【摘要】：在天然氣液化工業(yè)領(lǐng)域中,廣泛采用非共沸混合工質(zhì)低溫制冷技術(shù),其中,中小型天然氣液化流程的冷箱主要采用板翅式換熱器。冷箱內(nèi)工質(zhì)換熱的溫度滑移較大,可達(dá)到200°C左右。在其低溫蒸發(fā)段,混合制冷工質(zhì)干度低,流速慢,尤其在天然氣液化負(fù)荷較低的工況下,換熱器的換熱性能下降。然而,這種工況下板翅式換熱器的換熱、工質(zhì)液相積存等問題的理論和實(shí)驗(yàn)研究仍然欠缺。為了研究低溫低干度混合工質(zhì)在板翅式換熱器中的換熱性能,建立具有實(shí)際參考價(jià)值的理論預(yù)測模型,本文搭建了一套以板翅式換熱器為低溫段回?zé)嵫b置、以類似于天然氣液化流程所用的混合工質(zhì)(N2+CH4+C2H4+C3H8+iC4H10)為制冷劑、以普通商用渦旋壓縮機(jī)驅(qū)動的低溫制冷系統(tǒng),在獲取最低-160°C低溫的前提下,對低干度低流速混合工質(zhì)在板翅式換熱器中的換熱性能進(jìn)行了研究;同時(shí),整理了國內(nèi)外學(xué)者對混合工質(zhì)冷凝及沸騰換熱關(guān)聯(lián)式的研究,基于傳熱單元模型對板翅式換熱器的總傳熱系數(shù)進(jìn)行了理論預(yù)測,并與實(shí)驗(yàn)結(jié)果進(jìn)行了對比分析,同時(shí),基于實(shí)驗(yàn)數(shù)據(jù)對部分關(guān)聯(lián)式進(jìn)行了修正。在此基礎(chǔ)上,對應(yīng)用于天然氣液化流程的板翅式換熱器的優(yōu)化設(shè)計(jì)進(jìn)行改進(jìn),采用遺傳算法,以單位熵增數(shù)為目標(biāo)函數(shù),冷熱流壓降為約束條件對板翅式換熱器中通道和翅片參數(shù)進(jìn)行尋優(yōu),并提出板翅式換熱器設(shè)計(jì)的改進(jìn)方向�；旌瞎べ|(zhì)的傳熱特性與流動特性密切相關(guān),工質(zhì)在系統(tǒng)中的液相積存不僅影響換熱器的換熱性能,還會影響混合工質(zhì)的循環(huán)濃度,系統(tǒng)的總體性能也因此改變。因此,本文針對混合工質(zhì)低溫制冷循環(huán)中的降溫特性、回?zé)崞髦械睦錈崃鳒夭钭兓Q熱器負(fù)荷變化以及液相積存等問題進(jìn)行了分析討論。另外,基于混合工質(zhì)一維豎直流動方程建立了低溫制冷系統(tǒng)中混合工質(zhì)液相積存的數(shù)學(xué)模型,分析討論了低溫制冷循環(huán)中混合工質(zhì)的液相積存特性,以及混合工質(zhì)循環(huán)濃度隨降溫過程的變化規(guī)律。天然氣液化流程中混合工質(zhì)的循環(huán)濃度對系統(tǒng)效率和穩(wěn)定運(yùn)行起著至關(guān)重要的作用,然而,在實(shí)際運(yùn)行過程中很難將運(yùn)行循環(huán)濃度調(diào)節(jié)至與優(yōu)化濃度一致。通過對SMR液化流程的優(yōu)化發(fā)現(xiàn),在以系統(tǒng)(火用)效率為目標(biāo)函數(shù)的前提下對混合工質(zhì)進(jìn)行尋優(yōu),可以得到差異較大的不同混合工質(zhì)循環(huán)濃度,同時(shí),系統(tǒng)在獲取較高(大于0.4)的(火用)效率同時(shí),換熱器中溫差匹配均勻合理。另外,在不同的預(yù)冷溫度、系統(tǒng)高低壓的情況下,優(yōu)化的循環(huán)濃度均能使系統(tǒng)保持高效運(yùn)行,這說明混合工質(zhì)的濃度配比在天然氣液化流程中存在較強(qiáng)的魯棒性;另外,本文提出了SMR液化流程的調(diào)節(jié)方案,在實(shí)際工程應(yīng)用中具有一定的參考價(jià)值。
[Abstract]:In the field of natural gas liquefaction industry, non-azeotropic mixture cryogenic refrigeration technology is widely used, in which plate-finned heat exchanger is mainly used in the cold box of small and medium-sized natural gas liquefaction process. The temperature slip of working fluid heat transfer in cold box is large, which can reach about 200 擄C. In the low temperature evaporation section, the dryness of the mixed refrigerating fluid is low and the flow rate is slow, especially under the condition of low liquefaction load of natural gas, the heat transfer performance of the heat exchanger decreases. However, the theoretical and experimental research on the heat transfer and liquid phase accumulation of plate-wing heat exchanger under this condition is still lacking. In order to study the heat transfer performance of low temperature and low dryness mixed working fluid in plate-wing heat exchanger and establish a theoretical prediction model with practical reference value, a set of plate-wing heat exchanger is built in this paper. Using a mixture (N2 CH4 C2H4 C3H8 iC4H10) similar to that used in a natural gas liquefaction process as a refrigerant and a cryogenic refrigeration system driven by a commercial scroll compressor, on the premise of obtaining a minimum temperature of-160C, The heat transfer performance of low dryness and low velocity mixed working fluid in plate-finned heat exchanger was studied. At the same time, the correlation of condensation and boiling heat transfer of mixed working