Scholarly article on topic 'Experimental Evaluation of Fluid Dynamic and Thermal Behaviors in Compact Heat Exchanger with Aluminum Foam'

Experimental Evaluation of Fluid Dynamic and Thermal Behaviors in Compact Heat Exchanger with Aluminum Foam Academic research paper on "Materials engineering"

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{"metal foam" / "heat exchanger" / "heat transfer" / PPI}

Abstract of research paper on Materials engineering, author of scientific article — Giusi Cicala, Luca Cirillo, Alessandra Diana, Oronzio Manca, Sergio Nardini

Abstract In this paper an experimental investigation on forced convection in a compact heat exchanger made up with an aluminum foam plate of 212.5 mm x 212.5 mm with a thickness of 40 mm and a single array with five circular tubes is presented. The foam has a porosity of 0.93 with 10, 20 and 30 pores per inch and the tubes in aluminum have internal and external diameters equal to 9.5 mm and 12.5 mm. The test rig consists of an open air channel and a closed water cycle and the aluminum foam plate is placed inside the channel. The performances of the compact heat exchanger are evaluated for assigned hot water mass flow rate and different hot water inlet temperatures and air mass flow rate. Results are given in terms of heat transfer rates and pressure drops as a function of air velocity and Reynolds numbers. The evaluation of dimensionless, Colburn factor and Nusselt number is performed for different air mass flow rates and hot water inlet temperatures.

Academic research paper on topic "Experimental Evaluation of Fluid Dynamic and Thermal Behaviors in Compact Heat Exchanger with Aluminum Foam"

Procedía

71st Conference of the Italian Thermal Machines Engineering Association, ATI2016, 14-16

September 2016, Turin, Italy

Experimental evaluation of fluid dynamic and thermal behaviors in compact heat exchanger with aluminum foam

Giusi Cicalaa, Luca Cirilloa*, Alessandra Dianaa, Oronzio Mancaa, Sergio Nardinia

aSeconda Università degli Studi di Napoli, Via Roma 29, Aversa 81031, Italy

Abstract

In this paper an experimental investigation on forced convection in a compact heat exchanger made up with an aluminum foam plate of 212.5mm x 212.5mm with a thickness of 40 mm and a single array with five circular tubes is presented. The foam has a porosity of 0.93 with 10, 20 and 30 pores per inch and the tubes in aluminum have internal and external diameters equal to 9.5 mm and 12.5 mm. The test rig consists of an open air channel and a closed water cycle and the aluminum foam plate is placed inside the channel. The performances of the compact heat exchanger are evaluated for assigned hot water mass flow rate and different hot water inlet temperatures and air mass flow rate.

Results are given in terms of heat transfer rates and pressure drops as a function of air velocity and Reynolds numbers. The evaluation of dimensionless, Colburn factor and Nusselt number is performed for different air mass flow rates and hot water inlet temperatures.

© 2016 The Authors.Publishedby Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Scientific Committee of ATI 2016.

Keywords: metal foam; heat exchanger; heat transfer; PPI.

1. Introduction

Metal foams are a new class of materials with low densities and novel thermal and mechanical properties. The open porosity, low relative density and high thermal conductivity of the cell edges, the large accessible surface area per unit volume, and the ability to mix the cooling fluid by promoting eddies all make metal foam heat exchangers efficient, compact and light weight. Aluminum foams combine low weight with good rigidity, strength, damping of vibrations and noise, shock resistance and low thermal conductivity [1].

