Heat transfer and pressure drop characteristics of wet air flow in metal foam under dehumidifying conditions

Accepted Manuscript Title: Heat transfer and pressure drop characteristics of wet air flow in metal foam under dehumidifying conditions Author: Haitao Hu, Xiaomin Weng, Dawei Zhuang, Guoliang Ding, Zhancheng Lai, Xudong Xu PII: DOI: Reference:

S1359-4311(15)00939-4 http://dx.doi.org/doi: 10.1016/j.applthermaleng.2015.09.019 ATE 7011

To appear in:

Applied Thermal Engineering

Received date: Accepted date:

11-5-2015 8-9-2015

Please cite this article as: Haitao Hu, Xiaomin Weng, Dawei Zhuang, Guoliang Ding, Zhancheng Lai, Xudong Xu, Heat transfer and pressure drop characteristics of wet air flow in metal foam under dehumidifying conditions, Applied Thermal Engineering (2015), http://dx.doi.org/doi: 10.1016/j.applthermaleng.2015.09.019. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Heat transfer and pressure drop characteristics of wet air flow in metal foam under dehumidifying conditions Haitao Hu, Xiaomin Weng, Dawei Zhuang, Guoliang Ding*, Zhancheng Lai, Xudong Xu (Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China) * Corresponding Author, Tel: 86-21-34206378; Fax: 86-21-34206814; E-mail: [email protected]

Keywords: Metal foam; Wet air; Dehumidifying condition; Heat transfer; Pressure drop

Highlights 

Heat and mass transfer characteristics of wet air in metal foam were analyzed.



Pressure drop characteristics of wet air in metal foam were analyzed.



Effects of metal foam PPI under dehumidifying conditions were analyzed.



Performances between metal foam and fin-and-tube heat exchangers were compared.

Abstract: In order to know the possibility of applying metal foam to improve performance of air conditioners, heat transfer and pressure drop characteristics of wet air flow in metal foam under air conditioning conditions were experimentally investigated and compared with those of a louvered fin-and-tube heat exchanger with the same volume. The experimental conditions cover the inlet air velocity from 0.5 to 2 m∙s-1, Reynolds number from 100 to 500, air temperature from 27 to 32oC, relative humidity from 50% to 90%, and PPI (numbers of pores per inch) of metal foam from 5 to 15. The research results show that, as the relative humidity of inlet air increases, both the total heat transfer rate and pressure drop in metal foam increase, and the maximal increments are 67%and 62%,

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respectively; with the increase of metal foam PPI, the heat transfer rate initially decreases slightly and then increases; the heat transfer capacity of copper foam heat exchanger is enhanced by 56%~196% compared to that of the fin-and-tube heat exchanger.

Nomenclature Cp

Specific heat, J/kg K

Greek symbols

d

humidity ratio, kg/kg

γ

latent heat of vaporization, kJ∙K-1

D

Diameter, mm

ε

porosity

H

height, mm

φ

relative partial pressure of water vapour

i

Enthalpy, kJ∙kg-1

Subscripts

L

length, mm

a

air (vapour)

m

mass flux rates, kg∙s-1

cw

cooling water

P

Pressure, Pa

dehumid

Dehumidifying

Pl

tube longitudinal pitch, mm

dry

Dry air

Pt

tube transverse pitch, mm

in

inlet

PPI

number of pores per inch

Latent

Latent heat

Q

heat transfer, W

out

outlet

t

thickness of metal foam sample, mm

v

Water vapor

T

Temperature, oC

w

Water (liquid)

V

volumetric flow rate , m3∙s-1

Sensible

sensible heat

W

Width, mm

1. Introduction Open-cell metal foam is one kind of porous media with high porosity (up to 98%), high effective thermal conductivity, high specific surface area (up to 10000m2m-3) and a tortuous flow path to promote mixing, and is widely used in heat exchangers [1-4]. Using metal foam to replace the conventional fins in the heat exchanger would enhance the dry air convection flow heat transfer characteristics characteristic, resulting in the improvement of heat exchanger performance [5-7]. The

