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Analysis of Air-To-Air Rotary Energy Wheels ANALYSIS OF AIR-TO-AIR ROTARY ENERGY WHEELS A dissertation presented to the faculty of the Russ College of Engineering and Technology of Ohio University In partial fulfillment of the requirements for the degree Doctor of Philosophy Abdulmajeed S. Al-Ghamdi June 2006 © 2006 Abdulmajeed S. Al-Ghamdi All Rights Reserved This dissertation entitled ANALYSIS OF AIR-TO-AIR ROTARY ENERGY WHEELS by ABDULMAJEED S. AL-GHAMDI has been approved for the Department of Mechanical Engineering and the Russ College of Engineering and Technology by Khairul Alam Moss Professor of Mechanical Engineering Dennis Irwin Dean, Russ College of Engineering and Technology ABSTRACT Al-GHAMDI, ABDULMAJEED S., Ph.D., June 2006. Integrated Engineering ANALYSIS OF AIR-TO-AIR ROTARY ENERGY WHEELS (269 pp.) Director of Dissertation: Khairul Alam Integration of the energy recovery ventilator (ERV) with a traditional HVAC system has gained recent attention because of the potential for energy savings. The energy recovery ventilator selected for this research employs a rotary air-to-air energy wheel with a porous matrix as the heat and moisture transfer medium. The wheel is symmetrical and balanced, operating with counter-flow pattern. The primary goal of this research is to develop a mathematical and numerical model to predict the effectiveness of this energy wheel. Three models were developed for the energy recovery ventilator (ERV) containing a porous exchange matrix: a sensible model, a condensation model, and an enthalpy model. The sensible model describes the heat transfer in a rotary wheel with a non-desiccant porous matrix. Two energy conservation equations were used to model this ERV. The condensation model describes the heat and mass transfer due to condensation and evaporation in a rotary wheel designed with non-desiccant porous matrix. The enthalpy model describes the heat and mass transfer in a rotary wheel using a desiccant porous media matrix in which adsorption and desorption can take place. Both the condensation and enthalpy models can be mathematically represented by two energy conservation equations with four basic thermodynamic relationships, and two mass conservation equations. A non-dimensional representation for each model was derived and solved using the finite difference method with the integral-based method formulation scheme. The study examined the effect of wheel design parameters such as: rotational speed, number of transfer units, heat capacity, and porosity of the matrix. Operating parameters such as volume flow rate, inlet air temperatures and humidity ratios were studied, and the numerical results are compared with experimental results. Experimental data of effectiveness obtained from a commercial unit are found to be consistent with the numerical results. Approved: Khairul Alam Moss Professor of Mechanical Engineering ACKNOWLEDGMENTS First of all, I would like to thank almighty God for helping and guiding me during my life and through my study and research. Then, I would like to express my sincere gratitude and appreciation to my adviser Professor M. K. Alam. Thank you for providing ideas, guidance, assistance, and editing throughout this research. I especially thank him for his thoughtful insights not only in scientific research but also in life issues. I would like to extend my sincere appreciation to my Ph.D. committee members, Professor H. Pasic, Professor D. Gulino, Professor L. Herman, and Professor N. Pavel, for supporting and encouraging me to finish this work successfully. Special thanks go to my wife, Fattmah, for all her patience, understanding, support, and love. I also wish to thank my family and friends for their support and encouragement. Finally, I would like to acknowledge the help of Stirling Technology, Inc. for letting me use their experimental facility to conduct the experimental works required for my research. Thanks also go to their mechanical engineer, Jason Morosko. vii TABLE OF CONTENTS ABSTRACT………………………………..………………………………………….…iv ACKNOWLEDGMENTS………………...…………………………………………….vi LIST OF TABLES……………………………………………...……………………….xi LIST OF FIGURES…………………………………………...……………………….xiii NOMENCLATURE…………………………………………...………………………xix 1. INTRODUCTION…..……………………………………………………………….1 1.1 Background……………………………………………..…………………….…1 1.2 Rotary Air-to-Air Energy Wheels…………………………………………….…3 1.2.1 Principles of Operations…………………………………........................3 1.2.2 Classification……………………………………………….....................5 1.2.3 Typical Wheel….………………………...……………….......................6 1.2.4 Application……………………………………………………………..7 1.3 Review of Analysis of ERVs ………………………………………………….11 1.3.1 Non-Desiccant Wheels…………………………………………………11 1.3.1.1 Sensible Wheels……….