Li-Xia Pei

South China University of Technology, Shengcheng, Guangdong, China

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Publications (21)44.6 Total impact

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    ABSTRACT: The flow maldistribution and the consequent performance deteriorations in a cross flow hollow fiber membrane module used for air humidification are investigated. The effects of structure-induced flow maldistributions on the deteriorations in humidification efficiencies are studied. As a first step, a CFD code is used to calculate the flow distribution in the exchanger, by treating the hollow fiber membrane core as a porous media. Then a coupled heat and moisture transfer model between the air flow and the water flow is set up. The shell-and-tube core is equivalently converted to a cross-flow parallel-plates heat mass exchanger. Using the CFD predicted flow distributions on the core face, the sensible cooling and humidification efficiencies are calculated with a finite difference scheme. The results indicate that the packing fraction affects the flow maldistribution substantially. The consequent sensible cooling and humidification efficiencies are influenced significantly. Depending on air flow rates, the sensible cooling efficiencies can be deteriorated by 3–30%, and the humidification efficiencies can be deteriorated by 26–58%.
    Journal of Membrane Science 07/2014; 427:1–9. DOI:10.1016/j.memsci.2012.09.030 · 4.91 Impact Factor
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    ABSTRACT: It is common to use fillers to increase the thermal conductivity of polymer materials. Filler conductivity and filler content have been identified as the dominating factors influencing the overall effective thermal conductivity of composite materials. The interactions between the fillers and the base materials have been seldom considered. This research addresses this problem by solving the heterogeneous two-dimensional conjugate heat conduction problem in the composite materials. The interactions between the fillers and the base materials are considered. It is found that the filler shapes and their orientation angles have great impacts on the effective thermal conductivity of the composite materials. Of the various shapes, the I shaped fillers are the best choice for composite polymer materials. The influences of filler shapes and angles explain the reason why there are large discrepancies between the measured data and the available correlations in literature, which were searched for many years for the accurate predictions of the effective thermal conductivity of composite materials. The slices fillers are effective only when they are distributed parallel to heat flow directions. Currently the I shaped and T shaped fillers are promising and they should be used for future materials fabrications.
    International Journal of Heat and Mass Transfer 09/2013; 64:735-742. DOI:10.1016/j.ijheatmasstransfer.2013.05.021 · 2.52 Impact Factor
  • Si-Min Huang, Li-Zhi Zhang, Li-Xia Pei
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    ABSTRACT: The transport phenomena in a hollow fibre membrane bundle for liquid desiccant air dehumidification were investigated. In the bundle, the liquid desiccant solution flows inside the fibres and the process air flows across the fibre bundle; where air is dehumidified by moisture permeation through the membrane. This study investigated the fluid flow and conjugate heat and mass transfer in the cross-flow hollow fibre membrane bundle by considering the interactions between the neighbouring fibres. Two regularly packed arrangements: in-line and staggered, were considered. Due to the periodicity of the fluid flow and heat and mass transfer across the bundle, two representative periodic unit cells, which simultaneously include 2 to 3 neighbouring fibres, were selected as the calculation domains. The equations governing the fluid flow and heat and mass transfer in the two cross-flow streams (solution and air) in the membranes, were solved together with the heat and mass diffusion equations. The friction factor and the Nusselt and Sherwood numbers on both the air and the solution sides were then calculated and experimentally validated. The results were compared to those available data calculated from the free surface model.
    Indoor and Built Environment 06/2013; 22(3):559-574. DOI:10.1177/1420326X12452881 · 1.72 Impact Factor
  • Li-Zhi Zhang, Si-Min Huang, Li-Xia Pei
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    ABSTRACT: The fluid flow and conjugate heat and mass transfer in a cross-flow hollow fiber membrane contactor are investigated. The shell-and-tube like contactor is used for liquid desiccant air dehumidification, where numerous fibers are packed into the shell and air flows across the fiber bank. To overcome the difficulties in the direct modeling of the whole contactor, a representative cell, which comprises of a single fiber, a liquid solution inside the fiber, and an air stream across the fiber, is selected as the calculation domain. The air stream in the cell is surrounded by an assumed outer free surface. The equations governing the fluid flow and heat and mass transfer in the two cross-flow streams are solved together with the heat and mass diffusion equations in the membrane. The friction factor and the Nusselt and Sherwood numbers on the air and stream sides are then calculated and experimentally validated.
