Sept. 13, 2019
Springer Link: https://link.springer.com/journal/11630/28/4
1. Review of the Working Fluid Thermal Stability for Organic Rankine Cycles
DAI Xiaoye, SHI Lin, QIAN Weizhong
Journal of Thermal Science, 2019, 28(4): 597-607
Keywords: organic Rankine cycle (ORC), working fluid, thermal stability
Abstract: The organic Rankine cycle (ORC) is an efficient power generation technology and has been widely used for renewable energy utilization and industrial waste heat recovery. Thermal stability is a significant property of ORC working fluids and is the primary limitation for working fluid selection and system design. This paper presents a review of the working fluid thermal stability for ORCs, including an analysis of the main theoretical method for thermal stability, a summary of the main experimental method for thermal stability, a summary of the decomposition experimental results for working fluids, and a discussion of the decomposition influence on ORC systems. Further research trends of thermal stability are also discussed in this paper.
2. Study on Unstable Characteristics of Centrifugal Pump under Different Cavitation Stages
DONG Liang, SHANG Huanhuan, ZHAO Yuqi, LIU Houlin, DAI Cui, WANG Ying
Journal of Thermal Science, 2019, 28(4): 608-620
Keywords: centrifugal pump, cavitation development stage, cavitation bubble distribution, pressure pulsation, radial force
Abstract: In order to reveal the regularity of unsteady flow of centrifugal pump under different cavitation stages, avisual closed test-bed is built to collect signals such as the distribution of cavitation bubbles at the impeller inlet and external characteristics, etc. in the process of cavitation of centrifugal pumps. Combined with the shape and distribution of bubbles captured by high-speed photography, the cavitation stage of the centrifugal pump is divided. In addition, the variation of vorticity distribution, pressure pulsation and radial force of centrifugal pump under different cavitation stages are studied using the standard k-ε turbulence model and the Kunz cavitation model. Main contributions are as follows: The cavitation bubbles can absorb the energy of vortex core to a certain extent and increase the volume of vortex core. Cavitation bubbles can also block the flow-path and induce the distortion of the internal flow field, resulting in unstable pressure waves that cause a significant increase inpressure pulsation rate. Besides, with the development of cavitation, the radial force on the impeller tends to remain invariable first and then decrease, and trajectory of the radial force changes from closed to open.
3. Performance Analysis of a Multistage Centrifugal Pump Used in an Organic Rankine Cycle (ORC) System under Various Condensation Conditions
YANG Yuxin, ZHANG Hongguang, TIAN Guohong, XU Yonghong, WANG Chongyao, GAO Jianbing
Journal of Thermal Science, 2019, 28(4): 621-634
Keywords: waste heat recovery, organic Rankine cycle, multistage centrifugal pump, operating characteristics, various condensation conditions, backwork ratio (BWR)
Abstract: In an organic Rankine cycle (ORC) system, the working fluid pump plays an important role in the system performance. This paper focused on the operating characteristics of a multistage centrifugal pump at various speeds and condensation conditions. The experimental investigation was carried out to assess the influence of the performance of the pump by the ORC system with special attention to actual net power output, thermal efficiency as well as back work ratio (BWR). The results showed that an increase in the pump speed led to an increase in the mass flow rate and expand in the operating range of the outlet pressure. The mass flow rate decreased nonlinearly with the increase of the outlet pressure from 0.22 to 2.41 MPa; the electric power consumption changed between 151.54 and 2409.34 W and the mechanical efficiency of the pump changed from 7.90% to 61.88% when the pump speed varied from 1160 to 2900 r/min. Furthermore, at lower pump specific speed the ORC system achieved higher thermal efficiency, which suggested that an ultra-low specific speed pump was a promising candidate for an ORC system. The results also suggested that the effects of condensation conditions on the pump performance decreased with the pump speed increasing and BWR was relatively sensitive to the condensation conditions, especially at low pump speed.
