Behzad
Experiments conducted on a commercial energy exchanger show that an applied pressure differential between the supply and exhaust flows leads to variations in the pressure drop along each flow path, which are caused by the membrane deflection. Membrane deflection in energy exchangers affects their performance. This is because of a change in passage geometry due to the deflection of the membrane. This study examines the relationship between the membrane deflection and the corresponding variation in the pressure and flow distribution. The membrane deflection is a strong function of the applied pre-tension. Thus, one needs to know the value of pre-tension to estimate the membrane deflection. This information might not be available from the manufacturer, or it might not be accurate because of the subsequent manufacturing steps or structural damage. This study describes a novel contact-based method to measure the pre-tension for applications, such as membrane energy exchangers, where optical access to the membrane is limited. The technique is used to measure the tension of membranes pre-tensioned to set values. The RMS deviation between the measured tension and the set pre-tension is 0.55 N/m, which is approximately 3% of the pre-tension used in a typical membrane energy exchanger. Finally, a numerical model is proposed to find the deflection pattern in an ERV. The model is validated against experimental data. The model is consistent with experiments to within 0.15 and 0.08 inches of water RMS error, which corresponds to approximately 15% and 8% of the pressure drop under typical working conditions of the exchanger. For conditions typical of a commercial exchanger core, the membrane deflections can lead to changes in flow in the open channels of about 16%. The influence of unbalanced flow on the pressure drop is also investigated and shown to be fairly minor if the flow rate difference is small. For larger flow differences, the unbalance flow could change the pressure drop up to 20%.
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