Nucleate Boiling simulation
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The simulation of nucleate boiling was conducted using ANSYS Fluent for two cases with different operating pressures and heat fluxes. The study found that in both cases, the temperature difference between wall and saturation temperatures was below critical levels, avoiding departure from nucleate boiling. Additionally, the steam generation rate was significantly higher in the first case due to higher heat flux and pressure.
Nucleate Boiling simulation
- 1. NUCLEATE BOILING SIMULATION USING ANSYS FLUENT
- 2. INTRODUCTION TO NUCLEATE BOILING Nucleate boiling is a phenomenon that occurs when the wall temperature is higher than the saturation temperature of the fluid while the temperature of the bulk fluid is below saturation temperature. 𝐓𝐛𝐮𝐥𝐤 ≤ 𝐓𝐬𝐚𝐭𝐮𝐫𝐚𝐭𝐢𝐨𝐧 ≤ 𝐓 𝐰𝐚𝐥𝐥 Nucleate boiling regime is desirable for boiler tubes because it involves higher heat transfer rate.
- 3. DEPARTURE FROM NUCLEATE BOILING (DNB) If nucleate boiling regime is exceeded than it results into sudden increase in the wall temperature and may cause its melting which is not desirable. This is due to film of steam (low thermal conductivity) acting as insulation between bulk fluid and heating wall. This phenomenon is called Departure From Nucleate Boiling (DNB). Therefore it is very important to avoid DNB in tubes where heat transfer occurs with phase change (Boiling).
- 4. PROBLEM STATEMENT The pipe is 15.4mm in diameter & 2000mm in length. The flow is vertically upward through the heated pipe. Two cases are made, 1. p45_q570 & 2. p15_q380, for which heat flux at the wall are 570KW/m2 & 380KW/m2 & the operating conditions are 45Bar & 15Bar respectively. CASE OBJECTIVE Objective of this case study is to understand the concept of heat transfer through boiling and simulating it using Ansys Fluent. To check whether departure from nucleate boiling will occur or not. Inlet Outlet Heated Wall r = 7.7mm
- 5. Outlet Heated Flux 380W/m2 r = 7.7mm Properties Water Steam Density (kg/m3) 787.6 22.6 Dynamic Viscosity (kg/m-s) 0.000103 0.0000177 Specific Heat Capacity (J/kg-k) 4949.2 4227.9 Thermal Conductivity (W/m-k) 0.612 0.0533 Latent Heat (KJ/kg) 1122.1 2798.0 Saturation Temperature At 45 Bar 257.4°C or 530.6°K Therefore Inlet Temperature Will Be 197.4°C Properties At 45 Bar CASE 1 (p45_q570) The operating pressure for this case is 45 Bar Heat flux at the heated wall is 570 W/m2 Inlet sub-cooling 60°C Mass flux is 900 Kg/m2/s Inlet Velocity 1.14m/s
- 6. CASE 2 (p15_q380) The operating pressure for this case is 15 Bar Heat flux at the heated wall is 380 W/m2 Inlet sub-cooling 60°C Mass flux is 900 Kg/m2/s Inlet Velocity 1.04m/s Outlet Heated Flux 380W/m2 r = 7.7mm Properties Water Steam Density (kg/m3) 866.6 7.6 Dynamic Viscosity (kg/m-s) 0.000136 0.0000157 Specific Heat Capacity (J/kg-k) 4485.5 2964.5 Thermal Conductivity (W/m-k) 0.6643 0.03978 Latent Heat (KJ/kg) 844.72 2791.01 Saturation Temperature At 15 Bar 198.3°C or 471.4°K Therefore Inlet Temperature Will Be 138.3°C Properties At 15 Bar
- 7. GEOMETRY & MESH The geometry considered for this study was made in design modeler and axis-symmetric assumption was taken into account to simplify the 3D pipe into 2D. A high quality structured grid was generated in the ANSYS Mesh. The mesh count is 16000 in this grid. Following names were assigned to the boundaries in ANSYS Mesh. Inlet Outlet Wall Axis
- 8. The given flow boiling problem was simulated using RPI Boiling Model Under Eulerian Multiphase Model. (EMM) Recommended Turbulence Model was k-w SST. Gravity was enabled. Water-liquid & Water-vapor were selected as primary & secondary phase respectively. Properties were given according to the operating pressure. FLUENT CASE SETUP Models & Materials