4. Development of a New-Type Multiple-Source Heat Pump with Two-Stage Compression
ZHONG Xiaohui, YANG Hailong, YANG Ke, XU Jianzhong
Journal of Thermal Science, 2019, 28(4): 635-642
Keywords: multiple-source, heat pump, two-stage compression, solar collector, simulation
Abstract: A new-type multiple-source heat pump cycle with two-stage compression was established on the basis of the problems of similarly existing heat pumps. The equivalent temperature levels of typical evaporators are applied to the different heat sources of the proposed cycle, and the high-temperature heat sources are shown to enhance vapor injection. Then, the mathematical model and prototype are developed, and the results from experimental simulation and validation showed that the solar collector can improve the heating performance of the proposed heat pump system. In the middle-temperature heating period, the outdoor temperature is less than ？25°C, and the average coefficient of performance (COP) value of the proposed heat pump was 4.2, which was greater than the COPs of conventional ground source heat pumps.
5. Selection and Evaluation of Dry and Isentropic Organic Working Fluids Used in Organic Rankine Cycle Based on the Turning Point on Their Saturated Vapor Curves
ZHANG Xinxin, ZHANG Congtian, HE Maogang, WANG Jingfu
Journal of Thermal Science, 2019, 28(4): 643-658
Keywords: dry fluid, isentropic fluid, turning point, saturated vapor curve, near-criticalregion triangle, Organic Rankine Cycle
Abstract: The organic Rankine cycle (ORC) is a popular technique used in the utilization of low-grade thermal energy. Among wet, dry, and isentropic organic working fluids, the latter two types are more appropriate for ORC systems. In this paper, the definition of turning point on saturated vapor curve of dry fluid and isentropic fluid was given according to the shape of the saturated curve of working fluids in a T-s diagram. On this basis, the model of near-critical region triangle was established. Using this model, the thermodynamic performance of 57 kinds of dry and isentropic organic working fluids in ORC was evaluated. The performance includes the relation between turning point temperature and cycle thermal efficiency, the relation between near-critical region triangle area and cycle thermal efficiency, the relation between near-critical region triangle area and exergy at turning point temperature, the relation between near-critical region triangle area and reciprocal value of slope of saturated vapor curve. Moreover, working fluid selection was also conducted in terms of heat source type. It was found through theoretical analysis results that the popular R123 is an acceptable choice especially for the utilization of closed type heat source. Considering it will be phased out in near future, then cis-butene, butane, trans-butene, and isobutene are worth studying as its successor. Dodecane is worthy of attention and further research and it can be a good choice for utilization of open type heat source.
6. Thermodynamic Analysis and Optimization of an Irreversible Maisotsenko-Diesel Cycle
ZHU Fuli, CHEN Lingen, WANG Wenhua
Journal of Thermal Science, 2019, 28(4): 659-668
Keywords: Maisotsenko-Diesel cycle, finite-time thermodynamics, power output, thermal efficiency, performance optimization
Abstract: So far, Maisotsenko cycle has been applied to many fields such as heating ventilation and air-conditioning, power industry, chemical production, and so on. A lot of researches about classical thermodynamic analyses of Maisotsenko cycle have been made. A new cycle model of combined Diesel and Maisotsenko cycles considering heat transfer loss (HTL), piston friction loss (PFL) and internal irreversible loss (IIL) was proposed in this paper. By using the finite time thermodynamic (FTT) theory, the power and efficiency performances of the Maisotsenko-Diesel cycle (MDC) were studied. Effects of mass flow rate (MFR) of water injection in the Maisotsenko air saturator (MAS) and the other parameters related to the design of Diesel engine on the optimal cycle performances were analyzed. Furthermore, it was testified that irreversible MDC was superior than conventional irreversible Diesel cycle in both power output and thermal efficiency. The results can expand the application of Maisotsenko cycle (M-cycle) and provide some theoretical guidelines for the practical devices.
