ASHRAE IJHVAC 9-2: 国际空调热工研究

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ASHRAE（美国采暖、制冷与空调工程师学会）是一个全球领先的组织，在建筑环境控制领域拥有深厚的专业知识和权威。《ASHRAE International Journal of Heating, Ventilation, Air-conditioning and Refrigerating Research》（HVAC研究国际期刊）是该协会出版的一本专业期刊，旨在发表与供热、通风、空气调节及制冷技术相关的最新研究成果。该期刊第九卷第二期，于2003年4月出版，其版权归属于ASHRAE，并受IHS许可提供，供Naval Aviation Depot使用，但禁止未经许可的复制或网络传播。期刊由Reinhard Radermacher博士担任编辑，他拥有机械工程博士学位，任马里兰大学环境能源工程中心的主任。他的团队包括多位知名学者，如Michael J. Brandemuehl博士、James E. Braun博士、Alberto Cavallini博士等，他们分别在各自的学术机构教授相关课程并进行研究工作，如工程学、物理学和技术领域，以及建筑设计、工程科学和生物医学建模等。 ASHRAE的期刊内容涵盖了广泛的HVAC技术，包括但不限于热能系统的设计与优化、节能技术、空气质量控制、暖通空调设备的性能评估、以及新兴的可持续建筑策略。每一期的论文可能探讨的主题可能包括但不限于： 1. **高效能建筑系统**：介绍新的能源效率标准和创新设计，以减少能耗和碳排放。 2. **智能控制系统**：探讨如何通过自动化和人工智能技术提高建筑环境的舒适度和能源管理效率。 3. **可再生能源集成**：研究太阳能、地源热泵等可再生能源在HVAC中的应用和潜力。 4. **室内环境质量**：关注空气质量监测、污染物控制和人体健康的影响。 5. **绿色建筑材料**：新材料和施工方法对室内气候控制的影响和环保性。这些文章不仅提供了学术交流的平台，也对行业实践者、政策制定者以及研究者提供了最新的行业动态和前沿理论，有助于推动HVAC领域的持续发展和创新。通过阅读和研究这样的期刊，读者可以了解到全球范围内对暖通空调技术的最新进展和挑战，从而在各自的工作中做出更科学、更有效的决策。

120

HVAC&R

RESEARCH

1600

1400

1200

1000

800

I-

600

100

150

200 250 300 350

Surfactant Concentration

[moi

ppm]

Figure 6. Heat Transfer Coefficient

vs.

Surfactant Concentration for Three Alcohols,

from Beutler et al. (1996a)

ified correspond approximately to the range 0.0171r10.044 kg/m-s).

the surfactant concen-

tration increases to about 40 ppm, the heat transfer coefficient continues to increase slightly to a

maximum improvement of about 60% to

85% depending on film flow rate, but beyond 40 ppm,

the heat transfer coefficient is relatively constant.

Kim

et al.

996) found very similar behavior

with a vertical tube absorber, although the maximum augmentation of the heat transfer coeffi-

cient was about 200%. The solubility limit of 2EH in aqueous LiBr is around

100

ppm

(Kim et al. 1996; Kulankara et al. 1999) but varies with concentration and temperature (Kim et

al. 1994; Kulankara and Herold 2002). Even with the apparent uncertainty in the solubility limit

in the literature (Ya0 et al. 1991), the results of Hoffman et al. (1996) indicate that the heat trans-

fer enhancement occurs at concentrations well below the solubility limit. The test apparatus was

enclosed in a glass container, which allowed visual inspection of the film flow patterns. The

authors noted that the largest improvement due to the 2EH was in the low film-flow rate cases

where the surfactant eliminated the incomplete wetting that occurred when

surfactant was

pïeseiit.

They

akü

iiüied

kicabcd

iio&onrai

movement

Kim

fne

tubes in the regions

of high absorption, which agrees with the observations of Cosenza and Vliet

990) and others.

