This article briefly reviews the recent works related to small-scale combustion and its potential impact into combustion science and engineering is presented. Followed by a simple description of the scale effect on combustion to highlight its “unique” feature, past related works are then summarized. The impact of heat recirculation appearing in combustion systems, which is the most prominent feature of a micro- or small-scale combustion system, is focused upon and is understood that exactly the same strategy promising better combustion performance is confirmed irrespective of flame type (either premixed or non-premixed). With respect to this, paying attention to the entire combustor design, to optimize within the target working range is a crucial matter when micro-scale combustion is adopted. Potential subjects to be covered to further promote these aspects in this field are then presented.
A finite volume based three dimensional numerical studies on the heat transfer due to natural convection inside the inclined cubical cavity filled with CNT-water nanofluid is solved by vorticity-vector potential formalism. The enclosure contains an Ahmed body at its middle with differentially heated vertical walls. The other walls are adiabatic. The effect due to Rayleigh number (103 ≤ Ra ≤ 105), volumetric fraction (0 ≤ φ ≤ 0.05) of CNT particles, angle of incidence of enclosure (0° ≤ γ ≤ 180°) and thermal conductivity ratio (0.01 ≤ Rc ≤ 100) are analyzed. The results of fluid flow with single phase model are elucidated with Particle trajectories, Velocity vectors, Iso-surfaces of temperature and Nusselt number. CNT-particles enhanced the heat transfer in all the considered cases. Maximum average Nusselt number is reported when the angle of inclination is 30° and 150°. The variation in thermal conductivity ratio has a least effect on convection.
The dissimilarity between the momentum and heat transfer due to streamwise traveling wave-like wall deformation in turbulent channel flows is investigated through direct numerical simulations. The flow rate is kept constant, and the bulk Reynolds number is Reb = 5600. A constant temperature difference condition is imposed on the channel walls. The parametric study shows that the heat transfer is enhanced when the wave travels in the upstream direction. The maximum analogy factor is found to be 1.13, i.e., 13% enhancement of heat transfer under a given pressure gradient, when the wall deformation amplitude is large and the wall deformation period is short. An analysis using the identity equations for the drag and the heat transfer with a three component decomposition reveals that the random component plays an important role in the enhancement of the heat transfer.
Three-dimensional numerical simulations are carried out to investigate the effects of film-hole arrangement and blowing ratio on the squealer tip leakage flow field and tip film-cooling performance. Six film-hole arrangements with 13 holes are designed in the current study for comparison. In type-A and type-B, the film-cooling holes are arranged in a single row, located at the middle camber line or close to the suction-side squealer. The four modified film-hole arrangements are realized by placing two rows of total 7 film-cooling holes at the leading edge (type-C, type-D, type-E and type-F) and remaining the rest film-cooling holes in a row at the middle chord zone. The results show that the leakage flow entering the tip gap from the leading edge of suction side, the leading edge of pressure side and the middle chord and trailing edge of pressure side behaves different flow feature inside the tip cavity, inducing complicated swirling flow filed. The modified film-hole arrangements yield more reasonable film coverage on the tip surface by comparing with the single row film-hole arrangement under relatively high blowing ratios. In addition, the modified film-hole arrangements also show different rules on the film-cooling effectiveness distributions over some specific surfaces, such as tip cavity bottom surface and squealer top surface, as well as PS squealer inner surface and SS squealer inner surface. Among the presented four modified film-hole arrangements, type-D and type-F gain the most favorable film-cooling improvement.
In the present work, the generalized thermoelastic interactions in a hollow cylinder with one relaxation time are considered. The modulus of elasticity are taking as function of temperature. Due to the nonlinearity of the governing equations, finite element method is adopted to solve such problem. The exact solution in the case of temperature-independent is discussed explicitly. Numerical results for the temperature distribution, displacement and radial and hoop stresses represented graphically. The accuracy of the finite element method validated by comparing between the finite element and exact solutions for temperature-independent.
A parametric investigation of secondary flow control using endwall vortex generator jet (VGJ) is performed in a high turning compressor cascade. The mechanisms for the variations of VGJ performance with different jet parameters including the pitch angle, yaw angle and jet-to-inflow total pressure ratio are discussed in detail. And then the potential of VGJ at the off-design points is further validated. The results show that the influence of VGJ on the cascade performance depends on the combination of the loss increase in the upwash region and the loss reduction in the corner region. A jet pointing upstream of the cascade or with too large jet-to-inflow total pressure ratio would enhance the mixing losses in the upwash region remarkably. Whereas a jet with negative yaw angle, too large or too small pitch angle would reduce the effects of VGJ on the endwall cross flow and the low energy fluid accumulation in the corner region due to the weakened reverse vortex. In this work, the optimal performance is obtained by the jet with a pitch angle of 20° and a yaw angle of 0°. For the off-design points, the potential of VGJ on the loss reduction is enhanced with the increase of the incidence. The loss reductions at i=-5° and i=5° are 13.2% and 32.5% respectively, whereas the beneficial effect on the flow turning decreases slightly.
