【導讀】mmmmmrrrar/)/(???93DU1,95DU1].

　　

【正文】 experienced most recirculation with results as shown in figure . Recirculation appears to be a maximum at Vw/Vp ＝ . 內蒙古工業(yè)大學本科畢業(yè)論文 17 Figure : Recirculation in sixcell cooling tower. Figure : Recirculation in a counterflow cooling tower consisting of two threecell rows, two cells apart. As shown in figure recirculation is considerably reduced when the two rows of three cells each are separated by a distance of two cells. Further separation does not reduce recirculation much. By placing two rows of three cells each side by side, recirculation is relatively high as shown in figure . 內蒙古工業(yè)大學本科畢業(yè)論文 18 Figure : Recirculation in cooling tower consisting of two rows of three cells located side by side. If the two rows of three cells are separated by one cell width only a relatively small reduction in maximum recirculation is experienced as is shown in figure . Based on these results it may be concluded that a row of induced draft cooling tower cells should be arranged inline with the prevailing wind direction. A high air outlet velocity and the addition of a diffuser will also tend to reduce recirculation. Bender et al. [97BE1] numerically analyzed the air flow into a counterflow induced draft cooling tower consisting of two adjacent cells of the type shown in figure (b) with a view to reducing or eliminating ice formation at the tower inlet during windy periods in winter. The dimensions of the tower they studied were m (width), m (length) and m (height) with an intake height of m. The stack or diffuser diameter was m and its height was m. Ice buildup tends to be most prevelant at the windward facing intake where the entering air flow rate is higher than on the leeward intake. By placing a 10 percent porous wall m in height, m in front of the cooling tower inlet, the air flow entering on either intake was found to be essentially the same. Tesche [96TEl] conducted model tests to determine the effect of recirculation and interference on the performance of rows of induced draft hybrid cooling tower cells (similar to the unit or cell shown in figure ). His results are applicable in cases where the wind velocity distribution is given by Vw/Vwr = (Z/Zr). It is found that the recirculation of individual cells in a row consisting of twelve ceils varies as shown in figure . All wind speeds are specified at 10 m above ground level. 內蒙古工業(yè)大學本科畢業(yè)論文 19 Figure : Recirculation in cooling tower consisting of two rows of three cells separated by one cell width. Figure : Re, circulation in a row consisting of twelve hybrid cooling tower cells. The lowest recirculation is observed when the wind blows in the direction of the major axis of the cell row. The influence of the number of cells under these conditions is shown in figure . 內蒙古工業(yè)大學本科畢業(yè)論文 20 Figure : Recirculation as a function of number of cells in row. The influence of the ratio of wind speed to plume exhaust speed Vw/Vp on recirculation is shown in figure . A maximum recirculation occurs at a wind speed of 5 m/s. Figure : Recirculation as function of speed ratio. When two rows of six ceils each are placed next to each other with their major axes in parallel, the resultant average interference for different spacings between them is shown in figure . The interference for rows of twelve cells are shown in figure . 內蒙古工業(yè)大學本科畢業(yè)論文 21 Figure : Interference for two rows of six cells at different spacings. Figure : Interference for two rows of twelve ceils at different spacings. Recirculating plume air increases the effective wetbulb temperature at the inlet to the cooling tower as shown in figure . Since this increase is not only a function of the wetbulb temperature of the ambient air, but also of the thermodynamic state of the plume air, figure is at best an indication of the trend in wetbulb temperature change. 內蒙古工業(yè)大學本科畢業(yè)論文 22 Figure : Increase in wetbulb temperature due to recirculation. 內蒙古工業(yè)大學本科畢業(yè)論文 1 空冷熱交換器和空冷塔 (本文譯自 MR KROGER 的空冷熱交換器和空冷塔一書 包括 ， ， ） 回流： 熱空氣在空冷換熱器中會出現(xiàn)回流現(xiàn)象，因此，會降低冷卻效率，圖 所示，為一個“ X”型空冷熱交換器，在無風的情況下，有浮力的水蒸氣在換熱器中垂直上升。盡管如此，還有一部分熱蒸汽會返回空冷塔的入口處，這種現(xiàn)象就是回流。 圖 強制空冷器氣流流動方式 由于實際性能的影響，回流會有許多不同的形式，這些影響的因素中包括換熱器的結構、布 置位置、周圍設備、外界空氣條件以及焓值得影響，此外，回流現(xiàn)象幾乎不受結構設計的制約，總會出現(xiàn)。 Lichtensein 定義了回流， mmmmmr rrar /)/( ??? () 如公式 ()其中 mr是回熱空氣的質量流動速率， ma是外界空氣流入空氣的換熱器的流動速率。 現(xiàn)在，盡管有很多研究關于回流的文章，但它們大部分還是依據(jù)幾何形狀試驗特性得到的結論，并沒有一個完整的理論根據(jù)。 Gunter 和 Shipes 在回流的流動方面定義了一個限制，它同時也包含了最新的關于此領域的研究測試結果?，F(xiàn)在報道的一些文章，是應用數(shù)字分析的方式來解決回熱內蒙古工業(yè)大學本科畢業(yè)論文 2 流動的問題。 Kroger 發(fā)現(xiàn)了另一種分析的方法，即用實驗、數(shù)字、推導了一個較為合理的公式，在這個公式中包含了換熱器回熱一些主要的特性參數(shù)都是可以預計的。 回流分析： 考慮到空冷熱交換器中回流現(xiàn)象的發(fā)生，對此，要進行系統(tǒng)的分析。分析的目的在于找到一個合理解決辦法。由于海拔高度的不同，不同的換熱器擁有自己不同的特性。如圖 (a)所示。 圖 熱交 換器工質流動方式 假設空氣沿著換熱器的邊緣進入的速度與水平位置的入口速率相同，為 vi，出口速度為 vo。 ，在豎直方向也是相同的?？紤]到換熱器在出口處會形成一條特殊的蒸汽線，它會從 1處和回流蒸汽的出口處分開，蒸汽現(xiàn)進入 2 位置，還有一些進入換熱器下 Hr處，如果考慮黏度的影響，蒸汽在混合和換熱時，外界環(huán)境可以忽略， Bernoulli的公式可以在 1與 2之間應用 ()。 )(2/)(2/ 222211 rioorioo HHgvpHHgvp ??????? ???? () 假設 1處的所有壓力相等，與外界相通水平面下的大氣壓 力相比較，是很合理的如公式 ()所示。 1211 2/ ao pvp ?? ? () 2處的凈壓可以表示為 ()， 2/2222 vpp aa ??? () 此時， ()。 21 2 araa pgHp ?? ? () 把公式 ()、 ()、 ()代入公式 ()可以得到公式 ()。 內蒙古工業(yè)大學本科畢業(yè)論文 3 gv