Finally, they suggest that the effect of surfactant cannot be easily correlated simply with tradi-

tional nondimensional numbers and that understanding the effects requires determining the

details of the film flow and droplet formation in the presence of surfactant.

The results of Hoffman et al. (1996) did not show the level of improvement found in the

results of Nagaoka et al.

(1987b),

although the

two

groups were using different surfactants in

their investigations. J3eutler et al. (1996a) investigated the effect of three surfactants, 2EH,

I-octanol, and 1-decanol, using the same experimental apparatus as Greiter et al. (1993) and

Hoffman et al. (1996). The heat transfer coefficients shown in Figure 6 illustrate that the

improvement with octanol is greater (about

80%)

than with 2EH (about 60%) for a nominal

operating condition. The improvement with octanol shown in Figure 6 is somewhat lower than

that reported by Fujita (1993) (Figure 3), but the film flow rate is about 0.043 kg/m-s, which is

higher than the highest flow rate used by Fujita (1993). Since the improvement in Figure 3

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VOLUME

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121

appears to be decreasing with increasing film flow rate, these results seem to corroborate each

other. This may suggest that the results shown by Fujita (1993) were also influenced by incom-

plete surface wetting at the low flow rates without surfactant. Beutler et al. (1996a) note that all

three alcohols studied lower the surface tension of aqueous LiBr and that this reduction

increases steadily up to their respective solubility limits. This trend in surface tension does not,

however, adequately explain the trends shown in Figure 6. Moreover, they point out that the

sur-

face tension is nearly the same with 1-octano1 as with 1-decano1 at high concentrations. There-

fore, they conclude that surface tension reduction is not a sufficient parameter for explaining the

absorption enhancement. Kulankara and Herold (2000) suggest that this might be because the

vapor pressures of 2EH and octano1 are 12 and

times higher than 1-decanol, resulting in rela-

tively little 1 -deconal in the vapor to be adsorbed onto the film surface. This phenomenon, there-

fore, provides one possible explanation of the results shown in Figure 6. Unfortunately, Beutler

et al. (1996a) do not estimate the mass transfer coefficients,

there is no corroboration of

results shown in Figure 4.

investigators such as Kim et al. (1994) and Yuan and Herold (2001) have pointed out that the

surface tension in the presence of surfactants depends on the age of the surface and the constitu-

ents of the surrounding vapor, and that transient measurements of surface tension yield signifi-

cantly different results than steady-state measurements. The reduction of surface tension due to

surfactants increases with the age of the surface, although the duration of this delay depends on

the surfactant. Moreover, Kulankara and Herold (2000), Yuan and Herold (2001), and Kyung

and Herold (2002) suggest that the arrival of surfactant to the surface from the vapor phase is

more influential than the concentration in the liquid phase as will be discussed below. Also, Zie-

gler et al. (1 999) use the apparatus of Greiter et al. (1 993) to show that the heat transfer coeffi-

cient actually increases as the temperature driving force increases, which suggests a two-way

coupling between the effect of surfactant

the absorption rate; that is, the enhancement of

absorption rates due to the surfactant depends on the rate of absorption. Kyung and Herold

(2000) show similar trends using an entire single-effect absorption system. Additionally, Ziegler

et al. (1999) show that almost all of the enhancement associated with 2EH occurs with concen-

trations as low as a fraction of a ppm. As expected, since surfactants primarily influence the

behavior at interfaces, the bulk concentration of surfactant is not as relevant as its concentration

at the film interface in determining the effect of surfactant on absorption (Kulankara et al. 1999).

Atchley et al. (1998) and Miller (1999) both investigate the effect of 2EH on absorption over

six horizontal tubes; these two reports are based on data taken from the same test apparatus.

