Robert B. Keller

Transient pool boiling in microgravity

Abstract Transient nucleate pool boiling experiments using R113 are conducted for short times in microgravity and in earth gravity with different heater surface orientations and subcoolings. The heating surface is a transparent gold film sputtered on a quartz substrate, which simultaneously provides surface temperature measurements and permits viewing of the boiling process from beneath. For the microgravity experiments, which have uniform initial temperatures and no fluid motion, the temperature distribution in the R113 at the moment of boiling inception is known. High speed cameras with views both across and through the heating surface record the boiling spread across the heater surface, which are classified into six (6) distinct categories.

Effects of heater surface orientation on the critical heat flux—I. An experimental evaluation of models for subcooled pool boiling

An experimental study of boiling at high heat fluxes in low-velocity subcooled forced convection boiling is presented, demonstrating the effects of subcooling and buoyancy orientation on the critical heat flux (CHF) and the bubble residence time. At the low velocity of 0.04 m s−1 used, the flow forces acting on the vapor are insignificant compared with buoyancy and the CHF behaves as in pool boiling, depending primarily on the orientation of the heater surface with respect to gravity. This dependence is related to the bubble residence time, which is found to be inversely proportional to the CHF for all heater orientations at a given operating condition. This relationship, that the product of the CHF and the corresponding bubble residence time is a constant, suggests that the mechanism for dryout is independent of the heater surface orientation, despite changes in the vapor departure velocity and the CHF with the orientation angle. Furthermore, increases in the bulk liquid subcooling substantially reduce the net rate of vapor generation such that the bubble residence times at the CHF are independent of subcooling. The work concludes by proposing the energy per unit area leaving the heater surface during the bubble residence time as the CHF mechanism, which is more generalized than the mechanism assumed in the macrolayer dryout models.

Orientation and Related Buoyancy Effects in Low-velocity Flow Boiling

This work is an extension of experimental results reported previously, which might provide design guidance for approximating certain aspects of the flow boiling process in microgravity but taking place in Earth gravity. In that research the buoyancy effects on the bubble dynamics were minimized by the imposition of a liquid velocity parallel to a flat heater surface in the inverted horizontal position, or nearly horizontal (within ±5°), thus holding the heated liquid and vapor formed close to the heater surface. For the fluid used, liquid velocities in the range U= 5–10 cm/s were judged to be critical for changes in the behavior of the flow boiling process. Using the hydraulic diameter of the rectangular duct used, with the heater surface embedded in one side, this velocity range gives rise to flow Reynolds numbers on the order of 4400–8800. It was subsequently judged to be of interest to extend the range of orientation of the flat heater surface relative to gravity to the full circular range of 0–360°, in increments of 45°, and the results of this work are presented here. A solid massive copper heater with a gold‐plated boiling heat transfer surface 19 × 38 mm in size, previously used for critical heat flux measurements with boiling, provided a near‐uniform surface temperature. Only steady measurements of heat flux and surface temperature were possible with the copper heater. R‐113 was the fluid used; the velocity was varied over the interval of 4–28 cm/s; bulk liquid subcooling was varied over 5–11°C; and heat flux varied over 0–10 w/cm2.

Criteria for approximating certain microgravity flow boiling characteristics in Earth gravity.

Abstract: The forces governing flow boiling, aside from system pressure, are buoyancy, liquid momentum, interfacial surface tensions, and liquid viscosity. Guidance for approximating certain aspects of the flow boiling process in microgravity can be obtained in Earth gravity research by the imposition of a liquid velocity parallel to a flat heater surface in the inverted position, horizontal, or nearly horizontal, by having buoyancy hold the heated liquid and vapor formed close to the heater surface. Bounds on the velocities of interest are obtained from several dimensionless numbers: a two‐phase Richardson number, a two‐phase Weber number, and a Bond number. For the fluid used in the experimental work here, liquid velocities in the range U= 5‐10cm/sec are judged to be critical for changes in behavior of the flow boiling process. Experimental results are presented for flow boiling heat transfer, concentrating on orientations that provide the largest reductions in buoyancy parallel to the heater surface, varying ±5 degrees from facing horizontal downward. Results are presented for velocity, orientation, and subcooling effects on nucleation, dryout, and heat transfer. Two different heater surfaces were used: a thin gold film on a polished quartz substrate, acting as a heater and resistance thermometer, and a gold‐plated copper heater. Both transient and steady measurements of surface heat flux and superheat were made with the quartz heater; only steady measurements were possible with the copper heater. R‐113 was the fluid used; the velocity varied over the interval 4‐16cm/sec; bulk liquid subcooling varied over 2‐20°C; heat flux varied over 4‐8W/cm2.

Electrical Hazards Posed by Graphite Fibers

The direct electrical resistance and the electrical contact resistance of graphite fibers released from both burned and unburned Fiberite T300/1034 composites were measured. The voltage at which arcing oc curs due to fibers settling on electrical conductors was also determined. On the basis of results obtained, an assessment was made of the hazards posed by graphite fibers to electrical equipment.