A critical review of filmwise natural and forced convection condensation on enhanced surfaces

The use of enhanced surfaces is an efficient method to improve filmwise condensation. Due to their immense potential, many enhanced structures were developed and investigated with the aim of improving natural and forced convection condensation heat transfer coefficients. This has resulted in large collections of predictive models and experimental data being reported. In this review, the developments in this field of research in the past few decades and the recent advances are collated and examined. This paper focuses on the review of natural convection condensation on the external surfaces of enhanced flat plates and tubes and forced convection condensation in enhanced tubes. The various models predicting the heat transfer coefficients on plain and enhanced surfaces are evaluated. For natural convection condensation, the liquid film-based models, semi-empirical models and numerical modes are reviewed whereas, for forced convection condensation, the gravity-dominated and vapor shear-dominated models are discussed. The effects of these enhanced structures on the liquid film and two-phase flow characteristics are analyzed and the various types of enhanced tubes and flat plates investigated are categorized. The manufacturing techniques employed to fabricate these surfaces are identified. A detailed evaluation of the heat transfer and pressure drop performances of the various enhanced surfaces is performed. In addition, their thermal performances are summarized and compared, and their associated heat transfer mechanisms are elucidated. For external condensation on a single tube row, three-dimensional fin structures were found to provide better thermal performance than two-dimensional structures, with some three-dimensional fin structures exhibiting more than 6 times the heat transfer coefficients of a plain surface. However, in a tube bundle, the heat transfer coefficient of three-dimensional fin tubes decreases more significantly with increasing tube row as compared to two-dimensional fin tubes. For convective condensation in circular tubes, the herringbone and pin fin tubes demonstrated better thermal and pressure drop performances than other internally enhanced tubes. Their efficiency indices were between 1.25 and 1.28. Based on the literature surveyed, the various experimental results are compared, existing research gaps are identified and frameworks for future research work are provided. Ministry of Education (MOE) Nanyang Technological University The first author would like to acknowledge the financial support for his research appointment at the University of Illinois at Urbana Champaign, USA under the College of Engineering (CoE) International Postdoctoral Fellowship Scholarship (IPS) provided jointly by the Ministry of Education, Singapore and Nanyang Technological University, Singapore.