查看更多>>摘要：It is critical to eliminate the difficulties of using li-ion batteries at low-temperature environments for acceptable performance by preheating strategy. Hence, an experimental study is reported on the performance of a new hybrid preheating unit integrated to graphite matrix composite with phase change thermal management for small-scale li-ion module exposed to low-temperature conditions. The graphite matrix with 75 g/L is saturated with organic paraffin. Cartridge heating is mounted in graphite matrix composite with phase change for preheating period. The performance tests of the li-ion module for a low-temperature environment (T = -15 degrees C) are conducted at 1C and 1.6C discharge rates. For the low-temperature performance of graphite matrix composite with and without preheating, temperature history, discharge, and energy capacity variations are reported comprehensively. The results show that the proposed preheating unit integrated to graphite matrix composite with phase change has an important contribution on operating temperature, discharge and energy capacity values for better life span and safety compared to the non-preheating case for extreme low-temperature environment. The PCM/graphite matrix with preheating is more effective for higher discharge rates (1.6C). For the non-preheating case at 1.6C-rate, 100% capacity loss is occurred depending on instant voltage drop at T = -15 degrees C. However, the capacity loss is decreased to 22% with hybrid preheating of the li-ion module. The energy capacity value of the li-ion module reaches 44Wh at a higher discharge rate (1.6C-rate), while energy capacity is 0 Wh in the non-preheating case. Moreover, heating efficiency is obtained as 58%.
查看更多>>摘要：Road tunnel fires can jeopardize the life safety of occupants and the structural integrity of the tunnel. Such fires are usually characterized by the heat release rate (HRR) parameter. Most tunnel fire safety systems are designed based on this parameter. The tunnel structure (walls and ceilings) are usually protected with a passive protections such as magnesium oxide boards, cementitious sprays, polypropylene fiber mix or calcium silicate boards. Amongst these materials, calcium silicate boards are the most popular choice due to their easy installation and capability to protect the tunnel at very high temperatures for several hours. The HRR in a tunnel fire depends on various parameters such as type of vehicle involved in the fire, amount of fuel, location of fuel and ignition source, tunnel geometry, presence of fixed firefighting systems/ventilation, and several other factors. However, no evidence of structural linings (passive protection) affecting the heat release rate is found in the literature. To investigate the role of structural linings (calcium silicate) on the HRR, a numerical study using Fire Dynamic Simulator (FDS) is conducted for a road tunnel involving fire in a heavy goods vehicle. Two scenarios are investigated; one considering concrete tunnel linings without any passive protection while in the second scenario the tunnel is protected with calcium silicate linings. Comparison of the results from the two abovementioned scenarios reveal different patterns of HRR and tunnel temperatures and suggests that in large fires, structural linings such as calcium silicate lining can alter the overall HRR by enhancing the heat feedback and fuel burning rates inside the tunnel.
查看更多>>摘要：A numerical investigation is performed to explore the thermal-fluid characteristics of corrugated surfaces with different wave shapes cooled by a vibrating piezoelectric fan. Totally-nine different corrugated surfaces and one baseline flat surface are taken into consideration. Meanwhile, two non-dimensional parameters (A* and T*, representing the amplitude and period of the waves, respectively) are defined to describe the geometric char-acteristics of these corrugated surfaces quantitatively. Then the effects of A* and T* on flow heat transfer characteristic are investigated in detail. Time-averaged Nusselt number and instantaneous velocity vector are calculated. It is confirmed that the time-averaged Nusselt number distributions on corrugated surfaces is pri-marily determined by its geometrical shape, and the minimum and maximum values usually appear at the trough and peak, respectively. Compared to that of the flat surface, the corrugated surfaces could trigger the velocity boundary layer separation more easily, and the size of the corrugation directly limits the scale of the counter-rotating vortices. Of all these corrugated surfaces we studied, only the one with the smallest A* of 0.0625 and the largest T* of 2.0 is demonstrated advantageously. It is revealed that the wave shape influences the flow and heat transfer performance substantially.
