Sridhar Hari

Improvement of the subcooled boiling model for low-pressure conditions in thermal-hydraulic codes

Abstract The functioning of the subcooled boiling model adopted in a thermal-hydraulic computer program has been investigated in detail, for low-pressure conditions, and necessary refinements have been incorporated into the code. The investigation has been carried out in two stages; in the first stage, the performance of the interfacial heat transfer/condensation is studied. Necessary refinements to the vertical flow map for the transition from bubbly to slug flow regimes and the interpolation with the ‘umbrella’ limitation that bounded the interfacial heat transfer values are carried out. Simulations of low-pressure subcooled boiling experiments were performed with the refined code version and a reasonable agreement with the experimental void fraction data was obtained. In addition, a high-pressure experiment was also simulated with the refined code version to check if these revisions do not affect the code performance at high pressures. No significant adverse effects were observed. In the second stage of the study, the performance of the wall heat flux partitioning model adapted in the code was investigated. In particular, the effectiveness of the ‘pumping factor’ formulation in the above model and its functioning at low-pressure conditions was investigated. Different ‘pumping factor’ formulations available in the literature were implemented into the code. Simulations of low-pressure subcooled boiling experiments were performed with the refined code version and the appropriate ‘pumping factors’ to be used for low-pressure conditions were determined.

Analysis of transient events without scram in a research reactor using the RELAP5/MOD3.2 computer code

Simulations of two different events without scram were conducted for a hypothetical research reactor, based on the High-Flux Australian Reactor (HIFAR) moderated and cooled by heavy water circulating under atmospheric pressure. The simulations were performed with the RELAP5/MOD3.2 computer program. Although the simulations neglected reactivity feedback effects, the focus on the thermal-hydraulic aspects represents a step toward full analyses of hypothetical events in HIFAR. Two simulations focused on events associated with the failure of the primary coolant circulation pumps, and three simulations focus on the events associated with the reduced heat removal via the nonavailability of heat exchangers. The critical heat flux subroutine of the RELAP program was modified to account for the concentric annular fuel element geometry of HIFAR fuel elements.

Aerosol Deposition on Electroformed Wire Screens

Insect screens, which are usually an integral component of an air sampling inlet, can cause inadvertent deposition of larger aerosol particles. Numerical and experimental studies were performed to characterize aerosol deposition on commercially available electroformed wire screens for aerodynamic particle diameters between 3 and 20 μ m, Stokes numbers between 0.49 and 20, wire widths between 35 and 160 μ m, and screen open area fractions of 0.56 to 0.90. With increasing values of Stokes numbers, the actual collection efficiency increases gradually to a maximum value that is asymptotic to the fraction of open area. Deposition is characterized in terms of a standardized screen efficiency, which is the actual efficiency divided by the areal solidarity (1–fraction of open area). A correlation equation has been developed for the electroformed mesh screens, which relates the standardized efficiency to the fraction of open area, the Stokes number, the interception parameter, and the Reynolds number based on wire size. Data obtained from experimental studies with two screen types and numerical studies with those, plus two additional screen types, over a wide range of Stokes numbers and wire Reynolds number (Re w ) from about 0.5 to 30, collapse to a single correlation curve, which is valid for the range of variables tested. The pressure drop across the screens is low, on the order of 1 Pa for face velocity values that are on the order of 1 m/s. A regression analysis was used to obtain coefficients that fit the results of the numerical experiments to an existing pressure loss model.

CFD Study on the Effects of the Large Particle Crossing Trajectory Phenomenon on Virtual Impactor Performance

Two-dimensional CFD simulations were performed on a full-section numerical model of an as-built slot virtual impactor prototype and its completely symmetric ideal counterpart. The simulations reproduce the trends of the experimentally observed performance including verification of a third region in the transmission efficiency curve, which is a drop-in transmission efficiency for large particle sizes. Visualization of simulated particle tracks show this decrease is attributed to a crossing trajectory phenomenon, whereby larger particles that acquire enough inertia in a chamfered acceleration nozzle, crossover the vertical mid-plane and impact on the opposite-side wall, particularly on the wall of the receiver section. Some experimental data presented in the literature for rectangular slot and round-nozzle virtual impactors with chamfered 45° half-angle acceleration nozzles (similar to the geometry tested herein), show a similar drop-in transmission efficiency that commences at a particle Stokes numbers of about 6. However, many studies do not demonstrate the drop in transmission efficiency because wall losses are not taken into account. The Reynolds number, based on the acceleration nozzle size and velocity, does not noticeably affect the onset of the phenomenon. The crossing trajectory phenomenon can severely restrict the size range over which a virtual impactor can be used as an efficient particle concentrator. Geometrical asymmetry from dimensional tolerance considerations in the construction of a virtual impactor exacerbates the impact of the crossing trajectory phenomenon.

Optimization Studies on a Slit Virtual Impactor

This study reports the results of numerical investigations performed on a proposed slit virtual impactor configuration. Simulations were performed using the computational fluid dynamics program CFX-4.4. A detailed numerical investigation was carried out to determine the critical geometrical parameters of the proposed configuration that would optimize the performance for the anticipated operation conditions (specified Reynolds number and major-to-minor flow split ratio). A detailed sensitivity analysis was performed on the optimized configuration to determine its performance characteristics (impactor efficiency and wall loss curves) for (a) operation at different major-to-minor flow split ratios for a constant Reynolds number and (b) operation at different Reynolds numbers but at the same major-to-minor flow split ratio. Finally, the effect of modifying the radius of curvature of the receiving section of the optimized configuration on the impactor performance was studied for the anticipated operation conditions, and the results are reported.

A Circumferential Slot In-Line Virtual Impactor

An In-line Virtual Impactor is presented, which has an application as a pre-separator for sampling inlets, where the device scalps large particles from the aerosol size distribution. Numerical simulation was the principal tool employed in the design process, with physical experiments used to verify computational predictions. Performance investigations were primarily carried out for a configuration that provides a nominal cutpoint particle size of 10 μ m aerodynamic diameter at an inlet flow of 111 L/min and a major flow exhaust of 100 L/min; however, the concept is scalable in terms of both flow rates and cutpoint sizes. An inverted dual cone configuration contained within a tube provides a characteristic circumferential slot of width 2.54 mm (0.100 inches) and a slot length of 239 mm (9.42 inches) at the critical zone. The upper cone causes the flow to accelerate to an average throat velocity of 3.15 m/s, while the lower cone directs the major flow toward the exit port and minimizes recirculation zones that could cause flow instabilities in the major flow region. The cutpoint Stokes number is 0.73; however, the cutpoint can be adjusted by changing the geometrical spacing between the acceleration nozzle exit plane and a flow divider. When the system is operated at the major exhaust flow rate of 100 L/min, the pressure drop is 45 Pa. Good agreement is obtained between numerically predicted and experimentally observed performance.