Cheng Yongpan
Email Address
checyp@nus.edu.sg
Organizational Units
SPECIALTY RESEARCH INST/CTRS
faculty
33 results
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Publication Experimental and numerical investigations on the electrostatics generation and transport in the downer reactor of a triple-bed combined circulating fluidized bed(2012-10-31) Cheng, Y.; Lau, D.Y.J.; Guan, G.; Fushimi, C.; Tsutsumi, A.; Wang, C.-H.; CHEMICAL & BIOMOLECULAR ENGINEERINGElectrostatics is an inevitable phenomenon in fluidization processes and granular flow systems where collisions between particulates and walls with different materials occur. In this study, the electrostatic performance in the downer reactor of a triple-bed combined circulating fluidized bed was investigated through both experiments and numerical simulations. In numerical simulation the Discrete Element Method (DEM) was adopted to simulate the electrostatic charge generation and transfer occurring in the downer. Both experimental and numerical results showed that the averaged induced currents caused by electrostatics increased with increasing solids mass flux in the downer, and the electrostatics was the strongest near the entrance of the downer because of the highest frequency of collisions between particles and the wall caused by the largest solids holdup at that location. Numerical studies also revealed that the averaged induced currents increased with increasing initial particle velocity and particle size, and the averaged induced current caused by the charge transfer was much larger than those by charge generation through tribocharging. These results may help us better understand the mechanism of electrostatic phenomena, and better cope with challenges and problems caused by electrostatics. © 2012 American Chemical Society.Publication Studies of solid-solid mixing behaviors in a downer reactor(2011) Cheng, Y.; Lim, E.W.C.; Wang, C.-H.; Guan, G.; Nakamura, Y.; Ishizuka, M.; Fushimi, C.; Tsutsumi, A.; CHEMICAL & BIOMOLECULAR ENGINEERINGDowner reactor has the very short residence times, narrow residence time distributions and uniform solids holdup distributions with the aid of gravitational acceleration. As a result, it has become an attractive alternative to its counterpart, the riser reactor, for certain classes of chemical reactions. These include gasification reactions, fluid catalytic cracking, Fischer-Tropsch reactions and others. In particular, in downer reactor, high reaction efficiencies and low tar formation can be achieved in fast pyrolysis of coal or biomass. The flow behavior of solids is a key to the successful operation of the reactor within the downer, because it determines the contacting pattern as well as the extents of mixing between different phases of materials. Despite the research efforts that have been invested towards both experimental and numerical studies of hydrodynamics and flow behaviors in downer reactor systems, current understanding of such systems is far from complete. In particular, relatively little attention has been given towards the studies of mixing behaviors between two or more types of solids within a downer reactor. Our work will focus on the characterization of the mixing between two types of solids through both experiment and numerical studies.Publication 3D numerical simulation on fluid flow and heat transfer characteristics in multistage heat exchanger with slit fins(2007-11) Tao, W.Q.; Cheng, Y.P.; Lee, T.S.; MECHANICAL ENGINEERINGIn this paper, a numerical investigation is performed for three-stage heat exchangers with plain plate fins and slit fins respectively, with a three-dimensional laminar conjugated model. The tubes are arranged in a staggered way, and heat conduction in fins is considered. In order to save the computer resource and speed up the numerical simulation, the numerical modeling is carried out stage by stage. In order to avoid the large pressure drop penalty in enhancing heat transfer, a slit fin is presented with the strip arrangement of "front coarse and rear dense" along the flow direction. The numerical simulation shows that, compared to the plain plate fin heat exchanger, the increase in the heat transfer in the slit fin heat exchanger is higher than that of the pressure drop, which proves the excellent performance of this slit fin. The fluid flow and heat transfer performance along the stages is also provided. © 2007 Springer-Verlag.Publication Numerical simulations and experiments on heat transfer around a probe in the downer reactor of a triple-bed combined circulating fluidized bed(2012) Cheng, Y.; Guan, G.; Ishizuka, M.; Fushimi, C.; Tsutsumi, A.; Wang, C.