The capture and utilization of CO2 is aimed at conserving fossil fuels and reducing greenhouse gas emissions by providing an alternative carbon feedstock and closing the carbon cycle. This paper combines chemical-looping combustion (CLC) and methane reforming with CO2 to accomplish CO2 capture and utilization and proposes a novel polygeneration system to produce syngas, electricity, chilled water for cooling, and hot water. In this cycle, CLC, assisted by solar thermal energy, is employed to drive turbines to produce electricity and to separate CO2 after the recovery of waste heat from high temperature gas. The reaction in the reformer of separated CO2 and methane is driven by solar thermal energy to produce syngas. A part of the produced syngas fuel is sent to CLC such that CLC and methane reforming are coupled in this polygeneration system. The thermodynamic performance of the polygeneration system, including the energy and exergy efficiencies, are analyzed and compared on the basis of design conditions. In addition, the energy utilization diagram (EUD) analysis reveals the mechanism for the improvement and enhancement of the system performance in the novel system. The results indicate that the energy and exergy efficiencies approach 71% and 56%, respectively. Meanwhile, the instantaneous solar share is approximately 46%, and the net solar-to-exergy efficiency approaches 25%. The maximum exergy destruction of the system occurred in the chemical reaction sub-system.
The concept of isothermal chambers filled with GPMs (gradient porous materials) was presented to homogenize the temperature distribution between the center and outer wall of the chambers with disc-shaped cross-section. An analytical heat transfer model was first developed to study the thermal behaviors of the GPMs chamber. Besides, numerical simulation was conducted to obtain the temperature profiles of the chamber with different GPMs. The effects of various gradient parameters on the heat resistance, temperature profiles and filling mass of the chamber were investigated in details. The results indicated that the gradient direction and gradient magnitude have a significant influence on the comprehensive thermal performances of the chamber filled with GPMs. Increasing the porosity gradient of GPMs along radius direction can effectively enhance the heat conduction. Moreover, the optimum gradient magnitude was obtained analytically to further improve the heat conduction of the GPMs chamber.
The non-axisymmetric geometry structure of the volute leads to the uneven circumferential distribution of the flow field inside centrifugal compressors. It is difficult to describe the whole flow field for few measuring points installed around the centrifugal compressor casing wall. In this paper, seventy-two static pressure measuring probes including the six circumferential position and the twelve meridional position were installed around the casing wall, and the circumferential distribution of the static pressure was compared under the different speeds at the full flow rate. Some especial circumferential distributions of the static pressure were founded through analyzing the experimental results. At the small flow rate, the circumferential distribution of the static pressure not only has the peak static pressure point induced by the volute tongue but also has the bulge phenomenon, which also appears near the design flow rate. The stall inception most likely occurs at the static pressure peak or the static pressure bulge region. In each speed, there is a flow rate corresponding to the lowest circumferential variation of the static pressure and the peak efficiency. The amplitude of the circumferential static pressure variation does not decrease with the reducing flow rate, on the contrary, the amplitude has the increasing tendency along the meridional direction. The circumferential static pressure distribution at the leading edge of the splitter blade is almost the same at different small flow rates. Meanwhile, the circumferential static pressure distribution has the two peaks phenomenon, which has the approximately 180° circumferential difference between the two peaks points.
Turbulent temperature profiles in the quasi-fully developed region of a micro-tube were obtained numerically by solving the energy equation including the substantial derivative of pressure and viscous dissipation terms for the case of constant wall temperature. The fluid was assumed to be an ideal gas with constant density over the cross-section. The turbulent velocity profile was approximated by the three-layer model of Von Kármán. The temperature profiles were compared with the laminar flow and previous numerical solutions. The total temperature was higher than the wall temperature depending on the Mach number. The static temperature in the quasi-fully developed region agrees well with the temperature results for an ideal gas flow obtained by solving the Navier-Stokes and energy equations.
The energy and power densities of lithium-ion batteries (LiBs) are bound to improve in the future. To use such high-performance battery systems as the power source of electric vehicles, the safety and the stability of these systems need to be guaranteed and abnormal heat emissions should not lead to thermal runaway. In this study, we have developed a prototype hybrid cooling system combined with a phase-change material and heat pipes to control abnormal heat emissions in LiBs. The system was built using paraffin wax as the phase change material, heat pipes, and an electric heater modeled on A4-sized laminated-type LiBs pack. We conducted some experiments using this system under conditions that would result in abnormal heating and thermal runaway. As a result, we were able to confirm that the time needed to reach temperatures leading to thermal runaway in the modeled battery pack was extended to 708 seconds by adopting our proposed cooling system, from 104 seconds in the case with no cooling device. A numerical analysis of the heat balance and thermal distribution was also calculated. The calculation results confirmed the thermal behavior in the experimental system, and we investigated the effect of the heat pipes and PCM. The issues that need to be solved to make practical use of the cooling system were also clarified by those discussions.
