磁致冷復熱器之性能提升設計

關於磁致冷復熱器（AMR）的相關研究過去多未考量熱能在熱端與冷端之間直接經由金屬磁熱材料進行傳遞造成的效能下降，因此，本研究由阻斷材料中的熱能傳遞著手，藉由設置低熱傳導率的區域作為絕熱段並配置於磁熱材料之中，使熱能無法經由磁熱材料在冷、熱端之間傳遞，以此提升AMR之效能。同時，研究過程中改變絕熱段之長度、數量與位置，並在不同的週期、溫差等參數條件下進行測試，藉以了解性能改善之狀況以及各參數間的相互影響，使AMR的性能最佳化。研究以COMSOL Multiphysics模擬軟體進行，並簡化AMR之結構，以一維暫態模型進行模擬分析。模擬採用的磁致冷裝置為往復式AMR，磁熱材料為Gd平板，工作流體為水，磁場強度1 T。　　研究分別在固定孔隙率和固定材料厚度的設置下進行，結果顯示，在各種參數條件下，絕熱段的設置皆可使AMR的性能提升，提升的幅度則與絕熱段本身的設計和參數條件有關。多數情況下，絕熱段長度越長、數量越多，AMR的性能提升幅度越大；而將絕熱段之位置設在冷端與復熱器中點的中間附近，通常會有較大的提升效果。另外，當AMR的週期較長、溫差較大時，絕熱段的提升效果較明顯；而AMR溫差越大，最佳操作週期則越長，並會受到材料片數（總表面積）的影響。固定孔隙率與固定材料厚度的結果相似，惟固定厚度下磁熱材料總質量隨材料片數增加而增加，因此冷凍能力與其提升量隨著片數變化的趨勢與固定孔隙率不同。研究結果並顯示，在固定厚度下，質量流率若配合材料總質量改變，使利用因數維持在1 ~ 1.6附近，則定溫差下AMR的冷凍能力最大。

關鍵字

AMR ；磁致冷復熱器；絕熱段；磁熱材料

並列摘要

Most of previous researches about active magnetic regenerators (AMR) didn’t pay much attention to the effects of conduction heat transfer between the hot end and the cold end through the magnetocaloric material in AMR on the efficiency of cooling. For an increase in efficiency, the work here investigates this point with an introduction of adiabatic segments inserted into the magnetocaloric material to prevent the heat conduction through it. The length, number and positon of adiabatic segments under various system parameters are analyzed numerically to improve the cooling performance of AMR. A simplified 1-D transient model of AMR is adopted and solved by COMSOL Multiphysics. The AMR model is a reciprocating type with Gd plates as magnetocaloric material and water as working fluid. The maximum magnetic field is 1 T. The numerical simulation is done under the conditions of constant porosity and constant thickness of Gd plates. The results show that the adiabatic segments can improve the cooling performance of AMR. For most cases, longer and more segments lead to a better performance. In addition, the position of segments in the middle region between the cold end and the center of Gd plates usually provides a higher cooling power. Further, the adiabatic segments will be more effective for high temperature difference and long operating period. The parameters such as the temperature difference, the period and the number of Gd plates will interact with one another and have coupling effects on the AMR performance. The two conditions (constant porosity and constant thickness) have similar results except that the trend of the cooling power versus the number of Gd plates is different due to the total Gd mass of the latter one increasing with the number of Gd plates. Under the constant-thickness condition, a better AMR performance is achieved for the magnitude of utilization factor around 1-1.6 by adjusting the mass flow rate.

並列關鍵字

active magnetic regenerator ； adiabatic segment ； magnetocaloric material

參考文獻

[1] K. A. Gschneidner, Jr. and V. K. Pecharsky, “Thirty years of near room temperature magnetic cooling: Where we are today and future prospects,” International Journal of Refrigeration, Vol. 31, pp. 945-961, 2008.

[2] V. K. Pecharsky and K. A. Gschneidner, Jr., “Giant Magnetocaloric Effect in Gd5(Si2Ge2),” Physical Review Letters, Vol. 78, pp. 4484-4497, 1997a.

[3] B. Yu, M. Liu, P. W. Egolf and A. Kitanovski, “A review of magnetic refrigerator and heat pump prototypes built before the year 2010,” International Journal of Refrigeration, Vol. 33, pp. 1029-1060, 2010.

[4] A. Rowe and A. Tura, “Experimental investigation of a three-material layered active magnetic regenerator,” International Journal of Refrigeration, Vol. 29, pp. 1286-1293, 2006.

[5] A. Rowe and A. Tura, “Active magnetic regenerator performance enhancement using passive magnetic materials,” Journal of Magnetism and Magnetic Materials, Vol. 320, pp. 1357–1363, 2008.

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磁致冷復熱器之性能提升設計

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