Boiling and Condensation (ME742)

Transcription

1 Indian Institute of Technology Kanpur Department of Mechanical Engineering Boiling and Condensation (ME742) PG/Open Elective Credits: Updated Syllabus: Introduction: Applications of boiling and condensation. Difference between evaporation and boiling. Comparison of Nucleate and Convective (or Flow) boiling. Pool boiling: Nukiyama Experiment. Theory of vapour bubble formation: Homogeneous and Heterogeneous Nucleation. Bubble Growth Models. Mechanism of Critical Heat Flux (CHF). Various models and correlations. Pool Boiling of Binary Mixture. Flow Boiling: homogeneous and heterogeneous models. Flow Boiling in Microchannels. Flow Boiling of Binary Mixtures. Boiling enhancement techniques. Film and dropwise condensation. Nusselt s analysis of laminar film condensation on vertical plate, single horizontal tube and vertical array of tubes. Laminar-wavy and turbulent film condensation. Film condensation inside horizontal tubes. Condensation enhancement techniques. Special topics (Suggested): Boiling in Microgravity Environment. Boiling of Nanofluids. Liquid Metals Boiling. Boiling on Structured Surfaces. Effect of Non-condensable Gases in Vapour on Condensation. Numerical Modelling of Boiling and Condensation Heat Transfer. Special topics: Boiling of Nanofluids. Computer Simulation of Pool Boiling by the Coupled Map Lattice Method. Lecture-by-Lecture Break-up (Three 50 min lectures per week) Lecture # 1: Introduction: Applications of Boiling and Condensation: Thermal Power Plant Cycle. Ideal Vapour-Compression Refrigeration Cycle. Immersion Cooling of Computers. Heat Pipe. Lecture # 2: Difference between Evaporation and Boiling. Evaporation: Benard Convection. The typical temperature profile in the evaporating liquid. Definition of h. Why Tsat is used in the definition of h? Nucleate Boiling: Bubble Nucleation. Temperature Profile in Water. Comparison of h vs.q // plots for evaporation and nucleate boiling of water at 1 atm. Lecture # 3: Convective Boiling: Various flow regimes in a vertical heated tube. Comparison of nucleate boiling and convective boiling. Lecture # 4: Review of Thermodynamics of Phase Change of Pure Substances: subcooled liquid, saturated liquid, saturated liquid-vapour mixture, saturated vapour, superheated vapour, critical point. Psat vs. Tsat plot: Clapeyron Equation and Clausius- 1

2 Clapeyron Equation. Triple Point. Superheated liquid: Definition. Application in boiling. Concept of metastable equilibrium of vapour bubble and superheated liquid. Lecture # 5: Interfacial Tension: Explanation from Molecular Point of View. Formation of Vapour Bubbles: Derivation of the expression for the diameter of a spherical vapour bubble in thermal and mechanical equilibrium with its superheated liquid. Theory of heterogeneous nucleation (to be continued). Lecture # 6: Heterogeneous Nucleation. Expression of Mikic and Rohsenow (1969) for Activation Superheat of a Cavity. Homogeneous Nucleation. Spinodal lines and metastable states on a p-v diagram. Bubble Growth near a Heated Surface: Liquid Inertia Controlled Growth and Heat Transfer Controlled Growth. Lecture # 7: Mathematical Modelling of heat-transfer controlled bubble growth in the non-uniform temperature field near a superheated wall: Model of Mikic and Rohsenow (1969): Basic assumptions. Expression for the waiting time. Expression for the rate of growth of bubble (to be continued). Lecture # 8: Expression for the rate of bubble growth (completed). Example Problem. Bubble Departure Diameter and the Frequency of Bubble Release: Definition of the frequency of bubble release. Various mechanisms of bubble release. Sample Correlations for Departure Diameter: Fritz (1935), Zuber (1959) and Cole (1967). Lecture # 9: Frequency of Bubble Release: Influencing factors. Correlations for frequency of bubble release (valid in the intermediate regime between inertia and heattransfer controlled growth): Jacob and Fritz (1931), Peebles and Garber (1953), Zuber (1963). Basic forms of correlations given by Ivey (1967) for inertia controlled growth and heat-transfer controlled growth. Favourable conditions for inertia controlled growth and heat-transfer controlled growth. Example Problem. Lecture # 10: Pool Boiling: Definition. Applications. Temperature Controlled Saturated Pool Boiling Curve for a Large Horizontal Surface: Various regimes. Maximum or Critical heat flux (CHF). Minimum heat flux. Points of difference between heating and cooling curves: (a) Contact angle hysteresis in transition regime; (b) near onset of nucleation condition. Heat Flux Controlled Curve: Various regimes. Points of difference between heating and cooling curves: hysteresis effect (Missing transition regime). Lecture # 11: Single Composite Pool Boiling Curve showing heating and cooling for both temperature and heat flux controlled modes. Definition of CHF for temperature controlled and heat flux controlled modes. Definition of Minimum Heat Flux. Experiment of Nukiyama (1934): Saturated Pool Boiling of Water at Atmospheric Pressure (to be continued). Lecture # 12: Details of Nukiyama Experiment: Equipments used. Experimental Procedure. Sources of Experimental Error. Results. Maximum and Minimum heat 2