Although their test setup has a comparatively low coolant-side heat transfer coefficient (similar

to Cosenza and Vliet

[

1990]), their results corroborate some of the findings mentioned above. In

addition, the authors present photographs of the change in flow pattern when

500

ppm 2EH is

added. The improvement in heat transfer coefficient due to the addition of surfactant reported by

the authors is nominally about 200% to 300%, which is higher than reported above. However,

this result is obtained with a lower solution flow rate (0.009 kgím-s) and a higher inlet LiBr con-

centration (60% to 62%) than the previous results. Nagaoka et al. (1987b) showed that the

improvement due to surfactant increases as the film flow rate decreases. This trend can also be

seen indirectly in some of the results of Atchley et al. (1998) and Miller (1999). Kim et al.

(1 994) suggest that for a given concentration of surfactant, the surface tension reduction

increases as the LiBr concentration increases. Thus, it is plausible that the larger improvement

measured by Atchley et al. (1 998) and Miller (1999) is a combined result of the lower solution

flow rate and higher inlet concentration. By varying the coolant inlet temperature and, thus, the

total absorption duty, the authors show that without surfactant, the heat transfer coefficient

remains nearly constant irrespective of absorber load. However, with surfactant, the heat trans-

fer coefficient increases steadily with increasing absorption rate, the same coupling observed by

Ziegler et al. (1 999). The authors also note that the effect of the 2EH on the flow pattern changes

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122

HVAC&R

RESEARCH

with time. The photographs they present show that shortly after the surfactant is introduced, the

film exhibits a wavy texture with a large amount of motion in the axial direction of the tube as

noted by other investigators (Cosenza and Vliet 1990; Beutler et al. 1996a; Hoffman et al.

1996). They also show that the behavior of the droplets between the tubes is changed from the

case with no surfactant; there appear to be more droplet formation sites, although the size of the

droplets and the trail of solution following them is about the same as with no surfactant.

pho-

tograph 16 minutes later shows that the film interface is largely smooth and that the droplets are

significantly smaller, spaced closer together, and have much shorter trails than either the case

without 2EH or shortly after its introduction. The authors report that the absorber performance

during this transient was nearly constant, but they provide no explanation for this transient

change in the film behavior, although these observations may corroborate the theory of Kulan-

kara and Herold (2000) and Kyung and Herold (2002) that the vapor-borne surfactant is impor-

tant. Other researchers have also noted that the droplets become smaller and more closely

spaced in the presence of surfactant (Beutler et al. 1996b; Hoffman et al. 1996).

Kyung and Herold (2000,2002) investigate the effect of 2EH on the absorption process using

a bank of four and eight horizontal tubes in a complete single-effect absorption heat pump cycle.

Besides using an entire cycle, they also purposely incline the horizontal tubes 0.45" from hori-

zontal to promote wetting. The authors note that at the low flow rates, there are still

dry

patches

when no additive is present, but because of the inclination, the

dry

patches continuously move

along the tube

that no portion of the surface remains dry for a period of one second. The bank

of four tubes was tested at the same system conditions as the bank of eight tubes. Significantly

higher heat fluxes were seen in the bank of four tubes. For the bank of four tubes, the authors

show that the heat transfer coefficient enhancement due to the presence of surfactant (100 ppm)

was about

67%

and basically independent of solution flow rate for the conditions shown in

Table

It is plausible that this lack of dependence

the flow rate is due to the improved wet-

ting at low solution flow rates from the tube inclination compared to the results discussed above.

With the bank of eight tubes and thus a lower heat and mass flux, the improvement due to

2EH

was reported to be only 20%. This corroborates the fact that the enhancement due to surfactant

depends on the absorption rate and also supports the hypothesis about the flow of surfactant to

the film surface from the vapor phase. The authors show that the heat transfer coefficient

increases with the concentration of 2EH only up to 80 ppm, with further increases in 2EH con-

centration not affecting the heat transfer coefficient, similar to the findings of Hoffman et al.

