白通通，南曉紅，金寶紅，初正鵬

豎壁貼附送風改善冷藏庫內(nèi)流場特性

白通通，南曉紅※，金寶紅，初正鵬

（西安建筑科技大學建筑設(shè)備科學與工程學院，西安 710055）

流場；冷藏庫；豎壁貼附送風；數(shù)值模擬；不均勻系數(shù)；能量利用系數(shù)

0 引 言

冷藏庫內(nèi)溫度和速度的均勻性對果蔬的貯藏品質(zhì)具有重要的影響。其溫度的不均勻性將加劇局部果蔬的變質(zhì)，風速的不均勻性將引起果蔬的干耗，從而縮短果蔬的貯藏期，降低果蔬的經(jīng)濟價值。而冷藏庫內(nèi)溫度與速度分布主要取決于冷庫的氣流組織方式。

流場均勻性的問題一直是冷庫環(huán)境調(diào)控研究的熱點問題，CFD（computational fluid dynamics）模擬論證了氣流組織方式對冷藏庫和冷藏車內(nèi)的流場分布具有重要性[1-3]。Akdemir等[4]采用CFD模擬的方法研究了冷風機送風和孔板送風方式下冷藏庫內(nèi)溫度與相對濕度的分布，闡明了孔板送風能夠有效地提高冷藏庫內(nèi)流場分布的均勻性。Moureh等[5]為了改善冷藏車內(nèi)的換氣效率和氣流分布而設(shè)計了一種具有新型結(jié)構(gòu)的側(cè)向風口，并研究了其流場的分布特性，通過對比中央風口所形成的流場分布特性，發(fā)現(xiàn)該側(cè)向風口可以實現(xiàn)更高的換氣效率和更為均勻的流場。杜子崢等[6]應(yīng)用計算流體動力學的方法，研究了風機的擺放位置對冷藏庫內(nèi)流場和溫度場分布的影響。劉曉菲等[7]通過數(shù)值模擬的方法闡明了均勻送風可以有效地改善冷藏庫內(nèi)溫度和相對濕度的分布。由此可見，合理的氣流組織方式能夠有效地改善冷藏庫內(nèi)的流場分布特性。豎壁貼附送風是在混合通風的基礎(chǔ)上發(fā)展起來的一種新型的通風方式。該送風方式不僅能夠消除圍護結(jié)構(gòu)傳熱形成的負荷，而且可以實現(xiàn)置換通風的效果。為此，本研究將豎壁貼附送風方式引入果蔬冷藏庫，探索其在低溫貯藏環(huán)境應(yīng)用的特性。本文以西安市某50 t蘋果冷藏庫為研究對象，研究豎壁貼附送風模式下冷藏庫內(nèi)的流場分布特性，并與傳統(tǒng)的冷風機直吹送風方式進行對比，以此來闡明豎壁貼附送風能夠有效地改善冷藏庫內(nèi)的流場分布特性。

1 模型與方法

1.1 物理模型

本文以50 t蘋果冷藏庫為研究對象，該冷藏庫的尺寸為8.0 m（長）×4.6 m（寬）×6.5 m（高）。根據(jù)冷庫設(shè)計規(guī)范計算該冷庫的冷卻設(shè)備負荷，并確定冷風機送風量。該冷風機尺寸為1.6 m（長）×0.5 m（寬）×0.5 m（高）。為了實現(xiàn)豎壁貼附送風，設(shè)計了一個尺寸為4.0 m（長）×0.5 m（寬）×0.5 m（高）的靜壓箱，該靜壓箱底部開設(shè)長度為4.0 m，寬度為0.1 m的條形風口用于實現(xiàn)向下的貼附送風?；仫L口設(shè)置在冷風機的側(cè)面，回風口的長度為1.6 m，寬度為0.5 m，其物理模型如圖1所示；作為對比，選擇冷負荷相同、送風溫差一致的冷風機直吹式冷庫作為比較對象。在該冷藏庫中，冷風機的送風口是2個直徑為0.4 m的圓形風口，回風口位于冷風機背面，距離墻壁0.5 m，其物理模型如圖2所示。

在2種送風方式下，4堆貨物平行擺放。其中，每個堆垛的尺寸為4.0 m（長）×1.2 m（寬）×5.0 m（高）。堆垛間距為0.3 m，距側(cè)墻分別為0.3 m，距地面0.5 m。

