19 research outputs found
An Experimental Investigation of the Air-Side Convective Heat Transfer Coefficient on Wire and Tube Refrigerator Condenser Coils
This thesis presents the results of an experimental investigation of the convective airside
heat transfer from wire and tube condensers. The ftrst law of thermodynamics is applied
to the "refrigerant", water in this investigation, flowing through the tubes in order to
determine the total heat loss from the condenser. The test section is 910 mm (36 in) wide by
300 mm (12 in) tall; thus the coil is tested in an essentially inftnite stream. During the course
of the experiments, the influence of the free stream air velocity ranging from 0.15 rn/s to 2.0
rn/s (0.49 ftls to 6.56 ftls) is established. The angle of attack, n, was varied from - 40 degrees
to 40 degrees with the air flow always normal to the tubes ('11= 1t/2) and varied from -20
degrees to 20 degrees with the air flow normal to the wires ('II = 0). A method for
,
calculating view factors and the radiation heat transfer for wire and tube condensers is
derived. The effect of the length of the coil is measered at 0 and -5??angle of attack. In
addition, the influence of the ftn efftciency on the heat transfer is investigated and accounted
for in the deftnition of the heat transfer coefftcient. The heat transfer data in the inertia
dominated regime (Richardson number less than 0.0013) are correlated assuming
NUcoil = t(Re, n, 'II)' g( S: ) with the Reynolds number based on the wire diameter. The
range of Reynolds numbers covered is 15.7 < Rew < 207.5. The ranges of coil geometric
parameters (nondimensionlized by dividing by the wire diameter) covered in this study are:
3.022 < nondimensional tube diameter < 5.134, 18.84 < nondimensional tube spacing <
40.94,2.819 < nondimensional wire spacing < 4.427,53.80 < nondimensional tube length<
143.6, and 207.2 < nondimensional wire length < 500.2. The function is represented by
tl(a)??Reh (a) for 'II = 0 and h(a).Rei4 (a) for'll=1t/2. Approximately 1700 tests were
performed in this investigation using seven different coils. The ftnal correlation is capable of
predicting the data with 2cr equal to 16.7% for Ri < 0.0013. A limited natural convection
study is also presented.Air Conditioning and Refrigeration Center Project 4
Ring Substituents Mediate the Morphology of PBDTTPD-PCBM Bulk-Heterojunction Solar Cells
Among Ï-conjugated polymer donors for efficient bulk-heterojunction (BHJ) solar cell applications, poly(benzo[1,2-b:4,5-bâČ]dithiopheneâthieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) polymers yield some of the highest open-circuit voltages (VOC, ca. 0.9 V) and fill-factors (FF, ca. 70%) in conventional (single-cell) BHJ devices with PCBM acceptors. In PBDTTPD, side chains of varying size and branching affect polymer self-assembly, nanostructural order, and impact material performance. However, the role of the polymer side-chain pattern in the intimate mixing between polymer donors and PCBM acceptors, and on the development of the BHJ morphology is in general less understood. In this contribution, we show that ring substituents such as furan (F), thiophene (T) and selenophene (S)âincorporated into the side chains of PBDTTPD polymersâcan induce significant and, of importance, very different morphological effects in BHJs with PCBM. A combination of experimental and theoretical (via density functional theory) characterizations sheds light on how varying the heteroatom of the ring substituents impacts (i) the preferred side-chain configurations and (ii) the ionization, electronic, and optical properties of the PBDTTPD polymers. In parallel, we find that the PBDT(X)TPD analogs (with X = F, T, or S) span a broad range of power conversion efficiencies (PCEs, 3â6.5%) in optimized devices with improved thin-film morphologies via the use of 1,8-diiodooctane (DIO), and discuss that persistent morphological impediments at the nanoscale can be at the origin of the spread in PCE across optimized PBDT(X)TPD-based devices. With their high VOC âŒ1 V, PBDT(X)TPD polymers are promising candidates for use in the high-band gap cell of tandem solar cells
BAs and boride III-V alloys
Boron arsenide, the typically-ignored member of the III-V arsenide series
BAs-AlAs-GaAs-InAs is found to resemble silicon electronically: its Gamma