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Available online at www.sciencedirect.com

ScienceDirect

Energy Procedia 101 (2016) 1103- 1110

* Corresponding author. Tel.:+39 335 5878553; E-mail address: lcirillo87@gmail.com

1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Scientific Committee of ATI 2016.

doi:10.1016/j.egypro.2016.11.150

Nomenclature

A area of the duct (m2) St transverse pitch in a bank

C heat capacity (W/K) T temperature (K)

Cp specific heat at costant pressure (J/kg K) u air velocity (m/s)

D pipe diameter (m) Z depth (m)

Dhyd hydraulic diameter (m)

f friction factor Greek symbols

h convection heat transfer coefficient (W/m2 K) AP pressure loss (Pa)

j Colburn factor e effectiveness

kc thermal conductivity (W/m K) P density of the air (kg/m3)

L length of the channel (m)

m mass flow rate (kg/s) Subscripts

N number of cylinders in a bank C cold fluid

Nu Nusselt number in,c inlet of cold fluid

P wetted perimeter of the duct (m) out,c outlet of cold fluid

PPI number of pores per inch h hot fluid

Pr Prandtl number in,h inlet of hot fluid

Re Reynolds number out,h outlet of hot fluid

The possible applications are dependent on many parameters such as the type of metal, the porosity (open or closed), the process by which the foam is obtained (temperature, pressure, residence time, melt flow etc.), density and overall costs. The advantages of metal foams reside in low density and high strength of the structure. The low thermal inertia allows a transfer of heat faster than in the ceramic materials. The material has a high ratio between surface area and pressure drop, and with uniform lower density. The pressure drop is lower than in ceramic structures when considering the unit of volume [2]. Kim et al. [3] carried out an experimental analysis on heat transfer and pressure drop of aluminum foam brazed between two flat tubes (porous fin) of a plate-fin water-air heat exchanger. Comparison of performance between the porous fins and the conventional louvered fins was provided. In the investigation, six types of foam were considered with varying porosity (0.89 -0.96) and varying the pores per inch, PPI, (10-40). The porous fins showed similar thermal performance to the conventional louvered fin although the louvered fin presented a little better performance in terms of pressure drop. However, for compactness of the heat exchanger, the porous fins with high pore density and low porosity are preferable. Aluminum foam as a small compact heat exchangers was investigated by Boomsma et al. [4]. It was employed as a heat sink in electronic cooling with the foam brazed onto a heat spreader plate and water was used as working fluid through the metal foam. The impact of the compression on the resulting thermo-hydraulics was studied and it was found that increasing the compression rate resulted in both a larger pressure drop and heat transfer. The increase in metal foams compression determines an heat transfer rate increase but compressing the foams too much an overall performance worsen is detected due to the very sharp increase in pressure drop. Sumithra Raju and Narasimham [5] carried out a numerical analysis on heat transfer and pressure drop of an inline tube bundle (5 rows) with porous fins and air as fluid. The thickness of the porous fin varied from 0 to the half of the transversal tube pitch. The results showed that for a porosity equal to 0.58 and fin thickness equal to the tube radius an increase of Nusselt number and in pressure drop with respect the system without porous fin of about 30% and 25%, respectively. An experimental investigation on a single row of aluminum tubes, covered with layers of aluminum foams, was carried out by T'joen et al. [6]. A range of foam layer thickness, Reynolds number tube spacing and different type of foam were considered and compared with compact helically finned tube heat exchangers. Results indicated that thermal contact resistances have a strong effect on the performance of the metal foam heat exchangers. It was observed that for stream velocity higher than 4 m/s, a good metallic bonding, small tube spacing and thin foam layer thickness potentially improve the performance of metal foam heat exchangers compared to a conventional finned-tube heat exchanger. An experimental investigation was carried out by Sertkaya et al. [7] to compare three metal foam heat exchangers (10, 20 and 30 PPI) to three finned heat exchangers with the same tube layout and overall dimensions. Results showed that the finned heat exchangers furnished a higher heat transfer and a lower pressure drop than the foamed heat

