Three-Phase and Unidirectional Heat Transfer

Smart thermal management by which ultra-high heat fluxes (i.e., q''> 100 W/cm²) are dissipated efficiently, is increasingly desirable for many applications in aerospace, electronic packaging, metallurgy, as the existing cooling solutions are highly constrained. For example, the cooling strategy for aircraft must be executed in such a way that will operate independently of orientation while also screen out external heat loads coming from the neighboring electronic boxes and/or external sources. Therefore, it is crucial to develop heat transfer devices which could effectively dump heat away while additionally shield against external heat loads. Thermal diodes, by definition, accomplish this desirable unidirectional heat transfer functionality. Nonetheless, the existing thermal diodes are currently constrained by either a low diodicity (i.e., heat transfer ratio), gravitational dependence, a one-dimensional configuration, or poor durability. Further example for the necessity of smart thermal management would be in firefighting and nuclear reactor safety. Above a critical temperature referred to as the ``Leidenfrost temperature'', the highly effective nucleate boiling is completely replaced by insulating film boiling, causing a dramatic decrease in the essential cooling rate of water pool boiling and spray quenching. In chapter 2, after noting the mechanism and shortcomings of each existing solid-state and phase-change thermal diode, we develop a unique thermal diode, called bridging-droplet thermal diode, which operates independent of orientation, is planar and durable. Our diode is comprised of two opposing copper plates separated by an insulating gasket of micrometric thickness; one plate contains a superhydrophilic wick structure while the other is smooth and hydrophobic. In the forward mode of operation, water evaporates from the heated wicked plate and condenses on the opposing hydrophobic plate. The large contact angle of the dropwise condensate enables bridging across the gap to replenish the wicked evaporator, providing sustained phase-change heat transfer. Conversely, in the reverse mode the heat source is now on the hydrophobic plate, resulting in dryout and excellent thermal insulation across the gap. An orientation-independent heat transfer ratio (i.e. diodicity (η)) of approximately 85 was experimentally measured. In chapter 3, after highlighting that our experimental proof-of-concept discussed in chapter 2, was limited to only a narrow parameter space, we develop a comprehensive thermal circuit model for both the forward and reverse modes of operation to theoretically characterize the bridging-droplet thermal diode over a broad parameter space. Parameters that are varied include the gap height, input heat flux, effective thermal conductivity of the wetted wick structure, height of the wicking micropillars, wettability of the opposing smooth surface, and heat sink temperature. Our findings show that a vapor space height of Hᵥ≈ 250 μm, short and densely packed micropillars, a higher applied heat flux in the forward mode, and a hotter heat sink temperature result in optimal diodicities of η~ 100. In chapter 4, we discuss that the Leidenfrost effect has been a two-phase phenomenon thus far: either an evaporating liquid or a sublimating solid levitates on its vapor. Here, we demonstrate that an ice disk placed on a sufficiently hot surface exhibits a three-phase Leidenfrost effect, where both liquid and vapor films emanate from under the levitating ice. Curiously, the critical Leidenfrost temperature was over three times hotter for ice than for a water drop. As a result, the effective heat flux was an order of magnitude larger when quenching aluminum with ice rather than water over a wide temperature range of 150--550 °C. An analytical model reveals the mechanism for the delayed film boiling: the majority of the surface's heat is conducted across the levitating meltwater film due to its 100 °C temperature differential, leaving little heat for evaporation. In chapter 5, we note that nucleate boiling achieves dissipative heat fluxes as high as q''~ 100 W/cm² and is widely used for power plants, spray quenching metal alloys, desalination, and electronics cooling. However, above a Leidenfrost temperature of about 150 °C for water, an insulating vapor film massively degrades the heat flux by two orders of magnitude. Here, we demonstrate that robust nucleate boiling can be maintained even at temperatures as high as 400 °C by using ice particles in place of water droplets. Ice pellets are periodically released onto a superheated stage and compared to spray quenching at an equivalent mass flow rate. Ice quenching was twice as fast as spray quenching at low superheats, and at large superheats, only ice quenching is successful. Our results demonstrate that ice quenching can maintain groundbreaking heat fluxes of q''~ 100--1,000,W/cm² over a broad range of superheats, far superior than classical spray quenching.

Name thesis:degree_name

Doctor of Philosophy

Level thesis:degree_level

doctoral

Discipline thesis:degree_discipline

Mechanical Engineering

Department dc:contributor.department

Mechanical Engineering

Grantor dc:publisher

Virginia Tech

Year dc:date.issued

2022

Author dc:creator

• Edalatpour, Mojtaba

Chair dc:contributor.committeechair

• Boreyko, Jonathan B.

Committee members dc:contributor.committeemember

• Staples, Anne E.
• Paul, Mark R.
• Cheng, Jiangtao

• Thermal diode
• vapor chambers
• phase-change
• interfacial phenomena
• Leidenforst effect
• boiling

Statement dc:rights

• Creative Commons Attribution-NonCommercial 4.0 International

Licence dc:rights.uri

• http://creativecommons.org/licenses/by-nc/4.0/

Language dc:language.iso

en

Dc Identifier Other

vt_gsexam:35633

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/112361