Biomedical subjects

Herman Merte

Publications and source records attributed to Herman Merte.

3 recordsLinked to original sources

PubMed2006

Some parameter boundaries governing microgravity pool boiling modes.

Pool boiling experiments were conducted in microgravity on five space shuttle flights, using a flat plate heater consisting of a semitransparent thin gold film deposited on a quartz substrate that also acted as a resistance thermometer. The test fluid was R-113, and the vapor bubble behavior at the heater surface was photographed from beneath as well as from the side. Each flight consisted of a matrix of three levels of imposed heat flux and three levels of initial bulk liquid subcooling. In many of the total of 45 experiments, steady nucleate boiling was observed from 16-mm movie films, where a large vapor bubble formed and remained slightly removed from the heater surface, with small vapor bubbles growing on the heater surface, and on contact coalescing with the large bubble. Computations of the forces associated with the momentum transfer in this process, which counters the Marangoni convection effects tending to impel the large bubble toward the heater surface, have been completed for all cases where applicable. The modes of pool boiling observed with successive increases in levels of heat flux in microgravity are categorized as: (i) minimum or incipient nucleate boiling; (ii) nucleate boiling with vigorous motion of the bubbles adjacent and parallel to the heater surface, impelled by Marangoni convection effects; (iii) nucleate boiling followed by coalescence with a neighboring large vapor bubble; (iv) partial dryout of the heater surface, in parallel with nucleate boiling; (v) complete dryout. The boundaries between these modes are delineated graphically as a function of the imposed heat flux and initial bulk liquid subcooling, together with the levels of the forces holding the large bubbles, acting as vapor reservoirs, away from the heater surface for the steady nucleate boiling mode.

Journal Article↗

PubMed2004

Momentum effects in steady nucleate pool boiling during microgravity.

Pool boiling experiments were conducted in microgravity on five space shuttle flights, using a flat plate heater consisting of a semitransparent thin gold film deposited on a quartz substrate that also acted as a resistance thermometer. The test fluid was R-113, and the vapor bubble behavior at the heater surface was photographed from beneath as well as from the side. Each flight consisted of a matrix of three levels of heat flux and three levels of subcooling. In 26 of the total of 45 experiments conditions of steady-state pool boiling were achieved under certain combinations of heat flux and liquid subcooling. In many of the 26 cases, it was observed from the 16-mm movie films that a large vapor bubble formed, remaining slightly removed from the heater surface, and that subsequent vapor bubbles nucleate and grow on the heater surface. Coalescence occurs upon making contact with the large bubble, which thus acts as a vapor reservoir. Recently, measurements of the frequencies and sizes of the small vapor bubbles as they coalesced with the large bubble permitted computation of the associated momentum transfer. The transient forces obtained are presented here. Where these arise from the conversion of the surface energy in the small vapor bubble to kinetic energy acting away from the solid heater surface, they counter the Marangoni convection due to the temperature gradients normal to the heater surface. This Marangoni convection would otherwise impel the large vapor bubble toward the heater surface and result in dryout and unsteady heat transfer.

Convection↗

PubMed2002

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

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 degrees C; heat flux varied over 4-8W/cm(2).

Journal Article↗