exchangers. Chumpia and Hooman [8] compared in an experimental investigation five foam wrapped tubes to a finned tube as benchmark. The results show that the foam heat exchangers with thicker foam layer perform better than those with thinner foam layer. Moreover, the foam wrapped heat exchanger with suitable foam thickness gives heat transfer benefit while keeping the pressure drop at the same level of the finned tube. A strong dependence on the foam's morphology and foam material of thermal hydraulic performance of metal foam heat exchangers was provided by of Huisseune et al. [9]. A numerical study on a metal foam heat exchanger was provided by Mao et al. [10]. Their results showed that the existence of an optimal metal foam porosity corresponding to the wall heat transfer coefficient and pressure drop reach the design target. An experimental investigation on two sets of three tubular heat exchangers were performed by Chumpia and Hooman [11]. Two tubular heat exchangers were constructed by wrapping aluminum foam of different thickness around cylindrical tubes. The effects of foam thickness and transversal pitch distance were discussed and thermo-hydraulic results were benchmarking against those of a conventional finned tube array. Results showed that the heat exchanger array with thicker foam layer enhances heat transfer compared with the one having thinner foam. Heat transfer improved at narrower pitches for an array with fixed foam thickness. A numerical analysis to evaluate the performance of metal foam heat exchangers and compare it to the performance of a bare tube bundle and of an existing conventional louvered fin heat exchanger was presented by Huisseune et al. [12]. It was found that, at the same fan power, the foamed heat exchangers show up to 6 times higher heat transfer rate than the bare tube bundle. Whereas, for the same overall dimensions, the finned heat exchanger showed the best performance in comparison with the metal foam heat exchanger. However, a metal foam heat exchanger can outperform the finned heat exchanger if the frontal area is changed. From the previous short review it is clear that the thermal and fluid dynamic performance parameters of metal foam heat exchangers present several uncertainties and more numerical and experimental investigations are necessary. The aim of this experimental investigation on a air-water aluminum foam heat exchanger is to evaluate its thermal and fluid dynamic characteristics. Results are given for different pores per inch (10,20 and 30 PPI) air mass flow rate in a range of laminar flow in terms of heat transfer rate, performance evaluation coefficient, thermal resistance and air difference temperatures.

2. Experimental apparatus

The evaluation of heat transfer behavior related to the aluminum foam heat exchanger, in terms of convective average coefficient, Nusselt number and effectiveness was carried out by means of the apparatus sketched in Fig. 1 a. The hot fluid is water and it flows in the internal tubes placed inside the metal foam whereas the cold fluid is air. The water enters in the heat exchanger at an assigned temperature and its value is monitored through a thermocouple. The maximum volumetric flow rate of water in the circuit was about 106.2 l/h. The air enters inside a duct where is placed the heat exchanger, through a convergent channel, as reported in Fig. 1 on the right of the upper part. The duct has a square transversal section of 220 mm x 220 mm and 760 mm long, whereas the convergent duct is 454 mm long and it has the squared transversal inlet and outlet sections equal to 490 mm x 490 mm and 220 mm x 220 mm, respectively. Air motion is obtained by means of a fan, which is modulated by an AGILENT E3633A power supply system, in order to obtain different air flow rates. The air volumetric flow rate ranges between 1.0 m3/h and 6.0 m3/h. At the end of this duct there is the heat exchanger, as shown in Fig. 1.

Beyond the test section, the air flow enters a second duct of trapezoidal shape, having a larger section and smaller section of 220 mm x 220 mm and 126 mm x 126 mm, respectively. A series of pipes allows to reduce the section from 160 mm, downstream the fan, to 36 mm at the exit section of apparatus. in order to obtain a more accurate measurement of the velocity by means of a Pitot tube. The exit tube has a diameter equal to 36 mm and 2.550 m long. The tubes downstream the fan were in PVC and the junctions between two consecutive tubes were made using a silicone sealant.

1.1 Heat exchanger

The heat exchanger is made of two plates of aluminum foam produced by M-PORE [13] with a porosity of 0.95 and 10, 20 and 30 pores per inch (PPI). The plates have dimensions equal to 212.5 mm x 212.5 mm x 20 mm, as shown in Fig. 1b. Grooves were made in such a way as to allow the location of five aluminum pipes 12.5 mm diameter and length equal to 270 mm. Flexible steel tubes were used in order to connect the aluminum tubes inside the metal foam. The side surfaces were insulated with mineral wool to avoid heat losses between the heat exchanger and the external ambient.