-H.; CHEMICAL & BIOMOLECULAR ENGINEERINGDue to very short residence time in the downer, heat transfer performance inside it has great significance for coal gasification. In this study heat transfer around a heating probe in the downer of a circulating fluidized bed was studied through both experimental and numerical work. Both experimental results and numerical results revealed that average heat transfer coefficients decreased with the increasing superficial air velocity or decreasing solids mass flux, which could be attributed to the decreasing solids holdup, as particle convection was dominant in the heat transfer mechanism under current cases. Numerical simulation also revealed that heat transfer may deteriorate with the increasing particle size. Finally it was found that the heat transfer performance with constant temperature boundary condition around the probe was much better than that with constant heat flux boundary condition. These results may help us better understand the heat transfer in the downer and design downer with high efficiency.Publication Experimental investigation on the heat and mass transfer between air and liquid desiccant in a cross-flow dehumidifier(2012-01) Gao, W.Z.; Liu, J.H.; Cheng, Y.P.; Zhang, X.L.; CHEMICAL & BIOMOLECULAR ENGINEERINGDehumidifier is the core component in a liquid desiccant air-conditioning system, whose efficiency directly affects the whole performance. In this study a cross-flow dehumidifier was established in which LiCl solution and Celdek structured packing were used. Enthalpy efficiency and moisture efficiency were adopted to describe the heat and mass transfer performance of the dehumidifier. Influence of desiccant and air inlet parameters on the dehumidifier performance was experimentally investigated, and efficiency comparison in different air outlet regions was also carried out, as well as influence of packing structure size on dehumidifier performance. The results show that the outlet parameter values of air and solution can be easily gained with enthalpy efficiency and moisture efficiency which was obtained from experimental regression. Efficiency has obvious regional characteristics. By increasing the thickness, width or height simultaneously, better performance of Dehumidifier can be achieved without increasing the pressure loss. © 2011 Elsevier Ltd.Publication Influence of electrostatics on instrumentation in fluidized bed measurements(2012) Cheng, Y.; Wang, C.-H.; CHEMICAL & BIOMOLECULAR ENGINEERINGIn fluidized bed, electrostatics is a common phenomenon due to the frequent collisions among solid particles, as well as between solid particles and the wall. It can cause serious problems for fluidized beds, altering hydrodynamics, causing agglomeration, interfering with instrumentation, generating nuisance discharges and even creating the danger of explosions. Despite these negative effects of electrostatics, current understanding on this subject is far from completion. The present study will focus on electrostatics in fluidized bed. Through our experimental study, it is found that the strong electrostatics can greatly affect the instrumentation and can easily lead to the terminations of data acquisitions. With increasing fluidizing air velocity, the generated electrostatics is increased rapidly. In order to reduce the influence of electrostatics, the non-conductive tape is adhered to the inner walls of fluidized bed, and the generated electrostatics is greatly decreased, as such the instrumentation can also work continuously. © 2012 American Institute of Physics.Publication Flow behaviors in the downer of a large-scale triple-bed combined circulating fluidized bed system with high solids mass fluxes(2011-09-15) Guan, G.; Fushimi, C.; Ishizuka, M.; Nakamura, Y.; Tsutsumi, A.; Matsuda, S.; Suzuki, Y.; Hatano, H.; Cheng, Y.; Chuan Lim, E.W.; Wang, C.-H.; CHEMICAL & BIOMOLECULAR ENGINEERINGThe flow behaviors in the downer of a large-scale triple-bed circulating fluidized bed (TBCFB) gasifier cold model, which is composed of a downer (φ 0.1m×6.5m), a bubbling fluidized bed (BFB, 0.75×0.27×3.4m3), a riser (φ 0.1m×16.6m) and a gas-sealing bed (GSB, φ 0.158m×5m), were investigated. Sand particles with a density of 2600kg/m3 and an average particle size of 128γm were used as bed materials. Solids mass fluxes were in the range 113-524kg/m2s. Average solids holdup in the developed region of the downer increased with increasing solids mass flux. The gas seal between the riser and the downer had a large effect on the solids holdup distribution in the downer. Compared with the solids holdup in the riser, a relatively low solids holdup was formed in the downer even at high solids loadings. A pressure balance model was set up to predict the solids mass flux for this TBCFB system. It was found that the static bed height in the GSB had a great effect on the solids mass flux. The possibilities of achieving a high density solids holdup in a downer were discussed. © 2011 Elsevier Ltd.Publication Numerical simulations and experiments on heat transfer around a probe in the downer reactor for coal gasification(2013-02) Cheng, Y.; Guan, G.; Ishizuka, M.; Fushimi, C.; Tsutsumi, A.; Wang, C.