查看更多>>摘要：A Pool Boiling (PB) is crucial to safe and efficient operations in various applications such as thermoelectric power conversion, refrigeration, electronics, transportation, microfluidics, biomedical engineering, metallurgical industry, and space exploration. However, the poor counter-current flows near the heated surface limit the maximum PB cooling efficiency via a premature surface dryout, i.e., poor Critical Heat Flux (CHF) and Heat Transfer Coefficient (HTC). The two-phase flow substantially changes as the surface orientation deviates from the upward facing, leading to the CHF and HTC changes. This paper reviews the effects of surface orientation on the two-phase flows and their PB performance using various coolants for plain and engineered surfaces. This review includes both the experimental and theoretical approaches to understand HTC and CHF, including tailored two-phase flow for enhanced HTC and CHF. This review also discusses future research directions for engineered surface under different surface orientation.
查看更多>>摘要：The objective of this study is to experimentally evaluate two residential split heat pumps using low-GWP refrigerants, i.e., R452B and R454B, and compare the results against R410A. Each heat pump used the same model of two-speed scroll compressor, the same indoor blower and outdoor fan. However, they were distinguished by the indoor and outdoor heat exchanger types: tube-fin heat exchangers (TFHXs) in one case, and microchannel heat exchangers (MCHXs) in the other. The two heat pumps were extensively evaluated following the test conditions for two-speed SEER (cooling seasonal energy efficiency rating)/HSPF (heating seasonal performance factor) ratings. In general, these results show that R452B and R454B are good replacement candidates for R410A for heat pump using tube-fin heat exchangers and microchannel heat exchangers. R452B and R454B led to lower capacities. In the TFHX heat pump, on average, compared to the performance of R410A, the cooling capacity of R452B is 96%; the heating capacity of R452B is 91%; the cooling capacity of R454B is 95%, the heat capacity of R454B is 89%. In the MCHX heat pump, the cooling capacity of R452B is 97%; the heating capacity of R452B is 97%; the cooling capacity of R454B is 98%, the heat capacity of R454B is 97%. With respect to cooling and heating seasonal performances, in the TFHX heat pump, the SEERs of R452B and R454B are 1.5% lower than R410A. HSPFs of the three refrigerants are identical. In the MCHX heat pump, R454B has 7% higher SEER, and R452B has 2% higher SEER than R410A; in heating mode, R454B has the same HSPF as R410A, while R452B is 4% more efficient. The compressor lubricant overheating issue of R452B and R454B, at low ambient heating condition, high stage, should be noticed.
查看更多>>摘要：Marine diesel engines can be exposed to high backpressure conditions in the scenario of maritime, because the outlet of its exhaust manifold is normally below the sea surface to reduce the emission level. The performance of the engine has been confirmed to drastically deteriorate due to the high exhaust backpressure. The current methods for turbo-engine matching and power recovery at high backpressure conditions are specific instead of general. This article aims to understand the mechanism of influence on turbocharged engine performance by exhaust backpressure, and to develop a performance evaluation method as well as optimized methods of turbo-engine matching at high backpressure conditions. Firstly, numerical method is established for a 16-cylinder V-type turbocharged diesel engine and calibrated/validated based on experimental results. The influence of the backpressure on engine and turbocharger performance is discussed by the numerical method. The power recovery efforts are founded to be constrained by the maximum cylinder pressure and exhaust temperature. The influence of the effective flow area of the turbine and turbocharger efficiency on the operational constraints are discussed. It is concluded that the influence on intake air mass flow and fuel margin is the source of the influence on the constraints. Based on the findings, a capability region of engine power recovery is proposed, which is bounded by curves of maximum cylinder pressure and maximum exhaust temperature, respectively. Furthermore, the availability of exhaust gas is the root of influencing the boundaries of the region. According to the mechanism, a universal evaluation method based on the available energy of the exhaust gas is proposed which is tailored for turbo-engine matching at different exhaust backpressures.