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application status of metal foam in heat exchangers has been exhaustively reviewed in the literatures [2,4,8]. Compared to the commercially available plate heat exchangers, the compressed aluminum foam heat exchanger performed well in heat transfer enhancement, and its thermal resistance is decreased by nearly half [8]; metal foam heat exchangers under the high speed laminar jet confined by two parallel walls are superior compared to conventional finned surfaces at no pressure drop and material weight [7]; the performance of the metal foam-wrapped tube bundle heat exchanger can noticeably be better than that of the conventional design of finned tube heat exchangers, and a very thin layer of metal foam wrapped around a bare tube bundle can significantly improve the area goodness factor [9]; the performance of the foam filled annular tube is approximately three times higher than that of the longitudinally finned tube [10]. Therefore, the use of metal foams can greatly enhance the heat transfer [10-12], and metal foams have significant potential in the manufacture of compact heat exchanger in refrigeration and air conditioning systems. In the practical application of air conditioning systems, the evaporators mostly operate under dehumidifying conditions, and the water vapor in the wet air is condensed as the wet air is cooled to below its dew point [13]. Condensation water would easily adhere to the interconnected metallic fibres, and may accumulate inside metal foam, which would have great influence on the heat transfer and pressure drop characteristics. In order to design and optimize the metal foam heat exchanger for air conditioning systems, it is necessary to investigate the heat transfer and pressure drop characteristics of wet air in metal foam during the dehumidifying process. For the heat transfer and pressure drop characteristicsin metal foam, the existing research is mainly focused on the dry air.The metal foam performances under dry conditions were extensively reviewed by Zhao [2] and Han et al. [4]. The research results on the dry air in metal foam show that,

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the effective thermal conductivity at ambient pressure is significantly improved due to the contribution of natural convection compared to that at vacuum condition [14]; the heat transfer rate are significantly affected by porosity of the porous fins [15,16] as well as the surface area and foam finned surface efficiency [17-19], and the pressure drop is directly proportional to the pore density and inversely proportional to the metal foam porosity [20-22]; the phenomenon of non-local thermal equilibrium enhances with the increase of the porosity and the pore density [23]; compared to the louver fin metal foam has the same or better heat transfer performance at low velocity but has slightly worse performance at high velocities [24,25]. The above existing researches are specialized in dry conditions, and the dehumidifying characteristics were not included. For the heat transfer and pressure drop characteristics under dehumidifying conditions, the existing research mainly focuses on the fin-and-tube exchangers with different types of conventional fins, covering the plain fin [26-28], the wavy fin [29-34], the slit fin [35-36] and the louvered fin [37-39]; while there is only one published paper on wet air in metal foam [40]. For the wet air in fin-and-tube heat exchangers, the research results show that, the fin pattern and fin structure parameters have significant effects on the heat transfer and pressure drop characteristics [29-31,37,39], and the effect of inlet relative humidity on the heat transfer performance is different from diverse fins [32,33,38]. For the wet air in metal foam, the water-drainage behavior of the metal foam through dynamic dips tests was investigated, and the heat transfer and pressure drop of metal foam heat exchanger under dry and wet surface conditions were evaluated experimentally [40]; the drainage from metal foams is as good as or better than that from louver-fin heat exchangers, and the heat transfer coefficient under wet condition is slightly higher than that under dry condition. The existing research on the wet air in metal foam focused on the water-drainage behavior and the comparison between dry

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and wet conditions under a certain condition of constant inlet relative humidity and temperature, while the influencing factors on the heat transfer and pressure drop characteristics, such as the relative humidity, the operation conditions and metal foam structures, were not discussed. The purpose of the present study is to experimentally obtain the heat transfer and pressure drop characteristics of wet air in metal foam during the dehumidifying process under the air conditioning conditions, to analyze the influencing principles of different factors on heat transfer and pressure drop characteristics, and to know the heat transfer performance differing from a fin-and-tube heat exchanger.