查看更多>>摘要：The article addresses the characterization of laminar flame speed of Hythane generated from non thermal plasma reforming of Methane using a dielectric barrier discharge reactor. Hythane with 5%, 10% and 15% Hydrogen fraction is generated by controlling the Methane residence time in the plasma zone. Planar laminar flame speed for the Hythane so generated is estimated using a flame tube and compared with flame speed of bottled Hythane over a range of operating stoichiometry. It is observed that Hythane from plasma reforming of Methane has substantially higher laminar flame speed as compared to bottled Hythane. At the peak flame speed equivalence ratio of Phi = 1.1, Hythane generated from plasma activation of Methane has laminar flame speed higher than corresponding bottled Hythane to the extent of 13.3%, 34.1% and 38.9% for 5%, 10% and 15% Hydrogen in Hythane respectively. It is also observed that flame speed difference is muted for lean mixtures as compared to rich mixtures. Thermo-chemical analysis indicates that the enhanced flame speed can neither be addressed based on thermal effects nor due to the presence of neutral intermediate species and non thermal reforming of Methane to generate Hythane potentially results in additional reaction kinetics influences which accelerate the combustion process. Such influences could arise from presence of long lived species formed (charged or otherwise) not detected in conventional analysis and changes to the energy levels of the reaction species, principally Methane and Hydrogen. In terms of terminal utility, non thermal plasma reforming of Methane can have path breaking influence considering that enhanced flame speeds can significantly improve the thermodynamic efficiency of a spark ignited engine with lower than pure thermo-chemical Hydrogen requirement in Hythane.
查看更多>>摘要：A transient numerical simulation was carried out to investigate the convection heat transfer enhancement of heated asymmetrical concave surfaces using multi piezoelectric fans. And the numerical methodology has been validated through the comparison with the results of validation tests. The effects of the relative curvature (RK), dimensionless fan-to-fan pitch (P/W-PF), vibration phase difference between adjacent piezoelectric fans (phi) and the dimensionless offset distance of multi-fans (Delta y/App) on flow and heat transfer performance were reported. RK was varied from 6 to 2. P/W-PF was varied from 0.25 to 1.0. The value of phi was 0 degrees and 180 degrees. Delta y/App was varied from 0 to 1.0. The heat transfer of the heated surface was characterized by evaluating a time-averaged convection heat transfer coefficient over a complete vibration cycle of the multi-piezoelectric fans after the flow and thermal fields reached a quasi-steady state. In addition, transient and time-averaged flow fields were captured to study the underlying mechanism of the heat transfer enhancement. It was found that higher heat transfer coefficients were observed in stagnation zone with small P/W-PF at smaller RK value of asymmetrical concave surfaces whereas the P/W-PF is observed to affect the data only marginally at larger RK values. The influence of phi on the heat transfer coefficients was primarily reflected in the clearance region of adjacent fans, where operating in-phase could produce stronger flow in clearances than out-of-phase. Finally, it was also observed that proper adjustment of the fan position can reduce its equivalent relative curvature, resulting in improvement in the heat performance of the asymmetrical concave surface with large RK value.
查看更多>>摘要：The cryogenic filling process is a complex phenomenon with a combination of thermal and flow transients, especially in the case of inert gas purge during the start-up of liquid rocket engines. A 7-Equation two-fluid model is proposed in this work by adding an extra mass conservation equation for the purging gas compared with the conventional 6-Equation model. And the conservation equations for mass and energy are in the differential form, while the momentum equations are given in quasi-steady state. The model is programmed in Modelica language based on the idea of modular modeling. A two-step rising pressure in the supply system of a real engine shown by the experimental data is also predicted by the model. Then the reason for the pressure surge is analyzed by the numerical results. Finally, the numerical study is carried out to examine the effect of inlet pressure and purging pressure on filling characteristics in the supply system. The results show that within the range of parameter considered, the magnitude of pressure surge in the oxygen dome increases with increase in inlet pressure and purging pressure. The filling rate of liquid oxygen in the dome changes in a positive way with inlet pressure while in a negative way with purging pressure.