查看更多>>摘要：In the current paper, we present the influence of adding high thermal conductivity materials on the thermal management performance of low-melting-point alloys in different aspects. We analyze the phase change process of 47 alloy and n-tricosane before and after compounding with copper foam based on numerical simulation. Moreover, we calculate the temperature management curve of low-melting-point alloy composite materials with different foam porosity. Our results indicate that the addition of copper foam significantly improves the thermal management performance of n-tricosane. Due to the difference in latent heat, the thermal management time of copper foam/47 alloy is 1.21 times higher than the former. The addition of copper foam mainly improves its thermal response and inhibits the temperature rise in the phase transition stage. Just 5% of copper foam has a significant effect. For copper foam/47 alloy, the higher the porosity, the stronger the temperature management ability. This feature is more prominent when the upper limit is the temperature below the melting point.
查看更多>>摘要：Thermal energy storage (TES) is an indispensable part of solar energy utilization system. This paper intends to explore the role of heat conduction, natural convection and secondary flow on the charging performance of phase change material in vertical smooth/finned-tube TES respectively. It was found that heat conduction is dominant in smooth-tube TES. The role of natural convection is to cause non-uniformity of melting rate of the lower and upper part. Meanwhile, secondary flow is dominant in finned-tube TES due to the generation of vortices between adjacent fins. Fin material has little effect on the melting characteristic of phase change material when fin pitch is larger than 40 mm in this study. Therefore, nonmetallic fin and cheap metallic fin could be used in finned-tube TES to increase thermal storage density and reduce cost of investment. Compared to conduction model, the total melting time was reduced by 9.7% and 34.2% in smooth-tube TES and aluminum finned-tube TES when convection model is considered in the computation. Besides, the heat conduction intensity and secondary flow intensity were explored in aluminum finned-tube TES. The heat conduction enhancement rate φ was defined to characterize the heat conduction intensity. With the fin pitch decreasing from 40 mm to 8 mm, the heat conduction enhancement rate increases from 15.2% to 39.6%, indicating that heat conduction plays a crucial role in the finned-tube TES. The aluminum finned TES exhibit the best thermal storage characteristic as a result of the best heat transfer enhancement and smaller density.
查看更多>>摘要：Supersonic ejectors have many applications across engineering, but their broader utilization is prevented by their generally low efficiency, which is related to a lack of understanding of the processes within them. This paper presents experimental and numerical analyses of the choking phenomenon within an axisymmetric supersonic air ejector with an adjustable motive nozzle. The effects of the operating and geometric conditions on the flow within the ejector were studied in detail. Five motive nozzle configurations were considered, and the results showed that the specific nozzle position (xp) could be changed to influence the choking of the secondary flow and hence achieve different working regimes while keeping the operating conditions constant. At xp = 1 or 2 mm, the secondary flow reached sonic conditions before even entering the mixing chamber and therefore before mixing with the primary flow. At xp = 3–5 mm, choking occurred within the mixing chamber, and two different choking mechanisms were identified. These results may serve as background for better understanding of flow choking in supersonic ejectors.
查看更多>>摘要：Elastocaloric cooling, also known as thermoelastic cooling, has been identified as a promising alternative to current state-of-the-art vapour compression cooling systems which tend to use environmentally unfriendly refrigerants. In this study, five different types of rubbers including natural rubber, silicone rubber, cis-butadiene rubber, styrene-butadiene rubber and chlorosulfonated polyethylene were investigated as working materials for thermoelastic cooling applications. A setup and protocol was developed to investigate and quantify the cooling effect of the rubbers during stretching and releasing cycles. The results show that for all five rubbers, the mechanical response of the stress–strain curve depends on the maximum loading. This is previously studied and named as Mullins effect. Such effect was negated after nine cycles of loading andunloading. The Coefficient Of Performance (COP) for the studied materials were quantified to compared to the performance of the investigated working materials. The natural rubber was found to be the working material with the best performance worth further exploration with a COP of more than 2. The cycle analysis of this material shows that the temperature difference of such system is around 7 K which shows a great potential for applications in cooling systems.