7. Numerical Investigation of the Effect of Hydrogen Enrichment on an Opposed-Piston Compression Ignition Diesel Engine
ZHOU Jianhao, SHENG Xueshuang, HE Longqiang
Journal of Thermal Science, 2019, 28(4): 669-681
Keywords: two-stroke, opposed-piston compression ignition, hydrogen, combustion
Abstract: High power-to-weight and fuel efficiency are bounded with opposed-piston compression ignition (OPCI) engine, which makes it ideal in certain applications. In the present study, a dynamic three-dimensional CFD model was established to numerically investigate the combustion process and emission formation of a model OPCI engine with hydrogen enrichment. The simulation results indicated that a small amount of hydrogen was efficient to improve the indicated power owing to the increased in-cylinder pressure. Hydrogen tended to increase the ignition delay of diesel fuel due to both dilution and chemical effect. The burning rate of diesel fuel was apparently accelerated when mixing with hydrogen and premixed combustion became dominated. NOx increased sharply while soot was sufficiently suppressed due to the increase of in-cylinder temperature. Preliminary modifications on diesel injection strategy including injection timing and injection pressure were conducted. It was notable that excessive delayed injection timing could reduce NOx emission but deteriorate the indicated power which was mainly attributed to the evident decline of hydrogen combustion efficiency. This side effect could be mitigated by increasing the diesel injection pressure. Appropriate delay of injection coupled with high injection pressure was suggested to deal with trade-offs among NOx, soot and engine power.
8. Experimental Study on Thermal Balance of Regulated Two-Stage Turbocharged Diesel Engine at Variable Altitudes
LIU Ruilin, YANG Chunhao, ZHANG Zhongjie, JIAO Yufei, ZHOU Guangmeng
Journal of Thermal Science, 2019, 28(4): 682-694
Keywords: regulated two-stage turbocharged, diesel engine, VGT vane opening, variable altitudes, thermal balance
Abstract: It is significant to study thermal balance of diesel engine under different variable geometry turbocharger (VGT) vane openings at variable altitudes, which is helpful to assess the heat distribution, control the heat load and improve the heat efficiency of the diesel engine. A thermal balance test system was built to study the influence of the VGT vane opening angles on a regulated two-stage turbocharged (RTST) diesel engine’s thermal balance performance. The experiment was conducted under full load operating conditions at different altitudes (0 m, 3500 m and 5500 m). Results indicated that the heat load of engine increased and the thermal efficiency decreased with the increase of altitudes under all operating conditions. As the VGT vane openings increased, the exhaust and maximum combustion temperature increased, while the maximum cylinder combustion pressure decreased. Inparticular, the maximum combustion temperature was more than 2000 K when the VGT vane openings were greater than 70% at the altitude of 5500 m, and the maximum combustion pressure exceeded 17 MPa when the opening of VGT vane was 70% at 0 m. The thermal efficiency of the engine decreased with the increase of VGT vane openings at the altitudes of 0 m and 5500 m, but the thermal efficiency increased and then decreased at the altitude of 3500 m. It was finally obtained that the best openings of VGT vane was 80%, 60% and 50% under the engine speed of 2100 r/min at 0 m, 3500 m and 5500 m, respectively.
9. Thermodynamic Analysis of a Modified Ejector-Expansion Refrigeration Cycle with Hot Vapor Bypass
LI Yunxiang, YU Jianlin
Journal of Thermal Science, 2019, 28(4): 695-704
Keywords: compressor pressure ratio, discharge temperature, ejector, refrigeration cycle, vapor bypass
Abstract: In this study, a modified ejector-expansion refrigeration cycle (MERC) is proposed for applications in small refrigeration units. A vapor bypass circuit is introduced into the standard ejector expansion refrigeration cycle (ERC) for increasing the ejector pressure lift ratio, thereby lowering the compressor pressure ratio in the MERC. A mathematical model has been established to evaluate the performances of MERC. Analysis results indicate that since a two phase vapor-liquid stream is used to drive the ejector in the MERC, a larger ejector pressure lift ratio can be achieved. Thus, the compressor pressure ratio decreases by 21.1% and the discharge temperature reduces from 93.6°C to 82.1°C at the evaporating temperature of -55°C when the vapor quality of two phase vapor-liquid stream increases from 0 to 0.2. In addition, the results show that the higher ejector component efficiencies are effective to reduce the compressor pressure ratio and the discharge temperature. Actually, the discharge temperature reduces from 91.4°C to 82.1°C with the ejector component efficiencies increasing from 0.75 to 0.85 at the two phase stream vapor quality of 0.2. Overall, the proposed cycle is found to be feasible in lower evaporating temperature cases.
