Effect of Inlet Humidity Condition on the Air Side Performance of an Inclined Brazed Aluminum Evaporator

Effect of inlet humidity condition on the air-sideperformance of an inclined brazed aluminum evaporator

Man-Hoe Kim*, Sumin Song, Clark W. Bullard

Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign,

1206 West Green Street, Urbana, IL 61801, USA

Received 15 March 2001; accepted 13 June 2001

Abstract

The effect of air inlet humidity condition on the air-side heat transfer and pressure drop characteristics for aninclined brazed aluminum heat exchanger has been investigated experimentally. For a heat exchanger with a louverangle of 27�, fin pitch of 2.1 mm and flow depth of 27.9 mm, a series of tests are conducted for the air-side Reynolds

numbers of 80–400, with variation of inlet humidity condition. The heat transfer data are obtained for wet conditiononly and the pressure drop data are measured for both dry and wet conditions. The inlet air temperature and relativehumidity range are 12 �C and 60–90%, respectively. The inclination angles (�) from the vertical position are 0, 14, 45,

and 67� clockwise (leeward direction). The inclination angles affect moderately the sensible heat transfer coefficient forwet condition, and the pressure drops for both dry and wet conditions increase systematically with the inclinationangle. The heat transfer and pressure drop characteristics under wet condition are not influenced substantially by theair inlet humidity for � 445�. The effect of the louver directions at the inlet and outlet of the inclined heat exchanger on

the performance is also addressed. # 2002 Elsevier Science Ltd and IIR. All rights reserved.

Keywords: Heat transfer; Mass transfer; Air cooler; Fin; Design; Humid air; Performance; Measurement

Impact de l’humidite a l’entree d’un evaporateur incline braseen aluminium sur la performance cote air

Mots cles : Transfert de chaleur ; Transfert de masse ; Refroidisseur d’air ; Ailette ; Geometrie ; Air humide ; Performance ; Mesure

1. Introduction

The effect of inlet humidity conditions and inclination

angle on the air-side thermal hydraulic performance of abrazed aluminum heat exchanger has been investigatedexperimentally. Inclined one slab air-cooled heat

exchangers have been used in air-conditioning and heatpump applications for the cost-effective compact sys-tems, although these configurations may deteriorate the

system performance. There are some publications on theeffect of inclination angle and inlet humidity conditionson the heat transfer and pressure drop of the heat

exchangers. However, most of the published data haveconsidered bare-tube banks, high-fin tube banks andconventional finned tube heat exchangers [1–10]. Mirth

and Ramadhyani [7,8] presented that inlet humidityconditions affected the heat exchanger performance. Onthe other hand, Wang et al. [9,10] reported that they didnot influence significantly the sensible heat transfer

coefficients, while their effect on the pressure dropsdepended on the heat exchanger configurations, espe-cially the longitudinal tube pitch. They found the effect

0140-7007/02/$22.00 # 2002 Elsevier Science Ltd and IIR. All rights reserved.

PI I : S0140-7007(01 )00061-5

International Journal of Refrigeration 25 (2002) 611–620

www.elsevier.com/locate/ijrefrig

* Corresponding author. Tel.: +1-217-244-1531; fax: +1-

217-333-1942.

E-mail address: [email protected] (M.-H. Kim).

of inlet conditions was negligible when the longitudinal

tube pitch was 22 mm, while for the longitudinal pitchof 19.05 mm the friction factors for RH1 =90% were 5–25% larger than those for RH1=50%.

A microchannel tube heat exchanger is one of thepotential alternatives for replacing the conventional fin-ned tube heat exchangers and has been considered asboth evaporator and gas cooler for prototype CO2 air-

conditioning systems [11]. Many investigators havestudied the air-side heat transfer and pressure dropcharacteristics of the louvered fin and flat tube heat

exchangers [12–19]. However, only small amount of

published data on the effect of the inclination angle onthe performance of the brazed aluminum heat exchan-gers is available in the open literature. Recently, Osada

et al. [20] studied the effect of inclination on the heattransfer and pressure drop characteristics of the louv-ered fin automotive evaporators with larger flow depth(Fd=58 and 70 mm) and conducted condensate visuali-

zation tests. They reported that both the leewardand windward inclinations improved heat exchangerperformance. Kim et al. [21] investigated the effect of

Nomenclature

Ac Minimum free-flow area for air side, m2

Af Fin surface area, m2

Afr Frontal area, m2

Ao Total air-side surface area (Af+At), m2

At Tube surface area, m2

Aw Tube wall area, m2

bi0, bp

0, bw Defined in Eq. (4)cp Specific heat, J/kgKCi Glycol-side capacity for wet surface, kg

