EXPERIMENTAL STUDY OF SATURATED NUCLEATE POOL BOILING IN AQUEOUS POLYMERIC SOLUTIONS A Thesis Submitted to the Graduate School University of Cincinnati in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE (M.S.) in the Department of Mechanical Engineering of the School of Dynamic Systems 2011 By Advait Dattatraya Athavale B.E., University of Pune, India, 2005 Committee Co-Chairs: Dr. R. M. Manglik Dr. M. A. Jog ABSTRACT Saturated nucleate pool boiling experiments are conducted in de-ionized, distilled water and in aqueous polymeric solutions over a horizontal, cylindrical heater. Boiling characteristic ( vs. ∆Tsat) of water, at atmospheric pressure, is first established by conducting experiments over an extended period of time and confirming repeatability of the experimental results. Aqueous solutions of three grades of HEC (Hydroxyethyl Cellulose) polymer viz. 250-HR (1000 kg/mol), 250-MR (750 kg/mol) and QP-300 (600 kg/mol), are then used at varied concentrations in a series of nucleate pool boiling experiments, so as to study the effect of pseudoplasticity on boiling heat transfer. Polymers, when dissolved in water, change the rheological and interfacial properties of the solution and affect the ebullient boiling behavior. This viscous non-Newtonian, shear- thinning solution also displays interfacial tension relaxation, which tends to be both concentration dependent and temporal. A corresponding increase in surface wettability (smaller contact angle) is also observed. The boiling behavior in aqueous polymer solutions is found to be significantly influenced by changes in the wetting, vapor-liquid interfacial tension, and shear- thinning viscosity of the polymeric solutions. Both the concentration of the polymer and its degree of polymerization (which is reflected in its molecular weight and rheology) have an effect on the heat transfer and associated bubble dynamics. The ebullient dynamics is captured in photographic records to characterize changes in bubble shape, size, frequency, and coalescence in boiling in both polymer solutions and distilled de-ionized water. i The effect of concentration (1.0 × 10-9 ≤ C ≤ 4.0 × 10-9 mol/cc) are seen in the nucleate boiling characteristics of aqueous solutions of HEC QP-300 (M ~ 600 kg/mol). The measured pool boiling heat transfer from the electrically heated horizontal cylinder in C = 1.0 × 10-9 mol/cc (~ critical polymer concentration, C*, for HEC QP-300) aqueous solution is found to be 2 enhanced by ~ 20 % over the entire heat flux range (4.0 < < 200 kW/m ). In higher concentration solutions, however, heat transfer deteriorates at low hear fluxes (or in the incipience and partial boiling regime). At high heat fluxes or in the fully-developed nucleate boiling regime, on the other hand, heat transfer enhancement (~ 45 % maximum) is obtained. This anomalous boiling behavior in the two regimes is characterized by respectively different ebullience signature (as depicted by photographic imaging). Also, it is shown to be scaled with changes in the liquid-solid interface wetting, vapor-liquid interfacial tension, and shear-thinning viscosity of the polymeric solutions. The effects of liquid pseudoplasticity and dynamic interfacial tension are seen in the Saturated nucleate pool boiling of aqueous solutions of three grades of the HEC polymer, namely, 250-HR (1000 kg/mol), 250-MR (750 kg/mol), and QP-300 (600 kg/mol). The experiments are conducted at constant molar concentration of C = 2.5 × 10-9 and 4.0 × 10-9 mol/cc, which is 2.5 – 7 times higher than critical polymer concentration, for all the three polymer grades, the latter are in the range 0.35 × 10-9 ≤ C* ≤ 1.0 × 10-9 mol/cc. With C = 2.5 × 10-9 mol/cc solution, boiling heat transfer coefficient is found to decrease even 2 below that of water in the partial boiling regime (4.0 ≤ ≤ 20 kW/m ). The reduction increases with viscosity. However, at higher heat flux, the shear-thinning of the polymeric solutions diminishes the viscous effects giving enhancement up to 32 %. The lower molecular mass HEC ii QP-300 has a better performance than the large-chain 250-MR and 250-HR because of its lower dynamic surface tension in the high-frequency ebullience regime of fully-developed boiling. The interplay of pseudoplasticity of the solution and dynamic surface tension is further seen in the results for C = 4.0 × 10-9 mol/cc solutions of HEC QP-300 and 250-MR. The characteristics of the consequent bubbling activity for each of these cases are identified in respective photographic records. iii iv ACKNOWLEDGEMENT I would like to thank my academic advisors and mentors Dr. Raj M. Manglik and Dr. Milind A. Jog for their excellent support and guidance throughout my M.S. program. Generous help was also offered by the mechanical