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Design of Plate-Fin Tube Dehumidifiers for Humidification–Dehumidification Desalination Systems

2014; Taylor & Francis; Volume: 36; Issue: 3 Linguagem: Inglês

10.1080/01457632.2014.916153

1521-0537

Martin Sievers, John H. Lienhard,

Heat Transfer and Optimization

AbstractA two-dimensional numerical model of a plate-fin tube heat exchanger for use as a dehumidifier in humidification–dehumidification (HDH) desalination systems is developed, because typical heating, ventilating, and air conditioning (HVAC) dehumidifier models and plate-fin tube dehumidifier geometries are not intended for the considerably higher temperature and humidity ratio differences that drive heat and mass transfer in HDH desalination applications. The experimentally validated model is used to investigate the influence of various heat exchanger design parameters. Potential improvements on common plate-fin tube dehumidifier designs are identified by examining various methods of optimizing tube diameter and longitudinal and transverse tube spacing to achieve maximum heat flow for a given quantity of fin material at a typical HDH operating point. Thicker fins are recommended than for HVAC geometries, as the thermal conductive resistance of HVAC fins restricts dehumidifier performance under HDH operating conditions. NOMENCLATUREA=heat transfer area, m2Ac=cross-sectional area, m2At,b=surface area of tubes without fins, m2cp=specific heat capacity at constant pressure, J kg−1 K−1D=diffusion coefficient of water in dry air, m2 s−1dc=fin collar diameter, mdf=fin thickness, mdh=hydraulic diameter, md1=inner tube diameter, md2=outer tube diameter, mg=acceleration due to gravity g = 9.80665 m s−2H=height in air flow direction, mh=specific enthalpy, J kg−1hfg=specific latent heat of vaporization, J kg−1ht=heat transfer coefficient, W m−2 K−1j=Colburn j-factor, j = ht ρ−1 w −1 cp−1 Pr2/3K=total pressure loss coefficientk=thermal conductivity, W m−1 K−1L=finned tube length (one pass), mLe=Lewis number, Le = k cp−1 ρ −1 D −1M=molar mass, g mol−1m=Prandtl number exponent, fin parameter=mass flow rate, kg s−1n=number of longitudinal tube rowsNu=Nusselt number, Nu = ht dh k −1p=pressure, PaPr=Prandtl number, Pr = cp μ k-1=heat flow, WRex=Reynolds number with characteristic length x, Rex = w x ρ μ −1Rf=thermal fouling resistance, m2 K W−1S=salinity, kg kg−1s=fin spacing, mSc=Schmidt number, Sc = μ ρ −1 D−1Sh=Sherwood number, Sh = hm dh D−1t=temperature, °Cta=air temperature in bulk flow, °Ctsw=saline water or seawater temperature in bulk flow, °Ctpw=product water temperature, condensate temperature, °CW=width normal to airflow direction, mw=flow velocity, m s−1w0=air face velocity, m s−1wc=velocity at minimum flow area, m s−1Xl=longitudinal tube spacing (see Figure 7), mXt=transverse tube spacing (see Figure 7), mx=coordinate (see Figure 2), my=coordinate in air flow direction (see Figure 2), mz=coordinate in tube axis direction (see Figure 2), mGreek Symbolsζ=Ackermann correctionηf=fin efficiencyηs=surface efficiencyμ=dynamic viscosity, kg m−1 s−1ξ=Darcy-Weissbach friction factorρ=mass density, kg m−3ϕa=relative humidityω=humidity ratio, kg kg−1Subscripts0=wall surface on condensate sidea=humid airda=dry airdec=decelerationf=fin, liquidg=vapor, gaseousI=at the interfacial boundary between air and condensatel=laminarpw=product watersw=saline water, seawatert=tube, turbulentw=waterx=in x directiony=in y direction, at position yz=in z direction, at position zAdditional informationNotes on contributorsMartin SieversMartin Sievers works in the advanced engineering department at MAHLE Behr GmbH & Co. KG, Stuttgart, Germany. He obtained a B.Sc. in general engineering science and a Dipl.-Ing. in mechanical engineering from Hamburg University of Technology, Germany. In 2008/2009 he was a visiting graduate student in the Department of Mechanical Engineering at the University of California at Berkeley and in 2010 in the Department of Mechanical Engineering at Massachusetts Institute of Technology. His research interests are heat transfer with phase change, power electronics cooling, and heat exchanger design and simulation.John H. Lienhard VJohn H. Lienhard V is the Samuel C. Collins Professor of Mechanical Engineering at MIT. During more than 25 years on the MIT faculty, his research and educational efforts have focused on heat transfer, desalination, thermodynamics, fluid mechanics, and instrumentation. He has also filled a number of administrative roles at MIT. Professor Lienhard received his bachelor's and master's degrees in thermal engineering at UCLA from the Chemical, Nuclear, and Thermal Engineering Department, and his Ph.D. from the Applied Mechanics and Engineering Science Department at UC San Diego. He has been the director of the Rohsenow Kendall Heat Transfer Laboratory since 1997, and he is the director of the Center for Clean Water and Clean Energy at MIT and KFUPM.