People with a transtibial amputation using passive-elastic prostheses exhibit reduced prosthetic ankle power and push-off work compared to non-amputees and compensate by increasing their affected leg (AL) hip joint work and unaffected leg (UL) ankle, knee, and hip joint and leg work during level-ground walking. Use of a powered ankle-foot prosthesis normalizes step-to-step transition work during level-ground walking over a range of speeds for people with a transtibial amputation, but the effects on joint work during level-ground, uphill, and downhill walking have not been assessed. We investigated how use of passive-elastic and powered ankle-foot prostheses affect leg joint biomechanics during level-ground and sloped walking. 10 people with a unilateral transtibial amputation walked at 1.25 m/s on a dual-belt force-measuring treadmill at 0°, ±3°, ±6°, and ±9° using their own passive-elastic and a powered prosthesis (BiOM T2, BionX Medical Technologies, Inc., Bedford, MA, USA) while we measured kinematic and kinetic data. We calculated AL and UL prosthetic, ankle, knee, hip, and individual leg positive, negative, and net work. Use of a powered compared to passive-elastic anklefoot prosthesis resulted in greater AL prosthetic and individual leg net work on uphill and downhill slopes. Over a stride, AL prosthetic positive work was 23-30% greater (p < 0.05) during walking on uphill slopes of +6°, and +9°, prosthetic net work was up to 10 times greater (more positive) (p ≤ 0.005) on all uphill and downhill slopes and individual leg net work was 146 and 82% more positive (p < 0.05) at uphill slopes of +6° and +9°, respectively, with use of the powered compared to passive-elastic prosthesis. Greater prosthetic positive and net work through use of a powered ankle-foot prosthesis during uphill and downhill walking improves mechanical work symmetry between the legs, which could decrease metabolic cost and improve functional mobility in people with a transtibial amputation.

During level-ground walking, mechanical work from each leg is required to redirect and accelerate the center of mass. Previous studies show a linear correlation between net metabolic power and the rate of step-to-step transition work during level-ground walking with changing step lengths. However, correlations between metabolic power and individual leg power during step-to-step transitions while walking on uphill/downhill slopes and at different velocities are not known. This basic understanding of these relationships between metabolic demands and biomechanical tasks can provide important information for design and control of biomimetic assistive devices such as leg prostheses and orthoses. Thus, we compared changes in metabolic power and mechanical power during step-to-step transitions while 19 subjects walked at seven slopes (0°, +/−3°, +/−6°, and +/−9°) and three velocities (1.00, 1.25, and 1.50 m/s). A quadratic model explained more of the variance (R 2 =0.58-0.61) than a linear model (R 2 =0.37-0.52) between metabolic power and individual leg mechanical power during step-to-step transitions across all velocities. A quadratic model explained more of the variance (R 2 =0.57-0.76) than a linear model (R 2 =0.52-0.59) between metabolic power and individual leg mechanical power during step-to-step transitions at each velocity for all slopes, and explained more of the variance (R 2 =0.12-0.54) than a linear model (R 2 =0.07-0.49) at each slope for all velocities. Our results suggest that it is important to consider the mechanical function of each leg in the design of biomimetic assistive devices aimed at reducing metabolic costs when walking at different slopes and velocities.

Sloped walking is challenging for individuals with transtibial amputation (TTA) due to the functional loss of the ankle plantarflexors. Prostheses that actively generate ankle power may help to restore this lost function. The purpose of this study was to use musculoskeletal modeling and simulation to quantify the mechanical power delivered to body segments by passive and powered prostheses and the remaining muscles in the amputated and intact legs during sloped walking. We generated walking simulations from experimental kinematic and kinetic data on slopes of 0, ±3 deg and ±6 deg in eight people with a TTA using powered and passive prostheses and eight nonamputees. Consistent with our hypothesis, the amputated leg hamstrings generated more power to both legs on uphill slopes in comparison with nonamputees, which may have implications for fatigue or overuse injuries. The amputated leg knee extensors delivered less power to the trunk on downhill slopes (effect size (ES) ≥ 1.35, p ≤ 0.02), which may be due to muscle weakness or socket instability. The power delivered to the trunk from the powered and passive prostheses was not significantly different (p > 0.05), However, using the powered prosthesis on uphill slopes reduced the contributions from the amputated leg hamstrings in all segments (ES ≥ 0.46, p ≤ 0.003), suggesting that added ankle power reduces the need for the hamstrings to compensate for lost ankle muscle function. Neither prosthesis replaced gastrocnemius function to absorb power from the trunk and deliver it to the leg on all slopes.

