Received 10 September 2013Received in revised form 24 November 2013Accepted 9 January 2014Available online 20 January 2014
Keywords:Heat exchangerAxial conductionFlow maldistribution
When a compact microchannel heat exchanger is operated at cryogenic environments, it has potentialproblems of axial conduction and ow maldistribution. To analyze these detrimental effects, the heat
First, the heat transfer area is increased due to small hydraulicdiameter of the channel, therefore, the area density is large withinsame volume. Since the heat transfer coefcient is larger than that
ers . The owcomposed of bun-l heat excmaldistr
problem should be considered. Moreover, the axial condproblem appears when large temperature difference exists igle heat exchanger. Compact (short) heat exchangers areaccompanied with larger temperature gradient than conventional(long) heat exchangers due to small geometry. Axial conductioneffect, therefore, is not negligible, but sometimes critical for itsthermal performance.
The owmaldistribution problems have been treated in variouscounter-ow heat exchanger geometries. Because actual ow dis-tribution is hard to measure, some simplied ow maldistribution
Corresponding author. Tel.: +82 42 350 3079; fax: +82 42 350 8207.E-mail addresses: [email protected] (S. Baek), [email protected] (C. Lee),
[email protected] (S. Jeong).1 Tel.: +82 42 350 3079; fax: +82 42 350 8207.2 Tel.: +82 42 350 3039; fax: +82 42 350 8207.
Cryogenics 60 (2014) 4961
Contents lists availab
journal homepage: www.elseThe compact cryogenic liquefaction process inevitably requiressmall components due to space limitation of a ship, as well as highperformance heat exchanger (e > 0.90 NTU > 10) for efcient oper-ation. Microchannel heat exchangers satisfy these requirements.
commonly treated in conventional heat exchangmaldistribution occurs when a heat exchanger isdles of parallel channels. Since a microchannedoes have parallel channels, therefore, the owhttp://dx.doi.org/10.1016/j.cryogenics.2014.01.0030011-2275/ 2014 Elsevier Ltd. All rights reserved.hangeributionuctionn a sin-usuallyDemand of high performance compact heat exchangers isincreasing for volume-limited cryogenic processes. The most rep-resentative example of the volume limited cryogenic process isthe natural gas liquefaction process for Liqueed Natural Gas-Floating Production Storage and Ofoading (LNG-FPSO).
achieved within small volume of heat exchanger.A design method of compact microchannel heat exchanger for
cryogenic environment is not different from that of conventionalheat exchanger. When designing a high effectiveness heat exchan-ger, however, one must consider some particular problems such asow maldistribution and axial conduction effects that are notMicrochannelCryogenic
1. Introductionexchanger model that includes both axial conduction and owmaldistribution effect is developed in con-sideration of the microchannel heat exchanger geometry. A dimensionless axial conduction parameter (k)is used to describe the axial conduction effect, and the coefcient of variation (CoV) is introduced toquantify the ow maldistribution condition. The effectiveness of heat exchanger is calculated accordingto the various values of the axial conduction parameter and the CoV. The analysis results show that theheat exchanger effectiveness is insensitive when k is less than 0.005, and effectiveness is degraded withthe large value of CoV. Three microchannel heat exchangers are fabricated with printed circuit heatexchanger (PCHE) technology for validation purpose of the heat exchanger model. The rst heat exchan-ger is a conventional heat exchanger, the second heat exchanger has the modied cross section to elim-inate axial conduction effect, and the third heat exchanger has the modied cross section and the crosslink in parallel channel to mitigate ow maldistribution effect. These heat exchangers are tested in cryo-genic single-phase, and two-phase environments. The third heat exchanger shows the ideal thermal char-acteristic, while the other two heat exchangers experience some performance degradation due to axialconduction or ow maldistribution. The impact of axial conduction and ow maldistribution effectsare veried by the simulation results and compared with the experimental results.
