查看更多>>摘要：With the development of high-performance multi-core microprocessors, the increase in hotspots density poses a severe challenge to chip thermal management. Embedded cooling can improve the cooling performance at the chip level. However, the cooling efficiency will be deteriorated by the interaction of hotspots. This paper proposes the concept of thermal superposition effect based on the relationship between the cooling efficiency of two hotspots and their relative positions. The effective cooling area and superposition area describe the thermal characteristics and interaction of hotspots, and the superposition factor beta evaluates the intensity of interaction. By optimizing the superposition factor, the average temperature reduces 22%. Moreover, the factor can predict the average temperature of hotspots within the error under14%. Therefore, the thermal superposition effect and factor beta is an effective analysis tool in multi-hotspot cooling, simplifying the analysis process and making it possible to predict and optimize the interaction by experiment and finite element simulation.
查看更多>>摘要：Liquid air energy storage technology is a technology that stores liquid air in case of excess power supply and evaporates the stored liquid air to start a power generation cycle when there is an electric power demand. When liquid air is stored for a long-time during operation, safety and performance degradation can be caused or mitigated by the tank stratification. To investigate the tank stratification phenomenon and associated issues, an experimental facility is constructed. The heat ingress is controlled with respect to changing vacuum level in the experiment. Furthermore, the conditions under which stratification occurs are defined in terms of temperature and concentration, and based on this, the stratification stability ratio and the stability map are defined and evaluated experimentally. The results show that the time required for destratification is 8-29% shorter for liquid air mixture cases than for liquid nitrogen. Moreover, the time required for destratification is 2.4 times longer for the high tank pressure cases, and it is 39% shorter for the case of high heat ingress. From experimental observations, an operation strategy utilizing stratification inside the liquid air storage tank is newly suggested that can minimize the boil-off gas of liquid air in the tank.
查看更多>>摘要：The China VI natural gas engine is based on the theory of the air-fuel ratio method, leading to a high exhaust temperature, large heat load and difficult cooling. The paper carried out a thermal balance experiment of the China VI natural gas engine under different working conditions. A dual-loop organic Rankine cycle system is designed to reduce the difficulty of cooling system and improve the thermal efficiency. The high-temperature loop uses water as working fluid to recover the heat of engine exhaust gas and recirculation exhaust gas. The low-temperature loop recovers the heat from cooling water and waste heat of the high-temperature loop. The high-temperature and low-temperature loops are coupled through a shared heat exchanger. The results show that the China VI natural gas engine has a higher and more stable waste heat temperature than traditional engines and is more adaptable to waste heat recovery under variable operating conditions. For fixed evaporation temperature, with the increase of inlet temperature of the expander, the net output power of high-temperature loop increases first and then decreases, reaching the maximum value at about 800 K. In the low-temperature loop working fluids R601a, R601b, R600, R600a and R12333zd, The maximum net output power of the waste heat recovery system based on the combination of water and R12333zd is approximately 75.4 kW under the rated condition, which is 22.24% of the rated power of the engine.
查看更多>>摘要：Heat transfer deterioration (HTD) of supercritical CO2 heated in a tube influences the efficiency and safe operation of the system due to the occurrence of local high temperature. To suppress and delay the HTD, the characteristics and mechanisms of self-excited oscillation pulsating flow on HTD of supercritical CO2 are studied by experiment and simulation at pressure 8 MPa, mass fluxes from 350 to 800 kg/m2.s, heat fluxes from 30 to 200 kW/m2. The Helmholtz oscillator is introduced into the inlet of the vertical tube for generating a pulsating flow of supercritical CO2. The heat transfer performance is compared with that of without Helmholtz oscillator at the inlet of the tube. The results show that the self-excited oscillation pulsating flow improve the heat transfer performance significantly. The heat transfer parameters present oscillations with small amplitude along flow direction before pseudo-critical point (Tpc). The average heat transfer coefficient can be up to 3.4 times and the enhancement takes place mainly at the HTD region which is located at the entrance section of the heated tube. The effect of suppressing HTD is more significant at higher heat flux, and the peak of wall temperature can be reduced by 100 K at a heat flux of 200 kW/m2. Compared to the steady flow, the mechanism analysis of the selfexcited oscillation pulsating flow based on radial distributions of velocity, turbulent kinetic energy (TKE), and density reveals that the velocity distribution of "M- shape" appears later and gentler. The production and diffusion of TKE are improved at log layer (30 < y+ < 0.2r). In addition, the period and the amplitude do not show monotonous trends on heat transfer performance. The effects of pulsating parameters on HTD are optimized that the heat transfer performance with the period of 0.016 s and the amplitude of 100 kg/m2.s is the best in calculated cases.