2. Design of experiment 2.1 Experimental conditions needed for analyzing the performance of wet air flow in metal foam In order to quantitatively analyze the performance of wet air flow in metal foam, the experimental data should be obtained under various test conditions responding to the practical operation conditions of air conditioners, covering various inlet air velocity, air temperature, relative humidity and evaporation temperature. In the present study, the experimental conditions of the wet air include the inlet air velocity from 0.5 to 2 m∙s-1, Reynolds number from 100 to 500, temperature from 27 to 32oC and relative humidity from 50 to 90%; the cooling water is used to simulate the operation conditions of refrigerant evaporation in heat exchangers, covering the cooling water temperature from 6 to 18 oC, as commonly used in the literatures [31, 33]. The experimental conditions are listed in Table 1. Moreover, the experimental data for different metal foams with various structures should also be obtained to analyze the performance variations with the structures. In the present study, three open-cell metal foams with PPIs of 5, 10 and 15 will be used to analyze the influence of structure parameters. In order to compare the performance between the metal foam heat exchanger and conventional

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fin-and-tube heat exchanger under dehumidifying conditions, the experiments on a louvered fin-and-tube heat exchanger with the same volume as that of metal foam heat exchanger will also be performed. For the louvered fin sample, the width (W), length (L) and height (H) are 38 mm, 90 mm and 14 mm, respectively; the tube diameter (D), the tube transverse pitch (Pt) and the tube longitudinal pitch (Pl) of the louvered fin are 5 mm, 12 mm and 21 mm, respectively, and the fin pitch is 1.4 mm.

2.2 Experimental apparatus The schematic diagram of the experimental rig is shown in Fig. 1, and it can realize the adjustment of the experimental conditions shown in Table 1. The experimental rig can be subdivided into two main parts, including wet air side system and cooling water system. In the wet air side system, an air compressor (Greeloy GA-82Y) with a dryer is used to provide the low-humidity air (Relative humidity<10%) with the required volume flow rate at a constant gauge pressure of 2 Bar. The low-humidity air passes through an air conditioner box, and then mixes with the water vapor evaporated by an electrical heater in the humidifier. The amount of pure water vapor was adjusted by the electrical power of the heater in humidifier and the open degree of the valve. The pure water vapor from the humidifier is guided into the air conditioner box, where it mixes with the low-humidity air from the air compressor to become the wet air with a certain relative humidity. The wet air flows into the volume flow meter (HTLZD-15/F10/RR1/JSE) with an accuracy of ±2L/min, and then flows through the air deflector and enters into the test section. The wet air is cooled in the test section, and the condensing water forms on the cooled metal fibre, therefore the humidity of the air decreases. Finally, the wet air out of the test section is discharged into the atmosphere. In the cooling water system, it consists of a micro water pump, a thermostatic water tank, a

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magnetic flow meter (Omega FLR1008-BR-P) with ±4%precision, and two T-type thermocouples. The thermocouples were calibrated to a NIST-traceable mercury-in-glass thermometer using a thermostatic bath, and the calibrated thermocouples have the uncertainty of 0.1 oC. As the cold water in the thermostatic water tank reaches the required temperature, it will be conducted through the back of the test sample by the micro water pump in order to cool the metal foam, and then return back to the water tank. The pressure drop of the wet air flow through the test sample was measured directly by the differential pressure transducer (OMEGA PX655-0.25DI) with a high precision of 0.3 Pa under the measuring range of 0-60 Pa. The dry bulb temperature and relative humidity of the wet air at the inlet and outlet of the test section are obtained by two temperature and humidity transducers with ±0.1 oC and ±1.4% precisions. The inlet air temperature and humidity in the test section are controlled by the power regulator of the electrical heaters for air and water vapor, respectively, and the accuracies of the air temperature and humidity transducer are ±0.2 oC and ±3%, respectively. All signals obtained from the measuring instruments are recorded by a data acquisition system and finally averaged over the elapsed time. The outlet air temperature and humidity are used for checking whether the experiment is steady or not. As the time averaged outlet air temperature and humidity change within 0.1 oC and 2% respectively, the experimental condition is considered as a steady state, and the experimental data will be recorded. In the experiments, only those data that satisfy the ASHRAE standard [41] requirements (namely, the energy balance conditions, |Qw−Q|/Q≤0.05) are considered in the final analysis.

