Conjugate Heat and Mass Transfer in Heat Mass Exchanger Ducts Li-Zhi Zhang AMSTERDAM • BOSTON • HEIDELBERG • LONDON NEW YORK • OXFORD • PARIS • SAN DIEGO SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO Academic Press is an imprint of Elsevier Academic Press is an imprint of Elsevier 225 Wyman Street, Waltham, MA 02451, USA The Boulevard, Langford Lane, Kidlington, Oxford, OX5 1GB, UK First edition 2014 © 2014 Elsevier Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher. Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone (+44) (0) 1865 843830; fax (+44) (0) 1865 853333; email: [email protected]. 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Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-407782-9 For information on all Academic Press publications visit our website at http://store.elsevier.com Printed and bound in United States of America 13 14 15 16 17 9 8 7 6 5 4 3 2 1 Preface In modern society, new materials and new processes are required to solve the emerging new energy and environmental problems. Increasingly, novel heat and mass exchangers are being invented and used in various industries, for instance, in energy recovery, gas separations, air purification, air cleaning, waste water treatment, liquids separations, air dehumidification, and so on. Adsorbent beds, desiccant wheels, membrane contactors, membrane total heat exchangers, and air cleaners all belong to this type of heat and mass exchangers. Ducts are the basic elements in these heat and mass exchangers. In this book, a systematic descrip- tion of the conjugate heat and mass transfer in such ducts is presented. The basic physical phenomena in these ducts are described. The methods to treat the conjugate boundaries, either steady-state or transient, are introduced. The detailed transport data for ducts of various heat and mass exchangers are presented: adsorbent beds, desiccant wheels, membrane total heat exchangers, membrane contactors, either of parallel-plate type or of hollow fiber bundle type. The book also illustrates some examples of the applications of novel heat and mass exchangers, with theoretical analysis combined with experi- mental work: for instance, air dehumidification by honeycomb adsorbent beds, air dehumidification and energy recovery by desiccant wheels, heat and moisture recovery by membrane total heat exchang- ers, air humidification by hollow fiber membrane contactors, air dehumidification by salt solutions with parallel-plate, or hollow fiber membrane contactors. Both the working theories and the heat and mass transfer problems are addressed. The exchangers are constructed and their performances are optimized with the obtained heat and mass transfer data. This book provides a reference book for the design of these novel heat and mass exchangers. The book combines theoretical analysis with engineering practices. It covers a wide range of knowl- edge from fundamental heat mass transfer to novel system design and performance analysis. It also gives deep insights and design guidelines for the heat and mass exchangers used in other industries such as air conditioning, energy engineering, mechanical engineering, chemical engineering, environ- mental engineering, food, and drug engineering, etc. This is the first book systematically presenting conjugated heat and mass transfer in the ducts of various heat and mass exchangers. Various duct cross-sectional shapes are included. The book may also serve as a reference book for scientists, engineers, practitioners, and students in energy, chemical, production, and environmental industries for component design. xi Acknowledgments After 15 years accumulating knowledge and expertise, I began to write this book. After 8 months of preparation and hard work, I am relieved that it is now completed. It is like a new born son to me. I am full of joy, but more, a sense of responsibility. I hope readers can share with me the knowledge in this book, as well as the heart of curiosity and the feeling of accomplishment in finishing this work. This book is a cross-disciplinary endeavor which relies heavily on numerical heat mass transfer. I hope peer engineers, scientists, and research students will benefit from the methodologies exhibited in this analysis and extend them to analyses of other energy and environmental systems. Your feedback of suggestions and comments, as well as criticisms is also valuable. Readers’ support is critical to the success of my continuing writings. I am deeply grateful to the Academician of the Chinese Academy of Sciences, Prof. Wenquan Tao at, Xi’an Jiaotong University, for the precious time that he squeezed from his tense schedule to write a Foreword for this book which has become an introduction to the content. I would also like to thank the Natural Science Foundation of China (NSFC) for their continuing financial support. I am indebted to my colleagues at South China University of Technology, and others all over China and throughout the world who have provided suggestions and ideas which, in no small way, have contributed to the fabric of this text. They all contributed a great deal to the results described in this book. I am also grateful to my colleagues at South China University of Technology and elsewhere, who have provided positive reinforcement for my efforts. Li-Zhi Zhang South China University of Technology Guangzhou, China Email: [email protected] xiii Foreword Energy and environment are two critical issues today. Mankind is facing increasing energy and envi- ronmental problems. New materials and new processes are required to solve these problems. More frequently than before, novel heat and mass exchangers are being invented and applied in various industries; for instance, in energy recovery, gas separations, air purifications, air cleaners, waste water treatment, liquid separations, air dehumidification, toxic gas and liquid disposals, and so on. Depending on the application fields, these exchangers can be adsorbent beds, desiccant wheels, membrane contac- tors, membrane total heat exchangers, or cleaners. Regardless of the names, they all belong to these types of exchangers. For these new exchangers, the ducts are the basic elements where the processes take place. Heat and mass transfer usually accompany each other. Simultaneous heat and mass transfer in the ducts are the fundamental and basic phenomena in these novel heat and mass exchangers. Therefore, the data on heat and mass transfer in these ducts are the fundamentals for component design, system optimization, and performance evaluation. Compared to traditional metal-type heat exchangers, there are special features in these new heat and mass transfer ducts: 1. The solid walls have new functional materials. The heat and mass transfer in fluids are closely related to those in the solid walls. So, they are conjugate heat and mass transfer problems. The governing equations for the fluids and the solids should be solved together; simultaneously. 