No description available.

An investigation was carried out to study the process of rotating stall inception in a low hub-tip ratio fan. Such fans are expected, based on an elementary analysis, to stall from the root. However, experimental evidence had led to the belief that the fans stalled from the tip. The effects of streamtube contraction were first studied and this was followed by an experimental investigation on an isolated rotor, with successive build modifications to increase the likelihood of rotating stall inception occurring at the root. A computer based streamline curvature method was used to study the effects of streamtube contraction and streamtube diffusion that commonly occur when a fan is operated at flows below its' design flow rate. The results indicated a reduced expectation for the root to stall first when compared to a simple 2-D flow analysis. Experimental measurements were then carried out to determine how the experimental local characteristics differed from the predicted characteristics. It was apparent that real fluid effects tended to steepen the root characteristic, thus enhancing the stability of the root. The tip characteristics tended to droop and become less stable. The enhancement of the root stability was also seen in the profiles of deviation angle. The axial Velocity contours at the rotor exit supported the conclusion that the root stability enhancement was caused by "centrifuging". To determine the actual radial location of rotating stall inception, an array of hot wires was used to record events during the inception transient. Inception was first detectable at the tip. This tip stalling behaviour persisted for all the build modifications. Measurements of unsteady pressure were also made to study the movement of the overall operating point since it was felt that this could continuously alternate between a pair of closely spaced characteristics. The results indicated that the fan operated along a unique characteristic. The overall conclusion was that a low hub-tip ratio fan shows a strong reluctance to stall at the root due to "centrifuging" of the blade boundary layer. The inception process appears to be dominated by events in the tip region.

No description available.

No description available.

In this paper, a 16-cell direct methanol fuel cell (called DMFC) stack was developed to power or charge a mobile phone without any voltage transformer. The various types of the performance decay of DMFCs are studied before a 16-cell DMFC stack is made. The decays due to improper storage are found and avoided. The influences of the MEA treatments on the performance are also studied. Eventually, we try to find the best storage and treatment methods to keep stacks in a good condition all the way. In order to solve the problem of methanol crossover lead to the cathode poisoned, it is necessary to operate under the proper methanol concentration and to discharge before finishing the whole experiment. It is also necessary to maintain MEAs in proper wetness so that the performance will not decline during storage. Additionally, the catalyst in the cathode will use Pt/Ru to replace Pt. This 16-cell DMFC stack is composed of two 8-banded MEAs and 16 carbon fiber bunches. Each MEA is made with 8 sets of electrodes on a piece of membrane. The stack with 16 cells will be connected in series outside of the reaction chamber. The weight and volume of this 16-cell DMFC stack are 55 g (not including 20 c.c. methanol solution) and 99 cm3. The total electrode is 50 cm2 (16-cell¡Ñ3.15 cm2 per cell). The power at voltage 4V is 1680mW when it is operating at room temperature and air breathing. The maximum power density can reach 33 mW/cm2. The specific power density is 22 mW/g and the volumetric power density is 16.9 mW/cm3. This stack can power or charge a mobile phone directly.

This work focuses on the concept of sustainable propellant collection. The concept consists of gathering ambient gas while on-orbit and using it as propellant. Propellant collection could potentially enable operation in very-low Earth orbits without compromising spacecraft lifetime. This work conducts a detailed analysis of propellant collection from a physics perspective in order to test the assertions of previous researchers that propellant collection can dramatically reduce the cost of propellant on-orbit. Major design factors for propellant collection are identified from the fundamental propellant collection equations, which are derived in this work from first principles. A sensitivity analysis on the parameters in these equations determines the relative importance of each parameter to the overall performance of a propellant-collecting vehicle. The propellant collection equations enable the study of where propellant collection is technically feasible as a function of orbit and vehicle performance parameters. Two case studies conducted for a very-low Earth orbit science mission and a propellant depot-type mission serve to demonstrate the application of the propellant collection equations derived in this work. The results of this work show where propellant collection is technically feasible for a wide range of orbit and vehicle performance parameters. Propellant collection can support very-low Earth operation with presently available technology, and a number of research developments can further extend propellant-collecting concepts' ability to operate at low altitudes. However, propellant collection is not presently suitable for propellant depot applications due to limitations in power.

There are several disadvantages in conventional unipolar/bipolar plates such as cost expensive, weight heavy and volume large. Therefore, it is difficult in making use conventional unipolar/bipolar plates to portable fuel cells. With a new heterogeneous carbon fiber bipolar plate, pumpless and air-breathing design and in cooperating with a special MEA, portable fuel cell stacks developed in our lab have made portable applications to be possible. The structure of the DMFC stack made with the new carbon fiber bipolar plate is much more simple and weight-light than the other designs. A two-layer 16-Cell DMFC Stack had been designed and made by using the heterogeneous carbon fiber monopolar plates developed in our fuel cell laboratory. With this design, the methanol solution can be directly stored in the anode chamber which can store fuel 17 ml and does not need any auxiliary equipment, so it easy to apply to the portable power source. Not including fuel, total weight of stack is only 50g and the volume is 75 cm3. The 16 cell stack includes two pieces of 117 membrane, 16 anode electrodes loading Pt/Ru 5 mg/cm2 and 16 cathode Pt loading 5 mg/cm2. Each single cell electrode area is about 3.5 cm², so the total electrode area of the 16-cell stack is 56cm2. With methanol concentration 3 M, pumpless, air-breathing, and room temperature, the largest output power density of the fuel cell can reach 10.3 mW/cm², and the total power can reach 578 mW in this stage. The performance of the stack will be further improved in the next stage.