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2.3 Test section Figure 2 shows the photos of the test section. The test section consists of two parts, i.e. wet air and cooled water cycles. The wet air part is used for guiding the wet air through the metal foam of the test sample, and the cooled water part is a closed cavity, which is used for guiding the cold water through the back of the test sample to supply the cooling capacity. The closed cavity consists of an acrylic jacket and a seal, which are used to form an enclosed space in order to circulate the cold water at the back of the test sample, as shown in Figs. 2(a) and 2(b). In the present study, open-cell copper foams are used, and they are manufactured by frothing method. The structure of the copper foam includes the porosity of 85% and pore density of 5~15 PPI, as shown in Fig. 2(c). The copper foam has solid struts, and its composition is 100-wt% Cu. PPI and porosity were used for classifying the foam in the present study, as did by Dukhan et al. [42] and De Jaeger et al. [43], and they were provided by manufacturers. PPI was measured using the method proposed by Bhattacharya et al. [44]; porosity was measured using the method proposed by De Schampheleire et al. [45], and it is the function of copper mass and metal foam volume. The test sample consists of a copper plate with the thickness of 5mm and a piece of copper foam with the length of 38mm, thickness of 14mm and height of 90mm, which is the same as those for fin-and-tube heat exchanger sample used in the present study. In order to reduce the thermal contact resistance, the brazing bonding method was adopted due to its lowest thermal contact resistance compared to the other bonding methods [46-49]. The copper plate and the copper foam are welded together by copper-based amorphous solder (melting point 640 oC), as shown in Fig. 3. The solder has a similar thermal conductivity as the copper. Compared to the other influencing factors, the thermal contact resistance caused by brazing bonding method was smaller [46],

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which accounts for lower than 1% of total thermal resistance. Therefore, the contact resistance for the metal foam test sample can be neglected. On the polymethyl methacrylate plate, the test sample is installed by several screws, as shown in Fig. 3(a). The composition and size of the solder are listed in Table 2. The solder is placed between the copper plate and the copper foam, and then the test sample is then placed into the furnace, which is filled with inert gas. The temperature in the furnace is raised to 700 oC to melt the solder [50], and the copper plate and copper foam are burnt together. Besides the copper part covered by metal foam, the other part was covered by a plate of polymethyl methacrylate with low thermal conductivity, which is used to restrain the heat transferred from the wet air to the copper plate and to avoid the water vapor condensation on the copper plate. The pictures of the metal foam test samples are shown in Fig. 3(b).

3. Data reduction and uncertainties 3.1 Data reduction The flow characteristics of the wet air in metal foam include the pressure drop and heat transfer. The pressure drop is directly measured by the differential pressure transducer. The heat transfer rate is a deduced parameter, which is calculated by the measured parameters. The total heat transfer rate of the wet air flow through the metal foam can be expressed as the average value of the airside heat transfer rate Qa and the cooling water side heat transfer rate Qcw, as shown in Eq. (1). The maximum difference between Qa and Qcw is smaller than 5%, which satisfied the ASHRAE standard [41] requirements. Q

1 2

( Q a  Q cw )

(1)

The cooling water heat transfer rate (Qcw) can be calculated based on the water flow rate (mcw) and the inlet and outlet water temperatures (Tcw,in and Tcw,out), as shown in Eq. (2):

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Q cw  m cw C p cw (Tcw ,in Tcw ,out ) _

(2)

Airside heat transfer rate (Qa) consists of the latent heat transfer rate (Qa,Latent) and the sensible heat transfer rate (Qa,Sensible) [50]: Q a  Q a ,Latent  Q a,S ensible

(3)

Q a ,Latent  m w ,dehum id  w

(4)