10. Analyses of an Improved Double-Absorber Absorption Refrigeration System at Low Temperatures
HE Yijian, CHEN Guangming
Journal of Thermal Science, 2019, 28(4): 705-713
Keywords: absorption refrigeration, auto-cascade refrigeration, low refrigeration temperature, double-absorber
Abstract: An auto-cascade absorption refrigeration (ACAR) system could achieve a ？60°C refrigeration temperature by low-grade heat. For an ACAR system, its performance is mainly affected by energy and mass coupling of the auto-cascade processes. Anovel ACAR system with double-absorber was proposed to get higher-efficient refrigeration as low as ？60°C in this context, which used R23-R134a-DMF (N,N-Dimethylformamide) as its working fluids. Theoretical calculation and analyses were conducted under different working conditions. From the calculated results, the new system gained a COP value 20% higher than that of an ACAR system with single-absorber under the same generating, condensing, absorbing and refrigerating temperatures. Compositions of a refrigerant mixture showed key influences on energy and mass coupling ofthe auto-cascade processes, and an optimal composition of the mixed refrigerants was obtained for the new ACAR system. In addition, it was clearly found that absorbing processes of the new system had great effects on energy and mass coupling of the auto-cascade processes. Based on the difference of absorbing characteristics among R23, R134a and DMF, the absorbing processes were intensified under the different absorbing pressures. As a result, an optimal matching pressure was obtained for the new ACAR system. Energy and mass coupling of the auto-cascade processes were further optimized, and the highest COP value was obtained. The theoretical analyses showed that performance of the innovative ACAR system could be superior to that of an ACAR system with single-absorber at a refrigeration temperature from ？55°C to ？60°C.
11. Investigation on Precooling Effects of 4 K Stirling-Type Pulse Tube Cryocoolers
CAO Qiang, LI Zimu, LUAN Mingkai, SUN Zheng, TANG Xiao, LI Peng, JIANG Zhenhua, WEI Li
Journal of Thermal Science, 2019, 28(4): 714-726
Keywords: stirling-type pulse tube cryocooler, precooling effects, liquid-helium temperatures
Abstract: Stirling-type pulse tube cryocoolers (SPTCs) working at liquid-helium temperatures are appealing inspace applications because of their promising advantages such as high reliability, compactness, etc. Worldwide efforts have been put in to develop SPTCs operating at liquid-helium temperatures especially with helium-4 as the working fluid. Staged structure is essential to reach such low temperatures. Generally, both the regenerator of the last section and the pulse tube together with the phase shifter are precooled by its upper stage or by external cold source to a low temperature of around 20 K. However, the precooling effects on the regenerator and the pulse tube are synthetic in previous studies, and their independent effects have not been studied clearly. In this manuscript, the precooling effects on the regenerator and on the pulse tube together with the phase shifter are tested independently on aunique-designed precooled SPTC. The tested precooling temperature is between 13.3 K and 22 K, and the no-load refrigeration temperature gets down to 3.6 K. Further analyses and numerical calculations have been carried out. It is found that the influence on there generator is remarkable, which is different from previous conclusions. It is also found that the precooling effects on the pulse tube are relatively weak because of the large pressure-induced enthalpy flow of a real gas working at the temperatures near to the critical point. Furthermore, the phase shifting capacity is analyzed with two cases and with both helium-4 and helium-3 asworking fluids, and it keeps quite constant after optimizing the frequency and the precooling temperature for each case. The investigation on these independent effects will provide valid reference on the precooling mechanism study of SPTCs working down to liquid-helium temperatures.