3 fluxes. High speed video camera pictures of heat flux controlled saturated pool boiling showing regimes till CHF. Lecture # 13: h vs. Tw plot in Pool Boling: Explanation. Heat Transfer Mechanism during Nucleate Boiling: Rohsenow s Model and its basis. Rohsenow (1952) Correlation. Lecture # 14: Stephan and Abdelsalam Correlation (1980): Basis. Correlations for water, hydrocarbons, cryogenic fluids, and refrigerants. Unified correlation valid for all fluids (lower accuracy). Simpler and easy-to-use correlations. Surface roughness // correction. Comparison of q vs. Tw graphs on log-log plot for boiling of water at 1 bar using Rohsenow (1952) and, Stephan and Abdelsalam (1980) correlations. Lecture # 15: Basic approaches used in the making of pool boiling correlations. Applicability of pool boiling correlations for horizontal tubes/surfaces to other geometries/orientations. Example Problems. Liquid Metals Pool Boiling: Physics. Basic differences with respect to boiling of normal fluids. Subbotin et al. (1970) Correlation. Lecture # 16: Transition Boiling and Taylor Instability. Helmholtz Instability. Mechanism of CHF: Derivation of Zuber (1959) Correlation using Taylor instability and Helmholtz instability theories. Lecture # 17: Summary of Zuber s Hypothesis. Drawbacks of Zuber s Model. Effect of geometry on CHF: Correlations given by Lienhard and co-workers for square and round heated surfaces of finite size. Example Problem. Lecture # 18: Accuracy of CHF Correlations given by Lienhard and co-workers. CHF Correlation for Liquid Metals. CHF as a Function of Pressure: Explanation for the nature of the curve. Stephan (1992) Correlation for organic fluids and water. Example Problem. Lecture # 19: Criticisms on Zuber s Model of CHF. Effect of Surface Wetting Characteristics: Pool boiling on a completely non-wetted surface. Alternative Model of Kandlikar (2001) for predicting CHF: Postulate. Basic Mechanism. Model Description. Derivation of CHF. Kandlikar s expression of CHF for saturated pool boiling of pure liquids. Validation with earlier experiments. Lecture # 20: Kandlikar s Model of CHF (Contd.): Effect of bubble contact angle and plate inclination on CHF. Difficulties in the use of Kandlikar s correlation. Lecture # 21: Minimum Heat Flux: Zuber(1959) theory and correlation, and modification by Berenson (1961) for Infinite Plate. Lienhard and Wong (1963) Correlation for Horizontal Cylinder. Film Boiling: Bromley (1950) Correlations for horizontal cylinder and sphere. Berenson (1961) Correlation for Infinite Horizontal Surface. Effect of Radiation on Film Boiling: Correction by Bromley (1950). Film Boiling on Finite Horizontal Surface. 3