查看更多>>摘要：An artificial neural network (ANN) model for a steam-centered ejector was established and the effect of different training algorithms on the prediction effectiveness of the ANN model was discussed, which found that the ANN model produces better results than the conventional thermodynamic model on the fitting and prediction of experimental data. The Levenberg-Marquardt(LM) trained model yielded the best results among three chosen ANN models, with the experimental accordance improvement of 68% and the prediction error within 15% under given operating conditions. The LM model made the prediction for a steam ejector in a certain system that the outlet area ratio exhibits a smaller effect on the system operation, compared with the entrainment ratio and throat area ratio, which assists to optimize system design and maintain operation stability.
查看更多>>摘要：In this study, multiple regression and artificial neural network methods were applied to desiccant wheels to calculate the dimensionless outlet temperatures and humidity ratios of the processed air (Apro) and regeneration air (Areg). First, coupled heat and mass transfer equations expressing the relationship between the air and desiccant wheel were nondimensionalized to obtain seven criterion numbers, including three basic criterion numbers. The three basic criterion numbers were related to the structural and operating parameters of desiccant wheels and the physical parameters of the air and desiccants. Subsequently, the value ranges of the three basic criterion numbers were obtained based on the ranges of the above parameters. Based on a wide range of air inlet conditions and value ranges of the three basic criterion numbers, 2500 cases were designed. The nondimensionalized equations were used to calculate the dimensionless outlet temperatures and humidity ratios of Apro and Areg for the 2500 cases. Next, a multiple regression method was used to regress formulas for a fixed air inlet condition based on the original calculated results. The formulas were related to the three basic criterion numbers and could be used to calculate the dimensionless outlet temperatures and humidity ratios of Apro and Areg. Compared with the original data, the prediction errors of the formulas ranged from ±5% to ±15% for the analyzed working conditions. Finally, based on the 2500 original data groups, the backpropagation neural network was used to predict the dimensionless outlet states of Apro and Areg, with the three basic criterion numbers and air inlet conditions as the inputs. Compared with the original data, the prediction errors of the four dimensionless outputs mainly ranged from ±5% to ±10%. The difference between the two prediction methods was within ±10%.
查看更多>>摘要：Thermal stratification is the main reason for thermal fatigue failure in the rectangular T-junction. An impeller is set in the mixing zone in the rectangular T-junction to improve the mixing behaviour of hot and cold fluids under the deflecting jet with the inflow momentum ratio of MR = 0.49. Blade numbers and blade diameters are investigated for the range of Np = 2 ～ 4 and Dp* = 0.33 ～ 0.8, respectively. By the application of large-eddy simulation, the flow fields and temperature fields are obtained in this work. The flow fields show the mixing behaviour between hot and cold fluids can be promoted by setting the impeller with a constant rotation speed, and three flow patterns including the wall jet, deflecting jet and impinging jet will be observed in the T-junction, which is effective for breaking the thermal stratification. Temperature fluctuation and temperature gradient calculated by mean temperature are compared with that of no impeller. It is found that by adding the impeller the temperature gradient is reduced while the temperature fluctuations almost keep the same. The criterion of mixing length is proposed according to the mass mean temperature and the mixing lengths for different conditions are obtained, as well as the temperature uniformity parameter is proposed to evaluate the mixing degree in the T-junction. The results indicate that the relationship between the mixing length and blade diameter is not monotonic and the optimal operating point is around Dp* = 0.5 for the impeller of 2 blades. By combining the temperature fields and pressure drops, the results indicate among Np = 2 ～ 4, the impeller of 3 blades is the best choice. Temperature power spectrum density is also investigated and it is possible to control the critical frequency of temperature fluctuation in the duct by changing the blade numbers.
NSTL
Elsevier