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An experimental study of passive regenerator geometries
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 8 1 7 e1 8 2 2
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An experimental study of passive regenerator geometries K. Engelbrecht*, K.K. Nielsen, N. Pryds Fuel Cells and Solid State Chemistry Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark - DTU, Building 779, P.O. Box 49, DK-4000 Roskilde, Denmark
article info
abstract
Article history:
Active magnetic regenerative (AMR) systems are being investigated because they represent
Received 14 February 2011
a potentially attractive alternative to vapor compression technology. The performance of
Received in revised form
these systems is dependent on the heat transfer and pressure drop performance of the
22 July 2011
regenerator geometry. Therefore this article studies the effects of regenerator geometry on
Accepted 30 July 2011
performance for flat plate regenerators. This paper investigates methods of improving the
Available online 5 August 2011
performance of flat plate regenerators for use in AMR systems and studies how manufacturing variation affects regenerator performance. In order to eliminate experi-
Keywords:
mental uncertainty associated with magnetocaloric material properties, all regenerators
Magnetic refrigerator
are made of aluminum. The performance of corrugated plates and dimpled plates are
Regenerator
compared to traditional flat plate regenerators for a range of cycle times and utilizations.
Experimentation
Each regenerator is built using 18 aluminum plates with a 0.4 mm thickness, which allows their performance to be compared directly. ª 2011 Elsevier Ltd and IIR. All rights reserved.
Etude expe´rimentale sur les ge´ome´tries des re´ge´ne´rateurs passifs Mots cle´s : Re´frige´rateur magne´tique ; Re´ge´ne´rateur ; Expe´rience
1.
Introduction
Active magnetic regenerative (AMR) refrigeration systems represent an attractive alternative to vapor compression refrigeration and air-conditioning systems. AMR systems use a solid magnetocaloric refrigerant rather than a fluorocarbon working fluid, and it interacts with the environment via a heat transfer fluid. The heat transfer fluid will likely be aqueous and will therefore have minimal environmental impact. In theory, a well-designed AMR system can be competitive with or even more efficient than vapor compression systems, provided that the volume of the active magnetic regenerator is
sufficiently large (Engelbrecht et al., 2006). Some of the most important factors that affect AMR system performance are the magnetocaloric effect in the solid refrigerant, pumping losses associated with the regenerator geometry, and heat transfer performance of the regenerator (Tishin and Spichkin, 2003). Heat transfer between the fluid and the magnetocaloric material is the major loss mechanism in many AMR systems. In order to maximize AMR performance, it is critical to understand heat transfer processes in the regenerator. This paper investigates methods of improving the performance of regenerators based on flat plates for use in AMR systems. In order to eliminate experimental uncertainty associated with
* Corresponding author. Tel.: þ45 46775649; fax: þ45 46775858. E-mail address: [email protected] (K. Engelbrecht). 0140-7007/$ e see front matter ª 2011 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2011.07.015
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Nomenclature Variables c cH k t T u U
specific heat (J kg1 K1) specific heat at constant magnetic field (J kg1 K1) thermal conductivity W m2 K1) time (s) temperature (K) fluid velocity (m s1) utilization
magnetocaloric material properties, all regenerators are made of aluminum. A simple and flexible passive regenerator test setup has been developed where a heater is applied to the hot reservoir and the steady state reservoir temperatures are measured. The performance of each regenerator is determined by the regenerator effectiveness defined by the heat load and resulting temperature span. Theoretically, flat plate regenerators offer the best heat transfer to pressure drop ratio Sarlah (2008) for common regenerator designs. Flate plate regenerators have been used in AMR systems with gadolinium by Vasile and Muller, (2006), Tu sek et al., (2009), and Clot et al., (2003). However, highperformance flat plate regenerators require a very small plate thickness and plate spacing, making fabrication difficult Mitchell et al. (2007). Theoretically, heat transfer performance for parallel plates increases as the plate spacing decreases, until dimensions become so small the fluid can no longer be modeled as a continuum (Gamrat et al., 2005). However, experimental microchannel results have shown that stacks of multiple rectangular flow channels show a degradation of performance compared to theory as the plate spacing decreases (Rosa et al., 2009). This paper investigates the effectiveness of flat plate passive regenerators with varying plate spacing to investigate the performance of microchannel heat exchangers used as passive regenerators as the geometry is scaled down. Several alternative regenerator geometries based on flat plates were also tested. Regenerator geometries that may improve thermal performance including corrugated, or chevron, plates were constructed and compared to flat plate regenerators experimentally. There are several other promising alternative regenerator geometries that were not studied here, such as the wavy-structure regenerator (Sarlah et al., 2006). The conclusion of this paper is a suggested geometry based on flat plates for a prototype AMR.
2.
