Development of Models for Validation of Boiling and CHF Indo-UK Collaboration

Relevance of the field Design margins of nuclear reactors depend on CHF; CHF occurs in annular flow regime in BWRs and subcooled boiling in PWRs. Mechanistic modelling of CHF requires accurate boiling flow models from subcooled flow regime to annular flow regime Development of boiling flow models is still in pre-matured stage; CHF simulation thus requires empirical models backed by experiments Conducting experiments for reactor conditions are difficult and very expensive. Thus, development of boiling and CHF models are important for evaluation of design margins of nuclear reactors.

Motivation for collaboration Water cooled reactors continue to dominate the nuclear power production Both India and the UK have plans to set up many water cooled reactors. In view of this, modeling and validation of boiling models and CHF are very important. BARC has constructed many experimental facilities. Imperial College London is widely known for development of computational models, tools and CFD codes. Thus we formed a symbiotic collaboration where the experiments and computations will augment each other.

Background The collaborative work on boiling and CHF began after formal approval by DAE and EPSRC in June 2010. Following this a research programme was finalized and conceived to provide a better understanding of flow boiling with emphasis on annular two-phase flow. Following projects have been undertaken: Verification and Validation of CHF and CFD Thermal Hydraulics of Boiling and Passive Systems Premature Oscillation Induced CHF (OICHF)

Background -Technical Critical Heat flux is a condition which in general refers to deterioration of surface heat transfer leading to inordinate temperature rise in heat flux controlled scenarios. The mechanism for heat transfer deterioration however depends on the operating flow regime. CHF may occur in Bubbly, slug and annular flow. Bubbly flow Slug flow Increasing quality Annular flow Peak exit quality in PWRs BWRs

Roadmap for the studies In view of the previous, two broad directions were chosen for work under the present collaboration: Microscopic Boiling (for low quality CHF) Studies in bubbly flow regime Bubble generation, growth and departure are the governing parameters Annular flow (for high quality CHF) Studies in annular flow regime Annular liquid film, its development, thickness, flow rate etc. are the important parameters.

Microscopic boiling- facilities Primary quantities of interest Bubble departure diameter Bubble departure frequency Flow boiling facility Effect of flow and inlet subcooling. How does flow affect the bubble dynamics? Pool boiling facility Effect of orientation, surface finish, subcooling etc.

Microscopic boiling- snapshots Growth of a typical vapor bubble Pool boiling Sliding of a bubble on vertical rod Flow boiling

Microscopic boiling - Research Outputs Pool Boiling: Bubble Departure diameter: Increases with increase in: Surface heat flux Heater surface area Inclination from horizontal Wall superheat Decreases with increase in: Subcooling Heater surface roughness Bubble Departure frequency: Increases with increase in Decreases with increase in Flow boiling: General trends similar to pool boiling. Further studies are in progress. Effect of inlet subcooling and heat flux on departure diameter

Annular flow studies Background The thin, wall adhering annular film plays the most important role in heat transfer and pressure drop characteristics of annular flow. CHF occurs when the film vanishes. Deposition and entrainment rates are important quantities for modelling It was decided to study the characteristics of the film: Film thickness Film flow rate Waves on the film Based on these studies the existing models will be revisited and improved if necessary Additional studies on oscillatory CHF relevant to NC systems in reactors will be performed.

Annular flow studies – Facilities Transparent Air-water facility, FFTF A test bed for development and testing of film flow rate and film thickness measurement techniques developed as a part of the collaboration. Studies on film flow rate and film thickness Studies on waves in churn and annular flow regimes Steam-water facility, CHIL Film flow rate measurement in boiling flows Evaluation of heat flux effect on entrainment and deposition processes Studies on oscillatory CHF (OICHF)

Air-water facility (Film flow rate, film thickness, visualization)

Steam-water facility (HPHT, Film flow rate, CHF) Extraction chamber Presently only film flow rate measurements are possible. It is planned to also put film thickness sensor in the loop. If successful, this will be the first such high temperature-high pressure measurement in the world. Design pressure: 100 bar Design temp.: 300 oC Mass flux: 600 – 2000 kg/m2s

