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Min Zeng - One of the best experts on this subject based on the ideXlab platform.
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effect of non condensable Gas on laminar film condensation of steam in horizontal minichannels with different cross sectional shapes
International Communications in Heat and Mass Transfer, 2016Co-Authors: Yining Wu, Qiuwang Wang, Min ZengAbstract:Abstract In the present study, a 3-D numerical simulation of laminar film condensation of steam in the presence of Non-Condensable Gas is performed in horizontal minichannels with six cross-sectional shapes based on the volume of fluid (VOF) method. Mixture of steam and oxygen enters the channel with uniform temperature, while the inlet volume fraction of oxygen increases from 0% to 3%. It is shown that the existence of Non-Condensable Gas results in significant reduction in the mass transfer rate from vapor to liquid along the interface in the axial direction. And then the heat transfer coefficient of condensation with oxygen is demonstrated to decline sharply compared with that of the pure vapor condensation.
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numerical simulation of laminar film condensation in a horizontal minitube with and without non condensable Gas by the vof method
Numerical Heat Transfer Part A-applications, 2015Co-Authors: Bengt Sundén, Qiuwang Wang, Min ZengAbstract:Based on the volume of fluid (VOF) method, a steady three-dimensional numerical simulation of laminar film condensation of water vapor in a horizontal minitube, with and without Non-Condensable Gas, has been conducted. A user-defined function defining the phase change is interpreted and the interface temperature is correspondingly assumed to be the saturation temperature. An annular flow pattern is to be expected according to a generally accepted flow regime map. The heat-transfer coefficient increases with higher saturation temperature and a smaller temperature difference between the saturation and wall temperatures, but varies little with different mass flux and degree of superheat. The existence of a Non-Condensable Gas will lead to the generation of a Gas layer between vapor and liquid, resulting in a lower mass-transfer rate near the interface and higher vapor quality at the outlet. In consequence, the heat-transfer coefficient of condensation with a Non-Condensable Gas drops sharply compared with th...
Junde Li - One of the best experts on this subject based on the ideXlab platform.
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cfd simulation of water vapour condensation in the presence of non condensable Gas in vertical cylindrical condensers
International Journal of Heat and Mass Transfer, 2013Co-Authors: Junde LiAbstract:This paper presents the simulation of the condensation of water vapour in the presence of Non-Condensable Gas using computational fluid dynamics (CFD) for turbulent flows in a vertical cylindrical condenser tube. The simulation accounts for the turbulent flow of the Gas mixture, the condenser wall and the turbulent flow of the coolant in the annular channel with no assumptions of constant wall temperature or heat flux. The condensate film is assumed to occupy a negligible volume and its effect on the condensation of the water vapour has been taken into account by imposing a set of boundary conditions. A new strategy is used to overcome the limitation of the currently available commercial CFD package to solve the simultaneous simulation of flows involving multispecies and fluids of Gas and liquid in separate channels. The results from the CFD simulations are compared with the experimental results from the literature for the condensation of water vapour with air as the Non-Condensable Gas and for inlet mass fraction of the water vapour from 0.66 to 0.98. The CFD simulation results in general agree well with the directly measured quantities and it is found that the variation of heat flux in the condenser tube is more complex than a simple polynomial curve fit. The CFD results also show that, at least for flows involving high water vapour content, the axial velocity of the Gas mixture at the interface between the Gas mixture and the condensate film is in general not small and cannot be neglected.
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condensation of vapor in the presence of non condensable Gas in condensers
International Journal of Heat and Mass Transfer, 2011Co-Authors: Junde Li, Mohammad Saraireh, Graham R ThorpeAbstract:Abstract This paper presents a set of differential and algebraic equations that model heat and mass transfer in condensers in which a mixture of water vapor and Non-Condensable Gas is cooled. The model has been used to predict the condensation rate, the bulk temperatures of the coolant and the Gas–vapor mixture, and the surface temperatures of the condenser wall. The predicted results for counter flow tube condensers are compared with three sets of published experimental data for system in which air is the Non-Condensable Gas. It is found that the predicted condensation rates and coolant bulk temperatures agree very well with all the three sets of experimental data, the predicted wall temperatures agree reasonably well with the experimental results, and the agreement between the predictions and the experimental results on the bulk temperature of the air–vapor mixture is excellent for one set of the experimental data, reasonable for the second set of experimental data, but poor for the third set of experimental data. It is suggested that the poor agreement between the predicted and measured bulk temperatures of the mixture for the third set of experimental data arises from the experimental errors. The results from this study show that when modeling vapor condensation in the presence of a Non-Condensable Gas, a simple model for the mixture channel alone may not be sufficient since neither the temperature nor the heat flux at the wall can be assumed to be constant. The results also show that the wall temperature in the coolant channel can be quite high, and careful modeling of the heat transfer in the coolant channel is needed in order to achieve good agreement between the model predictions and the experimental results.