Experimental apparatus
The experiment is comprised of a single regenerator in contact with hot and cold fluid reservoirs. Fluid flow is provided by a displacer in the hot reservoir, which is also equipped with a heater. The device is the same described for AMR experiments in the past by Engelbrecht et al. (2011) and Bahl et al. (2008). The device can be used for passive experiments by removing the magnetic field variation from the regenerator and forcing a temperature span across the passive regenerator by adding a heat load to one end. A resistive
v ε r s
velocity (m s1) regenerator effectiveness density (kg m3) total cycle time (s)
Subscripts C cold f fluid H hot s solid regenerator material
heater is placed in the hot reservoir and provides a heat load to the hot reservoir. The maximum heater power is approximately 1.5 W. A displacer provides alternating fluid flow through the regenerator and in each experiment, the system is cycled until steady state has been reached. The entire regenerator and both reservoirs are isolated from the environment by foam insulation. The displacer has a variable speed and stroke length, giving the test apparatus flexibility regarding fluid flow rate, regenerator utilization and cycle frequency. The cold reservoir communicates thermally with the environment through a heat exchanger and a secondary heat transfer fluid. The performance of the regenerator is determined by the temperature difference between the hot and cold reservoirs, which is a measure of the regenerator effectiveness.
2.1.
Regenerator fabrication
Experiments were performed on a total of seven regenerators, each comprised of 18 aluminum plates 0.4 mm in thickness. By holding the mass of the regenerator material constant, the utilization is held constant for the same displacer stroke while the porosity varies with the spacing between plates. The plates were laser cut to the desired length and width in order to keep them as flat as possible during the cutting process. Four flat plate regenerators with different plate spacing, two corrugated plate regenerators, and one dimpled plate regenerator were fabricated. The flat plate regenerator stacks were fabricated using thin wire spacers to regulate the plate spacing. Sections of wire of varying diameter were stretched slightly to produce a straight wire with no sharp bends. The regenerator was stacked with two wires between each plate. After all the plates were stacked, the stack was compressed slightly to reduce the effects of slight bending of the wires and the plates were bonded with epoxy on both sides along the entire length of the plates in the flow direction. The resulting regenerator stack height was measured to determine the average effective plate spacing. Neither the variation in plate spacing nor the nonuniformity of the flow channels are reported for any regenerators in this paper. However, the effective average plate spacing is always slightly larger than the wire spacers, most likely due to non-uniform flatness and thickness of the plates, slight bending in the wire spacers, or possibly from variations introduced when the epoxy was applied. The corrugated plates were formed by pressing the plate between interspaced cylinders 0.3 mm in diameter. The
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orientation of the cylinders in relation to the flow direction was controlled. The orientation of alternating plates was reversed, such that troughs in the plates created by the cylinders were never parallel and the plates could not nest on each other. Once all plates were stacked, the stack was compressed and the plates were sealed with epoxy on both sides. The dimpled plates were formed with a special tool consisting of half spheres 1 mm in radius on both halves of the tool. The half spheres are arranged on each half of the tool such that after the plate is formed to a shape similar to an egg crate pattern by the tool. The height of the dimpled pattern can be controlled by how far the halves of the tool are pressed together. Metal stops were placed between the pressing tool halves to yield pressed plates that were approximately 0.65 mm from peak to peak. The regenerator was assembled by alternating flat plates and dimpled plates, which gives an average fluid flow channel that is 0.25 mm. Each regenerator stack was placed in an acrylic housing and sealed around the periphery of the stack with silicone to prevent heat transfer fluid from bypassing the regenerator stack. A photograph of the dimpled plate regenerator assembled in the housing is shown in Fig. 1. A summary of the characteristics of each regenerator tested in this paper is given in Table 1.
3.
Numerical model
The governing equations and fluid interactions in a passive regenerator are discussed in detail in the literature, (Bird et al., 2006) for example. The experimental data for flat plate regenerators were compared to predictions from a 2D numerical regenerator model (Nielsen et al., 2009). The model treats the regenerator as a repeating cell of a half plate and half channel with an alternating fluid flow. The thermal governing equations solved by the model are given below. vTf kf 2 vTf ¼ V Tf u vt rf cf vx
(1)
vTs ks 2 ¼ V Ts vt rs cH;s
(2)
Table 1 e A summary of the passive regenerators that were tested. Regenerator 1 2 3 4 5 6 7
Type Flat plate Flat plate Flat plate Flat plate Corrugated plate Corrugated plate Dimpled plate
Description
Porosity
0.74 mm spacing 0.31 mm spacing 0.20 mm spacing 0.10 mm spacing 120 included angle 90 included angle 0.23 mm spacing
0.64 0.43 0.33 0.20 0.64 0.64 0.35
The subscripts f and s indicate fluid and solid, respectively. The thermal properties are denoted k, r and c, respectively, and they represent the thermal conductivity, mass density and specific heat. Their values are assumed constant for the experiments conducted here, and the properties used are given in Table 2. The heat transfer between the solid and fluid domains is done through an internal boundary condition of the form ks
vTs vTf ¼ kf vy vy
(3)
The numerical model converges to a steady state as a function of prescribed temperatures at the cold and hot sides, respectively. Modeling several temperature spans, keeping the hot side fixed, yields a heater power versus temperature span curve. From this, it is possible to intersect with the applied heater load from the experiment in order to find the resulting model temperature span.
4.
