Critical heat balance error for heat exchanger experiment based on entropy generation method

• Proposed a new method – critical heat balance error method for heat exchanger design, evaluation, and experiment.
• Analytically expressed the critical heat balance error to filter the invalid entropy generation cases.
• Obtained simplified expression of critical heat balance error criterion for engineering use.
• Experimentally validated the critical heat balance error criterion for counter-flow heat exchanger.

This research is inspired by one interesting question: whether entropy generation measured (indirected measurement) in a heat exchanger experiment is always positive, and what criterion is needed to keep a positive entropy generation measurement. As is well-known, the entropy generation method is a second-law method in heat exchanger design and evaluation. However, according to authors' knowledge, most design and evaluation ignores the influence of error. In this paper, the relation of the entropy generation number and heat balance error is investigated through both analytical and experimental methods. Based on this relation, the concept of critical heat balance error is proposed. Being zero entropy generation in the experiment, the critical heat balance error is a perfect criterion to filter the invalid negative entropy generation in the heat exchanger experiment. The analytical form of critical heat balance error is expressed as: −(1 − τ)(1 − ε), where ε and τ are heat exchanger efficiency and inlet temperature ratio, respectively. In following experiment section, out of 975 experimental cases, 39 invalid cases are found. After verification of analytical relations, the filtration efficiency of the analytical criterion is reported to be as high as 82%. The present paper proposed a new definition called critical heat balance error and provided its application on heat exchanger experiment.