1.吊頂式冷風機 2.靜壓箱 3.條形送風口 4.回風口 5.貨物

1.吊頂式冷風機 2.貨物

1.2 數(shù)學模型

為了較為準確地反映冷藏庫內(nèi)的流場分布，本文采用三維不可壓的雷諾平均方程對冷藏庫進行建模。考慮到貨物區(qū)數(shù)學模型的復(fù)雜性，相關(guān)學者將其簡化為多孔介質(zhì)[8-9]，即在原有數(shù)學模型的基礎(chǔ)上附加了黏性阻力和慣性阻力引起的動量源項。經(jīng)過這樣的簡化后，該數(shù)學模型在穩(wěn)態(tài)工況下的表達式如下所示[10-12]

假設(shè)空氣密度僅是溫度的函數(shù)，即空氣密度的變化遵循Boussinesq假設(shè)，由此動量方程可以表示為

表1 多孔介質(zhì)區(qū)的孔隙率及阻力系數(shù)

假設(shè)空氣和蘋果處于局部熱平衡狀態(tài)（空氣和多孔固體基質(zhì)的溫度被假定為相等），則能量方程可以表示為

為了保證數(shù)值模擬的準確性和數(shù)學模型的可靠性，必須對其計算結(jié)果進行一定的驗證。數(shù)值模擬的準確性主要取決于離散步長[19]。為了避免離散步長對數(shù)值計算的影響，本文進行了網(wǎng)格無關(guān)性驗證。而穩(wěn)態(tài)數(shù)學模型的合理性主要取決于湍流模型的選擇以及邊界條件的設(shè)置。本文通過選取恰當?shù)耐牧髂Ｐ?，設(shè)置合理的邊界條件，并通過試驗擬合公式對其進行了驗證，來確保數(shù)學模型的可靠性。

1.2.1 網(wǎng)格無關(guān)性分析

a. 豎壁貼附線上的速度分布b. 冷藏庫中心線上的速度分布 a. Velocity distribution on vertical wall attachment lineb. Velocity distribution on centerline of refrigerator

1.2.2 湍流模型的選擇

在豎壁貼附送風中，空氣在射流沖擊區(qū)會發(fā)生較大的彎曲現(xiàn)象。而RNG、SST模型均對流體流動過程的強彎曲現(xiàn)象進行了考慮與修正[20]。但是，通過比較預(yù)測精度可以發(fā)現(xiàn)SST模型具有較好的預(yù)測性能[21]。于是，本文選取了SST湍流模型，并采用Simple算法[22]對該數(shù)學模型進行了求解。

1.2.3 邊界條件的設(shè)置

2）回風口邊界條件：回風口采用自由出流（outflow）邊界條件，即速度在流線方向的梯度為零。

1.2.4 模型驗證

為了驗證數(shù)值模擬計算的可靠性，該研究將空庫狀態(tài)下豎壁貼附軸線處的速度分布與尹海國[21]提出的豎壁貼附軸線處無因次速度擬合式進行了對比，其對比結(jié)果如圖4所示。

由圖4可知；數(shù)值模擬的無因次速度變化趨勢與擬合公式的無因次速度變化趨勢基本一致，其中無因次軸線速度的平均偏差為8.86%。引起該偏差的原因主要有2點：1）當無因次距離在10時，不同入口風速對應(yīng)的無因次軸線速度曲線是略高于擬合曲線的，這部分的誤差主要受送風速度、風口尺寸及貼附距離等因素的影響；2）當時，速度的無因次曲線是低于擬合曲線的，這部分的誤差主要受限于擬合公式擬合精度。由于偏差小于9%，因此，該結(jié)果可以驗證數(shù)值模擬的可靠性。

2 結(jié)果與分析

2.1 代表性截面與監(jiān)測線的選取

圖5 監(jiān)測面的三維示意圖

圖6 監(jiān)測線的平面布置圖

2.2 流場的速度監(jiān)測

2.2.1 監(jiān)測面的速度矢量

a. 豎壁貼附送風方式y(tǒng)=2.3 m截面 a. Vertical wall attached air supply mode y=2.3 m sectionb. 冷風機直吹送風方式y(tǒng)=1.85 m截面 b. Cooling fan direct blowing air supply mode y=1.85 m section

2.2.2 監(jiān)測線上的速度分布

a. 豎壁貼附送風方式a. Vertical wall attached with air supplyb. 冷風機直吹送風方式b. Cooling fan direct blowing air supply mode

2.3 流場的溫度監(jiān)測

2.3.1 代表截面的溫度監(jiān)測

圖9-10顯示了2種不同送風方式下典型截面的溫度分布圖，從圖中可以看出，2種不同的送風射流都能及時帶走冷庫壁面的熱流量，有效抑制了壁面的傳熱對冷藏庫貯藏環(huán)境的破壞。但是，相比冷風機直吹的送風方式，豎壁貼附送風方式形成的溫度場更為均勻。在冷風直吹送風方式下，氣流受送風射流的卷吸作用，使得一部分換熱后的氣流未被冷卻又隨射流進入循環(huán)，進而導致貨物上部區(qū)域的溫度較高。