(1 996). From visual observation of the film, the authors note that solution slinging, or the ten-

dency of the solution to fall from the underside of a tube at an angle other than vertical due to

imbalanced flow on the two sides of the tube, was more severe with 2EH present. Because when

directly beneath, solution slinging can decrease the amount of solution flowing over subsequent

tubes in a single-column arrangement. The resulting lower solution flow rate on subsequent

tubes due to slinging would decrease the heat transfer coefficient, as reported by many research-

ers including Kyung and Herold (2002). This is discussed further in the section addressing the

effects

solution flow rate.

From the above literature, it is clear that surfactants play a major role in the performance of

horizontal tube absorbers. Most researchers agree that effective surfactants create significant

Marangoni convection (induced by

surface tension gradients) that causes mixing

the bound-

ary layers within the film and exposes fresh surface to the vapor. Hoffman et al.

996) and Zie-

gler and Grossman (1996) point out that the trends and theories reported about surfactants by

different researchers to date are somewhat conflicting. For instance, Kashiwagi et al. (1985) and

others have suggested that Marangoni convection is induced by the presence of so-called surfac-

tant islands. Others (Hozawa et al. 1991; Hihara and Saito 1993; Ziegler and Grossman 1996)

so~ù:iun

s!in,ing

GCCuïS,

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falLiig

from

illig;it

íloi

iíllpact

tiliie

tubiie

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have proposed that the change in surface tension with LiBr concentration andor temperature in

the presence of surfactant is what causes the instabilities in the liquid film. The numerical analy-

sis of Koenig et al. (1999) suggested that, in addition to the change in surface tension with LiBr

concentration and temperature, the diffusivity of the surfactant may be important. Fujita and

Hihara (1999) suggested that the effect of temperature may actually be more important than that

of concentration for the surfactant 1-octanol. The work of Kulankara and Herold

(2000)

and

Kyung and Herold

(2002)

points to the importance of surfactant in the vapor phase, providing

some explanation of the differences in the effects of surfactant at different absorption rates.

Also, they explain why additives with low volatility may not enhance absorption rates signifi-

cantly in spite of their surfactant properties. Visual observations of the film during absorption

have identified significant motion in the axial direction of the tubes in the form of wavy rivulets

and the change in droplets between the tubes as the predominant mechanism by which the liquid

film is agitated and absorption rates increased. In addition to inducing film instabilities, the sur-

factant has been shown to alter the behavior of the droplets between the tubes, which some

researchers believe contributes significantly to the overall mass transfer process (Nomura et al.

1993; Kirby and Perez-Blanco 1994; Atchley et al. 1998; Jeong and Garimella

2002).

However,

this basic understanding reveals several areas where more research is needed to further under-

stand and quantify the effect of surfactants

on the overall absorption rate, including the quantifi-

cation of the effects on film waviness, heat and mass transfer coefficients, droplet behavior, the

coupling between absorption rate and interfacial instabilities, and surface wetting.

EFFECTS OF ABSORBER GEOMETRY

Primary design features that affect the performance of a horizontal tube absorber include:

tube diameter

vertical space between tubes

number of tubes per column

tube surface structure

The effects of these parameters are summarized in Table 3 and discussed below. Other parame-

ters, such

tube length and number of columns, basically scale the size of the absorber. These

could affect the performance by changing the film flow rate per tube length, which will be dis-

cussed in the section on operating conditions. Other practical design considerations that must be

considered in a complete design but that will not be discussed here include: vapor flow area (to

minimize vapor pressure drop), film distribution method (to ensure even distribution and pro-

mote surface wetting), vapor inlet, solution collection, purge system for non-absorbable gases,

etc.

Tube Diameter

The literature is remarkably bereft of investigations into the effect of this parameter. The

range of tube diameters shown in Table 1 is fairly narrow, with most falling between 15 and 20

mm.