1.2 Measurement procedure

An AGILENT 34980A multifunction measurement unit and a computer were used for the data acquisition. The air

and water temperatures were measured by means of 0.50 mm OD ungrounded iron-constantan (J-type) thermocouples. In the air, one thermocouple was placed at the inlet section, close to the surface of the convergent channel of the duct, and nine thermocouples were allocated in the center line of the section at 30 mm behind the heat exchanger.

Fig. 1 (a) Sketch of experimental apparatus; (b) Heat Exchanger with metal foam

They were at different positions from 40 mm to 200 mm with a step of 20 mm with respect to the lower horizontal surface of the duct. To evaluate the water temperature at the inlet and outlet sections of the heat exchanger one thermocouple was placed inside an adiabatic mixer at the inlet section and two thermocouples were inside an adiabatic mixer at the outlet section. Moreover, surface temperature of aluminum tubes were measured at inlet and outlet of each five tubes. Average surface temperature, Tsa, was evaluated by the average among these ten temperature values. An Isotech instrument mod. 938 ice point, with an accuracy of ±0.04 °C and 50 channels, was used as thermocouple reference junctions. The thermocouples were calibrated in a range from 19 °C to 75 °C and calibration of the temperature measuring system showed an estimated precision of ±0.1 °C. Mass flow rate was evaluated by measuring the velocity at the exit tube, with a diameter of 36 mm, by a Pitot tube Fluke mod. 922 Airflow meter. The Pitot tube was located at the center of the section at 0.100 m from the outlet section of the apparatus. In this section a fully developed flow was attained and in the air mass flow rate, considered in the present work, a laminar flow was achieved. Pitot tube was located at 60 tube diameters from the inlet of the exit tube. To evaluate the pressure drop a digital manometer, Fluke mod. 922 Airflow meter with an accuracy ±1%, was used. The measurements were obtained estimating the pressure upstream and downstream the heat exchanger.

3. Data reduction

The inlet water temperature was set equal to 50°C, 60°C and 70°C. The metal foams 10, 20 and 30 PPI were investigated. The heat transfer rate from water is:

Q = mhcp (Tin,h - Tout,h ) (1)

where mh is mass flow rate, cp is specific heat of water,Tin h is the inlet temperature of hot fluid (water) and Tout h is the outlet temperature of hot fluid (water).

hDhyd (2)

where Dhyd is hydraulic diameter, defined as

Dhyd = (3)

kc is the thermal conductivity of cold fluid (air) and h is heat transfer coefficient can be evaluated [7]:

PN,S,uCpc (Tout,c - Tic,c)

ttNDZI

Where Nt (=5) is number of tubes in the transverse direction of a bank, St(=2) is transverse pitch in a bank of tubes, N is number of tubes in a bank, D is pipe diameter and Z is depth. Friction factor f is defined as:

f AP (5)

The heat exchangers are commonly characterized by Colburn factor j, which provides a further estimate of heat exchanger performance and it is calculated using the equation as follow:

Nu (6)

Effectiveness is the maximum possible heat transfer rate fraction of the actual heat transfer rate. The effectiveness can be given as below [7]: C (t - T "l

c y c,out c,in /

C (t - T

min \ h,in c,in /

where Tc,out is the outlet temperature of cold fluid, Tcin is the inlet temperature of cold fluid and Thin is the inlet temperature of hot fluid. Here Cc is the heat capacity rate of cold fluid, Cmin can be espressed as follows:

Cc = mc Cpc Cmin = mh Cph

(8) (9)

Uncertainty

The uncertainty of Nusselt number, Reynolds number, efficiency are evaluated with the Kline and McClintock methodology. It is estimated that the uncertainty on the Nusselt number is 3.7%, the Reynolds number is 2.6%, efficiency is 4.3 %.