    International Journal of Heat and Mass Transfer 12/2012; 55(s 25–26):8061–8072. DOI:10.1016/j.ijheatmasstransfer.2012.08.041 · 2.52 Impact Factor
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    ABSTRACT: Adding fillers to materials of low thermal conductivity has been recognized as an efficient way to increase the thermal conductivity of composite materials. The filler shapes have great impacts on heat conduction in the materials, which can be analyzed by a direct heat conduction problem. However what is the best shape? This study answers this inverse problem by a nonlinear optimization programming technique. The problem is first standardized to the optimization of a nonlinear objective function with nonlinear constraints. Then it is solved by SQP (sequence quadratic nonlinear programming) numerical scheme. The optimum filler shape parameters are obtained. The results are that the best shapes are “I” shapes. Further, the optimization process can be regarded as a filler growth process. Depending on the fillers thermal conductivity and filler volume contents, the finally formed optimum shapes can be classified into three categories: the handicapped dumbbells, the infant less developed “I” shapes, and adult fully developed “I” shapes. Composite materials made of paraffin wax and steel fillers of six shapes are prepared to validate the optimization results.
    International Journal of Heat and Mass Transfer 11/2012; 55(23-24):7287-7296. DOI:10.1016/j.ijheatmasstransfer.2012.07.059 · 2.52 Impact Factor
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    ABSTRACT: Conjugate heat and mass transfer in a hollow fiber membrane module used for liquid desiccant air dehumidification is investigated. The module is like a shell-and-tube heat exchanger where the liquid desiccant stream flows in the tube side, while the air stream flows in the shell side in a counter flow arrangement. Due to the numerous fibers in the shell, a direct modeling of the whole module is difficult. This research takes a new approach. A representative cell comprising of a single fiber, the liquid desiccant flowing inside the fiber and the air stream flowing outside the fiber, is considered. The air stream outside the fiber has an outer free surface (Happel’s free surface model). Further, the equations governing the fluid flow and heat and mass transfer in the two streams are combined together with the heat and mass diffusion equations in membranes. The conjugate problem is then solved to obtain the velocity, temperature and concentration distributions in the two fluids and in the membrane. The local and mean Nusselt and Sherwood numbers in the cell are then obtained and experimentally validated.
    International Journal of Heat and Mass Transfer 06/2012; 55(s 13–14):3789–3799. DOI:10.1016/j.ijheatmasstransfer.2012.03.034 · 2.52 Impact Factor
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    ABSTRACT: Fluid flow and convective heat mass transfer in membrane-formed parallel-plates channels are investigated. The membrane-formed channels are used for liquid desiccant air dehumidification. The liquid desiccant and the air stream are separated by the semi-permeable membrane to prevent liquid droplets from crossing over. The two streams, in a cross-flow arrangement, exchange heat and moisture through the membrane, which only selectively permits the transport of water vapor and heat. The two flows are assumed hydrodynamically fully developed while developing thermally and in concentration. Different from traditional method of assuming a uniform temperature (concentration) or a uniform heat flux (mass flux) boundary condition, the real boundary conditions on membrane surfaces are numerically obtained by simultaneous solution of momentum, energy and concentration equations for the two fluids. Equations are then coupled on membrane surfaces. The naturally formed boundary conditions are then used to calculate the local and mean Nusselt and Sherwood numbers along the channels. Experimental work is performed to validate the results. The different features of the channels in comparison to traditional metal-formed parallel-plates channels are disclosed.
    International Journal of Heat and Mass Transfer 04/2012; 55(s 9–10):2571–2580. DOI:10.1016/j.ijheatmasstransfer.2012.01.003 · 2.52 Impact Factor
  • Li-Xia Pei, Zhi-Mei Lv, Li-Zhi Zhang
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    ABSTRACT: Selective adsorption of a novel material is analyzed. To improve indoor air quality, a high selective desiccant, which can adsorb more water vapor and less volatile organic compounds (VOCs), is developed for rotary wheels type heat and moisture recovery. This novel desiccant was successfully prepared by amine-functionalization and subsequent self-assembly with malic acid on silica gel. The BET surface area, pore volume and average pore diameter of the prepared desiccant were characterized by N2-adsorption method. The adsorption performances were evaluated by the breakthrough test, and the regeneration ability was also tested. Compared with commercial silica gel, the proposed desiccant exhibits much higher selectivity and acceptable adsorption capacity for water vapor. The selectivities of moisture/toluene and moisture/acetaldehyde are improved by 3.6 and 2.3 times, respectively. The novel desiccant is promising for future use in rotary wheels for heat and moisture recovery with improved indoor air quality.