查看更多>>摘要：High infrastructure expenditure and large operating costs of hydrogen refueling stations reduce the competitiveness of fuel cell vehicles over conventional fuel vehicles. The present study tends to explore the potential of reducing the precooling cost in refueling stations through a composite hydrogen refueling process integrating a turbo-expander. Accordingly, a thermodynamic model of the process following the requirements of SAE J2601 protocol is established to evaluate its feasibility. Based on the model, dynamic simulations are carried out to compare the performances of the proposed refueling process and conventional one. Obtained results reveal that applying the turbo-expander could reduce the precooling energy consumption by 52.6%. Moreover, the infrastructure expenditure of the proposed process is about 210,000$ lower than that of the conventional one. From a feasible investigation of the proposed process, it is concluded that refueling process integrating a turbo-expander has a remarkable potential in hydrogen fueling stations.
Liu, PengfeiKandasamy, RanjithHo, Jin YaoXie, Jinlong...
15页
查看更多>>摘要：An experimental study is conducted to characterize the effects of surface roughness and the combined effects of surface roughening and macro-structure topology on the spray cooling heat transfer. It was found that the spray cooling thermal performances increase as the surface roughness increases. From the experimental results, a power law relationship is established between the heat flux and the magnitude of normalized roughness. On this basis, an empirical correlation is developed for spray cooling heat transfer on structured flat surfaces with varying roughness. The correlation is found to have an accuracy of 15%. In addition, the experimental results on the straight finned surfaces are compared with a region-based model with the surface roughness effect incorporated in the developed empirical correlation. The analysis reveals a decoupling relationship between the micro-roughness and macro-structure enhancing mechanisms. Furthermore, a decoupling analysis suggests that the micro-roughness enhancement and the macro-structure enhancement dominate the 0.5 mm pin finned surface and the 1.0 mm pin finned surface, respectively. However, the two types of enhancing mechanisms have a comparable contribution to spray cooling heat transfer enhancement on the straight finned surfaces. In general, spray cooling heat transfer can be enhanced by around 116% by increasing surface micro-roughness whereas heat transfer enhancement can reach as high as 136% and 288% on the surfaces with macro straight fin and pin fin structures containing micro-roughness, respectively.
查看更多>>摘要：Falling film evaporation is a promising technology widely applied in refrigeration, desalination and beyond applications owing to the great advantages. However, the liquid film hydrodynamics and heat transfer performance are studied insufficiently. Compered with experiments, numerical simulations are economical and efficient especially in the study of liquid film, through which some microscopic understandings are expected to be extracted. In this paper, a comprehensive review for a large pool of computational studies about falling liquid film flow and heat transfer on the horizontal tube and tube bundle is presented. Computational methods of liquid-gas two-phase flow and mass transfer model, as well as relevant researches were first introduced. Then, the studies on the falling film hydrodynamics, film thickness, sensible heat transfer, flow pattern, evaporation and boiling outside the single tube, tube bundle, special-shaped tube and entire evaporator studied with 2D and 3D models are respectively reviewed in order. Then, the investigations on the falling film breakout and dryout are reviewed. And then, the numerical predictions of falling film thickness and heat transfer coefficient are involved. Afterthat, the benchmark data for falling film numerical simulation are summarized. Finally, some future needs and recommendation relating to crucial technologies that must be solved are proposed.
Daghooghi-Mobarakeh, HoomanSubramanian, VarunPhelan, Patrick E.