department staff members, especially Mr. Larry Schartman, departmental IT manager, Mr. Bo Westheider, departmental electronics coordinator, and Mr. Doug Hurd of the machine shop. I specially thank my parents for their constant encouragement and support throughout my studies. I would like to thank my fellow graduate students Deepak Veettil and Gabriel Wickizer, and all the students in the Thermal-Fluids and Thermal Processing Laboratory who showed a great support for all these years and contributed to my studies in immeasurable ways. My friends and roommates, especially Prashant Patel, Abir Sengupta, Sagar Bhamare, Anup Khinvasara, Abhinav Pande, and Samrish Variyath made my life outside campus enjoyable and unforgettable. Finally, I would like to thank the University of Cincinnati, for giving me this research opportunity and to make all the resources easily available from time to time. v TABLE OF CONTENTS Page ABSTRACT I ACKNOWLEDGEMENTS v TABLE OF CONTENTS vi LIST OF FIGURES viii NOMENCLATURE x 1. INTRODUCTION 1 1.1 Nucleate Pool Boiling – An Overview 1 1.2 Nucleate Pool Boiling with Polymers 4 1.3 Scope of study 7 2. RHEOLOGY AND INTERFACIAL PROPERTIES OF AQUEOUS 8 POLYMERIC SOLUTIONS Introduction 8 2.1 Polymer Additives 9 2.2 Viscosity Measurement 11 2.2.1 Rheometer ‘AR-2000’ 12 2.2.2 Capillary Viscometer 13 2.2.3 Data Analysis 15 2.2.4 Results and Discussion 17 2.3 Dynamic and Equilibrium Surface Tension measurements 21 2.3.1 Maximum Bubble Pressure Method 21 2.3.2 Equilibrium Surface Tension Measurements 23 2.3.3 Dynamic Surface Tension Measurements 25 2.4 Surface Wettability 29 vi 2.4.1 Contact angle measurement 30 3. NUCLEATE POOL BOILING HEAT TRANSFER IN WATER 33 3.1 Nucleate Pool Boiling Experiment 33 3.1.1 Experimental Setup 33 3.1.2 Heater Design 35 3.1.3 Data Acquisition System 36 3.1.4 Photographic Record of Boiling 38 3.1.5 Experimental Procedure 40 3.2 Nucleate Pool Boiling of Water and Boiling Correlations 41 3.2.1 Literature Survey of Pool Boiling Correlations 41 3.2.1.1 Rohsenow Correlation 41 3.2.1.2 Borishanskii Correlation 42 3.2.1.3 Cooper Correlation 45 3.2.1.4 Cornwell-Houston Correlation 45 4. POOL BOILING OF AQUEOUS POLYMERIC SOLUTIONS 47 4.1 NPB of Aqueous HEC QP-300 Solutions 48 4.2 NPB of Aqueous solutions of Different HEC grades 54 5. CONCLUSION 64 BIBLIOGRAPHY 67 APPENDIX A. UNCERTAINTY ANALYSIS 70 APPENDIX B. DATA COMPILATION 73 vii LIST OF FIGURES Page 1.1 Schematic of typical boiling curve with different regimes 3 2.1 Idealized molecular structure of HEC 8 2.2 Ubbeholde Viscometer 13 2.3 Usual viscous response of HEC to applied shear 16 2.4 Shear dependent viscous behavior of HEC 250-HR, 250-MR and QP-300 at 17 different concentrations 2.5 Intrinsic viscosity of HEC 250 HR, 250 MR and QP 300 18 2.6 Schematic of ‘Sendadyne – QC6000’ used for equilibrium and dynamic surface 22 tension measurements 2.7 Equilibrium surface tension variation in HECs with concentration 23 2.8 Dynamic surface tension of HEC grades at 2.5 × 10-9 mol/cc 25 2.9 Dynamic surface tension of aqueous HEC QP-300 solutions at different 27 concentrations 2.10 Interfacial Stresses 29 2.11 Contact angle variation with concentration of aquous HEC solutions 31 3.1 Schematic of Pool Boiling Apparatus 34 3.2 Schematic diagram of cylindrical heater 34 3.3 Schematic of electrical circuit used to vary heat input during boiling experiment 37 3.4 Schematic of setup for CCD camera capturing bubbling activity during boiling 39 viii 3.5 Pool boiling data of distilled de-ionized water in comparison with different pool 43 boiling correlations 3.6 Sweeping vapor mass along cylindrical heater surface 46 4.1 Pool boiling curves for aqueous HEC QP-300 solutions at different concentrations, 48 and distilled, deionized water 4.2 Variation of dimensionless enhancement of heat transfer coefficient with heat flux in 50 aqueous solutions of HEC QP-300 with different concentration 4.3 Visual characteristics of bubbling behavior during boiling of aqueous HEC QP-300 52 solutions of different concentrations 4.4 Pool boiling curves for aqueous HEC solutions at 2.5 × 10-9 mols/cc 54 4.5 Variation of dimensionless enhancement of heat transfer coefficient with heat flux in 56 aqueous HEC solutions at 2.5 × 10-9 mols/cc 4.6 Visual characteristics of bubbling behavior during boiling of aqueous HEC grades at 58 2.5 × 10-9 mols/cc 4.7 Pool boiling curves for aqueous HEC solutions at 4.0 × 10-9 mols/cc 60 4.8 Variation of dimensionless enhancement of heat transfer coefficient with heat flux in 61 aqueous HEC solutions at 4.0 × 10-9 mol/cc 4.9 Visual characteristics of bubbling behavior during boiling of aqueous HEC grades at 62 4.0 × 10-9 mol/cc ix NOMENCLATURE 2 Wall heat flux at heater surface [kW/m ] Tw Temperature of heater surface [K]