fluid is studied by scholars at home and abroad. Based on the heat transfer unit model, the total heat transfer coefficient of plate-finned heat exchanger is predicted theoretically, and the results are compared with the experimental results. Based on the experimental data, part of the correlation is modified. On this basis, the optimal design of plate-wing heat exchanger applied to natural gas liquefaction process is improved, and the genetic algorithm is adopted, and the unit entropy increase is taken as the objective function. The pressure drop of cold and hot flow is used as the constraint condition to optimize the channel and finned parameters of the plate-finned heat exchanger, and the improvement direction of the design of the plate-finned heat exchanger is put forward. The heat transfer characteristics of the mixed working fluid are closely related to the flow characteristics. The liquid phase accumulation of the working fluid in the system not only affects the heat transfer performance of the heat exchanger, but also affects the circulating concentration of the mixed working fluid, and the overall performance of the system is also changed. Therefore, the cooling characteristics in the low temperature refrigeration cycle of mixed working fluid, the temperature difference of cold and heat flow in the regenerator, the load change of the heat exchanger and the liquid phase accumulation are analyzed and discussed in this paper. In addition, based on the one-dimensional vertical flow equation of mixed working fluid, the mathematical model of liquid phase accumulation of mixed working fluid in low temperature refrigeration system is established, and the liquid phase accumulation characteristics of mixed working fluid in low temperature refrigeration cycle are analyzed and discussed. And the variation of the concentration of mixed working fluid cycle with the cooling process. The circulating concentration of mixed working fluid in natural gas liquefaction process plays an important role in the efficiency and stable operation of the system. However, it is difficult to adjust the operating cycle concentration to be consistent with the optimized concentration in the actual operation process. Through the optimization of SMR liquefaction process, it is found that the cycle concentration of different mixtures can be obtained by optimizing the mixture under the premise of system (exergy) efficiency as the objective function. At the same time, the temperature difference matching in the heat exchanger is uniform and reasonable when the exergy efficiency is higher than 0.4. In addition, under the condition of different pre-cooling temperature and high and low pressure of the system, the optimized cycle concentration can keep the system running efficiently, which indicates that the concentration ratio of the mixture has strong robustness in the natural gas liquefaction process. In addition, this paper puts forward the adjustment scheme of SMR liquefaction process, which has certain reference value in practical engineering application.
【學(xué)位授予單位】：華南理工大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2016
【分類號】：TB657

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