4. Results

Tests were carried out with distilled water as working fluid. Results are presented in terms of heat transfer rate, pressure

drop, efficiency as a function of air mass flow. Fig. 2 (a-d) shows heat transfer rate as a function of air mass flow rate at

different temperatures and PPI of aluminum foam.

50°C clean

o 60°C clean

▼ 70°C clean

m [kg/h]

350 300 250 200 150 100 50 0

-•— 50°C 10PPI 0 ■ 60°C 10PPI -T- 70°C 10PPI

m [kg/h]

—50°C 20PPI o 60°C 20PPI -f- 70°C 20PPI

m [kg/h]

—•— 50°C 30PPI o 60°C 30PPI -T- 70°C 30PPI

m [kg/h]

Fig. 2 Heat transfer rate as a function of air mass flow at different temperatures: a) clean; b) 10 PPI; c) 20 PPI; d) 30 PPI

Heat transfer rate increases with air mass flow as expected. Each value is obtained from the average of ten different measurements. At the same inlet temperature heat transfer rate increases with air mass flow particularly for low values whereas for high values profiles tend to be nearly constant. Heat transfer rate is higher for metal foam of 20 PPI at 70°C of hot fluid, in fact, the higher value is 463 W

2000 1800 1600 1400 1200 £ 1000 800 600 400 200 0

50°C clean 60°C clean 70°C clean

1000 1500

1000 1500

1000 1500

4000 g 3000 2000 1000

-•--50°C 30PPI

.......o................60"C 30PPI

-----70°C 30PPI

----A--------60°C 30PPI

1000 1500

Fig. 3 Nusselt number as a function of Reynolds number at different temperatures: a) clean; b) 10 PPI; c) 20 PPI; d) 30 PPI

—•— 50°C clean o 60°C clean -T- 70°C clean

m [kg/h]

—•— 50°C 10PPI o 60°C 10PPI -■70°C 10PPI

0 10 20 30 40 50

m [kg/h]

—•— 50°C 20PPI o 60°C 20PPI 70°C 20PPI

m [kg/h]

—•— 50°C 30PPI o 60°C 30PPI 70°C 30PPI

m [kg/h]

Fig. 3 Efficiency as a function of air mass flow at different temperatures: a) clean; b) 10 PPI; c) 20 PPI; d) 30 PPI

400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 Re

50°C 10PPI

........O........ 60°C 10PPI

---T--- 70°C 10PPI

----A----- 50°C 20 PPI

--m — 60°C 20 PPI

---□--- 70°C 20 PPI

--♦-- 50°C 30 PPi

-O- 60°C 30PPI

........▲........ 70°C 30 PPI

-•— 10 PPI o 20 PPI 30 PPI

m [kg/h]

Fig.4 a) Colburn factor as a function of Reynolds number; b) friction factor as a function of air mass flow rate.

For metal foams the Nusselt number is always increasing and it reaches the maximum value of 8257 for the maximum investigated Reynolds number (fig.3 a-d). Without metal foams the Nusselt number increases until it reaches a costant value. Figure 3 (a-d) shows the efficiency as a function of air mass flow at three different temperatures. It is obtained that the metal foams improve the heat transfer. In particular, the higher efficiency it reaches at 50 °C with metal foam of 20 PPI.The highest Colburn factor was seen on 20 PPI of the aluminum foam while the lowest values was recorded on 10 PPI of the aluminum foam. In this study , for all the heat exchangers, it was found that at low air mass flow, the friction factor is high but as the air mass flow increases friction factor tends to decrease [fig.4 a-b].

5. Conclusions

In this experimental investigation the thermodynamic of a metal foam 10, 20 and 30 PPI was considered. The investigation was carried out on a metal foam with 10, 20 and 30 PPI and porosity 0.95 metal foam. Steady state for a heat exchanger is reached later for hot fluid at 50°C than for 60°C and 70°C. Performances of open-cell aluminium foam heat exchanger were experimentally compared and the conclusions drawn in this paper are summarized as follows:

• In this study, it was found that, for all heat exchangers, effectiveness is high at low air mass flow rate values and vice versa. It is found that the best performance was exhibited by aluminum foams whereas the least effective heat exchanger was that made at clean configuration.