圖9 兩種不同送風方式下z=3 m截面的溫度（K）分布

a. 豎壁貼附送風方式 a. Vertical wall attached with air supplyb. 冷風機直吹送風方式 b. Cooling fan direct blowing air supply mode

2.3.2 監(jiān)測線上的溫度分布

圖11分別描繪了2種送風方式下堆垛中心高度方向的溫度分布。從圖中可以看出：2種送風方式下堆垛中心處溫度變化趨勢存在較大的差異。在豎壁貼附送風方式下，貨物區(qū)中心線處的溫度分布比較集中，主要處于273～273.4 K。而在冷風直吹的送風方式下，由于堆垛上部區(qū)域溫度較高，導致貨物區(qū)溫度分布范圍為272.8～274.4 K。主要是由于頂部射流對氣流有較強的卷吸作用，使得換熱后溫度較高的氣流向上運動形成的。

a. 豎壁貼附送風方式a. Vertical wall attached air supplyb. 冷風機直吹送風方式b. Cooling fan direct blowing air supply

3 氣流組織的評價

根據(jù)通風（空調(diào)）的目的，可以從3個方面來描述和評價氣流組織：送風的有效性、污染物排除的有效性及熱舒適性等相關(guān)的參數(shù)。對于冷藏庫而言，貨物所處的溫度場、速度場以及換熱的有效性應(yīng)為關(guān)注重點。因此，本文選用能量利用系數(shù)和不均勻系數(shù)對2種送風方式進行綜合評價。

3.1 能量利用系數(shù)

不同送風方式下庫內(nèi)熱量移除的有效性，可以用能量利用系數(shù)來評價[23-24]，其定義如下

表2 兩種送風方式下的能量利用系數(shù)

注：t為送風溫度，t回風溫度，t貨物區(qū)平均溫度，能量利用系數(shù)。

Note:tis the supply air temperature,tis the return air temperature,tis the average temperature of the cargo area,is the energy utilization coefficient.

3.2 不均勻系數(shù)

冷藏庫內(nèi)氣流組織的均勻性對果蔬的貯藏品質(zhì)有著重要的影響，為此引用“不均勻系數(shù)”指標對冷藏庫內(nèi)的氣流組織進行評價。

圖12 堆垛測點分布示意圖

Fig 12 Distribution diagram of stacking measurement points

圖13 兩種送風方式下不均勻系數(shù)的對比

表3 兩種送風方式下的不均勻系數(shù)

4 結(jié) 論

本文以某蘋果冷藏庫為研究對象，采用數(shù)值模擬的方式，研究了豎壁貼附送風方式下冷藏庫內(nèi)流場的分布特性。通過與冷風機直吹送風方式的對比，證明了豎壁貼附送風模式可以形成更為均勻的速度場和溫度場，并能夠有效地提高送風的能量利用率。其詳細結(jié)論如下：

[1]Chourasia M K, Goswami T K. Steady state CFD modeling of airflow, heat transfer and moisture loss in a commercial potato cold store[J]. International Journal of Refrigeration, 2007, 30(4): 672－689.

[2]Ho S H, Rosario L, Rahman M M. Numerical simulation of temperature and velocity in a refrigerated warehouse[J]. International Journal of Refrigeration, 2010, 33(5): 1015－1025.

[3]Tapsoba M, Moureh J, Flick D. Airflow patterns in a slot-ventilated enclosure partially loaded with empty slotted boxes[J]. International Journal of Heat and Fluid Flow, 2007, 28(5): 963－977.

[4]Akdemir S, Ozturk S, Edis F O, et al. CFD modelling of two different cold stores ambient factors[J]. IERI Procedia, 2013(5): 28－40.

[5]Moureh J, Flick D. Airflow characteristics within a slot-ventilated enclosure[J]. International Journal of Heat and Fluid Flow, 2005, 26(1): 12－24.