Co Air-side capacity for wet surface, kgCr Capacity ratioD Heat exchanger core depth, m

Dh Hydraulic diameter, mf Fanning friction factorFp Fin pitch, mFd Flow depth, m

H Fin height or heat exchanger width, mHX Heat exchangerh Heat transfer coefficient, W/m2K

i Enthalpy, J/kgj Colburn j-factork Thermal conductivity, W/mK

Kc Abrupt contraction coefficientKe Abrupt expansion coefficientL Heat exchanger length, m

La Louver angle, degLl Louver length, mLp Louver pitch, mm:

Mass flow rate, kg/s

NTU Number of transfer unitNu Nusselt number (hDh/k)�P Pressure drop, Pa

Pr Prandtl number (�/�)Q Heat transfer rate, WRei Glycol-side Reynolds number (ViDhi/�i)ReLp Air-side Reynolds number based on

louver pitch (VcLp/no)RH Relative humidity, %

t Temperature, �C

Td Tube depth, mu Air face velocity, m/sUA Overall transport coefficient, based on

enthalpy difference, kg/sVc Maximum air velocity (u Afr/Ac), m/sVi Glycol velocity, m/sx* Dimensionless length defined in Eq. (5)

yw Thickness of Condensation water film, m

Greek letters

� Thermal diffusivity (k/�cp), m2/s

� Constant in Eq. (11)�f Fin thickness, m

�t Tube thickness, m�w Tube wall thickness, m" Effectiveness Aspect ratio of tube

f Fin efficiencyo Surface effectiveness� Viscosity, m2/s

� Inclination angle,� Density, kg/m3

� Contraction ratio of the fin array (Ac/Afr)

Subscripts1 Inlet

2 Outletf Fini Glycol-sidem Mean value

max Maximum valuemin Minimum valueo Air-side

p Tube surfaces Saturation statet Tube

w Wet condition or waterwall Tube wall

612 M.-H. Kim et al. / International Journal of Refrigeration 25 (2002) 611–620

inclination angle (0, �30, �45, and �60� clockwise) onthe heat transfer and pressure drop of a brazed alumi-num heat exchanger with Fd=20 mm under dry andwet conditions. They found that the heat transfer per-

formance for both dry and wet conditions was notinfluenced significantly by the inclination angle(�60�<� <60�), while the pressure drops increased

consistently with the inclination angle. For the effectof inlet humidity condition on the thermal hydraulicperformance of brazed aluminum multi-louvered fin

heat exchangers under wet conditions, however, there isno published data in the open literature.The purpose of this study is to provide experimental

data on the effect of inlet humidity condition on the air-side thermal hydraulic performance for an inclinedbrazed aluminum heat exchanger under dehumidifyingconditions. A series of tests are conducted for the air-

side Reynolds number range of 80–400 (based on louverpitch) with variation of the inclination angles (0, 14,45, and 67� clockwise) from the vertical position. The

effect of inlet humidity condition and inclination onthe heat transfer and pressure drop characteristics isaddressed.

2. Experiments

2.1. Test apparatus

Fig. 1 shows a schematic diagram of the test appara-

tus used in the study. It consists of a ducted airflowsystem, heat transfer fluid (glycol) circulation loop anddata acquisition systems. It is situated in a constant

temperature and humidity chamber that can maintaintemperature within �0.5 �C and absolute humidity�2%. The wind tunnel entrance is 1.0 m wide and 0.6 m

high. The air inlet conditions of the heat exchanger aremaintained by controlling the chamber temperature andhumidity. The exit wet-bulb temperature was deter-mined from weight measurement of condensate water in

the test sample. The air-side pressure drop through theheat exchanger is measured using a differential pressuretransducer and the airflow rate is determined from the

nozzle pressure difference.

2.2. Test heat exchanger

Fig. 2 indicates the geometrical configuration and theterminology of the test heat exchanger. As shown in

Table 1, the heat exchanger is an automotive radiatorwith no surface coating. It has flat coolant tubes with asingle rectangular channel and corrugated multi-louverfins with 27�-louver angle, flow depth of 27.9 mm, fin

pitch of 2.1 mm, tube pitch of 9.9 mm. The louver pitch,louver length and fin height are 1.4, 6.6, and 8.3 mm,respectively, and the core size tested is 394�381 mm.