Dengyu Chen - One of the best experts on this subject based on the ideXlab platform.
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pyrolysis polygeneration of poplar wood effect of heating rate and pyrolysis temperature
Bioresource Technology, 2016Co-Authors: Dengyu Chen, Kehui Cen, Min LuoAbstract:The pyrolysis of poplar wood were comprehensively investigated at different pyrolysis temperatures (400, 450, 500, 550, and 600°C) and at different heating rates (10, 30, and 50°C/min). The results showed that BET surface area of biochar, the HHV of Non-Condensable Gas and bio-oil reached the maximum values of 411.06m(2)/g, 14.56MJ/m(3), and 14.39MJ/kg, under the condition of 600°C and 30°C/min, 600°C and 50°C/min, and 550°C and 50°C/min, respectively. It was conducive to obtain high mass and energy yield of bio-oil at 500°C and higher heating rate, while lower pyrolysis temperature and heating rate contributed towards obtaining both higher mass yield and energy yield of biochar. However, higher pyrolysis temperature and heating rate contributed to obtain both higher mass yield and energy yield of the Non-Condensable Gas. In general, compared to the heating rate, the pyrolysis temperature had more effect on the product properties.
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torrefaction of biomass stalk and its effect on the yield and quality of pyrolysis products
Fuel, 2015Co-Authors: Dengyu Chen, Zhongcheng Zheng, Kexin Fu, Ze Zeng, Jiajia Wang, Mengting LuAbstract:Abstract Torrefaction is a promising pretreatment technology for biomass and its pyrolysis products upgrading. In this study, the effect of torrefaction temperature (220, 250 and 280 °C) on the fuel properties of cotton stalk was investigated. And then pyrolysis experiments of torrefied cotton stalk were performed using a fixed-bed pyrolysis reactor. The effect of torrefaction on the yield and quality of pyrolysis products was also discussed. The results show that after torrefaction, the grinding performance and hydrophobicity of cotton stalk are improved, and the number of functional groups containing oxygen is decreased. Compared with the dried cotton stalk, oxygen content of torrefied cotton stalk reduce greatly while carbon content and heating value increase. The pyrolysis results show that the yield of bio-oil decreases, while the yield of biochar rapidly increases with increased torrefaction temperature. Torrefaction has little effect on the chemical composition of biochar. However, it has important effect on the quality of Non-Condensable Gas and bio-oil. It enhances CH 4 and H 2 formation and increases the heating value of Non-Condensable Gas. For bio-oil, it reduces acids but increases phenols content. In addition, the water content of bio-oil deceased obviously, leading to a noticeable increase in the heating value of bio-oil.
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bamboo pyrolysis using tg ftir and a lab scale reactor analysis of pyrolysis behavior product properties and carbon and energy yields
Fuel, 2015Co-Authors: Dengyu Chen, Hongru Zhang, Yong Chen, Qian LiAbstract:Abstract Slow pyrolysis of moso bamboo was performed using thermogravimetry–Fourier transform infrared analysis (TG–FTIR) and a laboratory-scale pyrolysis reactor. The effects of temperature on slow pyrolysis behavior, product properties, and carbon and energy yields at pyrolysis temperatures of 300–700 °C were discussed. Typical compounds such as CO 2 , H 2 O, CH 4 , and CO were identified by their characteristic absorbance. The changes in the absorbance of volatiles during pyrolysis agreed with the weight loss in the derivative thermogravimetric curve. With increased temperature, biochar yield decreased while Non-Condensable Gas yield increased. The bio-oil achieved the highest yield of 36.57% at 500 °C. Biochar was considered the primary product of pyrolysis because it had 41% or higher carbon and energy yields. Bio-oil and Non-Condensable Gas contained 50–60% of carbon and energy content when the pyrolysis temperature was >400 °C. Therefore, these three types of products as valuable resources should be developed together for utilization.
Jiang He - One of the best experts on this subject based on the ideXlab platform.
