Investigation of transients important for heat pipes in microreactor applications
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Authors
Yilgor, Ilyas
Issue Date
2024-12
Type
Electronic thesis
Thesis
Thesis
Language
en_US
Keywords
Mechanical engineering
Alternative Title
Abstract
Heat pipes are two-phase heat transfer devices that employ the cyclicevaporation and condensation of a working fluid to transfer heat between two
interfaces. Due to their safety, efficiency, and passive operation, heat pipes
have found diverse applications that include electronics, aerospace, and nuclear
systems. In particular, the use of heat pipes with liquid metal working fluids
in nuclear microreactors is of interest due to unique advantages compared to
conventional, as well as other novel advanced reactor concepts. Heat pipes can
enable increased reliability in microreactors as they eliminate the need for
reactor coolant pumps and their associated auxiliary systems, which also results
in greatly reduced spatial footprint. Experimental work is needed to aid and
expedite the design and licensing of future heat pipe microreactors (HPMRs),
especially on the validation of heat pipe performance as key heat transfer
components. High operating temperatures and chemical reactivity of working fluids such as liquidmetals dictate stringent safety precautions and require high startup costs.
Thus, the present work develops fluid-to-fluid and geometric scaling laws for
heat pipes and two-phase closed thermosyphons that can be used to quantify
similarities between a model system that uses low-temperature working fluids and
a prototypical system using liquid metals. Similarity parameters which were
obtained from the non-dimensionalized governing equations and constitutive
relations were tabulated and discussed. A case study involving the scaling of a
microreactor heat pipe for the purpose of investigating the pressure and
temperature profiles was given. In addition, a parametric study was conducted on
the effects of the liquid and vapor Prandtl numbers on the steady state pressure
and temperature profiles. Furthermore, a Low-Temperature Heat Pipe Test Facility (LTHPF) was designed andconstructed according to the developed scaling laws to bypass the difficulties
of experimenting with liquid metal working fluids by using surrogate fluids. The
design, instrumentation, and experimental capabilities of the facility were
described. The testing conditions including various operating limits and the
ranges of the non-dimensional parameters used for scaling analysis were
reported. Due to the complex flow structures within heat pipes and thermosyphons, acomprehensive experimental database consisting of temperatures, pressures,
pressure drops, flow visuals, and film thicknesses, is needed for model
development and verification, along with the robust identification of
operational characteristics, potential instabilities, and oscillatory
phenomena. Thus, the present work develops an extensive heat pipe
experimental database using water working fluid under wickless (two-phase closed
thermosyphon), annulus-screen, and wrapped-screen configurations. The
investigated experimental cases include quasi-steady states, condenser heat
transfer coefficient transients, and power transients. Emphasis is placed on
describing oscillatory phenomena, disturbances, and other observed
irregularities and instabilities. From thermosyphon experiments, it was found that high fill ratios and condensercoolant flow rates diminish isothermal operation through liquid holdup in the
condenser in the form of increased subcooled liquid presence and the formation
of a liquid plug at the condenser endcap. The sustained liquid plug effectively
decouples that portion of the condenser from the rest of the thermosyphon.
Furthermore, intermittent or geyser boiling oscillations observed at low powers
were characterized based on their amplitudes and frequencies, which were found
to be strongly influenced by the fill ratio. Lastly, high frequency temperature
and pressure oscillations were identified at the onset of flooding, along with a
lower secondary frequency for higher fill ratios. Two sets of heat pipe tests were conducted using annulus-screen andwrapped-screen wicks. The quasi-steady state characteristics are described
through time series, axial temperature and pressure profiles, thermal
resistances, and operating parameters. The quasi-steady experiments allowed the
investigation of the effects of input power, condenser coolant flow rate, and
fill ratio on important operational parameters and limit conditions.
Additionally, heat transfer coefficient and evaporator input power transients
are investigated, and important parameters are discussed along with the response
times of liquid film temperatures and operating pressures. Overall, it was found
that the performance of the annulus-screen wick was seriously limited due to
perceived vapor generation within the annulus in the evaporator, whereas a
relatively coarse wrapped-screen wick yielded a considerably larger operating
envelop. Important phenomena observed includes subcooled liquid presence near
the condenser endcap and distinct behavior of heat pipes with different fill
ratios near limit conditions.
Description
December2024
School of Engineering
School of Engineering
Full Citation
Publisher
Rensselaer Polytechnic Institute, Troy, NY