    Building and Environment 03/2012; 49:124–128. DOI:10.1016/j.buildenv.2011.09.031 · 2.70 Impact Factor
  • Journal of Heat Transfer 01/2012; 134(8):082001. DOI:10.1115/1.4006208 · 2.06 Impact Factor
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    ABSTRACT: A one-step approach is employed to prepare an asymmetric cellulose acetate (CA) membrane that is used for total heat recovery (heat and moisture recovery). The process only involves a cheap raw material of CA, an environmental friendly solvent acetic acid and an additive deionized water. A porous support layer and a dense skin layer are formed simultaneously during the solvent and coagulant medium exchange. Investigations found that the approach is successful in making membranes for heat and moisture recovery. The new membranes have high moisture permeability. Moreover, the permeation of other unwanted gases like CO2 is effectively prohibited. Analysis found that the optimum solution compositions for casting membranes are: acetic acid to deionized water ratio 70:30. The approach provides an environmental friendly yet economical solution for preparing membranes for heat and moisture recovery.
    Fuel and Energy Abstracts 01/2011; 366(1):158-165. DOI:10.1016/j.memsci.2010.09.054
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    ABSTRACT: A direct expansion (DX) air dehumidification system is an efficient way to supply fresh and dry air to a built environment. It plays a key role in preventing the spread of respiratory disease like Swine flu (H1N1). To improve the efficiency of a conventional DX system in hot and humid regions, a new system of DX in combination with a membrane-based total heat exchanger is proposed. Air is supplied with dew points. A detailed mathematical modeling is performed. A cell-by-cell simulation technique is used to simulate its performances. A real prototype is built in our laboratory in South China University of Technology to validate the model. The effects of inlet air humidity and temperature, evaporator and condenser sizes on the system performance are investigated. The results indicate that the model can predict the system accurately. Compared to a conventional DX system, the air dehumidification rate (ADR) of the novel system is 0.5 times higher, and the coefficient of performance (COP) is 1 times higher. Furthermore, the system performs well even under harsh hot and humid weather conditions.
    Energy 09/2010; 35(9-35):3891-3901. DOI:10.1016/j.energy.2010.06.002 · 4.16 Impact Factor
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    ABSTRACT: Membrane-based energy recovery ventilators (or total heat exchangers) are key equipments to fresh air ventilation, which is helpful for the control of respiratory diseases like Swine flu (H1N1) and SARS. Parallel-plates narrow channels are common structure for membrane-based energy recovery ventilators. In practice, the exchanger channel lengths are limited due to the confinement in pressure drops and noises. In these channels, the hydraulically, thermally and concentrationally entry regions account for a large fraction of the total duct length. However, previous investigations neglected the entry issues for simplicity. Either hydraulically fully developed, or thermally or/and concentrationally fully developed flow were assumed, which would underestimate equipments performances seriously. This study provides a more accurate methodology: fluid flow, heat and mass transport equations were solved directly as they enter into the channel. In other words, both the fluid flow and the heat and mass transport are in simultaneously developing regions. The membrane and the two neighboring flows are considered as a conjugate problem. The conjugate heat transfer problem is solved with a commercial CFD code. Then the conjugate mass transfer problem is solved by transferring it to another conjugate heat transfer problem by heat mass analogy. The Nusselt and Sherwood numbers in the entry regions are calculated. The effects of three typical flow arrangements: cocurrent, counter and cross flow, on the boundary conditions and the consequent Nusselt and Sherwood numbers in the channels are evaluated.
    International Journal of Heat and Mass Transfer 02/2010; 53(5):815-824. DOI:10.1016/j.ijheatmasstransfer.2009.11.043 · 2.52 Impact Factor
  • Li-zhi Zhang, Li-xia Pei
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    ABSTRACT: Porous polyvinylidene fluoride (PVDF) membranes with various structures were prepared via phase inversion using DMF as solvent. Polyethylene glycols (PEG) with three different concentrations and molecular weights were used. Effects of PEG on the structure and performance of porous membranes were investigated. The morphology and structure of the prepared membrane were characterized by scanning electron microscopy (SEM). The water vapor permeability through membranes was evaluated by permeation test. The results showed that PEG was used as pore forming agent in PVDF/DMF/H2 O system. Moreover, it has great influences on morphology and water vapor permeability. The membranes becomes more porous with PEG additives. Water vapor transfer resistance was reduced, which leads to improved water vapor permeability.