2.3. Test conditions and methods

Fig. 3 shows a simple schematic of a heat exchangerinstallation. The heat exchanger is installed in the test

section, surrounded by insulation to protect it from heatloss and air leakage. For leeward (�=0, 14, 45, and 67�

clockwise) inclinations, a series of tests for wet condi-

tion are performed in the range of the Reynolds numberof 80–400. As shown in Fig. 3, air enters normal to thecoil and is then turned through a duct, except for �=67

which has an upstream duct. The inlet air temperatureand relative humidity ranges for wet condition are 12 �Cand 60–90%, respectively, and glycol inlet temperatures

are 0–2.5 �C. The heat transfer data are obtained for thewet surface condition and the pressure drop data aremeasured for both dry (adiabatic) and wet conditions,separately. The pressure drops for the dry condition are

measured with isothermal condition, while for wet con-dition the pressure drops were measured with variationof inlet humidities of 60–90%.

2.4. Data reduction

The data reduction process for wet condition is thesame as Kim et al. [17,21] used in their studies, so only abrief description is given here. The effectiveness-NTU

method is used for obtaining the air-side heat transfercoefficient.The "-NTU equation for both fluids unmixed condi-

tions is

" ¼ 1� expNTU0:22

Crexpð�CrNTU0:78Þ � 1� ��

ð1Þ

The effectiveness-NTU for the wet condition wasdetermined using the arithmetic average value (Q) of the

measured air- and glycol-side heat transfer rates; theenergy balance between them is within �3%.

" ¼Q

m:oðio1 � is;i1Þ

;NTU ¼UowAow

Cmin; Cr ¼

Cmin

Cmax

Co ¼ m:o; Ci ¼

m:icp;ib0i

ð2Þ

The air-side heat transfer coefficient can be obtained

from the following equation, based on enthalpy difference.

1

UowAow¼

b0ihiAi

þb0P�wall

kwallAwallþ

bwowhowAow

ð3Þ

Where bi0, bp

0 and bw are the slopes of the saturatedmoisture air enthalpy-temperature curves.

b0i ¼is;Pi � is;itPi � ti

; b0P ¼is;Po � is;PitPo � tPi

; bw ¼�is;w�ts;w

; ð4Þ

M.-H. Kim et al. / International Journal of Refrigeration 25 (2002) 611–620 613

For the heat transfer coefficients on the glycol-side,two-dimensional duct flow (thermally developing and

hydrodynamically developed flow) is assumed since theReynolds number (Rei=106–127) is small and the ductaspect ratio (=22.4) is extremely large [Fig. 2(a) andTable 1] and x* >0.006 [22].

Nui ¼ 7:541þ 0:0235=x

x ¼L

DhiReiPrið5Þ

The surface effectiveness and the fin efficiency for thewet surface [23] are

ow ¼ 1�Af

Aowð1� fwÞ ð6Þ

fw ¼tanhðmlÞ

ml; m ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2howkf�f

1þ�fFd

� �s; l ¼ H=2� �f ;

ð7Þ

The heat transfer coefficient for the wet surface is

how ¼1

cp;obwho

þywkw

ð8Þ

where ho is the sensible heat transfer coefficient for thewet surface, and yw and kw are the thickness and con-

ductivity of condensation water film, respectively. Theterm yw/kw is neglected here because it is very smallcompared to cp,o/(bwho) in practice. Note that how is

usually about twice ho in common operating conditionsand so the air-side fraction of the total thermal resis-tance is 50–75% in this study.

The heat transfer coefficient and the pressure dropcan be expressed as j and f factors

j ¼ho

�mVccp;oPr2=3o ð9Þ

Fig. 1. Schematic diagram of test apparatus.

Fig. 1. Schematic diagram of test apparatus.


Fig. 2. Schematic diagram of a brazed aluminum heat exchanger. (a) Definition of geometric parameters; (b) cross-section of louvered

fin geometry.

Fig. 2. Schematic diagram of a brazed aluminum heat exchanger. (a) Definition of geometric parameters; (b) cross-section of louvered fin

geometry.

Table 1

Specification of the test heat exchanger

Heat exchanger Core size, H�L�D (mm) 394�381�28

Air-side area, Ao (Afin+Atube) (m2) 4.08 (3.33+0.75)