• Heat transfer rate in 20 PPI aluminum foam heat exchangers is higher than in the clean configurations. Maximum heat transfer occurs with 20 PPI corresponding value of 463 W at 70°C.

In this study, the effectiveness of 20 PPI aluminum foams is about twice as higher as that of clean configurations. It is hoped that more interesting results will follow in further studies. Heat transfer performance of aluminum foam heat exchangers can be compared with those of other metal foam heat exchangers such as nickel, lead, zinc, magnesium and copper carbon.

References

[1] Zhao C.Y., "Review on thermal transport in high porosity cellular metal foams with open cells", Int. J. of Heat and Mass Trans., vol. 55, pp. 3618-3632, (2012).

[2] Han, X.-H., Wang, Q., Park, Y.-G., T'Joen, C., Sommers, A., Jacobi, A., "A review of metal foam and metal matrix composites for heat exchangers and heat sinks", Heat Transfer Engineering, vol. 33, pp. 991-1009, (2012).

[3] Kim, S.Y., Paek, J.W., Kang, B.H., "Flow and heat transfer correlations for porous fin in a plate-fin heat exchanger", ASME J. of Heat Trans., vol. 122, pp. 573-578, (2000).

[4] Boomsma, K., Poulikakos, D., Zwick, F., "Metal foams as compact high performance heat exchangers", Mechanics of Materials, vol. 35, pp. 1161-1176, (2003).

[5] Sumithra Raju, K., Narasimhan, A., "Porous medium interconnector effects on the thermo hydraulics of near-compact heat exchangers treated as porous media", ASME J. of Heat Transfer, vol. 129, pp. 273-281, (2007).

[6] T'Joen, C., De Jaeger, P., Huisseune, H., Van Herzeele, S., Vorst, N., De Paepe, M., "Thermo-hydraulic study of a single row heat exchanger consisting of metal foam covered round tubes", Int. J. of Heat and Mass Trans., vol. 53, pp. 3262-3274 (2010).

[7] Sertkaya, A. A., Altinisik, K., Dincer, K., "Experimental investigation of thermal performance of aluminum finned heat exchangers and open-cell aluminum foam heat exchangers", Experimental Thermal and Fluid Sciences, vol. 36, pp. 86-92 (2012).

[8] Chumpia, A., Hooman, K., "Performance evaluation of single tubular aluminum foam heat exchangers", Applied Thermal Engineering, vol. 66, pp. 266-273 (2014).

[9] Huisseune, H., De Schampheleire, S., Ameel, B., De Paepe M., "Evaluation of the thermal hydraulic performance of round tube metal foam heat exchangers for HVAC applications", in: Proceedings of the 15th Int. Heat Transfer Conf., IHTC-15, Kyoto, Japan, IHTC15-8831 (2014).

[10] Mao, S., Love, N., Leanos, A., Rodriguez-Melo, G., "Correlation studies of hydrodynamics and heat transfer in metal foam heat exchangers", Applied Thermal Engineering, vol. 71, pp. 104-118, (2014).

[11] Chumpia, A., Hooman, K., 2015, "Performance evaluation of tubular aluminum foam heat exchangers in single row arrays", Applied Thermal Engineering, vol. 83, pp. 121-130 (2015).

[12] Huisseune, H., De Schampheleire, S., Ameel, B., De Paepe, M., "Comparison of metal foam heat exchangers to a finned heat exchanger for low Reynolds number applications", Int. J. of Heat and Mass Trans., vol. 89, pp. 1-9, (2015).

[13] M-Pore, Dresden, Germany. www.m-pore.de/index.html (accessed 14.09.12).