11页
查看更多>>摘要：The phase change process of freezing water is an important application in several fields such as ice making, food freezing technologies, pharmaceuticals, etc. Due to the widespread usage of ice-related products, process improvements in this technology can potentially lead to substantial energy savings. It is well known that supercooling has a negative effect on the overall time and energy consumption of the freezing process. Therefore, ultrasound is proposed as a technique to improve the freezing process by eliminating the supercooling effect and the resulting energy savings is investigated. An experimental study was conducted to analyze the energy expenditures in the freezing process with and without the application of ultrasound. After a set of preliminary experiments, an intermittent application of ultrasound at 3.52 W & 8.25 W power levels was found to be more effective than constant-power application. The supercooling phenomenon was thoroughly studied through iterative experiments. It was also found that the application of ultrasound during the freezing process led to the formation of shard-like ice crystals. From the intermittent ultrasound experiments performed at 3.52 W & 8.25 W power levels, energy savings relative to no-ultrasound processes of 12.4% and 10.8% were observed, respectively.
查看更多>>摘要：Adsorption cooling is a sustainable technology, since it can utilize solar energy or waste heat, while employing substances without ozone depletion and global warming potential. The adsorption reactor design is determinant for the system performance. An underexplored geometry hitherto - the hexagonal honeycomb adsorption reactor - was numerically investigated. An in-house, validated, three-dimensional computational model based on unstructured meshes was employed. The Specific Cooling Power (SCP) and Coefficient of Performance (COP) were quantified for several geometrical and operational parameters. The cell inradius creates a dichotomy between SCP and COP, being 218.9 W/kg(s) and 0.356 for 1 mm, while being 80.4 W/kg(s) and 0.606 for 6 mm. The cell height influences prominently the SCP, being 159.5 W/kg(s) and 86.1 W/kg(s) for 5 mm and 30mm, respectively. The fin thickness impacts mostly the COP, being 0.599 and 0.364 for 0.5 mm and 3 mm, respectively. Higher COP is achieved for higher evaporator, lower adsorption and lower condenser temperatures. Higher SCP is achieved for lower adsorption and condenser, and higher evaporator and desorption temperatures. Shorter cycles result in high SCP and low COP, whereas the inverse occurs for longer cycles. Aluminum heat exchanger yields 7.7% higher COP than copper. The results are discussed from a physical, as well as, an engineering perspective.
查看更多>>摘要：The present paper investigates the influence of using two grooved channel geometries on the hydrothermal performance of a helical microchannel heat sink. The results were compared with those obtained for the plain channel geometry. Based on the results, the highest flow mixing is observed for the staggered grooved configuration. Moreover, the convective heat transfer coefficient enhances by nearly 70% and 17% for the MCHS with the staggered and parallel grooved channels, respectively, as compared to that with the plain channel. The influence of Reynolds number (Re) escalation on the convective heat transfer coefficient is insignificant, while a 21% increase is obtained when NF concentration is increased from 0% to 1%. Furthermore, the increase of phi in the grooved configurations is more effective than that in the plain configuration. In addition, the lowest thermal resistance and temperature uniformity factors were obtained for the staggered grooved configuration. Besides, the parallel grooved configuration represents the highest frictional entropy generation rates among the three studied geometries due to the considerable reduction of the fluid flow cross-section area between the groove ribs. The highest heat transfer coefficient and pressure drop are associated with the staggered grooved configuration. For the plain, parallel grooved, and staggered grooved configurations, the increase of Re from 500 to 2000 escalates the NF pressure drop by 2730%, 4420%, and 4460%, respectively. Therefore, the highest performance evaluation criterion of more than unity was obtained for the MCHS with the staggered grooved channels for the studied range of Re (500-2000). The numerical analysis was also performed to evaluate the influence of groove pitch on the hydrothermal performance of the staggered groove channel for the pitch range of 0.15 mm to 0.75 mm. The results demonstrated that the highest PEC is associated with the groove pitch of 0.75 mm.
NSTL
Elsevier