Experimental results
Passive regenerators are generally defined by the effectiveness of the regenerator, which is defined below Dragutinovic and Baclic (1998). ZT ε¼
TH Tf;exit dt
0
sðTH TC Þ
(4)
Fig. 1 e The dimpled plate regenerator assembled in the regenerator housing (a) and a single dimpled aluminum plate (b).
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Table 2 e Material properties used by the numerical model. Parameter Fluid Fluid Fluid Fluid Fluid Fluid
specific heat density thermal conductivity specific heat density thermal conductivity
Value 4200 J kg1 K1 1000 kg m3 0.6 W m1 K1 903 J kg1 K1 2702 kg m3 237 W m1 K1
Where s is the blow period and Tf,exit is the temperature of the fluid exiting the regenerator. TH and TC are defined experimentally as the average temperature of the hot reservoir and cold reservoir, respectively once cyclical steady state has been achieved. Therefore Eq. (4) can be interpreted as one minus the heater power necessary to maintain a reservoir temperature divided by the maximum energy required to heat the fluid from the cold reservoir temperature to the hot temperature. For the experiments considered here, the heater power in the hot reservoir is held constant and the cold reservoir temperature is fixed. Therefore, the temperature span achieved by each regenerator is a direct gauge of its effectiveness, and regenerator performance is reported in terms of temperature span in this paper. A higher temperature span indicates a regenerator with higher effectiveness. The performance of a regenerator is generally presented as a function of the utilization, U, which is defined in Eq. (5). The utilization is the ratio of the thermal capacity of the fluid flowing into the regenerator during one fluid flow cycle to the thermal capacity of the solid regenerator material. U¼
vf Af s2 rf cf Vs rs cs
(5)
In each experiment, a heater power of 1.2 W was applied to the hot reservoir. The heater power was chosen such that the resulting temperature span across the passive regenerator would be in a range that could be accurately measured but would not result in large changes in the fluid or solid due to temperature variations. In one set of experiments, the heat transfer fluid was water and the second set the heat transfer fluid was a mixture of 75% water and 25% ethylene glycol. Therefore, the experiments using different heat transfer fluids are not directly comparable. The dimpled plate regenerator,
both corrugated plate regenerators and one flat plate regenerator were all tested with pure water as the heat transfer fluid and selected results are shown in Fig. 2. Fig. 2 shows that the corrugated plates with a 90 included angle generally exhibit the highest effectiveness while the flat plate regenerator generally exhibits the worst regenerator performance. The plate spacing for the flat plate regenerator was chosen to correspond to the average plate spacing for the corrugated plate regenerators. However, the dimpled plate spacing was dictated by the tool used to form the plates and the resulting plate spacing was smaller than the other regenerators shown in Fig. 2. Therefore, the dimpled plates have an advantage over the other regenerators, but a significant increase in performance was not measured. These experiments suggest that a corrugated plate regenerator can offer increased performance over a flat plate regenerator and that a 90 included angle relative to the flow direction performs better than plates with a 120 angle of corrugation. The next set of experiments was performed with a mixture of water and ethylene glycol as the heat transfer fluid for four regenerators with plate spacing of approximately 0.1 mm, 0.2 mm, 0.3 mm and 0.7 mm. The heater power applied to the hot reservoir was 1.2 W. Experiments were run for a range of fluid flow rates and utilizations, and the results for two flow rates are shown in Fig. 3. The temperature span achieved by each regenerator is plotted as a function of plate spacing for two utilizations in Fig. 4. The experimental data were also compared to predicted data from the 2D model. Selected experimental and predicted data are plotted in Fig. 5.
5.
Discussion
The purpose of the research presented here is to determine the optimum regenerator geometry for a prototype AMR. Flat plate, corrugated plate, and dimpled plate regenerators were tested and compared. The flat plate results were also compared to predictions by a detailed 2D numerical model. Examination of Fig. 2 shows that corrugated plate regenerators show improved heat transfer performance over a flat plate regenerator with approximately the same effective plate spacing. The data presented here suggest that the angle of the corrugation pattern affects regenerator performance, and a 90 included angle pattern outperformed a 120 corrugation
Fig. 2 e Temperature span as a function of utilization for four different regenerator geometries for a fluid flow rate of 0.7 g sL1 (a) and 2.7 g sL1 (b).
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 8 1 7 e1 8 2 2
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Fig. 3 e Temperature span as a function of utilization for flat plate regenerators with four different plate spacings for a fluid flow rate of (a) 0.7 g sL1 and (b) 2.7 g sL1.
