Energy Effectiveness of Simultaneous Heat and Mass Exchange Devices

Narayan, G. Prakash; Mistry, Karan H.; Sharqawy, Mostafa H.; Zubair, Syed M.; Lienhard, John H.

doi:10.5098/hmt.v1.2.3001

Cited by 73 publications

(38 citation statements)

References 17 publications

(23 reference statements)

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“…In contrast to the effectiveness defined by Narayan et al [32] for simultaneous heat and mass exchangers, by which bubble columns can be compared to other (generally counterflow) dehumidifier types, the parallel-flow effectiveness acknowledges that as long as the column fluid is well-mixed, each bubble column stage acts like a parallel-flow device.…”

Section: Parallel-flow Effectivenessmentioning

confidence: 96%

Experiments and modeling of bubble column dehumidifier performance

Tow

Lienhard

2014

International Journal of Thermal Sciences

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Humidification-dehumidification is a promising technology for decentralized, small-scale desalination, but conventional dehumidifiers are expensive due to the large surface area required. Direct-contact dehumidification in bubble columns has been shown to significantly decrease dehumidifier size and cost. In this paper, the heat flux and parallel-flow effectiveness of a bubble column dehumidifier are investigated experimentally using significantly smaller cooling coils than in previous work. In addition, a model is developed which predicts the heat transfer rate with an average error of less than 3%.It is found that heat flux rises and effectiveness decreases with decreasing coil area. Increasing air flow rate and air temperature both lead to increased heat flux but decreased effectiveness. Neither bubble-on-coil impact nor column height are found to significantly affect heat flux or effectiveness. The conflicting findings of previous research on bubble-on-coil impact are explained by the other trends identified in this work. Modeling results for salt water * Address all correspondence to lienhard@mit.edu April 24, 2014 temperature and tube diameter are presented. Additional heat transfer in the air gap above the column liquid is explored, but found to be minimal for well-designed columns with low temperature pinch. These findings will inform the design of bubble column dehumidifiers for high heat recovery and low capital cost. Keywords: condensation, direct contact heat transfer, HDH desalination

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Section: Parallel-flow Effectivenessmentioning

confidence: 96%

Experiments and modeling of bubble column dehumidifier performance

Tow

Lienhard

2014

International Journal of Thermal Sciences

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“…The air effectiveness defined as the ratio of the actual to maximum possible air-side heat transfer that would occur if the outlet air stream was saturated at the incoming water temperature is given by [12] , , …”

Section: Fig 4 Temperature Variation Versus Time For Seawater Coolinmentioning

confidence: 99%

Thermal Performance Evaluation of Seawater Cooling Towers

Sharqawy

Husain

Zubair

et al. 2011

Volume 1: Advances in Aerospace Technology; Energy Water Nexus; Globalization of Engineering; Posters

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Seawater has been used for long time as a cooling fluid in heat exchangers to reduce fresh water usage in industry and power plants. The thermophysical properties of seawater are different from those of fresh water due to the salt content or salinity. This difference is sufficient to affect the heat and mass transfer processes which in turn change the thermal performance. Thermal design of fresh water cooling towers is described in detail in many textbooks and handbooks. However, only a rule of thumb is frequently used for designing of seawater cooling towers. This rule recommends degrading the tower performance by approximately 1% for every 10,000 ppm of salts in the feed water. In this paper, the thermal performance of seawater cooling towers is presented using a detailed model of counterflow wet cooling towers which takes into consideration the coupled simultaneous heat and mass transfer processes and uses state-of-the-art seawater properties from the literature. The model governing equations are solved numerically and the validity of this model is checked using new experimental data that has been measured using a bench top counterflow seawater cooling tower. The effect of the variation of seawater salinity as well as other operating conditions on the effectiveness and Merkel number is investigated.

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“…Several recent studies have highlighted the importance of entropy generation minimization in maximizing the performance of humidification-dehumidification (HDH) desalination systems [1][2][3][4][5][6]. Of particular relevance here is the conclusion that the greatest source of entropy generation in an HDH system is usually the condenser, or dehumidifier, where large fractions of noncondensable gas (typically 80 to 90%) control the overall heat transfer and condensation rates.…”

Section: Introductionmentioning

confidence: 99%

Entropy generation in condensation in the presence of high concentrations of noncondensable gases

Thiel

Lienhard

2012

International Journal of Heat and Mass Transfer

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The physical mechanisms of entropy generation in a condenser with high fractions of noncondensable gases are examined using scaling and boundary layer techniques, with the aim of defining a criterion for minimum entropy generation rate that is useful in engineering analyses. This process is particularly relevant in humidification-dehumidification desalination systems, where minimizing entropy generation per unit water produced is critical to maximizing system performance. The process is modeled by a consideration of the vapor/gas boundary layer alone, as it is the dominant thermal resistance and, consequently, the largest source of entropy production in many practical condensers with high fractions of noncondensable gases. Most previous studies of condensation have been restricted to a constant wall temperature, but it is shown here that for high concentrations of noncondensable gases, a varying wall temperature greatly reduces total entropy generation rate. Further, it is found that the diffusion of the condensing vapor through the vapor/noncondensable mixture boundary layer is the larger and often dominant mechanism of entropy production in such a condenser. As a result, when seeking to design a unit of desired heat transfer and condensation rates for minimum entropy generation, minimizing the variance in the driving force associated with diffusion yields a closer approximation to the minimum overall entropy generation rate than does equipartition of temperature difference.

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Energy Effectiveness of Simultaneous Heat and Mass Exchange Devices

Cited by 73 publications

References 17 publications

Experiments and modeling of bubble column dehumidifier performance

Experiments and modeling of bubble column dehumidifier performance

Thermal Performance Evaluation of Seawater Cooling Towers

Entropy generation in condensation in the presence of high concentrations of noncondensable gases

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