Q a ,Sensible  m a,dry C p a (Ta,in Ta,out )  ( m a,dry d in m w ,dehum id ) C p v (Ta,in Ta,out ) _

_

_

(5)

where, mw,dehumid is the mass of the water dehumidified in the test section, kg/s; γ w is the specific latent heat of vaporization, kJ/K; ma,dry is the mass flow rate of dry air, kg/s; Cpa and Cpv are the specific heats at constant pressure specific of dry air and water vapour, respectively, J/kg K; Ta,in and Ta,out are the temperatures of the air at the inlet and outlet of the test section, respectively, K; d is the humidity ratio. In Eq. (4), the mass of the water dehumidified in the test section mw,dehumid can be calculated based on the inlet and out humidity ratios, as shown below. m w ,deh u m id  m a,dry ( d in d o u t ) _

d  0 .6 2 2

 ps

(6) (7)

p - pa

where, the humidity ratio d is equal to the water vapour mass in unit mass of dry air, kg/kg; ps is the saturated vapour pressure of water vapour, Pa; pa is the partial pressure of dry air, Pa; φ is the relative partial pressure of water vapour, and is equal to the ratio of the partial pressure of water vapour to the saturated vapour pressure of water vapour.

3.2 Experimental uncertainties The uncertainties of instruments, pressure drop and heat transfer rates are listed in Table 3. Experimental uncertainties are estimated according to the analysis method of error propagation

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proposed by Moffat [52]. The maximum uncertainties of the latent heat transfer rate, the sensible heat transfer rate and total heat transfer rate and ±3.9%, ±9.6% and ±13.5%, respectively, and the maximum uncertainty of pressure drop is ±0.3 Pa.

4. Experimental results and discussion 4.1 Effect of inlet air relative humidity on characteristics of heat transfer and pressure drop Figure 4 shows the influence of inlet air relative humidity on heat transfer and pressure drop characteristics in the copper foam with PPI of 15 and porosity of 85% under the conditions of the inlet moist air temperature of 27oC and cooling water of 6 oC. Figure 4(a) illustrates that, as the relative humidity increases, the latent heat transfer rates increase, while the sensible heat transfer rates decrease, resulting in the increase of the total heat transfer rate. At a constant cooling water temperature, the condensing water was found more at higher humidity, leading to a higher latent heat transfer rate. The condensing water accumulating on the metal fiber surface restrains the sensible heat transfer, resulting in the decrease of sensible heat transfer. While the relative humidity increases from 50% to 90%, the total heat transfer rate increasesby67%. Figure 4(b) illustrates that, the pressure drop increases with the increase of inlet air relative humidity due to the presence of the condensing water. As the relative humidity increases from 50% to 90%, the pressure drop increases by 62%.

4.2 Effect of inlet air temperature on characteristics of heat transfer and pressure drop The variations of heat transfer rate and pressure drop with different inlet air temperatures are shown in Fig. 5. During the experiments, the inlet air temperature changes from 25 oC to 35 oC, and the

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other operation parameters keep as constant values, including the inlet air humidity of 70%, the velocity of 1m/s and the cooling water temperature of 6 oC. Figure 5(a) illustrates that, as the air temperature increases, both the latent and sensible heat transfer rates increase, and the increase of latent heat transfer rate is more pronounced; the total heat transfer rate increases linearly. The humidity ratio of inlet air is higher at high temperature than that at low temperature under the condition of the same relative humidity. Under the condition of high humidity ratio, more water droplets will condensate in metal foam, causing the increase of the latent heat transfer. Moreover, thetemperaturedifference is greater at higher temperature under the condition of the same cooling water temperature, leading to the increase of the sensible heat transfer. Figure 5(b) shows that, with the increase of inlet air temperature, the pressure drop increases because the condensing water accumulates in metal foam. When the temperature increases from 25oC to 35oC, only 1.3 Pa of pressure difference is observed. Therefore, the air temperature has little effect on pressure drop.