Glycol-side area, Ai (m2) 0.75

Air-side hydraulic diameter, Dho (mm) 3.12

Glycol-side hydraulic diameter, Dhi (mm) 2.16

Tube Tube pitch, Tp (mm) 9.9

Tube spacing (mm) 7.9

Tube depth (major axis), Td (mm) 25.4

Tube thickness (minor axis), �t (mm) 1.9

Aspect ratio, 22.4

Wall thickness, �w (mm) 0.4

Number of tubes 38

Fin Fin pitch, Fp (mm) 2.1

Fin height, H (mm) 8.3

Flow depth, Fd (mm) 27.9

Fin thickness, �f (mm) 0.1

Louver pitch, Lp (mm) 1.4

Louver length, Ll (mm) 6.6

Louver angle, L� (�) 27

Number of louvers 18


f ¼Ac

Ao

�m�1

½2�1�Po

ð�mVcÞ2� ðKc þ 1� �2Þ

� 2�1�2

� 1

� �þ ð1� �2 � KeÞ

�1�2�

ð10Þ

WhereKc andKe are coefficients for pressure loss at theinlet and outlet of the heat exchanger [24]. Accounting forall instrument errors, property uncertainties, uncertain-

ties for the heat transfer coefficients and pressure dropswere estimated to be �13 and �11%, respectively [25].

3. Results and discussion

Figs. 4–9 present the test results for the heat transfer

and pressure drops. Fig. 4 shows how the sensible heattransfer coefficients for wet surface vary with face velo-city, inclination angle, and inlet relative humidity. As

expected, heat transfer coefficients increase with face airvelocity. For a fixed inlet humidity (80% RH), the heattransfer coefficients reach a maximum when �=14� and

then decrease with the increase of inclination angle. Thisinitial increase may be due to the effect of gravitationalforce promoting condensate drainage. The cause of thesubsequent decrease is less clear, as will be discussed

below. Osada et al. [20] reported that both the leewardand windward inclinations increased the heat transfercoefficients and inclination toward windward direction

had better performance. Recently, Kim et al. [21]reported also a modest inclination toward the leewardpromoted drainage and so the heat transfer coefficients

increased, but the windward inclination decreased theheat exchanger performance because the condensate hadto drain by flowing against the airflow direction. This

may be due to the different geometry of the heatexchangers used in their studies. Osada et al. [20] used asingle row test piece with larger flow depth, louver angle(Fd=58 mm, La=35�, and Fp/Lp= 1.5/1.2>1) and

three redirection louvers, while Kim et al. [21] used afull-scale heat exchanger (Fd=20 mm, La=27�, and Fp/Lp= 1.4/1.7 <1) with one redirection louver. Another

possibility can be due to the louver directions at the inletand outlet of the heat exchangers. Osada et al. [20]installed the heat exchangers so that the inlet and outlet

louvers turned the flow upward and downward, respec-tively, while Kim et al. [21] installed their heat exchan-ger in the opposition direction as shown in Fig. 5. This

indicates the louver directions at the inlet and outlet ofthe heat exchanger affect significantly the thermalhydraulic performance for both dry and wet conditions

when the heat exchanger is inclined from the verticalposition. The heat transfer coefficients decrease with theincrease of air inlet humidity, since higher inlet humidity

causes more condensate accumulation on the coil sur-face, which in turn acts as another thermal resistance forlow Reynolds number flow case studied here [17]. How-ever, the inlet humidity effect on the heat transfer coef-

ficient is not significant for the small inclination angles(� 445�). These results are similar to those of Wang etal. [9,10] who reported the effect of inlet humidity on the

Fig. 4. Effect of inlet humidity on the heat transfer coefficients.

Fig. 4. Effect of inlet humidity on the heat transfer coefficients.

Fig. 3. Schematic diagram of a heat exchanger installation (unit of length: mm). (a) �=0, 14, and 45�; (b) �=67�.

Fig. 3. Schematic diagram of a heat exchanger installation [unit of length: mm]. (a) y=0, 14, and 45�; (b) y=67�.


Fig. 5. Louver array at the inlet and outlet of the heat exchanger. (a) Present study and Kim et al. [21]; (b) Osada et al. [20].

Fig. 5. Louver array at the inlet and outlet of the heat exchanger. (a) Present study and Kim et al. [21]

Fig. 6. Air-side pressure drops for both dry and wet conditions

(80% RH). Fig. 7. Effect of air inlet humidity on pressure drops.

Fig. 7. Air-side pressure drops for both dry and wet conditions


heat transfer was negligible for the conventional finnedround tube heat exchangers. As shown in Fig. 4, itseffect increases with inclination angle and for �=67�,

the heat transfer coefficients decrease significantly withthe increase of air inlet humidity.Fig. 6 presents the air-side pressure drops vs. face

velocity with variation of inclination angle. As expected,

pressure drops for both dry and wet conditions increasesystematically with face velocity and inclination angle.The pressure drops for wet conditions are 3–14% larger

than those for dry condition at the same face velocity.

However, for �=67� a significant pressure drop increasewas observed. Unfortunately it is not possible to sepa-rate this effect from that due to the reduction of ductdiameter, because the constriction had to be installed

for structural and leakage reasons. However this resultis similar to that of Kim et al. [21] who reported that thepressure drop increased significantly when � 560�.