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effect of non condensable Gas on the startup of a loop heat pipe
Applied Thermal Engineering, 2017Co-Authors: Jiang He, Jianyin Miao, Hongxing ZhangAbstract:It is essential to address the startup issues prior to the wide application of loop heat pipes (LHPs) in both space and terrestrial surroundings. As Non-Condensable Gas (NCG) is an important factor affecting the startup behavior, its effects on the startup performance of an ammonia-stainless steel LHP with and without preconditioning were experimentally investigated in this work. Nitrogen with controlled amounts was used to simulate the NCG, and the temperature overshoot, liquid superheat and startup time were employed as the evaluation criteria. Four situations relating to initial liquid/vapor distribution in the evaporator were examined: (1) both evaporator core and vapor grooves are filled with liquid, (2) vapor exists in vapor grooves and the evaporator core is filled by liquid, (3) vapor grooves are filled by liquid and vapor exists in the evaporator core, and (4) vapor exists in both evaporator core and vapor grooves. Experimental results showed that with NCG presence in the LHP, the startup could only proceed in situation 1 with preconditioning, while it could proceed in situations 1, 3 or 4 without preconditioning. For the startup in situation 1, a larger NCG inventory led to much degraded startup performance, and a higher startup heat load could benefit the startup. For the startup in situation 3, the most difficult startup situation, NCG resulted in a very high temperature overshoot, which may even exceed the maximum allowable value. For the startup in situation 4, the existence of NCG in the vapor grooves could facilitate the evaporation there, leading to a very desirable startup.
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investigation on the effect of thermoelectric cooler on lhp operation with non condensable Gas
Applied Thermal Engineering, 2017Co-Authors: Rui Yang, Jiang He, Jianyin MiaoAbstract:Abstract Non-Condensable Gas (NCG) is one of the main causes affecting adversely the thermal performance, operating reliability and lifespan of two-phase heat transfer devices. In this work, nitrogen was charged into two ammonia-stainless steel loop heat pipes (LHPs) with controllable amounts to simulate the NCG, and extensive experiments were conducted to investigate the effect of thermoelectric cooler (TEC) on the LHP operation with NCG. Experimental results show that the TEC can effectively improve the thermal performance of the LHP with NCG, i.e., decreasing the steady-state operating temperature, expanding the allowable heat load range, realizing a successful startup with much reduced temperature overshoot, and eliminating or inhibiting the possible temperature oscillations. However, some unexpected phenomena or operation anomalies such as startup failure and operating temperature rise were also observed during the experiments with the application of the TEC. Through analyzing these experimental results, the design considerations for the TEC assembly are proposed to better play the role of TEC in improving the thermal performance of LHPs with NCG.
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effect of non condensable Gas on steady state operation of a loop thermosyphon
International Journal of Thermal Sciences, 2014Co-Authors: Jiang He, Jianyin Miao, Hongxing Zhang, Lu WangAbstract:Abstract Non-Condensable Gas (NCG) generated inside two-phase heat transfer devices can adversely affect the thermal performance and limit the lifetime of such devices. In this work, extensive experimental investigation of the effect of NCG on the steady-state operation of an ammonia-stainless steel loop thermosyphon was conducted. In the experiments, nitrogen was injected into the loop thermosyphon as NCG, and the thermal performance of the loop thermosyphon was tested at different NCG inventories, heat loads applied to the evaporator and condenser cooling conditions, i.e. natural air cooling or circulating ethanol cooling. Experimental results reveal that NCG elevates the steady-state operating temperature of the evaporator, especially when the loop thermosyphon is operating in the low temperature range; meanwhile, the more NCG exists in the loop thermosyphon, the higher the operating temperature of the evaporator, and the lower the reservoir temperature. In addition, the existence of NCG results in the decrease of the overall thermal conductance of the loop thermosyphon, and the overall thermal conductance under the ethanol cooling condition may be even lower than that under the air cooling condition when the heat load is smaller than a certain value. Finally, the experimental results are theoretically analysed and explained.
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effect of non condensable Gas on the operation of a loop heat pipe
International Journal of Heat and Mass Transfer, 2014Co-Authors: Jiang He, Jianyin Miao, Hongxing ZhangAbstract:Abstract Non-Condensable Gas (NCG) is one of the main causes affecting the thermal performance and lifespan of the two-phase heat transfer devices. In this work, the effect of NCG on the operating performance of an ammonia–stainless steel loop heat pipe (LHP) was experimentally investigated. Nitrogen was selected as NCG, and the steady-state operating characteristics of the LHP with NCG was systematically studied for different NCG inventories, heat loads applied to the evaporator, heat load cycles and heat sink temperatures. Experimental results reveal that NCG elevates the operating temperature of the evaporator, and the more NCG exists, the higher the operating temperature of the evaporator and the larger the heat load range corresponding to the variable conductance mode. Meanwhile, the effect of NCG is notable for a smaller heat load or lower heat sink temperature. The increase of the operating temperature attributes to that NCG breaks the original energy/pressure balance of the components of the LHP, which leads to further change of heat and mass transfer as well as the redistribution of the working fluid in the loop. With NCG in the LHP, the temperature hysteresis is a common phenomenon for different heat load cycles. In addition, unique temperature oscillation was observed in the experiments, and it may be associated with the frequent repeated processes of the generation/collapse of the bubbles in the evaporator core.