Fig. 4 e Temperature span as a function of plate spacing for different utilizations and a fluid flow rate of 0.9 g sL1.
pattern. The dimpled plate regenerator slightly outperformed the flat plate regenerator, but the effective plate spacing of the dimpled plate regenerator was approximately half that of the flat plate regenerator. Therefore, dimpled plate regenerators were not found to be an attractive alternative to flat plate regenerators. Although the corrugated plate regenerators were shown to have better heat transfer performance than flat plate regenerators with similar plate spacing, the pump losses associated with corrugated plates may be significantly higher than for the
and Svaic (2007) report that corrugated plates flat plates. Dovic will have noticeably higher pressure drop than flat plates, and pump losses may make corrugated plate regenerator less efficient than flat plates in an AMR device. Fabrication of corrugated plates for common magnetocaloric materials may also be a challenge. Gadolinium is a malleable metal and may be well-suited to being formed into corrugated plates, but more brittle materials may pose a challenge. Ceramic materials such as LCSM may be shaped before sintering, but development of the process is necessary. It was expected that the regenerators with smaller plate spacing would exhibit higher heat transfer coefficients between the fluid and solid and increase regenerator performance. However, results shown in Figs. 3 and 4 suggest that the performance of the flat plate regenerators tested for this paper was not highly dependent on plate spacing. Generally, the 0.2 mm or 0.3 mm regenerators performed best, but for some operating conditions the other regenerators produced better experimental results. The reduced dependence on plate spacing may be partly due to variation in plate spacing. Each regenerator was built from the same aluminum plates using the same fabrication technique, and the absolute variation in plate spacing and flatness is likely similar, meaning that the relative variation increases as the plate spacing decreases. The increased variation in the regenerators with smaller plate spacing may erode the performance increase from the enhanced heat transfer between the plate and solid. Based on experimental data generated in this paper, a plate spacing between 0.2 mm and 0.3 mm is optimum for the regenerator fabrication techniques used.
Fig. 5 e Experimental regenerator temperature span and predicted temperature span as a function of (a) utilization for 0.1 mm and 0.3 mm plate spacings and (b) fluid flow rate for a 0.1 mm plate spacing.
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The 2D regenerator model was able to capture general trends in the regenerator performance, but did not show excellent agreement for the range of experiments presented in this paper. In Fig. 5, the model under-predicts the regenerator performance for a utilization of 0.21 but over-predicts the performance when the utilization increases to 0.62. The discrepancy between experiment and model may be caused by the method of measuring the hot reservoir temperature. In this experiment, the heater is placed on the wetted side of the fluid displacer piston and the hot reservoir thermocouple is attached to the piston and measures the fluid temperature very close to the heater surface. The thermocouple may read an artificially high temperature due to its proximity to the heater. This effect is likely maximized for low utilizations because the lower piston stroke reduces mixing between the warm fluid near the heater and the cooler fluid exiting the regenerator. This effect has not been quantified presently.
6.
Conclusions
Experimental passive regenerator results were presented for three different types of geometry (flat plate, corrugated plate and dimpled plate). The best performance was achieved by the 90 corrugated plate regenerator, making it an attractive possible alternative regenerator geometry. The performance of flat plate regenerators in four different plate spacing was also compared. The experimental results did not show a significant increase in performance as the plate spacing decreased, although the nominal heat transfer coefficient was expected to increase. In some experiments regenerators with smaller channel heights exhibited lower performance than those with larger channel heights. This may be caused by variations due to the fabrication process of each regenerator. Using the fabrication technique described in this paper, the optimal plate spacing was determined by be 0.2 mme0.3 mm.
Acknowledgments The authors would like to acknowledge the support of the Programme Commission on Energy and Environment (EnMi) (Contract No. 2104-06-0032) which is part of the Danish Council for Strategic Research and from the Danish Council for Independent Research/Technical and Production Sciences (contract no. 10-092791). The technical support of Jørgen Geyti is greatly appreciated.
references
Bahl, C.R.H., Petersen, T.F., Pryds, N., Smith, A., 2008. A versatile magnetic refrigeration test device. Rev. Sci. Instrum. 79, 093906. Bird, R.B., Stewart, W.E., Lightfoot, E.N., 2006. Transport Phenomena. Wiley and Sons. Clot, P., Viallet, D., Allab, F., Kedous-Lebouc, A., 2003. A magnetbased device for active magnetic regenerative refrigeration. IEEE Trans. Magn. 39, 3349. , D., Svaic , S., 2007. Influence of chevron plates geometry on Dovic performances of plate heat exchangers. Tehnicki Vjesnik 14 (1e2), 37e45. Dragutinovic, G.D., Baclic, B.S., 1998. Operation of Counterflow Regenerators. Computational Mechanics Inc., Billerica, MA. Engelbrecht, K., Bahl, C.R.H., Nielsen, K.K., 2011. Experimental results for a magnetic refrigerator using three different types of magnetocaloric material regenerators. Int. J. Refrigeration 30 (4), 1132e1140. Engelbrecht, K.L., Nellis, G.F., Klein, S.A., 2006. Predicting the performance of an active magnetic regenerator refrigerator used for space cooling and refrigeration. HVAC R Res. 12 (4), 1077e1095. Gamrat, G., Favre-Marinet, M., Asendrych, D., 2005. Conduction and entrance effects on laminar liquid flow and heat transfer in rectangular microchannels. Int. J. Heat Mass Transfer 48 (14), 2943e2954. Mitchell, M.P., Gedeon, D., Wood, G., Ibrahim, M., 2007. Results of tests of etched foil regenerator material. Cryocoolers 14, 381e387. Nielsen, K.K., Bahl, C.R.H., Smith, A., Bjørk, R., Pryds, N., Hattel, J., 2009. Detailed numerical modeling of a linear parallel-plate active magnetic regenerator. Int. J. Refrigeration 32 (6), 