Marek Kapitz

Influence of Thermophysical Wall Properties During Pool Boiling on Large Diamond and SiC Heaters

The results of pool boiling experiments with synthetic diamond and silicon carbide (SiC) heaters are presented for water as the boiling liquid. The diamond and SiC heaters varied considerably in thermal conductivity, but they had smooth, nearly identical surfaces, which was also the case in regard to their contact angles for water. Temperature sensors and electric heating wires were directly vapor-deposited underneath the surfaces. The experiments were carried out with comparable large heaters (15 mm × 15 mm) for pure water under atmospheric pressure (1 bar) in a pool boiling cell. The heat transfer characteristics including the corresponding boiling curves were obtained. In prior work, it was found that the influence of the thermophysical wall properties might be substantial in the case of special heater geometries, leading to trapped bubbles, but no significant differences between both materials were observed in the case of conventional heater configurations.

Boiling Under Hele-Shaw Flow Conditions: The Occurrence of Viscous Fingering

Boiling and bubble dynamics were experimentally investigated in a Hele-Shaw flow cell using pure water at atmospheric pressure as working fluid. The resulting vapor bubble shapes were recorded by means of a high-speed camera for several plate spacings and heating power levels. It was found that viscous fingering phenomena of vapor bubbles occurred only under very special boiling conditions and cell parameters. The evaporation front velocity was identified as a major parameter for the onset of viscous fingering. The observed basic viscous fingering dynamics was in reasonable agreement with theoretical analyses. In addition to that classical viscous large fingering, small-scale evaporation instability was observed leading to microscopic roughening of accelerating evaporation fronts. This instability might be explicitly related to evaporative heat and mass transfer effects across the fast-moving phase interface.Copyright

Confined Boiling Heat Transfer, Two-Phase Flow Patterns, and Jet Impingement in a Hele-Shaw Cell

ABSTRACT Experiments were carried out to study heat transfer and two-phase flow patterns during boiling in a Hele-Shaw cell filled with pure water vapor at atmospheric pressure and with a central inlet of a liquid jet. The Hele-Shaw cell was based on a circular copper rod surface and a polycarbonate plate permitting optical access and thus high-speed cinematography. The diameter of the heated copper rod was 10 mm, the jet diameters were 0.5 and 1 mm, and spacing was varied between 50, 100, and 200 μm. The heat was applied through 4 cartridge heaters with a maximum heat flux of 327 W/cm2. Results showed how high-volume flow rates for the liquid jet led to jet impingement heat transfer while low flow rates led to a Hele-Shaw flow boiling system. The relationship between the volume flow rate and the temperature difference differed significantly between these two regimes. Different flow patterns and evaporation fronts were observed using high-speed cinematography. They strongly depended on jet properties, applied heat flux, and gap spacing. The efficiency of the Hele-Shaw flow boiling system during high heat flux levels was attributed to high interface velocities, combined with viscous fingering at the interface. This combination led to high wetting rates with substantial microlayer evaporation. Good results regarding the heat transfer and the pressure drop were obtained with the final configuration of a 10-mm copper rod diameter, a jet diameter of 1 mm, and a spacing of 0.1 mm. A rather surprising observation was the existence of a stable rotation of an evaporating liquid jet in the Hele-Shaw boiling chamber. The driving mechanism for the rotation with a frequency of 105 Hz was the rapid microlayer evaporation at the rear side of the rotating liquid jet.

Numerical Simulation of Single Bubble Dynamics During Flow Boiling Conditions on a Horizontal Surface

Complete three-dimensional numerical simulations of single bubble dynamics during flow boiling conditions are carried out using the computational fluid dynamics code FLOW3D based on the volume-of-fluid method. The analyses include a numerically robust kinetic phase-change model and transient wall heat conduction. The simulation approach is calibrated by comparison with available experimental and theoretical data. It is found that the observed hydrodynamics (i.e., bubble shape, departure, and deformation) are simulated very well. The comparison with high-resolution transient temperature measurements during a heating foil experiment indicates that the modeling of the spatiotemporal heat sink distribution during bubble growth requires major attention. The simulation tool is employed for single bubble dynamics during flow boiling on a horizontal heating wall, and the agreement is excellent with published experimental data. The numerical results indicate how bulk flow velocity and wall heat transfer influence the bubble dynamics and heat transfer characteristics.

Influence of Wall Thermal Conductivity During Pool Boiling on Large Diamond and SiC Heaters

The results of pool boiling experiments with synthetic (CVD) diamond and silicon carbide (SiC) heaters are presented for water as boiling liquid. The diamond and SiC heaters varied considerably in thermal conductivity, but they had smooth, nearly identical surfaces, also in regard to their contact angles for water. Temperature sensors and electric heating wires were directly vapor-deposited underneath the surfaces. The experiments were carried out with comparable large heaters (15mm × 15mm) for pure water under atmospheric pressure (1bar) in a pool boiling cell. The heat transfer characteristics including the corresponding boiling curves were obtained. In a prior work, it was found that the influence of the thermophysical wall properties might be substantial in case of special heater geometries leading to trapped bubbles, but no significant differences between both materials were observed in case of conventional heater configurations.© 2012 ASME

Numerical Simulation of Single Bubble Dynamics During Pool and Flow Boiling Conditions

Complete three-dimensional numerical simulations of single bubble dynamics under pool and flow boiling conditions are carried out using the CFD code FLOW3D© based on the volume-of-fluid (VOF) method. The analyses include a numerically robust kinetic phase change model and transient wall heat conduction. The simulation approach is calibrated by comparison with available experimental and theoretical data. It is found that the observed hydrodynamics (i.e. bubble shape, departure, and deformation) are simulated very well. The comparison with high-resolution transient temperature measurements during a heating foil experiment indicates that modeling of the spatio-temporal heat sink distribution during bubble growth requires major attention. The simulation tool is employed for single bubble dynamics during flow boiling, and the agreement is excellent with published experimental data. The numerical results indicate how bulk flow velocity and wall heat transfer influence the bubble and heat transfer characteristics.© 2012 ASME

Effect of the Nucleation Site Location on the Thermalhydraulic Stability of a Short-Tube Natural Circulation Loop

For efficient cooling applications in power plants, the use of small two-phase natural circulation loops becomes attractive. An experimental study was carried out to examine how thermal hydraulic stability and operation conditions of these devices are affected by nucleation sites. A very smooth glass tube with artificial nucleation sites have been employed as boiling channel. The mass flow rate has been determined as function of heat flux and nucleation site location. Particular for low heat flux levels, the nucleation sites have a strong impact to the stability behavior. The observed flow instabilities have been analyzed with regard to non-linear effects and chaotic behavior.Copyright