Fig. 7 shows the effect of air inlet humidity on thepressure drops. The inlet humidity has no significantinfluence on pressure drops, a result similar to that for

the conventional finned round tube heat exchangerswith fully wet surface [9]. However, the data obtainedearlier in a nearly identical wind tunnel with a micro-

channel tube heat exchanger having a smaller fin-to-louver pitch ratio (Fp/Lp=1.4/1.7<1) and larger flowdepth (Fd=41.8 mm) revealed a significant effect of airinlet humidity on the air-side pressure drops at �=0�

[26]. This difference could be attributed to the heatexchanger geometry as pointed out by Wang et al. [10]who found the effect of inlet humidity on the friction

depended on the heat exchanger geometry, especially thelongitudinal tube pitch. The heat exchanger tested inthis study has a larger fin-to-louver pitch ratio (Fp/

Lp=2.1/1.4>1) and smaller flow depth (Fd=27.9 mm),so the effect of condensate amount on the surface maybe smaller compared to the heat exchanger with smaller

fin pitch and greater flow depth, suggesting the inlethumidity effect on the pressure drops depends on heatexchanger configuration, such as Fp/Lp and Fd.Fig. 8 shows j and f factors for wet condition in case

of inlet humidity of 80% RH. The j factors increase withinclination angle, reach a maximum at �=14�, anddecrease again when � 545�. The j factors for inclined

heat exchangers are always larger than those for the

Fig. 8. Sensible j and f factors for wet conditions (80% RH).

Fig. 8. Effect of air inlet humidity on pressure drops.

Fig. 9. Upstream and downstream configurations for �=67�.

Fig. 9. Upstream and downstream configurations for �=67�.


vertical heat exchanger (�=0�) in the range consideredhere, reinforcing the observation that the alignment ofgravitational and the air flow inertia forces are impor-tant in condensate drainage, and affect the thermal

hydraulic performance of the inclined heat exchanger[21]. As expected, the friction factors decrease withReynolds number and increase monotonically with

inclination angle. The downstream (exit turning) lossesfor larger inclination angle are higher than those forsmaller inclination angle [21]. In the case of �=67�, a

significant increase of the f factor is observed, suggestingthat a recirculating flow region may develop at thedownstream corner at large inclination angles. Also at

�=67�, the j factor is smaller than those for �=14 and45�. One possible explanation of this phenomenon isthat there are additional losses due to constriction in theupstream duct, causing another recirculating region as

shown in Fig. 9.Kim et al. [21] proposed pressure drop correlations

under dry and wet conditions in the absence of an

upstream duct, of the form:

f� ¼ f0ð1��j j

90Þ�

ð11Þ

where fo is the friction factor [15,17] at 0� inclination

angle for the microchannel tube heat exchangers withFp/Lp<1, and � is a correlation constant [21]. Howeverthose correlations do not predict well the present dataobtained in these experiments, where an upstream duct

was present and Fp/Lp=2.1/1.4>1. Therefore, furtherstudy is needed for the pressure drop characteristics ofthe heat exchangers with Fp/Lp>1.

4. Concluding remarks

The effect of air inlet humidity on the heat transfercoefficients and pressure drops of a multi-louvered finheat exchanger under dehumidifying conditions has

been investigated experimentally with variation of incli-nation angle. The heat transfer characteristics are influ-enced moderately by the inclination angle. The pressure

drops for wet condition are 3–14% larger than those fordry condition, and increase monotonically with inclina-tion angle. The effect of air inlet humidity on the heat

transfer and pressure drop is negligible for �445� incase of the larger fin-pitch heat exchanger studied here.When the heat exchanger is inclined from the vertical

position, it is important to carefully record and investi-gate the fin orientation, because the louver directions atthe inlet and outlet of the heat exchangers can influencesignificantly the heat transfer and frictional pressure

drop. In one case the louver and inclination angles areadditive, and in the other case they must be subtractedto determine the angle of attack of the louvers. Further

study is also required to investigate the effect of inlethumidity on the air-side thermal hydraulic performanceof several different brazed aluminum heat exchangerconfigurations.

Acknowledgements

This study was supported by Hydro Alunova, A. S.and The Trane Company. We gratefully acknowledge

the guidance and assistance of our colleagues P. Hrnjak,J. Yin and M. Richter at the Air Conditioning andRefrigeration Center (ACRC) at the University of Illi-

nois at Urbana-Champaign.

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Documents

Effect of Inlet Humidity Condition on the Air Side Performance of an Inclined Brazed Aluminum Evaporator