1478e1486. Rosa, P., Karayiannis, T., Collins, M., 2009. Single-phase heat transfer in microchannels: the importance of scaling effects. Appl. Thermal Eng. 29 (17e18), 3447e3468. Sarlah, A., 2008. Thermo-hydraulic properties of heat regenerators in magnetic refrigerator. Ph.D. thesis, University of Ljubljana, Slovenia. Sarlah, A., Kitanovski, A., Poredos, A., Egolf, P.W., Sari, O., Gendre, F., Besson, C., 2006. Static and rotating active magnetic regenerators with porous heat exchangers for magnetic cooling. Int. J. Refrigeration 29, 1332e1339. Tishin, A.M., Spichkin, Y.I., 2003. The Magnetocaloric Effect and Its Applications. Institute of Physics Publishing, Temple Back, Bristol, UK. Tu sek, J., Zupan, S., Sarlah, A., Prebil, I.A.P., 2009. Development of a rotary magnetic refrigerator. In: Proceedings of the 3 Int. Conference on Magnetic Refrigeration at Room Temperature, pp. 409e414. Des Moines, Iowa, USA. Vasile, C., Muller, C., 2006. Innovative design of a magnetocaloric system. Int. J. Refrigeration 29 (8), 1318e1326.
available at www.sciencedirect.com
w w w . i i fi i r . o r g
journal homepage: www.elsevier.com/locate/ijrefrig
An experimental study of passive regenerator geometries K. Engelbrecht*, K.K. Nielsen, N. Pryds Fuel Cells and Solid State Chemistry Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark - DTU, Building 779, P.O. Box 49, DK-4000 Roskilde, Denmark
article info
abstract
Article history:
Active magnetic regenerative (AMR) systems are being investigated because they represent
Received 14 February 2011
a potentially attractive alternative to vapor compression technology. The performance of
Received in revised form
these systems is dependent on the heat transfer and pressure drop performance of the
22 July 2011
regenerator geometry. Therefore this article studies the effects of regenerator geometry on
Accepted 30 July 2011
performance for flat plate regenerators. This paper investigates methods of improving the
Available online 5 August 2011
performance of flat plate regenerators for use in AMR systems and studies how manufacturing variation affects regenerator performance. In order to eliminate experi-
Keywords:
mental uncertainty associated with magnetocaloric material properties, all regenerators
Magnetic refrigerator
are made of aluminum. The performance of corrugated plates and dimpled plates are
Regenerator
compared to traditional flat plate regenerators for a range of cycle times and utilizations.
Experimentation
Each regenerator is built using 18 aluminum plates with a 0.4 mm thickness, which allows their performance to be compared directly. ª 2011 Elsevier Ltd and IIR. All rights reserved.
Etude expe´rimentale sur les ge´ome´tries des re´ge´ne´rateurs passifs Mots cle´s : Re´frige´rateur magne´tique ; Re´ge´ne´rateur ; Expe´rience
1.
Introduction
Active magnetic regenerative (AMR) refrigeration systems represent an attractive alternative to vapor compression refrigeration and air-conditioning systems. AMR systems use a solid magnetocaloric refrigerant rather than a fluorocarbon working fluid, and it interacts with the environment via a heat transfer fluid. The heat transfer fluid will likely be aqueous and will therefore have minimal environmental impact. In theory, a well-designed AMR system can be competitive with or even more efficient than vapor compression systems, provided that the volume of the active magnetic regenerator is
sufficiently large (Engelbrecht et al., 2006). Some of the most important factors that affect AMR system performance are the magnetocaloric effect in the solid refrigerant, pumping losses associated with the regenerator geometry, and heat transfer performance of the regenerator (Tishin and Spichkin, 2003). Heat transfer between the fluid and the magnetocaloric material is the major loss mechanism in many AMR systems. In order to maximize AMR performance, it is critical to understand heat transfer processes in the regenerator. This paper investigates methods of improving the performance of regenerators based on flat plates for use in AMR systems. In order to eliminate experimental uncertainty associated with
* Corresponding author. Tel.: þ45 46775649; fax: þ45 46775858. E-mail address: [email protected] (K. Engelbrecht). 0140-7007/$ e see front matter ª 2011 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2011.07.015
1818
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Nomenclature Variables c cH k t T u U
specific heat (J kg1 K1) specific heat at constant magnetic field (J kg1 K1) thermal conductivity W m2 K1) time (s) temperature (K) fluid velocity (m s1) utilization
magnetocaloric material properties, all regenerators are made of aluminum. A simple and flexible passive regenerator test setup has been developed where a heater is applied to the hot reservoir and the steady state reservoir temperatures are measured. The performance of each regenerator is determined by the regenerator effectiveness defined by the heat load and resulting temperature span. Theoretically, flat plate regenerators offer the best heat transfer to pressure drop ratio Sarlah (2008) for common regenerator designs. Flate plate regenerators have been used in AMR systems with gadolinium by Vasile and Muller, (2006), Tu sek et al., (2009), and Clot et al., (2003). However, highperformance flat plate regenerators require a very small plate thickness and plate spacing, making fabrication difficult Mitchell et al. (2007). Theoretically, heat transfer performance for parallel plates increases as the plate spacing decreases, until dimensions become so small the fluid can no longer be modeled as a continuum (Gamrat et al., 2005). However, experimental microchannel results have shown that stacks of multiple rectangular flow channels show a degradation of performance compared to theory as the plate spacing decreases (Rosa et al., 2009). This paper investigates the effectiveness of flat plate passive regenerators with varying plate spacing to investigate the performance of microchannel heat exchangers used as passive regenerators as the geometry is scaled down. Several alternative regenerator geometries based on flat plates were also tested. Regenerator geometries that may improve thermal performance including corrugated, or chevron, plates were constructed and compared to flat plate regenerators experimentally. There are several other promising alternative regenerator geometries that were not studied here, such as the wavy-structure regenerator (Sarlah et al., 2006). The conclusion of this paper is a suggested geometry based on flat plates for a prototype AMR.