4.3 Effect of inlet air velocity on heat transfer and pressure drop characteristics Figure 6 shows the effect of inlet air velocity on the heat transfer and pressure drop characteristics. During the measurement process, the Reynolds number varies from 100 to 500 with the increasing inlet air velocity, while the air temperature, the humidity and the cooling water temperature are kept as constant of 27 oC, 70% and 6 oC, respectively. As Reynolds number increases, both the latent and sensible heat transfer rates increase, and the sensible heat transfer rate increment is more obvious due to the increase of convective heat transfer [23]. The pressure drop increases as Reynolds number increases from 100 to 500, which is consistent

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with the results of dry air in aluminum foam heat exchanger [12].

4.4 Effect of cooling water temperature on heat transfer and pressure drop characteristics The effects of cooling water temperature on heat transfer and pressure drop characteristics are shown in Fig. 7. During the experiments, the inlet air temperature, the humidity and the velocity are 27oC, 70% and 1m/s, respectively. Figure 7 illustrates that, the latent and sensible heat transfer rates decrease with the increase of cooling water temperature, and the decrement of the latent heat transfer rate is greater than that of the sensible one. It is found that the highest pressure drop is achieved when the cooling water temperature is at 6 oC. It has pronounced increment when the cooling water temperature decreases from 12 oC to 6oC, and the increment is more obvious for the higher relative humidity. The possible reason for the phenomenon is that, as the cooling temperature increases, the subcooling degree decreases, leading to the decrease of condensing water amount and then the decrease of latent and sensible heat transfer rates.

4.5 Effect of PPI on heat transfer and pressure drop characteristics The effects of PPI on the heat transfer and pressure drop characteristics of wet air flow in metal foam are shown in Fig. 8. During the experiments, PPI changes from 5 to 15, while the inlet air temperature, the velocity and the cooling water temperature keep as constant of 27oC, 1 m/s and 6oC, respectively. Figure 8 illustrates that, the influence of metal foam PPI on the sensible, latent and total heat transfer are different under different humidity conditions, as discussed below in detail.

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As the relative humidity is 40%, the sensible and latent heat transfer rates increase with the increasing PPI. The possible reason is that, under the condition of low humidity, there is very little water condensation in metal foam, and the latent heat transfer is much smaller than the sensible heat transfer, which is proportional to the heat transfer surface area. As PPI increases, the heat transfer surface area and the sensible heat transfer increase, resulting in the increase of the total heat transfer with increasing PPI under the condition of low humidity. As the relative humidity reaches up to 70%, the latent heat transfer decreases with the increasing PPI, while the sensible heat transfer slightly increases and then decreases, resulting in the decrease of overall heat transfer with the increasing PPI; the sample with 5 PPI has the best heat transfer performance under higher humidity conditions. The possible reason is explained as follows. The latent heat transfer is extremely affected by the inlet air humidity. As the humidity increases, there will be more condensing droplets occur and adhere on metallic fibre. The metal foam sample with higher PPI retains much more water than does the sample with lower PPI [41], resulting in the increasing negative effect on sensible and latent heat transfer rates with the increase of PPI. Figure 8 also shows that, as the PPI increases, the pressure drop always increases; the maximum increment of pressure drop is up to144%as the PPI increases from 5 to 15. Moreover, there is boundary effect of the confining wall on heat transfer and pressure drop characteristics due to the incomplete cells and randomly chopped ligaments near the confining wall [42, 51]. As PPI of metal foam decreases from 15 to 5, the cell number decreases from 9 to 3, and the boundary effect becomes more significant. The area goodness factor decreases with the foam layer thickness [9, 53]; for low air velocities through the foam, the heat transfer from the copper plate is pretty much conduction-dominated and flow penetrates to a certain depth, which is 3 pores deep for the

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single row heat exchanger consisting of metal foam covered round tubes [53]. In the present study, the frontal area of the test section keeps as constant, and the quantitative boundary effect was not analysed, which should be further investigated in the future study.

