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 π-conjugated polymer donors for efficient bulk-heterojunction (BHJ) solar cell applications, poly(benzo[1,2-b:4,5-b′]dithiophene–thieno[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)—incorporated into the side chains of PBDTTPD polymers—can 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, 3–6.5%) 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 ∼1 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