Annular flow studies - Research output Film flow rate and film thickness evaluation has been done and available correlations have been compared against present data. Film flow rate evaluation has been performed in high pressure boiling scenarios. Film flow rate in unheated conditions Experiments (points) and predictions (lines) with Deposition-Entrainment correlation Film flow rate in steam-water conditions Extrapolation of film flow measurements gives value near to CHF

Annular flow studies - Research output Large waves play an important role in deposition and entrainment processes. Wave velocities, frequencies and wavelengths were measured Image processing technique to identify churn and annular flow regimes was developed. The condition for onset of entrainment was measured and also found to relate to the wavelength of large waves. Wave velocity/frequency data were generated in the low flow regime applicable to accident scenarios. Correlations available for wave velocity/ frequency were found to predict present data reasonably. The study of waves in churn and annular regime also led to a new methodology for estimation of entrained fraction at onset of annular flow. This is an important quantity for annular flow modeling. Annular flow Churn flow

Ongoing and future work Film flow rate evaluation experiments at higher pressure and higher power allowing flow conditions similar to BWRs, i.e., 70 bar, 285oC and 1000-2000 kg/m2s. OICHF : Critical Heat Flux experiments with flow oscillations. This is relevant to fault conditions in reactors as well as safety systems employing NC. The CHF-LUT is unable to predict OICHF reliably Typical OICHF data point (CHIL measurements) Sudden rise in surface temperature upon sustained flow oscillations A mechanistically based model for OICHF is one of the objectives of the phase-III project

Research output - papers Development and validation of a model for predicting direct heat transfer from the fuel to droplets in the post dryout regime, Arnab Dasgupta, D K Chandraker, A.K. Nayak, P.K. Vijayan, A. Rama Rao, S.P. Walker, ANS Embedded Topical meeting on Advances in Thermal Hydraulics 2016, New Orleans, LA, [2016] Visualization of large waves in churn and annular two-phase flow, Arnab Dasgupta, D.K. Chandraker, A.K. Nayak, P.K. Vijayan, S.P. Walker, Paper no. 589, Proc. 23rd National and 1st International ASTFE-ISHMT HMTC , [2015] Validation of the Dryout Modelling Code, FIDOM, Dinesh Kumar Chandraker, Arnab Dasgupta, A. K. Nayak, P. K. Vijayan, Kaushik Deshpande, S. P. Walker , Proc. NURETH-16, Chicago, USA, [2015] Quenching of a heated rod: physical phenomena and heat transfer, Arnab Dasgupta, P. P. Kulkarni, G.J. Gorade, D.K. Chandraker, A.K. Nayak, P.K. Vijayan, paper no. 13918, Proc. NURETH-16, Chicago, USA, [2015] Validation and cross verification of three mechanistic codes for annular two-phase flow simulation and dryout prediction, L. Sanmiguel Gimeno, S. P. Walker, G. F. Hewitt, J.-M. Le Corre, A. Dasgupta, M. Ahmad, paper no. 13529, Proc. NURETH-16, Chicago, USA, [2015] Experimental investigation on dominant waves in upward air-water two-phase flow in churn and annular regime Arnab Dasgupta, D.K Chandraker, Suhasith Kshirasagar, B. Raghavendra Reddy, R. Rajalakshmi, A.K. Nayak, S.P. Walker, P.K. Vijayan, G.F. Hewitt, submitted to Experimental Thermal and Fluid Science An Assessment of the Correlations for Entrainment and Deposition Rates in Annular Flow for Dryout Prediction Arnab Dasgupta, D.K. Chandraker, S.P. Walker, P.K. Vijayan, submitted to Multiphase Science and Technology Measurement of film flow rate and estimation of dryout power in annular flow, Arnab Dasgupta, et al., submitted to Proceedings of the 6th International and 43rd National Conference on Fluid Mechanics and Fluid Power December 15-17, 2016, MNNITA, Allahabad, U.P., India

Visits as a part of the collaboration Ms. V. Archana, HBNI to University of Sheffield for a period of 6 months (Jul. 14-Dec. 14). Mr. Arnab Dasgupta, BARC to Imperial College London for a period of 3 months (Nov. 14-Jan. 15). Ms. Parul Goel, HBNI to Imperial College London for a period of 6 months (Dec. 14-May 15). Prof. S.P. Walker and his team have made many short visits to BARC for the experimental collaboration.

Thank you !