圍護結構升溫傳熱特性

    對 高低溫試驗箱圍護結構由低溫升至高溫時的非穩態傳熱建立了數學模型，使用T D MA 方法進行了圍護結構傳熱數值計算，對3種不同升溫工況下的圍護結構進行傳熱特性分析，使用圍護結構質童系數概念來簡化計算其在升溫時的耗熱量。
    高低溫試驗箱是一種小型的環境試驗裝置，使用該裝置可以對產品進行高溫或低溫環境試驗以及溫度沖擊試驗，來檢驗產品承受脹縮、老化等性能。通常這類裝置具有溫 變速率要求，箱內溫度變化較大，整個箱體的熱量傳遞屬于非穩態過程。試驗箱在升溫過程中由于圍 護結構耗熱量占總耗熱量的比例甚，，因此研究圍護結構升溫 過程傳熱特性及變化規律對合理確定加熱系統設備容量、提高試驗箱的節能性十分必要。

    試驗箱簡介及升溫工況的選取

    試驗箱使用東莞瑞凱儀器生產的微電腦可控式高低溫試驗箱，溫度范圍 -70～100℃；試驗箱內空氣加熱方式采用電熱絲加熱，該方式可以使空氣快速升溫至設定的工作溫度，加熱絲額定功率為1500w。
    1）試驗箱由低溫穩 態到高溫穩態的升溫傳熱要復雜于從環境溫度到高溫穩態的傳熱；
    2）試驗箱內的裝、空載情況也可能對圍護結構產生不同的影響；
    圍護結構傳熱分析

    試驗箱圍(wei)護(hu)結構由3層(ceng)(ceng)材料組(zu)成，外層(ceng)(ceng)是鍍鋅鋼板(ban)，厚度(du)(du)1.2 mm，中間層(ceng)(ceng)是聚氨醋發(fa)泡材料，厚度(du)(du)100mm；里層(ceng)(ceng)是圍(wei)護(hu)結構模型如圖1SUS 304鋼板(ban)，厚度(du)(du)1.2 mm。

    數值計算與分析

    在計算(suan)中箱內(nei)空(kong)氣溫(wen)(wen)度(du)用升溫(wen)(wen)工況(kuang)下的(de)箱內(nei)空(kong)氣溫(wen)(wen)度(du)擬(ni)合函數(shu)的(de)離(li)散點來表(biao)示(shi)，3個工況(kuang)的(de)溫(wen)(wen)度(du)擬(ni)合函數(shu)詳見表(biao)2 

    從圖可以看出，不同(tong)(tong)工(gong)況(kuang)下的(de)節(jie)(jie)點(dian)溫(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)度(du)(du)(du)(du)(du)(du)變化(hua)是(shi)(shi)相(xiang)似(si)的(de)，其(qi)共(gong)同(tong)(tong)點(dian)是(shi)(shi):各(ge)節(jie)(jie)點(dian)溫(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)度(du)(du)(du)(du)(du)(du)都是(shi)(shi)在(zai)(zai)(zai)(zai)升(sheng)(sheng)高(gao)的(de)，升(sheng)(sheng)溫(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)過(guo)程中圍護結構(gou)(gou)外(wai)壁(bi)始終在(zai)(zai)(zai)(zai)吸(xi)熱(re)；在(zai)(zai)(zai)(zai)外(wai)壁(bi)面溫(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)度(du)(du)(du)(du)(du)(du)升(sheng)(sheng)到環(huan)境溫(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)度(du)(du)(du)(du)(du)(du)之前，圍護結構(gou)(gou)內部始終存在(zai)(zai)(zai)(zai)著溫(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)度(du)(du)(du)(du)(du)(du) 的(de)一點(dian)，并且(qie) 溫(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)度(du)(du)(du)(du)(du)(du)點(dian)的(de)大小(xiao)隨時間(jian)(jian)(jian)在(zai)(zai)(zai)(zai)不斷地上升(sheng)(sheng)同(tong)(tong)時又在(zai)(zai)(zai)(zai)向外(wai)壁(bi)側(ce)推移，在(zai)(zai)(zai)(zai)這期間(jian)(jian)(jian)圍護結構(gou)(gou)從內、外(wai)兩側(ce)空氣(qi)吸(xi)熱(re)，只(zhi)有(you)外(wai)壁(bi)面溫(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)度(du)(du)(du)(du)(du)(du)高(gao)于(yu)環(huan)境溫(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)度(du)(du)(du)(du)(du)(du)之后(hou)，圍護結構(gou)(gou)傳熱(re)方向才是(shi)(shi)單向朝(chao)箱外(wai)的(de)；在(zai)(zai)(zai)(zai)保(bao)溫(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)4 h之后(hou)節(jie)(jie)點(dian)溫(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)度(du)(du)(du)(du)(du)(du)基(ji)本呈(cheng)線(xian)性分布，通 過(guo)計算可確(que)定在(zai)(zai)(zai)(zai)保(bao)溫(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)6 h之后(hou)圍護結構(gou)(gou)完全進入傳熱(re)穩態(tai)。這3個工(gong)況(kuang)不同(tong)(tong)點(dian)在(zai)(zai)(zai)(zai)于(yu)升(sheng)(sheng)溫(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)時間(jian)(jian)(jian)的(de)長(chang)短不同(tong)(tong)，這是(shi)(shi) 由裝、空載(zai)情況(kuang)和溫(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)變區間(jian)(jian)(jian)所(suo)決定的(de)。裝載(zai)要比空載(zai)需要的(de)升(sheng)(sheng)溫(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)時間(jian)(jian)(jian)長(chang)，溫(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)變區間(jian)(jian)(jian)跨度(du)(du)(du)(du)(du)(du)越(yue)(yue)小(xiao)則升(sheng)(sheng)溫(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)(wen)時間(jian)(jian)(jian)越(yue)(yue)短。

    結語

    1）圍護結構由低溫升至高溫時，其內部溫度分布存在著一個溫度點，且溫度點的大小隨著時間的推進在不斷地上升同時又在向外壁側推移，在這期間 圍護結構從內、外兩側空氣吸熱。只有外壁面溫度高于環境溫度之后，圍護結構傳熱方向才是單 向朝箱外的。
    2）升溫過程中，高低溫試驗箱內空氣與圍護結構內壁面之間的溫差一直在增大，只有箱內空氣溫度達到設定值后 溫差才開始逐步回落。
    3）圍護結構非穩態導熱時在不同時刻、不同節點的溫度變化速率是不相同的，計算其 自身耗熱量十分困難，引人圍護結構質量系數就可以很容易地求出圍護結構的吸熱量和升溫過程平均耗熱量了。