The problem of the performance decay and the factors affecting on the DMFC performance for long-term operation are studied in this thesis. First, the influence of the initial treatments of MEA and the exposure of MEA in the atmosphere on the water content are measured. In addition, the effects of the pressure of the MEA hot press conditions, the treatments and preservation of MEA, and the operative conditions on the performance are also examined. Eventually, we expect that the best way to increase the DMFC performance and avoid the performance can be found. These can provide for references when a portable DMFC need to be designed and manufactured in future. In order to solve the problem of methanol crossover leading to the cathode poisoned, cells are operated only under the proper methanol concentration and discharged thoroughly before finishing the whole experiment. It is also necessary to maintain MEAs in proper wetness so that the performance of stack will not decay too quickly. In the initial treatment, firstly, a MEA is immersed in 3M MeOH and then boiled with 80oC DI water for an hour, respectively. The experimental conditions of this passive single-cell DMFC are pumpless in anode chamber, air-breathing, and room temperature. The power density of this DMFC with these test conditions can reach a value about 33mW/cm2. This value is about 106% higher than that of the untreated MEA. If MEA boiled with 0.5M H2SO4 for an hour and then boiled with 80oC DI water for an hour, its power density is about 75% higher than that of the untreated MEA.

Abstract Traditional unipolar/bipolar plates, such as the metal and the graphite unipolar/bipolar plates, are expensive, weight heavy and volume large, so that it is hard to be used in the portable application. A high efficiency, low cost and lightweight portable proton exchange membrane fuel cell (called PEMFC or called HFC when using pure hydrogen fuel), which is made with a new heterogeneous composite carbon fiber bipolar plate and a MEA, is developed in our lab. There are many advantages of the new carbon fiber unipolar/bipolar plates, such as low contact resistance, low cost, lightweight and small volume. We hope that the new unipolar/bipolar plate will be able to replace the conventional metal and graphite unipolar/bipolar plates in the future. The characteristics of a portable PEMFC in different operational conditions are studied in this research. From our experimental result, we find that the factors which affect the HFC performance include the gas temperature, humidity ratio, inlet gas pressure in anode, the geometry of inlet ports, the flow channels within cell, and the oxidant flow rate etc. In addition, the contact resistances between different materials within each cell all strongly influence HFC performance. The ribs of the carbon fiber unipolar/bipolar plates is pored structure, and the gas diffusion layer is no deformation because of only slight compression in stack assembly; therefore, the reactive gas can easily flow into the most of active area. In addition, the contact resistance between the carbon fiber unipolar plate and the gas diffusion layer is lower than that between the traditional unipolar plate and the gas diffusion layer, so that the electrons in active layer is easily to exit or enter this region. The experimental result at 1.15 atm and 40 oC displays that the current density with the new unipolar plate is about twice higher than that with the graphite unipolar plate at overpotential 0.6 V. With air as an oxidizer, we find that increasing the fan rotation speed can avoid output-voltage decay in high current density, but the design with fan is unfavorable for portable application. So a front open unipolar plate and air-breathing design is adopted on the cathode. The power density of this design is slightly lower than that with fan, but it still can reach a value 160 mW/cm2 without any heating and humidification in the anode. Because this design needs little supplement device, the application in portable fuel cells of the new design will be wider than that of a traditional design.

Several disadvantages in general unipolar/bipolar plates are that cost is expensive, weight is heavy and the volume is large. The high compressing pressure is also necessary to reduce the contact resistance in making up a fuel cell stack. Therefore, it is difficult in making use general unipolar/bipolar plates to portable fuel cells. With a new heterogeneous carbon fiber bipolar plate, pumpless and air-breathing design and in cooperating with a special MEA, a portable fuel cell stacks developed in our lab have made portable applications to be possible. The structure of the DMFC stack made with the new carbon fiber bipolar plate is much more simple and weight-light than the other designs. The three portable DMFC stacks flat type, cylinder type (I), and cylinder type (II) are developed in series in our lab. The methanol solution can be stored directly in the flow channel of the anode, and does not need the extra auxiliary equipment, so it easy to apply to the portable fuel cell. The developed portable DMFC of cylinder type (II), weight is only 20g, volume is 30cm3, and the fuel stored capacity is 7.5ml. In the flat type DMFC, In anode Pt-Ru loading 3 mg/cm2, and cathode Pt loading 1 mg/cm2, methanol concentration 3 M, pumpless, air-breathing, and room temperature, the largest of output power density of the fuel cell can reach 5.27 mW/cm2, and the total power can reach 71 mW. The weight of DMFC of cylinder type (II) is far lower than DMFC of flat type in addition, so its power density 1.33mW/g is about 1.68 times of flat type 0.79 mW/g.