……………………………………..12 1.3.1.2 Condensation Wheels………………………………………...15 1.3.2 Desiccant Wheels………………………………………………………16 1.3.2.1 Sorption Wheels…………………..………………………..17 1.3.2.2 Enthalpy Wheels……………………………………………..18 1.4 Research Objectives……………………………………………………………23 1.5 Scope of Research……………………………………………………………...24 1.6 Overview of Thesis…………………………………………………………….26 2. THEORETICAL BACKGROUND……………………………………….………27 2.1 Energy Wheel Models………………………………………………………….27 2.1.1 Sensible Wheel: Willmott’s Model…………………………………….28 2.1.2 Desiccant Wheel: Maclaine-Cross’s Model……………………………37 2.1.3 Enthalpy Wheel: Simonsons’s Model………………………………..41 2.1.4 Discussion of Models…………………………………………………..45 2.2 Effectiveness Correlations.…………………………………………………….46 2.2.1 The ε-NTU Method…….…...………………………………………....46 2.3 Performance Criteria…………………………………………………………...50 viii 2.4 Desiccants Overview…………………………………………………………..53 2.4.1 Desiccant Isotherms…………………………………………………....53 2.4.2 Desiccant Materials…………………………………………………….54 2.4.3 Desiccant General Sorption Curve……………………………………..56 2.4.4 Desiccant Isotherm Model……………………………………………..57 2.5 Thermodynamics Relationships………………………………………………..62 2.5.1 Properties of Moist Air………………………………………………...62 2.5.2 Properties of Matrix……………………………………………………66 3. MODELING OF ROTARY ENERGY WHEELS……………………………….68 3.1 Model Description.…………………………………………………………….68 3.1.1 Coordinates System and Nomenclature………………………………..69 3.1.2 Surface Geometrical Properties………………………………………..71 3.1.3 Dimensionless Parameters……………………………………………..73 3.2 Assumptions……………………………………………………………………75 3.3 Heat Transfer Model (Sensible Wheel Model)………………………………...77 3.3.1 Governing Equations…………………………………………………..77 3.3.2 Dimensionless Representation..………………………………………..81 3.3.3 Effectiveness Correlation for Limiting Cases……………………….....86 3.3.4 Finite Difference Equations.…………………………………………...92 3.4 Heat and Mass Transfer Model (Condensation Model)………………………102 3.4.1 Governing Equations…………………………………………………102 3.4.2 Modeling of Condensation and Evaporation…………………………116 3.4.3 Dimensionless Representation..………………………………………118 3.4.4 Finite Difference Equations.…………...……………………………..124 3.4.5 Control Criteria for Condensation Model Computer Program……….128 3.5 Heat and Mass Transfer Model (Enthalpy Model)…………………………...129 3.5.1 Governing Equations…………………………………………………129 3.5.2 Dimensionless Representation..……………………………………....134 3.5.3 Finite Difference Equations.…………………………...……………..136 3.5.4 Modeling Mass Transfer in the Enthalpy Wheel...…………………...140 3.6 Computer Codes………………………………………………………………142 3.6.1 Structure of Computer Programs………….………………………….142 3.6.2 Convergence and Stability..…………………………………………..145 4. MODELING RESULTS…………………………………………………………..147 4.1 Energy Wheel Characteristics and Boundary Conditions…………………….148 4.1.1 Energy Wheel…………………………………………………………148 4.1.2 Boundary Conditions…………………………………………………152 4.2 Sensible Wheel Model………………………………………………………..153 ix 4.2.1 Temperatures Profiles………………………………………………...154 4.2.2 Effect of Volume Flow Rate Q& on the Wheel Effectiveness………163 4.2.3 Effect of Rotational Speed Ω on the Wheel Effectiveness………165 4.2.4 Influence of NTU on the Effectiveness of Wheel…………………...167 4.2.5 Influence of Porosity ϕ on the Effectiveness of Wheel..…………….169 4.2.6 Summary……………………………………………………………...173 4.3 Condensation Wheel Model .............................................................................174 4.3.1 Heating Mode Where the Condensation Might Take Place…………..174 4.3.2 Comparing the Sensible Effectiveness with Sensible Model Effectiveness………………………………………………………….177 4.3.3 Effect of the Rotational Speed Ω on the Effectiveness.……………..179 4.3.4 Influence of the Inlet Conditions on the Wheel Performance……….182 4.3.5 Summary……………………………………………………………...184 4.4 Enthalpy Wheel Model……………………………………………………….185 4.4.1 Enthalpy Wheel Behavior…………………………………………….185 4.4.2 Effect of the Volume Flow Rate Q& on the Effectiveness..…………...192 4.4.3 Effect of the Rotational Speed Ω on the Effectiveness….…………..195 4.4.4 Effect of the Porosity ϕ on the Effectiveness of Wheel.…………….197 4.4.5 Effect of the Number of Transfer Unit NTU and Lewis Number Le on the effectiveness………………………………………………………200 4.4.6 Effect of the Isotherm Shape on the Effectiveness..……………….....204 4.4.7 Effect of the Supply Humidity on the Effectiveness.………………...207 4.4.8 Summary……………………………………………………………...208 5. EXPERIMENTAL DATA AND COMPARISON WITH THE ENTHALPY MODEL……………………………………………………………………………211 5.1 Description of the Experimental Facility……………………………………..212 5.1.1 Experimental Setup…………………………………………………...212 5.1.2 RecoupAerator………………………………………………………..214 5.1.3 Air Flow Measurement……………………………………………….216 5.1.4 Temeprature and Humidity Measurement……………………………217 5.1.5 Pressure Measurement………………………………………………..217 5.1.6 Data Acquisition and Dasylab………………………………………..217 5.2 Energy Wheel Under Investigation (RecoupAerator)………………………...219
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