2. The Biot numbers of these ducts are high. They are large Biot number ducts. So, material properties have decisive impacts on the conjugate heat and mass transfer in ducts. The thermo- chemical and interfacial characteristics of wall materials, which are new and always nonlinear, should be considered, in addition to thermal and fluid-dynamics problems, in the fluids. 3. The duct geometries are always irregular. The simple analytical solutions of fluid flow for common round or rectangular tubes are no longer applicable. 4. There are multi-variables that need to be solved. Velocity, temperature, concentrations are usually coupled together. The complexities also include chemical reactions, sometimes in the solids. These special features have led to many new phenomena: 1. The boundary conditions on duct walls have neither uniform values nor uniform fluxes. Rather, they are formed by the coupling of the fluids and materials. 2. It is often difficult to find analytical solutions. Therefore numerical schemes are highly depended on. Sometimes, new numerical schemes are required to solve a specific problem. 3. Transport data in the entry regions should be considered. 4. The final Nusselt and Sherwood numbers are highly dependent on materials, duct geometries, and operating conditions. The traditional way of estimating mass transfer coefficients from heat transfer coefficients with analogy correlations is not valid anymore. Due to these new features and new difficulties, a comprehensive analysis and data collection of the conjugate heat and mass transfer in ducts are highly desired, both for industries and for academics. xv xvi Foreword Regretfully, such studies are still very limited now. It is ridiculous that nowadays, transport data in old textbooks that are proposed for traditional heat transfer-only in simple cases, are usually adopted in the design and optimization of emerging novel heat and mass exchangers. This is a dilemma facing us. Over the past 15 years, the author of this book, Professor Li-Zhi Zhang, has conducted a sys- tematic study of the conjugate heat and mass transfer in the ducts of heat and mass exchangers. The basic transport phenomena in the ducts were investigated and new data on Nusselt and Sherwood numbers for these ducts were disclosed. He published many papers in this area. However, the data on this topic is quite sporadic and a systematic introduction is still not available. From this background Professor Zhang wrote this book: Conjugate Heat and Mass Transfer in Heat Mass Exchanger Ducts. This is the first book that systematically presents the conjugated heat and mass transfer in ducts of various heat and mass exchangers. The basic physical phenomena in these ducts are introduced. The governing equations for mass, momentum, and energy are presented and solved. Detailed numerical schemes to solve these conjugate problems are given. The detailed finite difference forms to solve the equations are specifically presented. The techniques of coordinate transformations from the physical domain to computational domain on various duct cross-sections are elucidated. The final algebraic forms of the partial differential equations are presented. The iterative procedures to solve these multi- elements and non-linear equations are also introduced. The methods to treat the conjugate boundaries, either steady state or transient, are introduced. Most of them are from the experiences of the author in solving them, besides the knowledge from classical textbooks. Experimental procedures and results are presented to validate the calculation results with various practical heat and mass exchangers. To some extent, this is also a reference book for numerical heat and mass transfer. Finally the detailed transport data for duct of various heat and mass exchangers is presented: adsorbent beds, desiccant wheels, mem- brane total heat exchangers, membrane contactors, either in parallel-plates type, plate-fin type, plate-fin and tube type, or in hollow fiber bundles type. Based on the fundamental heat mass transfer data, this book illustrates some examples of the applications of novel heat and mass exchangers. For instance, air dehumidification by honeycomb adsorbent beds, air dehumidification and energy recovery by desiccant wheels, heat and moisture recovery by membrane total heat exchangers, air humidification by hollow fiber membrane contac- tors, air dehumidification by solutions with parallel-plates or hollow fiber membrane contactors. Both the working theories and the heat and mass transfer problems are discussed. The exchangers are constructed and their performances are optimized with the obtained heat and mass transfer data. This book combines theoretical analysis with engineering practices. It covers a wide range of knowledge from fundamental heat mass transfer, to novel systems design and performance analysis, from materials introduction, characterization, to heat and mass exchanger thermodynamics and fluid dynamics. It can also provide insights and design guidelines for the other heat and mass exchangers, in various industries like air conditioning, energy engineering, mechanical engineering, chemical engi- neering, environmental engineering, food and drug engineering, etc. This book is a cross-disciplinary endeavor which relies heavily