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Research Papers: Heat Exchangers

Heat Transfer During High Temperature Gas Flow Through Metal Foam Heat Exchangers

[+] Author and Article Information
Pakeeza Hafeez

Department of Mechanical
and Industrial Engineering,
University of Toronto,
5 Kings College Road,
Toronto, ON M5S3G8, Canada
e-mail: hafeez.pakeeza@gmail.com

Sanjeev Chandra

Department of Mechanical
and Industrial Engineering,
University of Toronto,
5 Kings College Road,
Toronto, ON M5S3G8, Canada
e-mail: chandra@mie.utoronto.ca

Javad Mostaghimi

Department of Mechanical
and Industrial Engineering,
University of Toronto,
5 Kings College Road,
Toronto, ON M5S3G8, Canada
e-mail: mostag@mie.utoronto.ca

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received October 3, 2016; final manuscript received June 2, 2017; published online July 19, 2017. Assoc. Editor: Danesh K. Tafti.

J. Heat Transfer 139(12), 121801 (Jul 19, 2017) (11 pages) Paper No: HT-16-1623; doi: 10.1115/1.4037082 History: Received October 03, 2016; Revised June 02, 2017

An experimental study of heat transfer in metal foam heat exchangers fabricated from 10 and 40 pores per inch (PPI) was conducted. Heat exchangers were made by either brazing Inconel sheets to foam or plasma spraying Inconel skins on the foam. A burner test rig was built to produce high temperature combustion gases at either 550 °C or 750 °C that were passed over the exposed surface of heat exchangers that were cooled by passing air through them at rates of up to 200 SLPM. Both pressure drop and temperature rise of the air were measured. Friction factors and volumetric heat transfer coefficients were calculated for air velocities varying from 0.1 to 5 m/s and dimensionless correlations to predict these derived. The heat exchangers with 40 PPI foam were measured to have higher heat transfer rates and larger pressure drop than those with 10 PPI foam. Thermal sprayed heat exchangers were found to perform better than those that were brazed since they had lower thermal contact resistance between the external shell and foam struts. An analytical model was developed assuming local thermal nonequilibrium (LTNE) and predictions from model were found to be in good agreement with experimental results.

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References

Figures

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Fig. 1

(a) Thermally sprayed and (b) conventional brazed nickel foam heat exchangers and (c) foam core of heat exchanger only 40 PPI foam core is shown here. (a) Plasma spray heat exchanger with foam core, (b) conventional braze heat exchanger with foam core, and (c) section A-A foam core of the heat (40 PPI).

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Fig. 2

(a) Thermally sprayed skin attached to foam struts and (b) conventional heat exchanger. A gap is visible between sheet and foam struts.

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Fig. 3

(a) Experimental setup for heat transfer test and (b) test section with thermocouple locations: 1—combustion chamber, 2—divergent section, 3—quartz window, 4—test section, 5—exhaust, 6—heat exchanger, 7—thermocouples attached to the wall of heat exchanger exposed to hot gas, and 8—thermocouple measuring air inside the heat exchanger

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Fig. 4

Length normalized pressure drop curves based on 200 mm length of 10 and 40 PPI nickel foam heat exchanger versus velocity

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Fig. 5

Friction factor versus Reynolds number for 10 and 40 PPI Ni foams. Results from Beavers and Sparrow [31] for Ni foam and Kim et al. [7] for Al foam are also shown

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Fig. 6

Local wall temperature of 40 PPI plasma sprayed and conventional brazed heat exchangers at 5 SLPM (Pe = 0.4), 20 SLPM (Pe = 1.2), and 200 SLPM (Pe = 9.3)

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Fig. 7

Local air temperature of 40 PPI plasma sprayed and conventional brazed heat exchangers at 5 SLPM (Pe = 0.4), 20SLPM (Pe = 1.2), and 200 SLPM (Pe = 9.3)

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Fig. 8

Air temperature difference of 10 and 40 PPI plasma sprayed and conventional brazed heat exchangers at different Pe number

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Fig. 9

Heat transferred to air through 10 and 40 PPI plasma sprayed and conventional brazed heat exchangers at different flow rate

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Fig. 10

Variation of volumetric heat transfer coefficient with air velocity for 10 and 40 PPI foams

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Fig. 11

Nusselt number as a function Reynolds, Prandtl, and Darcy number for Ni foam of 10 and 40 PPI pore densities

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Fig. 12

Schematic of the analytical model for foam heat exchanger

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Fig. 13

Predicted and measured dimensionless solid and fluid temperatures for 10 PPI foam with air flowing through it at (a) 15SLPM, (b) 90 SLPM, and (c) 160 SLPM

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Fig. 14

Model predictions and experimentally measured air temperature difference (Ta,o−Ta,i) for plasma sprayed 10 and 40PPI heat exchangers at different flow rates

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