4.6 Comparison between metal foam and finned tube heat exchangers under dry and dehumidifying conditions For the dry conditions, the performance comparison between the metal foam heat exchanger and louvered fin-and-tube heat exchanger with the same volume was shown in Fig. 9. Under the conditions of low air velocities, metal foam slightly outperforms louvered fin; while under the conditions of high air velocities (1-1.5 m/s), metal foam has worse heat transfer performance than louvered fin, which is the same as the results in existing literatures [20, 21, 24]. The possible reason is that metal foam has smaller heat transferring area than louvered fin. The louvered fin pitch of the fin-and-tube heat exchanger used in the present study is 1.4 mm, and the surface-to-volume ratio is 910 m2/m3; while the surface-to-volume ratio of the metal foam with 15PPI and 85% porosity is about 700 m2/m3 [43], which is smaller than that of louver fin-and-tube heat exchanger with the same volume. For the dehumidifying conditions, the performance of metal foam is better than louvered fin-and-tube heat exchanger with the same volume, as shown in Fig. 10. Compared to the louvered fin-and-tube heat exchanger, the copper foam heat exchanger with the same volume can enhance the heat transfer capacity by 69.2%~127.7% under the dehumidifying conditions. The mass transfer rate for metal foam heat exchanger is 38%~86% larger than that for the louvered fin-and-tube heat exchanger under the humidity of 60%-80%, and the enhancement effect is more significant under low

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relative humidity (50%) because the mass transfer rate for louvered fin-and-tube heat exchanger is negligible under the condition of 50% humidity. Figure 10 also illustrates that, as the relative humidity increases, the latent heat transfer and total heat transfer for both metal foam and louvered fin increase, while the sensible heat transfer increases initially and then decreases due to the deterioration effect of retention condensation. The heat transfer enhancement effect of metal foam compared to louvered fin under dehumidifying conditions is more significant than that under dry conditions obtained in the literatures [20-22, 24, 25]. The possible reason is explained as follows. The dehumidification performance mainly lies on the nucleation site density, which is greatly influenced by the topography of condensation surface [54]. For the metal foam, the irregularity and complexity of metal fiber are considerably larger than those of louvered fin, leading to more nucleation sites and higher heat transfer enhancement effect for metal foam than those for louvered fin. The conclusion can be deduced that, the metal foam has better heat and mass transfer performance than louvered fin but with higher pressure drop under dehumidifying conditions. For achieving the same heat capacity, metal foam heat exchanger has lighter weight and smaller volume than fin-and-tube heat exchanger, which makes metal foam have more application prospect in compact heat exchangers.

5. Conclusions The heat transfer and pressure drop characteristics of wet air flow in metal foam during the dehumidifying process under air conditioning conditions were experimentally investigated, and were compared with those in a fin-and-tube heat exchanger with the same volume. The experimental

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conditions cover the inlet air velocity from 0.5 to 2 m∙s-1, air temperature from 27 to 32oC, relative humidity from 50% to 90%, and PPI of metal foam from 5 to 15. The research results show that: 1)

With the increase of relative humidity, latent heat transfer rate increases and sensible heat transfer rate decreases, while both the total heat transfer and pressure drop increase. As the relative humidity of inlet air increases from 50% to 90%, the total heat transfer rate increases by a maximum of 67%, and the pressure drop increases by 62%.

2)

As the wet air temperature increases, both the latent and sensible heat transfer rates increase due to the great temperature difference between the air and the cooled water, making total heat transfer increase significantly. When the temperature increases from 25 oC to 35oC, the maximum increment of heat transfer rate is 82%.

3)

As PPI of metal foam increases from 5 to 15, the total heat transfer decreases initially, and then increases due to the influence of condensing water.

4)

Compared to the louvered fin-and-tube heat exchanger with the same size, the heat transfer capacity of copper foam heat exchanger is enhanced by 69.2%~127.2% under dehumidifying condition, and mass transfer rate is increased by 38%~86%, meaning that the heat and mass transfer performance of metal foam is superior to that of louvered fin with the same volume.