[6]杜子崢，謝晶，朱進林. 數(shù)值模擬技術(shù)預(yù)測風機兩種擺放方式對冷庫堆垛貨物的影響[J]. 食品與機械，2015，31(3)：145－149. Du Zizheng, Xie Jing, Zhu Jinlin. Effects of two different fans arrangement on stacking cargo in cold store based on numerical simulation[J]. Food and Machinery, 2015, 31(3): 145－149. (in Chinese with English abstract)

[7]劉曉菲，南曉紅. 裝設(shè)均勻送風管道對冷藏庫氣流流場特性的改善[J]. 農(nóng)業(yè)工程學報，2016，32(1)：91－96. Liu Xiaofei, Nan Xiaohong. Improvement on characteristics of air flow field in cold storage with uniform air supply duct[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2016, 32(1): 91－96. (in Chinese with English abstract)

[8]Thanh V T, Van Brecht A. Vranken E. Modelling of three-dimensional air temperature distributions in porous media[J]. Biosystems Engineering, 2007, 96(3): 345－360.

[9]Ambaw A, Bessemans N, Gruyters W. Analysis of the spatiotemporal temperature fluctuations inside an apple cool store in response to energy use concerns[J]. International Journal of Refrigeration, 2016, 66: 156－168.

[10]陳寶明，劉芳，云和明. 多孔介質(zhì)自然對流傳熱傳質(zhì)[M]. 北京：科學出版社，2016.

[11]Antohe B V, Lage J L. A general two-equation macroscopic turbulence model for incompressible flow in porous media[J]. International Journal of Heat and Mass Transfer, 1997, 40(13): 3013－3024.

[12]Nakayama A, Kuwahara F. A Macroscopic turbulence model for flow in a porous medium[J]. Journal of Fluids Engineering, 1999, 121(2): 427－433.

[13]馮坤旋，南曉紅，楊巧銀. 果蔬冷庫進貨期間貨物溫度穩(wěn)定性的影響因素[J]. 農(nóng)業(yè)工程學報，2015，31(20)：294－300. Feng Kunxuan, Nan Xiaohong, Yang Qiaoyin, et al. Factors on goods temperature stability in fruits & vegetables cold storage during the loading process[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2015, 31(20): 294－300. (in Chinese with English abstract)

[14]Ansys Fluent, User’s Guide Southpointe[M]. ANSYS Inc, 2015.

[15]楊昭，徐曉麗，李喜宏. 保鮮庫三維非穩(wěn)態(tài)流場模擬及實驗[J]. 天津大學學報，2007，40(2)：157－162. Yang Zhao, Xu Xiaoli, Li Xihong. Simulation and Experiment on the unsteady 3D flow field of cool store[J]. Journal of Tianjin University, 2007, 40(2): 157－162. (in Chinese with English abstract)

[16]Lisowa H, Wujec M, Lis T. Influence of temperature and variety on the thermal properties of apples[J]. International Agrophysics, 2002, 16(1): 43－52.

[17]Delele M A, Schenk A, Tijskens E, et al. Optimization of the humidification of cold stores by pressurized water atomizers based on a multiscale CFD model[J]. Journal of Food Engineering, 2009, 91(2): 228－239.

[18]Hoang H M, Duret S, Flick D, et al. Preliminary study of airflow and heat transfer in a cold room filled with apple pallets: Comparison between two modelling approaches and experimental results[J]. Applied Thermal Engineering, 2015, 76: 367－381.

[19]Cheng Y, Niu J, Liu X, et al. Experimental and numerical investigations on stratified air distribution systems with special configuration. Thermal comfort and energy saving[J]. Energy and Buildings, 2013, 64(5): 154－161.

[20]王燁. 封閉腔內(nèi)湍流自然對流換熱及工程應(yīng)用[M]. 北京：科學出版社，2015.

[21]尹海國. 條縫型送風口豎壁貼附射流氣流組織設(shè)計方法研究[D]. 西安：西安建筑科技大學，2012. Yin Haiguo. Study on Design Procedures of Air Distribution by Air Curtain Ventilation with A Linear Slot Diffuser[D]. Xi’an: Xi′an University of Architecture and Technology, 2012. (in Chinese with English abstract)

[22]謝晶，瞿曉華，徐世瓊. 冷藏庫內(nèi)氣體流場數(shù)值模擬與驗證[J]. 農(nóng)業(yè)工程學報，2005，21(2)：11－16. Xie Jing, Qu Xiaohua, Xu Shiqiong. Numerical simulation and verification of air flow in cold-store[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2005, 21(2): 11－16. (in Chinese with English abstract)

[23]Rhee K, Shin M, Choi S. Thermal uniformity in an open plan room with an active chilled beam system and conventional air distribution systems[J]. Energy and Buildings, 2015, 93: 236－248.

[24]Michal Kraj?ík, Angela Simone, Bjarne W. Air distribution and ventilation effectiveness in an occupied room heated by warm air[J]. Energy and Buildings, 2012, 55: 94－101.