12. Experimental Investigation on Ejector Performance Using R134a as Refrigerant
DAI Zhengshu, YU Bo, LIU Pengpeng, CHEN Guangming, ZHANG Hua
Journal of Thermal Science, 2019, 28(4): 727-735
Keywords: ejector performance, refrigeration, R134a, experiment
Abstract: Ejector refrigeration has the advantage of low capital cost, simple design, reliable operation, long lifespan and almost no maintenance. The only weakness is the low efficiency and its intolerance to deviations from design operation condition. R134a used inejector refrigeration system gives better performance in comparison with many other environmental friendly refrigerants as the generation temperature is from 75°C to 80°C. The present work experimentally investigated the on-design and off-design performance of the ejector with fixed geometry using R134a as refrigerant, and cycle performance of the ejector refrigeration system. The experimental prototype was constructed and the effects of primary flow inlet pressure, secondary flow inlet pressure and ejector back pressure on ejector performance and cycle performance were investigated respectively. The operation conditions are: primary flow inlet pressure from 2.2 MPa to 3.25 MPa, secondary flow inlet pressure from 0.36 MPa to 0.51 MPa, ejector back pressure from 0.45 MPa to 0.67MPa. Conclusions were drawn from the experimental results, and the experimental data can be used for validation of theoretical model for both critical and subcritical mode.
13. Research on the Heat Transfer Characteristics of a Loop Heat Pipe Used as Mainline Heat Transfer Mode for Spacecraft
WANG Lu, MIAO Jianyin, GONG Mingming, ZHOU Qiang, LIU Chang, ZHANG Hongxing, FAN Hanlin
Journal of Thermal Science, 2019, 28(4): 736-744
Keywords: LHP, mainline heat transfer mode, heat transfer characteristics
Abstract: An experimental research is conducted on the heat transfer characteristics of a loop heat pipe (LHP) used in the “mainline” heat transfer mode for spacecraft platform thermal control. The heat from multiple instruments scattered in different locations is collected by thermal control techniques such as axially grooved heat pipes and then transmitted to the radiant surface for dissipation through the LHP in an unified way. The research contents include the start-up characteristics, the operational stability characteristics, the operational blocking characteristics, the continuous blocking characteristics, the heat transfer capability, the thermal resistance, and the dynamic response characteristics under the change of the heat sink temperature. The results show that the higher the auxiliary starting power is, the easier it is to start the LHP; the higher the input power of the thermoelectric cooler is, the more beneficial it is to speed up the stabilization of the vapor-liquid interface in the condenser; the higher the blocking power, the shorter the blocking time of the LHP; the LHP can be operated stably within the heat sink temperature alteration process; the heat transfer ability is higher than 500 W with a systematic thermal resistance of 0.037°C/W.
14. Thermal Conductivity of Low Density Polyethylene Foams Part I: Comprehensive Study of Theoretical Models
REZGAR Hasanzadeh, TAHER Azdast, ALI Doniavi, RICHARD Eungkee Lee
Journal of Thermal Science, 2019, 28(4): 745-754
Keywords: thermal conductivity, polymeric foams, theoretical models, radiation
Abstract: Polymeric foams are one of the most applicable thermal-insulation materials due to their low thermal conductivity, high mechanical properties,and low cost. Optimization of thermal-insulation performance of polymeric foams needs a theoretical model in order to predict the overall thermal conductivity. So far, several theoretical approaches are presented in this regard but to the best knowledge of the authors, there is no comprehensive investigation on comparing the proposed models. Therefore, the study of validity of the theoretical models in comparison with the experimental results is one of themain goals of the present study. Low density polyethylene (LDPE) foams are selected as the case study due to the wide application range. Different models to predict the overall conductivity of the foam based on conduction through the combined gas and solid phases (lgs) as well as radiation thermal conductivity (lr) are presented. The results indicate that the best model is a model in which lgs is calculated using Gibson and Ashby model and lr is obtained using Williams and Aldao model based on the root mean square (RMS) parameter. The results show that the theoretical error of this model is smaller than 10%.