conduction band minimum is p-like (Gamma_15), not s-like (Gamma_1c), it has an
X_1c-like indirect band gap, and its bond charge is distributed almost equally
on the two atoms in the unit cell, exhibiting nearly perfect covalency. The
reasons for these are tracked down to the anomalously low atomic p orbital
energy in the boron and to the unusually strong s-s repulsion in BAs relative
to most other III-V compounds. We find unexpected valence band offsets of BAs
with respect to GaAs and AlAs. The valence band maximum (VBM) of BAs is
significantly higher than that of AlAs, despite the much smaller bond length of
BAs, and the VBM of GaAs is only slightly higher than in BAs. These effects
result from the unusually strong mixing of the cation and anion states at the
VBM. For the BAs-GaAs alloys, we find (i) a relatively small (~3.5 eV) and
composition-independent band gap bowing. This means that while addition of
small amounts of nitrogen to GaAs lowers the gap, addition of small amounts of
boron to GaAs raises the gap (ii) boron ``semi-localized'' states in the
conduction band (similar to those in GaN-GaAs alloys), and (iii) bulk mixing
enthalpies which are smaller than in GaN-GaAs alloys. The unique features of
boride III-V alloys offer new opportunities in band gap engineering.Comment: 18 pages, 14 figures, 6 tables, 61 references. Accepted for
publication in Phys. Rev. B. Scheduled to appear Oct. 15 200
The Interaction Between the Substrate and Frost Layer Through Condensate Distribution
Microscopic observations of frost deposition on a variety of substrates having
different contact angles, (polytetrafluoroethylene PTFE, kapton, glass and others) allow
the quantification of substrate effects on frost structure during inception and growth. The
deposition of water vapor at the beginning of the frosting process on a clean glass
substrate is found to be as condensate (condensation frosting) rather than as ice
(ablimation frosting) for a substrate temperatures above -33??C and an absolute humidity
above 0.15 g/kg. The inception of "condensation frosting" (the condensation period and
early frost growth period) is further examined microscopically as a function of air and
substrate temperatures, absolute humidity, and substrate contact angle. The water
distribution on the substrate at the end of the condensation period is found to be strongly
dependent on substrate temperature, humidity ratio, and substrate contact angle. Colder
substrates result in smaller more uniform droplets and substrates with lower contact
angles result in shorter, larger diameter droplets with a larger percentage of the substrate
covered. The effective density of the condensate on hydrophobic substrates is found to
be lower than that on hydrophilic substrates. The structure and form of the ice
immediately after freezing is substrate dependent. High-speed imaging of the freezing
process is used to study the propagation of the freezing front in a droplet. The images
show that a protrusion is formed at the top of the droplets during freezing. From
observations, this protrusion is hypothesized to result from the convective condition at
the droplet surface and the difference in specific volume between liquid and solid water.
Additionally, the apparent ejection of water vapor during freezing of a droplet on a hydrophobic substrate was observed. This ejection of water vapor is thought to be caused
by the wanning of the droplet caused by the release of latent heat. In contrast to trends
observed during the early growth period, the growth rate of mature frost is found to
decrease with substrate contact angle while frost density is found to increase. This
behavior is explained in terms of the effect of substrate contact angle on the structure and
form of the incipient frost, which constitutes the initial condition for further (mature)
frost growth. A higher conductivity layer is formed on the hydrophilic than on the
hydrophobic substrate. A model relating crystal orientation to conductivity is used to
simulate the frost growth rate and density on the two different substrates and match the
experimental data. Using similar reasoning, the higher conductivity frost formed on
colder substrates is also explained.Air Conditioning and Refrigeration Project 10