    ASME 2009 Second International Conference on Micro/Nanoscale Heat and Mass Transfer; 01/2009
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    ABSTRACT: Parallel-plates enthalpy exchangers are one of the most commonly encountered energy recovery devices that are used to simultaneously transfer both sensible heat and moisture between fresh air and exhaust ventilation air. For such equipments, the water vapor sorption properties of the plate materials have tremendous impacts on system performance. In this investigation, three different materials, namely, common paper, CA (cellulose acetate) membrane and a modified CA membrane) are selected as the plate materials for three enthalpy exchangers. Sorption curves and contact angles of these three materials are measured to reflect their hydrophilicity. The steady-state sensible and latent effectiveness of the three exchangers are tested in a special test rig, and the test results are compared with the model predictions. A heat and moisture transfer model for the enthalpy exchangers is proposed. The effects of the varying operating conditions like air flow rates, temperature, and humidity on the sensible and latent effectiveness are evaluated. Both the numerical and experimental results indicate that the moisture resistance through plates is co-determined by thickness, sorption slope, and sorption potential. Moisture diffusivity in various materials is in the same order. So when the plate thickness is fixed, the higher the sorption slopes are, the higher the latent performance is. Of the three exchangers, the exchanger with the modified CA membrane material has the highest performance due to small thickness, steep sorption slope, and large sorption potentials. The paper exchanger has a latent effectiveness of 0.4, while the membranes have latent effectiveness of greater than 0.7.
    Journal of Membrane Science 12/2008; 325(2):672-682. DOI:10.1016/j.memsci.2008.08.041 · 4.91 Impact Factor
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 09/2008; 39(40). DOI:10.1002/chin.200840087
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    ABSTRACT: Low amount of manganese meso-tetraphenyl porphyrin [Mn(TPP)] was used for highly efficient selective oxidation of sulfide to sulfone by hydrogen peroxide at room temperature. Sulfones were produced directly with yields generally around 90% while the catalyst concentration was only 4×10−5 mol·L−1. In a large-scale experiment of thioanisole oxidation, the isolated yield of sulfone (87%) was obtained and the turnover number (TON) reached up to 8×106, which is the highest TON for the oxidation systems of sulfide to sulfone catalyzed by metalloporphyrins.
    Chinese Journal of Chemistry 06/2008; 26(6):1114-1118. DOI:10.1002/cjoc.200890198 · 1.04 Impact Factor
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 02/2008; 39(9). DOI:10.1002/chin.200809089
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    ABSTRACT: Tin promoted RuB amorphous alloy supported on SiO2 was prepared by a novel reductant impregnation method for the hydrogenation of carbonyl compounds to the corresponding alcohol under atmospheric H2 and ambient temperature. The as-prepared Ru–Sn–B/SiO2 catalyst presented high activity for hydrogenation of carbonyl compounds even under atmospheric H2. The TEM images showed that addition of tin improved the dispersion of RuB particles, and then significantly increased the conversion of carbonyl compounds. Based on the XPS spectra and catalytic performance, tin in Ru–Sn–B/SiO2 catalyst was supposed to play a role of adsorbing and activating the CO bond of carbonyl group during the hydrogenation.
    Catalysis Communications 01/2008; 9(1):27-34. DOI:10.1016/j.catcom.2007.05.012 · 3.32 Impact Factor
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    ABSTRACT: Efficient selective oxidation of alcohols to carbonyl compounds by molecular oxygen with isobutyraldehyde as oxygen acceptor in the presence of metalloporphyrins has been reported. Ruthenium (III) meso-tetraphenylporphyrin chloride (Ru(TPP)Cl) showed excellent activity and selectivity for oxidation of various alcohols under mild conditions. Moreover, different factors influencing alcohols oxidation, for example, catalyst, solvent, temperature, and oxidant, have been investigated. In large-scale oxidation of benzyl alcohol, the isolated yield of benzaldehyde of 89% was observed.
    Bioorganic & Medicinal Chemistry Letters 12/2007; 17(22):6364-8. DOI:10.1016/j.bmcl.2007.08.063 · 2.33 Impact Factor
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    ABSTRACT: Highly efficient selective oxidation of sulfides to sulfoxides by molecular oxygen catalyzed by ruthenium (III) meso-tetraphenylporphyrin chloride (Ru(TPP)Cl) with isobutyraldehyde as oxygen acceptor has been reported. In large-scale experiment of thioanisole oxidation, the isolated yield of sulfoxide of 92% was obtained and the turnover number reached up to 92,000.
    Bioorganic & Medicinal Chemistry Letters 09/2007; 17(16):4650-3. DOI:10.1016/j.bmcl.2007.05.073 · 2.33 Impact Factor