Acknowledgments This study is supported by National Nature Science Foundation of China (No. 51576122) and Nature Science Foundation of Shanghai (No. 15ZR1422000).

References

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Figure captions Fig. 1 Experimental rig for wet air flow in metal foam Fig. 2 Photos of transparent test section Fig. 3 Metal foam test samples Fig. 4 Effect of air inlet relative humidity on the heat transfer and pressure drop characteristics of wet air in 15 PPI metal foam with the porosity of 85% for T m-air = 27 oC, Tc-water = 6 oC, Vm-air =1 m/s Fig. 5 Effect of air inlet temperature on the heat transfer and pressure drop characteristics of wet air in 15 PPI metal foam with the porosity of 85% for V m-air =1 m/s,Tc-water = 6 oC, RH= 70% Fig. 6 Effect of air inlet velocity on the heat transfer and pressure drop characteristics of wet air in 15 PPI metal foam with the porosity of 85% for T m-air = 27oC, Tc-water = 6 oC, RH= 70% Fig. 7 Effect of cooling water temperature on the heat transfer and pressure drop characteristics of wet air in 15 PPI metal foam with the porosity of 85% for T m-air = 27 oC, Vm-air =1 m/s, RH= 70% Fig. 8 Effect of PPI on the heat transfer and pressure drop characteristics of wet air in metal foam with the porosity of 85% for T m-air = 27oC, Tc-water = 6 oC, Vm-air =1 m/s, RH= 70% Fig. 9 Comparison between metal foam and louvered fin-and-tube heat exchangers under dry conditions Fig. 10 Comparison between metal foam and louvered fin-and-tube heat exchangers under dehumidifying conditions

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Table 1 Experimental conditions in the present study Heat exchanger

Parameter

value

Metal foam heat exchanger

PPI of metal foam

5, 10, 15

Porosity of metal foam

85%

Air velocity (m/s)

0.5, 0.8, 1.0, 1.3, 1.55 o

Inlet air temperature ( C)

25, 27, 30, 32, 35

Relative humidity

50%, 60%, 70%, 80%, 90% o

Louver fin-and-tube heat

Cooling water temperature ( C)

6, 12, 18

Air velocity (m/s)

0.5, 0.8, 1.0, 1.3, 1.55 o

exchanger

Inlet air temperature ( C)

25, 27, 30, 32, 35

Relative humidity

50%, 60%, 70%, 80%, 90% o

Cooling water temperature ( C)

6, 12, 18

Table 2 Composition and size of the solder Type OURI HPCU

o

Composition

Melting point ( C)

Thickness (mm)

Width (mm)

640

0.1

20

Cu 73.6, Ni 9.6, Sn 9.7, P 7.0

Table 3 Uncertainties of instruments and heat transfer coefficient Parameters

Major source of uncertainty

Instrument

Range

Max. uncertainty

Air temperature

Instrumentation calibration

Temperature and humidity

-50~100oC

±0.1 oC

0–100%

±0.8%RH

sensor (ROTRONICHC2-SH) Air relative humidity

Instrumentation calibration

Temperature and humidity sensor (ROTRONICHC2-SH)

Air flow rate

Instrumentation calibration

Float flow meter (LZ series)

1.2~12m3/h

±0.3 L/min

Water flow rate

Instrumentation calibration

Volumetric flow meter (OMEGA

0.2~2L/min

±0.15L/min

FLR1011) Water temperature

Instrumentation calibration

T-type thermocouple

0~60oC

±0.1 oC

Airside pressure drop

Instrumentation calibration

Differential pressure transducer

0~60 Pa

±0.3Pa

Calculated

0-50 W

±3.9%

Calculated

0-50 W

±9.6%

Calculated

0-50 W

±13.5%

(OMEGA PX655-0.25DI) Latent heat transfer

Air flow rate, air temperature, air relative humidity

Sensible heat transfer

Air flow rate, air temperature, air relative humidity

Total heat transfer

Water

flow

rate,

water

temperature, air flow rate, air temperature, air relative humidity

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