[25]Zhang Y, Liu J, Pei J. Performance evaluation of different air distribution systems in an aircraft cabin mockup[J]. Aerospace Science and Technology, 2017, 70: 359－366.

[26]Li A G, Liu Z J, et al. Reduced-scale model study of ventilation for large space of generatrix floor in Hohhot underground hydropower station[J]. Energy and Buildings, 2011, 43(4): 1003－1010.

Improvement on characteristics of air flow field in cold storage with vertical wall attached ventilation

Bai Tongtong, Nan Xiaohong※, Jin Baohong, Chu Zhengpeng

(710055)

During the storage of fruits and vegetables, the uniformity of air distribution inside the cold storage store is crucial to affect both the storage quality of fruits or vegetables and energy efficiency of supply air. In the traditional cooling fan direct blowing air supply mode, some local air velocities and temperatures are easily to be larger than those required for cargo area, which would greatly reduce the uniformity of air distribution and the storage quality of fruits and vegetables. Therefore, the reasonable airflow organization is critical to the air distribution of the cold storage and must be carefully considered. In order to improve the uniformity of indoor air distribution, the study introduced the vertical wall attached ventilation which originated applied for indoor climate control of public buildings. To investigate the flow characteristics of the vertical wall attached ventilation, a three-dimensional SSTsolution model was established to study the distribution characteristics of the flow field and the cooling effect of the stored cargo. According to the practical array of the stored apples, the cargo area was regarded briefly as porous medium zone. In order to ensure the accuracy of the numerical calculation and the rationality of the mathematical model, grid independence verification and experimental verification were carried out. The accuracy of numerical calculation was studied by comparison with some accepted correlations and the rationality of the mathematical model was proved. The velocity distribution and temperature distribution of typical sections were monitored to illustrate the air flow pattern and temperature distribution characteristics under the vertical wall attached ventilation mode. Meanwhile, compared with the traditional air supply by cooling fan for direct blowing, it was clarified that the vertical wall attached ventilation could form a more uniformly temperature distribution and velocity distribution on the monitoring section and monitoring line. However, the air distribution of monitoring section and monitoring line could not fully reflect the overall temperature and velocity distribution inside the cold storage. Furthermore, the air distribution evaluation index was introduced and calculated in order to fully understand the temperature distribution characteristics of the studied cold storage room. First of all, in order to reflect the uniformity of air distribution, the non-uniformity coefficient of temperature and non-uniformity of velocity were introduced. According to the temperature and velocity values of 40 measuring points in each stack, the velocity non-uniformity coefficient and temperature non-uniformity coefficient were calculated under two air distribution modes. It was manifested that the vertical wall attached ventilation enabled non-uniformity coefficient of temperature and non-uniformity of velocity to decrease by 31% and 47%, respectively at the same air supply flow rate and temperature. Secondly, to evaluate the energy utilization of the air supply, the energy utilization efficiency of the two modes was calculated from monitoring air supply temperature, air return temperature and average temperature in the cargo area. It was indicated that the vertical wall attached ventilation made the energy utilization increase by 19% due to the more sufficient heat exchange between the air and cargo. Since the vertical wall attached ventilation can form a more uniform air distribution under a higher energy utilization rate, it can effectively improve indoor air distribution characteristics and well meet the storage requirements of fruits and vegetables.

flow field; cold storage store; vertical wall attached ventilation; numerical simulation; non-uniformity coefficient; energy efficiency

白通通，南曉紅，金寶紅，初正鵬. 豎壁貼附送風改善冷藏庫內(nèi)流場特性[J]. 農(nóng)業(yè)工程學報，2019，35(22)：331－337. doi：10.11975/j.issn.1002-6819.2019.22.039 http://www.tcsae.org

Bai Tongtong, Nan Xiaohong, Jin Baohong, Chu Zhengpeng. Improvement on characteristics of air flow field in cold storage with vertical wall attached ventilation[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2019, 35(22): 331－337. (in Chinese with English abstract) doi：10.11975/j.issn.1002-6819.2019.22.039 http://www.tcsae.org

2019-04-14

2019-10-05

陜西省自然科學基礎(chǔ)研究計劃項目（2018JM3038）

白通通，研究方向為冷藏庫內(nèi)氣流組織的數(shù)值模擬。Email：1910686979@qq.com

南曉紅，教授，博士，主要從事制冷技術(shù)領(lǐng)域的科研與教學工作。Email：nanxh@xauat.edu.cn

10.11975/j.issn.1002-6819.2019.22.039

TB61+1

A

1002-6819(2019)-22-0331-07