- 9. In the Phase Interaction Panel, following pre-defined correlations were used to model boiling. The saturation temperature was also set according to the operating condition. Surface tension between the interacting phases is important and must be carefully entered. (0.038 N/m for current case) FLUENT CASE SETUP Phase Interactions Model Eulerian Boiling Model Option RPI Boiling Model Wall Lubrication Antal-at-al Interfacial Drag Force Ishii Interfacial Lift Force Tomiyama Turbulent Dispersion Lopez-De-Bertodano Turbulent Interaction Troshko-Hassan Interfacial Heat Transfer Coefficient Ranz-Marshall Interfacial Area Particle Bubble Departure Diameter Tolubinski-Kostanchuk Frequency Of Bubble Departure Cole Nucleation Site Density Lemmart-Chawla Area Influence Coefficient Delvalle-Kenning
- 10. In the solution methods, “Coupled” pressure-velocity coupling scheme was used. First Order Discretization schemes for all the equations was used. In the solution controls, Flow Courant Number was set to 10 & following under relaxation factors were used as shown in picture. FLUENT CASE SETUP Solution Methods & Controls
- 11. RESULTS CASE 1 : p45_q570
- 12. RESULTS: Steam Volume Fraction (Case p45_q570) In the contour below, the blue colored area indicates zero volume fraction of steam which means water has still not converted into steam in that area. The red color indicates maximum conversion of water into steam in that area which is about 0.45.
- 13. RESULTS: Steam Volume Fraction (Case p45_q570) From the chart, we can clearly observe that the steam has started forming near the wall at a distance half the length of the pipe whereas it appears at the center at about 1.8m from the inlet. The maximum conversion of water into steam is 0.45 by volume fraction near the outlet walls.
- 14. RESULTS: Steam Temperature (Case p45_q570) For this case, the Tsat was 530.6°K & the Twall,max reached is 540.1°K. Therefore 𝛁Twall is not more than 9.5°k anywhere along the pipe length. Hence the heat transfer will occur in nucleate boiling regime & departure from nucleate boiling will not occur for these conditions.
- 15. RESULTS: Case p45_q570 Cross Sectional Area Of Pipe (Ac):- 3.14x(0.0077)^2=0.0001862 m2 Mass Flow Rate:- Ac X Mass Flux = 0.0001862 X 900 = 0.16755 kg/s or 603.2 kg/h From the Flux Report for mass flow rate of mixture, mass conservation can be seen.
- 16. RESULTS: Case p45_q570 Also the water content at the outlet mixture can be seen in the flux report. It is about 0.164 kg/s or 590.12 kg/h. So we can say that the steam content in the outlet mixture is 603.20-590.12 =13.08 kg/hr.
- 17. RESULTS CASE 2 : p15_q380
- 18. RESULTS: Case p15_q380 For this case, it can be clearly observed that the steam has just started forming at the wall near the outlet and the volume fraction is still very low. The nucleate boiling has just initiated at the end of the pipe.
- 19. RESULTS: Case p15_q380 Cross Sectional Area Of Pipe (Ac):- 3.14x(0.0077)^2=0.0001862 m2 Mass Flow Rate:- Ac X Mass Flux = 0.0001862 X 900 = 0.16755 kg/s or 603.2 kg/h From the Flux Report for mass flow rate of mixture, mass conservation can be seen.
- 20. RESULTS: Case p15_q380 Also the water content at the outlet mixture can be seen in the flux report. It is about 0.167106 kg/s or 601.58 kg/h. So we can say that the steam content in the outlet mixture is 603.20-601.58 =1.62 kg/hr.
- 21. CONCLUSION Nucleate boiling was simulated for both the cases in ANSYS Fluent Solver using the RPI Boiling Model & the k-w SST Turbulence Model. In both the cases, the difference between wall temperature and saturation temperature is well below the critical temperature. So the departure from nucleate boiling will not occur in both the cases. As expected, the steam generation rate in case 1 (p45_q580) is much higher than in the case 2 (p15_q370). It is about 12.4% higher due to high operating pressure and high heat flux rate. The steam generation has just initiated at the end of the pipe in second case.