We propose a method for inferring the condition of factors influencing the temperature of heat-generating components in electronic equipment. Here, “factor” includes cooling components (fans, heat sinks, etc.), intake temperature, and the caloric value of the heat-generating component. The performance of these cooling components is reduced by unusual use or deterioration due to aging such as clogging or evaporation of grease. Such factors raise the temperature of heat-generating components. High temperatures of heat-generating components lead to system failure or a shortened life of the electronic equipment. Detecting decreases in the performance of cooling components allows timely provision of the required maintenance, possibly leading to increased product reliability. However, monitoring temperature alone is insufficient. The temperature of heat-generating components is also affected by factors such as intake temperature, and caloric value. In this study, we develop a method for inferring the condition of each factor influencing temperatures by using sensors installed at multiple locations. In this method, the effect of each factor on temperature depends on location, and the condition of each factor is expressed as a multivariate function using temperatures at multiple locations. The condition of each factor can then be inferred by measuring only temperatures. Furthermore, we can develop the method simply by mounting several temperature sensors on the board, making the method easy to implement. System failures can thus be prevented by performing maintenance based on the monitored factors. We conducted verification experiments using a computer to confirm the benefits of this method. In the case of inferring the conditions of four factors using four temperature sensors, the mean accuracy exceeded 90%.
Polymers containing ceramics filler are used in various fields that require good thermal conductivity and electric insulation. For material design, prediction of the thermal conductivity is thus critical. There are several prediction methods to determine the thermal conductivity of filler-dispersed composites, such as the Hatta-Taya and Bruggeman models. However, these models do not consider the effect of interaction between fillers; thus, the actual thermal conductivity is larger than the predicted thermal conductivity. In this research, the thermal conductivity of polymers containing ceramic filler was numerically simulated, and the results were compared with the values predicted using the Hatta-Taya model to investigate the effect of filler orientation angle and aspect ratio. The trend of the thermal conductivity change was similar to that observed for the Hatta-Taya model, however, the simulated thermal conductivities were 30% larger than those determined using the Hatta-Taya model for a filler thermal conductivity and volume content of 40 W/(mK) and 25%, respectively. The simulated thermal conductivity results prominently deviated from the Hatta-Taya model values when the filler content ratio was large, the filler aspect ratio was small, or the filler orientation was along the heat-transfer direction.
Water injection is a technique that has been used for decades to control the combustion and emissions in diesel engines. The effects of water injection at the intake and exhaust manifolds on the combustion and emission characteristics of a direct injection diesel engine are studied in this work. Water injection in the intake manifold increases engine heat losses. Therefore, waste heat in exhaust gases is used to vaporize water before combustion and prevent the water-cooling effect. The injection of 40 mg/cycle of water into the intake and exhaust manifolds were tested with exhaust gas recirculation (EGR) ratios of 10% and 25%. The fuel injection timing, quantity, and engine speed were maintained at constant values to keep the cylinder combustion condition constant. The results show that the exhaust manifold water injection improves engine performance and combustion characteristics and reduces emissions compared to intake manifold water injection. The peak improvement is achieved at exhaust manifold water injection at 25% EGR where the reduction in the brake specific fuel consumption (bsfc) is about 5%, while the increase in the indicated mean effective pressure (IMEP) and the indicated thermal efficiency (ITE) is by 7% and 3%, respectively. On the other hand, the maximum reduction in soot was obtained with exhaust manifold water injection at 10% EGR ratio with reduction ratios of 55%. The intake manifold water injection gives the lowest NOx emissions with a 88% reduction ratio. The exhaust manifold injection is the recommended technique for water injection to improve engine performance and reduce emissions to avoid the disadvantages of the previously applied techniques.
The effects of unburned-gas temperature on the characteristics of cellular premixed flames generated by hydrodynamic and diffusive-thermal instabilities were numerically investigated. Two-dimensional reactive flow was calculated in large space, based on the compressible Navier-Stokes equations including a one-step irreversible chemical reaction. The dynamic behavior of cellular premixed flames, i.e. the coalescence and division of cells, appeared in large space owing to intrinsic instability. The behavior of flame fronts became more unstable with a decrease in unburned-gas temperature, even though the burning velocity of a planar flame reduced. This was due to the strength of thermal-expansion effects and to the enlargement of Zeldovich numbers. We found that the average burning velocity of a cellular flame normalized by that of a planar flame increased as the unburned-gas temperature became lower and the space size became larger. To elucidate the increase of burning velocity, we proposed the new model and showed that the normalized increment factor of burning velocity became larger under low unburned-gas temperature. In addition, we performed fractal analysis to consider the fractal dimension for three-dimensional flow. The obtained fractal dimension corresponding to laminar flames was nearly identical to the experimental and numerical results of turbulent flames.
Edited and published by : The Japan Society of Mechanical Engineers and The Heat Transfer Society of Japan Produced and listed by : Showa Joho Process Co., Ltd.(Vol.8 No.3-) Sanbi Printing Co., Ltd.(Vol.1 No.1-Vol.8 No.2)