2.
Experimental apparatus
The experiment is comprised of a single regenerator in contact with hot and cold fluid reservoirs. Fluid flow is provided by a displacer in the hot reservoir, which is also equipped with a heater. The device is the same described for AMR experiments in the past by Engelbrecht et al. (2011) and Bahl et al. (2008). The device can be used for passive experiments by removing the magnetic field variation from the regenerator and forcing a temperature span across the passive regenerator by adding a heat load to one end. A resistive
v ε r s
velocity (m s1) regenerator effectiveness density (kg m3) total cycle time (s)
Subscripts C cold f fluid H hot s solid regenerator material
heater is placed in the hot reservoir and provides a heat load to the hot reservoir. The maximum heater power is approximately 1.5 W. A displacer provides alternating fluid flow through the regenerator and in each experiment, the system is cycled until steady state has been reached. The entire regenerator and both reservoirs are isolated from the environment by foam insulation. The displacer has a variable speed and stroke length, giving the test apparatus flexibility regarding fluid flow rate, regenerator utilization and cycle frequency. The cold reservoir communicates thermally with the environment through a heat exchanger and a secondary heat transfer fluid. The performance of the regenerator is determined by the temperature difference between the hot and cold reservoirs, which is a measure of the regenerator effectiveness.
2.1.
Regenerator fabrication
Experiments were performed on a total of seven regenerators, each comprised of 18 aluminum plates 0.4 mm in thickness. By holding the mass of the regenerator material constant, the utilization is held constant for the same displacer stroke while the porosity varies with the spacing between plates. The plates were laser cut to the desired length and width in order to keep them as flat as possible during the cutting process. Four flat plate regenerators with different plate spacing, two corrugated plate regenerators, and one dimpled plate regenerator were fabricated. The flat plate regenerator stacks were fabricated using thin wire spacers to regulate the plate spacing. Sections of wire of varying diameter were stretched slightly to produce a straight wire with no sharp bends. The regenerator was stacked with two wires between each plate. After all the plates were stacked, the stack was compressed slightly to reduce the effects of slight bending of the wires and the plates were bonded with epoxy on both sides along the entire length of the plates in the flow direction. The resulting regenerator stack height was measured to determine the average effective plate spacing. Neither the variation in plate spacing nor the nonuniformity of the flow channels are reported for any regenerators in this paper. However, the effective average plate spacing is always slightly larger than the wire spacers, most likely due to non-uniform flatness and thickness of the plates, slight bending in the wire spacers, or possibly from variations introduced when the epoxy was applied. The corrugated plates were formed by pressing the plate between interspaced cylinders 0.3 mm in diameter. The
1819
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 8 1 7 e1 8 2 2
orientation of the cylinders in relation to the flow direction was controlled. The orientation of alternating plates was reversed, such that troughs in the plates created by the cylinders were never parallel and the plates could not nest on each other. Once all plates were stacked, the stack was compressed and the plates were sealed with epoxy on both sides. The dimpled plates were formed with a special tool consisting of half spheres 1 mm in radius on both halves of the tool. The half spheres are arranged on each half of the tool such that after the plate is formed to a shape similar to an egg crate pattern by the tool. The height of the dimpled pattern can be controlled by how far the halves of the tool are pressed together. Metal stops were placed between the pressing tool halves to yield pressed plates that were approximately 0.65 mm from peak to peak. The regenerator was assembled by alternating flat plates and dimpled plates, which gives an average fluid flow channel that is 0.25 mm. Each regenerator stack was placed in an acrylic housing and sealed around the periphery of the stack with silicone to prevent heat transfer fluid from bypassing the regenerator stack. A photograph of the dimpled plate regenerator assembled in the housing is shown in Fig. 1. A summary of the characteristics of each regenerator tested in this paper is given in Table 1.
3.
