•We study nucleate boiling on substrates of small thermal capacity using IR camera.•Microlayer does not fully evaporate due to limited thermal capacity of the heater.•Microlayer evaporation and ...rewetting are less important compared to thick heaters.•Most energy is removed by convective effects created by bubble growth and departure.•We re-derived heat flux partitioning model to quantify each heat transfer mechanism.
In this work, we studied the wall heat flux partitioning during the pool boiling of water on thin metallic surfaces. We conducted boiling experiments on surfaces where we engineered nucleation sites by nanosecond-fiber-laser texturing. These nucleation sites form triangular lattice patterns with different pitches. We measured the time-dependent temperature and heat flux distributions on the boiling surface using an infrared camera. We developed post-processing algorithms to measure, based on these distributions, all the fundamental boiling parameters used in heat flux partitioning models (e.g., nucleation site density, bubble wait and growth time, and bubble footprint radius) and the actual partitioning of the heat flux, i.e., how much heat is transferred by evaporation of the microlayer, rewetting of the surface, and convective effects.
This work reveals that the mechanisms of heat transfer on substrates of small thermal capacity are very different compared to substrates of large thermal capacity. With water, the bubble microlayer typically does not dry out and the surface temperature at rewetting is practically the same as the rewetting fluid temperature. These effects limit the efficiency of microlayer evaporation and rewetting heat transfer. Instead, convective effects generated by the bubble growth process remove most of the energy from the heated surface. This behavior is captured by a heat flux partitioning model that we re-derived from first principles to describe the heat transfer mechanisms on substrate of small thermal capacity.
We investigated propagation of a sharp crack in a thin metallic conductor with an edge crack due to electric current induced electromagnetic forces. Finite element method (FEM) simulations showed ...mode I crack opening in the edge-cracked conductor due to the aforementioned (i.e., self-induced) electromagnetic forces. Mode I stress intensity factor due to the self-induced electromagnetic forces,
K
IE
,
was evaluated numerically as
K
IE
=
μ
l
2
j
2
(
π
a
)
0.5
f
(
a
/
w
)
, where
μ
is the magnetic permeability,
l
is the length of the conductor,
a
is the crack length,
j
is the current density,
w
is the width of the sample and
f
(
a
/
w
) is a geometric factor. Effect of dynamic electric current loading on edge-cracked conductor, incorporating the effects of induced currents, was also studied numerically, and dynamic stress intensity factor,
K
IE
,
d
, was observed to vary as
K
IE
,
d
∼
f
d
(
a
/
w
)
j
2
(
π
a
)
1.5
. Consistent with the FEM simulation, experiments conducted using
12
μ
m
thick Al foil with an edge crack showed propagation of sharp crack due to the self-induced electromagnetic forces at pulsed current densities of
≥
1.85
×
10
9
A/m
2
for
a
/
w
=
0.5
. Further, effects of current density, pulse-width and ambient temperature on the fracture behavior of the Al foil were observed experimentally and corroborated with FEM simulations.
In this investigation a microtensile machine in combination with a non-contacting laser-optical speckle correlation sensor to determine strain with high resolution was used to study the stress-strain ...behavior of thin metallic foils of Cu and Al with varying thickness ranging between 10 and up to 250 μm. The grain sizes varied between 2 and up to 250 μm. A size effect was detected resulting in an influence mainly on the fracture strain. This effect will be explained on the basis of texture differences, the number of activated gliding systems as a dependence on the ratio of grain size to foil thickness. To support these experimental findings the fracture topography has also been investigated. In addition, the fatigue crack propagation properties of the above mentioned Cu foils were studied as function of thickness. Using a specially designed fatigue testing set-up it was feasible to determine crack growth curves from free-standing foils. Depending on thickness, an unexpected crack growth behavior was detected. Using the ECCI technique it was feasible to study the interaction of the global dislocation arrangement with the crack tip.
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