15. Influence of the Addition of Cotton Stalk during Co-pyrolysis with Sewage Sludge on the Properties, Surface Characteristics, and Ecological Risks of Biochars
WANG Zhipu, WANG Jian, XIE Like, ZHU Henan, SHU Xinqian
Journal of Thermal Science, 2019, 28(4): 755-762
Keywords: co-pyrolysis, sewage sludge, cotton stalk, biochar
Abstract: Sewage sludge produced by municipal sewage treatment plants can potentially be used as a biomass energy source because of its high organic content. Presently, the conversion and utilization of rapidly growing amounts of sewage sludge represent an urgent challenge in China. Thermal conversion of sewage sludge to biochar through pyrolysis is a promising solution to this problem. However, biochar produced by pyrolysis of sewage sludge alone has apoor pore structure as a result of its low C content and high ash content. Furthermore,it is enriched in heavy metals that may pose high ecological risks. In this study, we addressed these issues through co-pyrolysis of sewage sludge and cotton stalks (1:1, wt./wt.) at different pyrolysis temperatures ranging from 350°C to 750°C. The properties and surface characteristics of the biochars were investigated. Meanwhile, the transformation behavior of heavy metals during theco-pyrolysis process was studied, and the potential ecological risks of heavy metals in biochars were assessed. The results showed that elevated pyrolysis temperatures reduced the biochar yield and C content of the biochars, where as such temperatures increased the pH value and ash content of the biochars. The biochars prepared at different pyrolysis temperatures were all mesoporous materials. The elevated temperatures promoted the transformation of heavy metals from mobile fractions to stable ones, thus resulting in a significant decrease in the ecological risks. In summary, co-pyrolysis of sewage sludge with cotton stalks proved to be a feasible method for the conversion and utilization of sewage sludge.
16. Insights into Pyrolysis of Nano-Polystyrene Particles: Thermochemical Behaviors and Kinetics Analysis
DING Li, ZHAO Jianping, PAN Yong, GUAN Jin, JIANG Juncheng, WANG Qingsheng
Journal of Thermal Science, 2019, 28(4): 763-771
Keywords: nano-polystyrene, thermal decomposition, kinetics, mechanism
Abstract: The thermal degradation kinetics of nano-polystyrene particles with diameters of 60, 90, 160, and 225 nm were investigated in nitrogen atmosphere using thermogravimetric analysis (TGA). Various kinetic models were employed to determine the thermal degradation mechanism and kinetics. Nano-polystyrene particles have relatively lower thermal stability when compared to micro-polystyrene. Both differential thermo–gravimetric (DTG) data and apparent activation energies indicate that the thermal degradation of nano-polystyrene particles at 60 nm is a two-step reaction process where the second step plays a dominant role, while nano-polystyrene particles with diameter greater than 60 nm exhibit single-step degradation. Similar to most micro/macro polystyrene particles, DTG peaks of nano-polystyrene particles shift towards higher temperatures with increasing heating rates. Thermal degradation of nano-polystyrene particles under nitrogen atmosphere follows the first-order reaction model. However, the apparent activation energies increase (162-181 kJ·mol–1) with the increase of particle sizes (60-225 nm). This study could provide some insights into pyrolysis of nano-polystyrene particles and a safer process of manufacturing, storage and handling of nano-polystyrene particles.
17. Preheating Combustion Characteristics of Ultra-low Volatile Carbon-based Fuel
ZHU Shujun, LYU Qinggang, ZHU Jianguo, LI Jiarong, MAN Chengbo, LIU Wen
Journal of Thermal Science, 2019, 28(4): 772-779
Keywords: preheating combustion, circulating fluidized bed, ultra-low volatile carbon-based fuel, air-staging, NOx emissions
Abstract: Pulverized coal combustion technology with preheating solid fuel in a circulating fluidized bed was used for the combustion test of ultra-low volatile carbon-based fuel. This paper first validated the feasibility and advantages of applying the combustion technology to this kind of fuel. The carbon-based fuel could achieve a stable preheating process in this test system. After the preheating, the apparent sensible heat of the fuel was significantly increased. This provided a necessary condition for the stable ignition and efficient combustion of the carbon-based fuel in the post-combustion chamber. The relative proportions of CO, H2, and CH4 in preheated coal gas were very low, and the effect of high-temperature coal gas at the entrance of the post-combustion chamber was greatly impaired, indicating that the combustion process in post-combustion chamber was mainly the combustion of preheated char. At the same time, the strong reducing atmosphere in the circulating fluidized bed also facilitated the reduction of fuel-nitrogen into N2, which resulted in low NOx emissions. On this basis, with the combination of preheating combustion technology and air-staging combustion technology, the NOx emissions had drastically decreased when the burnout air distribution position moved down or varied from a single-layer distribution to a multi-layer distribution system. The lowest original NOx emissions were 90.6 mg/m3 (at 6% O2), and the combustion efficiency exceeded 97%, which ultimately achieved efficient and clean combustion of ultra-low volatile carbon-based fuel.