查看更多>>摘要：In this paper the novelty is represented by comparison of optical and energy performances of spherical mirror, rarely used as primary optics in point-focus CPV systems, with those of refractive or parabolic reflective optics. First, concentration factor and optical efficiency are experimentally determined varying distance between Triple Junction (TJ) cell and spherical mirror, also with secondary optics that increases acceptance angle reducing precision required to solar tracker. Successively, the TJ cell temperature and electrical power values are evaluated under maximum concentration obtaining daily electrical producibility of 98Wh at Fisciano (Italy). Producibility is evaluated for correct and incorrect solar tracking, showing after 750 s decrease of 38.0% and 50.3% respectively with and without secondary optics. Hence, equations linking optical parameters are obtained by experimental data collected for three months and are adoptable for similar point-focus configurations with same TJ cell dimensions and ranges of concentration factor and focal distance. Finally, spherical optics performances are theoretically compared with those of parabolic mirror and experimentally with those of Fresnel lens. Spherical optics is preferable to parabolic from technical-economic point of view because it is easier to produce. Moreover, spherical mirror allows concentration factor and optical efficiency values three times higher than Fresnel lens. The electric power unitary costs of both optics are near but spherical optics requires a CPV system area about three times smaller with same power output. Electrical power and temperature values reached by single TJ cell with spherical optics are respectively about 10 W and 30 ? higher than those of Fresnel lens.
Venkateshwar, K.Tasnim, S. H.Gadsden, S. A.Mahmud, S....
10页
查看更多>>摘要：Thermal rectifiers have recently emerged as a field of interest because of their potential application in a wideranging field. Thermal rectifiers can be employed to shield heat-sensitive electronics components, building thermal management and thermal energy conversion. A thermal rectifier allows heat transfer in a preferred direction while curtailing heat transfer in the reverse direction. Recently, the thermal conductivity differential of Phase Change Materials (PCMs) in their different states has been employed to develop thermal rectifiers. However, these studies are limited to analyzing the effect of change in the thermal conductivity alone, neglecting the influence of change in other thermophysical properties. The difference in PCM density in different states leads to the development of void volume upon phase transition, which can significantly alter thermal rectification. Therefore, this study analyzes the interdependent influence of thermal conductivity and density on thermal rectification under a wide range of temperature biases. The presence of void volume under reverse bias augments thermal rectification, whereas it has an adverse effect when developed under the forward bias. A criterion is developed to identify when the influence of density negates that of thermal conductivity, which requires the design of the thermal rectifier to be altered. Furthermore, optimization criteria and expressions for optimal thermal rectification are developed, incorporating the influence of both thermal conductivity and density.
Elkady, M. S.Abdelaziz, Gamal B.Sharshir, Swellam W.Mohammed, Abdelkarim Y. A. M....
15页
查看更多>>摘要：A macroscopic-scale Darcy's equation is applicable for both single-phase and multi-phase flow through classical porous materials with a porosity of 0-100% at a low Reynolds number. However, it has more than restrictions at high Reynolds number flow therefore, it cannot be applied in these conditions. The fundamental Darcy's law assumes no inertial effect and friction between fluid and walls (channeling effect). In this research study, the influence of inertia "Forchheimer term " and macroscopic shear "Brinkman term " are pointedly considered. The influence of porosity and gravity parameters are also considered. Momentum equation considering viscous term, gravity, inertial term, and channeling effect for each phase beside energy equation are derived and solved by finite volume technique. Test section of a vertical, cylinder is filled with spherical beads and exposed to constant wall temperature. The two flowing fluids are assumed to move concurrently, downward, and steady with no variation in their properties along time and length. The numerical results are acquired for conditions of particle diameter to the pipe radius ratio 0.176 < D < 0.65, Reynolds number ranging 10(1) < Re < 10(6), saturation ranging 0 < S < 1, and dimensionless pressure gradient up to 10(10).& nbsp; & nbsp; & nbsp; & nbsp;Numerical prediction based on this formulation have been depicted to fair agree the experimental data. The novelty of this study is utilizing a general wide model to express two-phase flow through porous media considering the influences of inertial force, Brinkman macroscopic shear friction, gravity, and porosity variation. The newly acquired findings are compared to the past studies by El-Kady (1997) and Poulikakos and Renken (1987) for single-phase flow through porous materials and satisfactory agreements are achieved.
NSTL
Elsevier