4 Lecture # 22: Example Problem on Film Boiling. Summary of Saturated Pool Boiling. Lecture # 23: Parametric Effects on Pool Boiling: Effect of Subcooling. Kutateladze(1952) correlation for CHF in Subcooled Boiling. Effect of Gravity (to be continued). Lecture # 24: Effect of Gravity (Completed). Effect of Size and Wettability. Effect of Surface Roughness. Flow Boiling: Introduction. Lecture # 25: Basic Terms and Definitions in Two-Phase Flows. Basic Model and Governing Equations (Conservation of Mass and Momentum) for One-dimensional Two- Phase Flow: Assumptions and derivations. Expression of total axial pressure gradient in terms of frictional effect at the wall, gravitational head effect and acceleration (or deceleration) of the flow. Lecture # 26: Calculation of Frictional Pressure Gradient: Definition of Two-Phase Multipliers, φ l, φlo, φv. Homogeneous Flow: Basic definition and idealizations. 2 Derivation of the expression for φ lo. Heterogeneous Flow: Basic definition. Method of Lockhart and Martinelli (1949): Basic philosophy. Concept of Correction factor or Martinelli parameter, X. To be continued. Lecture # 27: Heterogeneous Flow (Completed): Details of the method and its accuracy and limitations. Profile of ( dp / dz) fr vs. z: Physical Explanation. Example Problem. (Homogeneous Flow). Lecture # 28: Variation of φ l and φ v as well as α with X: Physical Explanation. Basic Procedure for obtaining Pressure Drop in a Two-Phase Flow. Example Problem. (Heterogeneous Flow). To be continued. Lecture # 29: Example Problem (Completed). Lecture # 30: Regimes of Convective Boiling in Round Tubes. Method of Calculating Heat Transfer Coefficient in Two-Phase Flows: Chen (1966) Correlation for vertical tube. Example Problem (to be continued). Lecture # 31: Example Problem (Completed). Modification to Chen s Correlation: Bennett and Chen (1980) Correlation. Condition of applicability of Chen s Correlation to flow boiling in horizontal tubes. Correlation for stratified flow boiling in horizontal tubes. Gungor and Winterton (1986) Correlation for vertical tube flow boiling. Boiling Regime Map for Constant Wall Flux Condition: Departure from nucleate boiling and dryout. Why is thermodynamic quality negative in subcooled boiling and greater than 1 in drop flow regime? To be continued. 4

5 Lecture # 32: Real Quality vs. Thermodynamic Quality (Completed). Critical Boiling States: Definition of CHF in flow boiling. Boiling Crisis at low quality and at high quality. Plot of CHF vs. Quality. Condensation: Definition. Dropwise and Film Condensation. Nusselt s Analysis of Laminar Film Condensation on a Vertical Flat Plate: Basic Assumptions. To be continued. Lecture # 33: Nusselt s Analysis (Completed): Derivation of expression of average heat transfer coefficient and average Nusselt number. Lecture # 34: Expression for mass rate of condensation. Effect of Subcooling: Derivation of the expression for modified latent heat of condensation h / fg. Turbulent Film Condensation over a Vertical Plate: Definition of film condensation Reynolds number. Various regimes of film condensation flow such as laminar, laminar-wavy and turbulent and transition criteria. Experimental correlations for average heat transfer coefficient for laminar-wavy and turbulent regimes. Basic method of solution of film condensation problems: Why are iterations required? Example Problem (to be continued). Lecture # 35: Example Problem (Completed). Laminar Film Condensation over a Single Horizontal Tube (Nusselt s Approach): Derivation of the expression for Lecture # 36: Laminar Film Condensation on a Vertical Tier of n tubes: Nusselt s Analysis. Chen (1961) Correlation: Modification of Nusselt s correlation. Calculation of total condensation rate on a square array of tubes. Staggered Tube Arrangement. Effect of vapour velocity and non-condensable gases in the vapour. Lectures # 37-40: Special Topics: Boiling of Nanofluids. Computer Simulation of Pool Boiling by the Coupled Map Lattice Method h D. Reference Books 1. Stephan K, 1992, Heat Transfer in Condensation and Boiling, Springer-Verlag, Berlin. 2. Carey, V.P., 2008, Liquid-Vapor Phase-Change Phenomena, 2nd edn, Taylor & Francis, New York. 5