A study was conducted to experimentally characterize the pool boiling performance of hydrofluorocarbon (HFC)-245fa. The motivation for this research is to characterize the performance of candidate refrigerants for potential use in automotive power electronics two-phase cooling systems. The HFC-245fa pool boiling experiments were conducted using horizontally oriented 1-cm2 heated surfaces to quantify the effects of pressure and a microporous-enhanced coating on heat transfer coefficients and critical heat flux (CHF) values. Experiments were carried out at pressures ranging from 0.15 MPa to 1.1 MPa (reduced pressure range: 0.04–0.31). To enhance boiling heat transfer, a copper microporous coating was applied to the test surfaces. The coating was found to enhance heat transfer coefficients by as much as 430% and CHF by approximately 50%. Increasing pressure decreased the magnitude of the heat transfer coefficient enhancements but had minimal effect on CHF enhancements. The experimental data were then used to generate correlations for the boiling heat transfer coefficients and CHF values. Finally, the performance of HFC-245fa was compared to the performance of hydrofluoroolefin (HFO)-1234yf and HFC-134a at conditions of equivalent saturation temperatures and reduced pressures.

A study was conducted to experimentally characterize the pool boiling performance of hydrofluorocarbon HFC-245fa at pressures ranging from 0.15 MPa to 1.1 MPa (reduced pressure range: 0.04-0.31). Pool boiling experiments were conducted using horizontally oriented 1-cm2 heated surfaces to quantify the effects of pressure and a microporousenhanced coating on heat transfer coefficients and critical heat flux (CHF) values. Results showed that the coating enhanced heat transfer coefficients and CHF by 430% and 50%, respectively. The boiling heat transfer peiformance of HFC-245fa was then compared with the boiling performance of HFC-134a and hydrofluoroolefin HFO-1234yf.

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This study evaluates a jet impingement based cooling strategy combined with microfinned enhanced surfaces as a means of improving thermal management for power electronic devices. For comparison, a baseline channel flow heat exchanger and jet impingement on plain surfaces are characterized. The jets, augmented with enhanced microfinned surfaces, provide localized cooling to areas heated by the insulated-gate bipolar transistors and diode devices. Lighter materials and simpler manufacturing while managing required pumping power increase the overall performance while reducing weight, volume, and cost. Computational fluid dynamics modeling validated by experiments was used to characterize the baseline as well as jet-impingement-based heat exchangers at typical automotive flow rates using a 50%-50% mixture by volume of water and ethylene glycol. The three cooling configurations were tested at full inverter power (40 to 100 kW output power) on a dynamometer. An increased thermal performance was observed for the jet-impingement configurations. Experiments were also performed to investigate the long-term reliability of the jets impinging on enhanced surfaces.

Experiments were conducted to evaluate the use of a passive two-phase cooling strategy as a means of cooling automotive power electronics. The proposed cooling approach utilizes an indirect cooling configuration to alleviate some reliability concerns and to allow the use of conventional power modules. An inverter-scale proof-of-concept cooling system was fabricated and tested using the refrigerants hydrofluoroolefin HFO-1234yf and hydrofluorocarbon HFC-245fa. Results demonstrated that the system can dissipate at least 3.5 kW of heat with 250 cm 3 of HFC-245fa. An advanced evaporator concept that incorporates features to improve performance and reduce its size was designed. Simulation results indicate the concept's thermal resistance can be 58% to 65% lower than automotive dual-side-cooled power modules. Tests were also conducted to measure the thermal performance of two air-cooled condensers-plain and rifled finned tube designs. The results combined with some analysis were then used to estimate the required condenser size per operating conditions and maximum allowable system (i.e., vapor and liquid) temperatures.