2014 Elsevier Ltd. All rights reserved.
of macrochannel in laminar ow, the higher effectiveness can beArticle history:Effect of ow maldistribution and axial cmicrochannel heat exchanger
Seungwhan Baek , Cheonkyu Lee 1, Sangkwon JeongCryogenic Engineering Laboratory, #5119, Department of Mechanical Engineering, KoreDaejeon 305-701, Republic of Korea
a r t i c l e i n f o a b s t r a c tduction on compact
vanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu,
le at ScienceDirect
vier .com/locate /cryogenics
pDh hydraulic diameter, m
enicconditions have been assumed with the heat exchanger geometry.Fleming  and Jung [3,4] have investigated ow maldistributioneffect in plate type heat exchanger geometry. Their studies haveassumed that the ow distribution on one side is uniformly distrib-uted, and the other side is not. The fraction of FL is introduced todene the degree of ow maldistribution, where FL indicates thepercentage of layer with lower-than-average ow. Rao  investi-gated the effectiveness loss due to ow maldistribution in conven-tional plate heat exchangers. The ow distribution prole in U andZ type plate type heat exchanger header is used, and the effective-ness is calculated analytically. Pacio and Dorao  considered theimpact of ow maldistribution in a shell and tube heat exchangergeometry, and assumed that the ow maldistribution occurs in
G mass ux, kg/m2 sh heat transfer coefcient, W/m2 Kk thermal conductivity, W/m KL length of heat exchanger, m_m mass ow rate, kg/sN number of channels, number of dataNTU number of transfer unitsNu Nusselt numberP precision errorq heat transfer rate, WRe Reynolds numberS standard deviation of datat T-distribution for a condence levelT temperature, Kth thickness of separator, mU heat transfer conductance, W/m2 KNomenclature
A area, m2
B bias errorC heat capacity rate, W/Kc heat capacity, J/kg K
50 S. Baek et al. / Cryogcylindrical layers. However, the ow maldistribution in micro-channel heat exchanger geometry has not been carefully treatedyet.
Axial conduction problem in heat exchangers have been studiedby many researchers. Among those researchers, Kroeger  solvede-NTU relation analytically, and Nellis  investigated axial con-duction in counterow heat exchanger numerically. Since theseheat exchanger models were composed with only one hot and coldchannels, the ow maldistribution problem was not considered.
None of the preceding researchers studied the coupled problemof ow maldistribution and axial conduction effects at the sametime in the heat exchanger. The objective of this study is to quan-tify the ow maldistribution and axial conduction effect simulta-neously in the microchannel heat exchanger. The method tomitigate ow maldistribution and axial conduction are proposed,and the performance improvement is measured experimentallyin microchannel heat exchangers.
2. Heat exchanger modeling
2.1. Heat exchanger model with axial conduction effect
The one dimensional counterow heat exchanger model includ-ing axial conduction effect is rst developed by using MATLAB[8,9]. The model is composed with the hot uid channel, the colduid channel, and the metal separator as displayed in Fig. 1. Thegoverning equations are developed from the energy balance ofthe uid streams and the metal separator, as equations from (1)(3).
Th Tw 1
_mccp;c dTcdx hcAc;HT
Tw Tc 2
_mhcp;h dThdx _mccp;c
Since the numerical scheme of heat exchanger model is fully ex-plained in the literature , the important assumptions are only
x axial position, m
Greeka aspect ratio of square channele effectivenessk dimensionless axial conduction parameterl average value or viscosityr standard deviationH nondimensional temperature
Subscriptc cold uidh hot uidideal ideal conditionin inletmax maximummin minimumMR mixed refrigerantout outletw wall or wall cross sectional area
s 60 (2014) 4961highlighted in this paper. The model inputs are as followings:
constant heat transfer coefcients (hh, hc) on both sides thickness (thw) and constant thermal conductivity (kw) of thewall (or separator)
mass ow rate (mh,mc) on both sides inlet temperature (Th,in, Tc,in) on both sides constant heat capacity (cp,h, cp,c) on both sides
The pressure drop in the microchannel is neglected in thisstudy. The output results from the heat exchanger model are thetemperature prole in the heat exchanger. The heat transfer coef-cients and heat capacity values on both streams are assumed tohave constant values. The number of transfer unit (NTU) is denedas the following equation.
NTU UAHT _mcpmin4
The thermal resistance of metal separator between hot and colduids is neglected. Therefore, the overall heat conductance is de-ned with only the local heat transfer coefcients and the heattransfer area, excluding the thermal resistance of the metal separa-tor, as the following equation.
The effectiveness of heat exchanger is calculated with the fol-lowing equation
e q 6
exchanger. The metal separator receives heat from the hot uid,however, not all of the heat is transferred to the cold uid. Heatis partially transferred through the metal plate in axial direction.Therefore, t