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effect of non condensable Gas on startup of a loop thermosyphon
International Journal of Thermal Sciences, 2013Co-Authors: Jiang He, Jianyin Miao, Hongxing Zhang, Lu WangAbstract:Abstract Non-Condensable Gas (NCG) generated inside two-phase heat transfer devices can adversely affect the thermal performance and limit the lifetime of such devices. In this work, experimental investigation of the effect of NCG on the startup of an ammonia–stainless steel loop thermosyphon was conducted. In the experiment, nitrogen was injected into the loop thermosyphon as NCG. The effect of NCG inventory on the startup behavior was investigated by adjusting the injected amount of nitrogen. The experimental results reveal that NCG prolongs the startup time and increases the startup liquid superheat and temperature overshoot; the more NCG exists in the loop thermosyphon, the higher the liquid superheat and temperature overshoot. When NCG is present in the system, boiling usually occurs in the evaporator before startup, but it does not mean the system will start up instantly, which differs from the conditions without NCG. Under all the conditions, increasing the heat load can effectively shorten the startup time but leads to a large temperature overshoot; forced convection cooling of the condenser has almost no effect on shortening the startup time especially for large NCG inventory situations, but it can effectively limit the temperature overshoot. For large NCG inventory situations, the loop thermosyphon can start up at a small heat load (5 W) or even without a heat load when the condenser is cooled by forced convection of ethanol. No failed startups occurred during any of the tests.
Li Wang - One of the best experts on this subject based on the ideXlab platform.
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effect of non condensable Gas on the start up of a gravity loop thermosyphon with Gas liquid separator
Experimental Thermal and Fluid Science, 2016Co-Authors: Jian Huang, Li Wang, Jianbiao ShenAbstract:Abstract This study experimentally investigated the influence of Non-Condensable Gas (NCG) on the start-up time of gravity loop thermosyphon (GLT) developed for regenerative building heating exchangers. A Gas–liquid separator was added to the end of the condensate line in the GLT. Given characteristics of the NCG that accumulated in the Gas–liquid separator, and the NCG is prevented from circulating in the loop, thereby effectively lowering the effect of NCG on heat exchange. The presence of NCG not only influenced the local condensation heat transfer of the condenser but also affected the start-up of the loop thermosyphon. Results of the experimental investigation revealed that the presence of NCG increased the start-up time of GLT. A higher NCG level corresponds to, a longer start-up time. In particular, the start-up time was 2.8 times more than that in the evacuation when amount of NCG was 30% in volume. The NCG–vapor blocks zone hinders the vapor transmission from the evaporator to the condenser in the vapor line between the evaporator and condenser, further causing a rapid increase in system pressure during start-up. Moreover, obvious pressure peak curve phenomena occurred in a number of conditions, such as heat load was greater than or equal 2.0 kW and amount of NCG was less than or equal 30% in volume.
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review of vapor condensation heat and mass transfer in the presence of non condensable Gas
Applied Thermal Engineering, 2015Co-Authors: Jian Huang, J Y Zhang, Li WangAbstract:Abstract In 1929, Donald Othmer discovered that a small amount of Non-Condensable Gases (NCG) in pure vapor had a great effect on condensation heat transfer coefficient (HTC), reducing the efficiency of heat transfer equipment. Since then, a large amount of research has been performed. This paper reviews experimental, mechanism and model research progresses in condensation in the presence of NCG. Particular attention is given to research on physical models of heat transfer for filmwise condensation (FWC) with NCG, with a brief review of dropwise condensation (DWC). The models for FWC heat transfer in the presence of NCG can be divided into two categories: semi-theoretical models and theoretical models. The semi-theoretical models are based on hydraulics and thermodynamics, use some specific parameters of the correlation which determined by experiments, and are suitable for engineering design. The theoretical models are based on mass, momentum, and energy conservation equations, and divided into boundary layer models and diffusion layer models. Through research of experiments and models people found that, condensate film thickness, surface waves, interfacial shear strength and suction effect play an important role in the FWC heat transfer in presence of NCG.