Numerical model
The governing equations and fluid interactions in a passive regenerator are discussed in detail in the literature, (Bird et al., 2006) for example. The experimental data for flat plate regenerators were compared to predictions from a 2D numerical regenerator model (Nielsen et al., 2009). The model treats the regenerator as a repeating cell of a half plate and half channel with an alternating fluid flow. The thermal governing equations solved by the model are given below. vTf kf 2 vTf ¼ V Tf u vt rf cf vx
(1)
vTs ks 2 ¼ V Ts vt rs cH;s
(2)
Table 1 e A summary of the passive regenerators that were tested. Regenerator 1 2 3 4 5 6 7
Type Flat plate Flat plate Flat plate Flat plate Corrugated plate Corrugated plate Dimpled plate
Description
Porosity
0.74 mm spacing 0.31 mm spacing 0.20 mm spacing 0.10 mm spacing 120 included angle 90 included angle 0.23 mm spacing
0.64 0.43 0.33 0.20 0.64 0.64 0.35
The subscripts f and s indicate fluid and solid, respectively. The thermal properties are denoted k, r and c, respectively, and they represent the thermal conductivity, mass density and specific heat. Their values are assumed constant for the experiments conducted here, and the properties used are given in Table 2. The heat transfer between the solid and fluid domains is done through an internal boundary condition of the form ks
vTs vTf ¼ kf vy vy
(3)
The numerical model converges to a steady state as a function of prescribed temperatures at the cold and hot sides, respectively. Modeling several temperature spans, keeping the hot side fixed, yields a heater power versus temperature span curve. From this, it is possible to intersect with the applied heater load from the experiment in order to find the resulting model temperature span.
4.
Experimental results
Passive regenerators are generally defined by the effectiveness of the regenerator, which is defined below Dragutinovic and Baclic (1998). ZT ε¼
TH Tf;exit dt
0
sðTH TC Þ
(4)
Fig. 1 e The dimpled plate regenerator assembled in the regenerator housing (a) and a single dimpled aluminum plate (b).
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i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 8 1 7 e1 8 2 2
Table 2 e Material properties used by the numerical model. Parameter Fluid Fluid Fluid Fluid Fluid Fluid
specific heat density thermal conductivity specific heat density thermal conductivity
Value 4200 J kg1 K1 1000 kg m3 0.6 W m1 K1 903 J kg1 K1 2702 kg m3 237 W m1 K1
Where s is the blow period and Tf,exit is the temperature of the fluid exiting the regenerator. TH and TC are defined experimentally as the average temperature of the hot reservoir and cold reservoir, respectively once cyclical steady state has been achieved. Therefore Eq. (4) can be interpreted as one minus the heater power necessary to maintain a reservoir temperature divided by the maximum energy required to heat the fluid from the cold reservoir temperature to the hot temperature. For the experiments considered here, the heater power in the hot reservoir is held constant and the cold reservoir temperature is fixed. Therefore, the temperature span achieved by each regenerator is a direct gauge of its effectiveness, and regenerator performance is reported in terms of temperature span in this paper. A higher temperature span indicates a regenerator with higher effectiveness. The performance of a regenerator is generally presented as a function of the utilization, U, which is defined in Eq. (5). The utilization is the ratio of the thermal capacity of the fluid flowing into the regenerator during one fluid flow cycle to the thermal capacity of the solid regenerator material. U¼
vf Af s2 rf cf Vs rs cs
(5)
In each experiment, a heater power of 1.2 W was applied to the hot reservoir. The heater power was chosen such that the resulting temperature span across the passive regenerator would be in a range that could be accurately measured but would not result in large changes in the fluid or solid due to temperature variations. In one set of experiments, the heat transfer fluid was water and the second set the heat transfer fluid was a mixture of 75% water and 25% ethylene glycol. Therefore, the experiments using different heat transfer fluids are not directly comparable. The dimpled plate regenerator,
both corrugated plate regenerators and one flat plate regenerator were all tested with pure water as the heat transfer fluid and selected results are shown in Fig. 2. Fig. 2 shows that the corrugated plates with a 90 included angle generally exhibit the highest effectiveness while the flat plate regenerator generally exhibits the worst regenerator performance. The plate spacing for the flat plate regenerator was chosen to correspond to the average plate spacing for the corrugated plate regenerators. However, the dimpled plate spacing was dictated by the tool used to form the plates and the resulting plate spacing was smaller than the other regenerators shown in Fig. 2. Therefore, the dimpled plates have an advantage over the other regenerators, but a significant increase in performance was not measured. These experiments suggest that a corrugated plate regenerator can offer increased performance over a flat plate regenerator and that a 90 included angle relative to the flow direction performs better than plates with a 120 angle of corrugation. The next set of experiments was performed with a mixture of water and ethylene glycol as the heat transfer fluid for four regenerators with plate spacing of approximately 0.1 mm, 0.2 mm, 0.3 mm and 0.7 mm. The heater power applied to the hot reservoir was 1.2 W. Experiments were run for a range of fluid flow rates and utilizations, and the results for two flow rates are shown in Fig. 3. The temperature span achieved by each regenerator is plotted as a function of plate spacing for two utilizations in Fig. 4. The experimental data were also compared to predicted data from the 2D model. Selected experimental and predicted data are plotted in Fig. 5.