on numerical heat mass transfer. I hope peer researchers, engineers, and research students could benefit from the methodologies exhib- ited in this analysis and extend them to the analyses of other energy and environmental systems. Foreword xvii At last, let me extend my congratulations to Professor Zhang on his new book. I hope he can con- tinue his researches on this topic, which in no doubt, will benefit both industrial progress and academic developments. Professor Wenquan Tao Academician of the Chinese Academy of Science Xi’an Jiaotong University Xi’an, China Email: [email protected] CHAPTER 1 An Introduction to Conjugate Heat and Mass Transfer in Ducts 1.1 Heat and mass transfer ducts Energy and the environment are of increasing concern today, because mankind is facing greater energy and environmental problems than ever before. According to a report released recently [1], world energy consumption is projected to increase by 47% from 2010 through 2035. Most of the growth is projected for emerging economies outside the OECD (Organization for Economic Cooperation and Development), where robust economic growth is accompanied by increased demand for energy. Total non-OECD energy use grows by 72%, compared with an 18% increase in OECD energy use. Energy consumption in non-OECD Asia shows the most robust growth among the non-OECD regions, rising by 91% from 2010 to 2035. However, strong growth also occurs in many of the rest of the non-OECD regions: 69% in Central and South America, 65% in Africa, and 62% in the Middle East. Accompanying energy use, global carbon emissions from fossil fuels have significantly increased since 1900. Emissions increased by over 16 times between 1900 and 2008 and by about 1.5 times between 1990 and 2008. It is projected that the trend will continue for the next decades. These two factors have led to many energy and envi- ronmental problems such as fossil fuel depletion, global warming, ozone depletion, air pollution, new diseases, etc. The increased concerns with regard to energy and environmental problems have placed demands on new technologies. To date, many new technologies are emerging to help to solve these energy and environmental problems. New types of equipment are invented to reclaim waste heat and to increase energy utiliza- tion efficiency; for example, total heat exchangers, desiccant wheels, etc. Other new equipment is constructed to get rid of air pollution from our environment, from either air or water, for example air cleaners and water cleaners. The basic processes in these types of equipment are heat and mass transfer. The nature of these processes is that substances and energy are transported from one place to another, and from one medium to another. Energy and pollutants are treated, transferred and processed in terms of heat and mass transfer. Without heat and mass transfer, energy and environmental problems cannot be controlled. All these novel forms of equipment can be called heat and mass exchangers. Ducts are the basic units in these heat and mass exchangers. There is no site to accomplish heat and mass transfer without these ducts. They provide the interfaces for gases, solids and liquids to make Conjugate Heat and Mass Transfer in Heat Mass Exchanger Ducts. http://dx.doi.org/10.1016/B978-0-12-407782-9.00001-0 1 © 2014 Elsevier Inc. All rights reserved. 2 CHAPTER 1 An Introduction to Conjugate Heat and Mass Transfer in Ducts contact and be transported. These ducts are usually very fine and are numerous, to provide sufficient area for gas-solid or gas-liquid contact. So the basic transport phenomenon in these small ducts, or channels, is the key for system design and performance optimization. Here are some examples. Figure 1.1 shows a total heat exchanger unit for the reclamation of waste heat from buildings. The whole machine includes two fans, the ducting work for the two flows and the sealing walls sepa- rating the two flows, and the shell. The heart of the machine is the core. Figure 1.2 shows the core inserted in the total heat exchanger [2]. It is in either a parallel-plate ducts or a plate-fin ducts struc- ture. Figure 1.3 shows a schematic of the heat and moisture exchange between the outside air and the indoor air through these numerous ducts. It is a cross-flow membrane based total heat exchanger. Because the heat and mass transfer between a solid surface and an air stream is usually very low, to achieve the required magnitude of treatment capability, large contact areas between the two flows are required. Therefore the cores are made of numerous fine channels. With total heat exchangers, the efficiency of the existing HVAC systems can be improved sub- stantially. With them, 20–40% of the energy use for air conditioning can be saved. The reason is that normally the fresh air is dehumidified by through condensation in a cooling coil followed by a re- heating process, which is very energy intensive. This energy can be saved if total heat exchangers are installed to reduce the dehumidification load. Besides energy conservation, total heat exchangers have the additional benefits of ensuring a sufficient supply of fresh air, which is crucial for the prevention of epidemic respiratory diseases such as SARS and bird flu. As can be seen from Figure 1.2, the exchanger is composed of numerous ducts. Fresh air and exhaust air exchange heat and moisture simultaneously in these ducts. In this way, fresh air is dehumidified and cooled by the exhaust air in summer and heated and humidified by the exhaust air in winter. The hot and humid incoming fresh air becomes cool and dry in summer, thus decreasing the cooling load in summer. The cold and dry incoming fresh air is heated and humidified in winter, thus decreasing the heating load in winter. The ducts are the basic units for heat and moisture exchange between fresh air and exhaust air in a total heat exchanger. Two FIGURE 1.1 A total heat exchanger for heat and moisture recovery. 1.1 Heat and Mass Transfer Ducts 3 FIGURE 1.2 The core inserted in the total heat exchanger. FIGURE 1.3 Schematic of heat and mass transfer through ducts of a total heat exchanger.