18. Numerical Investigation on Porous Media Quenching Behaviors of Premixed Deflagrating Flame using RANS/LES Model
WEN Xiaoping, SU Tengfei, LIU Zhigang, XIE Maozhao, WANG Fahui, LIU Zhichao
Journal of Thermal Science, 2019, 28(4): 780-788
Keywords: RANS/LES model, porous media, quenching, flame propagation
Abstract: To understand the mechanism of premixed flame quenching by porous media, a zonal hybrid RANS/LES model was employed, in which the LES flow solver was used to resolve the large turbulent structures within the non-porous region, while RANS was applied to porous media zone. The predicted results were compared with previous experimental data. And it was evident that the premixed flame propagation rates and structure as well as quenching behaviors were reproduced by this numerical approach with a better accuracy. Due to the inherently higher heat transfer coefficients between the solid and gas phases in porous media, the gas phase temperature has been decreased rapidly. However, upstream obstacles can cause the flame propagating faster and thus reduce the axial gas temperature gradients, resulting in the invalidity of the operation of premixed flame quenching. By comparison with the case without upstream obstacle, the values of reaction rate attained in the case with three pairs of obstacles are higher, which makes a positive impact on the flame passing through the porous medium. In addition, the porous media with greater pore density has an excellent flame quenching property mainly owing to both the larger volumetric heat transfer and higher quenching temperature.
19. Experimental Investigation for NOx Emission Reduction in Hydrogen Fueled Spark Ignition Engine Using Spark Timing Retardation, Exhaust Gas Recirculation and Water Injection Techniques
JEERAGAL Ramesh, K. A. Subramanian
Journal of Thermal Science, 2019, 28(4): 789-800
Keywords: hydrogen, spark ignition engine, spark timing, exhaust gas recirculation, NOx emission, water injection
Abstract: The experimental tests were carried out on a single cylinder hydrogen fueled spark ignition (SI) generator set with different spark timings (4-20°CA bTDC), exhaust gas recirculation (EGR) up to 28% by volume and water injection up to 1.95 kg/h (maximum water to fuel mass ratio of 8:1). The engine speed was kept constant of 3000 r/min. The NOx emission and thermal efficiency of engine with gasoline and hydrogen fuel operation at 1.4 kW power output are 5 g/kWh and 12.1 g/kWh, and 15% and 20.9% respectively. In order to reduce the NOx emission at source level, retarding spark timing, exhaust gas recirculation (EGR), and water injection techniques were studied. NOx emission decreased with spark timing retardation, EGR, and water injection. NOx emission with hydrogen at 1.4 kW power output decreased from 12.1 g/kWh with maximum brake torque (MBT) spark timing (10°CA bTDC) to 8.1 g/kWh with retarded sparktiming (4°CA bTDC) due to decrease in the in-cylinder peak pressure and temperature. The NOx emission decreased to 6.1 g/kWh with 20% EGR due to thermal and chemical dilution effect. However, thermal efficiency decreased about 33% and 17% with spark timing retardation and 20% EGR respectively as compared to that of MBT spark timing. But, in the case of water injection, the NOx emission decreased significantly without affecting the thermal efficiency of the engine and it is 5.6 g/kWh with water-hydrogen ratio of 4:1 (water flow rate of 0.92 kg/h). Water injection is the best suitable method to reduce the NOx emission in a hydrogen fueled engine compared with the spark timing retardation and EGR technique.