5.
Discussion
The purpose of the research presented here is to determine the optimum regenerator geometry for a prototype AMR. Flat plate, corrugated plate, and dimpled plate regenerators were tested and compared. The flat plate results were also compared to predictions by a detailed 2D numerical model. Examination of Fig. 2 shows that corrugated plate regenerators show improved heat transfer performance over a flat plate regenerator with approximately the same effective plate spacing. The data presented here suggest that the angle of the corrugation pattern affects regenerator performance, and a 90 included angle pattern outperformed a 120 corrugation
Fig. 2 e Temperature span as a function of utilization for four different regenerator geometries for a fluid flow rate of 0.7 g sL1 (a) and 2.7 g sL1 (b).
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 8 1 7 e1 8 2 2
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Fig. 3 e Temperature span as a function of utilization for flat plate regenerators with four different plate spacings for a fluid flow rate of (a) 0.7 g sL1 and (b) 2.7 g sL1.
Fig. 4 e Temperature span as a function of plate spacing for different utilizations and a fluid flow rate of 0.9 g sL1.
pattern. The dimpled plate regenerator slightly outperformed the flat plate regenerator, but the effective plate spacing of the dimpled plate regenerator was approximately half that of the flat plate regenerator. Therefore, dimpled plate regenerators were not found to be an attractive alternative to flat plate regenerators. Although the corrugated plate regenerators were shown to have better heat transfer performance than flat plate regenerators with similar plate spacing, the pump losses associated with corrugated plates may be significantly higher than for the
and Svaic (2007) report that corrugated plates flat plates. Dovic will have noticeably higher pressure drop than flat plates, and pump losses may make corrugated plate regenerator less efficient than flat plates in an AMR device. Fabrication of corrugated plates for common magnetocaloric materials may also be a challenge. Gadolinium is a malleable metal and may be well-suited to being formed into corrugated plates, but more brittle materials may pose a challenge. Ceramic materials such as LCSM may be shaped before sintering, but development of the process is necessary. It was expected that the regenerators with smaller plate spacing would exhibit higher heat transfer coefficients between the fluid and solid and increase regenerator performance. However, results shown in Figs. 3 and 4 suggest that the performance of the flat plate regenerators tested for this paper was not highly dependent on plate spacing. Generally, the 0.2 mm or 0.3 mm regenerators performed best, but for some operating conditions the other regenerators produced better experimental results. The reduced dependence on plate spacing may be partly due to variation in plate spacing. Each regenerator was built from the same aluminum plates using the same fabrication technique, and the absolute variation in plate spacing and flatness is likely similar, meaning that the relative variation increases as the plate spacing decreases. The increased variation in the regenerators with smaller plate spacing may erode the performance increase from the enhanced heat transfer between the plate and solid. Based on experimental data generated in this paper, a plate spacing between 0.2 mm and 0.3 mm is optimum for the regenerator fabrication techniques used.
Fig. 5 e Experimental regenerator temperature span and predicted temperature span as a function of (a) utilization for 0.1 mm and 0.3 mm plate spacings and (b) fluid flow rate for a 0.1 mm plate spacing.
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The 2D regenerator model was able to capture general trends in the regenerator performance, but did not show excellent agreement for the range of experiments presented in this paper. In Fig. 5, the model under-predicts the regenerator performance for a utilization of 0.21 but over-predicts the performance when the utilization increases to 0.62. The discrepancy between experiment and model may be caused by the method of measuring the hot reservoir temperature. In this experiment, the heater is placed on the wetted side of the fluid displacer piston and the hot reservoir thermocouple is attached to the piston and measures the fluid temperature very close to the heater surface. The thermocouple may read an artificially high temperature due to its proximity to the heater. This effect is likely maximized for low utilizations because the lower piston stroke reduces mixing between the warm fluid near the heater and the cooler fluid exiting the regenerator. This effect has not been quantified presently.
6.
Conclusions
Experimental passive regenerator results were presented for three different types of geometry (flat plate, corrugated plate and dimpled plate). The best performance was achieved by the 90 corrugated plate regenerator, making it an attractive possible alternative regenerator geometry. The performance of flat plate regenerators in four different plate spacing was also compared. The experimental results did not show a significant increase in performance as the plate spacing decreased, although the nominal heat transfer coefficient was expected to increase. In some experiments regenerators with smaller channel heights exhibited lower performance than those with larger channel heights. This may be caused by variations due to the fabrication process of each regenerator. Using the fabrication technique described in this paper, the optimal plate spacing was determined by be 0.2 mme0.3 mm.
Acknowledgments The authors would like to acknowledge the support of the Programme Commission on Energy and Environment (EnMi) (Contract No. 2104-06-0032) which is part of the Danish Council for Strategic Research and from the Danish Council for Independent Research/Technical and Production Sciences (contract no. 10-092791). The technical support of Jørgen Geyti is greatly appreciated.
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