| 摘要： 本研究建立兩個數學模式描述室冷法冷卻時紙箱內甘藍的熱傳遞。模式係爲12個具有特定熱性質的等直徑圓球包含於理想氣體的控制體積內，控制體積的表面有熱流通過。分散模式考慮球體的熱傳導以偏微分方程式表示，用常微分方程描述控制體積內空氣的能量平衡，並以數值方法求得此模式的解。團塊模式假設球體內溫度均一，僅計算表面的熱傳。團塊模式爲兩個耦合的常微分方程式所組成，以解析法求得其解。 實驗在實驗室規模的冷藏庫進行，並記錄甘藍內溫度的變化與紙箱內空氣溫度。兩個模式以實驗數據驗證，模式的預測與實驗結果非常符合。紙箱內甘藍的降溫速率遠低於紙箱外未包裝的甘藍，紙箱內的空氣溫度高於冷藏庫內空氣。通過紙箱的熱通量因紙箱內外側的對流阻力與瓦楞紙板的熱傳導阻力而減少。研究中並以實驗與模式來探討紙箱上的開孔對於降溫過程中箱內空氣溫度與甘藍溫度分布的影響。Two mathematical models were developed to simulate heat transfer for cabbages packed in a carton subjected to room cooling proesses. The configuration of the system was considered as 12 equal diameter spheres of certain thermal properties confined with idealized air in a control volume and heat flow was allowed on the boundary. The control volume was surrounded with air at a fixed temperature. Heat conduction in the sphere was considered in the distributed model. The distributed model was represented with a partial differential equation coupled with an ordinary differential equation which accounted for energy balance of the air. The distributed model was solved by applying numerical methods. Uniform temperature distribution in the sphere was assumed in the lumped model and only convective heat transfer on the surface of spheres was taken account. The lumped model was represented with two coupled ordinary equations which were analytically solved. Cooling processes were conducted in a laboratory scale cold storage. Temperature history of cabbages and air inside the carton were recorded. Both models were justified by experimental data, very consistent results were obtained. The cooling rate of the cabbage inside the carton was much lower than that of the cabbage without barrier of the carton. The temperature of the air in the carton was higher than the temperature of surrounding air in the cold storage. Heat flux through the carton was hindered by surface convective resistances on both side of the carton as well as conductive resistance of the corrugated cardboard. Models and experiments were utilized to examine the influence of openings of the carton on the transient state temperature distribution in the cabbage and temperature of the air inside the carton. |
頁面載入中請稍後...