20. Effects of End-Bend and Curved Blades on the Flow Field and Loss of a Compressor Linear Cascade in the Design Condition
KAN Xiaoxu, WU Wanyang, YANG Ling, ZHONG Jingjun
Journal of Thermal Science, 2019, 28(4): 801-810
Keywords: compressor linear cascade, flow loss, vortex structure, curved blade, end-bend blade, design condition
Abstract: Curved blade and end-bend blade are considered as effective passive control methods for improving the aerodynamic performance of a compressor cascade, and their applicability also needs to be further studied. A highly-loaded compressor linear cascade in the design condition was taken as the research objective in this paper. Numerical simulation was used to study the positive and negative effects of these methods on the flow loss of the cascade by redistributing the vortex structures. Results show that: the curved blade could reduce the flow loss to 2.55%, while the end-bend blade and the end-bend+curved blade increase it to 10.58% and 2.19%, respectively. The positive effect of the curved blade weakens the strength and scale of Concentrated Shedding Vortex (CSV), which accounts for 59.2% of the total flow loss. The negative effect of the end-bend blade exacerbates the low-energy fluid clusters from the end wall and wake into CSV. Finally, the end-bend+curved blade can take their own strengths, but it enhances the twisting motion of Passage Vortex (PV) and CSV, which makes it fail to reduce the flow loss. Therefore, the positive and negative effects of these methods on the flow loss of a compressor cascade are much clearer. In addition, we also predict the potential danger of the end-bend blade in the negative incidence conditions and provides some suggestion for future study.
21. Numerical Investigation and Non-Axisymmetric Endwall Profiling of a Turbine Stage
REHMAN Abdul, LIU Bo
Journal of Thermal Science, 2019, 28(4): 811-825
Keywords: optimization, non-axisymmetric endwall, high pressure turbine stage, efficiency, secondary kinetic energy, steady, unsteady, off-design
Abstract: This paper presents an optimization of a high pressure turbine by constructing non-axisymmetric endwalls to the stator row and the rotor hub. The optimization was quantified by using optimization algorithms based on the multi-objective function. The objective was to increase total-to-total efficiency with the constraint on the mass flow rate equal to the design point value. In order to ensure that global optimum could be achieved, the function of parameters was first approximated through the artificial neural network, and then optimum was achieved by implementing the genetic algorithm. It was adopted through the design and optimization environment of FineTM/Design3D. Three individual treatments of the endwalls were presented. Firstly, the hub and the shroud of the stator were optimized together. Secondly, the hub of the rotor was optimized. Thirdly, the rotor hub was optimized in the presence of the optimized stator. The result of the investigation showed that the optimized shape of the endwalls can significantly help to increase the efficiency up to 0.18% with the help of a reduction of the transverse pressure gradient. The coefficient of secondary kinetic energy, entropy coefficient, spanwise mass averaged entropy were reduced. In order to investigate the periodic effects, the design of the optimized turbine under steady simulations was confirmed through unsteady simulations. The last part of the investigation made sure thatthe performance improvement remained consistent over the full operating line at off-design conditions by the implementation of non-axisymmetric endwalls.
22. Droplet Motion and Phase Change Model with Two-Way Coupling
ZHAO Fulong, LIU Qianfeng, YAN Xiao, BO Hanliang, ZENG Chen, TAN Sichao
Journal of Thermal Science, 2019, 28(4): 826-833
Keywords: droplet evaporate, heat and mass transfer, two-way coupling, local parameter
Abstract: The droplet interacts intensively with surrounding gas when moving and evaporating in the gas, of which the mutual effects of the gas and the evaporating droplet need to be taken into account. For the typical droplet model, the gas parameters are usually considered as that at infinity and the local parameter variation surrounding the droplet is neglected, consequently leading to some discrepancies. This research tries to develop a new moving droplet phase change model with two-phase coupling which characterizes the local parameter variation of gas phase surrounding the evaporating droplet. Firstly, the interaction mechanism of two phases is presented based on the droplet evaporation phenomena. Then, the droplet motion and phase change model is developed through the theoretical derivation. Subsequently, the analysis of the evaporation characteristics of the injected droplets in the hot air is conducted to simulate the operation process of the containment spray system in the nuclear power station. The numerical simulation indicates the refined droplet model is more capable for precise prediction of the situations with large quantities of evaporating droplets and with intensive interactions between two phases.