-H.; CHEMICAL & BIOMOLECULAR ENGINEERINGThe heat transfer performance of a downer reactor has great significance for coal gasification processes due to the very short residence time of coal. In this study both experimental and numerical works were carried out to study heat transfer around a heating probe in a downer. Both experimental results and numerical results revealed that average heat transfer coefficients decreased with increasing superficial air velocity or decreasing solid mass flux, which could be attributed to decreasing solid holdup, as particle-particle convection was dominant in the heat transfer mechanism for the current cases. Numerical simulations also revealed that heat transfer may deteriorate with increasing particle size. Finally it was found that the heat transfer performance with constant temperature boundary condition around the probe was much better than that with constant heat flux boundary condition. These results may help us better understand heat transfer in downers for improving the design of downers with higher efficiencies. © 2012 Elsevier B.V.Publication The influence of strip location on the pressure drop and heat transfer performance of a slotted fin(2007-01) Tao, W.Q.; Cheng, Y.P.; Lee, T.S.; MECHANICAL ENGINEERINGIn this article, a numerical study is conducted to predict the air-side heat transfer and pressure drop characteristics of slit fin-and-tube heat transfer surfaces. A three-dimensional steady laminar model is applied, and the heat conduction in the fins is also considered. Five types of slit fins, named slit 1, slit 2, slit 3, slit 4, and slit 5, are investigated, which have the same global geometry dimensions and the same numbers of strips on the fin surfaces. The only difference among the five slit fins lies in the strip arrangement. Slit 1 has all the strips located in the front part of the fin surface, then, following the order from slit 1 to slit 5, the strip number in the front part decreases and, correspondingly, the strip number in the rear part increases, so that all the strips of slit 5 are located in the rear part. Furthermore, slit 1 and slit 5, slit 2 and slit 4, have a symmetrical strip arrangement along the flow direction. The numerical results show that, following the order from slit 1 and slit 5, the heat transfer rate increases at first, reaching a maximum value at slit 3, which has the strip arrangement of "front coarse and rear dense"; after that, it begins to decrease, as does the fin efficiency. Although they have the symmetrical strip arrangement along the flow direction, slit 5 has 7% more Nusselt number than slit 1, and slit 4 also has 7% more Nusselt number than slit 2, which shows that strip arrangement in the rear part is more effective than that in the front part. Then the difference of heat transfer performance among five slit fins is analyzed from the viewpoint of thermal resistance, which shows that when the thermal resistances in the front and rear parts are nearly identical, the optimum enhanced heat transfer fin can be obtained. This quantitative rule, in conjunction with the previously published qualitative principle of "front sparse and rear dense," can give both quantitative and qualitative guides to the design of efficient slotted fin surfaces. Finally, the influence of fin material on the performance of enhanced-heat-transfer fins is discussed. © Taylor & Francis Group, LLC.Publication Droplet flash evaporation of mixed dehumidification solutions(2012-11) Gao, W.; Shi, Y.; Han, X.; Zhang, X.; Cheng, Y.; CHEMICAL & BIOMOLECULAR ENGINEERINGRegeneration of dehumidification solution is necessary to maintain the continuous dehumidifying process, whose efficiency directly affects the performance of whole system. The size, mass, and temperature of droplets during the flash evaporation were analyzed based on droplet flash theory, and the factors affecting the efficiency of flash evaporation were tested through preliminary experiments. The results show that pressure is the key factor affecting the efficiency of the flash evaporation. Flash evaporation may proceed only if the steam is removed timely. The mixed ratio of different solutions is relevant to the efficiency of flash evaporation. The initial temperature of droplet has little effect on the efficiency. The experimental values of temperature are higher than the theoretical values, so the physical properties need to be modified if the flash evaporation theory of droplet is adopted according to droplets flash evaporation of mixed dehumidification solutions. © All Rights Reserved.