Cosenza and Vliet (1990) compare the results with a bank of five 20 mm tubes to those

with a bank of three 42

tubes, both with about 500 ppm 2EH, and suggest the following

re1 ationship

(3)

where

19.5

mm,

4rlp, and

is the viscosity. Although the authors point out the lim-

ited basis for this correlation, this relationship suggests that as the tube diameter increases the

heat transfer coefficient decreases, although the sensitivity is not great. Because the tube spacing

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124

HVAC&R

RESEARCH

Table

Summary of the Effects of Absorber Geometry

Parameter

Tube

diameter

Vertical space between

tubes

Number of tubes

per

column

Tube

surface

structure

Effects

Cosenza and Vliet (1990) suggest

that

heat

transfer coefficients decrease

with

increasing

diameter.

Wassenaar

(1996) suggests that between-tube mixing improves up to

between tubes. Atchley

al. (1998)

and

Miller

(1999) suggest

that

results

depend on presence

surfactant,

and

wetting

may

improve with

increased tube spacing.

Results

from

Greiter

al.

(1 993) and Nomura

al.

(1993) indicate

that

the heat transfer

coefficient

increases farther

down the

tube bank,

but

absorption

rate

decreases due to driving

force

decrease with tube number.

Most

research indicates improved performance with enhanced surface

tubes. Wetting and transfer coefficients may

both

be improved, but

results

depend

presence of surfactant.

The

effects

heat

transfer coefficients

and

mass

transfer coefficients are often different. Some surface structures

may inhibit the augmentation of the mass transfer coefficient due

surfactant bv restricting axial motion of

the

film

(Fuiita 1993).

and number of tubes are different between the

two

test setups, there may be some confounding

effect in the results. Nevertheless, this trend seems reasonable considering the effect of droplets

and boundary layer development. Assuming a well-mixed film at the top of each tube due to the

action of the droplets, larger tubes cause the boundary layers in the film to become more fully

developed and, thus, decrease the heat and mass transfer rates (Cosenza and Vliet 1990). In the

case of a smaller tube, there is less distance for the film to flow between droplet mixing events

and so it might be anticipated that smaller tubes would lead to greater transfer coefficients.

Also,

some investigators (Nomura et al. 1993; Kirby and Perez-Blanco 1994) have suggested that the

droplets contribute significantly to the total mass transfer. For the same total surface area,

smaller tubes would lead to more droplets because more tubes would be required. Thus, the rela-

tionship in Equation 3 seems at least qualitatively correct, although these plausible interpreta-

tions must be confirmed through specifically designed experiments and analysis.

Vertical Spacing Between Tubes

Wassenaar (1996), Atchley et al. (1998), and Miller (1999) systematically investigate the

effect of tube spacing on the performance of a horizontal tube absorber. Wassenaar (1 996) uti-

lizes a bank of ten 12

tubes where the space between the tubes could be varied from

to 24

(center-centericiiameter from

3.û).

lie absoibent the author uses

mole ratio

mixture of LiBr and ZnBr2 with the refrigerant

CH30H. Experimental results are not presented.

Instead ratios of predicted outlet solution temperature and concentration to the measured values

are shown. Furthermore, the inlet conditions are specified in terms of only a few nondimen-

sional parameters, and, thus, it is difficult

determine the actual inlet conditions as reflected in

Table 2. The author does state that the amount of subcooling was large “in order to get a suffi-

cient change in temperature and concentration.” The author notes that depending on film flow

rate and tube spacing, the liquid may flow between the tubes as droplets, jets, or sheets; for the

larger tube spacing, the transition from droplet to jet flow occurred at Re

1 where the charac-

teristic length in the expression for Re is the Nusselt film thickness for a vertical wall. The

author also notes that as the tube spacing increases, the space between the droplets increases.

The testing is performed without any surfactant, and incomplete wetting of the tube surfaces

results. The author notes that the correlation between model and experimental results improves

if the actual wet surface area is accounted for. To account for tube spacing in the model, the

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